Membrane Excitation The Neuromuscular Junction

We have just seen that an action potential in the plasma membrane of a skeletal-muscle fiber is the signal that triggers contraction. The next question we must ask then is: How are these action potentials initiated? Stimulation of the nerve fibers to a skeletal muscle is the only mechanism by which action potentials are initiated in this type of muscle. As we shall learn, there are additional mechanisms for activating cardiac- and smooth-muscle contraction.

The nerve cells whose axons innervate skeletal-muscle fibers are known as motor neurons (or somatic efferent neurons), and their cell bodies are located in either the brainstem or the spinal cord. The axons of motor neurons are myelinated and are the largest-diameter axons in the body. They are therefore able to propagate action potentials at high velocities, allowing signals from the central nervous system to be transmitted to skeletal-muscle fibers with minimal delay.

Upon reaching a muscle, the axon of a motor neuron divides into many branches, each branch forming a single junction with a muscle fiber. A single motor neuron innervates many muscle fibers, but each muscle fiber is controlled by a branch from only one motor neuron. A motor neuron plus the muscle fibers it innervates is called a motor unit (Figure 11-17a). The muscle fibers in a single motor unit are located in one muscle, but they are scattered throughout the muscle and are not adjacent to each other (Figure 11-17b). When an action potential occurs in a motor neuron, all the muscle fibers in its motor unit are stimulated to contract.

Motor neurons

Motor neurons

Neuromuscular Junction Steps For Dummies

FIGURE 11-17

(a) Single motor unit consisting of one motor neuron and the muscle fibers it innervates. (b) Two motor units and their intermingled fibers in a muscle.

FIGURE 11-17

(a) Single motor unit consisting of one motor neuron and the muscle fibers it innervates. (b) Two motor units and their intermingled fibers in a muscle.

The myelin sheath surrounding the axon of each motor neuron ends near the surface of a muscle fiber, and the axon divides into a number of short processes that lie embedded in grooves on the muscle-fiber surface. The region of the muscle-fiber plasma membrane that lies directly under the terminal portion of the axon has special properties and is known as the motor end plate. The junction of an axon terminal with the motor end plate is known as a neuromuscular junction (Figure 11-18).

The axon terminals of a motor neuron contain vesicles similar to the vesicles found at synaptic junctions between two neurons. The vesicles contain the neuro-transmitter acetylcholine (ACh). When an action potential in a motor neuron arrives at the axon terminal, it depolarizes the nerve plasma membrane, opening voltage-sensitive calcium channels and allowing calcium ions to diffuse into the axon terminal from the extracellular fluid. This calcium binds to proteins that enable the membranes of acetylcholine-containing vesicles to fuse with the nerve plasma membrane thereby releasing acetylcholine into the extracellular cleft separating the axon terminal and the motor end plate.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Myelin sheath

Motor axon

Muscle plasma membrane

Acetylcholine -vesicle

Motor

Acetylcholine -vesicle

Motor

Neuromuscular Junction Steps

FIGURE 11-18

Neuromuscular junction. The motor axon terminals are embedded in grooves in the muscle fiber's surface. %

FIGURE 11-18

Neuromuscular junction. The motor axon terminals are embedded in grooves in the muscle fiber's surface. %

ACh diffuses from the axon terminal to the motor end plate where it binds to receptors [of the nicotinic type (Chapter 8)]. The binding of ACh opens an ion channel in each receptor protein. Both sodium and potassium ions can pass through these channels. Because of the differences in electrochemical gradients across the plasma membrane (Chapter 8), more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end-plate potential (EPP). Thus, an EPP is analogous to an EPSP (excitatory postsynaptic potential) at a synapse (Chapter 8).

The magnitude of a single EPP is, however, much larger than that of an EPSP because neurotransmitter is released over a larger surface area, binding to many more receptors and hence opening many more ion channels. For this reason, one EPP is normally more than sufficient to depolarize the muscle plasma membrane adjacent to the end-plate membrane, by local current flow, to its threshold potential, initiating an action potential. This action potential is then propagated over the surface of the muscle fiber by the same mechanism described in Chapter 8 for the propagation of action potentials along axon membranes (Figure 11-19). Most neuromuscular junctions are located near the middle of a muscle fiber, and newly generated muscle action potentials propagate from this region in both directions toward the ends of the fiber.

To repeat, every action potential in a motor neuron normally produces an action potential in each muscle fiber in its motor unit. This is quite different from synaptic junctions, where multiple EPSPs must occur, undergoing temporal and spatial summation, in order for threshold to be reached and an action potential elicited in the postsynaptic membrane.

A second difference between synaptic and neuromuscular junctions should be noted. As we saw in Chapter 8, at some synaptic junctions, IPSPs (inhibitory

Propagated action potential in muscle plasma membrane r

Motor neuron action potential

Propagated action potential in muscle plasma membrane

Site of muscle-fiber -action potential initiation

Motor neuron terminal

- Acetylcholine vesicle y— Released^ acetylcholine

Current between depolarized -end plate and adjacent muscle plasma membrane t t

Site of muscle-fiber action potential initiation

Acetylcholine receptor Acetylcholinesterase

Current between depolarized end plate and adjacent muscle plasma membrane

Essentials of Human Physiology

Essentials of Human Physiology

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.

Get My Free Ebook


Post a comment