The Nervous System
They are specialized to respond to physical and chem
Case Investigation stimuli, conduct electrochemical impulses, and release ch
ical regulators. Through these activities, neurons enable
Sandra’s grades have been improving, and she
perception of sensory stimuli, learning, memory, and the
treats herself to dinner at a seafood restaurant. However, after
trol of muscles and glands. Most neurons cannot divid
just beginning to eat some mussels and clams gathered from
mitosis, although many can regenerate a severed portio
the local seashore, she complains of severe muscle weak-
sprout small new branches under certain conditions.
ness. Paramedics are called, and when they examine Sandra,
Supporting cells aid the functions of neurons and
they notice that she has a droopy eyelid and that her purse
about five times more abundant than neurons. In the C
contains a prescription bottle for an MAO inhibitor. When
supporting cells are collectively called neuroglia, or sim
questioned, Sandra states that she had a recent Botox treat-
glial cells (from the Middle Greek glia = glue). Unlike
ment, and the medication was prescribed to treat her clinical
rons, which do not divide mitotically (except for partic
depression. Further investigation reveals that the shellfish
neural stem cells; chapter 8, section 8.1), glial cells are
were gathered from waters at the beginning of a red tide and
to divide by mitosis. This helps to explain why brain tum
that Sandra’s blood pressure was in the normal range.
in adults are usually composed of glial cells rather tha
Some of the new terms and concepts you will
neurons.
encounter include:
■ Voltage-gated channels and the action of saxitoxin
■ Neurotransmitter release and the action of botulinum Neurons
toxin
■ Monoamine neurotransmitters and monoamine Although neurons vary considerably in size and sh
oxidase (MAO) they generally have three principal regions: (1) a cell b
(2) dendrites, and (3) an axon (figs. 7.1 and 7.2). Dend
and axons can be referred to generically as processes
extensions from the cell body.
The cell body is the enlarged portion of the neuron
7.1 NEURONS AND contains the nucleus. It is the “nutritional center” of the
ron where macromolecules are produced. The cell body
SUPPORTING CELLS larger dendrites (but not axons) contain Nissl bodies, w
The nervous system is composed of neurons, which pro- are seen as dark-staining granules under the microsc
Nissl bodies are composed of large stacks of rough endop
duce and conduct electrochemical impulses, and sup-
mic reticulum that are needed for the synthesis of memb
porting cells, which assist the functions of neurons. Neurons proteins. The cell bodies within the CNS are frequently c
are classified functionally and structurally; the various types tered into groups called nuclei (not to be confused with
of supporting cells perform specialized functions. nucleus of a cell). Cell bodies in the PNS usually occu
clusters called ganglia (table 7.1).
LEARNING OUTCOMES Dendrites Axon hillock
Direction of
conduction
After studying this section, you should be able to:
✔ Describe the different types of neurons and supporting Collateral axon
cells, and identify their functions.
✔ Identify the myelin sheath and describe how it is formed (a)
Cell body
in the CNS and PNS.
✔ Describe the nature and significance of the Axon
blood-brain barrier.
The nervous system is divided into the central nervous Axon
Direction of
system (CNS), which includes the brain and spinal cord, and conduction
the peripheral nervous system (PNS), which includes the
cranial nerves arising from the brain and the spinal nerves (b)
arising from the spinal cord.
Dendrite
The nervous system is composed of only two principal
types of cells—neurons and supporting cells. Neurons are the Figure 7.1 The structure of two kinds of neurons
basic structural and functional units of the nervous system. A motor neuron (a) and a sensory neuron (b) are depicted her
fox78119_ch07_160-202.indd 161 2
, 162 Chapter 7
Nucleus Dendrite
Node of Ranvier
Schwann
cell nucleus
Cell
body Myelinated
region Axon
Axon
hillock Unmyelinated
region
Myelin
Figure 7.2 Parts of a neuron. The axon of this neuron is wrapped by Schwann cells, which form a myelin sheath.
Table 7.1 | Terminology Pertaining to the Nervous System
Term Definition
Central nervous system (CNS) Brain and spinal cord
Peripheral nervous system (PNS) Nerves, ganglia, and nerve plexuses (outside of the CNS)
Association neuron (interneuron) Multipolar neuron located entirely within the CNS
Sensory neuron (afferent neuron) Neuron that transmits impulses from a sensory receptor into the CNS
Motor neuron (efferent neuron) Neuron that transmits impulses from the CNS to an effector organ; for example, a muscle
Nerve Cablelike collection of many axons in the PNS; may be “mixed” (contain both sensory and motor fibers)
Somatic motor nerve Nerve that stimulates contraction of skeletal muscles
Autonomic motor nerve Nerve that stimulates contraction (or inhibits contraction) of smooth muscle and cardiac muscle and that
stimulates glandular secretion
Ganglion Grouping of neuron cell bodies located outside the CNS
Nucleus Grouping of neuron cell bodies within the CNS
Tract Grouping of axons that interconnect regions of the CNS
Dendrites (from the Greek dendron = tree branch) are 400 mm/day) mainly transports membranous vesicles (impor-
thin, branched processes that extend from the cytoplasm of tant for synaptic transmission, as discussed in section 7.3).
the cell body. Dendrites provide a receptive area that trans- One slow component (at 0.2 to 1 mm/day) transports micro-
mits graded electrochemical impulses to the cell body. The filaments and microtubules of the cytoskeleton, while the
axon is a longer process that conducts impulses, called other slow component (at 2 to 8 mm/day) transports over
action potentials (section 7.2), away from the cell body. 200 different proteins, including those critical for synaptic
Axons vary in length from only a millimeter long to up function. The slow components appear to transport their
to a meter or more (for those that extend from the CNS cargo in fast bursts with frequent pauses, so that the overall
to the foot). The origin of the axon near the cell body is rate of transport is much slower than that occurring in the
an expanded region called the axon hillock; it is here that fast component.
action potentials originate. Side branches called axon col- Axonal transport may occur from the cell body to the
laterals may extend from the axon. axon and dendrites. This direction is called anterograde
Because axons can be quite long, special mechanisms transport, and involves molecular motors of kinesin proteins
are required to transport organelles and proteins from the that move cargo along the microtubules of the cytoskeleton
cell body to the axon terminals. This axonal transport is (chapter 3, section 3.2). For example, kinesin motors move
energy-dependent and is often divided into a fast component synaptic vesicles, mitochondria, and ion channels from the
and two slow components. The fast component (at 200 to cell body through the axon. Similar anterograde transport
fox78119_ch07_160-202.indd 162
, The Nervous System
Central Nervous System (CNS) Peripheral Nervous System (PNS)
Association neuron (interneuron)
Sensory neuron
Receptors
Somatic motor neuron
Skeletal
muscles
Autonomic motor neurons
Smooth muscle
Cardiac muscle
Glands
Autonomic ganglion
Figure 7.3 The relationship between CNS and PNS. Sensory and motor neurons of the peripheral nervous system carry
information into and out of, respectively, the central nervous system (brain and spinal cord).
occurs in the dendrites, as kinesin moves postsynaptic recep- effectors—smooth muscle, cardiac muscle, and glands.
tors for neurotransmitters and ion channels along the micro- cell bodies of the autonomic neurons that innervate t
tubules in the dendrites. organs are located outside the CNS in autonomic gan
By contrast, axonal transport in the opposite direction— (fig. 7.3). There are two subdivisions of autonomic
that is, along the axon and dendrites toward the cell body—is rons: sympathetic and parasympathetic. Autonomic m
known as retrograde transport and involves molecular motor neurons, together with their central control centers, co
proteins of dyneins. The dyneins move membranes, vesicles, tute the autonomic nervous system, the focus of chapter
and various molecules along microtubules of the cytoskeleton The structural classification of neurons is based on
toward the cell body of the neuron. Retrograde transport can number of processes that extend from the cell body of
also be responsible for movement of herpes virus, rabies virus, neuron (fig. 7.4). Pseudounipolar neurons have a si
and tetanus toxin from the nerve terminals into cell bodies. short process that branches like a T to form a pair of lo
processes. They are called pseudounipolar (from the
Latin pseudo = false) because, although they originate
Classification of Neurons two processes, during early embryonic development t
two processes converge and partially fuse. Sensory
and Nerves rons are pseudounipolar—one of the branched proce
Neurons may be classified according to their function or receives sensory stimuli and produces nerve impulses;
structure. The functional classification is based on the other delivers these impulses to synapses within the b
direction in which they conduct impulses, as indicated in or spinal cord. Anatomically, the part of the process
figure 7.3. Sensory, or afferent, neurons conduct impulses conducts impulses toward the cell body can be consider
from sensory receptors into the CNS. Motor, or efferent, dendrite, and the part that conducts impulses away from
neurons conduct impulses out of the CNS to effector organs cell body can be considered an axon. Functionally, howe
(muscles and glands). Association neurons, or interneu- the branched process behaves as a single, long axon
rons, are located entirely within the CNS and serve the asso- continuously conducts action potentials (nerve impuls
ciative, or integrative, functions of the nervous system. Only the small projections at the receptive end of the
There are two types of motor neurons: somatic and auto- cess function as typical dendrites, conducting graded ele
nomic. Somatic motor neurons are responsible for both chemical impulses rather than action potentials. Bip
reflex and voluntary control of skeletal muscles. Autonomic neurons have two processes, one at either end; this typ
motor neurons innervate (send axons to) the involuntary found in the retina of the eye. Multipolar neurons, the m
fox78119_ch07_160-202.indd 163 2
, 164 Chapter 7
Pseudounipolar
Supporting Cells
Dendritic branches Unlike other organs that are “packaged” in connective tissue
derived from mesoderm (the middle layer of embryonic tis-
Bipolar sue), most of the supporting cells of the nervous system are
Dendrite
derived from the same embryonic tissue layer (ectoderm) that
produces neurons. The term neuroglia (or glia) traditionally
Multipolar Axon refers to the supporting cells of the CNS, but in current usage
Dendrites
the supporting cells of the PNS are often also called glial cells.
There are two types of supporting cells in the peripheral
nervous system:
1. Schwann cells (also called neurolemmocytes), which
form myelin sheaths around peripheral axons; and
2. satellite cells, or ganglionic gliocytes, which support
Figure 7.4 Three different types of neurons. neuron cell bodies within the ganglia of the PNS.
Pseudounipolar neurons, which are sensory, have one process that There are four types of supporting cells in the central
splits. Bipolar neurons, found in the retina and cochlea, have two nervous system (fig. 7.5):
processes. Multipolar neurons, which are motor and association
neurons, have many dendrites and one axon. 1. oligodendrocytes, which form myelin sheaths around
axons of the CNS;
2. microglia, which migrate through the CNS and
common type, have several dendrites and one axon extend- phagocytose foreign and degenerated material;
ing from the cell body; motor neurons are good examples 3. astrocytes, which help to regulate the external
of this type. environment of neurons in the CNS; and
A nerve is a bundle of axons located outside the CNS. Most 4. ependymal cells, which line the ventricles
nerves are composed of both motor and sensory fibers and are (cavities) of the brain and the central canal of
thus called mixed nerves. Some of the cranial nerves, however, the spinal cord.
contain sensory fibers only. These are the nerves that serve the
special senses of sight, hearing, taste, and smell. A bundle of Microglia are of hematopoietic (bone marrow) origin, and
axons in the CNS is called a tract. indeed can be replenished by monocytes (a type of leukocyte)
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular Axons
feet
Myelin sheath
Ependymal
cells
Cerebrospinal Microglia
fluid
Figure 7.5 The different types of neuroglial cells. Myelin sheaths around axons are formed in the CNS by oligodendrocytes.
Astrocytes have extensions that surround both blood capillaries and neurons. Microglia are phagocytic, and ependymal cells line the
brain ventricles and central canal of the spinal cord.
fox78119_ch07_160-202.indd 164
They are specialized to respond to physical and chem
Case Investigation stimuli, conduct electrochemical impulses, and release ch
ical regulators. Through these activities, neurons enable
Sandra’s grades have been improving, and she
perception of sensory stimuli, learning, memory, and the
treats herself to dinner at a seafood restaurant. However, after
trol of muscles and glands. Most neurons cannot divid
just beginning to eat some mussels and clams gathered from
mitosis, although many can regenerate a severed portio
the local seashore, she complains of severe muscle weak-
sprout small new branches under certain conditions.
ness. Paramedics are called, and when they examine Sandra,
Supporting cells aid the functions of neurons and
they notice that she has a droopy eyelid and that her purse
about five times more abundant than neurons. In the C
contains a prescription bottle for an MAO inhibitor. When
supporting cells are collectively called neuroglia, or sim
questioned, Sandra states that she had a recent Botox treat-
glial cells (from the Middle Greek glia = glue). Unlike
ment, and the medication was prescribed to treat her clinical
rons, which do not divide mitotically (except for partic
depression. Further investigation reveals that the shellfish
neural stem cells; chapter 8, section 8.1), glial cells are
were gathered from waters at the beginning of a red tide and
to divide by mitosis. This helps to explain why brain tum
that Sandra’s blood pressure was in the normal range.
in adults are usually composed of glial cells rather tha
Some of the new terms and concepts you will
neurons.
encounter include:
■ Voltage-gated channels and the action of saxitoxin
■ Neurotransmitter release and the action of botulinum Neurons
toxin
■ Monoamine neurotransmitters and monoamine Although neurons vary considerably in size and sh
oxidase (MAO) they generally have three principal regions: (1) a cell b
(2) dendrites, and (3) an axon (figs. 7.1 and 7.2). Dend
and axons can be referred to generically as processes
extensions from the cell body.
The cell body is the enlarged portion of the neuron
7.1 NEURONS AND contains the nucleus. It is the “nutritional center” of the
ron where macromolecules are produced. The cell body
SUPPORTING CELLS larger dendrites (but not axons) contain Nissl bodies, w
The nervous system is composed of neurons, which pro- are seen as dark-staining granules under the microsc
Nissl bodies are composed of large stacks of rough endop
duce and conduct electrochemical impulses, and sup-
mic reticulum that are needed for the synthesis of memb
porting cells, which assist the functions of neurons. Neurons proteins. The cell bodies within the CNS are frequently c
are classified functionally and structurally; the various types tered into groups called nuclei (not to be confused with
of supporting cells perform specialized functions. nucleus of a cell). Cell bodies in the PNS usually occu
clusters called ganglia (table 7.1).
LEARNING OUTCOMES Dendrites Axon hillock
Direction of
conduction
After studying this section, you should be able to:
✔ Describe the different types of neurons and supporting Collateral axon
cells, and identify their functions.
✔ Identify the myelin sheath and describe how it is formed (a)
Cell body
in the CNS and PNS.
✔ Describe the nature and significance of the Axon
blood-brain barrier.
The nervous system is divided into the central nervous Axon
Direction of
system (CNS), which includes the brain and spinal cord, and conduction
the peripheral nervous system (PNS), which includes the
cranial nerves arising from the brain and the spinal nerves (b)
arising from the spinal cord.
Dendrite
The nervous system is composed of only two principal
types of cells—neurons and supporting cells. Neurons are the Figure 7.1 The structure of two kinds of neurons
basic structural and functional units of the nervous system. A motor neuron (a) and a sensory neuron (b) are depicted her
fox78119_ch07_160-202.indd 161 2
, 162 Chapter 7
Nucleus Dendrite
Node of Ranvier
Schwann
cell nucleus
Cell
body Myelinated
region Axon
Axon
hillock Unmyelinated
region
Myelin
Figure 7.2 Parts of a neuron. The axon of this neuron is wrapped by Schwann cells, which form a myelin sheath.
Table 7.1 | Terminology Pertaining to the Nervous System
Term Definition
Central nervous system (CNS) Brain and spinal cord
Peripheral nervous system (PNS) Nerves, ganglia, and nerve plexuses (outside of the CNS)
Association neuron (interneuron) Multipolar neuron located entirely within the CNS
Sensory neuron (afferent neuron) Neuron that transmits impulses from a sensory receptor into the CNS
Motor neuron (efferent neuron) Neuron that transmits impulses from the CNS to an effector organ; for example, a muscle
Nerve Cablelike collection of many axons in the PNS; may be “mixed” (contain both sensory and motor fibers)
Somatic motor nerve Nerve that stimulates contraction of skeletal muscles
Autonomic motor nerve Nerve that stimulates contraction (or inhibits contraction) of smooth muscle and cardiac muscle and that
stimulates glandular secretion
Ganglion Grouping of neuron cell bodies located outside the CNS
Nucleus Grouping of neuron cell bodies within the CNS
Tract Grouping of axons that interconnect regions of the CNS
Dendrites (from the Greek dendron = tree branch) are 400 mm/day) mainly transports membranous vesicles (impor-
thin, branched processes that extend from the cytoplasm of tant for synaptic transmission, as discussed in section 7.3).
the cell body. Dendrites provide a receptive area that trans- One slow component (at 0.2 to 1 mm/day) transports micro-
mits graded electrochemical impulses to the cell body. The filaments and microtubules of the cytoskeleton, while the
axon is a longer process that conducts impulses, called other slow component (at 2 to 8 mm/day) transports over
action potentials (section 7.2), away from the cell body. 200 different proteins, including those critical for synaptic
Axons vary in length from only a millimeter long to up function. The slow components appear to transport their
to a meter or more (for those that extend from the CNS cargo in fast bursts with frequent pauses, so that the overall
to the foot). The origin of the axon near the cell body is rate of transport is much slower than that occurring in the
an expanded region called the axon hillock; it is here that fast component.
action potentials originate. Side branches called axon col- Axonal transport may occur from the cell body to the
laterals may extend from the axon. axon and dendrites. This direction is called anterograde
Because axons can be quite long, special mechanisms transport, and involves molecular motors of kinesin proteins
are required to transport organelles and proteins from the that move cargo along the microtubules of the cytoskeleton
cell body to the axon terminals. This axonal transport is (chapter 3, section 3.2). For example, kinesin motors move
energy-dependent and is often divided into a fast component synaptic vesicles, mitochondria, and ion channels from the
and two slow components. The fast component (at 200 to cell body through the axon. Similar anterograde transport
fox78119_ch07_160-202.indd 162
, The Nervous System
Central Nervous System (CNS) Peripheral Nervous System (PNS)
Association neuron (interneuron)
Sensory neuron
Receptors
Somatic motor neuron
Skeletal
muscles
Autonomic motor neurons
Smooth muscle
Cardiac muscle
Glands
Autonomic ganglion
Figure 7.3 The relationship between CNS and PNS. Sensory and motor neurons of the peripheral nervous system carry
information into and out of, respectively, the central nervous system (brain and spinal cord).
occurs in the dendrites, as kinesin moves postsynaptic recep- effectors—smooth muscle, cardiac muscle, and glands.
tors for neurotransmitters and ion channels along the micro- cell bodies of the autonomic neurons that innervate t
tubules in the dendrites. organs are located outside the CNS in autonomic gan
By contrast, axonal transport in the opposite direction— (fig. 7.3). There are two subdivisions of autonomic
that is, along the axon and dendrites toward the cell body—is rons: sympathetic and parasympathetic. Autonomic m
known as retrograde transport and involves molecular motor neurons, together with their central control centers, co
proteins of dyneins. The dyneins move membranes, vesicles, tute the autonomic nervous system, the focus of chapter
and various molecules along microtubules of the cytoskeleton The structural classification of neurons is based on
toward the cell body of the neuron. Retrograde transport can number of processes that extend from the cell body of
also be responsible for movement of herpes virus, rabies virus, neuron (fig. 7.4). Pseudounipolar neurons have a si
and tetanus toxin from the nerve terminals into cell bodies. short process that branches like a T to form a pair of lo
processes. They are called pseudounipolar (from the
Latin pseudo = false) because, although they originate
Classification of Neurons two processes, during early embryonic development t
two processes converge and partially fuse. Sensory
and Nerves rons are pseudounipolar—one of the branched proce
Neurons may be classified according to their function or receives sensory stimuli and produces nerve impulses;
structure. The functional classification is based on the other delivers these impulses to synapses within the b
direction in which they conduct impulses, as indicated in or spinal cord. Anatomically, the part of the process
figure 7.3. Sensory, or afferent, neurons conduct impulses conducts impulses toward the cell body can be consider
from sensory receptors into the CNS. Motor, or efferent, dendrite, and the part that conducts impulses away from
neurons conduct impulses out of the CNS to effector organs cell body can be considered an axon. Functionally, howe
(muscles and glands). Association neurons, or interneu- the branched process behaves as a single, long axon
rons, are located entirely within the CNS and serve the asso- continuously conducts action potentials (nerve impuls
ciative, or integrative, functions of the nervous system. Only the small projections at the receptive end of the
There are two types of motor neurons: somatic and auto- cess function as typical dendrites, conducting graded ele
nomic. Somatic motor neurons are responsible for both chemical impulses rather than action potentials. Bip
reflex and voluntary control of skeletal muscles. Autonomic neurons have two processes, one at either end; this typ
motor neurons innervate (send axons to) the involuntary found in the retina of the eye. Multipolar neurons, the m
fox78119_ch07_160-202.indd 163 2
, 164 Chapter 7
Pseudounipolar
Supporting Cells
Dendritic branches Unlike other organs that are “packaged” in connective tissue
derived from mesoderm (the middle layer of embryonic tis-
Bipolar sue), most of the supporting cells of the nervous system are
Dendrite
derived from the same embryonic tissue layer (ectoderm) that
produces neurons. The term neuroglia (or glia) traditionally
Multipolar Axon refers to the supporting cells of the CNS, but in current usage
Dendrites
the supporting cells of the PNS are often also called glial cells.
There are two types of supporting cells in the peripheral
nervous system:
1. Schwann cells (also called neurolemmocytes), which
form myelin sheaths around peripheral axons; and
2. satellite cells, or ganglionic gliocytes, which support
Figure 7.4 Three different types of neurons. neuron cell bodies within the ganglia of the PNS.
Pseudounipolar neurons, which are sensory, have one process that There are four types of supporting cells in the central
splits. Bipolar neurons, found in the retina and cochlea, have two nervous system (fig. 7.5):
processes. Multipolar neurons, which are motor and association
neurons, have many dendrites and one axon. 1. oligodendrocytes, which form myelin sheaths around
axons of the CNS;
2. microglia, which migrate through the CNS and
common type, have several dendrites and one axon extend- phagocytose foreign and degenerated material;
ing from the cell body; motor neurons are good examples 3. astrocytes, which help to regulate the external
of this type. environment of neurons in the CNS; and
A nerve is a bundle of axons located outside the CNS. Most 4. ependymal cells, which line the ventricles
nerves are composed of both motor and sensory fibers and are (cavities) of the brain and the central canal of
thus called mixed nerves. Some of the cranial nerves, however, the spinal cord.
contain sensory fibers only. These are the nerves that serve the
special senses of sight, hearing, taste, and smell. A bundle of Microglia are of hematopoietic (bone marrow) origin, and
axons in the CNS is called a tract. indeed can be replenished by monocytes (a type of leukocyte)
Capillary
Neurons
Astrocyte
Oligodendrocyte
Perivascular Axons
feet
Myelin sheath
Ependymal
cells
Cerebrospinal Microglia
fluid
Figure 7.5 The different types of neuroglial cells. Myelin sheaths around axons are formed in the CNS by oligodendrocytes.
Astrocytes have extensions that surround both blood capillaries and neurons. Microglia are phagocytic, and ependymal cells line the
brain ventricles and central canal of the spinal cord.
fox78119_ch07_160-202.indd 164
