The Autonomic Nervous System And The Cranial Nerves

Description: Preferred language style: English (U.S.).  I need 2 chapters.  One on the autonomic nervous system and the other on the cranial nerves.  MLA format only.  You must use at least one book for each and 1 journal for each; the others can be Internet sources.

The Cranial nerves are a dozen pairs of nerves that arise from the brain.  They pass through the opening (foramina) present in the base of the cranium.  The 12 cranial nerves include the olfactory nerve (1^st cranial nerve), optic nerve (2^nd cranial nerve), oculomotor nerve (3^rd cranial nerve), trochlear nerve (4^th cranial nerve), trigeminal nerve (5^th cranial nerve), abducent nerve (6^th cranial nerve), facial nerve (7^th cranial nerve), acoustic nerve (8^th cranial nerve), glossopharyngeal nerve (9^th cranial nerve), vagus nerve (10^th cranial nerve), accessory nerve (11^th cranial nerve) and hypoglossal nerve (12^th cranial nerve) (University of Washington, 2005, Fitzgerald, 1985 & Grays, 1918).

Several fibers of the cranial nerve arise from the gray mater of the brain (a special nucleus).   Usually, the motor cranial nerves that bring about certain movement functions in the body contain their nucleus within the brain.  The sensory cranial nerves which carry sensory signals to the brain have their nucleus located outside the CNS.  In the Nose and the eyes, the sensory ganglia are present (University of Washington, 2005 & Grays, 1918).

The first cranial nerve is the olfactory nerve.  It supplies the mucosa of the nose, the superior nasal concha and the nasal septum.  The Rhinencephalon is usually related to the olfactory center present in the cerebral cortex.  The ectoderm gives rise to the olfactory nerves.  The olfactory bulb plays an important role in the transmission of the nerve signals of the olfactory nerve.  The olfactory nerve plays a very important role in the smell sensation (University of Washington, 2005 & Grays, 1918).

The optic nerve plays a very important role in sight and vision.  The ganglion of the optic nerve is located near the retina.  The axons of these nerves end in the lower visual centers such as the superior colliculus, geniculate body, pulvinar, etc.  Certain fibers present in the optic nerve also help in maintaining pupillary reflexes.  The optic nerve and the tract that it follows are much peculiar compared to the other cranial nerves.  Frequently, the optic nerve is considered to be an extension of the brain rather than a separate nerve.  It is covered by a sheath made of the pia mater, arachnoid mater and the dura mater (University of Washington, 2005, Fitzgerald, 1985 & Grays, 1918).

The oculomotor nerve is a motor cranial nerve that supplies the muscles of the orbit (eyeball socket).  It does not supply the Obliquus superior and the Rectus lateralis.  It indirectly supplies the Sphincter pupillae and the ciliary muscles.  The nucleus of the oculomotor nerve is present in the gray substance present in the floor of the cerebral aqueduct.  It is made of smaller nuclei that are located in two groups.  These nuclei can also be divided into smaller groups, each of which controls a specific muscle present in the orbit (University of Washington, 2005 & Grays, 1918).

The Trochlear nerve is the fourth cranial nerve that supplies motor signals to the Obliquus superior oculi.  The nucleus is present in the cerebral aqueduct floor.  The nerve maintains a tortuous course in the cranium.  It moves in the lateral wall of the cavernous sinus, passing through the gap between the ophthalmic division of the trigeminal and the oculomotor nerve.  It enters the orbit via the superior orbital fissure.  The lacrimal division of the trochlear arises in the cavernous sinus (University of Washington, 2005 & Grays, 1918).

The fifth cranial nerve is the trigeminal nerve.  It is the largest of all the cranial nerves and forms an important sensory nerve in the head and face.  It also forms a motor nerve, helping in mastication and chewing.  The nerve has a large sensory root and a small motor root from the side of the pons.  The superior and inferior nucleus gives rise to the motor root.  The semilunar ganglion gives rise to the sensory root, from the cavity of the dura mater present in the temporal bone.  The trigeminal nerve has three divisions, namely, the ophthalmic division, the maxillary division and the mandibular division.  These three divisions have four ganglions, namely the ciliary ganglion (which is associated with the ophthalmic nerve), the sphenopalatine ganglion (associated with the maxillary nerve), the submaxillary nerve (associated with the mandibular nerve) and the otic ganglion (associated with the mandibular nerve).  The ophthalmic nerve and the maxillary nerves are predominantly sensory nerves, whereas the mandibular nerve is predominantly motor nerves (University of Washington, 2005, Fitzgerald, 1985 & Grays, 1918).

The sixth cranial nerve is the abducent nerve that supplies the rectus lateralis oculi.  Its nucleus arises from the upper portion of the rhomboid fossa.  Fibers also supply the rectus medialis.  The nerve passes through the cavernous sinus close to the oculomotor, trigeminal and ophthalmic nerves.  The nerve is also closely associated with the trochlear nerve and the ophthalmic nerve in the superior orbital fissure (University of Washington, 2005 & Grays, 1918).

The seventh cranial nerve is the facial nerve, which is a mixed nerve.  It arises from the side of the pons.  The sensory division of the facial nerve supplies the front two-thirds of the tongue and also portions of the middle ear.  The motor division of the facial nerve supplies the muscles of the scalp, face, ear lobe, buccinator, platysma, stapedius, stylohyoid, posterior belly of the digastric.  It also supplies the submandibular and the sublingual salivary glands.  The nucleus for the motor root is present in the reticular formation of the pons and the nucleus for the sensory root is present in the geniculate ganglion (University of Washington, 2005, Fitzgerald, 1985 & Grays, 1918).

The eighth cranial nerve is the statoacoustic nerve, which help in hearing and balance.  It is made up of two separate nerves, namely the Cochlear nerve and the vestibular nerve.  The Cochlear nerve arises from the spiral ganglion of the cochlea.  This has cental fibers and peripheral fibers.  The central fibers pass into the outer portion of the internal auditory meatus, whereas the peripheral portion passes to the Organ of Corti.  The Cochlear nerve has two nuclei, the accessory nucleus and the tuberculum acusticum.  The vestibular nerve arises in the vestibular ganglion from the bipolar cells.  The peripheral fibers of the vestibular nerve divide into three branches (University of Washington, 2005 & Grays, 1918).

The ninth cranial nerve is the glossopharyngeal nerve that contains both sensory and motor fibers.  It supplies the tongue and the pharynx (including the fauces, tonsils, posterior portion of the tongue, etc).  The petrous and the superior ganglia give rise to the sensory fibers.  These mainly help in taste sensation.  The motor fibers arise from the nucleus ambiguus.  This nucleus also supplies the vagus and the accessory nerves.  The vagus, facial and the sympathetic nerves also communicate with the glossopharyngeal (University of Washington, 2005 & Grays, 1918).

The tenth nerve is a vagus nerve, containing both sensory and motor fibers, and moving through various parts of the head and neck, and into the thorax.  It arises from the medulla oblongata.  The jugular ganglion and the nodosum ganglion give rise to the sensory fibers, whereas the motor fibers arise from the nucleus ambiguus.  In some patients suffering from medullary refractory seizures, vagus nerve stimulation has proven to be effective.  This involves surgical placement of a device that stimulates the vagus nerve.  Studies are being conducted to determine the potential benefits of the procedure on depressive disorders (Grays, 1918 & Ansari et al, 2007).

The accessory nerve or the eleventh cranial nerve has two portions, namely the cranial portion and the spinal portion.  The cranial portion arises from the nucleus ambiguus and is the smaller of the two portions.  The spinal portion arises from the gray substance of the medulla oblongata.  The accessory gives out a branch which supplies the Musculus uvulae and the levator veli palantine.  The spinal portion supplies several muscles of the neck such as the trapezius (University of Washington, 2005 & Grays, 1918).

The hyoglossal nerve is the 12^th cranial nerve and mainly provides motor function to the tongue.  The hypoglossal nucleus present in the medulla spinalis gives rise to the nerve.  The hypoglossal nerve has 4 branches of distribution, namely the meningeal, thyrohyoid, descending and the muscular (University of Washington, 2005 & Grays, 1918).

Individuals suffering from many systemic conditions may be involved with cranial nerve palsy.  Children suffering from leukemia and lymphomas are more prone to suffer from cranial nerve palsy.  The most frequently involved nerve would be the facial nerve, which supplies the head and neck region extensively.  This mainly occurs due to involvement of the brain with the malignancy.  Treatment with chemotherapy and/or radiotherapy may help to reduce the severity of the nerve palsy (Paryani et al, 1983).

The autonomic nervous system (ANS) is a portion of the nervous system the feeds the muscles of the internal organs (involuntary or smooth muscles), cardiac muscles (helping it to release the blood) and the exocrine glands (glands that release their secretion directly into the).  There are two components of the ANS, namely, the sympathetic nervous system and the parasympathetic nervous system.  The arrangement of the sympathetic nervous system includes the pre-ganglionic neuron (present before the ganglion that arises from the spinal cord), the sympathetic ganglion, and the sympathetic ganglion (that arises from the sympathetic ganglion).  In the sympathetic ganglion, the preganglionic nerve fibers meet the post-ganglionic nerve fibers.  The effector organ, such as a gland or a muscle is supplied by the post-ganglionic nerve fibers.  It receives a signal from the central nervous system and sends it to the effector organ.  The arrangement of the parasympathetic nervous system is much similar to that of the sympathetic nervous system.  It also contains a pre-ganglionic nerve fibers, ganglion and post-ganglionic nerve fibers (Grays, 1918, Fitzgerald, 1985, Low, 2006 & Ham, 1979).

The Ganglion present in the parasympathetic nervous system are usually known as ‘terminal ganglion’ as they are located either close to the structures they would be supplying (which is usually an involuntary muscle, exocrine gland or the cardiac muscle), or sometimes even within the structure they are supplying.  This is contrasting in relation to the sympathetic nervous system, in which the ganglion is located closer to the spinal cord, from which the preganglionic nerve fibers arise.  Compared to the sympathetic nervous system, the parasympathetic nervous system is much simpler and primitive (Grays, 1918, & Fitzgerald, 1985)

In each portion of the spinal cord, such as the cervical, thoracic, lumbar, etc, a collection of nerve cell bodies, known as the ‘nuclei’ are located.  These nuclei supply corresponding portions of the body.  One of the main portions of the sympathetic nervous system is the thoraco-lumbar portion.  The lateral horns of the spinal cord contain the preganglionic fibers of the sympathetic nerves.  These lateral horns appear spindle-shaped, and peculiar to the other neuronal bodies.  There are also smaller in size and are enclosed in a coating of proteins and fats, called as the ‘myelin sheath’.  This myelin sheath provides some amount of insulation for the nerve fibers.  At certain points on the nerve fibre, there is lack of myelin (node of Ranvier).  The nerve signal in fact jumps from one node of Ranvier to another.  The ventral nerve root of the spinal cord is that point at which the nerve fibre exits the spinal cord.  Following the exit of the sympathetic nerve from the spinal cord, a special nerve sheath encloses the nerve and the spinal nerve till the sympathetic ganglion.  It is known as ‘white rami commicantes’.  During the course of the nerve, the pre-ganglionic nerve fibers enters into the sympathetic ganglion, exits as the post-ganglionic nerve fibre and again enters into the spinal nerve.  It is protected by a small tunnel or pathway, known as ‘gray rami commicantes’ (Grays, 1918, & Fitzgerald, 1985)

The nuclei of the parasympathetic nervous system are located in the midbrain, medulla oblongata, brain stem, spinal cord (especially the pelvic portion).  For this reason, the parasympathetic nervous system is known as ‘cranio-sacral division’.  In the brain, the preganglionic nerve fibers usually arise from gray mater of the midbrain and the medulla oblongata.  These exit the brain via the cranial nerves (especially the 3^rd, 7^th, 9^th and 10^th cranial nerves).  The other preganglionic nerve fibers arise from the lateral horns of the spinal cord especially in the sacral region.  The 2^nd, 3^rd and the 4^th sacral nerves contain the preganglionic parasympathetic nerves (Grays, 1918, & Fitzgerald, 1985).

Schwann cells are cells that lay down myelin required to protect the nerve.  These Schwann cells surround the nerve cells, and are present in a thick layer of myelin.  In the thin nerves, the Schwann cells are presents surrounding the nerve cells, but do not contain any myelin sheath.  In most of the cell body and the axon of the pre-ganglionic nerve fibers, myelin sheath is found covering.  The post-ganglionic nerve fiber is usually not found having a myelin sheath covering.  The post-ganglionic nerve fibers are longer compared to the pre-ganglionic nerve fibers, because the sympathetic ganglion is located towards the spinal cord (Grays, 1918, Young, 2000 & NDRF, 2006).

From the spinal cord to the sympathetic ganglion, the preganglionic nerve fibre transmits the nerve signal.  In the ganglion of the sympathetic nervous system, the signal is relayed.  There are several cells present in the sympathetic nervous system that performs important functions.  The Schwann cells helps in laying down the myelin sheath, and helping the insulation of the nerve cell and the transmission of the nerve signal.  The speed at which the nerve signal transmits the signal varies depending on the thickness of the nerve fibre.  When a nerve signal is myelinated, the nerve signal is transmitted at a much faster rate, as the nerve signal would jump from one node of Ranvier to another.  On the other hand, in an unmyelinated nerve fiber, the nerve signals moves at a much lower rate compared to the myelinated nerve fiber (Grays, 1918, Fitzgerald, 1985, & Young, 2000).

The white rami communicantes and the gray rami communicantes protect the sympathetic nerve and the ganglion.  From the exit of the preganglionic nerve fiber from the exit of the spinal nerve to the sympathetic ganglion, the white rami communicantes protects the nerve fiber, the post-ganglionic nerve fiber is protected till the point it meets the spinal nerve by the gray rami communicantes (Grays, 1918).

In the parasympathetic nervous system, once the nerve leaves the spinal nerve, it enters short communicating link known as ‘visceral rami’.  In the ganglion of the parasympathetic nervous system, the cells are much smaller compared to the sympathetic. The post-ganglionic nerve fiber contains lesser myelin and is thinner in size compared to the preganglionic (Grays, 1918, & Young, 2000).

The nerve signal is transmitted across the physiological gap existent between one nerve and the other.  This physiological gap is known as the ‘synapses’.  At this location, certain chemicals that help to transmit the nerve signal play a very important role.  These chemical are known as ‘neurotransmitters’.  Between the pre-ganglionic nerve fiber and the post-ganglionic nerve fiber, the neurotransmitter that plays an important role is ‘acetylcholine’, whereas in the synapses located between the effector organ and the post-ganglionic nerve fiber, nor-epinephrine plays an important role (Grays, 1918, Fitzgerald, 1985, & NDRF, 2006).

In the parasympathetic nervous system, the main neurotransmitter is ‘acetylcholine’.  Compared to the sympathetic nervous system, the action of the parasympathetic nervous system is more localized and distinct.  The synapses present in the parasympathetic are frequently larger, leaving a greater amount of space for the neurotransmitter to travel (Grays, 1918, & NDRF, 2006)

In several portions of the body, such as the otic ganglion (located near the parotid gland), the parasympathetic nervous system is well-developed.  In other parts, the parasympathetic nervous system appears scattered.  The ganglia of the parasympathetic nervous are smaller in size and may not be distinct. The capsule that covers the ganglia may not appear well-defined (Grays, 1918, & NDRF, 2006).

The ganglion of the sympathetic nervous system is made of various types of cells including the nerve cells and the supporting cells.  The collagen fibers are present in the nerve cells, axons, etc.  The nerve cells of the sympathetic nerve appear star-shaped, and contain a large and rounded nucleus that is present in the cytoplasm.  This nucleus is usually located away from the center of the nerve cell.  A granular substance known as ‘Nissl’s granules’ is also present in the cytoplasm.  From the nerve cells, several branches project out, known as ‘dendrites’.  Certain cells known as Satellite cells are present in the ganglion, which are similar to the Schwann cells.  In the ganglion, the dendrites of the post-ganglionic nerve fiber enter into a synapse with the axon of the pre-ganglionic nerve fiber (Grays, 1918, & Fitzgerald, 1985).

Usually, the sympathetic ganglion is located near the spinal cord known as the paravertebral ganglion.  In certain occasions, it is located along the aortic nerve and is called as the ‘prevertebral ganglion’.  All the sympathetic that are present along the spine are related to one another through a link known as the ‘sympathetic chain’.  The prevertebral ganglion is also known as ‘collateral ganglion’ and is adjacent to the muscle or gland that is supplied.  They are often closely related to the blood vessels that supply the structures (Grays, 1918, & Fitzgerald, 1985)

Several important functions are enabled by the sympathetic nervous system.  Most of the involuntary muscles and the glands are supplied by the autonomic nervous system; contain a sympathetic portion and a parasympathetic portion.  These two components enable opposing actions, thus helping to comprehensively control the functioning.  the actions brought about by the sympathetic nervous system includes increase in the respiration rate, breathing, heart beat, blood pressure, dilation of the pupil and reduction of the activity of the internal organs.  It is frequently increased during flight and fight activities.  The conscious of the individual does not control the activity of the sympathetic nervous system (Grays, 1918, NDRF, 2006, & Low, 2006).

The parasympathetic activity helps the body to relax and rest.  It is usually active during the non-emergency situations, when digestion functions need to be performed.  It brings about a reduction in the heart rate, respiration, contraction of the pupil, increase in the kidney activity, decrease in the blood pressure, etc.  The conscious does not control the parasympathetic activity (Grays, 1918, NDRF, 2006, & Low, 2006).

Individuals affected with tetanus may have autonomic nervous system involvement.  Some of the symptoms of ANS involvement include hypotension, hypertension, tachycardia, tachyarrhythmias, bradycardia, abnormal ECG’s, etc.  The outcome of ANS involvement in severe tetanus is poor.  In about 30 % of the patients suffering from tetanus has ANS involvement (Wasay, 2004).

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