The collections of axons are called “tracts” in CNS and “nerves” in PNS

Tracts CNS

The collections of axons are called “nerves” in PNS

nerves” in PNS

The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In
describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system;
these structures are known as the enteric nervous system and are a special subset of the PNS.

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Ganglia

Ganglia (singular is “ganglion”) are neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions.

The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, extending into the CNS through the dorsal nerve root. A nerve root is
the part of a nerve that comes out from the spinal cord and carries signals between the body and the brain. The ganglion is an enlargement of the nerve root. Under microscopic
inspection, it can be seen to include the cell bodies of the neurons, and bundles of fibers that are the posterior nerve root (Figure 4.2).

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The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region
is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine).

nerve root is

A nerve root is
the part of a nerve that comes out from the spinal cord and carries signals between the body and the brain.

The cells of the dorsal root ganglion are unipolar cells (see Figure 2.9, p. 59 for “neuron classification by shape”), classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding the neuron cell bodies.

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cranial nerve ganglion.

Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone, whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells.

The other major category of ganglia is those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region.

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sympathetic chain ganglia

sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord.

Superior to the chain
ganglia

Superior to the chain
ganglia are three paravertebral ganglia in the cervical region

Three other autonomic ganglia related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these
autonomic ganglia are multipolar in shape, with dendrites radiating around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms.

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prevertebral
ganglia,

which are located outside of the chain but have similar functions. They are referred
to as prevertebral because they are anterior to the vertebral column.

Another group of autonomic ganglia is the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect
of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the heart
receives two inputs: one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck regions, whereas the terminal ganglia that receive sacral input are located in the lower abdominal
and pelvic cavities. We will explore the Autonomic Nervous System(ANS) in the next module

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autonomic ganglia

autonomic ganglia is the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms

These two sets of ganglia, sympathetic and parasympathetic,
often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ.

For example, the heart receives two inputs: one increases heart rate, and the other decreases it.

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terminal ganglia

The terminal ganglia that receive input from cranial nerves are found in the head and neck region

terminal ganglia that receive sacral

terminal ganglia that receive sacral input are located in the lower abdominal and pelvic cavities

Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as
a choroid plexus). For example, the enteric plexus is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is part of the enteric nervous system, along with the gastric plexus and the esophageal plexus. Though the
enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.

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plexus

A plexus, in a general sense, is a network of fibers or vessels. This can
apply to nervous tissue (as in this instance) or structures containing blood vessels (such as
a choroid plexus).

enteric plexus

For example, the enteric plexus is the extensive network of axons and
neurons in the wall of the small and large intestines. The enteric plexus is part of the enteric
nervous system, along with the gastric plexus and the esophageal plexus

Afferent and efferent divisions

Afferent and efferent divisions
The PNS is made up of afferent (sensory) and efferent (motor) components. The afferent division brings information from the periphery (receptors found within skin, joints, muscle,
visceral organs) to the spinal cord and brain via sensory nerves. The information brought in by afferent neurons is decoded and processed by the CNS. The efferent component, on the other hand, utilizes motor neurons that transmit information from the CNS to the periphery. This includes our muscle tissue (effectors) causing muscle contraction and thus voluntary body movements. In addition, the efferent division also controls smooth muscle in visceral organs, cardiac muscle and glands. It can function with or without consciousness (voluntary or involuntary).

afferent

The afferent
division brings information from the periphery (receptors found within skin, joints, muscle, visceral organs) to the spinal cord and brain via sensory nerves. The information brought in by afferent neurons is decoded and processed by the CNS

The efferent component

The efferent component, on
the other hand, utilizes motor neurons that transmit information from the CNS to the periphery. This includes our muscle tissue (effectors) causing muscle contraction and thus voluntary body movements. In addition, the efferent division also controls smooth muscle
in visceral organs, cardiac muscle and glands. It can function with or without consciousness (voluntary or involuntary).

Nerves

Nerves
Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different from the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well
as blood vessels supplying the tissues with nourishment.

The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are surrounded by the layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium (Figure 4.4). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

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Cranial Nerves: Classifications and Groups
Cranial Nerves
The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck. One of the 12 cranial nerves, Vagus nerve (CN X) extends into the abdomen. There are twelve cranial nerves, designated CN I
through CN XII for “Cranial Nerve,” using the Roman numerals for I through XII. CN I and CN II are the only cranial nerves attached to the forebrain. The remaining cranial nerves, CN III through CN XII, emerge from the brainstem. Specifically, CN III and IV are attached
to the midbrain; CN V, CN VI, CN VII, and CN VIII are attached to the pons; and CN IX, CN X, CN XI, and CN XII are attached to the medulla.

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Cranial nerves can be classified by their functions as sensory nerves, motor nerves, or a combination of both (mixed), meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter
the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck (most of the cranial nerves). Three of the nerves are solely composed of sensory fibers, five are strictly motor, and the remaining four are mixed nerves.

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Function

General

Target Area

Somatic (S): skin,
skeletal muscle

Direction

Afferent
(A)

Classification

General Somatic
Afferent (GSA)

Description

Carries general
sensory information
(e.g., touch, pain,
proprioception)

Associated CNs

CN V, VII, IX, X

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Function

General

Target Area

Somatic (S):
Skeletal (striated)
muscle

Direction

Efferent

Classification

General Somatic
Efferent (GSE)

Description

Sends motor
commands to
skeletal muscles

Associated CNs

CN III, IV, VI, XII

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Function

General

Target Area

Visceral (V):
Internal organs
(viscera

Direction

Afferent

Classification

General Visceral
Afferent (GVA)

Description

Conveys sensory
input from viscera

Associated CNs

CN IX, X

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Function

General

Target Area

Visceral (V):
Smooth muscle,
glands, cardiac

Direction

Efferent

Classification

General Visceral
Efferent (GVE)

Description

Autonomic motor
control of organs
and glands

Associated CNs

CN III, VII, IX, X

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Function

Special

Target Area

Somatic (S): Eye,
ear (vision,
hearing, balance)

Direction

Afferent

Classification

Special Visceral
Afferent (SVA)

Description

Special senses:
vision, hearing,
balance

Associated CNs

CN II, VIII

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Function

Special

Target Area

Visceral (V):
Tongue, nose
(taste, smell)

Direction

Afferent

Classification

Special Visceral
Afferent (SVA)

Description

Special senses
related to internal
structures: taste
and smell

Associated CNs

CN I, VII, IX, X

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Function

Special

Target Area

Visceral (V):
Muscles of face,
jaw, pharynx,
larynx (branchial
arch)

Direction

Efferent

Classification

Special Visceral
Efferent
(Branchial)
(SVE)

Description

Motor to muscles
from branchial
arches

Associated CNs

CN V, VII, IX, X,
XI

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4.1 organizes cranial nerve fibers by their function (general or special), target area (somatic or visceral), and Direction (afferent or efferent). Each fiber type is given a specific classification, such as GSA or SVE, based on these features. For example, GSA fibers carry
general somatic afferent (sensory) input from skin and skeletal muscles. Special classifications (like SSA or SVA) involve senses such as vision. The final column shows which carry each fiber type, helping learners understand how cranial nerves support both
basic and specialized body functions.

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GSA

For example, GSA fibers carry
general somatic afferent (sensory) input from skin and skeletal muscles.

Special
classifications (

Special
classifications (like SSA or SVA) involve senses such as vision

Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter
of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that
have personal significance. The names of the cranial nerves are listed in

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Another important aspect of the cranial nerves that lends itself to a mnemonic is each nerve's functional role. The nerves fall into one of three basic groups. They are sensory (S), motor (M), or both (B). The sentence, “Some Say Marry Money, But My Brother Says Brains Beauty
Matter More,” corresponds to the basic function of each nerve. See the summary below. • The first, second, and eighth nerves are purely sensory: the olfactory (CN I), optic (CN II), and vestibulocochlear (CN VIII) nerves.
• The three eye-movement nerves are all motor: the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI).
• The spinal accessory (CN XI) and hypoglossal (CN XII) nerves are also strictly motor.
• The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves.

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The nerves that convey both (mixed cranial nerves) are often related to each other. Cranial nerve V (CN V: trigeminal nerve) and cranial nerve VII (CN VII: facial nerve) concern the face; one concerns the sensations, and the other concerns the muscle movements. Cranial nerve VII (CN VII: facial nerve) and cranial nerve IX (CN IX: glossopharyngeal nerve) are both responsible for conveying gustatory (taste) sensations, as well as controlling salivary glands.
This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.

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