Excitable and Contractile Tissues 2017-2018 PDF

EXCITABLE AND CONTRACTILE TISSUES Dr. E. O. Aihie, Department of Physiology, School of Basic Medical Sciences, College of Medical Sciences, University of Benin. 1/29/2018 Jesus is Lord 1 ...

EXCITABLE AND CONTRACTILE TISSUES Dr. E. O. Aihie, Department of Physiology, School of Basic Medical Sciences, College of Medical Sciences, University of Benin. 1/29/2018 Jesus is Lord 1  INTRODUCTORY AND GENERAL PHYSIOLOGY (PHS212 + OPT218) PHARMACY/OPTOMETRY/NURSING SCIENCE 1. Introductory and Control System - Dr. C.O. Azubike 2. Excitable and Contractile Tissues – Dr. E.O. Aihie 3. Blood and Body Fluid Physiology - Mr. C.A. Inneh 1/29/2018 Jesus is Lord 2  basic Punctuality Feel free to ask questions on what has been taught so that I can explain better Please don’t get distracted during the lecture Contributions to the lecture is welcome, no one has the monopoly of knowledge 1/29/2018 Jesus is Lord 3  Contd. If you want to make a “Distinction”, the time to start “thinking about it” and “putting in your efforts” to achieve it is “NOW”. Above all these, God is the One who can sustain you through life. O LORD, I know that the path of [life of] a man is not in himself; It is not within [the limited ability of] man [even one at his best] to choose and direct his steps [in life]. (Jeremiah 10:23) 1/29/2018 Jesus is Lord 4  Importance of Physiology The goal of physiology is to explain the physical and chemical factors that are responsible for the origin, development, and progression of life (Chapter 1 of Guyton) Human Physiology. In human physiology, we attempt to explain the specific characteristics and mechanisms of the human body that make it a living being. “The physiology of today is the medicine of tomorrow”. Ernest H. Starling, Physiologist (1926) Physiology deals with life. 1/29/2018 Jesus is Lord 5  Revision The basic living unit of the body is the cell. Each organ is an aggregate of many different cells held together by intercellular supporting structures. Each type of cell is specially adapted to perform one or a few particular functions. For instance, the red blood cells, numbering about 25 trillion in each human being, transport oxygen from the lungs to the tissues. Although the red blood cells are the most abundant of any single type of cell in the body, about 75 trillion additional cells of other types perform functions different from those of the red blood cell. 1/29/2018 Jesus is Lord 6  contd. About 60 percent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. This extracellular fluid is in constant motion throughout the body. In the extracellular fluid are the ions and nutrients needed by the cells to maintain life. 1/29/2018 Jesus is Lord 7  contd. A typical cell has two major parts: the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the surrounding fluids by a cell membrane, also called the plasma membrane. The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates. 1/29/2018 Jesus is Lord 8  Cell Membrane The Cell Membrane Lipid Barrier Impedes Penetration by Water-Soluble Substances. Its basic structure is a lipid bilayer, which is a thin, double- layered film of lipids—each layer only one molecule thick— that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. 1/29/2018 Jesus is Lord 9 Structure of the cell membrane, showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. (Modified from Lodish HF, Rothman JE: The assembly of cell membranes. Sci Am 240:48, 1979. Copyright George V. Kevin.) 1/29/2018 Jesus is Lord 10  contd. Many of the 1integral proteins provide structural channels (or pores) through which i. water molecules and ii. water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. 1/29/2018 Jesus is Lord 11  contd. Most substances pass through the cell membrane by diffusion and active transport. Diffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance Very large particles enter the cell by a specialized function of the cell membrane called endocytosis. The principal forms of endocytosis are pinocytosis and phagocytosis. 1/29/2018 Jesus is Lord 12  INTRODUCTION TO EXCITABLE AND CONTRACTILE TISSUES What is irritability? An ability of all living tissues to respond to stimuli (either external or internal environment) What is excitability? An ability of specialized cells to respond to certain stimuli by producing electrical signals known as action potential at its membrane 1/30/2018 Jesus is Lord 13  contd. Excitable Tissues Tissues which are capable of generation and transmission of electrochemical impulses along the membrane e.g Nerve tissue and mucle tissue Contractile Tissues Tissues which are capable of shortening and generating a pulling force. e.g mucle tissue: skeletal, cardiac, and smooth. 1/30/2018 Jesus is Lord 14  Nerve cell Nerve tissue consists of nerve cells called neurons. Neuron or nerve cell is defined as the structural and functional unit of the nervous system. Neuron is similar to any other cell in the body, having nucleus and all the organelles in cytoplasm. However, it is different from other cells in two ways: 1. Neuron has branches or processes called axon and dendrites 2. Neuron does not have centrosome. So, it cannot undergo division. 1/30/2018 Jesus is Lord 15  Assignment no 1 Read up the classification of neurons 1/30/2018 Jesus is Lord 16  STRUCTURE OF NEURON Neurons occur in a wide variety of sizes and shapes. Structurally, the typical spinal motor neuron is made up of three parts: 1. The cell body (soma) 2. Dendrite 3. Axon. Dendrite and axon form the processes of neuron. 1/30/2018 Jesus is Lord 17  THE CELL BODY The cell body is also known as soma It is irregular in shape. As in other types of cells, a neuron’s cell body (or soma) contains the nucleus and ribosomes and thus has the genetic information and machinery necessary for protein synthesis. Like any other cell, it is constituted by a mass of cytoplasm called neuroplasm, which is covered by a cell membrane. 1/30/2018 Jesus is Lord 18  DENDRITES The dendrites are a series of highly branched outgrowths of the cell body. In the PNS, dendrites receive incoming sensory information and transfer it to integrating regions of sensory neurons. In the CNS, dendrites and the cell body receive most of the inputs from other neurons, with the dendrites generally taking a more important role. Branching dendrites increase a cell’s surface area—some neurons may have as many as 400,000 dendrites. The structure of dendrites in the CNS increases a cell’s capacity to receive signals from many other neurons. 1/30/2018 Jesus is Lord 19  AXON Sometimes also called a nerve fiber, is a long process that extends from the cell body and carries outgoing signals to its target cells. In humans, axons range in length from a few microns to over a meter. The region of the axon that arises from the cell body is known as the initial segment (or axon hillock ). The initial segment is the “trigger zone” where, in most neurons, 1/30/2018 propagated electrical signals are generated. 20 Jesus is Lord  Contd. These signals then propagate away from the cell body along the axon or, sometimes, back along the dendrites. The axon may have branches, called collaterals. Near their ends, both the axon and its collaterals undergo further branching. The greater the degree of branching of the axon and axon collaterals, the greater the cell’s sphere of influence. 1/30/2018 Jesus is Lord 21  Contd. 1/30/2018 Jesus is Lord 22  Contd. Fig. A diagram of a Motor neuron with a myelinated axon 1/30/2018 Jesus is Lord 23  Contd. Each branch ends in an axon terminal, which is responsible for releasing neurotransmitters from the axon. These chemical messengers diffuse across an extracellular gap to the cell opposite the terminal. Alternatively, some neurons release their chemical messengers from a series of bulging areas along the axon known as varicosities. The axons of many neurons are covered by sheaths of myelin, which usually consists of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby 1/30/2018 supporting cell. Jesus is Lord 24  Contd. In the brain and spinal cord, the myelin-forming cells are the oligodendrocytes. In the PNS, the myelin-forming cells are the Schwann cells. The spaces between adjacent sections of myelin where the axon’s plasma membrane is exposed to extracellular fluid are called the nodes of Ranvier. The myelin sheath speeds up conduction of the electrical signals along the axon and conserves energy. 1/30/2018 Jesus is Lord 25  Contd. To maintain the structure and function of the cell axon, various organelles and other materials must move as far as between the cell body and the axon terminals. This movement is termed axonal transport Movement from the cell body toward the axon terminals (anterograde) and is important in moving : nutrient molecules, enzymes, mitochondria, neurotransmitter-filled vesicles, and other organelles. Jesus is Lord 26 1/30/2018  Contd. Movement in the other direction (retrograde), carrying : recycled membrane vesicles, growth factors, and other chemical signals that can affect the neuron’s morphology, biochemistry, and connectivity. Retrograde transport is also the route by which some harmful agents invade the CNS, including tetanus toxin and the herpes simplex, rabies, and polio viruses. 1/30/2018 Jesus is Lord 27  Functions of Myelin Sheath 1. Faster conduction Myelin sheath is responsible for faster conduction of impulse through the nerve fibers. In myelinated nerve fibers, the impulses jump from one node to another node. This type of transmission of impulses is called saltatory conduction. 2. Insulating capacity Myelin sheath has a high insulating capacity. Because of this quality, myelin sheath restricts the nerve impulse within single nerve fiber and prevents the stimulation of neighboring nerve fibers. 1/30/2018 Jesus is Lord 28  Contd. Functionally, neurons generally have four important zones: 1. A receptor, or dendritic zone, where multiple local potential changes generated by synaptic connections are integrated; 2. A site where propagated action potentials are generated (the initial segment in spinal motor neurons, the initial node of Ranvier in cutaneous sensory neurons); 3. An axonal process that transmits propagated impulses to the nerve endings; and 4. The nerve endings, where action potentials cause the release of synaptic transmitters. The cell body is often located at the dendritic zone end of the axon, but it can be within the axon (eg, auditory neurons) or attached to the side of the axon (eg, cutaneous neurons). Its location makes no difference as far as the receptor function of the dendritic zone and the transmission function of the axon are concerned. 1/30/2018 Jesus is Lord 29  EXCITATION & CONDUCTION What is excitability? Excitability is the ability of specialized cells to respond to certain stimuli by producing electrical signals known as action potential at its membrane The energy or chemical that impinges upon and activates a sensory receptor is known as a stimulus. Electrical, chemical, mechanical stimulus To excite a tissue, the stimulus must possess two characters: 1. Intensity or strength 2. Duration. 1/30/2018 Jesus is Lord 30  Contd. 1. Intensity or strength of a stimulus is of five types: i. Subminimal stimulus ii. Minimal stimulus iii. Submaximal stimulus iv. Maximal stimulus v. Supramaximal stimulus. Stimulus whose strength (or voltage) is sufficient to excite the tissue is called threshold or liminal or minimal stimulus. 2. Duration Whatever may be the strength of the stimulus, it must be applied for a minimum duration to excite the tissue. However, the duration of a stimulus depends upon the strength of the stimulus. For a weak stimulus, the duration is longer and for a stronger stimulus, the duration is shorter. Nerve fibers have a low threshold for excitation than the other cells. 1/30/2018 Jesus is Lord 31  Contd. When a nerve fiber is stimulated, based on the strength of stimulus, two types of physicochemical responses develop: 1. Local, nonpropagated potentials: depending on their location, they can be called synaptic, generator, or electrotonic potentials. This develops when a stimulus with subliminal strength is applied. 2. Propagated potentials, the action potentials (or nerve impulses). Action potential develops in a nerve fiber when it is stimulated by a stimulus with adequate strength. Adequate strength of stimulus, necessary for producing the action potential in a nerve fiber is known as threshold or minimal stimulus. 1/30/2018 Jesus is Lord 32 1/30/2018 Jesus is Lord 33  MEMBRANE POTENTIALS The nerve and muscle cells generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along their membrane. 2/02/2018 Jesus is Lord 34  Chemical compositions of extracellular and intracellular fluids. The question mark indicates that precise values for intracellular fluid are unknown. The red line indicates the cell membrane. 2/02/2018 Jesus is Lord 35  DIFFUSION POTENTIALS is the potential difference generated across a membrane because of a concentration difference of an ion. A diffusion potential is caused by diffusion of ions. A diffusion potential can be generated only if the membrane is permeable to that ion. If the membrane is not permeable to the ion, no diffusion potential will be generated no matter how large a concentration gradient is present. 2/02/2018 Jesus is Lord 36  EQUILIBRIUM POTENTIALS The concept of equilibrium potential is simply an extension of the concept of diffusion potential. The equilibrium potential is the diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference. At electrochemical equilibrium, the chemical and electrical driving forces that act on an ion are equal and opposite, and no more net diffusion of the ion occurs. 2/02/2018 Jesus is Lord 37  NERNST EQUATION The Nernst equation is used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane. By definition, the equilibrium potential is calculated for one ion at a time. Thus, where EMF is electromotive force and z is the electrical charge of the ion (e.g., +1 for K+). 2/02/2018 Jesus is Lord 38  Contd. Approximate values for equilibrium potentials in nerve and muscle ENa+ +65 millivolts (mV) ECa2+ +120 millivolts (mV) EK+ –85 millivolts (mV) ECl- –85 millivolts (mV) It is useful to keep these values in mind when considering the concepts of resting membrane potential and action potentials. 2/02/2018 Jesus is Lord 39  The Resting Membrane Potential All cells under resting conditions have a potential difference across their plasma membranes, with the inside of the cell negatively charged with respect to the outside. This potential is the resting membrane potential. By convention, it is expressed as the intracellular potential relative to the extracellular potential. Thus, a resting membrane potential of –70 mV means 70 mV, cell negative. 2/02/2018 Jesus is Lord 40  (a) Apparatus for measuring membrane potentials. The voltmeter records the difference between the intracellular and extracellular electrodes. (b) The potential difference across a plasma membrane as measured by an intracellular microelectrode. The asterisk indicates the moment the electrode entered the cell. 2/02/2018 Jesus is Lord 41  Contd. 1. The resting membrane potential is established by diffusion potentials that result from concentration differences of permeant ions. 2. Each permeable ion attempts to drive the membrane potential toward its equilibrium potential. 3. Ions with the highest permeabilities, or conductances, will make the greatest contributions to the resting membrane potential, and those with the lowest permeabilities will make little or no contribution. 2/02/2018 Jesus is Lord 42  Contd. 4. For example, the resting membrane potential of nerve is – 70 mV, which is close to the calculated K+ equilibrium potential of –85 mV, but far from the calculated Na+ equilibrium potential of +65 mV. 5. At rest, the nerve membrane is far more permeable to K+ than to Na+. 6. The Na+–K+ pump contributes only indirectly to the resting membrane potential by maintaining, across the cell membrane, the Na+ and K+ concentration gradients that then produce diffusion potentials. 2/02/2018 Jesus is Lord 43  Contd. The resting membrane potential holds steady unless changes in electrical current alter the potential. The resting membrane potential exists because of a tiny excess of negative ions inside the cell and an excess of positive ions outside. The excess negative charges inside are electrically attracted to the excess positive charges outside the cell, and vice versa. Thus, the excess charges (ions) collect in a thin shell tight against the inner and outer surfaces of the plasma membrane, whereas the bulk of the intracellular and extracellular 2/02/2018 fluid remainsJesus electrically is Lord neutral. 44  The excess positive charges outside the cell and the excess negative charges inside collect in a tight shell against the plasma membrane. In reality, these excess charges are only an extremely small fraction of the total number of ions inside and outside the cell. 2/02/2018 Jesus is Lord 45  Contd. Of the ions that can flow across the membrane and affect its electrical potential, Na+, K+, and Cl- are present in the highest concentrations. The membrane permeability to each is independently determined. Na+ and K+ generally play the most important roles in generating the resting membrane potential, but in some cells Cl- is also a factor. 2/02/2018 Jesus is Lord 46  Contd. The concentration differences for Na+ and K+ are established by the action of the sodium–potassium ion pump (Na+-K+-ATPase) that pumps 3 Na+ out of the cell and 2 K+ into it. The magnitude of the resting membrane potential depends mainly on two factors: (1) differences in specific ion concentrations in the intracellular and extracellular fluids; and (2) differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane. 2/02/2018 Jesus is Lord 47  Contd. Most of the negative charge inside neurons are accounted for not by chloride ions (Cl-) but by impermeable organic anions—in particular, proteins and phosphate compounds. Thus, when there is a net flux of K+ out of a cell, these are the main ion species contributing to the negative charge on the inside of the membrane. The resting potential is generated across the plasma membrane largely because of the movement of K+ out of the cell down its concentration gradient through open K+ channels (called leak K+ channels ). This makes the inside of the cell negative with respect to the outside. 2/02/2018 Jesus is Lord 48  Movement of solutes across a typical plasma membrane involving membrane proteins. 2/02/2018 Jesus is Lord 49 Transport pathways through the cell membrane and the basic mechanisms of transport. 2/02/2018 Jesus is Lord 50  Terminologies The terms depolarize, repolarize, and hyperpolarize are used to describe the direction of changes in the membrane potential relative to the resting potential. The resting membrane potential is said to be “polarized,” simply meaning that the outside and inside of a cell have a different net charge. The membrane is depolarized when its potential becomes less negative (closer to zero) than the resting level (the cell interior becomes less negative). 2/02/2018 Jesus is Lord 51  Contd. Inward current is the flow of positive charge into the cell. Inward current depolarizes the membrane potential. Overshoot refers to a reversal of the membrane potential polarity—that is, when the inside of a cell becomes positive relative to the outside. When a membrane potential that has been depolarized is returning toward the resting value, it is repolarizing. 2/02/2018 Jesus is Lord 52  Contd. The membrane is hyperpolarized when the membrane potential is more negative than the resting level (the cell interior becomes more negative). Outward current is the flow of positive charge out of the cell. Outward current hyperpolarizes the membrane potential. Threshold potential is the membrane potential at which the action potential is inevitable. At threshold potential, net inward current becomes larger than net outward current. 2/02/2018 Jesus is Lord 53  Depolarizing, repolarizing, hyperpolarizing, and overshoot changes in membrane potential relative to the resting potential. 2/02/2018 Jesus is Lord 54  Local, nonpropagated potentials or Graded Potentials Are changes in membrane potential that are confined to a relatively small region of the plasma membrane. They are called graded potentials simply because the magnitude of the potential change can vary (is “graded”). Graded potentials are given various names related to the location of the potential or the function they perform e.g receptor potential, synaptic potential, and pacemaker potential are all different types of graded potentials. 2/02/2018 Jesus is Lord 55  Action Potential The action potential is a phenomenon of excitable cells, such as nerve and muscle, and consists of a rapid depolarization (upstroke) followed by repolarization of the membrane potential. Action potentials are the basic mechanism for transmission of information in the nervous system and in all types of muscle. Action potential are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. 2/02/2018 Jesus is Lord 56  Contd. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the axon terminal. The successive stages of the action potential are as follows: 1. Resting Stage. 2. Depolarization Stage. 3. Repolarization Stage. 2/02/2018 Jesus is Lord 57 Typical action potential 2/02/2018 Jesus is Lord 58  Gated Ion Channels There are many types of ion channels and several different mechanisms that regulate the opening of the different types. 1. Ligand-gated channels open in response to the binding of signaling molecules 2. Mechanically gated channels open in response to physical deformation (stretching) of the plasma membranes. 3. Voltage-gated channels open in response to changes in the membrane potential. 2/02/2018 Jesus is Lord 59  Assignment no 2 Read up the following 1. Voltage-gated Na+ channel 2. Voltage-gated K+ channel 2/02/2018 Jesus is Lord 60  Resting Stage This is the resting membrane potential before the action potential begins. The membrane is said to be “polarized” during this stage because of the –70 millivolts negative membrane potential that is present. The resting membrane potential is close to the K+ equilibrium potential because there are more open K+ channels than Na+ channels. These are Potassium leak channels At rest, the Voltage-gated Na+ channel are closed and Na+ conductance 2/02/2018 is low. Jesus is Lord 61 Functional characteristics of the Na+-K+ pump and of the K+ “leak” channels. ADP, adenosine diphosphate; ATP, adenosine triphosphate. The K+ leak channels also leak Na+ ions into the cell slightly but are much more permeable to K+ 2/02/2018 Jesus is Lord 62  Depolarization Stage The membrane suddenly becomes very permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon (Inward current). The normal “polarized” state of –70 millivolts is immediately neutralized by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction. Depolarization is an example of a positive feedback mechanism. 2/02/2018 Jesus is Lord 63  Contd. Rapid opening of the activation gates of the Voltage-gated Na+ channels (fast channels) , and the Na+ conductance of the membrane promptly increases. The Na+ conductance becomes higher than the K+ conductance, and the membrane potential is driven toward (but does not quite reach) the Na+ equilibrium potential of +65 mV. (Why does it not reach the Na+ equilibrium potential ?) Thus, the rapid depolarization during the upstroke is caused by an inward Na+ current. 2/02/2018 Jesus is Lord 64  Contd. In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually “overshoot” beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state. 2/02/2018 Jesus is Lord 65 Characteristics of the voltage-gated sodium channel, showing successive activation and inactivation of the sodium channels when the membrane potential is changed from the normal resting negative value to a positive value. 2/02/2018 Jesus is Lord 66  Repolarization Stage Depolarization also closes the inactivation gates of the Na+ channel (but more slowly than it opens the activation gates). Closure of the inactivation gates results in closure of the Na+ channels, and the Na+ conductance returns toward zero. Depolarization slowly opens K+ channels and increases K+ conductance to even higher levels than at rest. 2/02/2018 Jesus is Lord 67  Contd. The combined effect of closing the Na+ channels and greater opening of the K+ channels makes the K+ conductance higher than the Na+ conductance, and the membrane potential is repolarized. Thus, repolarization is caused by an outward K+ current. 2/02/2018 Jesus is Lord 68 Characteristics of the voltage-gated potassium channels, showing delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. 2/02/2018 Jesus is Lord 69 Characteristics of the voltage-gated sodium (top) and potassium (bottom) channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. 2/02/2018 Jesus is Lord 70  Undershoot (hyperpolarizing afterpotential) Voltage-gated K+ channels close relatively slowly immediately after an action potential. The K+ conductance remains higher than at rest for some time after closure of the Na+ channels. During this period, the membrane potential is driven very close to the K+ equilibrium potential. Once the voltage-gated K+ channels finally close, however, the resting membrane potential is restored. 2/02/2018 Jesus is Lord 71 Nerve action potential and associated changes in Na+ and K+ conductance. 2/02/2018 Jesus is Lord 72  Refractory Periods Refractory period is a period during which another normal action potential cannot be elicited in an excitable cell. Refractory periods can be absolute or relative. a. Absolute refractory period is the period during which another action potential cannot be elicited, no matter how large the stimulus. coincides with almost the entire duration of the action potential. The inactivation gates of the Na+ channel are closed when the membrane potential is depolarized. 2/02/2018 Jesus is Lord 73  Contd. They remain closed until repolarization occurs. No action potential can occur until the inactivation gates open. 2/02/2018 Jesus is Lord 74  Contd. b. Relative refractory period begins at the end of the absolute refractory period and continues until the membrane potential returns to the resting level. An action potential can be elicited during this period only if a larger than usual inward current is provided. The K+ conductance is higher than at rest, and the membrane potential is closer to the K+ equilibrium potential and, therefore, farther from threshold; More inward current is required to bring the membrane to threshold. 2/02/2018 Jesus is Lord 75  CHARACTERISTICS OF ACTION POTENTIALS Action potentials have three basic characteristics: 1. Stereotypical size and shape: Each normal action potential for a given cell type looks identical, depolarizes to the same potential, and repolarizes back to the same resting potential. 2. Propagation: An action potential at one site causes depolarization at adjacent sites, bringing those adjacent sites to threshold. Propagation of action potentials from one site to the next is nondecremental. 2/02/2018 Jesus is Lord 76  Contd. 3. All-or-none response.: An action potential either occurs or does not occur. If an excitable cell is depolarized to threshold in a normal manner, then the occurrence of an action potential is inevitable. On the other hand, if the membrane is not depolarized to threshold, no action potential can occur. 2/02/2018 Jesus is Lord 77  Contd. The generation of action potentials is prevented by local anesthetics such as procaine (Novocaine) and lidocaine (Xylocaine) because these drugs block voltage-gated Na+ channels, preventing them from opening in response to depolarization. Without action potentials, graded signals generated in sensory neurons—in response to injury, for example— cannot reach the brain and give rise to the sensation of pain. 2/02/2018 Jesus is Lord 78  SUMMATION When one submiminal stimulus is applied, it does not produce any response in the nerve fiber because, the submiminal stimulus is very weak. However, if two or more submiminal stimuli are applied within a short interval of about 0.5 millisecond, a response is produced. This is because the submiminal stimuli are summed up together to become strong enough to produce the response. This phenomenon is known as summation. 2/02/2018 Jesus is Lord 79  ADAPTATION While stimulating a nerve fiber continuously, the excitability of the nerve fiber is greater in the beginning. Later the response decreases slowly and finally the nerve fiber does not show any response at all. This phenomenon is known as adaptation or accommodation. When a nerve fiber is stimulated continuously, depolarization occurs continuously. Continuous depolarization inactivates the sodium pump and increases the efflux (the flowing out) of potassium ions. 2/02/2018 Jesus is Lord 80  INFATIGABILITY Nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason is that nerve fiber can conduct only one action potential at a time. At that time, it is completely refractory and does not conduct another action potential. 2/02/2018 Jesus is Lord 81 2/02/2018 Jesus is Lord 82  MUSCLE Muscle cells, like neurons, can be excited chemically, electrically, and mechanically to produce an action potential that is transmitted along their cell membranes. Unlike neurons, they respond to stimuli by activating a contractile mechanism. The contractile protein myosin and the cytoskeletal protein actin are abundant in muscle, where they are the primary structural components that bring about contraction. All muscle cells are specialized to generate mechanical force. 2/05/2018 Jesus is Lord 83  Types of Muscle Three types of muscle tissue can be identified on the basis of: i. structure, ii. contractile properties, and iii. control mechanisms Into i. skeletal muscle, ii. cardiac muscle and iii. Smooth muscle, although smooth muscle is not a homogeneous single category 2/05/2018 Jesus is Lord 84  Contd. Skeletal muscle makes up the great mass of the somatic musculature. they are attached through other structures to bones and produce movements of the limbs or trunk. they are also attached to skin, such as the muscles producing facial expressions. It has well-developed cross-striations, does not normally contract in the absence of nervous stimulation, is generally under voluntary control. 2/05/2018 Jesus is Lord 85  Contd. Cardiac muscle cells are found only in the heart. also has cross-striations, but it is functionally syncytial and, although it can be modulated via the autonomic nervous system, it can contract rhythmically in the absence of external innervation owing to the presence in the myocardium of pacemaker cells that discharge spontaneously 2/05/2018 Jesus is Lord 86  Contd. Smooth muscle cells surround many of the tubes in the body—blood vessels, or the tubes of the gastrointestinal tract lacks cross-striations can be further subdivided into two broad types: i. unitary (or visceral) smooth muscle and ii. multiunit smooth muscle. The multiunit type found in the eye and in some other locations is not spontaneously active and resembles skeletal 2/05/2018 muscle in graded contractile Jesus is Lord ability. 87  Skeletal muscle is made up of individual muscle fibers that are the “building blocks” of the muscular system in the same sense that the neurons are the building blocks of the nervous system. Most skeletal muscle, as the name implies, is attached to bone, and its contraction is responsible for supporting and moving the skeleton. Contraction of skeletal muscle is initiated by action potentials in neurons of the somatic motor division of the nervous system, and is usually under voluntary control. 2/05/2018 Jesus is Lord 88  Contd. Most skeletal muscles begin and end in tendons, and the muscle fibers are arranged in parallel between the tendinous ends, so that the force of contraction of the units is additive. Each muscle fiber is a single cell that is multinucleated, long, cylindrical, and surrounded by a cell membrane, the sarcolemma. There are no syncytial bridges between cells. The muscle fibers are made up of myofibrils, which are divisible into individual filaments 2/05/2018 Jesus is Lord 89  Contd. These myofilaments contain several proteins that together make up the contractile machinery of the skeletal muscle. The contractile mechanism in skeletal muscle largely depends on the proteins i. myosin-II, ii. actin, iii. tropomyosin, and iv. troponin. 2/05/2018 Jesus is Lord 90  Contd. Troponin is made up of three subunits: i. Troponin I, ii. Troponin T, and iii. Troponin C. Other important proteins in muscle are involved in maintaining the proteins that participate in contraction in appropriate structural relation to one another and to the extracellular matrix. 2/05/2018 Jesus is Lord 91  Structure of the skeletal muscle The most striking feature seen when viewing skeletal muscle through a microscope is a distinct series of alternating light and dark bands perpendicular to the long axis. The cardiac muscle shares this characteristic striped pattern, and so these two types are both referred to as striated muscle. The smooth muscle derives its name from the fact that it lacks this striated appearance. 2/05/2018 Jesus is Lord 92  Comparison of (a) skeletal muscle to (b) cardiac and (c) smooth muscle as seen with light microscopy (top panels) and in schematic form (bottom panels). Both skeletal and cardiac muscle have a striated appearance. Cardiac and smooth muscle cells generally have a single nucleus, but skeletal muscle fibers are multinucleated. 2/05/2018 Jesus is Lord 93  Contd. Due to its elongated shape and the presence of multiple nuclei, a skeletal muscle cell is also referred to as a muscle fiber. The Sarcolemma is a thin membrane enclosing a skeletal muscle fiber. The term muscle refers to a number of muscle fibers bound together by connective tissue. Each muscle fiber contains several hundred to several thousand myofibrils. 2/05/2018 Jesus is Lord 94  Contd. Structure of skeletal muscle 2/05/2018 Jesus is Lord 95  Contd. Each myofibril is composed of adjacent thick myosin filaments and thin actin filaments. These filaments are large polymerized protein molecules that are responsible for the actual muscle contraction. The myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands. 2/05/2018 Jesus is Lord 96  Contd. The light bands contain only actin filaments and are called I bands because they are isotropic to polarized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands because they are anisotropic to polarized light. There are small projections from the sides of the myosin filaments called cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction. 2/05/2018 Jesus is Lord 97  Contd. The ends of the actin filaments are attached to a Z disk (Z line). From this disc, these filaments extend in both directions to interdigitate with the myosin filaments. The Z disk is composed of filamentous proteins different from the actin and myosin filaments It passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber. Therefore, the entire muscle fiber has light and dark bands, as do the individual myofibrils. 2/05/2018 Jesus is Lord 98  Contd. These bands give skeletal and cardiac muscle their striated appearance. The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called a sarcomere (from the Greek sarco, “muscle,” and mer, “part”). Sarcomere is defined as the structural and functional unit of a skeletal muscle. It is also called the basic contractile unit of the muscle. 2/05/2018 Jesus is Lord 99  Assignment no 3 Read about contractile proteins 1. Myosin 2. Actin 3. Troponin 4. Tropomysosin Read about Titin Filamentous Molecule. 2/05/2018 Jesus is Lord 100  Sarcoplasm The many myofibrils of each muscle fiber are suspended side by side in the muscle fiber. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm. It contains large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria. Mitochondria supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP). 2/05/2018 Jesus is Lord 101  THE MOLECULAR STRUCTURE 1. THE THICK FILAMENTS are composed almost entirely of the protein myosin. The myosin molecule is composed of two large polypeptide heavy chains and four smaller light chains. These polypeptides combine to form a molecule that consists of two globular heads (containing heavy and light chains) and a long tail formed by the two intertwined heavy chains. 2/05/2018 Jesus is Lord 102 (a) The heavy chains of myosin molecules form the core of a thick filament. The myosin molecules are oriented in opposite directions in either half of a thick filament. (b) Structure of thin filament and myosin molecule. Cross-bridge binding sites on actin are covered by tropomyosin. The two globular heads of each myosin molecule extend from the sides of a thick filament, forming a cross-bridge. 2/05/2018 Jesus is Lord 103 A. Myosin molecule. 2/05/2018 Jesus is Lord 104  Contd. The tail of each myosin molecule lies along the axis of the thick filament The two globular heads extend out to the sides, forming cross-bridges. The cross-bridges make contact with the thin filament and exert force during muscle contraction. Each globular head contains two binding sites, one for attaching to the thin filament and one for ATP. The ATP binding site also serves as an enzyme—an ATPase that hydrolyzes the bound ATP, harnessing its energy for contraction. 2/05/2018 Jesus is Lord 105 B, Combination of many myosin molecules to form a myosin filament. Also shown are thousands of myosin cross-bridges and interaction between the heads of the cross-bridges with adjacent actin filaments. 2/05/2018 Jesus is Lord 106  Contd. 2. THE THIN FILAMENTS are about half the diameter of the thick filaments are principally composed of the protein i. actin (so it is called actin filament) as well as two other proteins— ii. troponin and iii. tropomyosin Each actin molecule contains a binding site for myosin. 2/05/2018 Jesus is Lord 107  Actin filament, composed of two helical strands of F-actin molecules and two strands of tropomyosin molecules that fit in the grooves between the actin strands. Attached to one end of each tropomyosin molecule is a troponin complex that initiates contraction. 2/05/2018 Jesus is Lord 108  Contd. 3. SARCOMERE Two successive Z lines define the limits of one sarcomere. The thick filaments are located in the middle of each sarcomere, where they create a wide, dark band known as the A band. Each sarcomere contains two sets of thin filaments, one at each end. One end of each thin filament is anchored to the Z line, whereas the other end overlaps a portion of the thick filaments. 2/05/2018 Jesus is Lord 109  Arrangement of the thick and thin filaments in the sarcomere 2/05/2018 Jesus is Lord 110  Structure of the sarcomere in skeletal muscle. A. Arrangement of thick and thin filaments. 2/05/2018 Jesus is Lord 111  Contd. Thus, thin filaments from two adjacent sarcomeres are anchored to the two sides of each Z line. A light band known as the I band lies between the ends of the A bands of two adjacent sarcomeres and contains those portions of the thin filaments that do not overlap the thick filaments. The I band is bisected by the Z line. 2/05/2018 Jesus is Lord 112  Contd. Two additional bands are present in the A-band region of each sarcomere. i. The H band (H zone) is a narrow, light band in the center of the A band. It corresponds to the space between the opposing ends of the two sets of thin filaments in each sarcomere. ii. The M line is a narrow, dark band in the center of the H zone. It corresponds to proteins that link together the central region of adjacent thick filaments. 2/05/2018 Jesus is Lord 113  Contd. Filaments composed of the elastic protein titin extend from the Z line to the M line and are linked to both the M- line proteins and the thick filaments. Both the M-line linkage between thick filaments and the titin filaments act to maintain the alignment of thick filaments in the middle of each sarcomere. 2/05/2018 Jesus is Lord 114  SARCOTUBULAR SYSTEM The myofibrils are surrounded by structures made up of membranes that appear as vesicles and tubules. These structures form the sarcotubular system, which is made up of a i. T system and ii. a sarcoplasmic reticulum. 2/05/2018 Jesus is Lord 115  I. The T system is made up of transverse tubules which are continuous with the sarcolemma of the muscle fiber. The space between the layers of the T system is an extension of the extracellular space. contain a voltage-sensitive protein called the dihydropyridine receptor The T system, which is continuous with the sarcolemma, provides a path for the rapid transmission of the action potential from the cell membrane to all the myofibrils in the muscle. 2/05/2018 Jesus is Lord 116 A single muscle fiber surrounded by its sarcolemma. The cut surface of the myofibrils shows the arrays of thick and thin filaments. The sarcoplasmic reticulum with its transverse (T) tubules and terminal cisterns surrounds each myofibril. The T tubules invaginate from the sarcolemma and contact the myofibrils twice in every sarcomere. Mitochondria are found between the myofibrils. 2/05/2018 Jesus is Lord 117  II. The Sarcoplasmic Reticulum is a specialized endoplasmic reticulum of skeletal muscle forms an irregular curtain surrounding the myofibrils of each muscle fiber is the site of storage and release of Ca2+ for excitation- contraction coupling. It has a special organization that is extremely important in regulating Ca2+ storage, release, and reuptake and therefore muscle contraction. 2/05/2018 Jesus is Lord 118  Contd. It is composed of two major parts: (1) large chambers called terminal cisternae (also called lateral sacs) (2) long longitudinal tubules that surround all surfaces of the actual contracting myofibrils. The terminal cisternae is in close contact with the T system at the junctions between the A and I bands. The arrangement of the central T system along with the terminal cisterns on either side is called the triad of skeletal muscle. 2/05/2018 Jesus is Lord 119 Transverse (T) tubule– sarcoplasmic reticulum system. Note that the T tubules communicate with the outside of the cell membrane, and deep in the muscle fiber, each T tubule lies adjacent to the ends of longitudinal sarcoplasmic reticulum tubules that surround all sides of the actual myofibrils that contract. This illustration was drawn from frog muscle, which has one T tubule per sarcomere, located at the Z disk. A similar arrangement is found in mammalian heart muscle, but mammalian skeletal muscle has two T tubules per sarcomere, located at the A-I band junctions. 2/05/2018 Jesus is Lord 120 Transverse tubules and sarcoplasmic reticulum in a single skeletal muscle fiber. 2/05/2018 Jesus is Lord 121  Contd. The sarcoplasmic reticulum contains a Ca2+-release channel called the ryanodine receptor channels (named for the plant alkaloid that opens this release channel). Action potential triggers the opening of the Ca2+-release channel in the terminal cisternae, as well as in their attached longitudinal tubules. Ca2+ is accumulated in the sarcoplasmic reticulum by the action of Ca2+ ATPase (SERCA or Ca2+ pump) in the sarcoplasmic reticulum membrane. 2/05/2018 Jesus is Lord 122  Assignament no 4 What kind of channel is the ryanodine receptor channels ? What is the full meaning of SERCA ? 2/05/2018 Jesus is Lord 123  Contd. The Ca2+ ATPase pumps Ca2+ from the ICF of the muscle fiber into the interior of the sarcoplasmic reticulum, keeping the intracellular Ca2+ concentration low when the muscle fiber is at rest. Ca2+ is bound to calsequestrin, a low-affinity, high-capacity Ca2+-binding protein within the sarcoplasmic reticulum Calsequestrin, by binding Ca2+, helps to maintain a low free Ca2+ concentration inside the sarcoplasmic reticulum, thereby reducing the work of the Ca2+ ATPase pump. 2/05/2018 Jesus is Lord 124  Contd. A large quantity of Ca2+ can be stored inside the sarcoplasmic reticulum in bound form, while the intrasarcoplasmic reticulum free Ca2+ concentration remains extremely low. There is a significance of the physical relationship between the T tubules (and their dihydropyridine receptor) and the sarcoplasmic reticulum (and its ryanodine receptor). 2/05/2018 Jesus is Lord 125 Excitation-contraction coupling in skeletal muscle. The top panel shows an action potential in the transverse tubule that causes a conformational change in the voltage- sensing dihydropyridine (DHP) receptors, opening the Ca++ release channels in the terminal cisternae of the sarcoplasmic reticulum and permitting Ca++ to rapidly diffuse into the sarcoplasm and initiate muscle contraction. During repolarization (bottom panel), the conformational change in the DHP receptor closes the Ca++ release channels and Ca++ is transported from the sarcoplasm into the sarcoplasmic reticulum by an adenosine triphosphate– dependent calcium pump. 2/05/2018 Jesus is Lord 126  Structure of the sarcomere in skeletal muscle. B. Transverse tubules and sarcoplasmic reticulum. 2/05/2018 Jesus is Lord 127 Structure of the sarcomere in skeletal muscle. A. Arrangement of thick and thin filaments. B. Transverse tubules and sarcoplasmic reticulum. 2/05/2018 Jesus is Lord 128  Peripheral Nervous System Therefore, fibers in a nerve may be classified as belonging to the efferent or the afferent division of the PNS Efferent neurons carry signals out from the CNS to muscles, glands, and other tissues. The efferent division of the PNS is more complicated than the afferent, being subdivided into a somatic nervous system and an autonomic nervous system. 2/05/2018 Jesus is Lord 129  Contd. The simplest distinction between the somatic and autonomic systems is that the neurons of the somatic division innervate skeletal muscle, whereas the autonomic neurons innervate smooth and cardiac muscle, glands, neurons in the gastrointestinal tract, and other tissues. The somatic portion of the efferent division of the PNS is made up of all the nerve fibers going from the CNS to skeletal muscle cells. 2/05/2018 Jesus is Lord 130  Contd. The neurotransmitter these neurons release is acetylcholine. Because activity in the somatic neurons leads to contraction of the innervated skeletal muscle cells, these neurons are called motor neurons. Excitation of motor neurons leads only to the contraction of skeletal muscle cells; there are no somatic neurons that inhibit skeletal muscles. 2/05/2018 Jesus is Lord 131 2/05/2018 Jesus is Lord 132  SYNAPTIC AND NEUROMUSCULAR TRANSMISSION 2/06/2018 Jesus is Lord 133  SYNAPSE A synapse is a site where information is transmitted from one cell to another. Impulses are transmitted from one nerve cell (the presynaptic cell) to another cell e.g neuron, a muscle or gland cell (the postsynaptic cell) at synapses. Cell-to-cell communication occurs across either a chemical or electrical synapse. 2/06/2018 Jesus is Lord 134  TYPES OF SYNAPSES 1. Electrical Synapses Electrical synapses allow current to flow from one excitable cell to the next through low resistance pathways between the cells called gap junctions. Gap junctions are found in cardiac muscle and in some types of smooth muscle and account for the very fast conduction in these tissues. For example, rapid cell-to-cell conduction occurs in cardiac ventricular muscle, in the uterus, and in the bladder, allowing cells in these tissues to be activated simultaneously and ensuring that contraction occurs in a coordinated manner. Jesus is Lord 135 2/06/2018  Contd. 2. Chemical Synapses In chemical synapses, there is a gap between the presynaptic cell membrane and the postsynaptic cell membrane, known as the synaptic cleft. Information is transmitted across the synaptic cleft through a neurotransmitter, a substance that is released from the presynaptic terminal and binds to receptors on the postsynaptic terminal. In contrast to electrical synapses, neurotransmission across chemical synapses is unidirectional (from presynaptic cell to postsynaptic cell). 2/06/2018 Jesus is Lord 136  NEUROMUSCULAR JUNCTION Stimulation of the neurons to a skeletal muscle is the only mechanism by which action potentials are initiated in this type of muscle. The neurons whose axons innervate skeletal muscle fibers are known as motor neurons (or somatic efferent neurons) A single motor neuron innervates many muscle fibers, but each muscle fiber is controlled by a branch from only one motor neuron. A motor neuron plus the muscle fibers it innervates is called a motor unit. 2/06/2018 Jesus is Lord 137  contd. Single motor unit consisting of one motor neuron and the muscle 2/06/2018 fibers it innervates Jesus is Lord 138  Contd. The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon is known as the motor end plate. The junction of an axon terminal with the motor end plate is known as a neuromuscular junction or The synapse between a motor neuron and a muscle fiber is called the neuromuscular junction. An action potential in the motor neuron produces an action potential in the muscle fibers it innervates. 2/06/2018 Jesus is Lord 139  Structure of a neuromuscular junction 2/06/2018 Jesus is Lord 140 PHYSIOLOGIC ANATOMY OF THE NEUROMUSCULAR JUNCTION The nerve fiber forms a complex of branching nerve terminals (terminal buttons or endfeet) that fit into junctional folds but lie outside the muscle fiber plasma membrane. Junctional folds are depressions in the membrane of the muscle fiber also called the synaptic gutter or synaptic trough. The axon terminal contains many small, clear vesicles that contain acetylcholine, the excitatory transmitter at these junctions 2/06/2018 Jesus is Lord 141  Contd. The axon terminal also contain many mitochondria that supply adenosine triphosphate (ATP), the energy source that is used for synthesis of acetylcholine. The membrane of the nerve ending is called the presynaptic membrane and the membrane of the muscle fiber is called the postsynaptic membrane. The space between these two membranes is called the synaptic space or synaptic cleft. In the synaptic space are large quantities of the enzyme acetylcholinesterase, which destroys acetylcholine a few milliseconds after it has been released from the synaptic vesicles. 2/06/2018 Jesus is Lord 142  Contd. At the bottom of the synaptic gutter of the muscle fibre are numerous smaller folds of the muscle membrane called subneural clefts, subneural clefts greatly increase the surface area at which the synaptic transmitter can act. Postsynaptic membrane contains the receptors called nicotinic receptors. 2/06/2018 Jesus is Lord 143 The contact point between a single axon terminal and the muscle fiber membrane 2/06/2018 Jesus is Lord 144 Sequence of Events at the Neuromuscular Junction 1. Action potentials are propagated down the motor neuron. 2. The presynaptic terminal is depolarized, and this depolarization causes voltage-gated Ca2+ channels in the presynaptic membrane to open. 3. When these Ca2+ channels open, the Ca2+ permeability of the presynaptic terminal increases, and Ca2+ flows into the terminal down its electrochemical gradient. 4. Ca2+ uptake into the terminal causes release of the neurotransmitter acetylcholine (ACh), stored in synaptic vesicles. 2/06/2018 Jesus is Lord 145  Contd. 5. To release ACh, the synaptic vesicles fuse with the presynaptic membrane and empty their contents into the synaptic cleft by exocytosis. 6. ACh diffuses across the synaptic cleft to the postsynaptic membrane. 7. This specialized region of the muscle fiber is called the motor end plate and contains nicotinic receptors for ACh. 8. It is important to note that the nicotinic receptor for ACh is an example of a ligand-gated ion channel: Also known as acetylcholine-gated ion channels 2/06/2018 Jesus is Lord 146 Release of acetylcholine from synaptic vesicles at the neural membrane of the neuromuscular junction. Note the proximity of the release sites in the neural membrane to the acetylcholine receptors in the muscle membrane, at the mouths of the subneural clefts. 2/06/2018 Jesus is Lord 147  Contd. 9. ACh binds to the α-subunits of the nicotinic receptor and causes a conformational change 10. When the conformational change occurs, the central core of the channel opens 11. It allows the important positive ions - sodium (Na+), potassium (K+), and calcium (Ca2+) - to move easily through the opening. 12. Patch clamp studies have shown that one of these channels, when opened by acetylcholine, can transmit 15,000 to 30,000 sodium ions in a millisecond. 2/06/2018 Jesus is Lord 148 Acetylcholine-gated channel. A, After acetylcholine (Ach) has become attached and a conformational change has opened the channel, allowing sodium ions to enter the muscle fiber and excite contraction. Note the negative charges at the channel mouth that prevent passage of negative ions such as chloride ions. 2/06/2018 Jesus is Lord 149  Contd. 13. Far more sodium ions flow through the acetylcholine- gated channels than any other ions. 14. This action creates a local positive potential change inside the muscle fiber membrane, called the end plate potential (EPP). 15. In turn, this end plate potential initiates an action potential that spreads along the muscle membrane 16. The EPP at the motor end plate is terminated when ACh is degraded to choline and acetate by acetylcholinesterase (AChE) on the motor end plate. 2/06/2018 Jesus is Lord 150  Neuromuscular junction. ACh = acetylcholine; AChR = acetylcholine receptor. 2/06/2018 Jesus is Lord 151 Events at the neuromuscular junction that lead to an action potential in the muscle fiber plasma membrane. Although K+ also exits the muscle cell when Ach receptors are open, Na+ entry and depolarization dominate, Jesus is Lord 152 2/06/2018  Assignment no 5 1. Read about agents that alter neuromuscular function 2/06/2018 Jesus is Lord 153  SKELETAL MUSCLE ACTION POTENTIAL The electrical events in skeletal muscle and the ionic fluxes that underlie them share distinct similarities to those in nerve, with quantitative differences in timing and magnitude. 1. The resting membrane potential of skeletal muscle is about –90 mV. 2. The action potential lasts 2–4 ms and is conducted along the muscle fiber at about 5 m/s. 3. 2/06/2018 The absolute refractory period Jesus is Lord is 1–3 ms long. 154  Contd. Action potential is transmitted deeply into the muscle fiber along transverse tubules (T system) that penetrate all the way through the muscle fiber from one side of the fiber to the other. The T tubule action potentials cause release of calcium ions inside the muscle fiber in the immediate vicinity of the myofibrils, and these calcium ions then cause contraction. This overall process is called excitation-contraction coupling. 2/06/2018 Jesus is Lord 155 2/06/2018 Jesus is Lord 156  CONTRACTILE PROTEINS 2/09/2018 Jesus is Lord 157  Myosin Molecule Myosin molecule has a molecular weight of about 480,000. The myosin molecule is composed of six polypeptide chains— two heavy chains, each with a molecular weight of about 200,000, and four light chains with molecular weights of about 20,000 each. The two heavy chains wrap spirally around each other to form a double helix, which is called the tail of the myosin molecule. 2/09/2018 Jesus is Lord 158  contd. One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head. Thus, there are two free heads at one end of the double- helix myosin molecule. The four light chains are also part of the myosin head, two to each head. These light chains help control the function of the head during muscle contraction. 2/09/2018 Jesus is Lord 159  contd. The myosin filament is made up of 200 or more individual myosin molecules. The total length of each myosin filament is uniform— almost exactly 1.6 micrometers. Note, however, that there are no cross-bridge heads in the center of the myosin filament for a distance of about 0.2 micrometer because the hinged arms extend away from the center. Another feature of the myosin head that is essential for muscle contraction is that it functions as an adenosine triphosphatase (ATPase) enzyme. 160 2/09/2018 Jesus is Lord  F-actin Molecule It is the backbone of the actin filament composed of two helical strands of F-actin molecules. Each strand of the double F-actin helix is composed of polymerized G-actin molecules, each having a molecular weight of about 42,000. Attached to each one of the G-actin molecules is one molecule of ADP. These ADP molecules are believed to be the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction. 2/09/2018 Jesus is Lord 161  Contd. The active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament about every 2.7 nanometers. Each actin filament is about 1 micrometer long. The bases of the actin filaments are inserted strongly into the Z disks; the ends of the filaments protrude in both directions to lie in the spaces between the myosin molecules. 2/09/2018 Jesus is Lord 162  Tropomyosin Molecule The actin filament also contains another protein, tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin filaments to cause contraction. 2/09/2018 Jesus is Lord 163  Troponin Molecule Interacts with both actin and tropomyosin They are attached intermittently along the sides of the tropomyosin molecules. They are actually complexes of three loosely bound protein subunits, each of which plays a specific role in controlling muscle contraction. The subunits are designated by the letters I (inhibitory), T (tropomyosin binding) and C (Ca2+ -binding). 164 2/09/2018 Jesus is Lord  Contd. troponin I has a strong affinity for actin troponin T has a strong affinity for tropomyosin troponin C has a strong affinity for calcium ions This complex is believed to attach the tropomyosin to the actin The strong affinity of the troponin for calcium ions is believed to initiate the contraction process. One molecule of troponin binds to each molecule of tropomyosin and regulates the access to myosin-binding sites on the actin molecule in contact with that tropomyosin. 165 2/09/2018 Jesus is Lord  INTRODUCTION It is important to distinguish between the electrical and mechanical events in skeletal muscle. Although one response does not normally occur without the other, their physiologic bases and characteristics are different. Muscle fiber membrane depolarization normally starts at the motor end plate, the specialized structure under the motor nerve ending. The action potential is transmitted along the muscle fiber and initiates the contractile response. 2/09/2018 Jesus is Lord 166  MUSCLE CONTRACTION 2/09/2018 Jesus is Lord 167  Contd. The process by which the contraction of muscle is brought about is a sliding of the thin filaments over the thick filaments. Contraction is not due to changes in the actual lengths of the thick and thin filaments, rather, by their increased overlap within the muscle cell. The width of the A bands is constant, whereas the Z lines move closer together when the muscle contracts and farther apart when it relaxes. 2/09/2018 Jesus is Lord 168 The sliding of thick filaments past overlapping thin filaments shortens the sarcomere with no change in thick or thin filament length. The I band and H zone are reduced. 2/09/2018 Jesus is Lord 169  GENERAL MECHANISM OF MUSCLE CONTRACTION The initiation and execution of muscle contraction occur in the following sequential steps. 1. An action potential travels along a motor nerve to its endings on muscle fibers. 2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine (neuromuscular junction) 3. The acetylcholine acts on a local area of the muscle fiber membrane to open “acetylcholine-gated” cation channels. 2/09/2018 Jesus is Lord 170  Contd. 4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. 5. This action causes a local depolarization that in turn leads to opening of voltage-gated sodium channels, which initiates an action potential at the membrane. 6. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes. 2/09/2018 Jesus is Lord 171  Contd. 7. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. 8. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum. 9. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process. 2/09/2018 Jesus is Lord 172  Contd. 10. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca2+ membrane pump and remain stored in the reticulum until a new muscle action potential comes along; 11. This removal of calcium ions from the myofibrils causes the muscle contraction to cease. 2/09/2018 Jesus is Lord 173  MOLECULAR MECHANISM OF MUSCLE CONTRACTION 2/09/2018 Jesus is Lord 174  Excitation-Contraction Coupling Excitation-contraction coupling is the mechanism that translates the muscle action potential into the production of tension. The action potential always precedes the rise in intracellular Ca2+ concentration, which always precedes contraction. 2/09/2018 Jesus is Lord 175  Relationship of the action potential, the increase in intracellular [Ca2+], and muscle contraction in skeletal muscle. 2/09/2018 Jesus is Lord 176  Contd. The steps involved in excitation-contraction coupling are as follows : 1. Action potentials in the muscle cell membrane are propagated to the T tubules. 2. Recall that the T tubules are continuous with the sarcolemmal and carry the depolarization from the surface to the interior of the muscle fiber. 3. Depolarization of the T tubules causes a critical conformational change in its voltage-sensitive dihydropyridine receptor. 2/09/2018 Jesus is Lord 177  Contd. 4. This conformational change opens the Ca2+-release channels (ryanodine receptors or Ryr) on the sarcoplasmic reticulum (SR) 5. When these Ca2+-release channels open, Ca2+ is released from its storage site in the SR into the intracellular fluid (ICF) of the muscle fiber, 6. resulting in an increase in intracellular Ca2+ conc. 7. Ca2+ binds to troponin C on the thin filaments, causing a conformational change inJesusthe 2/09/2018 is Lord troponin complex. 178  Contd. In resting muscle, troponin I is bound to actin and tropomyosin and covers the sites where myosin heads interact with actin. Also at rest, the myosin head contains tightly bound ADP. Following an action potential, cytosolic Ca2+ is increased and free Ca2+ binds to troponin C. This binding results in a weakening of the troponin I interaction with actin and exposes the actin binding site for myosin to allow for formation of myosin/actin cross- bridges. 2/09/2018 Jesus is Lord 179  Contd. 8. The conformational change in troponin causes tropomyosin (which was previously blocking the interaction of actin and myosin) to be moved out of the way so that cross-bridge cycling can begin. 9. When tropomyosin is moved away, the myosin-binding sites on actin, previously covered, are exposed. 10. Myosin heads bind to actin and form cross-bridges. 11. Formation of cross-bridges is associated with hydrolysis of ATP and generation of force. 2/09/2018 Jesus is Lord 180 Activation of cross-bridge cycling by Ca2+. (a)Without calcium ions bound, troponin holds tropomyosin over cross- bridge binding sites on actin. (b) When Ca2+ binds to troponin, tropomyosin is allowed to move away from cross-bridge binding sites on actin, and cross-bridges can bind to actin. 2/09/2018 Jesus is Lord 181  12. Cross-bridge cycling Cross-bridge cycle is the sequence of events that occurs between the time a cross-bridge binds to a thin filament, moves, and then is set to repeat the process. Each cycle consists of four steps: (1) attachment of the cross bridge to a thin filament; (2) movement of the cross-bridge, producing tension in the thin filament known as power stroke; (3) detachment of the cross-bridge from the thin filament; (4) energizing the cross-bridge so it can again attach to a thin filament and repeat the 2/09/2018 cycle. Jesus is Lord 182 Chemical (shown in brackets) and mechanical representations of the four stages of a cross-bridge cycle. Crossbridges remain in the resting state (pink box at left) when Ca2+ remains low. In the rigor mortis state (pink box at right), cross-bridges remain rigidly bound when ATP is absent. In the chemical representation, A 5 actin, M 5 myosin, dots are between bound components, and plus signs are between detached components. 2/09/2018 Jesus is Lord 183 The “walk-along” mechanism for contraction of the muscle. 2/09/2018 Jesus is Lord 184  Contd. Thus, muscle contraction occurs by a sliding filament mechanism. 2/09/2018 Jesus is Lord 185  Relaxation Occurs when Ca2+ is reaccumulated in the sarcoplasmic reticulum by the Ca2+ ATPase of the sarcoplasmic reticulum membrane (SERCA). When the intracellular Ca2+ concentration decreases, there is insufficient Ca2+ for binding to troponin C. When Ca2+ is released from troponin C, tropomyosin returns to its resting position, where it blocks the myosin- binding site on actin. As long as the intracellular Ca2+ is low, cross-bridge cycling cannot occur, and the muscle will be relaxed. 2/09/2018 Jesus is Lord 186 Release and uptake of Ca2+ by the sarcoplasmic reticulum during contraction and relaxation of a skeletal muscle fiber. 2/09/2018 Jesus is Lord 187  Contd. Note that ATP provides the energy for both i. contraction (at the myosin head) and ii. relaxation (via SERCA). If transport of Ca2+ into the reticulum is inhibited, relaxation does not occur even though there are no more action potentials; the resulting sustained contraction is called a contracture. 2/09/2018 Jesus is Lord 188  Changes in sarcomere during muscular contraction Thus, changes that take place in sarcomere during muscular contraction are: 1. Length of all the sarcomeres decreases as the ‘Z’ lines come close to each other 2. Length of the ‘I’ band reduces since the actin filaments from opposite side overlap 3. ‘H’ zone either reduces or disappears 4. Length of ‘A’ band remains unchanged. 2/09/2018 Jesus is Lord 189 The sliding of thick filaments past overlapping thin filaments shortens the sarcomere with no change in thick or thin filament length. The I band and H zone are reduced. 2/09/2018 Jesus is Lord 190  Functions of ATP in Skeletal Muscle Contraction 1. Hydrolysis of ATP by the Na+-K+ -ATPase in the plasma membrane maintains Na+ and K+ gradients, which allows the membrane to produce and propagate action potentials. 2. Hydrolysis of ATP by the Ca2+-ATPase in the sarcoplasmic reticulum provides the energy for the active transport of calcium ions into the reticulum, ending the contraction, and allowing the muscle fiber to relax. 3. Hydrolysis of ATP by myosin energizes the cross-bridges, providing the energy for force generation. 4. Binding of ATP to myosin dissociates cross-bridges bound to actin, allowing the bridges to repeat their cycle of activity. 2/09/2018 Jesus is Lord 191  Rigor Mortis The importance of ATP in dissociating actin and myosin during step 3 of a cross-bridge cycle is illustrated by rigor mortis, the gradual stiffening of skeletal muscles that begins several hours after death and reaches a maximum after about 12 hours. The ATP concentration in cells, including muscle cells, declines after death because the nutrients and oxygen the metabolic pathways require to form ATP are no longer supplied by the circulation. In the absence of ATP, the breakage of the link between actin and myosin does not occur. 2/09/2018 Jesus is Lord 192  Contd. The thick and thin filaments remain bound to each other by immobilized cross-bridges, producing a rigid condition in which the thick and thin filaments cannot be pulled past each other. The stiffness of rigor mortis disappears about 48 to 60 hours after death as the muscle tissue decomposes. 2/09/2018 Jesus is Lord 193  THREE SOURCES OF ENERGY FOR MUSCLE CONTRACTION 1. Phosphocreatine also called creatine phosphate 2. Glycolysis 3. Oxidative metabolism 2/09/2018 Jesus is Lord 194 Important metabolic systems that supply energy for muscle contraction. 2/09/2018 Jesus is Lord 195  Assignment no 6 Read about types of contraction. 2/09/2018 Jesus is Lord 196  THE MUSCLE TWITCH The mechanical response of a muscle fiber to a single action potential is known as a twitch. This response is called a muscle twitch. The contractile property of the muscle is studied by using gastrocnemius-sciatic preparation from frog. It is also called muscle-nerve preparation. When the stimulus with threshold strength is applied, the muscle contracts and then relaxes. 2/09/2018 Jesus is Lord 197  Contd. These activities are recorded graphically by using suitable instruments. Contraction is recorded as upward deflection from the base line Relaxation is recorded as downward deflection back to the base line. 2/09/2018 Jesus is Lord 198  Contd. Important Points in Simple Muscle Curve Four points are to be observed in simple muscle curve: 1. Point of stimulus (PS): The time when the stimulus is applied. 2. Point of contraction (PC): The time when muscle begins to contract. 3. Point of maximum contraction (PMC): The point up to which the muscle contracts. It also indicates the beginning of relaxation of the muscle. 4. Point of maximum relaxation (PMR): The point when muscle relaxes completely. 2/09/2018 Jesus is Lord 199  Contd. Periods of Simple Muscle Curve All the four points mentioned above divide the entire simple muscle curve into three periods: 1. Latent period (LP) 2. Contraction period (CP) 3. Relaxation period (RP). 2/09/2018 Jesus is Lord 200  Simple muscle twitch 2/09/2018 Jesus is Lord 201  Simple muscle twitch 2/09/2018 Jesus is Lord 202 Time relationship between a skeletal muscle fiber action potential and the resulting contraction and relaxation of the muscle fiber. The latent period is the delay between the beginning of the action potential and the initial increase in tension. 2/09/2018 Jesus is Lord 203  Summation Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units contracting simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization. Jesus is Lord 204 2/09/2018 A motor unit consists of a motor neuron and the group of skeletal muscle fibers it innervates. A single motor axon may branch to innervate several muscle fibers that function together as a group. Although each muscle fiber is innervated by a single motor neuron, an entire muscle may receive input from hundreds of different motor neurons. 2/09/2018 Jesus is Lord 205  Frequency Summation and Tetanization When there is an increase in the frequency of individual twitch contractions, there comes a point when new contraction occurs before the preceding one is over. As a result, the second contraction is added partially to the first, and thus the total strength of contraction rises progressively with increasing frequency. Jesus is Lord 206 2/09/2018  Contd. When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous. This process is called tetanization. Tetany occurs because enough calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials. Jesus is Lord 207 2/09/2018 Frequency summation and tetanization. 2/09/2018 Jesus is Lord 208  Staircase Effect, or Treppe When a muscle begins to contract after a long period of rest, its initial strength of contraction may be as little as one half its strength 10 to 50 muscle twitches later. That is, the strength of contraction increases to a plateau, a phenomenon called the staircase effect, or treppe. Although all the possible causes of the staircase effect are not known, it is believed to be caused primarily by increasing calcium ions in the cytosol because of the release of more and more ions from the sarcoplasmic reticulum with each successive muscle action potential and failure of the sarcoplasm to recapture the ions immediately. Jesus is Lord 209 2/09/2018  Skeletal Muscle Tone Even when muscles are at rest, a certain amount of tautness usually remains, which is called muscle tone. Because normal skeletal muscle fibers do not contract without an action potential to stimulate the fibers, skeletal muscle tone results entirely from a low rate of nerve impulses coming from the spinal cord. These nerve impulses, in turn, are controlled partly by signals transmitted from the brain to the appropriate spinal cord anterior motoneurons and partly by signals that originate in muscle spindles located in the muscle itself. Jesus is Lord 210 2/09/2018 Peripheral sensory fibers and anterior motor neurons innervating skeletal muscle. 2/09/2018 Jesus is Lord 211  Muscle Fatigue Prolonged and strong contraction of a muscle leads to the well-known state of muscle fatigue. Studies in athletes have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen. Fatigue results mainly from inability of the contractile and metabolic processes of the muscle fibers to continue supplying the same work output. Jesus is Lord 212 2/09/2018  Contd. However, experiments have also shown that transmission of the nerve signal through the neuromuscular junction, can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle contraction. Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue within 1 or 2 minutes because of the loss of nutrient supply, especially the loss of oxygen. The onset of fatigue and its rate of development depend on the type of skeletal muscle fiber that is active, the intensity and duration of contractile activity, and the degree of an individual’s fitness. Jesus is Lord 213 2/09/2018  Types of Skeletal Muscle Fibers Skeletal muscle fibers do not all have the same mechanical and metabolic characteristics. Every muscle of the body is composed of a mixture of so- called fast and slow muscle fibers, with still other fibers gradated between these two extremes. Jesus is Lord 214 2/09/2018  Slow Fibers (Type 1, Red Muscle) Characteristics of these fibers include: 1. smaller than fast fibers. 2. innervated by smaller nerve fibers. 3. Compared with fast fibers, they have a more extensive blood vessel system and more capillaries to supply extra amounts of oxygen. 4. They have greatly increased numbers of mitochondria to support high levels of oxidative metabolism. 5. They contain large amounts of myoglobin, an iron- containing protein similar to hemoglobin in red blood cells. The myoglobin gives the slow muscle a reddish appearance and hence the name red muscle. Jesus is Lord 215 2/09/2018  Fast Fibers (Type II, White Muscle) Characteristics of these fibers include: 1. They are large for great strength of contraction. 2. An extensive sarcoplasmic reticulum is present for rapid release of calcium ions to initiate contraction. 3. Large amounts of glycolytic enzymes are present for rapid release of energy by the glycolytic process. 4. They have a less extensive blood supply than do slow fibers because oxidative metabolism is of secondary importance. 5. They have fewer mitochondria than do slow fibers, also because oxidative metabolism is secondary. A deficit of red myoglobin in fast muscle gives it the name white muscle. Jesus is Lord 216 2/09/2018  Remodeling of Muscle to Match Function All the muscles of the body are continually being remodeled to match the functions that are required of them. Their diameters, lengths, strengths, and vascular supplies are altered, and even the types of muscle fibers are altered at least slightly. This remodeling process is often quite rapid, occurring within a few weeks. Indeed, experiments in animals have shown that muscle contractile proteins in some smaller, more active muscles can be replaced in as littleJesus asis 2Lordweeks. 217 2/09/2018  Muscle hypertrophy is the increase of the total mass of a muscle. occurs to a much greater extent when the muscle is loaded during the contractile process. Only a few strong contractions each day are required to cause significant hypertrophy within 6 to 10 weeks. The manner in which forceful contraction leads to hypertrophy is not known. It is known, however, that the rate of synthesis of muscle contractile proteins is far greater when hypertrophy is developing, leading also to progressively greater numbers of both actin and myosin filaments in the myofibrils. Jesus is Lord 218 2/09/2018  Muscle Atrophy is the decrease of the total mass of a muscle. When a muscle remains unused for many weeks, the rate of degradation of the contractile proteins is more rapid than the rate of replacement. The pathway that appears to account for much of the protein degradation in a muscle undergoing atrophy is the ATP-dependent ubiquitin-proteasome pathway. Muscle denervation causes rapid atrophy Jesus is Lord 219 2/09/2018  Skeletal Muscle Disorders A number of conditions and diseases can affect the contraction of skeletal muscle. Many of them are caused by defects in the parts of the nervous system that control contraction of the muscle fibers rather than by defects in the muscle fibers themselves. For example, poliomyelitis is a viral disease that destroys motor neurons, leading to the paralysis of skeletal muscle, and may result in death due to respiratory failure. Jesus is Lord 220 2/09/2018  Muscle Cramps Involuntary tetanic contraction of skeletal muscles produces muscle cramps. During cramping, action potentials fire at abnormally high rates, a much greater rate than occurs during maximal voluntary contraction. The specific cause of this high activity is uncertain, but it is probably related to electrolyte imbalances in the extracellular fluid surrounding both the muscle and nerve fibers. These imbalances may arise from overexercise or persistent dehydration, and they can directly induce action potentials in motor neurons and muscle fibers. Jesus is Lord 221 2/09/2018 2/09/2018 Jesus is Lord 222  SMOOTH MUSCLE 2/12/2018 Jesus is Lord 223  Contd. Smooth muscle lacks striations, which distinguishes it from skeletal and cardiac muscle. There are no striations because the thick and thin filaments, while present, are not organized in sarcomeres. The nerves to them are part of the autonomic division of the nervous system rather than the somatic division. Thus, smooth muscle is not normally under direct voluntary control. 2/12/2018 Jesus is Lord 224  Contd. Smooth muscle, like skeletal muscle, uses cross-bridge movements between actin and myosin filaments to generate force, and calcium ions to control cross-bridge activity. They are found in the walls of hollow organs, such as i. the gastrointestinal tract, ii. the bladder, and iii. the uterus, as well as in iv. the vasculature, v. the ureters, vi. the bronchioles, and vii. the muscles of the eye. 2/12/2018 Jesus is Lord 225  Contd. The functions of smooth muscle are twofold: i. to produce motility (e.g., to propel chyme along the gastrointestinal tract or to propel urine along the ureter) and ii. to maintain tension (e.g., smooth muscle in the walls of blood vessels). 2/12/2018 Jesus is Lord 226  TYPES OF SMOOTH MUSCLE The great diversity of the factors that can influence the contractile activity of smooth muscles in various organs has made it difficult to classify smooth muscle fibers. Many smooth muscles can be placed, however, into one of two groups, based on the electrical characteristics of their plasma membrane: 1. Single-unit (Unitary) smooth muscles 2. Multiunit smooth muscles 2/12/2018 Jesus is Lord 227  Single-Unit (Unitary) Smooth Muscle The muscle cells in a single-unit smooth muscle undergo synchronous activity, both electrical and mechanical the whole muscle responds to stimulation as a single unit. This occurs because each muscle cell is linked to adjacent fibers by gap junctions, which allow action potentials occurring in one cell to propagate to other cells by local currents. Therefore, electrical activity occurring anywhere within a group of single-unit smooth muscle cells can be conducted to all the other connectedJesus 2/12/2018 cellsis Lord 228  Contd. Uni