Anatomy and Physiology of the Nervous System
I Structure of the Nerve Fiber
A Each nerve fiber has three components: (Figure 16-1)
1. Cell body or soma: The primary metabolic area of the nerve, site of initial synthesis of transmitter substances.
2. Dendrite: The structures that carry impulses to the cell body. A single neuron may contain 10,000 dendrites.
3. Axon: The structure that conducts impulses away from the cell body. Usually only one axon leaves a cell body; however, it may branch extensively.
A Each neuron is classified by the direction of impulse transmission.
1. Sensory (afferent) neurons: Transmit nerve impulses to the spinal cord or brain.
2. Motor (efferent) neurons: Transmit nerve impulses from the brain or spinal cord to muscles or glands (effector organs).
3. Interneurons (internuncial neurons): Conduct impulses from sensory to motor neurons entirely within the central nervous system (CNS).
III Nerve Cell Membrane Potential (Figure 16-2)
A In the resting state, the inner surface of the cell membrane is negative compared with the positive outer surface. This sets up an electric potential across the cell membrane (normal polarity).
1. Intracellular: The primary cation, potassium (K+), is readily diffusible across the cell membrane.
2. Extracellular: The primary cation, sodium (Na+), is poorly diffusible across the cell membrane.
3. High extracellular Na+ (142 mEq/L) and low extracellular K+ (5 mEq/L) levels, along with high intracellular K+ (140 mEq/L) and low intracellular Na+ (10 mEq/L) levels, are maintained in the resting state by the Na+ pump (active transport).
4. Active transport (Na+ pump): The movement of Na+ from the intracellular space to the extracellular space and the movement of K+ from the extracellular space to the intracellular space. Adenosine triphosphate (ATP) provides the energy for the uphill transport of these ions.
A For a nerve impulse to be transmitted, an alteration in the cell’s resting membrane potential must be activated.
B An action potential is a stimulus that is capable of significantly increasing the cell membrane’s permeability to sodium.
C The action potential is an all-or-nothing phenomenon (i.e., the stimulus must be strong enough to allow for a reversal in the membrane potential); Na+ moves into the cells causing the intracellular side of the membrane to become positive. If the stimulus is not of sufficient strength, an action potential does not occur.
V Nerve Impulse Propagation (Figure 16-3)
A The nerve impulse is a self-propagating wave of electric charge that travels along the surface of the neuron’s membrane (i.e., the movement of the action potential along the cell membrane changing its charge). The nerve impulse travels in one direction only, from dendrite to cell body to axon.
B Depolarization: Stage 1 of the action potential, in which an impulse travels along the nerve fiber.
1. Sodium ions rush to the inside of the cell membrane.
2. A positive intracellular membrane charge is set up with a negative extracellular membrane charge.
3. There is a complete reversal of the resting membrane potential.
C Repolarization: Stage 2 of the action potential, in which the membrane returns to the normal resting membrane potential.
A The nerve synapse (synaptic cleft) is the junction between one neuron and another neuron, muscle, or gland. It is an actual space between nerve fibers.
B Transmission of an impulse from the axon of one nerve to a dendrite, soma, or effector organ is a chemical process occurring across the synapse. The distance across the synapse is approximately 200 Å.
C Presynaptic terminals are located on the axon and contain vesicles that synthesize, store, and secrete a transmitter substance into the synapse. The transmitter substance stimulates the dendrite or soma of the next nerve, causing an action potential.
D Postsynaptic terminals are located on dendrites, somas, or effector organs. After being stimulated, these terminals secrete a substance into the synapse to metabolize the transmitter substance.
E Acetylcholine as the transmitter substance (Figure 16-4)
1. Acetylcholine is synthesized in the terminal endings of cholinergic nerve fibers. After synthesis the acetylcholine is transported into the presynaptic vesicles where it is stored and released. It is formed from the reaction of acetyl coenzyme A with choline in the presence of choline acetyltransferase.
2. Vesicles storing acetylcholine are formed in the cell body and migrate to the surface of the presynaptic terminal.
3. When an action potential reaches the presynaptic terminal, acetylcholine is released into the synapse.
4. The acetylcholine moves across the synapse and stimulates a receptor on the postsynaptic terminal.
5. After stimulation the acetylcholine is released back into the synapse, and the postsynaptic terminal releases cholinesterase (acetylcholinesterase and acetylcholine esterase are terms synonymous with cholinesterase), which metabolizes acetylcholine, forming choline and acetate ion.
6. The choline is reabsorbed into the presynaptic terminal and is available to form more acetylcholine.
F Norepinephrine as the transmitter substance (Figure 16-5)
1. Norepinephrine is synthesized inside and outside the presynaptic vesicles, where it is stored and released.
2. The synthesis of norepinephrine proceeds as follows:
3. Norepinephrine is released via a mechanism identical to acetylcholine (see section VI-E).
4. After stimulation norepinephrine has three possible immediate fates:
a. Reabsorption into the presynaptic terminal (50% to 80% of secreted volume) for future secretion.
b. Metabolism in adjoining tissue by catechol-O-methyl-transferase (COMT) or in the nerve ending by monoamine oxidase (MAO).
c. Diffusion to surrounding tissues and eventually into the blood.
5. Complete metabolism of by-products formed in the synapse or norepinephrine entering the blood occurs in the liver via MAO and COMT.
6. In the adrenal medulla the formation of norepinephrine may proceed one step further to epinephrine by the process of methylation.
7. Duration of effect of norepinephrine in the synapse is only a few seconds. However, the norepinephrine and epinephrine released into the blood by the adrenal medulla remain active for up to several minutes.
VII Alteration of Nerve Impulse Transmission
A This junction is the site of transmission of impulses from nerves to skeletal muscle fibers.
B The axon of the nerve branches at its end to form a structure called the motor endplate, which invaginates into muscle fiber but does not penetrate the muscle membrane.
C The motor endplate’s terminal aspects are referred to as sole feet.
D Sole feet provide a large area of “contact” on the muscle surface. It is from the sole feet that the transmitter substance acetylcholine is secreted.
E After stimulation cholinesterase is secreted by the muscle fiber to metabolize the acetylcholine.
IX Reflex Arc or Reflex Action
A The reflex arc is a functional process of the nervous system. Most neuromuscular and neuroglandular mechanisms are controlled by a reflex arc.
