Peripheral Nervous System

Nerve fibers in the peripheral nervous system transmit signals between the central nervous system and receptors and effectors in all other parts of the body. As noted earlier, the nerve fibers are grouped into bundles called nerves. The peripheral nervous system consists of 43 pairs of nerves: 12 pairs of cranial nerves and 31 pairs that connect with the spinal cord as the spinal nerves. The cranial nerves and a summary of the information they transmit were listed in Table 8-8. In general, of the spinal nerves, eight cervical nerves control the muscles and glands and receive sensory input from the neck, shoulder, arm, and hand. The 12 thoracic nerves are associated with the chest and abdominal walls. The five lumbar nerves are associated with the hip and leg, and the five sacral nerves are associated with the genitals and lower digestive tract. (A single pair of coccygeal nerves brings the total to 31 pair.)

Each nerve fiber is surrounded by a Schwann cell. Some of the fibers are wrapped in layers of Schwann-cell

PART TWO Biological Control Systems

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

PART TWO Biological Control Systems

TABLE 8-9 Summary of Functions of the Major Parts of the Brain

TABLE 8-10 Divisions of the Peripheral Nervous System

I. Brainstem

A. Contains all the fibers passing between the spinal cord, forebrain, and cerebellum

B. Contains the reticular formation and its various integrating centers, including those for cardiovascular and respiratory activity (Chapters 14 and 15)

C. Contains nuclei for cranial nerves III through XII

II. Cerebellum

A. Coordinates movements, including those for posture and balance (Chapter 12)

B. Participates in some forms of learning (Chapter 13)

III. Forebrain

A. Cerebral hemispheres

1. Contain the cerebral cortex, which participates in perception (Chapter 9), the generation of skilled movements (Chapter 12), reasoning, learning, and memory (Chapter 13)

Contain subcortical nuclei, including those that participate in coordination of skeletal-muscle activity (Chapter 12) Contain interconnecting fiber pathways

B. Thalamus 1. Is a synaptic relay station for sensory pathways on their way to the cerebral cortex (Chapter 9) Participates in control of skeletal-muscle coordination (Chapter 12) Plays a key role in awareness (Chapter 13)

C. Hypothalamus 1. Regulates anterior pituitary gland (Chapter 10)

Regulates water balance (Chapter 16) Participates in regulation of autonomic nervous system (Chapters 8 and 18) Regulates eating and drinking behavior (Chapter 18)

Regulates reproductive system (Chapters 10 and 19)

Reinforces certain behaviors (Chapter 13) Generates and regulates circadian rhythms (Chapters 7, 9, 10, and 18) Regulates body temperature (Chapter 18) Participates in generation of emotional behavior (Chapter 13)

D. Limbic system

1. Participates in generation of emotions and emotional behavior (Chapter 13)

2. Plays essential role in most kinds of learning (Chapter 13)

membrane, and these tightly wrapped membranes form a myelin sheath (see Figure 8-3). Other fibers are unmyelinated.

A nerve contains nerve fibers that are the axons of efferent neurons or afferent neurons or both. Accordingly, fibers in a nerve may be classified as belonging to the efferent or the afferent division of the peripheral nervous system (Table 8-10). All the spinal nerves

I. Afferent division

II. Efferent division

A. Somatic nervous system

B. Autonomic nervous system

1. Sympathetic division

2. Parasympathetic division

3. Enteric division contain both afferent and efferent fibers, whereas some of the cranial nerves (the optic nerves from the eyes, for example) contain only afferent fibers.

As noted earlier, afferent neurons convey information from sensory receptors at their peripheral endings to the central nervous system. The long part of their axon is outside the central nervous system and is part of the peripheral nervous system. Afferent neurons are sometimes called primary afferents or firstorder neurons because they are the first cells entering the central nervous system in the synaptically linked chains of neurons that handle incoming information.

Recall that efferent neurons carry signals out from the central nervous system to muscles or glands. The efferent division of the peripheral nervous system is more complicated than the afferent, being subdivided into a somatic nervous system and an autonomic nervous system. These terms are somewhat misleading because they suggest additional nervous systems distinct from the central and peripheral systems. Keep in mind that the terms together denote the efferent division of the peripheral nervous system.

The simplest distinction between the somatic and autonomic systems is that the neurons of the somatic division innervate skeletal muscle, whereas the auto-nomic neurons innervate smooth and cardiac muscle, glands, and neurons in the gastrointestinal tract. Other differences are listed in Table 8-11.

The somatic portion of the efferent division of the peripheral nervous system is made up of all the nerve fibers going from the central nervous system to skeletal-muscle cells. The cell bodies of these neurons are located in groups in the brainstem or spinal cord. Their large diameter, myelinated axons leave the central nervous system and pass without any synapses to skeletal-muscle cells. The neurotransmitter released by these neurons is acetylcholine. Because activity in the somatic neurons leads to contraction of the innervated skeletal-muscle cells, these neurons are called motor neurons. Excitation of motor neurons leads only to the contraction of skeletal-muscle cells; there are no somatic neurons that inhibit skeletal muscles.

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

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

TABLE 8-11 Peripheral Nervous System:

Somatic and Autonomic Divisions

Somatic

1. Consists of a single neuron between central nervous system and skeletal-muscle cells

2. Innervates skeletal muscle

3. Can lead only to muscle excitation

Autonomic

1. Has two-neuron chain (connected by a synapse) between central nervous system and effector organ

2. Innervates smooth and cardiac muscle, glands, and GI neurons

3. Can be either excitatory or inhibitory

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.

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