Nervous System Organization

The nervous system integrates and monitors the countless actions occurring simultaneously
throughout the entire human body. Therefore, every task, no matter how menial, accomplished
by a person is a direct result of the components of the nervous system. These actions can be
under voluntary control, like touching a computer key, or can occur without your direct
knowledge, like digesting food, releasing enzymes from the pancreas, or other unconscious acts..

The nervous system consists of two parts, shown in Figure 1 :

 The central nervous system (CNS) consists of the brain and spinal cord.
 The peripheral nervous system (PNS) consists of nerves outside the CNS.

Figure Two parts of the nervous system.

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Nerves of the PNS are classified in three ways. First, PNS nerves are classified by how they are
connected to the CNS. Cranial nerves originate from or terminate in the brain, while spinal
nerves originate from or terminate at the spinal cord.

Second, nerves of the PNS are classified by the direction of nerve propagation. Sensory
(afferent) neurons transmit impulses from skin and other sensory organs or from various places
within the body to the CNS. Motor (efferent) neurons transmit impulses from the CNS to
effectors (muscles or glands).
Third, motor neurons are further classified according to the effectors they target. The somatic
nervous system (SNS) directs the contraction of skeletal muscles. The autonomic nervous
system (ANS) controls the activities of organs, glands, and various involuntary muscles, such as
cardiac and smooth muscles.

The autonomic nervous system has two divisions:

 The sympathetic nervous system is involved in the stimulation of activities that prepare
the body for action, such as increasing the heart rate, increasing the release of sugar from
the liver into the blood, and other activities generally considered as fight-or-flight
responses (responses that serve to fight off or retreat from danger).
 The parasympathetic nervous system activates tranquil functions, such as stimulating
the secretion of saliva or digestive enzymes into the stomach and small intestine.

Generally, both sympathetic and parasympathetic systems target the same organs, but often work
antagonistically. For example, the sympathetic system accelerates the heartbeat, while the
parasympathetic slows the heartbeat. Each system is stimulated as is appropriate to maintain
homeostasis. The brain can be subdivided into several distinct regions:

 The cerebral hemispheres form the largest part of the brain,

occupying the anterior and middle cranial fossae in the skull and
extending backwards over the tentorium cerebelli. They are made up
of the cerebral cortex, the basal ganglia, tracts of synaptic
connections, and the ventricles containing CSF.
 The Diencephalon (not shown above) includes the thalamus,
hyopthalamus, epithalamus and subthalamus, and forms the central
core of the brain. It is surrounded by the cerebral hemispheres.
 The Midbrain (not shown) is located at the junction of the middle and
posterior cranial fossae.
 The Pons sits in the anterior part of the posterior cranial fossa- the
fibres within the structure connect one cerebral hemisphere with its
opposite cerebellar hemisphere.
 The Medulla Oblongata is continuous with the spinal cord, and is
responsible for automatic control of the respiratory and cardiovascular
systems.
 The Cerebellum overlies the pons and medulla, extending beneath
the tentorium cerebelli and occupying most of the posterior cranial
fossa. It is mainly concerned with motor functions that regulate muscle
tone, coordination, and posture.
The cerebral hemispheres can be further divided into four lobes:

 Frontal (red)
 Parietal (yellow)
 Occipital (green)

 Temporal (blue) Cranial Nerves

 Cranial nerves are nerves of the PNS that originate from or terminate in the brain. There
are 12 pairs of cranial nerves, all of which pass through foramina of the skull. Cranial
nerves are either sensory nerves (containing only or predominately sensory fibers) or
mixed nerves (containing both sensory and motor fibers). Characteristics of the cranial
nerves, which are numbered from anterior to posterior as they attach to the brain, are
summarized in Table 1 .

TABLE 1 Characteristics of Cranial Nerves

Nerve
Cranial Nerve Type Major Functions
I Olfactory sensory Smell
II Optic sensory Vision
III Oculomotor primarily eyeball & eyelid movement; lens shape
motor
IV Trochlear primarily eyeball movement; proprioception
motor
V Trigeminal: sensory sensations of touch & pain from facial skin, nose, mouth,
ophthalmic branch teeth, & tongue; proprioception motor control of
chewing
V Trigeminal: maxillary sensory sensations of touch & pain from facial skin, nose, mouth,
 Nerve
Cranial Nerve Type Major Functions
branch teeth, & tongue; proprioception motor control of
chewing
V Trigeminal: mixed sensations of touch & pain from facial skin, nose, mouth,
mandibular branch teeth, & tongue; proprioception motor control of
chewing
VI Abducens primarily eyeball movement; proprioception
motor
VII Facial mixed movement of facial muscles; tear & saliva secretion;
sense of taste & proprioception
VIII Vestibulocochlear: sensory Hearing
cochlear branch
VIII Vestibulocochlear: sensory sense of equilibrium
vestibular branch
IX Glossophayrngeal mixed sensations of taste, touch & pain from tongue & pharynx;
chemoreceptors (that monitor O2 & CO2), blood pressure
receptors; movement of tongue & swallowing; secretion
of saliva
X Vagus mixed parasympathetic sensation & motor control of smooth
muscles associated with heart, lungs, viscera; secretion
of digestive enzymes
XI Accessory primarily head movement; swallowing; proprioception
motor
XII Hypoglossal primarily tongue movement, speech & swallowing; proprioception
motor
The Spinal Cord
The spinal cord has two functions:

 Transmission of nerve impulses. Neurons in the white matter of the spinal cord transmit
sensory signals from peripheral regions to the brain and motor signals from the brain to
peripheral regions.
 Spinal reflexes. Neurons in the gray matter of the spinal cord integrate incoming sensory
information and respond with motor impulses that control muscles (skeletal, smooth, or
cardiac) or glands.

The spinal cord is an extension of the brain stem that begins at the foramen magnum and
continues down through the vertebral canal to the first lumbar vertebra (L1). Here, the spinal cord
comes to a tapering point, the conus medullaris. The spinal cord is held in position at its inferior
end by the filum terminale, an extension of the pia mater that attaches to the coccyx. Along its
length, the spinal cord is held within the vertebral canal by denticulate ligaments, lateral
extensions of the pia mater that attach to the dural sheath.

The following external features on the spinal cord (see Figure 1 ).

 Figure External features of the spinal cord.
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 Spinal nerves emerge in pairs, one from each side of the spinal cord along its length.
 The cervical enlargement is a widening in the upper part of the spinal cord (C4 to T1).
Nerves that extend into the upper limbs originate or terminate here.
 The lumbar enlargement is a widening in the lower part of the spinal cord (T9 to T12).
Nerves that extend into the lower limbs originate or terminate here.
 The anterior median fissure and the posterior median sulcus are two grooves that run the
length of the spinal cord on its anterior and posterior surfaces, respectively.
 The cauda equina are nerves that attach to the end of the spinal cord and continue to run
downward before turning laterally to other parts of the body.

A cross section of the spinal cord reveals the following features, shown in Figure 2 :

 Roots are branches of the spinal nerve that connect to the spinal cord. Two major roots
form:
o A ventral root (anterior or motor root) is the branch of the nerve that enters the
ventral side of the spinal cord. Ventral roots contain motor nerve axons,
transmitting nerve impulses from the spinal cord to skeletal muscles.
o A dorsal root (posterior or sensory root) is the branch of a nerve that enters the
dorsal side of the spinal cord. Dorsal roots contain sensory nerve fibers,
transmitting nerve impulses from peripheral regions to the spinal cord.
o A dorsal root ganglion is a cluster of cell bodies of a sensory nerve. It is located
on the dorsal root.
 Gray matter appears in the center of the spinal cord in the form of the letter H (or a pair
of butterfly wings) when viewed in cross section.
o The gray commissure is the cross-bra of the H.
o The anterior (ventral) horns are gray matter areas at the front of each side of the
H. Cell bodies of motor neurons that stimulate skeletal muscles are located here.
o The posterior (dorsal) horns are gray matter areas at the rear of each side of the H.
These horns contain mostly interneurons that synapse with sensory neurons.
o The lateral horns are small projections of gray matter at the sides of H. These
horns are present only in the thoracic and lumbar regions of the spinal cord. They
contain cell bodies of motor neurons in the sympathetic branch of the autonomic
nervous system.
o The central canal is a small hole in the center of the H cross-bar. It contains CSF
and runs the length of the spinal cord and connects with the fourth ventricle of the
brain.
 White columns (funiculi) refer to six areas of the white matter, three on each side of the
H. They are the anterior (ventral) columns, the posterior (dorsal) columns, and the lateral
columns.
 Fasciculi are bundles of nerve tracts within white columns containing neurons with
common functions or destinations.
 o Ascending (sensory) tracts transmit sensory information from various parts of the
body to the brain.
o Descending (motor) tracts transmit nerve impulses from the brain to muscles and
glands.

Spinal Nerves
There are 31 pairs of spinal nerves (62 total). The following discussion traces a spinal nerve as it
emerges from the spinal column.

 A spinal nerve emerges at two points from the spinal cord, the ventral and dorsal roots.

 The ventral and dorsal roots merge to form the whole spinal nerve.
 The spinal nerve emerges from the spinal column through an opening (intervertebral
foramen) between adjacent vertebrae. This is true for all spinal nerves except for the first
spinal nerve (pair), which emerges between the occipital bone and the atlas (the first
vertebra).

o Outside the vertebral column, the nerve divides into the following branches:

o The dorsal ramus contains nerves that serve the dorsal portions of the trunk.

o The ventral ramus contains nerves that serve the remaining ventral parts of the
trunk and the upper and lower limbs.

o The meningeal branch reenters the vertebral column and serves the meninges and
blood vessels within.

o The rami communicantes contain autonomic nerves that serve visceral functions.

 Some ventral rami merge with adjacent ventral rami to form a plexus, a network of
interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal
nerves, which are now carried together to some target location.

o ord to skeletal muscles.

o A dorsal root (posterior or sensory root) is the branch of a nerve that enters the
dorsal side of the spinal cord. Dorsal roots contain sensory nerve fibers,
transmitting nerve impulses from peripheral regions to the spinal cord.
o A dorsal root ganglion is a cluster of cell bodies of a sensory nerve. It is located
on the dorsal root.
 Gray matter appears in the center of the spinal cord in the form of the letter H (or a pair
of butterfly wings) when viewed in cross section.
o The gray commissure is the cross-bra of the H.
o The anterior (ventral) horns are gray matter areas at the front of each side of the
H. Cell bodies of motor neurons that stimulate skeletal muscles are located here.
o The posterior (dorsal) horns are gray matter areas at the rear of each side of the H.
These horns contain mostly interneurons that synapse with sensory neurons.
o The lateral horns are small projections of gray matter at the sides of H. These
horns are present only in the thoracic and lumbar regions of the spinal cord. They
contain cell bodies of motor neurons in the sympathetic branch of the autonomic
nervous system.
o The central canal is a small hole in the center of the H cross-bar. It contains CSF
and runs the length of the spinal cord and connects with the fourth ventricle of the
brain.
  White columns (funiculi) refer to six areas of the white matter, three on each side of the
H. They are the anterior (ventral) columns, the posterior (dorsal) columns, and the lateral
columns.
 Fasciculi are bundles of nerve tracts within white columns containing neurons with
common functions or destinations.
o Ascending (sensory) tracts transmit sensory information from various parts of the
body to the brain.
o Descending (motor) tracts transmit nerve impulses from the brain to muscles and
glands.

Nerves Spinal
There are 31 pairs of spinal nerves (62 total). The following discussion traces a spinal nerve as it
emerges from the spinal column.

 A spinal nerve emerges at two points from the spinal cord, the ventral and dorsal roots.

 The ventral and dorsal roots merge to form the whole spinal nerve.

 The spinal nerve emerges from the spinal column through an opening (intervertebral
foramen) between adjacent vertebrae. This is true for all spinal nerves except for the first
spinal nerve (pair), which emerges between the occipital bone and the atlas (the first
vertebra).

o Outside the vertebral column, the nerve divides into the following branches:

o The dorsal ramus contains nerves that serve the dorsal portions of the trunk.

o The ventral ramus contains nerves that serve the remaining ventral parts of the
trunk and the upper and lower limbs.

o The meningeal branch reenters the vertebral column and serves the meninges and
blood vessels within.

o The rami communicantes contain autonomic nerves that serve visceral functions.

 Some ventral rami merge with adjacent ventral rami to form a plexus, a network of
interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal
nerves, which are now carried together to some target location.
An area of the skin that receives sensory stimuli that pass through a single spinal nerve is called a
dermatome. Dermatomes are illustrated on a human figure with lines that mark the boundaries of
the area where each spinal nerve receives stimuli.

A reflex arc involves the following components, shown in :

 The receptor is the part of the neuron (usually a dendrite) that detects a stimulus.
 The sensory neuron transmits the impulse to the spinal cord.
 The integration center involves one synapse (monosynaptic reflex arc) or two or more
synapses (polysynaptic reflex arc) in the gray matter of the spinal cord. In polysynaptic
reflex arcs, one or more interneurons in the gray matter constitute the integration center.
 A motor neuron transmits a nerve impulse from the spinal cord to a peripheral region.
 An effector is a muscle or gland that receives the impulse form the motor neuron. In
somatic reflexes, the effector is skeletal muscle. In autonomic (visceral) reflexes, the
effector is smooth or cardiac muscle, or a gland.

The Autonomic Nervous System

The peripheral nervous system consists of the somatic nervous system (SNS) and the
autonomic nervous system (ANS). The SNS consists of motor neurons that stimulate
skeletal muscles. In contrast, the ANS consists of motor neurons that control smooth
muscles, cardiac muscles, and glands. In addition, the ANS monitors visceral organs and
blood vessels with sensory neurons, which provide input information for the CNS.
 Figure 1A reflex arc.

Some examples of reflexes follow:

 A stretch reflex is a monosynaptic reflex that is a response to a muscle that has been
stretched (the knee jerk reflex is an example). When receptors in muscles, called muscle
spindles, detect changes in muscle length, they stimulate, through a reflex arc, the
contraction of a muscle. Stretch reflexes help maintain posture by stimulating muscles to
regain normal body position.
 A flexor (withdrawal) reflex is a polysynaptic reflex that causes a limb to be withdrawn
when it encounters pain (refer to Figure 1 ).

The ANS is further divided into the sympathetic nervous system and the parasympathetic
nervous system. Both of these systems can stimulate and inhibit effectors. However, the two
systems work in opposition—where one system stimulates an organ, the other inhibits. Working
in this fashion, each system prepares the body for a different kind of situation, as follows.

 The sympathetic nervous system prepares the body for situations requiring alertness or
strength or situations that arouse fear, anger, excitement, or embarrassment (“fight-or-
flight” situations). In these kinds of situations, the sympathetic nervous system stimulates
cardiac muscles to increase the heart rate, causes dilation of the bronchioles of the lungs
(increasing oxygen intake), and causes dilation of blood vessels that supply the heart and
skeletal muscles (increasing blood supply). The adrenal medulla is stimulated to release
epinephrine (adrenalin) and norepinephrine (noradrenalin), which in turn increases the
metabolic rate of cells and stimulate the liver to release glucose into the blood. Sweat
glands are stimulated to produce sweat. In addition, the sympathetic nervous system
reduces the activity of various “tranquil” body functions, such as digestion and kidney
functioning.
 The parasympathetic nervous system is active during periods of digestion and rest. It
stimulates the production of digestive enzymes and stimulates the processes of digestion,
urination, and defecation. It reduces blood pressure and heart and respiratory rates and
conserves energy through relaxation and rest.

In the SNS, a single motor neuron connects the CNS to its target skeletal muscle. In the ANS, the
connection between the CNS and its effector consists of two neurons—the preganglionic neuron
and the postganglionic neuron. The synapse between these two neurons lies outside the CNS, in
an autonomic ganglion. The axon (preganglionic axon) of a preganglionic neuron enters the
ganglion and forms a synapse with the dendrites of the postganglionic neuron emerges from the
ganglion and travels to the target organ (see Figure 1 ). There are three kinds of autonomic
ganglia:
  The sympathetic trunk, or chain, contains sympathetic ganglia called paravertebral
ganglia. There are two trunks, one on either side of the vertebral column along its entire
length. Each trunk consists of ganglia connected by fibers, like a string of beads.
 The prevertebral (collateral) ganglia also consist of sympathetic ganglia. Preganglionic
sympathetic fibers that pass through the sympathetic trunk (without forming a synapse
with a postganglionic neuron) synapse here. Prevertebral ganglia lie near the large
abdominal arteries, which the preganglionic fibers target.
 Terminal (intramural) ganglia receive parasympathetic fibers. These ganglia occur near
or within the target organ of the respective postganglionic fiber.

Figure The target organs of the different nervous systems.

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A comparison of the sympathetic and parasympathetic pathways follows (see Figure 2 ):

 Sympathetic nervous system. Cell bodies of the preganglionic neurons occur in the lateral
horns of gray matter of the 12 thoracic and first 2 lumbar segments of the spinal cord. (For
this reason, the sympathetic system is also called the thoracolumbar division.) Preganglionic
fibers leave the spinal cord within spinal nerves through the ventral roots (together with the
PNS motor neurons). The preganglionic fibers then branch away from the nerve through
white rami (white rami communicantes) that connect with the sympathetic trunk. White rami
are white because they contain myelinated fibers. A preganglionic fiber that enters the trunk
may synapse in the first ganglion it enters, travel up or down the trunk to synapse with
another ganglion, or pass through the trunk and synapse outside the trunk. Postganglionic
fibers that originate in ganglia within the sympathetic trunk leave the trunk through gray rami
(gray rami communicantes) and return to the spinal nerve, which is followed until it reaches
its target organ. Gray rami are gray because they contain unmyelinated fibers.
 Parasympathetic nervous system. Cell bodies of the preganglionic neurons occur in the gray
matter of sacral segments S2-S4 and in the brain stem (with motor neurons of their associated
cranial nerves III, VII, IX, and X). (For this reason, the parasympathetic system is also called
the craniosacral division, and the fibers arising from this division are called the cranial
outflow or the sacral outflow, depending upon their origin.) Preganglionic fibers of the
cranial outflow accompany the PNS motor neurons of cranial nerves and have terminal
ganglia that lie near the target organ. Preganglionic fibers of the sacral outflow accompany
the PNS motor neurons of spinal nerves. These nerves emerge through the ventral roots of
the spinal cord and have terminal ganglia that lie near the target organ.