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Origin and Functions of Spinal and Cranial Nerves

Nerves in the spinal cord send signals to the muscles, bones, and internal organs of the body via peripheral nerves.
Spinal nerves
Nerves in the spinal cord send signals to the muscles, bones, and internal organs of the body via peripheral nerves. There are different spinal nerves for different parts of the body.

A spinal nerve is formed when sensory and motor nerve roots merge together to form a relatively large nerve. The nerve roots emerge directly from the spinal cord both ways in both directions, the motor roots emerging from the back and the sensory roots emerging from the front. These spinal nerves join at the spinal cord's sides to form the spinal cord. Sensory messages are sent to the spinal nerves from small nerves found in different parts of the body, including the skin, internal organs, and bones. Sensory messages are sent by the spinal nerves to the sensory roots in the back portion of the spinal cord, which then send them to the sensory fibers in the posterior part of the spinal cord. Nerve signals are transmitted into the spinal nerves from the motor roots when they receive nerve messages from the anterior (ventral or front) part of the spinal cord. Spinal nerves then communicate with smaller nerve branches that control the muscles in the arms, legs and other parts of the body.

As many as 31 spinal nerves pass through intervertebral foramina created by adjacent vertebrae to leave the vertebral canal. For example, the cervical discs have 8, the thoracic discs have 12, the lumbar discs have 5, the sacral discs have 5, and the coccygeal disc has 1. There are only seven cervical vertebrae, but each cervical nerve has eight branches, the first of which leaves the spinal canal between the atlas and occipital bone, and the eighth group branches below the last cervical vertebra. Next, each of the nerves is named and numbered according to the vertebrae above it. There are three groups of nerves that leave the spinal cord at the level of the 1st lumbar vertebra and extend downwards into the subarachnoid space. These nerves resemble a horse's tail when they stand together in the subarachnoid space. The nerves leaving the vertebral canal depart at the appropriate level, depending on whether they are bound for the lumbar, sacral, or coccygeal region.

The spinal cord and the spine contain nerves that are distributed in approximately equal numbers. In addition to protecting and surrounding the spinal cord, the spine is composed of several vertebral bones. The spinal nerves leave the spine through the foramina, openings in the vertebral bones from each side. The spinal nerves are formed in the space between the vertebrae, just a few centimeters from the spine. An extensive plexus of spinal nerves appears when several spinal nerves combine. Nerves in the spine can split into branches without forming a plexus. There are nerves that form a plexus when they are joined together. From the spinal nerves, there are five plexi:

Cervical plexus - Throughout the neck and shoulders are muscle fibers that are controlled by sensory and motor nerves that emerge from the neck plexus, composed of spinal nerves C1 through 5.
Branchial plexus - Branched from the C5 through T1 spinal nerve bundles, the brachial plexus provides motor control to arm and upper back muscles through the transmission of sensory messages.
Lumbar plexus - It is formed by the convergence of the spinal nerves L1 through L4. In addition to sending signals to muscle groups in the abdomen and legs, this plexus also provides motor control.
Sacral plexus - There are two types of spinal nerves in the spinal cord: the spinal nerves L4 and S4. These nerves connect together and branch out into muscles that provide sensory signaling and motor control to the legs.
Coccygeal plexus - This plexus is controlled by S4 and S1 nerves and facilitates motor and sensory control of the genital organs and the muscle groups controlling defecation.

Functions of spinal nerves
Sensory nerves and motor nerves develop in the spinal column. There are several spinal nerves, each with a different function, which corresponds to different parts of the body. These include motor control, sensation, and a variety of other functions like autonomic functions which generally control internal organs.

Motor
The spinal nerves receive motor messages from the brain. Brain cell (homunculus) initiates movement control by sending signals to muscles. By sending nerve impulses to the spinal cord, this command can move down to the nerves in the spinal cord. A motor stimulation activates only one or a few muscles, based on commands from the brain. The entire spinal nerve may be activated, or only one or two of its branches, to activate a very small group of muscles. It is called a myotome that describes how spinal nerves are distributed throughout the body. Several muscles are required to perform each physical movement, which is activated by a nerve branch coming from the spinal cord. In the biceps and triceps muscles, the C6 nerve controls the motor function, while the C7 nerve controls the sensory function.

Autonomic
Among the body's internal organs, the spinal cord mediates an autonomic function, which governs the bladder and intestines. Compared with motor and sensory branches, spinal nerves have fewer autonomic branches.

Sensory
Small nerve endings in the skin, muscles, joints and internal organs of the body send messages on touch, temperature, position, vibration, and pain to the spinal cord. According to the spinal cord anatomy, each spinal nerve corresponds to a different area of skin. Just for example - Sensations from the belly button are transmitted through T10, whereas those from the hand are transmitted through the C6, C7, and 8. Motor myotomes do not perfectly match sensory dermatomes.

Origin and functions of Cranial nerves
Nerves in your brain are connected to your head, neck, and trunk through cranial nerves. 12 of them have been named based on their functions or structures. In the lower surface of the brain are 12 pairs of nerves that originate from different nuclei, some motor, some sensory. As they connect to the brain, they are numbered using Roman numerals, starting anteriorly, according to their names or functions.

Sensory functions are usually classified as motor functions. Your sensory nerves regulate your senses, including touch, hearing, and smell. Nerves controlling movement and glandular function make sense; they are involved in muscles and glands.

12 cranial nerves are listed below in the following:
I. Olfactory: sensory
II. Optic: sensory
III. Oculomotor: motor
IV. Trochlear: motor
V. Trigeminal: mixed
VI. Abducent: motor
VII. Facial: mixed
VIII. Vestibulocochlear (auditory): sensory
IX. Glossopharyngeal: mixed
X. Vagus: mixed
XI. Accessory: motor
XII. Hypoglossal: motor

I. Olfactory nerves
Using your olfactory nerve, you carry sensory information regarding smells from your nose to your brain. The olfactory epithelium is a sticky layer at the roof of your nasal cavity that dissolves aromatic molecules when they are inhaled. Your olfactory bulb receives these nerve impulses when receptors are stimulated. There are specialized groups of nerve cells inside your olfactory bulb, which has an oval shape. Located below the frontal lobe of your brain, olfactory bulbs are composed of nerve fibres leading to your olfactory tract. Nerve signals travel to the parts of your brain that are involved in memory and smell recognition.

II. Optic nerves
Optic nerves are sensory nerves that control vision. In your retina, light makes contact with special receptors known as rods and cones as it enters your eye. Although the rods and cones are somewhat sensitive to light, the rods are the most sensitive. Black and white or night vision may be utilized more often by rods. The cones are fewer in number. The rods are more involved with colour vision and have a lower light sensitivity than cones. It is the retina that receives the information from your rods and cones, which is transmitted to your optic nerve. Within the skull, you have both optic nerves which converge and form something called the optic chiasm. The optic chiasm consists of two separate optic tracts formed by the nerve fibres in each limb of the retina. Eventually, the nerve impulses pass through each optic tract and reach your visual cortex, where they are processed. The visual cortex resides in the brain's back.

III. Oculomotor nerves
Besides muscle function, the oculomotor nerve also controls pupil size.
Muscle activity - There are six muscles around your eyes whose motor function is provided by your oculomotor nerve. They help you focus and move your eyes.
Pupil response - Additionally, it assists you in controlling how large your pupil is in response to light.

Your nerve starts in a part of your brainstem known as the midbrain, which is located at the back of your head. After passing through that area, it moves towards the area around your eyes.

IV. Trochlear nerves
Your superior oblique muscle is controlled by the trochlear nerve. It is the muscles of the eye that move downwards, outwards, and inwards. This part of the midbrain rises from the back. In the same manner that your oculomotor nerve travels forward, it moves to your eye sockets and stimulates the superior oblique muscles.

V. Trigeminal nerves
As one of your cranial nerves, the trigeminal nerve is the largest and performs both sensory and motor functions. It consists of three divisions, namely:

Opthalmic - The ophthalmic division comprises the forehead, scalp, and upper eyelids, and it sends sensory information from the upper portion of your face.
Maxillary - This middle part of your face, which includes your cheeks, upper lip, and nasal cavity, is responsible for transmitting sensory information to your brain.
Mandibular - Sensory and motor functions are both present in the mandibular division. It sends information to your brain based on the senses of your ears, lower lip, and chin. Your jaw and ear are also controlled by this organ.

In the midbrain and medulla regions of the brain stem, several nuclei - which are groups of nerve cells - are responsible for forming the trigeminal nerve. The sensory root and motor root are eventually separated by these nuclei. There are separate sensory roots for your ophthalmic, maxillary, and mandibular regions of the trigeminal nerve. A trigeminal nerve's motor branch only extends to the mandibular division below its sensory branch.

VI. Abducent nerves
Abducens nerve is involved in the control of the lateral rectus muscle, which assists with eye movement. It controls eye movements in the outward direction. For instance, it’s used when looking sideways. The nerve has its origin in the brainstem, where it is known as the abducent nerve. You can see the lateral rectus muscle being controlled by the nerve when it enters your eye socket.

VII. Facial nerves
Sensory and motor functions are provided by the facial nerve, including:
  1. Mouth, facial, and jaw muscles that move in response to facial expressions.
  2. It provides a sense of taste to the majority of your tongue
  3. These glands, such as salivary glands and tear-producing glands, supply your head and neck
  4. Your ear's outer parts communicate sensations
There is a complex path taken by your facial nerve. The motor and sensory roots of the motor cortex originate in the pons region of the brainstem. Eventually, these two connections fuse and form the facial nerve. The facial nerve branches further both inside and outside your skull into smaller nerve fibres that provide sensory information or stimulate muscles and glands.

VIII. Vestibulocochlear nerves
Vestibule - cochlear nerves have sensory functions associated with hearing and balance. Both cochlea and vestibular components are involved:
Cochlear portion - Your ear is given information about sound vibrations based on the loudness and pitch of the sound. A nerve impulse is generated by this and transmitted to the cochlea.
Vestibular portion - Additionally, there are cells here that track both straight-line movements and rotations of the head. It is used to adjust your balance and equilibrium by transmitting this information to the vestibular nerve.

The vestibulocochlear nerve's cochlear and vestibular parts are produced in different brain areas. Throughout your brain, you have an area called the inferior cerebellar peduncle that serves as the cochlea. In your pons and medulla is where your vestibular portion begins. The vestibulocochlear nerve is a combination of both parts.

IX. Glossopharyngeal nerves
Glossopharyngeal nerves perform a number of functions, including:
  1. Sensory information is sent to your senses from the back of your throat and sinuses, along with your inner ear and back section of your tongue.
  2. The back portion of your tongue is provided with a sense of taste
  3. The stylopharyngeus muscle in your back of the throat is stimulated by the stimulation
The glossopharyngeal nerve originates in the medulla oblongata, which is located in the brainstem. This nerve eventually reaches the neck and throat.

X. Vagus nerves
Vagus nerves are diverse nerves. In addition to its sensory functions, it also has motor functions.
  1. Your ear canal and your throat communicate sensation information to your brain
  2. Your heart and intestines transmit sensory information to your brain through your chest and trunk
  3. Your throat muscles can be controlled by motor control
  4. By stimulating the muscles in your chest and trunk, including those involved in digestion (peristalsis), you stimulate your body's waste removal.
  5. Your tongue's root is where you get your sense of taste
There is one nerve in the brain with the longest pathway, and that is the vagus nerve. The journey begins at the base of your skull and goes all the way to your abdomen. This region of your brain stem is known as the medulla.

XI. Accessory nerves
A motor nerve controls your neck muscles through your accessory nerve. Your neck and shoulders are able to be rotated, flexed, and extended by these muscles. The spinal column and the cranium form two parts of the spine. The spinal portion is located in the beginning of your spinal cord. In the medulla oblongata is where the cranial part begins. Before either part of the nerve branches off, the spinal portion of the nerve supplies your neck muscles while the cranial portion follows your vagus nerve.

XII. Hypoglossal nerves
A large part of the tongue's movement is controlled by the hypoglossal nerve, which is the 12th cranial nerve. The nerve travels along the midline of the skull, from the medulla oblongata to the jaw, where it reaches the tongue.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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