University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

382

Pages:

7
Author:

coureurpk

Motor Units • Muscle cells of a Motor Unit are dispersed throughout a muscle – provides ability to sustain longterm contraction as motor units take turns resting • Small Motor Units provide Fine Control – small motor units contain as few as 3-6 muscle fibers per nerve fiber – example: eye muscles • Large Motor Units are for Strength – large motor units have as many as 1000 muscle fibers per nerve fiber – example: gastrocnemius muscle Neuromuscular Junction • Neuromuscular Junction (NMJ) is a synapse between a nerve fiber and a muscle cell. • Synapse is the functional connection between a nerve cell and its target cell. • Components of the NMJ – terminal boutton (axon terminal, synaptic knob, terminal button, axonal swelling, synaptic bulb, end bulb) is the swollen end of a nerve fiber and contains vesicles of the neurotransmitter acetylcholine (ACh) – motor end plate is the specialized region of muscle cell membrane under the terminal boutton – motor end plate membrane has ACh receptors on junctional folds which bind ACh released from the nerve – acetylcholinesterase (AChE) is an enzyme in the basal lamina in the synaptic cleft that breaks down ACh and causes relaxation – synaptic cleft is the gap between the nerve and muscle cells – Schwann cells cover the axon and the NMJ The Neuromuscular Junction Muscle and Nerve Electrochemical Communication • At rest, muscle cell and nerve cell membranes are polarized (charged). Changes in charge are relayed from one cell to another. • Membrane polarity or charge is measured in units of volts – car battery = 12 volts – flashlight battery = 1.5 volts – muscle cell membrane = .06 volts or 60 millivolts (mV) • Difference in charge across the membrane is called the membrane potential. – Resting Membrane Potential is established by a Na+/K+ exchange pump that results in high [Na+] outside of cell and high [K+] and anions inside of cell resulting in a slightly negative voltage inside the cell (-60 mV). • Sarcolemma depolarizes in response to motor neuron stimulation from release of ACh. • Voltage change spreads across the membrane as an action potential Action Potential Resting membrane potential of -60 mV is established by the Na+/K+ ATPase pump and nongated K+ channels (K+ leak channels) Opening of Na+ channels is triggered by ACh and allows Na+ to rush in depolarizing the membrane. Membrane depolarization triggers the opening of voltage activated K+ channels and closing of Na+ channels. K+ leaves the cell through the open channels. The Na+/K+ pump works to restore the membrane potential back to resting levels. +75 3 4 2 -90 1 5 1 2 3 4 5 Resting Membrane Potential End Plate Potential Action Potential Depolarization Action Potential Repolarization Hyperpolarization Na+/K+ pump Na+ out K+ in Na+ out K+ in Na+ out K+ in Na+ out K+ in Na+ out K+ in non-gated K+ leak channels K+ out K+ out K+ out K+ out K+ out ACh Receptor (a ligand-regulated ion channel) Closed Open letting Na+ in and K+ out Open as long as ACh is present Open as long as ACh is present Open as long as ACh is present voltage-gated Na+ channel Closed Closed Open and letting Na+ in Closed Closed voltage-gated K+ channel Closed Closed Slowly Opening Open and letting K+ out Slowly Closing Ion primarily responsible for the membrane voltage Na+ pumped out K+ leaking out Na+ in K+ out Na+ in K+ out K+ out