Clinical aspect of neuroanatomy
There are two types
In brain affection, truama may be a concussion, contusion or cerebral laceration with increasing degrees of severity of unconcsiousness and brain damage
The following can occur in the brain
The pyramidal system is that system represented by the corticospinal tracts. The term pyramidal is taken form the pyramids of the medulla. This system takes charge of voluntary activities in the body, that is those actions that are meditated and performed by will as opposed to those actions, which are autonomous and controlled by the autonomic nervous system.
The pyramidal system or tract therefore runs from the cerebral cortex from motor area in the frontal lobe to the lower levels of the neuraxis mainly at the spinal cord but also at the cranial nerves that control willful activities, eg eye movement by oculomotor and tongue movement by the hypoglossal nerve. The corticospinal tract descends into the spinal cord and synapses with motor nerves from the nucleus motorius lateralis and medialis. The second order neurons now issue from these synapses to reach the end plate or effector organs. They are called the following names
Upper motor neuron- this is the first order neuron. They arise from the cortex and synapse at lower levels in the brain or spinal cord.
When damaged they have features which are different form those of lower motor neurons. They are as follows
Lower motor neuron lesion
1. Olivo spinal tract
2. Rubrospinal tract
3. Vestibulospinal tract
4. Tectospinal tract
These are the major extrapyramidal descending tracts although other tracts which may not be as important as the above are available.
Another important tract is the reticulospinal tract. This tact takes off from the reticular formation, which may be from the medulla pons or midbrain. The fibers descend into the spinal cord.
The fibers act on the lower motor neuron so that when the upper motor neuron is damaged they provide a form of spastic response to the lower motor neuron. Ordinarily they keep the action of the lower motor neuron in check so that they do not over act or provide also balance for their action.
Neurology is the study of the nervous system. It is useful in making diagnosis of diseases of nerves and that of the central nervous system. The diseases usually provide symptoms and signs which help to determine the cause of the problem. The problem may be degenerative, ie the neurons of the pathway degenerate, and in most cases due to genetic factors- Alzheimer's disease, etc. It could be infectious- e.g. Kuru, it could be traumatic, due to accident which may cause injury to nerves and therefore lead to their dysfunction. This is the commonest cause of neurological defects- e.g. RTA (road traffic accident) leading to whiplash injury affecting the spinal cord. The commonest is fracture of the vertebral spine which then impinges on the spinal cord and damages it leading to permanent disability such as paraplegia (paralysis of lower extremity) or quadriplegia which is paralysis of all four limbs. Other injuries include nerve cut, cerebral lacerations, gut shot injuries which may produce Brown-Sequard syndrome (spinal cord hemisection) etc.
Another cause of neurological defect which arise from the central nervous system includes
These accidents are caused by vascular defect leading to ischaemia or infarction of various areas of the brain. They are categorised as follows
1. Thrombotic causes. The lodging of intravascular clot which blocks the blood supply to an area of the body; in this case, the nervous system. It is usually referred to as thrombo-embolic phenomenon, because it arises from embolism in many cases.
2. Intracerebral haemorrhage. In this condition, blood vessels break open and bleed. The blood which is to supply the area may then be cut off and this may compound a pressure problem on surrounding structures caused by the hemorrhage. The areas above may then have both pressure symptoms which is a major problem for nerves and also structures of the CNS. Nerves can be damaged as a result of pressure or direct injury which may then cause any of the following
When symptoms and signs which are usually called clinical features help to determine a particular central nervous system lesion, that is called lateralizing signs
Such signs include
Blood supply to medulla is via the vertebral artery and its branches
Vertebral artery has the following branches
These are the branches of the vertebral artery but the ones that concern us are the posterior inferior cerebellar artery and basilar artery which it forms at its termination. These arteries give supply to the medulla and also the pons and cerebellum
1. Medial medullary syndrome caused by blockage of vertebral artery or lower basilar artery
Signs and symptoms
On the side of lesion
Paralysis and atrophy of part of the tongue caused by damage to ipsilateral 12th cranial nerve
On the opposite side of lesion
Paralysis of arm and leg but sparing the face- anatomical cause- affects the pyramidal tract before the pyramidal decussation
Impaired tactile and proprioceptive sense over half of body- caused by lesion to the medial lemnicsus
2. Lateral medullary syndrome. This may be caused by blockage of one of the following arteries
1. Vertebral artery
2. Posterior inferior cerebellar artery
3. Superior lateral medullary artery
4. Middle lateral medullary artery
5. Inferior lateral medullary artery
Symptoms and signs
On the side of the lesion
a. Pain, numbness, impaired sensation over half of the face- caused by spinal nucleus and tract of V
b. Ataxia of limbs and falling on the side of lesion (Rombergism)- due to affection of inferior olive and inferior cerebellar peduncle
c. Vertigo nausea and vomitting- vestibular nucleus
d. Nystagmus and diplopia- vestibular nucleus
e. Horner's syndrome- descending sympathetic tract
f. Loss of taste- caused by nucleus of tractus solitarius
g. Numbness of ipsilateral arm, trunk, or leg caused by cuneate and gracile nuclei affection
h. Hiccup- unknown
On the opposite side
Pain and temperature sensation impaired on the half of body and possibly face- caused by affection of lateral spinothalamic tract
Paralysis of all extremities- caused by affection of corticobulbar and corticospinal tracts.
Other syndromes of the medulla
Total unilateral medullary syndrome (blockage of vertebral artery)
Lateral pontomedullary syndrome - vertebral artery
Basilar artery syndrome
Medial inferior pontine syndrome- blockage of the paramedian branch of basilar artery.
On the side of lesion
1 Paralysis of conjugate gaze to side of lesion -caused by parabducent nucleus affection
2. Nystagmus- vestibuar nucleus affection
3. Ataxia of limbs and gait- possibly middle cerebellar peduncle
4. Diplopia on lateral gaze - caused by abducens nucleus affection
On the opposite side
5. Impaired sensation and proprioception on one side of body-
6. Paralysis of face, arm and leg- caused by corticospinal and corticobulbar tracts involvement
Lateral inferior pontine syndrome. This is caused by blockage of anterior inferior cerebellar artery
On the side of lesion
Nystagmus- vestibular nerve
Facial paralysis- cranial nerve 7
Paralysis of conjugate gaze- parabucent nucleus
Deafness and tinnitus- Cochlear nucleus
Ataxia - middle cerebellar peduncle
Impaired sensation of face- spinal tract or even pontine tract of V
On the opposite side of lesion
Pain and temperature sensation impairement- lateral spinothalmic tract
Total unilateral inferior pontine syndrome.
Affection of the anterior inferior cerebellar artery.
Medial midpontine syndrome is caused by blockage of midbasilar artery
On the side of lesion we have
1. Ataxia of limbs and gait- middle cerebellar peduncle
2. On the opposite side
Paralysis of face, arm and leg- by corticospinal tract
Variable touch and proprioceptive defects caused by medial lemniscus.
Lateral midpontine syndrome caused by block in short circumferential arteries
On the side of lesion
Ataxia of limbs- middle cerebellar peduncle
Paralysis of muscles of mastication
Motor fibers of V
Impaired sensation on side of face- pontine fibers of V
Medial superior pontine syndrome, caused by blockage of paramedian branches of upper basilar artery
On the side of lesion
Cerebellar ataxia- superior and middle cerebellar peduncle
Internuclear ophthalmoplegia- medial longitudinal fasciculus
On the opposite side
Paralysis of face, arm and leg- corticospinal tract
Touch vibration and position sense affection- medial lemniscus
Lateral superior pontine syndrome- caused by blockage of superior cerebellar artery
Lateral superior pontine syndrome
On the side of lesion
Ataxia of limbs and gait and falling to the side of lesion- middle and superior cerebellar peduncles
Tremors, nausea, dizziness, Horner's syndrome- vestibular nuclei
On the opposite side
Impaired pain and temperature sensation- lateral spinothalamic tract
Impaired sensation o touch, vibration, position sense etc- medial lemniscus
Posterior cerebral artery
This artery supplies the visual cortex, the thalamus and internal capsule
Coronal section of brain at level of thalamus and basal ganglia-blood supply; red- middle cerebral artery, blue- posterior cerebral artery, green- anterior cerebral artery.
Anterior cerebral artery
Blockage of anterior cerebral artery or its branches
1. Paralysis of opposite foot and leg- motor leg area on homonculus
2. Opposite paresis (minimal)- corona radiata involvement
3. Sensory loss (cortical) on toes, foot and leg- sensory area for foot and leg on sensory cortex
4. Unwitting urinary incontinence- sensorimotor area in the paracentral lobule
5. Contralateral grasp reflex: medial surface of posterior frontal lobe
6. Abulia - lesion is uncertain
7. Impairement of gait and stance - Frontal cortex
8. Mental impariment and affection of memory- unknown
9. Dyspraxia of left limbs, Tactile aphasia pf left limbs - all via the corpus callosum
10. Cerebral paraplegia bilateral lesion of anterior cerebral artery
Middle cerebral artery syndrome
The nucleus of this nerve lies in the hypoglossal trigone at the extreme inferior edge of the floor of the 4th ventricle. It lies above the obex and lateral to the area postrema. But below the vagal trigone. It contains nucleus of this nerve, which carries GSE fibers to the intrinsic and extrinsic muscles of the tongue excepting the palatoglossus. It is a new addition to the cranial nerves of vertebrates for fishes and lower vertebrates lack the hypoglossal nerve.
It may also carry GSA fibers to the meninges of the posterior cranial fossa, but this is doubtful. When damaged, it will cause an ipsilateral hemiparesis of tongue. In the condition known as inferior alternating hemiplegia there is a contralateral hemiplegia and an ipsilateral hemiparesis of tongue presumably due to the closeness of the pyramids to the hypoglossal nucleus so that damage to one may also affect the other. Because the fibers of the hypoglossal nerve are not crossed, we then obtain an ipsilateral hemiparesis of tongue.
Rootlets of hypoglossal nerve emerge from the medulla between the olive and the pyramid. It then forms two roots which pass through the hypoglossal canal in order to form a single nerve within the canal. It emerges from the canal after piercing the hypoglossal canal. This canal is deep to the jugular foramen and hence the hypoglossal nerve enters the neck deep to the nerve (IX,X,XI) which emerge from the jugular foramen. As it descends in the neck it moves with a lateral convexity which makes it increasingly more superficial to reach the lateral side of the three arteries which it course below the posterior belly of digastric. The arteries are occipital, external carotid and lingual arteries in that order from posterior to anterior. The hypoglossal nerve then passes on the hyoglossus deep to the mylohyoid and runs on the superior aspect of the greater horn of hyoid where it splits into terminal branches that enter the intrinsic and extrinsic muscles of tongue (hyoglossus, genioglossus, styloglossus, except the palatoglossus).
The accessory nerve is the 11th cranial nerve. It has two components
The spinal accessory begins at the level of C2 and its fibers are from the ventral horn of these cervical levels. The fibers enter the cranial cavity via the foramen magnum and joins its counterpart, the cranial accessory as it emerges from the nucleus ambiguus, which is associated with the vagal trigone. The two meet at the cranial cavity and then separate at the jugular foramen. The cranial accessory joins the vagus and is distributed to the recurrent laryngeal nerve to supply the intrinsic muscles of the larynx. The spinal accessory moves inferiorly to enter the posterior triangle of the neck and disappears behind the trapezius and supplies that muscle and the sternocleidomastoid.
This nerve is the 10th cranial nerve. It is very extensive and it is provided for by a special trigone in the floor of the 4th ventricle. This trigone called the vagal trigone is placed lateral and superior to the hypoglossal trigone. It contains mainly the dorsal nucleus of the vagus, which is the main nucleus associated with the vagus nerve only. Other nuclei groups associated with the vagus are as follows
Dorsal nucleus of vagus
This nucleus is only associated with the vagus nerve. It is the specialized nucleus for myomotor function and therefore supplies all areas of the vagus distribution with myomotor fibers for smooth muscles in foodways and airways. The vagus extends its supply to the gastrointestinal tract after the laryngeal and pharyngeal supply to reach the splenic flexure. Parasympathetic supply below or inferior to the splenic flexure is via the sacral outflow and not the vagus. Hence the vagus is the most extensive in supply and distribution of all cranial nerves.
It also supplies the heart muscles with cardioinhibitory vagal fibers and hence when damaged may cause problem with the heart. It supplies all the smooth muscles of the larynx, and trachea together with the gastrointestinal tract up to the splenic flexure.
It is placed most medial in the area associated with the vagus at the floor of the fourth ventricle
Nucleus of the tractus solitarius.
This nucleus is placed lateral to the dorsal nucleus of vagus. It contains cell bodies of second order neurons of the taste pathway. They will carry SVE fibers to reach the gustatory nucleus of the cortex (temporal lobe) area 43. They have the cell bodies of their first order neurons placed in the ganglion nodosum, which is the inferior vagal ganglion. This ganglion is associated with GSA and SVA fibers while the superior vagal ganglion is associated with the GSA fibers
This is a special medullary nucleus, which is associated only with structures derived from the pharyngeal arches. Hence it carries SVE fibers. It is placed between the nucleus of tractus solitarius and the dorsal nucleus of vagus around the vagal trigone. It supplies all the muscles of the larynx through superior laryngeal nerve and also the recurrent laryngeal nerve. The superior laryngeal nerve supplies the cricothyroid and the cricopharyngeus while the recurrent laryngeal nerve, which was initially carried in the cranial accessory but is later distributed by the vagus, supplies all the intrinsic muscles of the larynx excepting the above two.
Nucleus of spinal tract of V.
This nucleus is the most lateral of all nuclei associated with the vagus. It carries general sensation of the somatic system from the area of the auricle and the external auditory meatus. These are very small fibers whose cell bodies are placed in the superior vagal ganglion. Their central processes then run to the medulla and synapse with their second order neurons at the nucleus of the spinal tract of V. The tract of the second order neurons form the trigeminothalamic tract. They will project into the thalamus and reach the ventral posteromedian nucleus of the thalamus (VPM) where the 3rd order neurons issue to reach the sensory cortex.
Inferior salivatory nucleus
This nucleus is the caudal extension of the pontine superior salivatory nucleus. As its name implies, it is secretomotor and therefore it carries GVE (general somatic efferent) fibers to the glands in the foodways and airways supplied by the vagus and also to the glands of the soft palate and epiglottis. It is also the areas (soft palate and epiglottis) that SVA fibers associated with the vagus supplies for they have scattered taste buds on their surfaces.
Functional components of the vagus
The vagus nerve emerges from the brain at the middle compartment of the jugular foramen where it forms the superior vagal ganglion. It forms the inferior vagal ganglion just below the base of he skull. It runs in the neck within the carotid sheath and placed between the carotid artery and jugular vein. It then pass anterior to the subclavian artery at the root of the neck in order to enter the mediastinum of the thorax
In the thorax it runs towards the midline. The left vagus lies on the great vessels while the right is separated from the midline by the trachea. The left and right vagi then passes the posterior aspect of the lung root giving branches to the pulmonary plexus. They reach the midline esophageal plexus where they become mixed and emerge as anterior and posterior vagi which then enter the esophageal opening in the diaphragm together with the esophagus.
Branches in the neck
Branches in the thorax
These enter as anterior and posterior vagi. The posterior vagus enters the celiac plexus from where it is distributed to all parts of the gut up to the junction between the distal 1/3 and proximal 2/3 of the transverse colon. The anterior vagus is distributed round the stomach and the porta hepatis.
Isolated injuries to the vagus nerve is uncommon although it is a common site of surgery to correct peptic ulcer disease in surgical vagotomy. Surgical vagotomy has however been overtaken in most cases with the use of drugs to reduce acid production in the stomach, called medical vagotomy.
Thyroidectomy operations may lead to damage to the recurrent laryngeal nerves causing voice loss
Total vagotomy involves the complete transection of the vagus nerve supply to the stomach. This is done in order to allow reduction in secretion of acid in stomach but it also leads to relative atony so that drainage of the stomach through the pylorus is reduced. Hence vagotomy is performed together with pyloroplasty (enlarged pyloric exit) or with a gastrojejunostomy.
In highly selective vagotomy, the vagal trunk is divided but the nerve of Latarjet is preserved.
1 Anterior vagus 2 celiac branch 3 anterior nerve of Latarjet
1 posterior vagus 2 hepatic branch 3 posterior nerve of Latarjet
This nerve does not have a special trigone but its nuclei are associated with the vagal trigone as follows
Functional components of IX
The IX escapes from the medulla at the upper aspect of the postolivary sulcus above the rootlets of escape of X which in turn is above the rootlets of escape of the XI.
It then enters the jugular foramen also accompanied by the X and XI. In the jugular foramen it lies on the lateral aspect of the inferior petrosal sinus. It then forms the petrous ganglion which is divided into the superior and inferior petrosal ganglia. Together the cranial nerves traverse this foramen in order to escape into the neck region. At the neck region, the IX seeks for its muscle of supply, after given off branches to the oropharynx and the posterior 1/3 of the tongue. It enters the stylopharyngeus in order to supply that muscle.
It has the following branches in the neck
Lesion or complete transection of the glossopharyngeal nerve alone is uncommon. It usually involves also the vagus nerve. But when present alone, it leads to the following features
Vestibulocochlear nerve (VIII)
This nerve essentially is two- the cochlear and the vestibular nerves. The nerve is the only cranial nerve (or indeed any nerve at all) that is SSA. It is therefore sensory to the inner ear apparatus such as the semicircular canals the vestibule and the cochlear.
The cochlear nerve has the cell bodies of its neurons in the spiral ganglia arranged along the cochlea apparatus of the osseous labyrinth, spiral laminae. The peripheral processes receive impulses from hair cells which lie in the organ of Corti. The central processes of the neurons then issue from the spiral ganglia. They are actually bipolar neurons rather than pseudounipolar. The central processes gather together to form the cochlear nerve which approaches the internal acoustic meatus accompanied by the facial nerve, the nervus intermedius and the labyrinthine artery. Its fibers sweep across the superior edge of the inferior cerebellar peduncle at rostral level of the medulla. The fibers then terminate on the dorsal and cochlear nuclei of the floor of the 4th ventricle at the so called tuberculum acousticum found in the vestibular area.
Second order neurons now issue from these nuclear groups to projects upwards, sweeping across the medial lemniscus and also the superior olive to reach the trapezoid body all of which are found in the pontine tegmentum. The fibers pass through the lateral lemniscus to reach the midbrain and terminate at the inferior colliculus of the midbrain tectum.
3rd order neurons now issue from the superior colliculus to project upwards. Their destination will be the medial geniculate body of the metathalamus. From the medial geniculate body the geniculotemporal tract arise and terminate on the temporal auditory cortex. This is at the transverse temporal gyrus of the temporal lobe.
It is possible for fibers to reach the medial geniculate body directly from the dorsal or ventral cochlear nuclei of the medulla or for fibers to run from the inferior colliculus to terminate in the temporal cortex. Whatever be the case, the fibers which reach the superior colliculus or the medial geniculate body are responsible for auditory reflexes. Those which reach the cortex are responsible for voluntary mechanisms for which man is well recognized.
Along the pathway of the auditory system we have several nuclei which occur
This nerve has its cell bodies placed in the vestibular ganglia. It is responsible for kinetic and static equilibrium. Its peripheral processes reach the receptors which are hair cells placed in the vestibular apparatus (maculae of utricle and saccule) and semicircular ducts (ampullae) of the inner ear. The central processes issue from the bipolar neurons of the vestibular ganglia and move centrally accompanied by the cochlear nerve and the facial nerve into the internal acoustic meatus. It then enters the lower part of the pons and the upper part of the medulla, also sweeping across the inferior cerebellar peduncle to terminate in four characteristic nuclei as follows
These nuclei have afferent fibers from all parts of the brain including the cerebral cortex. Fibers also reach from the spinal cord and higher cortical centers via the medial longitudinal fasciculus. Efferent projections now issue from these nuclei and project into the spinal cord via the vestibulospinal tract and other parts of the neuraxis. Some will project into the inferior cerebellar peduncle to reach the cerebellum.
This nerve has its nucleus buried in the facial colliculus, a little ventral to the nucleus of the abducent nerve.
It then forms a genu which sweeps across the dorsal surface of the abducent nucleus in order to reach the lateral aspect of the pontine tegmentum where it runs a ventral course to escape at the cerebellopontine angle, medial and inferior to the point of escape of the abducent nerve. It, immediately at its extrabulbar course, forms the external genu of the facial nerve before it expands to form the geniculate ganglion. Its nervus intermedius part then gives off two branches
The chorda tympani enters the lingual nerve and it carries secretomotor fibers which reach the submandibular ganglion and are distributed to oral glands and the submandibular and sublingual salivary glands. The greater petrosal nerve passes forwards to enter the pterygopalatine ganglion where it relays and its postganglionic fibers then enter nasal and palatine glands. The facial nerve itself proceeds through the stylomastoid canal and emerges at the stylomastoid foramen to enter the face and supply the muscles of facial expression.
The nucleus associated with the facial nerve is the motor nucleus of facial nerve. Other nuclei are the superior salivatory nucleus, and the nucleus of the tractus solitarius. Hence the facial nerve, together with its nervus intermedius component has the following functional components
The facial nerve supplies the muscles of facial expression and those also derived with facial expression muscles from the 2nd pharyngeal arch. They are buccinator, stapedius, posterior belly of digastric, stylohyoid in addition to muscles of facial expression. It gives off 4 branches within the petrous temporal bone and emerges from the sternomastoid foramen to enter the parotid gland superficial to the external carotid artery and the retromandibular vein. Within the parotid gland it forms venous plexus called pes anserinus. It gives off a branch called the posterior auricular branch before it enters the parotid gland. This branch supplies the occipitofrontalis. Within the parotid substance it divides into two branches
These two branches form the terminal branches of the facial nerve which supply the muscles of facial expression. They are as follows
Branches of VII in petrous temproal bone
This nerve is considered a separate nerve from the facial nerve and a bona fide cranial nerve on its own right. Although it does seem very distinct independent nerve, most anatomists still consider it as part of the facial nerve complex. It contains two main fiber components as follows
Nervus intermedius has three important branches
Greater petrosal nerve is purely secretomotor with all its fibers from nervus intermedius. It then proceeds towards foramen lacerum where it is joined by lesser petrosal nerve which is peeled off the sympathetic nervous plexus that surrounds the internal carotid artery. Both nerves move together for a short distance. The cell bodies of lesser petrosal nerve which are sympathetic are found in the superior cervical ganglion. The greater petrosal nerve moves towards the pterygoid fossa and emerges from it as nerve of pterygoid canal and then moves towards the pterygopalatine fossa where it relays in the pterygopalatine ganglion. Its second order neurons move from the ganglion towards the face and the nasal cavity. A branch proceeds towards the nasal cavity and supplies nasal septum, lateral nasal wall, the hard and soft palates and the nasopharynx. Another branch supplies the lacrimal gland, which has hitch hicked with one of the branches of the maxillary nerve, after which it leaves the maxillary nerve to enter the lacrimal nerve- a branch of the ophthalmic division of trigeminal nerve. During the course of greater petrosal nerve, it passes through the middle cranial fossa were it is liable to damage in surgery of this fossa where it traverses the cavum trigeminale.
Chorda tympani contains mainly a special visceral afferent fibers. The peripheral fibers of the cell bodies constitute the chorda tympani which supplies the tongue but joins the lingual nerve which then carries it to the tongue.
Tympanic branches relay in the otic ganglion which provide parasympathetic supply to parotid gland.
Clinical aspect of facial nerve
Facial nerve can b paralysed in Bell's palsy where it affects the muscles of facial expression and others supplied by VII
Two broad lesions are recognised
Lesion at A placed distal to stylomastoid canal will cause paralysis of muscles of facial expression on that side. It leads to drooping of lips, inability of the eye lids to close and this may lead to drying of cornea.
Lesion in B placed in the stylomastoid canal just distal to the geniculate ganglion will lead to all the affection of A above and also hyperacusis (which is unusual sensitivity to noise) because o he paralysis of stapedius whose branch is given off before facial nerve emerges from the stylomastoid foramen. It may include loss of taste at the anterior 2/3 of tongue. Sometimes it does not happen as some taste fibers may pass through the greater petrosal nerve. It also includes affection of secretion of saliva
Lesion in C placed just distal to the medial but proximal to the geniculate ganglion will cause all the above including abolition of lacrimation
This nerve has its nucleus placed within the facial colliculus. It is responsible for the formation of the colliculus at the floor of the 4th ventricle. The fibers of the internal genu of facial nerve sweep across its dorsal aspect and later lateral to it. It escapes from the brain stem above the pyramids at the pontomedullary junction.
The abducent nucleus is surrounded by the parabducent nucleus. This nucleus is thought to contain fibers which are part of the extrapyramidal system that control lateral gaze movement. Abducent nerve supplies the lateral rectus. Damage to this nerve will cause paralysis of the lateral rectus. Damage to the parabducent nucleus will lead to paralysis of lateral gaze movements.
This nerve emerges at the junction between the pons and the medulla just above the pyramid of medulla. It immediately enters the cisterna pontis and moves upwards between the pons and anterior inferior cerebellar artery to pierce the arachnoid and dura. It then enters the inferior petrosal sinus at the apex of petrous temporal bone after which it proceeds into the cavernous sinus lateral to the internal carotid artery. It passes forward to reach the medial aspect of superior orbital fissure and enters the tendinous ring, inferior to the inferior division of the oculomotor nerve. It entes the lateral rectus to supply it.
Damage to the abducent nerve is common in fractures of base of skull. It causes convergent squint and diplopia
The trigeminal nerve has the following nuclear groups in the pontine tegmentum
Sensory nuclear group which carry GSA fibers. They are
Sensory nuclear group is quite massive. It extends from cervical level C4 through the medulla to the mesencephalon. It may therefore be divided into two
Superior group. This is found in the pons and midbrain. It is the principal sensory nucleus and the mesencephalic nucleus of V. The pontine nucleus or principal sensory nucleus is placed lateral to the motor nucleus in the pontine tegmentum. They both lie in the dorsal aspect of the tegmentum, close to the floor of the 4th ventricle. The mesencephalic nucleus is slightly displaced more dorsally, medial to the superior cerebellar peduncle and extending upwards into the midbrain.
The mesencephalic nucleus contains first order neurons for proprioception of the face region supplied by the fibers of V. Principal sensory nucleus is responsible for fine sensation of the face region while the spinal nucleus is responsible for coarse sensation in the face region. the sensory neurons are psedounipolar with their cell bodies in the trigeminal ganglion placed at the trigeminal cave of the middle cranial fossa. Their peripheral processes pass through the corresponding nerve divisions as follows
The sensory nuclear group is arranged in a lamina fashion. Ophthalmic fibers are ventral, maxillary intermediate and mandibular dorsal
Motor nucleus of V passes through the portio minor being smaller than the sensory division, which passes through portio major. The fibers remain separate from the sensory division, but join the division distally by entering the mandibular nerve which carries them through e foramen ovale into the infratemporal fossa. The motor divisions of V will supply the muscles of mastication, medial pterygoid, lateral pterygoid, masseter and temporalis, and then also the other muscles derived from the first pharyngeal arch, which are mylohyoid, tensor tympani, tensor veli palatini, and anterior belly of digastric muscle with SVE fibers.
Ophthalmic nerve runs in the lateral wall of the cavernous sinus after leaving the upper part of the trigeminal ganglion. Sympathetic fibers which supply the dilator pupillae are added to the nerve from the cavernous plexus as it runs to the anterior edge of the cavernous sinus where it gives off the tentorial nerves, which are its meningeal branches. It s divided into three terminal branches which enter the superior orbital fissure. They are
Lacrimal nerve passes into the orbit lateral to the tendinous ring. At the upper part of the lateral wall of orbit it picks up the secretomotor branches of the zygomatic nerve which supply the lacrimal gland. The nerve also contains sensory fibers which supply the lateral edge of the upper eyelid and their corresponding conjunctivae.
Frontal nerve is the largest of the nerves of the ophthalmic division. It also enters the orbit by passing lateral to the tendinous ring and is placed between the lacrimal and the trochlear nerves. It runs superior to the levator palpebrae superioris and then divides into supraorbital and supratrochlear nerves . The surpaorbital nerve is transmitted by the supraorbital foramen in the orbital margin of the frontal bone and supplies the skin and conjunctiva of the upper eyelid. The supratrochlear neve emerges from the supratrochlear foramen and supplies also the upper eyelid and the skin of the midline region of the forehead.
Nasociliary nerve enters the orbit placed between the two divisions of the oculomotor nerve within the tendinous ring. It then enters the cone of muscles which are placed superior to the optic nerve where it divides into two terminal branches which are the infratrochlear and anterior ethmoidal nerves. All the branches it gives off in the orbit are as follows
Maxillary nerve is the nerve of the maxillary process of the first pharyngeal arch. It runs in the lateral wall of the cavernous sinus after leaving the middle part of the trigeminal ganglion. It then enters the pterygopalatine fossa by passing through the foramen rotundum. In the fossa, it runs a short course to reach the inferior orbital fissure and then breaks into its two terminal branches
Other truncal branches are
The zygomatic nerve is one of the terminal branches of the maxillary nerve. It immediately enters the inferior orbital fissure and runs on the lateral wall of the orbit to enter the zygomatic bone where it divides into its terminal branches- the zygomaticotemporal nerve and the zygomaticofacial nerve both of which pierce the zygomatic bone to supply the skin over the bone.
This nerve runs on the floor of the orbit and then enters the infraorbital groove and from there to the infraorbital canal. It emerges from the canal by passing through the infraorbital foramen to reach the skin of the face between the levator labii superioris and the levator anguli oris, where it divides into labial, palpebral and nasal branches to supply the skin of these areas. In the infraorbital groove it gives off the middle superior alveolar nerve which supplies the 2 premolars and in the infraorbital canal it gives off the anterior superior alveolar nerve which supplies the canine and two incisors to include the maxillary sinus.
Mandibular nerve supplies the first pharyngeal arch. It immediately enters the foramen ovale after emerging from the lower part of the trigeminal ganglion. It is accompanied by the motor root which passes through the foramen ovale and joins it in the foramen. It then enters the infratemporal fossa after emerging from the foramen ovale between the superior head of lateral pterygoid and the tensor veli palatini. It gives off two truncal branches before it divides into 9 branches which are divide divided into two groups
Anterior branches are as follows
Posterior branches are as follows
Auriculotemporal nerve emerges by two roots which embrace the middle meningeal artery. It contains secretomotor fibers which are from the otic ganglion; they will supply the parotid gland. It then gives a sensory branch to the temporomandibular joint and curves round the neck of mandible and supplies the parotid gland with secretomotor fibres from the optic ganglion. It then divides into its two terminal branches - auricular branch, which supplies the external acoustic meatus and the temporal branch, that supplies the skin of the temple which first turns gray in ageing.
Lingual nerve joins the chorda tympani below the base of skull and inferior to the lower border of lateral pterygoid. It passes in front of the inferior alveolar nerve and then above the mylohyoid. Just below the last molar tooth it gives a gingival branch which supplies the gingival gum. It then ascends on the hyoglossus after passing below the submandibular duct to reach the anterior 2/3 of tongue, which it supplies. It also supplies the mucous membrane of the floor of the mouth.
Inferior alveolar nerve is the only nerve amongst he posterior ones that carry motor fibers. It runs below the inferior head of lateral pterygoid and then lies on the medial pterygoid to enter the mandibular foramen, anterior to the inferior alveolar vessels. It gives off the nerve to mylohyoid which contains the motor fibers of the inferior alveolar nerve. It continues in the lower jaw and supplies all the lower jaw teeth and gum. The nerve to mylohyoid enters the neck and lies between the mylohyoid and anterior belly of digastric, accompanied by submental vessels.
This nerve has its nucleus placed in the floor of the cerebral aqueduct and at the level of the inferior colliculus. It then runs to the superior medullary velum where it decussates. It emerges from the brains stem at the dorsal aspect and is the only cranial nerve that does so. It then runs on the lateral aspect of the midbrain to reach the ventral aspect where it enters the cavernous sinus, by running on the undersurface of the free edge of tentorium cerebelli. A this point it pierces the arachnoid-dura posterior to the oculomotor nerve. It enters the superior orbital fissure by passing lateral to the tendinous ring and above the levator palpebrae superioris. It then supplies the superior oblique.
Diplopia (double vision) occurs when the patient attempts to looks downwards and laterally with damage to trochlear nerve.
Oculomotor nuclear complex
This consists of the following nuclear groups
This nerve emerges from the brain stem attached to the medial side of crus cerebri at the interpeduncular fossa. It then passes between the posterior cerebral and superior cerebellar arteries and finally pierces the arachnoid and dura at the roof of cavenous sinus posterior to the internal carotid artery, but anterior to the trochlear nerve, where it lies at the lateral wall of the cavernous sinus. It then divides into its terminal branches after collecting sympathetic fibers from the cavernous plexus, into superior and inferior divisions. The two divisions enter the tendinous ring on the superior orbital fissure to gain access into the orbit where the nasociliary nerve lies between the two divisions, with the abducens nerve inferior to them. The superior division immediately enters the ocular surface of superior rectus while the inferior one divides into three branches as follows-
Complete lesion of the oculomotor cause the following features
The visual fields can be divided into.
The temporal portion of the visual field projects on the nasal portion of the retina. Information then proceeds from this nasal portion through the optic nerve to the optic tract. Information from the nasal visual field projects onto the temporal retina. At the optic chiasm, fibers carrying impulses from the 2 fields of the retina decussates partially. Lateral fibers proceed along the same line while medial fibers cross to the other side. Hence the optic tract consists of contralateral fibers from the nasal retina carrying impulses from the temporal field and ipsilateral temporal fibers carrying information from the nasal field.
The optic tract then moves around the brain stem to terminate in the lateral geniculate body. 2nd order neurons move from the lateral geniculate body as optic radiations within the sublenticular part of the internal capsule. Some fibers proceed to the superior colliculus to form the brachium of the superior colliculus. The superior colliculus subserves primitive visual reflexes. The fibers of the optic radiation then terminate in the visual cortex to form the geniculocalcarine tract that course along the bank of the calcarine sulcus to terminate in the area 17 of the medio-occipital cortex. This is the visual cortex.
Clinically blockage can occur in any part of the tract. A complete transection of the tract before the optic chiasm causes total blindness of the eye of that side.
A lateral section at the optic chiasm will lead to an ipsilateral nasal hemianopsia. A section posterior to the optic chiasm at the optic tract causes a left sided homonymous hemianopsia if the section is on the right side.
A section across the optic chiasm in the sagittal plane leads to bitemporal heteronymous hemianopsia. This usually occurs in pituitary tumor because of the close relationship between the hypophysis cerebri and the optic chiasm.
The pathway for the olfactory nerves begins in the olfactory epithelium of the lateral wall of the nasal cavity. The receptors are attached to peripheral fibers of the bipolar olfactory neurons which transverse the cribriform plate of the ethmoid, they are about 20- in number. The cell bodies lie in the olfactory bulb. The central processes now synapse with second order neurons, which enter the olfactory pathway. This pathway is divided into two – medial and lateral olfactory striae. At the synapse, they may from a glomerulus with more than one neuron
Medial olfactory stria terminates in the subcallosal gyrus.
The lateral olfactory striae carry fibers which terminate in pyriform lobe and the periamygdaloid body. The two structures mentioned (subcallosal gyrus and the pyriform lobe) are part of what is called the primary olfactory cortex. The third order neurons now project into the secondary olfactory cortex, called the entorrhinal cortex. 4th or 3rd order neurons issue from the entorrhinal cortex to reach the highest centers for olfaction in the hippocampal formation and the insular cortex.
Pyriform lobe is made u p of
This surrounds the amygadaloid nuclei complex, and it mediates the sensation of smell around the amygdala. Amyadaloid nuclear complex is divided into
It has two main efferent projections
Because of the intimate connection of the amygdala with these diencephalic structures, its function is similar in the control of visceral activity e.g. stimulation of amygdala leads to ovulation. It is also said to control gonadotropin levels together with the hippocampus.
7 primary smell are recognized
Frontal lobe tumors can cause unilateral anosmia. Uncal tumors may cause uncinate fits. Bilateral anosmia occurs after head injuries. It is commoner with anterior cranial fractures which occurs with rhinorrhea.
This is also spinal shock . It causes flaccid paralysis, visceral paralysis and sensory loss below the lesion on the spinal cord. This is caused mainly by trauma to the spinal cord without direct damage. It leads to complete recovery in about 7-10 days
In this condition the spinal cord is either completely transected or only one side is cut (hemisection called Brown-Sequard syndrome.
Complete transection leads to flaccid paralysis and sensation loss below the lesion
. In this condition the sensation is affected below the lesion on the opposite side. But motor function is affected on the same side. Also there is Rombergism on the same side. These are related to the various tracts, whether descending or ascending
This cause completely flaccid paralysis
Cauda equina syndrome
This is affection of the space below L1 vertebrae containing cauda equina (nerves which surround the filium terminale below L1 or 2) with space occupying lesion such as tumor, prolapsed intervertebral disc or fracture-dislocation of lumbar spine.
Creator: Oluwole Ogunranti