Cranial Nerves & Spinal Cord - Neuroscience Made Easy- 3

It is found in the spinal [neural] cavity of vertebral column. Like the brain it is covered by meninges. In humans it is 40 to 50 cm in length and 2 cm in diameter. It lies inside neural canal of vertebral column and terminates into 1t lumbar vertebra of abdominal region.

T.S of spinal cord shows that it has internal grey matter and external white matter. Grey matter is arranged like the letter 'H'. The upper arms are called dorsal [posterior] horns and the lower arms are called ventral horns. The dorsal horns are connected to the sensory nerves and ventral horns to motor nerves which join and form single mixed nerve. The horizontal bar of H shaped grey matter is called grey commissure.

Credit: Queensland Brain Institute

In the centre of the spinal cord is a central canal filled with CSF. The bundle of spinal nerves in the posterior most part of the spinal cord is like a horsetail, and is called Cauda equina. The non nervous part of the spinal cord is filum terminale. It is the extension of Piamater beyond spinal cord.

Spinal nerves:

The spinal nerves receive sensory messages from tiny nerves located in areas such as the skin, internal organs, and bones. The spinal nerves send sensory messages to the sensory roots, then to sensory fibers in the posterior (back or dorsal) part of the spinal cord.

The motor roots receive nerve messages from the anterior (front or ventral) part of the spinal cord and send the nerve messages to the spinal nerves, and eventually to small nerve branches that activate muscles in the arms, legs, and other areas of the body.

There are 31 pairs of spinal nerves including:

  • Eight cervical spinal nerves on each side of the spine called C1 through C8

  • Twelve thoracic spinal nerves in each side of the body called T1 through T12

  • Five lumbar spinal nerves on each side called L1 through L5

  • Five sacral spinal nerves in each side called S1 through S5

  • One coccygeal nerve on each side, Co1



Spinal nerves are distributed approximately evenly along the spinal cord and spine. The spine is a column of vertebral bones that protects and surrounds the spinal cord. Each spinal nerve exits the spine by traveling through the foramen, which are openings at the right and left sides of the vertebral bones of the spine.

The spinal nerves are formed within a few centimeters of the spine on each side. Some groups of spinal nerves merge with each other to form a large plexus. Some spinal nerves divide into smaller branches, without forming a plexus.

A plexus is a group of nerves that combine with each other. There are five main plexi formed by the spinal nerves:

  • Cervical Plexus: Composed of the merging of spinal nerves C1 through 5, these divide into smaller nerves that carry sensory messages and provide motor control to the muscles of the neck and shoulders.

  • Brachial Plexus: Formed by the merging of spinal nerves C5 through T1, this plexus branches into nerves that carry sensory messages and provide motor control to the muscles of the arm and upper back.

  • Lumbar Plexus: Spinal nerves L1 through L4 converge to form the lumbar plexus. This plexus splits into nerves that carry sensory messages and provide motor control to the muscles of the abdomen and leg.

  • Sacral Plexus: Spinal nerves L4 through S4 join together, and then branch out into nerves that carry sensory messages and provide motor control to the muscles of the legs.

  • Coccygeal Plexus: Composed of the merging of nerves S4 through Co1, this plexus supplies motor and sensory control of the genitalia and the muscles that control defecation.


Cranial Nerves

I. Olfactory nerve

The olfactory nerve transmits sensory information to your brain regarding smells that you encounter. When you inhale aromatic molecules, they dissolve in a moist lining at the roof of your nasal cavity, called the olfactory epithelium. This stimulates receptors that generate nerve impulses that move to your olfactory bulb. Your olfactory bulb is an oval-shaped structure that contains specialized groups of nerve cells. From the olfactory bulb, nerves pass into your olfactory tract, which is located below the frontal lobe of your brain. Nerve signals are then sent to areas of your brain concerned with memory and recognition of smells.

II. Optic nerve The optic nerve is the sensory nerve that involves vision. When light enters your eye, it comes into contact with special receptors in your retina called rods and cones. Rods are found in large numbers and are highly sensitive to light. They’re more specialized for black and white or night vision. Cones are present in smaller numbers. They have a lower light sensitivity than rods and are more involved with color vision. III. Oculomotor nerve The oculomotor nerve has two different motor functions: muscle function and pupil response.

  • Muscle function. Your oculomotor nerve provides motor function to four of the six muscles around your eyes. These muscles help your eyes move and focus on objects.

  • Pupil response. It also helps to control the size of your pupil as it responds to light.

This nerve originates in the front part of your midbrain, which is a part of your brainstem. It moves forward from that area until it reaches the area of your eye sockets.

IV. Trochlear nerve The trochlear nerve controls your superior oblique muscle. This is the muscle that’s responsible for downward, outward, and inward eye movements. It emerges from the back part of your midbrain. Like your oculomotor nerve, it moves forward until it reaches your eye sockets, where it stimulates the superior oblique muscle.

V. Trigeminal nerve The trigeminal nerve is the largest of your cranial nerves and has both sensory and motor functions. The trigeminal nerve originates from a group of nuclei — which is a collection of nerve cells — in the midbrain and medulla regions of your brainstem. Eventually, these nuclei form a separate sensory root and motor root. The sensory root of your trigeminal nerve branches into the ophthalmic, maxillary, and mandibular divisions. The motor root of your trigeminal nerve passes below the sensory root and is only distributed into the mandibular division.

VI. Abducens nerve The abducens nerve controls another muscle that’s associated with eye movement, called the lateral rectus muscle. This muscle is involved in outward eye movement. For example, you would use it to look to the side. This nerve, also called the abducent nerve, starts in the pons region of your brainstem. It eventually enters your eye socket, where it controls the lateral rectus muscle.


VII. Facial nerve The facial nerve provides both sensory and motor functions, including:

  • moving muscles used for facial expressions as well as some muscles in your jaw

  • providing a sense of taste for most of your tongue

  • supplying glands in your head or neck area, such as salivary glands and tear-producing glands

  • communicating sensations from the outer parts of your ear

Your facial nerve has a very complex path. It originates in the pons area of your brainstem, where it has both a motor and sensory root. Eventually, the two nerves fuse together to form the facial nerve. Both within and outside of your skull, the facial nerve branches further into smaller nerve fibers that stimulate muscles and glands or provide sensory information.

VIII. Vestibulocochlear nerve Your vestibulocochlear nerve has sensory functions involving hearing and balance. It consists of two parts, the cochlear portion and vestibular portion:

  • Cochlear portion. Specialized cells within your ear detect vibrations from sound based off of the sound’s loudness and pitch. This generates nerve impulses that are transmitted to the cochlear nerve.

  • Vestibular portion. Another set of special cells in this portion can track both linear and rotational movements of your head. This information is transmitted to the vestibular nerve and used to adjust your balance and equilibrium.

The cochlear and vestibular portions of your vestibulocochlear nerve originate in separate areas of the brain. The cochlear portion starts in an area of your brain called the inferior cerebellar peduncle. The vestibular portion begins in your pons and medulla. Both portions combine to form the vestibulocochlear nerve.

IX. Glossopharyngeal nerve The glossopharyngeal nerve has both motor and sensory functions, including:

  • sending sensory information from your sinuses, the back of your throat, parts of your inner ear, and the back part of your tongue

  • providing a sense of taste for the back part of your tongue

The glossopharyngeal nerve originates in a part of your brainstem called the medulla oblongata. It eventually extends into your neck and throat region.

X. Vagus nerve The vagus nerve is a very diverse nerve. It has both sensory and motor functions, including:

  • communicating sensation information from your ear canal and parts of your throat

  • sending sensory information from organs in your chest and trunk, such as your heart and intestines

  • allowing motor control of muscles in your throat

  • stimulating the muscles of organs in your chest and trunk, including those that move food through your digestive tract (peristalsis)

  • providing a sense of taste near the root of your tongue

Out of all of the cranial nerves, the vagus nerve has the longest pathway. It extends from your head all the way into your abdomen. It originates in the part of your brainstem called the medulla.

XI. Accessory nerve Your accessory nerve is a motor nerve that controls the muscles in your neck. These muscles allow you to rotate, flex, and extend your neck and shoulders. It’s divided into two parts: spinal and cranial. The spinal portion originates in the upper part of your spinal cord. The cranial part starts in your medulla oblongata. These parts meet briefly before the spinal part of the nerve moves to supply the muscles of your neck while the cranial part follows the vagus nerve.

XII. Hypoglossal nerve Your hypoglossal nerve is the 12th cranial nerve which is responsible for the movement of most of the muscles in your tongue. It starts in the medulla oblongata and moves down into the jaw, where it reaches the tongue.


Next Up on Neuroscience Made Easy:

The Limbic System

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