Chapter 12
Fundamentals of the Nervous System
and Nervous Tissue
I. Basic Divisions of the Nervous System (pp. 346–348, Figs. 12.1–12.3)
A. The nervous system is the master control and communications system of the body; it has the chief functions of
sensory input, integration, and motor output.
B. The basic divisions of the nervous system are the central nervous system (brain and spinal cord) and the
peripheral nervous system (nerves and ganglia).
C. The information of sensory (afferent) input travels from a point of somatic reception or a point of visceral
reception to the CNS; integration of this information dictates a response, then motor (efferent) signals are
carried from the CNS to sites of somatic motor, or branchial motor, or visceral motor responses.
D. The four main subdivisions of the PNS are somatic sensory, visceral sensory, visceral motor, and somatic
motor. This organization breaks down the PNS into functionally related groupings.
II. Nervous Tissue (pp. 348–358, Figs. 12.4–12.15)
A. The human body contains billions of nondividing neurons or nerve cells.
B. Neurons are composed of three main parts: the cell body (soma), dendrites, and an axon. (Figs. 12.4–12.5)
1. The cytoplasm of the cell body contains all the usual organelles and chromatophilic bodies. Most neuronal
cell bodies are located within the CNS; those in the PNS are termed ganglia.
2. Dendrites are branching processes extending from the cell body. Dendrites function as receptive sites for
receiving signals from other neurons.
3. Neurons have only one axon. An axon is an “impulse generator,” which takes impulses away from the
neuronal cell body.
C. Several functions characterize neurons: ability to conduct electrical impulses, extreme longevity, do not divide,
and high metabolic rate.
D. A second main type of cell in nervous tissue is the supporting cell.
E. A synapse is the functional junction and the site of communication between neurons; explain structural
components and how a synapse works; identify the major categories. (Figs. 12.6–12.8)
F. Electrical and chemical transmission of impulses occurs along the plasma membrane of a neuron; types of
potentials are action, graded, and synaptic. (p. 352, Fig. 12.9)
G. Neurons are classed by structure and by function. (pp. 353–354, Figs. 12.10–12.11)
1. Structural classification groups neurons as multipolar, bipolar, or unipolar neurons.
2. Functional classification is based on the direction the nerve impulse travels; sensory (afferent) neurons
conduct impulses toward the CNS; motor (efferent) neurons carry impulses away from the CNS;
interneurons lie in the CNS between motor and sensory neurons.
H. Non-nervous supporting cells in the nervous system support, protect, nourish, and insulate neurons. The
support cells are capable of division throughout life. (pp. 355–356, Figs. 12.12–12.13)
1. The four types of support cells in the CNS are collectively called neuroglia. Astrocytes, microglia,
ependymal cells, and oligodendrocytes are neuroglial cells of the CNS.
2. The two types of support cells in the PNS are Schwann cells and satellite cells. (pp. 355–356, Figs. 12.12–
12.13)
I. Myelin sheaths cover axons and provide insulation and increased speed of impulse conduction; myelinated
axons are present in the CNS and PNS. (pp. 330–332, Figs. 12.14–12.15)
III. Gross Anatomy of the Nervous System (pp. 358–360, Fig. 12.16)
A. Gray matter is gray-colored and surrounds the central cavities of the CNS. Gray matter is primarily composed
of neuronal cell bodies, neuroglia, and unmyelinated axons. In the cerebral cortex and cerebellum, an additional
layer of gray matter surrounds the white matter of the CNS.
B. White matter lies external to the gray matter of the CNS. White matter is composed of myelinated axons
passing between specific regions of the CNS. Tracts are bundles of axons traveling to similar destinations.
C. Nerves are bundles of axons in the PNS.
1. Axons of the PNS are surrounded by Schwann cells, which are covered by endoneurium.
2. Perineurium is a connective tissue surrounding nerve fascicles.
3. Epineurium is a tough connective tissue surrounding the entire nerve.
IV. Neuronal Integration (pp. 360–364, Figs. 12.17–12.21)
A. The CNS and PNS are functionally interrelated. Nerves of the PNS serve as information pathways to and from
the periphery of the body. The CNS is composed of interneurons that process sensory information, direct
information to specific CNS regions, initiate appropriate motor responses, and transport information from one
region of the CNS to another.
B. Reflexes are rapid, automatic motor responses to external as well as internal stimuli.
C. The reflex arc is a simple chain of neurons that determines the basic structural plan of the nervous system; there
are five basic components to a reflex arc: the receptor, a sensory neuron, an integration center, the motor
neuron, and the effector. (pp. 360–361, Figs. 12.17–12.18)
D. The basic simplified plan of the nervous system emphasizes the reflex arcs associated with the spinal cord;
sensory neurons enter the spinal cord dorsally and motor neurons exit the spinal cord ventrally. (pp. 361–363,
Fig. 12.19)
E. Multiple and complex neuronal circuits are formed by interneurons; these circuits are termed diverging,
converging, and reverberating circuits. (p. 363, Fig., 12.20)
F. Types of neuronal processing are serial processing and parallel processing. (pp. 363–364, Fig. 12.21)
V. Disorders of the Nervous System (pp. 364–365)
A. The most common cause of neural disability in young adults is multiple sclerosis (MS); an autoimmune disease
that destroys myelin in the CNS, MS is characterized by periods of relapses and remissions.
VI. Nervous Tissue Throughout Life (pp. 365–368, Figs. 12.2–12.23)
A. Invagination of the ectoderm forms the neural tube and neural crest; the neural tube becomes the CNS; sensory
and motor neurons are derived from neuroblasts. (pp. 365–366, Fig. 12.22)
B. Neurons form rapidly until the sixth month of development. Neuroepithelium begins to produce astrocytes and
oligodendrocytes, which extend outward and provide pathways along which young neurons migrate to their
final destination.
C. Sensory neurons arise from neural crest cells, which explains why their cell bodies lie outside the CNS in
ganglia.
D. Neuroscientists are investigating how growing neurons meet and form synapses.
E. Fully differentiated neurons do not divide. There is no obvious replacement of dead neurons. The discovery of
neural stem cells in adult humans has overturned the “no-new-neurons” doctrine. (See A Closer Look, pp.
367–368)
F. Regeneration of injured axons is possible in the PNS, but not possible in the CNS. (p. 366, Fig. 12.23)
SUPPLEMENTAL STUDENT MATERIALS to Human Anatomy,
Fifth Edition
Chapter 12: Fundamentals of the Nervous System and Nervous
Tissue
To the Student
It is important to understand an overview of the human nervous system before focusing on the functional anatomy of
the nervous tissue, especially neurons. Without a complete understanding of sensory input, integration, and motor
output, basic principles of neural function are elusive. The nervous system shares important functions with the
endocrine system; you will explore the endocrine system in detail in a later chapter. The nervous system is the master
controller of the other systems and enables you to respond to internal and external environmental information.
Step 1: Explain the basic organization of the nervous system.
- List functions of the nervous system.
- Distinguish between afferent input and efferent output.
- Define integration.
- Define CNS.
- Define PNS.
- Explain the following types of sensory and motor information carried by the nervous system:
• Somatic sensory
• Visceral sensory
• Somatic motor
• Visceral motor
• Branchial motor
Step 2: Describe nervous tissue.
- Define neuron, and list its structural features.
- Relate each structural feature of a neuron to its functional role.
- Define and describe a synapse.
- Explain the generation and propagation of electrical signals along neurons.
- Classify neurons structurally.
- Classify neurons functionally.
- Name six types of supporting cells and describe them by shape and function.
- Describe the structure of myelin sheaths.
Step 3: Explain nerves and the basic neuronal organization of the nervous
system.
- Define nerve.
- Describe basic components of nerves.
- Define reflex, distinguishing between monosynaptic and polysynaptic reflexes.
- Draw a reflex arc, including its basic features: sensory neuron, interneuron, and motor neuron.
- Explain how the reflex arc relates to the basic organization of the nervous system.
- Distinguish between gray matter and white matter in the CNS.
Step 4: Understand the development and disorders of nervous tissue.
- Describe the symptoms of multiple sclerosis.
- Describe the embryological development of nervous tissue.