How do spinal nerves of the peripheral nervous system (pns) differ from cranial nerves (cns)?

The spinal nerves and the cranial nerves make up the peripheral nervous system (PNS). These nerves are the passages by which information travels to and from the central nervous system (CNS) and the rest of the body.

There are 31 pairs of spinal nerves and 12 pairs of cranial nerves. The cranial nerves emerge directly from the brain, while the spinal nerves emerge from the spinal cord.

This article will take a closer look at the spinal and cranial nerves of the peripheral nervous system.

Also, to find out more about the structures of the nervous system and their function read 11 Facts about the Nervous System Every Nursing Student Should Know.

You have seen a lot of great mnemonics about the cranial nerves as a guide to help remember the order of the nerves.

This article goes a little further into the functioning of each nerve. This will help answer those test questions not related to memorization. First, we will look at the spinal nerves.

Spinal nerves

Firstly there are 31 pairs of spinal nerves. The spinal nerves include eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves.

Moreover, each spinal nerve derives its name from the level of the vertebral column in which it exits.

The vertebral column contains 33 vertebrae. So, there are seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five sacral vertebrae, and four coccygeal vertebrae.

Additionally, the first cervical nerve exits above the first cervical vertebrae and, the rest of the spinal nerves exit below the vertebrae they correspond to.

When a nerve contains both motor and sensory neurons, they are called mixed nerves. The spinal nerves are all mixed nerves.

Each spinal nerve has a ventral root as well as a dorsal root. Furthermore, the ventral and dorsal root come together to form a single spinal nerve.

The dorsal roots are sensory and transmit impulses from specific areas of the skin known as dermatomes. These impulses are transmitted to the dorsal horn ganglia.

A dermatome is an area of skin in which the fibers of a single dorsal root of a spinal nerve innervate. And, a ganglia is a mass of nerve tissue.

The ventral roots are motor and transmit impulses from the spinal cord to the body. These fibers are also somatic or visceral. The visceral fibers control the cardiac muscle and glandular secretions.

When spinal nerves exit the vertebral column they form complex intersecting networks nerves called plexuses.

A plexus is just a network of nerve fibers. Four main plexuses are formed by the spinal nerves. These plexus include the cervical plexus, thoracic plexus, lumbar plexus, and sacral plexus.

Cranial Nerves

The CNS contains the brain and the spinal cord. The CNS communicates with the outside world via the nerves of the PNS.

Therefore, the spinal cord communicates with the outside world via the spinal nerves. And, the brain connects with the outside world via the cranial nerves.

There are 12 pairs of cranial nerves. Unlike the spinal nerves, not all cranial nerves are mixed nerves.

Some cranial nerves are mixed but some are purely sensory or motor. Cranial nerves exit from the lower portion of the brain through the opening at the base of the skull.

Three of these cranial nerves are sensory, 1 cranial nerve is purely motor, and the other eight are mixed sensory and motor nerves.

These eight nerves may be mostly sensory or motor but actually, function in both capacities.

• The 12 pairs of cranial nerves are numbered with Roman numerals.
• Also, these nerves are numbered in the order they arise from the brain.
• The three sensory nerves are I, II, VIII.
• The four motor nerves are III, IV, VI, VII.
• And, the five mixed nerves are V, IX, and X, XI, and XII.
• Cranial nerves XI and XII are considered mixed but are mostly motor.

The cranial nerves carry information to and from the CNS. And, they mostly supply the head and neck structures.

The sensory nerve fibers send information to the central nervous system such as pain. And the motor nerve fibers send instructions for movements such as facial expressions.

The cranial nerves are more specialized than spinal nerves and have names based on their specialty.

I – Olfactory Nerve

The olfactory nerve is the sensory nerve of smell. So, this nerve carries the sense of smell from receptors in the nasal cavity to the brain.

II – Optic Nerve

The optic nerve is the sensory nerve that carries information from the retina to the brain. It is responsible for visual processes.

III – Oculomotor Nerve IV – Trochlear NerveVI – Abducens Nerve

The next 3 cranial nerves are out of order, but they all work together. The oculomotor, trochlear and abducens nerves control the six muscle that move the eye.

• These nerves are considered motor nerves.
• The six cardinal directions of gaze are used to test these nerves.
• See the article Tips for Performing a Nursing Health Assessment of the Nervous System.
• These nerves are only responsible for eye movement.
• So, they have nothing to do with sight.

V – Trigeminal Nerve

The trigeminal nerve is the main nerve of the face. This nerve is a mixed nerve.

• The trigeminal nerve is important for chewing.
• This nerve is named trigeminal because it has three branches.
• Those branches are the ophthalmic, maxillary, and mandibular branch.
• Firstly, the ophthalmic branch supplies the upper part of the face and the scalp.
• And then, the maxillary branch supplies the middle part of the face on each side.
• And finally, the mandibular branch supplies the bottom of the face and jaw.

VII – Facial Nerve

The facial nerve is a motor nerve. This nerve supplies the motor fibers used for facial expressions and, also the salivary and lacrimal glands.

VIII – Vestibulocochlear Nerve

The vestibulocochlear nerve is a sensory nerve and is responsible for transmitting information about balance and hearing from the inner ear to the brain.

The vestibulocochlear nerve is comprised of two nerves.

• These are the vestibular and cochlear nerve.
• Firstly, the vestibular nerve carries information about the position and equilibrium.
• And so, the cochlear nerve carries information about sound from the cochlear hearing receptors.

IX – Glossopharyngeal Nerve

The glossopharyngeal nerve is a mixed nerve. The motor fibers carry motor information from the throat to the brain. And the sensory fibers carry impulses from the pharynx and tongue (taste buds).

X – Vagus Nerve

The vagus nerve is the largest of the cranial nerves. This is a mixed nerve that extends from the head to the abdomen. The vagus nerve controls breathing and also digestion.

This nerve provides sensation from the throat, as well as organs of the chest and abdomen, taste from the tongue and back of the throat, and muscle function of the palate.

XI – Accessory Nerve or Spinal Accessory Nerve

The accessory nerve is a mixed nerve but mostly motor of the sternocleidomastoid and trapezius muscles. Also, it has fibers that innervate the pharynx and larynx.

XII – Hypoglossal Nerve

The hypoglossal nerve is considered mixed but mostly motor. This nerve supplies the muscles of the tongue. Also, it sends sensory impulses from the tongue to the brain.

This nerve is important for chewing, swallowing and speaking.

Conclusion

The cranial and spinal nerves comprise the PNS. The PNS connects the CNS to the outside world. These nerves are necessary for sending sensory and motor information to the CNS.

This article gives information about the nerves of the PNS and their functions.

Reference

Ignatavicus D., Workman L., Rebar C., Medical-Surgical Nursing: Concepts for Interpersonal Collaborative Care. 9th ed. St Louis, MO: Elsevier Inc. 2018.

Lewis S., Bucher l., Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 10th ed, St Louis, MO: Elsevier Inc. 2017.

Mosby’s Medical Dictionary (2017). 10th ed. St Louis, MO. Elsevier Inc.

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