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Nervous Tissue
Chapter 17
CHAPTER SUMMARY
This chapter begins with an excellent description of the structure and functions as well as the organization of the nervous
system, its divisions, and subdivisions. The histology of the nervous system is portrayed in detail. The important
characteristics of neuroglia are described. The characteristics of neurons are described in extensive detail. The
classification of neurons is clearly explained. The major features of gray matter and white matter are outlined. The
characteristics of neuronal circuits are described. Regeneration and neurogenesis are portrayed. This chapter concludes
with a list of key medical terms associated with nervous tissue, a thorough study outline, an excellent self-quiz, critical
thinking questions, and answers to questions that accompany chapter figures.
STUDENT OBJECTIVES
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List the structures and describe the basic functions of the nervous system.
Describe the organization of the nervous system.
Contrast the histological characteristics and functions of neuroglia and neurons.
Describe the parts of a neuron and the functions of each part.
Classify neurons by structure and function.
Distinguish between gray matter and white matter.
Identify the structure and function of the various types of neuronal circuits in the nervous system.
Define regeneration and neurogenesis.
LECTURE OUTLINE
A. Overview of the Nervous System (p. 537)
1. The nervous system and the endocrine system are the body’s major control and integrating centers.
2. Neurology is the study of the normal functioning and disorders of the nervous system.
3. The major components of the nervous system include the brain, cranial nerves, spinal cord, spinal nerves,
and enteric plexuses; a nerve is a bundle of axons (plus associated connective tissue and blood vessels) located
outside the brain and spinal cord.
4. The nervous system has three major functions:
i. sensory function, i.e., sensory receptors detect stimuli in the internal and external environments,
resulting in sensory information being transmitted by sensory or afferent neurons to the brain or
spinal cord
ii. integrative function, i.e., interneurons play a role in analyzing the sensory information, storing some
of it, and making decisions regarding appropriate behaviors
iii. motor function, i.e., motor or efferent neurons respond to integration decisions by initiating actions
in effectors, including muscle fibers and glandular cells
B. Organization of the Nervous System (p. 538)
1. The nervous system consists of two major divisions:
i. central nervous system (CNS), which consists of the brain and spinal cord
ii. peripheral nervous system (PNS), which consists of [1] cranial nerves that emerge from the brain,
and [2] spinal nerves that emerge from the spinal cord; the PNS contains [a] sensory or afferent
neurons which transmit nerve impulses from sensory receptors to the CNS, and [b] motor or efferent
neurons which transmit nerve impulses from the CNS to muscles and glands. The PNS is divided into
three major subdivisions:
a. voluntary somatic nervous system (SNS), which consists of [1] sensory neurons that transmit
information from somatic and special sensory receptors to the CNS, and [2] motor neurons that
transmit messages from the CNS to skeletal muscles
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b.
c.
involuntary autonomic nervous system (ANS), which consists of [1] sensory neurons that
transmit information from visceral receptors to the CNS, and [2] motor neurons that transmit
messages from the CNS to smooth muscle, cardiac muscle, and glands; the motor portion of
the ANS consists of two branches:
I. sympathetic division which generally supports exercise and emergency actions, i.e.,
“fight-or-flight” responses
II. parasympathetic division which generally promotes “rest-and-digest” activities
involuntary enteric nervous system (ENS; the “brain of the gut”) which consists
of neurons in the enteric plexuses that extend the entire length of the GI tract
C. Histology of Nervous Tissue (p. 539)
1. The nervous system consists of two major types of cells:
i. neurons, which perform most of the specialized functions of the nervous system
ii. neuroglia, which support, nourish, and protect the neurons and maintain the interstitial fluid that
bathes neurons
2. Neurons: (p. 539)
i. Neurons (or nerve cells) have excitability, the ability to respond to a stimulus and convert it into a
nerve impulse (action potential).
ii. Neurons range in length from less than 1 mm to greater than 1 meter, and they transmit nerve impulses
at speeds that range from 0.5 to 130 meters per second.
iii. The junction between two neurons or between a neuron and an effector (muscle or gland)
cell is called a synapse.
iv. The synapse between a motor neuron and a muscle fiber is called a neuromuscular junction.
v. The synapse between a neuron and a glandular cell is called a neuroglandular junction.
3. Parts of a Neuron:
i. Most neurons have three parts:
a. cell body (or soma) contains the nucleus surrounded by cytoplasm that includes typical
organelles as well as:
- lipofuscin pigment granules
- Nissl bodies
- neurofibrils and microtubules
b. dendrites are usually short, tapering, unmyelinated, and highly branched processes that emerge
from the cell body; they are the receiving or input portion of a neuron
c. axon is a long, thin cylindrical process that may be myelinated and transmits nerve impulses
toward the synapse; it has several notable features:
- joins the cell body at the axon hillock
- the first portion of the axon is called the initial segment
- except in sensory neurons, nerve impulses are initiated at the trigger zone (at the junction
of the axon hillock and initial segment)
- axoplasm is surrounded by the axolemma
- axon collaterals may branch off the axon
- the axon and its axon collaterals end at many fine processes called axon
terminals
- the tips of some axon terminals are bulbous synaptic end bulbs, whereas
others exhibit a string of swollen bumps called varicosities
- synaptic end-bulbs and varicosities contain synaptic vesicles that store neurotransmitter
molecules
- most axons are myelinated, i.e., are surrounded by a myelin sheath
ii.. The junction between two neurons is a synapse
- the presynaptic neuron transmits nerve impulses toward the synapse and the
postsynaptic cell is a neuron or muscle cell or gland cell that receives the
signal
- the synapse between a motor neuron and a muscle fiber is called a
neuromuscular junction
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- the synapse between a neuron and a glandular cell is called a neuroglandular
junction
- the small gap between cells at a synapse is called the synaptic cleft; the
presynaptic neuron releases neurotransmitters into the synaptic cleft
which act on the postsynaptic cell
- there are numerous neurotransmitters including acetylcholine (ACh), glutamate,
aspartate, glycine, norepinephrine (NE), dopamine (DA), serotonin, endorphins, nitric
oxide (NO), etc.
Structural Diversity in Neurons:
i. There is great variation in the size and shape of neurons:
a. cell bodies range in diameter from 5 to 135 micrometers
b. the pattern of dendritic branching is quite variable and distinctive for neurons in different
regions of the nervous system
c. a few small neurons lack an axon and many others have very short axons; long neurons have
axons that may exceed 1 meter in length
Classification of Neurons:
i. Neurons may be classified according to both structural and functional features.
ii. Structural classification is based on the number of processes that extend from the cell body:
a. multipolar neurons usually have several dendrites and one axon; most neurons in the brain
and spinal cord are of this type
b. bipolar neurons have one main dendrite and one axon; these are located in the retina, inner
ear, and olfactory area of the brain
c. unipolar neurons are sensory neurons have just one process extending from the cell body; this
process is essentially an axon with dendrites at its peripheral end
iii. Among the many types of neurons are:
a. Purkinje cells in the cerebellum
b. pyramidal cells in the cerebral cortex
Neuroglia or Glia (p. 543)
i. Neuroglia occupy about half the volume of the CNS; they are generally smaller but are more
numerous than neurons.
ii. Unlike neurons, neuroglia do not transmit nerve impulses and they can divide in the mature nervous
system; brain tumors derived from glia are called gliomas.
iii. There are four types of neuroglia in the CNS:
a. astrocytes are star-shaped cells (with many processes) that perform several functions in
support of neurons
b. oligodendrocytes have few processes and produce a myelin sheath; each oligodendrocyte can
myelinate parts of several axons
c. microglia are small, phagocytic neuroglia that protect the nervous system by engulfing
microbes and removing debris of dead cells
d. ependymal cells line the brain ventricles and the central canal of the spinal cord; they secrete
and aid in the circulation of cerebrospinal fluid
iv. There are two types of neuroglia in the PNS:
a. Schwann cells (or neurolemmocytes) produce the myelin sheaths around PNS neurons;
- each Schwann cell wraps about 1 mm of a single axon’s length
- the outer nucleated cytoplasmic layer of the Schwann cell is the neurolemma (sheath
of Schwann)
- gaps in the myelin sheath are called nodes of Ranvier
b. satellite cells support neurons in PNS ganglia
v. Axons that lack a myelin sheath are said to be unmyelinated.
Gray and White Matter: (p. 546)
i. The CNS has some regions that appear white and others that appear gray.
ii. White matter contains neuronal process that have myelin (white color).
iii. Gray matter contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and
neuroglia, all of which are unmyelinated (therefore, gray color).
iv. In the spinal cord, white matter surrounds a butterfly-shaped (in cross section) core of gray matter.
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v.
In the brain, a thin layer of gray matter covers the cerebrum and cerebellum; the brain also contains
numerous masses of gray matter called nuclei which contain neuronal cell bodies.
vi. Most nerves and all tracts are composed of white matter.
D. Neuronal Circuits (p. 547)
1. The CNS contains billions of neurons organized into complex networks called neuronal circuits, each having
its own function.
i. In a simple series circuit, a presynaptic neuron transmits a message to a single postsynaptic neuron,
which in turn stimulates another neuron, and so on.
ii. Most neuronal circuits are more complex:
a. diverging circuit in which a presynaptic neuron forms synapses with several postsynaptic cells
(i.e., divergence)
b. converging circuit in which several presynaptic neurons form synapses with a single
postsynaptic neuron (i.e., convergence)
c. reverberating circuit in which once a presynaptic neuron is stimulated, it will cause the
postsynaptic neuron to transmit a series of nerve impulses
c. parallel after-discharge circuit in which a single presynaptic neuron stimulates a group of
neurons, all of which form synapses with a common postsynaptic neuron
E. Regeneration and Neurogenesis (p. 548)
1. The nervous system exhibits plasticity, the ability to change based on experience.
2. But mammalian neurons have very limited powers of regeneration, the ability to replicate or repair
themselves.
3. Neurogenesis, the formation of new neurons from stem cells, is known to occur in the adult hippocampus but
has not been shown to occur elsewhere in the brain or spinal cord.
F. Key Medical Terms Associated with the Nervous Tissue (p. 549)
1. Students should familiarize themselves with the glossary of key medical terms.