Each skeletal muscle fiber is stimulated to contract by a branch of a nerve cell, a neuron. The muscle fiber is said to be innervated by the neuron. The neuron and the muscle make contact at the neuromuscular junction. Normally, there is one junction at the middle of each muscle fiber, though one neuron may innervate more than one muscle fiber. The neuron and all the muscle fibers it innervates are together called the motor unit.
The neuromuscular junction is shown in Figure 13.4. Our knowledge of the junction has been obtained from electron microscopy and biochemical experiments. Do not expect to be able to see this much detail with your microscope.
The long branch of the neuron travelling to the muscle fiber is the axon. The axon branches into several axon terminals each of which is in close proximity to the sarcolemma of the muscle fiber. The portion of the sarcolemma which is adjacent to the axon terminal is the motor end plate. The axon terminal and the sarcolemma do not actually touch, they are separated by a space called the synaptic cleft.
The distal end of an axon terminal is expanded into a synaptic end bulb filled with mitochondria and synaptic vesicles. The vesicles store neurotransmitters, chemicals which are capable of initiating a muscular contraction when they are released by the neuron and received by the muscle cell.
The sarcolemma of the motor end plate is folded into many small invaginations called subneural clefts. These folds increase the surface area for the reception of the neurotransmitter.
Correctly label Figure 13-4. Exercise 135
Examine a microscopic slide of a neuromuscular junction. Identify the axon, axon terminal, muscle fiber, and synaptic end bulb.
Figure 13.4 Neuromuscular Junction
Axon of neuron Axon terminal Mitochondrion Motor end plate Sarcomere Sarcoplasm Subneural cleft Synaptic cleft Synaptic end bulb Synaptic vesicle
Skeletal muscle synaptic end
Neuromuscular junction synaptic end muscle fiber ■MK III
2. _ A tube continuous with the membrane of the muscle cell is the (2).
3. _ Another name for a muscle cell is a muscle (3) .
4. _ The cell membrane of a muscle cell is the (4).
6. _ Thick myofilaments are composed of (6).
7. _ The gap between neuron and muscle is the CD .
8. _ Invaginations of the motor end plate serve to increase the surface area for the purpose of receiving (8) .
9. _ The invaginations of question eight are called (9) .
10. What role do the great number of mitochondria in the axon terminals play in the physiology of the neuromuscular junction?
CHAPTER I t~f
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.