Excitation of Skeletal Muscle: Neuromuscular Transmission + Excitation-Contraction Coupling

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Physiology Ch 7 p83-89

Excitation of Skeletal Muscle: Neuromuscular Transmission + Excitation-Contraction Coupling
-Skeletal muscle fibers are innervated by large, myelinated nerves from anterior horns of cord

-Each fiber branches and stimulates from 3-100 muscle fibers at neuromuscular junctions

Physiologic Anatomy of Neuromuscular Junction – Motor End Plate – nerve fiber forms a complex of branching nerve terminals that invaginate into muscle but are outside membrane

-entire structure is called the motor end plate; covered by schwann cells for insulation

-invaginated membrane is called synaptic gutter or synaptic trough and the space between the terminal and fiber membrane is called the synaptic cleft (20-30nm wide)

-at the bottom of the gutter, there are folds of muscle membrane called subneural clefts which increase the surface area onto which neurotransmitters act

-the axon terminal has many mitrochondria to supply ATP for synthesis of acetylcholine, which excited the muscle fiber membrane

-Ach is synthesized in cytoplasm but is absorbed into synaptic vesicles

-in synaptic space, the enzyme acetylcholinesterase breaks down Ach after release
Secretion of Acetylcholine by Nerve Terminals – when nerve impulse reaches neuromuscular junctions, 125 vesicles of acetylcholine are released into synaptic cleft

-on inside of neural membrane, linear dense bars are found; on each side of dense bars are voltage-gated calcium channels which open after an action potential to allow influx of Ca ions from synaptic space

-Ca is attractant for acetylcholine vesicles to draw them to membrane and fuse
Effect of Acetylcholine on Postsynaptic Muscle Fiber Membrane to Open Ion Channels – acetylcholine receptors exist on muscle fiber membrane and are almost entirely located near mouths of subneural clefts immediately below dense bar areas where Ach is empties

-Each receptor is a complex composed of 5 subunits: 2 alpha, and one beta, delta, gamma to form a tubular channel

-Ach receptor channel is normally constricted until two acetylcholine molecules attach to the alpha subunits to cause a conformational change that opens the channel

-the ACh-gated channel allows Na, K, and Ca to flow through; but negative Cl can’t

-sodium flows through the most because Na in extracellular fluid at high concentration, and high potassium intracellularly, and negative potential of -80 to -90mV pulls sodium in and preventing K+ from going out

-when ACh channel opens, Na rushes in to create a positive potential change inside muscle fiber membrane, called the end-plate potential, to initiate action potential spreading along muscle

Destruction of Released Acetylcholine by Acetylcholinesterase – acetylcholine will continue to activate receptors on muscle until removed in two ways:

  1. Most of acetylcholine destroyed by acetylcholinesterase

  2. Small amount of acetylcholine diffuses out of synaptic space and can’t act on muscle

-the short time that ACh stays in synaptic cleft can excite fiber, and rapid removal prevents continued muscle excitation
End-Plate Potential and Excitation of Skeletal Muscle Fiber – sudden influx of Na through ACh channels causes electrical potential inside fiber at the local area of endplate to increase in a positive direction as much as 50-75mV to create a local potential called end-plate potential

-this is enough to stimulate an action potential

-curare is a drug that blocks gating of acetylcholine by competing for acetylcholine on receptor site to cause weakness in end-plate potential

-botulinum toxin – bacterial poison that decreases quantity of ACh release by nerve terminals

Safety Factor for Transmission at Neuromuscular Junction; Fatigue – each impulse carries 3x as much end plate potential to stimulate muscle fiber; therefore, normal neuromuscular junction is said to have high safety factor

-stimulation of nerve fiber at rates greater than 100x/s often diminishes number of ACh vesicles so much that impulses fail to pass into muscle; this is called fatigue

Molecular Biology of Acetylcholine Formation and Release – formation through these stages:

  1. small vesicles formed by Golgi in soma of motorneuron are transported by axoplasm (core of axon) from soma all the way to neuromuscular junction; 300,000 collect there

  2. ACh Is synthesized in cytosol of nerve fiber terminal but is immediately transported through membranes of vesicles inside, where it is concentrated to 10,000/vesicle

  3. Action potential causes influx of Ca ions through voltage gated channels, which increase rate of fusion of acetylcholine vesicles with terminal membrane, allowing exocytosis of acetylcholine into synaptic space

    1. In few milliseconds, ACh is cleaved by acetylcholinesterase  acetate + choline

    2. Choline reabsorbed into neuron terminal to form new ACh

  4. Number of vesicles in nerve ending is sufficient to allow transmission of a few thousand nerve-to-muscle impulses, and so new vesicles need to be formed rapidly

  5. After each action potential is over, coated pits appear in terminal nerve membrane caused by contractile proteins in nerve ending with clathrin, which is attached at vesicle membrane; and within 20 seconds the proteins contract and vesicles pinch off inside, where acetylcholine enters and the cycle repeats itself

Drugs that Enhance or Block Transmission at Neuromuscular Junction

  1. Drugs that Stimulate Muscle Fiber by Acetylcholine-like action – methacholine, carbachol, and nicotine have same effect as acetylcholine, but these drugs are not broken down by acetylcholinesterase, so their action persists

    1. Drugs work by causing localized areas of depolarization of muscle membrane where ACh receptors are located, can cause muscle spasm

  2. Drugs that Stimulate Neuromuscular Junction by Inactivating Acetylcholinesterase – three drugs: neostigmine, physostigmine, and diisopropyl fluorophosphate inactivate acetylcholinesterase so that ACh accumulates in synapse to continuously stimulate muscle fiber and can cause muscle spasm

    1. Neostigmine and physostigmine bind to inactive acetylcholinesterase reversibly

    2. Diisopropyl fluorophosphate inactivates the enzyme for weeks

  3. Drugs that Block Transmission at the Neuromuscular Junction – group of drugs known as curariform drugs prevent passage of impulses from nerve ending onto muscle; D-tubocurarine blocks action of ACh on muscle fiber ACh receptors to prevent increae in permeability required to initiate an action potential

Myasthenia Gravis Causes Muscle Paralysis – causes inability of neuromuscular junctions to transmit enough signals from nerve fibers to muscle fibers

-antibodies attack acetylcholine receptors (autoimmune disease) at neuromuscular junction

-end plate potentials are too weak to initiate muscle depolarization, patient can die of paralysis
Muscle Action Potential – resting membrane potential is about -80 to -90mV in skeletal muscle, and the duration of action potential is 1-5ms, about 5x as long as in large, myelinated nerves

-velocity of conduction is about 3-5m/s, about 1/13th the velocity of myelinated nerves

Spread of Action Potential to Interior by Transverse Tubules – skeletal muscle is so large that action potential along surface causes no current to flow deeper into fiber; which is solved by use of transverse (T) tubules that penetrate through muscle fiber from one fiber to another

-T tubule action potentials cause release of Ca inside muscle fiber in immediate vicinity of myofibrils, and these Ca ions cause contraction in process called excitation-contraction coupling

Excitation-Contraction Coupling

Transverse Tubule – Sarcoplasmic Reticulum System – T tubules and run transverse to myofibrils beginning in cell membrane and going from one side of muscle fiber to the other

-where T tubules originate from cell membrane, they are open to exterior of the muscle fiber and communicate with extracellular fluid (they are internal extensions of cell membrane)

-when action potential spreads over muscle fiber, a potential change also spreads along T tubules deep into muscle fiber to stimulate contraction

-sarcoplasmic reticulum composed of (1) terminal cisternae (large chambers that are adjacent to T tubules), and (2) long longitudinal tubules surrounding all surfaces of myofibrils

Release of Ca Ions by Sarcoplasmic Reticulum – sarcoplasmic reticulum has an excess of Ca in vesicles that are released during action potential in adjacent T tubule

-Action potential of T tubule causes current flow into sarcoplasmic reticular cisternae

-as action potential reaches T tubule, voltage change is sensed by dihydropyridine receptors linked to calcium release channels also called ryanodine receptor channels in adjacent sarcoplasmic reticular cisternae to open Ca channels = influx of Ca into sarcoplasm
Calcium Pump for Removing Ca Ions from Myofibrillar Fluid After Contraction – once Ca ions have been released and diffused into myofibrils, muscle contraction continues as long as Ca stays in high concentration; but Ca is pumped via a continually active Ca pump on walls of sarcoplasmic reticulum away from myofibril and into sarcoplasmic tubules

-a protein called calsequestrin can bind Ca to concentrate it inside sarcoplasmic reticulum

Excitatory “Pulse” of Ca ions – normal resting concentration of Ca in cytosol that bathes myofibrils is too low for contraction, so troponin-tropomyosin complexes keeps actin filaments inhibited

-full excitation of T tubule and sarcoplasmic reticulum causes enough Ca release to increase concentration in myofibrillar fluid 500x, 10x more than required for contraction

-immediately after, calcium pump depletes the Ca again

-total duration of Ca “pulse” in skeletal muscle fiber is 1/20th of a second

-during this pulse, muscle contraction occurs

-to continue contraction, numerous sequential pulses must occur

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