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The cranial nerve innervation is highly disproportionate, as far as humans are concerned. I am not sure of the advantage of being innervated by cranial nerve versus being innervated by a normal spinal nerve branches, if any.
If there is any advantage to be innervated by cranial nerves, why is there such a largely disproportionate distribution of these cranial nerves?
Only the Vagus nerve (
X) innervates parts other than the head of the animal. While there are 3 motor nerves just for the 6 eyeball muscles and the ciliary muscles (
III,IV,VI), there is only one mixed nerve (
X) innervating heart, gastro-intestinal tract and the respiratory apparatus and all smooth muscles therein. While there are three nerves for gustatory receptors, (
VII,IX,X), one for retina (
II), cochlea and vestibule (
VIII) and the olfactory receptors (
I) each, the entire plethora of intero-receptors in thoracic and abdominal viscera, aortic arch, external auditory meatus, tympanic membrane etc are carried by a single nerve (
X). I am not sure if the disproportion is limited to the disproportionate area of innervation or even to the density of innervation in the target tissue.
Below are a few possible explanations that I came across while surfing the net, all without any strong arguments.
- Is it because of the fineness of the senses in the head, i.e. the skin receptors in facial skin being higher than in the general body, providing a finer resolution of tactile reception? The special senses (vision, hearing, smelling and tasting) are also very fine in their resolution and might require more concentrated innervation and direct interpretation by the brain. But then the fingers and feet also have a high density of skin receptors, and the tactile sense can also be very fine (owing to several different kind of receptors) requiring dense innervation? Certain body movements (especially of hand) are very fine and detailed, requiring a very dense and co-ordinated motor neuron framework.
- Is it because the cranial nerves, owing to some developmental restraint or functional trade-off, can only innervate areas close to its place of origin effectively? This might explain why 11 pairs of nerves innervate only the head region and only 1 pair innervates the remaining body. I have no idea about the validity and the reason of this claim.
- Could it just be a random evolutionary outcome, there being absolutely no benefit of cranial innervation leading to random drift in the innervation patterns?
We can somewhat rule out reason 1 because fingertips are also highly sensitive to sensory stimulus. Moreover innervation is not solely about sensory inputs but also motor functions, visceral control etc. All the nerves required for the motor function and a significant portion of visceral control nerves connect to spinal cord.
Therefore in my opinion reason 2 is the most appropriate which automatically rules out reason 3. Spinal cord is an advancement of neural organization in the chordates. Spinal cord can be viewed as a central cable in which all power lines merge (they still maintain their individuality [the spinal tracts]; they just get clustered to form an organized bundle). Evolution of spinal reflexes must have been a secondary event.
The case of vagus nerve and cranioaccessory nerve suggests that this is an ancient pathway to control the heart. Heart with a closed circulatory system is found in molluscs as well. Apart from the functional significance of the heart (which might to some extent justify direct cranial innervation), it is decently proximal to the brain as well.
The cranial nerves of reptiles, birds, and mammals consist of twelve pairs of numbered peripheral nerves that originate in the cerebrum and brainstem and have their own specific sensory and motor pathways. Lower vertebrates such as fish and amphibians only have ten pairs. Cranial nerves are motor, sensory, or mixed neurons that bring motor and sensory messages to and from the face, neck, and shoulders, as well as many internal organs such as the heart, lungs, and gastrointestinal tract.
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 these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, 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. Three of the nerves are solely composed of sensory fibers five are strictly motor and the remaining four are mixed nerves.
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 the rhyming couplet, “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 Table 13.3 along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve (Figure 13.3.2).
The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerve is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.
Figure 13.3.2 – The Cranial Nerves: The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.
Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.
Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?
Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table 13.3). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.
|Cranial Nerves (Table 13.3)|
|Mnemonic||#||Name||Function (S/M/B)||Central connection (nuclei)||Peripheral connection (ganglion or muscle)|
|On||I||Olfactory||Smell (S)||Olfactory bulb||Olfactory epithelium|
|Old||II||Optic||Vision (S)||Hypothalamus/thalamus/midbrain||Retina (retinal ganglion cells)|
|Olympus’||III||Oculomotor||Eye movements (M)||Oculomotor nucleus||Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic)|
|Towering||IV||Trochlear||Eye movements (M)||Trochlear nucleus||Superior oblique muscle|
|Tops||V||Trigeminal||Sensory/motor – face (B)||Trigeminal nuclei in the midbrain, pons, and medulla||Trigeminal|
|A||VI||Abducens||Eye movements (M)||Abducens nucleus||Lateral rectus muscle|
|Finn||VII||Facial||Motor – face, Taste (B)||Facial nucleus, solitary nucleus, superior salivatory nucleus||Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic)|
|And||VIII||Auditory (Vestibulocochlear)||Hearing/balance (S)||Cochlear nucleus, Vestibular nucleus/cerebellum||Spiral ganglion (hearing), Vestibular ganglion (balance)|
|German||IX||Glossopharyngeal||Motor – throat Taste (B)||Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus||Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic)|
|Viewed||X||Vagus||Motor/sensory – viscera (autonomic) (B)||Medulla||Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines)|
|Some||XI||Spinal Accessory||Motor – head and neck (M)||Spinal accessory nucleus||Neck muscles|
|Hops||XII||Hypoglossal||Motor – lower throat (M)||Hypoglossal nucleus||Muscles of the larynx and lower pharynx|
Passes through perforations in the cribiform plate of the ethmoid bone and terminate in the upper part of the nasal cavity.
Contains the afferant nerve fibers of the olfactory receptor neurons.
Test: coffee and other smells.
Optic nerves from the right and left join to form the optic chiasma
2. Inferior Branch:
a. Medial rectus
b. Inferior rectus
c. Inferior oblique
Visceral Motor: Parasympathetic to ciliary & pupillary constrictor muscles
1. Intorsion (Internal rotation)
2. Depression (primarily in the adducted position)
3. Abduction (lateral rotation)
Smallest in terms of the number of axons
Greatest intracranial length
Only cranial nerve that exits from the dorsal (rear) aspect of the brainstem
Only cranial nerve that innervates a muscle on the contralateral side from its origin
Branchial motor: muscles of mastication
General sensory: sensory for head/neck, sinuses, meninges, and external surface of tempanic membrane (Touch, pressure, pain, thermal)
General sensation from the anterior two-thirds of tongue are supplied by afferent fibers of the third division of the fifth cranial nerve (V-3).
A lesion in the abducent nerve results in paralysis of the lateral rectus muscle that normally abducts the eye. The eye will then deviate
medially as a result of the unopposed action of the medial rectus
Functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity.
It also supplies preganglionic parasympathetic fibers to several head and neck ganglia.
The facial nerve also supplies parasympathetic fibers to the submandibular gland and sublingual glands to increase the flow of saliva.
It also supplies parasympathetic innervation to the nasal mucosa and the lacrimal gland.
Perform a quick hearing assessment by holding your fingers a few inches away from the patient's ear and rubbing them together softly or use an analog watch.
Weber and Rinne tests for distinguishing conductive hearing loss from sensorineural
Special Visceral Afferent: scant taste buds on epiglottis
General Visceral Afferent: mucous membranes of the soft palate, and those lining the pharynx, larynx, esophagus, and trachea. Chemoreceptor
fibers (GVA also) terminate in the carotid body where they monitor blood carbon dioxide concentration.
General Somatic Afferent: conveying pain, temperature, and touch sensation reside in the superior ganglion and send their peripheral processes to the pinna, external auditory meatus, skin of the ear, and tympanic membrane.
Special Visceral Efferent: The fibers of these neurons
innervate all of the laryngeal and pharyngeal muscles with the exception of the stylopharyngeus and the tensor veli palatini muscles.
General Visceral Efferent: It supplies parasympathetic innervation to the laryngeal mucous glands and all of the thoracic and most of the abdominal organs. Parasympathetic innervation decreases the heart rate, reduces adrenal gland secretion, activates peristalsis, and stimulates glandular activity of various organs.
The nerves that arise from the brain are called cranial nerves. In human body, there are 12 pairs of cranial nerves. Among these, 1st, and 2nd nerves are connected to the forebrain 3rd and 4th nerves to midbrain 5th, 6th, 7th and 8th nerves to pons and 9th. 10th, 11th and 12th nerves to medulla oblongata. Some of these nerves such as 1st, 2nd and 8th nerves are purely sensory while some others such as 3rd, 4th, 6th, 11th and 12th nerves are purely motor and the remaining ones i.e., 5th, 6th, 9th and 10th nerves of mixed type.
1st cranial nerve
It is also called olfactory nerve. It is a purely sensory nerve which carriers the sensation of smell from the receptor of smell in the nose to the limbic lobe of cerebrum. So it is the nerve for smell sensation.
2nd cranial nerve
Its name is optic nerve and it is also a sensory nerve. It carries visual sensation from the retiria-of eye to the brain.
3rd cranial nerve
It is called oculomotor nerve. Iit is a purely motor nerve. It contains both somatic and autonomic type of nerve fibers. The somatic fibers innervate some external ocular muscles and thereby control the movements of eyeball. The autonomic fibers of this nerve belong to the parasympathetic system. They innervate the smooth muscles present within the eyeball namely the ciliary muscles for increasing the curvature of lens during near vision, and the circular muscles of iris for constriction of pupil.
4th cranial nerve
It is called trochlear nerve which is also a motor nerve that innervates some external ocular muscles and controls eye movements.
5th cranial nerve
5th cranial nerve is called trigeminal nerve. It is a mixed nerve. Its sensory fibers carry the sensation of touch, pressure, temperature and pain from the mouth cavity, nasal cavity, cornea and iris of eye and skin of face to the brain. Its motor fibers control the movement of masticatory muscles and help in chewing.
6th cranial nerve
It is known as abducens nerve. It is a motor nerve supplying some external ocular muscles and controls eye movements.
7th cranial nerve
It is called facial nerve. It is a mixed nerve. Its sensory fibers carry the sensation of taste from the anterior two-third part of the tongue as well as general sensations from the skin of external ear. It contains somatic and autonomic motor fibers. Somatic motor fibers supply the facial muscles and control facial expressions. The parasympathetic fibres innervate salivary (submaxillary and sublingual) glands and lacrimal (tear) glands thus, they control secretion of saliva and tear.
8th cranial nerve
It is known as auditory or vestibulo-cochlear nerve. It is a purely sensory nerve. It has two branches, vestibular and cochlear. The vestibular branch carries the information related to rotation of head originating from the vestibular apparatus of the internal ear. The cochlear division carries auditory sensations from the cochlea of the internal ear.
9th cranial nerve
The 9th cranial nerve is called glossopharyngeal nerve. It is a mixed nerve. Its sensory division carries conscious sensation of taste from the posterior one-third part of the tongue and sensation of touch and pressure from the pharynx. It also carries unconscious sensations related to blood flow, from the carotid sinus and carotid body receptors. Its motor division consists of somatic as well as parasympathtic fibers. The somatic motor fibers supply the pharyngeal muscles and help in swallowing. The parasympathetic fibers control secretion of salivary gland.
10th cranial nerve
Its name is vagus nerve. It is a mixed nerve and is most widely distributed among the cranial nerves. The sensory fibers of vagus nerve are distributed on pharynx, larynx, oesophagus, and various visceral organs present in chest and abdomen and carry different sensations from these organs. The motor fibers of vagus are mainly of parasympathetic type and are supplied to the muscles and glands of various thoracic and abdominal viscera. So, this nerve takes part in swallowing, voice production, movements of alimentary canal, secretion of digestive juices inhibition of heart etc.
11th cranial nerve
It is called spinal accessory nerve. It is a motor nerve having dual origin in medulla and upper five or six segments of spinal cord. Its cranial division controls the movements of larynx, pharynx and soft palate while the spinal division controls the muscles of neck. Thus, it controls voice production and movement of head.
12th cranial nerve
12th cranial nerve is known as hypoglossal nerve. It is a motor nerve supplying the voluntary muscles of tongue and thereby controls tongue movements.
Trigeminal Nerve Function
Trigeminal nerve function is also split into these three divisions or branches as each has its own roles. The ophthalmic nerve branch, or CN V1, the maxillary branch (CN V2), and the mandibular nerve branch, hardly surprisingly called CN V3 all do different things.
The trigeminal nerve originates from four nuclei or groups of CNS nerve cells that begin at the midbrain and end at the medulla oblongata. Three of these nuclei are sensory (the mesencephalic, principal sensory, and spinal nuclei). The fourth is known as the motor nucleus and sends out nerves that help with jaw movement. Everything you feel on and in your face, the front of your scalp, and the mucous membranes of your mouth, nose, and sinuses is due to the different branches of the trigeminal nerve.
The image below shows the thick trigeminal nerve ganglion at the end of the main trunk of the trigeminal nerve, just in front of the outer ear. It is from the trigeminal ganglion that the three branches or divisions begin. The branches (yellow) and the areas which these branches serve are also depicted – the ophthalmic (green), maxillary (pink), and mandibular (purple) zones.
CN V1 Function
Ophthalmic branch function is sensory (afferent) – afferent in this case means that sensory stimuli are sent towards the trigeminal nerve. The ophthalmic branch nerve is the smallest of the three branches but plays a substantial role.
The ophthalmic branch begins at the trigeminal ganglion – just like all of the trigeminal branches – and eventually divides into three smaller nerves known as the lacrimal nerve, frontal nerve, and nasociliary nerve. Knowing the names of these smaller branches is not necessary, but as they have specific functions it is worth listing them accordingly.
The lacrimal nerve innervates the lacrimal gland, upper eyelid, and conjunctiva. The frontal nerve further splits into supraorbital and supratrochlear branches the first of these innervates the upper eyelid, conjunctiva, and scalp the latter the upper eyelid, conjunctiva, and forehead. The nasociliary nerve divides into four divisions – they provide sensory innervation of the mucous membranes in the sinuses and nose. Finally, the long nasociliary nerve sends sensory information to the brain from the iris, cornea, and ciliary bodies that control the shape of the eye lens (see image below). Although some sources say the ophthalmic nerve dilates the pupils and produces tears, it is other nerve fibers that travel alongside CN V1 that perform these functions.
CN V2 Function
The maxillary branch (CN V2) enables sensation in the mid-region of the face (nasal cavity, sinuses, and maxilla). It has four divisions and these divisions also split so the entire face, as well as the membranes of the brain, are well innervated and extremely sensitive. For example, the superior alveolar nerves ensure we feel the uncomfortable pain of toothache when the teeth in the upper jaw become infected.
CN V3 Function
The mandibular nerve or CN V3 is a mixed nerve composed of efferent motor fibers and afferent sensory fibers that innervate the lower face, upper neck, oral cavity mucosa, and the gums and teeth of the lower jaw. This branch also splits, for example, into the inferior alveolar nerves that innervate the teeth of the lower jaw and, unlike the superior alveolar nerve, it also has motor fibers.
Mastication or the chewing of food involves using the powerful muscles that envelop the mandible and maxilla. Damage to this area of the trigeminal nerve can give a strange sensation when eating and completely change how we chew our food.
An example of trigeminal nerve function in the mandible is the innervation of the lateral pterygoid muscle by one of the motor branches of the mandibular branch. You can test this nerve by moving your lower jaw forward to produce an underbite, or open your mouth only by dropping the bottom jaw. This motor nerve also allows you to move your jaw from side to side. Another trigeminal nerve function example is the sensory buccal nerve branch of the mandibular nerve that allows us to feel a sensation on the cheek.
Objectives-3, BIO 2310, Cranial Nerves
Exits skull through olfactory foramina in cribriform plate of ethmoid.
Function: Sensory only. Sense of smell.
Exits skull through optic foramen of sphenoid.
Function: Sensory only. Vision.
Exits skull through superior orbital fissure of sphenoid.
Function: Mixed nerve. Motor to intrinsic and extrinsic eye muscles. Sensory for muscle sense (proprioception) of same area.
Exits skull through superior orbital fissure of sphenoid.
Function: Mixed nerve. Motor to extrinsic eye muscles. Sensory for muscle sense of same area.
Ophthalmic branch exits skull through superior orbital fissure.
Maxillary branch exits skull through foramen rotundum of sphenoid.
Mandibular branch exits skull through foramen ovale of sphenoid.
Function: Mixed nerve. Motor to mastication muscles. Sensory to face, scalp, tear glands, mucous membranes of the nasal cavity and mouth.
Exits skull through superior orbital fissure.
Function: Mixed nerve. Motor to extrinsic eye muscles. Sensory for muscle sense of same area.
Exits skull through stylomastoid foramen of temporal bone.
Function: Mixed nerve. Motor to muscles of facial expression, salivation, and lacrimation. Sensory for taste and muscle sense.
VIII. ACOUSTIC = VESTIBULOCOCHLEAR NERVE
Exits skull through internal auditory meatus of temporal bone.
Function: Sensory only. Vestibular branch is sensory for equilibrium cochlear branch is sensory for hearing.
IX. GLOSSOPHARYNGEAL NERVE
Exits skull through jugular foramen of temporal bone.
Function: Mixed nerve. Motor to swallowing and salivation. Sensory for taste from posterior part of tongue and muscle sense.
Exits skull through jugular foramen.
Function: Mixed nerve. Motor to pharynx, larynx, viscera of thorax and abdomen (Parasympathetic Nervous System). Sensory for muscle sense to same areas and taste.
Comprised of 2 nerves cranial one arises from brain stem, spinal nerve arises from cervical spinal cord.
Exits skull through jugular foramen.
Function: Mixed nerve. Motor to muscles of pharynx and larynx, and to head movement muscles (Sternocleidomastoid, Trapezius). Sensory for muscle sense to the same area.
XII. HYPOGLOSSAL NERVE
Exits skull through hypoglossal canal of occipital bone.
Function: Mixed nerve. Motor to tongue. Sensory for muscle sense of tongue.
Nerves of the Head and Neck
The nerves of the head and neck include the most vital and important organs of the nervous system — the brain and spinal cord — as well as the organs of the special senses. In addition, in this region we also find the major cranial and spinal nerves that connect the central nervous system to the organs, skin, and muscles of the head and neck. These structures all work together to control every part of the body and receive sensory messages from the environment and the body’s internal structures.
The brain is a grayish, highly convoluted organ found within the skull’s cranial cavity. Continue Scrolling To Read More Below.
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- The base of the brain that connects to the spinal cord is the brain stem. Lower brain functions related to breathing, heart rate, and reflexes are controlled by the brain stem.
- The cerebellum is a round, wrinkled mass of neurons posterior to the brain stem that controls coordination and balance.
- Superior to the brain stem is the diencephalon, which controls the endocrine system, relays messages to and from the higher regions of the brain, and regulates feelings of hunger and thirst.
- The largest, most superior, and highest functioning region of the brain is the cerebrum. All of the voluntary functions of the body, along with memory, creativity, and emotions are products of the neurons in the gray matter of the cerebrum. A large crease splits the cerebrum into left and right hemispheres, which monitor and control opposite sides of the body and maintain slightly varied but parallel functions within the brain.
Extending from many different regions on the inferior side of the brain, twelve pairs of cranial nerves provide direct connections between the brain and important structures of the head, neck, and trunk. The sensory organs of the head use the cranial nerves for signal transmission, including smell (olfactory nerve), vision (optic, oculomotor, abducens, and trochlear nerves), taste (facial and glossopharyngeal nerves) and hearing (vestibulocochlear nerve). The muscles of the head and neck are also controlled by various cranial nerves including the facial nerve (facial expression) and accessory nerve (head and neck movements). Wandering through the neck and torso, the vagus nerve communicates vital information from the brain to the heart and intestines.
The spinal cord is a thick nerve trunk that forms the brain’s most important connection to the body and carries all signals to and from the brain that are not provided by the cranial nerves. The spinal nerve extends from the inferior end of the brain stem and passes through the foramen magnum of the skull into the neck. In the neck, the spinal cord passes through the vertebral foramen of the cervical vertebrae, which surround and protect its delicate nervous tissue. Eight spinal nerves branch off from the spinal cord in the neck to form a network of nerves called the cervical plexus. The cervical plexus forms many connections between the brain and the skin and muscles of the head and neck, similar to the cranial nerves. A vital connection from the cervical plexus to the diaphragm is formed by the phrenic nerve, allowing the brain to control breathing.
Damage to any part of the spinal cord may cause a loss of sensation and/or motor function below the injury however, such injuries are most dangerous within the neck as they are likely to affect a greater area of the body and are more likely to result in death. The fatal result of damage to the pathway between the brain and the diaphragm is respiratory arrest, a condition where the diaphragm stops moving, thereby failing to move fresh air into the lungs.
Vagus Nerve Stimulation
Vagus nerve stimulation is nothing new vagal maneuvers have been used for centuries to lower the heart rate and induce a sense of relaxation. The ancient Greeks called the carotid artery the ‘site of sleep’ as – unbeknown to them – massage to this area stimulates the vagus nerve that lies alongside the much more visible and palpable artery. Massaging the right carotid region lowers the heart rate and blood pressure. Hippocrates made it very clear that every physician should be extremely adept in the art of ‘rubbing’.
In today’s medical world, the vagus nerve is stimulated using two methods. The first is a group of actions known as vagal maneuvers. The second is electrical stimulation of the vagus nerve.
Patients with supraventricular tachycardia suffer from a heart rhythm disorder associated with a suddenly-occurring rapid heart rate. Symptoms are palpitations, shortness of breath, lightheadedness, and sweating. While medications help, patients are also taught various vagal maneuvers as a first-line therapy that can often negate the need for a hospital visit. These actions include coughing, holding the breath, and splashing the face with icy water. Such activities temporarily increase intrathoracic pressure and in doing so, increase the blood pressure. In reaction to this artificial high blood pressure, the vagus nerve kicks in by slowing down the heart – a parasympathetic signal that also brings the blood pressure down. Cold water to the face also provokes the Diving reflex that results in a slower heart rate, a temporary holding of the breath, and the constriction of peripheral blood vessels that increases oxygen supply to the most important organs.
Other methods are also effective. Touching the back of the throat encourages the gag reflex which also stimulates the vagus nerve. Carotid massage just under the angle of the jaw is only done in patients without atherosclerosis as plaque can break off and cause a stroke. Carotid massage places immediate pressure (stimulation) on the cranial nerve X. This treatment should only ever be done by a medical professional.
Electrical vagus nerve stimulation (VNS) uses electrical impulses applied to the nerve and these can be either be applied with an implanted device (see the image below) or be given as a portable, temporary therapy. While not all medical journal contributions agree that electrical vagal stimulation is worthwhile, many patients and physicians have seen improvements during and after VNS in the treatment of depression, multiple sclerosis, epilepsy, Alzheimer’s disease, metabolic syndrome, cardiovascular disorders, Parkinson’s disease, migraine, tinnitus (ringing in the ears) and cluster headaches. Newer, wireless devices are being developed to help people rehabilitate after a stroke.
Recently, a study about transcutaneous vagus nerve stimulation (tVNS) of the outer ear for fifteen minutes a dag increased cardiovascular health, mood, and sleep in the over 55s. But stimulation of the ear to achieve parasympathetic results is far from new. Professional ear-pickers in China have used pressure, tuning forks, and ear-tickling methods for centuries – a dying national pastime that has a very calming effect.
Cranial Nerves in Man: Origin, Nature Distribution and Function
There are twelve pairs of cranial nerves in man. They are numbered by Roman numeral I to XII. A cranial nerve arises from the brain by two roots, a dorsal and a ventral root.
These two roots do not unite but appear like separate nerves. The cranial nerves are generally medullated (having a myelin sheath).
The following table shows the origin, nature and distribution of cranial nerves:
The nerve fibres which carry impulses or sensations or stimuli are of 3 kinds. They are sensory, motor and mixed nerve fibres. The sensory or afferent nerve fibres carry impulses or sensations from the receptors like skin, eye, ear etc. to the central nervous system (brain and spinal cord).
The motor or efferent nerve fibres carry impulses or sensations from the central nervous system to the effectors like muscles and glands. Certain nerves contain sensory and motor fibres, and hence are mixed type. All the body functions can be conveniently divided into two categories: somatic functions and visceral functions.
Somatic functions are performed by the help of the body wall (skin and muscles) and visceral functions are carried on by internal organs like the digestive, circulatory, urinogenital, and respiratory or endocrine glands. Accordingly the nervous system has four functional components and four kinds of nerves.
(a) Somatic sensory nerves carry impulses from somatic receptors such as skin, eyes, nose, body walls and joins to central nervous system.
(b) Somatic motor nerves carry impulses from central nervous system to voluntary muscles.
(c) Visceral sensory nerves carry sensations from the viscera to the central nervous system.
(d) Visceral motor nerves carry impulses from the central nervous system to the involuntary muscles of alimentary canal, glands and other visceral organs.
There are 31 pairs of spinal nerves in man — arising in pairs from the spinal cord. Out of these 8 pairs are Cervical, 12 pairs Thoracic, 5 pairs Lumbar, 5 pairs Sacral and 1 pair Coccygeal nerve (Fig. 1.14 and 1.16).
The spinal nerves are of mixed type and arise in pairs from the spinal cord by two roots, a dorsal root and a ventral root (Fig. 1.14 and 1.16). The dorsal or sensory root consists of afferent fibres which may be somatic sensory or visceral, sensory. It also bears a ganglion. The ventral root consists of efferent fibres which may be somatic motor or visceral motor fibres. These two roots unite to form a spinal nerve which comes out through a small aperture between vertebrae called inter-vertebral foramen guarded by a silvery- white calcareous gland known as glands of Swammerdam.
Soon after its emergence, the spinal nerve trunk divides immediately into three branches as follows:
(a) Ramus dorsalis contains somatic sensory fibres and supplies the skin and muscles of dorsal body wall.
(b) Ramus ventralis is the thick, main nerve and contains somatic motor fibres.
(c) Ramus communicans contains visceral sensory and visceral motor fibres and later joins autonomic nervous system and spinal cord.