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Peripheral Nervous System: Structures & Function
Peripheral Nervous System: Structures & Function
Includes labeled diagrams, downloadable notes, anatomy quizzes, and interactive learning tools
The peripheral nervous system (PNS) includes everything outside the central nervous system (CNS). It is not contained in one place like the brain or spinal cord. Instead, its structures are spread throughout the body and are often part of other organs.
Some parts of the PNS are built into organ systems. The enteric nervous system, for example, is found in the digestive system and works as a special part of the PNS.
Ganglia
A ganglion is a group of neuron cell bodies located in the peripheral nervous system. Most ganglia are either sensory or autonomic, depending on their function.
Sensory ganglia contain the cell bodies of neurons that receive information from the body. The most common type is the dorsal (posterior) root ganglion. These neurons have sensory endings in the periphery, such as in the skin, and their axons enter the central nervous system through the dorsal nerve root.
The ganglion appears as a swelling along the nerve root. The neurons in these ganglia are unipolar in shape, and they are surrounded by small satellite cells.
Another type of sensory ganglion is associated with cranial nerves. These are similar to dorsal root ganglia, but they are linked to cranial nerves instead of spinal nerves. The roots of cranial nerves are located inside the skull, while the ganglia are outside. The neurons in these ganglia are also unipolar and are surrounded by satellite cells.
Autonomic ganglia belong to the autonomic nervous system, which is divided into the sympathetic and parasympathetic systems. The sympathetic chain ganglia form a row along the vertebral column. They receive signals from the spinal cord and send signals to organs in the head, chest, abdomen, and pelvis to control sympathetic functions. There are also paravertebral ganglia in the neck region and prevertebral ganglia located in front of the vertebral column. The neurons in these ganglia are multipolar and have many branching processes.
The parasympathetic system contains terminal ganglia. These receive input from cranial nerves or sacral spinal nerves and control parasympathetic functions. In many organs, both sympathetic and parasympathetic ganglia send signals, often with opposite effects. For example, one input may increase heart rate, while the other decreases it. Many terminal ganglia are located within the walls of organs, where they form networks called plexuses.
A plexus is a network of nerve fibers. The enteric plexus is a large network found in the walls of the intestines. It is part of the enteric nervous system, which also includes gastric and esophageal plexuses. Although it receives input from the central nervous system, it can function independently to control the digestive system.
OpenStax College, CC BY 3.0, via Wikimedia Commons, changes were made.
Bundles of axons in the peripheral nervous system are called nerves. These are different from tracts in the central nervous system. Nerves are made up not only of nerve fibers but also of connective tissue and blood vessels. The outer layer of a nerve is called the epineurium.
Inside the nerve, axons are grouped into bundles called fascicles, which are surrounded by the perineurium. Each individual axon is surrounded by a layer called the endoneurium. These layers are similar to the connective tissue coverings found in muscles.
Cranial Nerves
The nerves attached to the brain are called cranial nerves. They are mainly responsible for the sensory and motor functions of the head and neck. One exception is a nerve that also connects to organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system.
There are twelve cranial nerves in total. They are numbered from I to XII using Roman numerals. These nerves can be sensory, motor, or a combination of both.
Sensory fibers come from sensory ganglia outside the skull and enter the brain, where they connect to nuclei. Motor fibers begin in the brain stem and connect to muscles in the head and neck.
Cranial nerves are divided into three groups based on their function:
three of them are purely sensory
five are purely motor
four are mixed nerves that carry both sensory and motor fibers.
Sensory nerves carry information into the brain. Motor nerves send commands from the brain to muscles. Mixed nerves do both.
Each cranial nerve has a specific role. The olfactory nerve (I) is responsible for the sense of smell. The optic nerve (II) is responsible for vision.
The oculomotor nerve (III) controls most eye movements. It also lifts the upper eyelid and causes the pupil to constrict.
The trochlear nerve (IV) and the abducens nerve (VI) also control eye movements, but each acts on different eye muscles. The trigeminal nerve (V) provides sensation to the face and controls the muscles used for chewing.
The facial nerve (VII) controls facial expressions. It also plays a role in taste and helps produce saliva. The vestibulocochlear nerve (VIII) is responsible for hearing and balance. The glossopharyngeal nerve (IX) controls muscles in the throat and contributes to taste and saliva production.
The vagus nerve (X) helps regulate organs in the thoracic and upper abdominal cavities and plays a role in maintaining internal balance. The spinal accessory nerve (XI) controls muscles of the neck. The hypoglossal nerve (XII) controls muscles of the tongue and lower throat.
Spinal Nerves
There are 31 pairs of spinal nerves, and they are named according to the level of the spinal cord from which they emerge.
There are eight cervical nerves (C1-C8), twelve thoracic nerves (T1-T12), five lumbar nerves (L1-L5), five sacral nerves (S1-S5), and one coccygeal nerve. These nerves are numbered from top to bottom.
Most spinal nerves exit the vertebral column through intervertebral foramina at the level matching their name.
However, there are some differences in the cervical region. The first cervical nerve exits between the skull and the first vertebra, and the eighth cervical nerve exits between the seventh cervical vertebra and the first thoracic vertebra.
Thoracic and lumbar nerves exit below the vertebra with the same number, and sacral nerves exit through openings in the sacrum.
After leaving the spinal cord, spinal nerves extend into the body to supply different regions. However, they do not simply continue as single nerves: their fibers mix and reorganize into networks called nerve plexuses. In these plexuses, fibers from different spinal nerves combine and then redistribute into new nerves that travel to specific body regions.
There are four major nerve plexuses in the body. Two are located in the cervical region, one in the lumbar region, and one in the sacral region.
The cervical plexus is formed by nerves C1 to C5 and supplies the neck and head. It also gives rise to the phrenic nerve, which controls the diaphragm.
The brachial plexus, formed from C4 to T1, supplies the upper limbs. Important nerves from this plexus include the radial, ulnar, and median nerves.
The lumbar plexus is formed from the lumbar spinal nerves and supplies the pelvic region and the front of the leg. One of its main branches is the femoral nerve, which gives rise to the saphenous nerve.
The sacral plexus is formed from L4, L5, and S1 to S4. Its largest branch is the sciatic nerve, which travels through the hip and leg. This nerve is commonly associated with sciatica, a condition caused by compression or irritation of the nerve.
The fibers within spinal nerves and plexuses serve both sensory and motor functions. Sensory fibers carry information from the body to the spinal cord, while motor fibers carry signals from the spinal cord to muscles, causing them to contract. For example, the radial nerve contains both sensory fibers from the skin of the arm and motor fibers that control arm muscles.
Not all spinal nerves form plexuses. The thoracic nerves from T2 to T11 remain separate and form intercostal nerves. These nerves run between the ribs and supply the muscles and skin of the chest.
Need a recap? Take a look at the peripheral nervous system anatomy notes here.
Need a recap? Take a look at the peripheral nervous system anatomy notes here.
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References
References
Gray, H. (1918). Anatomy of the human body (W. H. Lewis, Ed.; 20th ed.). Lea & Febiger.
J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. (2013, April 25). Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology-2e/pages/13-4-the-peripheral-nervous-system.
Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.
Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.
Content paraphrased; adaptations were made.
Image Credis
Image Credis
Images used in this guide (not including the first and second images) and games are by Dr. Johannes Sobotta. They are in the public domain; modifications have been made to the originals.
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