Electrical Biophysical Approach in Nervous System PDF

Electrical Biophysical Approach in Nervous System 1-The Nervous System It is a communication and controlling system that regulates all the vital Operations of the human body. Importance: 1- it receives information from the surrounding environment and from the body, then it interprets thi...

Electrical Biophysical Approach in Nervous System 1-The Nervous System It is a communication and controlling system that regulates all the vital Operations of the human body. Importance: 1- it receives information from the surrounding environment and from the body, then it interprets this information and makes the body responds to it. 2- it is responsible for knowing if things are ( hot or cold, sweet or bitter. Rough or smooth). 3- it adjusts the responses that require emotions, so it makes you( sad or happy, angry or calm). 4- it regulates multiple human functions in human body such as moving, feeding, digestion, breathing, thinking. 1-The Nervous System The building unit of the nervous system is the nerve cell that is called “ Neuron”. 1-The Nervous System Functions of central nervous system: Brain: Spinal cord  1-The Nervous System Functions of peripheral nervous system: peripheral nerves: 1- afferent nerves: nerve fibers that transmit sensory information to brain or spinal cord. 2- efferent nerves: nerve fibers that transmit information from the brain or spinal cord to the appropriates muscles and glands. 2-Neuron 2-Neuron 2-Neuron Dendrites These are branch-like structures that receive messages from other neurons and allow the transmission of messages to the cell body. Cell body cell body that receives electrical messages from other neurons through contacts, called, synapses located on the dendrites or, on the cell body Axon is a tube-like structure. If the stimulus is enough it carries electrical impulse from the cell body to the axon terminals that pass the impulse to muscles, glands or other neurons. Synapse It is the chemical junction between the terminal of one neuron and the dendrites of another neuron. 3-Neural Communication All body cells display a membrane potential: which is a separation of positive and negative charges across the membrane. This potential is related to the: 1- uneven distribution of Na+, K+, Cl- and large intracellular protein anions between the intracellular fluid (ICF) and extracellular fluid (ECF). 2- differential permeability of the plasma membrane to these ions. 3-Neural Communication Two types of cells have developed a specialized use for membrane potential : 1- neurons (nerve cells) 2- muscle cells. They can undergo transient, rapid fluctuations in their membrane potentials, which serve as electrical signals. Nerve and muscle are considered excitable tissues because they produce electrical signals when excited due to changes in channel permeability. Neurons use these electrical signals to receive, process, initiate, and transmit messages and muscle use these electrical signals to contraction. 3-Neural Communication Changes in ion movement are brought about by changes in membrane permeability in response to triggering events. A triggering event might be: (1) an interaction of a chemical messenger with a surface receptor on a nerve or muscle cell membrane. (2) a stimulus, such as sound waves stimulating specialized neurons in the ear. (3) a change of potential caused by changes in channel permeability. 4- Electrical signals Electrical signals can be classified into two basic types: 1- graded potentials. 2- action potentials. 4.1. Action potential 4.1. Action potential 4.1. Action potential 4.1. Action potential Ion permeability and movement: Action potentials occur when voltage-gated ion channels open, altering membrane permeability to Na+ and K+. Action potential phases: Resting potential Hyperpolarization Depolarization Repolarization 4.1. Action potential 4.1. Action potential action potentials moving from trigger zone to axon terminal cannot overlap and cannot travel backward. 4.1. Action potential 1- A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. 2- If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. 3- At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. 4- The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. 5- The K+ channels close and the Na+/K+ transporter restores the resting potential. 4.2. Voltage and ion permeability changes during the action potential 1- Resting membrane potential. 2- Depolarization stimulus. 3- Membrane depolarize to threshold. Voltage gated Na channels open quickly and Na enter the cell. The voltage gated K channels begin to open slowly. 4- Rapid Na entry depolarize the cell. 5- Na channels close and slower voltage gated K channels open. 6- K moves from cell to extracellular fluid. 7- voltage gated K channels remain open and additional K leaves the cell ( Hyperpolarization). The threshold 8- voltage gated K channels close, less K leaks is the value of the membrane potential out of the cell. 9- Cell returns to resting ion permeability and which, if reached, leads to the initiation of resting membrane potential. an action potential. 4.1. Action potential 4.1. Action potential Rising phase Falling phase is a rapid depolarization is a rapid repolarization followed by the overshoot, followed by the undershoot, when the membrane potential when the membrane potential becomes positive. hyperpolarizes past rest 4.2. Refractory period The refractory period of a neuron: is the time after an action potential is generated, during which the excitable cell cannot produce another action potential. OR, is the time in which a nerve cell is unable to fire another action potential (nerve impulse). ( recovery time) lasts 2 millisecond Two subsets exist in terms of neurons: 1-absolute refractory period 2- relative refractory period 4.2. Refractory period absolute refractory period relative refractory period - occurs during depolarization - occurs after the absolute and repolarization refractory period during hyperpolarization -represents the time required - K+ channels are still open for the Na+ channel gates to reset to their resting positions -it is impossible for the cell to.- it is possible for the cell generate an additional action to fire another action potential. potential; it just requires more excitatory stimulus than usual to do so 4.3.Na+ Channels Gates have two gates to regulate Na ion movement down its electrochemical gradient. The two gates, known as: 1-activation gate 2- inactivation gate 4.3.Na+ Channels Gates activation and inactivation gates move in response to depolarization. but the inactivation gate delays its movement for 0.5 msec. During that delay the Na+ channel is open, allowing enough Na+ influx to create the rising phase of the action potential. When the slower inactivation gate finally closes, Na+ influx stops, and the action potential peaks. 4.4.K+ Channels Gates 4.5.The Na+–K+ ATPase pump This carrier transports Na+ out of the cell, concentrating it in the ECF, and picks up K+ from the outside, concentrating it in the ICF. the Na+–K+ pump moves three Na+ out of the cell for every two K+ it pumps in. 5. Graded Potentials 5. Graded Potentials A graded potential is produced when a ligand opens a ligand- gated channel in the dendrites, allowing ions to enter (or exit) the cell. -- Graded potentials are changes in membrane potential that vary according to the size of the stimulus. They do not typically involve voltage-gated sodium and potassium channels, but rather can be produced by neurotransmitters that are released at synapses which activate ligand-gated ion channels. They occur at the postsynaptic dendrite in response to presynaptic neuron firing and release of neurotransmitter. 5. Graded Potentials Ca2+ enters the axon terminal via voltage-dependent calcium channels and causes exocytosis of synaptic vesicles, causing neurotransmitter to be released. The transmitter diffuses across the synaptic cleft and activates ligand-gated ion channels that mediate the EPSP (Excitatory postsynaptic potentials). EPSPs are caused by the influx of Na+ or Ca2+ from the extracellular space into the neuron or muscle cell. Graded potentials that make the membrane potential more negative, and make the postsynaptic cell less likely to have an action potential, are called inhibitory post synaptic potentials (IPSPs). Hyperpolarization of membranes is caused by influx of Cl− or efflux of K+. 5. Graded Potentials conducted only short distances. As the signal spreads from the site of stimulation, it loses strength and eventually dies out completely; 5. Graded Potentials Graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone. The trigger zone is the integrating center of the neuron and contains a high concentration of voltage-gated Na+ channels in its membrane. If graded potentials reaching the trigger zone depolarize the membrane to the threshold voltage, voltage-gated Na+ channels open, and an action potential is initiated. If the depolarization does not reach threshold, the graded potential simply dies out as it moves into the axon. A hyperpolarizing graded potential moves the membrane potential farther from the threshold value and makes the neuron less likely to fire an action potential. Consequently, hyperpolarizing graded potentials are considered to be inhibitory. 5. Graded Potentials are changes in membrane potential They cannot spread over long distances away from the stimulation. Graded potentials occur in the dendrites cell body A large stimulus causes a strong graded potential. a small stimulus results in a weak graded potential. 5. Graded Potentials Why do graded potentials lose strength as they move through the cytoplasm? There are two reasons: 1. Current leak. Some of the positive ions leak back across the membrane as the depolarization wave moves through the cell. 2. Cytoplasmic resistance. The cytoplasm itself provides resistance to the flow of electricity. The combination of current leak and cytoplasmic resistance means that the strength of the signal inside the cell decreases over distance. Graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone (center of the neuron ). 5. Graded Potentials graded potential that is above threshold triggers a burst of action potentials. As graded potentials increase in strength (amplitude), they trigger more frequent action potentials The amount of neurotransmitter released at the axon terminal is directly related to the total number of action potentials that arrive at the terminal per unit time.  6. Neurons Conduction Velocity in unmyelinated axon Two key physical parameters influence the speed of action potential conduction in a mammalian neuron: (1) the diameter of the axon larger the diameter of the axon faster an action potential will move (2) the resistance of the axon membrane to ion leakage out of the cell. more leak-resistant the membrane the faster an action potential will move. 7. Conduction in Myelinated Axons Myelinated axons limit the amount of membrane in contact with the extracellular fluid. The myelin sheath creates a high resistance wall that prevents ion flow out of the cytoplasm. As an action potential passes down the axon from trigger zone to axon terminal, it passes through alternating regions of myelinated axon and nodes of Ranvier. 7. Conduction in Myelinated Axons The conduction process is similar to that described previously for the unmyelinated axon, except that it occurs only at the nodes in myelinated axons. Each node has a high concentration of voltage- gated Na+ channels, which open with depolarization and allow Na+ into the axon. Sodium ions entering at a node reinforce the depolarization and keep the amplitude of the action potential constant as it passes from node to node. 7. Conduction in Myelinated Axons The apparent jump of the action potential from node to node is called saltatory conduction,  8. Conduction Velocity in Myelinated Axons Two primary factors affect the speed of propagation of the action potential: 1- the resistance within the core of the axon A decrease in resistance will increase the propagation velocity. 2- the capacitance (or the charge stored) across the membrane. A decrease in capacitance will increase the propagation velocity As Axon diameter increases The internal resistance of an axon decreases as the the propagation velocity increases  Myelinated nerve fiber Unmyelinated nerve fiber present in the present in the brain and also in the spinal autonomic nervous system. cord have axons have axons of large diameter of small diameter.  9. Examples of conduction (action potential)velocity A myelinated 8.6 μm mammalian conducts action potentials at 120 m/sec while action potentials in a smaller unmyelinated 1.5 μm pain fiber (axon) only travel 2 m/s  9. Examples of conduction (action potential)velocity A myelinated frog axon 10 μm in diameter conducts action potentials at the same speed as an unmyelinated squid axon 500-μm  In summary action potentials travel through different axons at different rates, depending on the two parameters of axon diameter and myelination. 10.Nerve Disease In demyelinating diseases the loss of myelin from vertebrate neurons can have devastating effects on neural signaling. slows the conduction of action potentials. In addition, when ions leak out of the now-uninsulated regions depolarization that reaches a node of Ranvier may not be above threshold and conduction may fail. 10.Nerve Disease Ex. Multiple sclerosis is the most common and best-known demyelinating disease. It is characterized by a variety of neurological complaints, including fatigue, muscle weakness, difficulty walking, and loss of vision.