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THEME 2: NEURAL CONTROL AND MUSCLE CONTRACTION Summary Notes (Lectures 6-10) Nervous system verse endocrine system Nervous system Endocrine system Mechanism of contr...

THEME 2: NEURAL CONTROL AND MUSCLE CONTRACTION Summary Notes (Lectures 6-10) Nervous system verse endocrine system Nervous system Endocrine system Mechanism of control AP releases Neurotransmitters Blood delivers hormones to tissue Cells affected Muscle, gland cells, neurons Virtually all body cells Resulting action Muscle contraction, gland secretion Changes in metabolic activity Time to action milliseconds Seconds, hours or days Duration of action Generally brief Generally long Organisation of the nervous system Organisation of neurons and support cells: 100 billion human neurons 10x more neuroglia cells! 90% of central nervous system (CNS) consists of neuroglia  Functions: - Supports neurons physically, and nourishes them - Forms a myelin sheath around axons of neurons - Cleans up waste or damaged cells In the Central Nervous System (CNS) neuroglia consist of:  Oligodendrocytes form myelin sheath  Astrocytes: remove K+, neurotransmitters, form the blood-brain barrier (BBB) 1   Ependymal cells (Ependymocytes): builds barriers between compartments  Microglia: phagocytes In the Peripheral Nervous System (PNS) neuroglia consist of:  Schwann cells form the myelin sheath  Satellite glial cells: thought to provide nutrient support and protection to neurons Structure of the myelin sheath Myelin sheath formed from special types of support cells (oligodendrocytes and Schwann cells) Insulation of the axon Alternated with gaps (nodes) = Nodes of Ranvier Membrane ion channels found within the nodes Impulse conduction occurs in a “jump-wise” fashion, leaping from node to node During embryology, membrane layers (= phospholipids) wrap around axons in a unique way INTRODUCTION TO ELECTRICAL POTENTIALS The resting membrane potential All living cells have an electric potential (resting potential) The resting potential of a neuron is approximately -70 mV This implies that they are ready for chemical / electrical work They are therefore in dynamic equilibrium Dynamic equilibrium  For neurons at rest: o ions are not equally distributed across the cell membrane o not only are there ion differences across the cell membrane, but also charge differences (potential differences)  Principles regarding electric potentials are: Chemical / concentration gradients can be used for electrical work (ions / molecules flow down the concentration gradient) Electrostatic gradients enable electrical work (positive ions will move to negative charges) 2  Membranes are selectively permeable (are more permeable to some ions; or only allow certain ions to pass through) Ion pumps use energy [ATP] and pump ions selectively across the membrane (pumped against the concentration / chemical gradient) Breakthrough studies on membrane potentials: was studied by Hodgkin & Huxley in the squid, Loligo pealleii they published a standard model for membrane and action potentials in 1952 why squid? They could study action potentials because of the star-shaped ganglion sending impulses via relatively enormous (“giant”) axons (up to 1 mm in diameter) of the squid. What does a neuron at rest look like? Inside there are large negatively charged proteins that cannot leave the cell across the membrane and high concentrations of K+. Outside the neuron we have high concentrations of Na ions that bind to Cl ions (also found outside cells). Inside is negatively charged (due to the proteins) and outside is positively charged due to excess Na+ How is the resting membrane potential generated for a neuron at rest? Important to understand the concept of equilibrium potential of a single ion first (important in neurons are potassium K+ and sodium Na+). Cells don’t achieve equilibrium potential because individual ions don’t function in isolation (only under experimental conditions). Let us imagine K+ is the only ion moving across the membrane. K+ moves down its chemical gradient (from inside to outside) but it is attracted back into the cell along the electric gradient (inside of cell is negative). When the chemical gradient is equal but opposite to the electric gradient there is no longer a net movement of K+ and the ion has reached its equilibrium potential. For K+ this is -90mV (it is minus 90 because it is standard to use the charge of the inside of the cell to denote the potential). 3 But K+ does not function alone because Na+ will also move across the membrane (however membrane is much more permeable to K+, about 50-75 times more permeable to K+) down its chemical and electric gradient, which are both into the cell (Can you see the problem here?). This movement of positive ions into the neuron cancels some of the negative charge left behind (in the form of the proteins) due to K+ leaving the cell. This leaves a resting neuron with a resting membrane potential of -70mV. It is clear that K+ contributes more to the resting membrane potential than Na+ (since the potential is closer to the equilibrium potential of K+, which is -90mV compared to Na+ which is +60mV). This is because there are many more leakage channels for K+ than for Na+. Membranes have leakage channels (always open) and are leaky to varying degrees to all ions (note that membranes also have voltage-gated ion channels and chemically- gated channels). Na+ has a chemical and electric gradient INTO the cell. This would result in Na+ flowing into the cell and collapsing the resting membrane potential but this does not happen because of the energy driven Na+/K+ pumps. These pumps pump Na+ out of cells (against the gradient). It acts to transport sodium and potassium ions across the cell membrane in a ratio of 3 sodium ions out for every 2 potassium ions brought in. Stimulating the neuron Graded potentials Resting potential is primarily a function of leakage channels Some ion channels however are gated and open and close in response to various stimuli (light, sound, chemicals, mechanical change etc) These stimuli can either stimulate or inhibit neurons This results in a deviation from the resting membrane potential = Graded potentials Referred to as graded because they have different “sizes” - amplitudes (also called amplitude codes) 4 Main membrane gated channels you will be introduced to this year 1) chemically-gated channels (also called ligand channels). The channel opens when a chemical (neurotransmitter) binds to the receptor on the channel and closes once the neurotransmitter detaches. 2) Voltage-gated channels (electric channels). This channel opens once there is a change in the potential difference across the membrane i.e. it is closed at -70mV but is triggered to open when the potential increases to -60mV. if membrane channels open it can lead to depolarization if, for example, Na+ channels open. This means Na+ flows INTO cells down their elctrochemical gradient reducing the membrane potential i.e. bringing the inside closer to 0mV. or it can lead to hyperpolarization if, for example, K+ channels open. This means K+ flows OUT of call down their chemical gradient increasing the membrane potential i.e. bringing the inside closer to -90mV (further away from 0mV). Hyperpolarization can also occur if Cl- ions flow into the cell bringing excess negative charge. Graded potentials occur on the dendrites and soma (cell body) of neurons (NOT axons) Do not involve voltage channels! May show depolarization or hyperpolarization Does not need a threshold potential to arise (compare this to Action Potentials – AP) Can be large or small, and fade over distance Can sum algebraically Does not have an absolute or relative refractory period (compare this to the AP) Why does the signal diminish with distance? 5 Graded potentials are conducted passively (local current). Many neurons communicate with each other in networks. This incoming information must be integrated and this is done on cell bodies (and dendrites) and can only happen with graded potentials (these codes can be summed). Summation of graded potentials leads to Threshold Potential (-55mV) needed for the Action Potential a single stimulus leads to a small graded potential single stimuli are therefore usually too weak to cause Action potentials (AP) if stimuli are too far apart, APs can also not arise because stimuli lead to excitation or inhibition, one needs enough excitatory stimuli to reach threshold potential for APs 6  if stimuli are above the threshold potential, AP will be generated at full strength the threshold potential is approximately -55 mV for neurons from here we refer briefly to threshold potential simply as ‘threshold’ The generation of action potentials is a function of voltage gated channels! specifically voltage gated Na+ channels and K+ channels membrane must depolarize sufficiently at the axon hillock to reach threshold as soon as the threshold is reached, the voltage-gated channels are triggered ion permeability of the membrane now increases dramatically Na+ ions and K+ ions then flow en masse across the membrane (with Na+ inflow preceding the outflow of K+). study the accompanying figure: Must understand the voltage-gated Na+ and K+ channels before we look at the generation of the AP 7 Triggering an action potential the graded potential must reach threshold (-55mV) at the axon hillock for an action potential taco be initiated at threshold three actions are triggered 1) The fast responding activation gate of the Na+ channel opens 2) The slower responding inactivation gate of Na+ begins closing 3) The sluggish K+ channel is triggered to open This results in a rapid influx on Na+ (depolarization), which brings the membrane closer to zero and even overshoots zero and reaches +30mV. By the time the membrane potential reaches +30mV, the inactivation gates of Na+ have closed and the channel is now closed and not able to open. Na+ stops flowing into the cell down these voltage channels. Simultaneously at 30mV, the sluggish K+ channel has opened. K+ rushes out of the cell (down its chemical gradient - repolarization) and the membrane potential starts becoming negative again (as positive charge leaves the cell). During this phase of repolarization, as the membrane approaches threshold, the Na+ activation channel closes and the inactivation channel opens (the channel is now closed but capable of opening). The K+ channel also closes but because of their sluggishness they do not close abruptly at -70mV (resting potential) and more K+ leaves the cell than required. The neuron becomes more negative (hyperpolarization) as K+ approaches its equilibrium potential. The Na+/K+ pumps pump the ions (Na+ and K+) back across the membrane and the membrane returns to rest. See figure on page 9. 8 What is the function of the dual gates in Na+ channels? During depolarization the gates are both open and the channel is activated. During repolarization, the inactivation gate is closed (and activation gate open) and this renders the Na+ channel closed and NOT capable of opening (it is in the incorrect configuration). That part of the membrane is in a refractory period (we refer to this as the absolute refractory period) – it cannot be stimulated because the Na+ gates are not capable of responding. The area of hyperpolarization is referred to as the relative refractory period. This is because the Na+ gates can respond (they have returned to the correct configuration i.e. the activation gate is closed and the inactivation gate is open) but the membrane is below resting potential (more negative than -70mV) and so the graded potentials needed to initiate action potentials would need to be of greater magnitude. Amplitude of graded potentials and frequency of action potentials: Graded potentials increase the frequency and not size (amplitude) of action potentials Frequency can increase to a maximum of ~ 1000 / sec. Absolute Refractory Period is the reason for this 9 Propagation of action potentials Action potentials are self-propagating (unlike graded potentials that spread through local current) and this means that once an action potential is initiated at the axon hillock that individual action potential will produce an exact replica of itself and so on and so on. Na+ flows into the neuron during depolarization. These ions are attracted to the negative ions further along the inside of the membrane that is still at rest. Na+ flows on the inside of the membrane (local current) and brings that section of the membrane to -55mV. This triggers the Na+ voltage gates to open and a new action potential is generated. This is a self-propagating process. Why have a refractory period? It is to ensure that impulses travel in one direction only. So action potentials can only move down the axon and not travel backwards because the membrane just having undergone an action potential is in a refractory state (the Na+ channels are in the incorrect configuration) and this ensures that the membrane ahead of an action potential is triggered to initiate another action potential. 10  Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of AP along myelinated axons from one node of Ranvier to the next node, increasing the conduction speed (velocity) of action potentials. Arriving at the axon terminals, how is information conveyed to the next neuron or effector? 1) Neurotransmitter stored in vesicles in terminal end bulbs (presynaptic neuron) 2) Action potential arrives and depolarizes the end bulb 3) This depolarization opens voltage-gated calcium channels 4) Ca2+ flows into the end bulb and acts as a 2nd messenger triggering the vesicles to move to the synaptic cleft and fuse with the plasma membrane 5) The Neurotransmitter is released into the cleft 6) The neurotransmitters bind to receptors on chemical gated channels of the postsynaptic cell 7) This opens the channel and allows the movement of ions (often Na+ flows into the postsynaptic cell) and an excitatory postsynaptic potential is initiated (excitatory because the inside becomes less negative) and if it reaches threshold AP will result. Examples of Neurotransmitters: Acetylcholine −more of Ach in muscle contraction Norepinephrine (noradrenaline) Brain and autonomic system: causes depolarization (stimulates) Dopamine Brain: stimulates or inhibits Serotonin Autonomous system: stimulation; work on emotions (too little gives rise to depression) Gamma-amino-butyric acid (GABA) - inhibitory −work against ‘anxiety’ Stimulating input = "Excitatory Post-Synaptic Potential" (EPSP) and results in the depolarization of the post- synaptic membrane e.g. Acetylcholine on skeletal muscle. Inhibitory input = "Inhibitory Post-Synaptic Potential" (IPSP) and results in the hyperpolarization of the post-synaptic membrane i.e. either opening of K+ channels or Cl- channels 11 How are nerve action potentials converted to muscle contraction? Let us first have a quick look at the basic structure of skeletal (striated) muscle One muscle fibre (= one cell): fused muscle cells and therefore multi-nucleated includes bundles of myofibrils note organelles in muscle have special names: sarcolemma = cell membrane sarcoplasm = cytoplasm sarcoplasmic reticulum (SR) = endoplasmic reticulum Myofibrils in turn consists of myofilaments:  actin (thin filaments)  myosin (thick filaments) by the nature of the arrangement of myofilaments, myofibrils have a banded pattern consisting of:  Z-lines  A-bands - where actin and myosin overlap (the A-band is as long as myosin)  I-bands – actin only  H-bands – myosin only  M-line – middle of the sarcomere  Sarcomere: from Z-line to Z-line = smallest functional unit of muscle contraction Actin and myosin are the contractile proteins of muscle (yet neither of them shorten – simply the two filaments slide between each other shortening the sarcomeres). 12 Myosin molecules Consist of globular heads and a long tail. The molecules orient themselves in such a way with their tails pointing towards the centre of the filament. The heads of each half orient opposite those of the other half and project out at the ends (called cross bridges). The head has two binding sites. 1) an actin binding site and 2) a mysoin ATPase site (ATP splitting site). Myosin is linked to the Z lines by elestic proteins called titin. Actin Molecules Actin molecules are spherical and form two strands that twist around each other into a helix. Each actin molecule has a myosin binding site. Associated with actin are two other regulatory proteins: troponin and tropomyosin. These two regulatory proteins are responsible for blocking the binding sites on actin. Actin and myosin have high binding affinity and so when muscle is at rest, actin and mysosin are prevented from binding. Tropomyosin is a threadlike molecule that covers the binding sites on actin when muscle is at rest. Tropomyosin is held in place by troponin (a protein complex that has Ca2+ binding sites) when muscle is relaxed. When Ca2+ binds to troponin (during muscle contraction) the troponin-tropomyosin complex changes conformation and the tropomyosin moves away from the binding sites allowing myosin to bind to actin (i.e. muscle contraction). Where does the Ca2+ come from? Ca2+ is stored in the sarcoplasmic reticulum and is released when a muscle cell is excited by activity in the motor neuron that stimulates it. Sarcomere is the smallest functional unit for muscle shortening and amounts to the ‘sliding’ of myofilaments (actin and myosin) over each other the theoretical model with which muscle shortening is explained is then called the ‘sliding filament theory’ the placement of the sarcomere in the myofibril, specifically in relation to the other organelles in the muscle fiber, is decisive for muscle shortening the T-tube system (indentations of the sarcolemma) and the sac-like, branched nature of the sarcoplasmic reticulum are of fundamental importance 13 Stimulus for Contraction: Motor unit  the sliding of filaments does not happen by itself  muscle contraction is controlled and motor units explain how the muscle works as a whole What is the motor unit then?  it is the motor neuron and all the muscle fibers that it innervates  when a motor neuron fires, all the fibers in the motor unit contract  the ‘all-or-nothing’ principle applies  motor unit can include up to 2000 fibers (usually ~ 150)  each fibre has a neuromuscular junction that innervates it What does the neuromuscular junction consist of? Functions the same as the chemical synapse discussed earlier. Acetylcholine is broken down by acetylcholinesterase. Muscle contraction following an action potential in the motor neuron See chemical synapse for details of neurotransmitter release from the axon terminal. The motor end plate contains the acetylcholine chemical gated channels. The Ach released from the axon terminal binds to the receptors, the channel opens and Na+ flows into the muscle cell producing an end plate potential (similar to the EPSP). The positive charge of Na+ is attracted to the negative charge of the membrane on either side of the end plate (this is still at rest and negatively charged). The local current flow opens voltage gated Na+ channels in the adjacent membrane. This brings the membrane to threshold, initiating an action potential which is then propagated throughout the muscle. Ach is destroyed by acetylcholinesterase. The action potential moves along the membrane and down the T-tublules and this depolarization of the T- tubules triggers the release of Ca2+ from the sarcoplasmic reticulum (down its chemical concentration). The Ca2+ binds to troponin which undergoes a conformational change pulling tropomyosin out of the way. The 14  myosin heads bind to the binding sites on actin (the heads are already energised having split ATP from the previous cycle –see slides) and the cross bridges swivel (power stroke) towards the M-line. This pulls actin inwards reducing the size of the H-zone. The heads detach, are re-energised, bind and swing again (cross bridge cycling) and this continues pulling actin further in (sliding filament theory). Muscle relaxes once action potentials no longer depolarize the membrane and Ca2+ is then pumped back into the sarcoplasmic reticulum (requires ATP) and the troponin-tropomyosin complex shifts back and covers the binding sites on actin and mysosin no longer attached – muscle is relaxed (see summary slide). Energy required for cross bridge cycling At rest the myosin head is energised, and has ADP and Pi attached (ATP has been split). When binding sites become available on actin, myosin binds and Pi is released. This initiates the power stroke, causing the filaments to slide, and ADP is released. The ATPase site on the myosin head is now empty. Before myosin can detach from actin, a new ATP molecule must bind to the ATPase site and then myosin is released from actin. The ATP is hydrolysed (split) and this “energises” the cross bridge and returns the myosin cross bridge to its original orientation. It can then re-attach and start the cycle again (see slide). ATP in muscle: Use of ATP in muscle contraction: o Energy for cross-bridge movement These two processes are using ONE ATP molecule o Detachment of actin and myosin o Pump back from Ca2+ to SR ATP production: o Creatine phosphate During rest, energy is stored to synthesize ATP o Anaerobic respiration Occurs in the absence of oxygen breakdown of glucose to form ATP and lactic acid (lactate) o Aerobic respiration Uses oxygen and breaks down glucose to form ATP, CO2 and water more effective than anaerobic respiration Muscle fibre response: ‘twitch’ Active (contracted) phase terminates rapidly as Ca2+ returns to SR after it is initially released ‘summation’ - a second AP follows first before all Ca2+ is removed Extends to further contractions and so one gets an extended active phase ‘quick successive’ contractions = ‘tetanus’ The active phase has been extended and several APs are short on each other, but one does not see relaxation between successive APs 15 Factors determining muscle types:  Myosin ATPase activity = speed of contraction  Density of mitochondria (rate of ATP production) = fatigue resistance Motor unit recruitment:  recruitment of motor units depends on the amount of tension that must be generated in the muscle for a specific task  can increase tension by recruiting more motor units (the number recruited), larger motor units (the number of muscle fibres within that motor unit) and by frequency of stimulation (but this can lead to muscle fatigue). Bone muscle interaction:  muscle groups work antagonistically for controlled performance  work can be isometric or isotonic builds up tension, but does not elongate, in isometric contraction muscle shortens in isotonic contraction 16