Nervous system

8 Nervous system




ANATOMY



CRANIAL CAVITY


The cranium, or the skull, consists of the cranial cavity and the facial skeleton. Most bones of the cranial cavity are flat bones having two plates of compact bone separated by a thin layer of trabecular bone, or the diploe. Both the inner and outer surfaces are lined by periosteum, the inner periosteum being the endosteal layer of dura mater. The bones of the cranial cavity are the frontal, occipital, sphenoid, ethmoid and the paired temporal and parietal bones.


The cranial cavity has a cranial vault and the base of the cranium with the three cranial fossae.



Cranial vault


The cranial vault, or the roof of the cranial cavity, is formed by the frontal bone anteriorly, the paired parietal bones laterally and the occipital bone posteriorly (Fig. 8.1). In about 8% of cases a metopic suture pre-sents in the midline during early stages of development between the two halves of the frontal bones and persists in adulthood. A midline sagittal groove marks the position of the superior sagittal sinus. The sinus and its groove widen as they pass posteriorly. The falx cerebri is attached to the lips of this groove. Irregular depressions along the groove lodge the arachnoid granulations.



The sagittal suture separates the two parietal bones in the midline. The coronal suture divides the frontal from the parietal bones, and the lambdoid suture divides the two parietal bones from the occipital and the temporal bones. Posterior to the coronal suture the middle meningeal vein and its tributaries, accompanied by the middle meningeal artery, groove the vault of the skull. The bony vault is thin in the temporal and the lower part of the occipital regions, where there are thick muscular attachments. A blow on the skull vault may cause internal injuries without fracture because of the plasticity of the skull bones.


The lambda is the junction between the lambdoid suture and the sagittal suture. It is the area of the posterior fontanelle in the infant. The bregma, where the anterior fontanelle was in the infant, is at the junction between the coronal and the sagittal sutures. The glabella is the prominence above the nasion which is the depression between the two supraorbital margins. The pterion is a thin part of the skull at the junction of the parietal, frontal and temporal bones and the greater wing of the sphenoid in the temporal region of the skull. The anterior branch of the middle meningeal artery and the accompanying vein traverse the pterion.


Vascular markings of the meningeal vessels, sutures and diploic vessels may be confused as fracture lines. At birth the anterior and posterior fontanelle are open and are palpable. Blood can be taken by puncturing the anterior fontanelle in the midline. CSF can be aspirated by passing a needle obliquely through it into the subarachnoid space. The posterior fontanelle fuses by about three months after birth and the anterior by about 18 months.



Three cranial fossae


The floor of the cranial cavity has three cranial fossae – the anterior, middle, and the posterior cranial fossae – each progressively lower than the one in front. The anterior cranial fossa overlies the orbit and the nasal cavities. The frontal lobe of the brain lies in the anterior cranial fossa. The middle cranial fossa lies below and behind the anterior and contains the temporal lobes. Most posteriorly the posterior cranial fossa lies at the lowest level and contains the brainstem and the cerebellum.




Middle cranial fossa


The body of the sphenoid lies in the middle forming the floor of the pituitary (hypophyseal) fossa (Fig. 8.2). Laterally are the greater wings of the sphenoid and the squamous parts of the temporal bones. The petrous part of the temporal bone containing the middle and inner ear forms the posterior boundary of the fossa.


The pituitary fossa is bounded in front and behind by the anterior and posterior clinoid processes. It contains the pituitary gland and is roofed by the diaphragma sellae, a fold of dura mater.


Anteriorly the middle cranial fossa has the optic canal and the supraorbital fissure communicating with the orbit. The optic canal transmits the optic nerve and the ophthalmic artery. The supraorbital fissure transmits the:







Lateral to the pituitary fossa the middle cranial fossa has a few important foramina:






Fractures of the middle cranial fossa are common, as the bone is weakened by the foramina and canals. Fracture involving the tegmen tympani, the thin anterior surface of the petrous temporal bone, results in bleeding into the middle ear. Excessive bleeding ruptures the tympanic membrane, discharging blood from the ear. This can be associated with CSF otorrhoea. The seventh and eighth nerves also may be involved, as they run in the petrous temporal bone.



Posterior cranial fossa


The posterior cranial fossa has an anterior wall formed by the petrous temporal bone laterally and the body of the sphenoid and the basilar part of the occipital bone medially. The latter two form the clivus which extends from the foramen magnum to the dorsum sellae. The occipital bone mostly forms the floor and lateral walls of the fossa. The internal occipital protuberance is in the midline on the posterior wall. Above this the skull is grooved by the superior sagittal sinus. Running ante-rolaterally on either side from the internal occipital protuberance are the grooves for the transverse sinuses, which continue down beneath the petrous temporal bone as the sigmoid sinuses. The sigmoid sinus passes through the jugular foramen to become the internal jugular vein. The ninth, tenth and eleventh nerves as well as the inferior petrosal sinus pass through the jugular foramen anterior to the sigmoid sinus. The hypoglossal or anterior condylar canals transmitting the hypoglossal nerves lie on the anterior rim of the foramen magnum. Through the foramen magnum the medulla oblongata continues into the vertebral canal as the spinal cord. The vertebral arteries and the spinal accessory nerves enter the skull via the foramen magnum.


Anteriorly in the fossa on the medial aspect of each petrous temporal bone is the internal acoustic meatus conveying the seventh and eighth nerves and the labyrinthine arteries into the internal ear. Below the internal acoustic meatus in the anterior aspect of the jugular foramen is the cochlear canaliculus into which opens the aqueduct of the cochlea (perilymphatic duct) which brings the perilymph of the internal ear into communication with the CSF.


Fractures of the posterior cranial fossa may involve the basilar part of the occipital bone which separates the pharynx from the posterior cranial fossa. Bleeding may then occur into the pharynx. More lateral fractures can bleed into the back of the neck.



BRAIN AND MENINGES



Brain


The brain is subdivided into the forebrain, midbrain and hindbrain, comprising the major parts listed in Table 8.1.


Table 8.1 Major subdivisions and parts of the brain


image



Cerebral hemisphere


The cerebral hemisphere has a layer of grey matter on its external surface, the cerebral cortex, and white matter in the interior in which there are nuclei forming the basal ganglia. The cavity of the cerebral hemisphere is the lateral ventricle.


The cerebral hemisphere (Fig. 8.3) is divided into four lobes for descriptive purposes:







The cerebral cortex has a large number of sulci (clefts) and gyri (folds). The lateral sulcus is the largest sulcus on the superolateral surface and separates the temporal lobe from the parietal and frontal lobes (Fig. 8.3).


The central sulcus separates the precentral and postcentral gyri which contain the primary motor and sensory areas of the cortex. On the medial surface of the hemisphere the parieto-occipital sulcus separates the occipital lobe from the parietal lobe. The calcarine and postcalcarine sulci concerned with visual centres are also seen on the medial surface.


The corpus callosum which is seen between the two hemispheres carries commissural fibres linking one hemisphere to the other. Its anterior enlargement is the genu and the posterior end the splenium.






Midbrain


The midbrain connects the diencephalon to the pons of the hindbrain and contains a small canal, the cerebral aqueduct. The cerebral aqueduct extends from the third ventricle to the fourth ventricle. The part behind the aqueduct is the tectum containing the superior and inferior colliculi, which are respectively connected to the visual and auditory pathways. The two cerebral peduncles lying in front of the aqueduct are further divided into tegmentum and basis pedunculi by the substantia nigra. The basis pedunculi contain the descending fibre tracts which are continuations of the internal capsule. The tegmentum of the midbrain has the ascending tract as well as nuclei for the oculomotor and the trochlear nerves. The oculomotor nerve nuclei are situated at the level of the superior colliculus and the trochlear nerve nucleus at the level of the inferior colliculcus. The substantia nigra is connected to the corpus striatum, providing the latter with its dopaminergic innervation. Vascular lesions of midbrain may cause nystagmus and even ophthalmoplegia, and hemiparesis.


The midbrain is contained in the gap between the free border and the tentorium cerebelli (the tentorial notch). An increase in cranial pressure above or below the tentorium can displace the midbrain and compress the structures surrounding it against the unyielding tentorium. The temporal lobe can be compressed and the uncus can herniate through the tentorial notch. A supratentorial lesion raising the intracranial pressure often compresses the oculomotor nerves at this level.


The pineal gland is situated in the midline between the two superior colliculi, towards the posterior end of the third ventricle. Its function in man is not clearly known. In lower animals it converts serotonin to melatonin which maintains the circadian rhythm. The human pineal gland normally becomes calcified and as it is normally in the midline its lateral displacement may be a sign of displacement of the hemisphere by a space occupying lesion.





Cerebellum


The cerebellum (Fig. 8.8) is the largest part of the hindbrain. It is made up of two lateral cerebellar hemispheres separated by the vermis. The cerebellum is connected to the brainstem by the three pairs of cerebellar peduncles.






The most anterior and caudal part of the lateral lobe is the flocculus attached to the nodule in the midline. The flocculonodular lobe is an important part in the vestibular system, which maintains balance. The bulge of the lateral lobe that projects inferiorly posterolateral to the medulla is the tonsil. In cases where there is raised intracranial tension, the tonsils can herniate into the foramen magnum and compress the medulla oblongata following a lumbar puncture.


The structural organisation of the cerebellum is uniform and is similar to that of the cerebral hemisphere, i.e. a thin layer of cortex outside and the deeper white matter containing various cerebellar nuclei.


Lesions of cerebellum are characterised by ataxia and intention tremors (tremor during movements, unlike the tremor in Parkinson’s disease which is seen at rest).



Meninges


The three layers of the meninges are:





The three meningeal spaces are:







Falx cerebri


The falx cerebri lies between the two cerebral hemispheres, and is attached anteriorly to the crista galli and posteriorly to the tentorium cerebelli. The superior sagittal sinus lies along its superior border, and the inferior sagittal sinus lies along its inferior free margin. The straight sinus is seen where the falx cerebri meets the tentorium cerebelli.











Ventricular system and cerebrospinal fluid


Cerebrospinal fluid is produced in all four ventricles by the choroid plexus. It flows from the lateral ventricles into the third ventricle, from there through the cerebral aqueduct into the fourth ventricle and thence into the subarachnoid space. It is reabsorbed into the venous system through the arachnoid granulations along the dural venous sinuses. The total volume of CSF is about 100–150 mL in the adult; its pressure is about 8–10 cm H2O.


The general shape of the ventricular system is shown in Fig. 8.11. The lateral ventricles, larger than the others, are contained in the cerebral hemispheres. Each lateral ventricle has a body which is floored by the thalamus and the caudate nucleus. The corpus callosum forms its roof. The anterior horn projects forward in front of the interventricular foramen. The posterior horn projects into the occipital lobe, and the inferior horn projects into the temporal lobe. The choroid plexuses, which are found in the inferior horn and the body, are continuous with those on the roof of the third ventricle through the interventricular foramen. The interventricular foramen (foramen of Monro) is bounded by the anterior end of the thalamus and the fornix. It connects the lateral ventricle to the third ventricle.



The third ventricle is a narrow slit-like space between the two thalami and the hypothalami. It is roofed by the tela choroidea, a double layer of pia mater, containing choroid plexus. The third ventricle is connected to the fourth ventricle by the cerebral aqueduct.


The fourth ventricle is tent shaped with a diamond shaped floor or anterior wall formed by the pons and the medulla. It is roofed by the superior and inferior medullary vela connected to the superior and inferior cerebellar peduncles, respectively. The cerebellum lies posterior to the fourth ventricle. The fourth ventricle has three openings on its roof, which connect it to the subarachnoid space. The single foramen of Magendie is in the midline, and the paired foramen of Luschka more laterally. Through these, CSF from the ventricular system enters the subarachnoid space.


The circumventricular organs are midline structures bordering the 3rd and 4th ventricles where the blood-brain barrier is deficient. They include the pineal gland, median eminence, neurohypophysis, area postrema of the fourth ventricle and the choroid plexus. These barrier-deficient areas are recognized as important sites for communicating with the CSF and between the brain and peripheral organs via blood-borne products. (See under physiology.)



BLOOD SUPPLY TO THE BRAIN



Arterial supply


The two vertebral arteries and the two internal carotid arteries supply the brain (Fig. 8.12).




Vertebral arteries


After entering the cranial cavity through the foramen magnum, the two vertebral arteries lie in the subarachnoid space and ascend on the surface of the medulla to the lower border of the pons where they unite to form the basilar artery. The basilar artery lies in the groove on the anterior surface of the pons and, at its upper border, divides into the two posterior cerebral arteries.


The following branches supplying the brain and spinal cord arise from the vertebral artery:





The posterior spinal artery arises from the lower part of the vertebral artery, descends along the line of attachment of the dorsal roots of the spinal nerves and supplies the dorsal column of the white mater and the dorsal horn of the grey mater of the spinal cord. The artery often arises as a branch of the posterior inferior cerebellar artery.


The anterior spinal artery descends in front of the medulla and unites with the artery of the opposite side, forming a single artery lying in the anterior median fissure of the spinal cord. It supplies the ventral two-thirds of the spinal cord as well as the anteromedial aspect of the medulla, including the pyramid and the medial lemniscus. Anterior spinal artery syndrome or Beck’s syndrome is characterized by ischemia or infarction of the spinal cord in the distribution of the anterior spinal artery. This condition is usually associated with atherosclerosis of the aorta and may result from an acute aortic dissection or rarely dissection of the anterior spinal artery. Clinical features include weakness and loss of pain and temperature sensation below the level of injury, with relative sparing of position and vibratory sensation perceived by the posterior columns.


The posterior inferior cerebellar artery winds backward deep to the rootlets of the hypoglossal, vagus and the glossopharyngeal nerves to reach the cerebellum. The artery supplies the posterolateral aspect of the medulla, besides the cerebellum, and its blockage compromises the nucleus ambiguus and the nucleus of the spinal tract of the trigeminal, resulting in ipsilateral paralysis of the muscles of the palate and pharynx and anaesthesia for pain and temperature on the face.




Internal carotid arteries


The branches of the internal carotid artery supplying the brain are as follows:






The posterior communicating artery is a small artery running backwards from the internal carotid to join the posterior cerebral to form the circle of Willis.


The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid artery. It crosses over the optic nerve and, near the midline, is connected to the opposite artery by the anterior communicating artery. The anterior cerebral artery supplies the medial part of the inferior surface of the frontal lobe, and courses along the upper surface of the corpus callosum, supplying the medial surface of the frontal and parietal lobes and the corpus callosum. It also supplies a narrow strip on the upper part of the lateral surface. The motor and sensory areas of the lower extremity, located in this area (Fig. 8.5), are supplied by the anterior cerebral artery, resulting in characteristic paralysis when the artery is occluded.


The middle cerebral artery is the larger of the terminal branches of the internal carotid artery. It lies in the lateral sulcus, and its branches supply the lateral surface of the frontal, parietal and temporal lobes, except the narrow strip in the upper part supplied by the anterior cerebral. Occlusion of the artery results in contralateral motor and sensory paralysis of the face and arm.


The anterior choroid artery is given off from the internal carotid near its termination. It may also arise from the middle cerebral. It courses backward along the optic tract and supplies the interior of the brain, including the choroid plexus in the inferior cornu of the lateral ventricle.




Venous drainage of the brain


The veins of the brain, lying along with the arteries in the subarachnoid space, are thin-walled vessels without valves. They pierce the arachnoid and drain into the dural venous sinuses. The major veins of the brain are as follows:






The superior cerebral veins drain the lateral surface of the cerebral hemisphere. They open into the superior sagittal sinus. Veins lying posteriorly in this group are directed forward and join the sinus against the direction of the blood flow.


The superficial middle cerebral vein lies in the lateral sulcus. It runs downward and forward and drains into the cavernous sinus.


The basal vein is formed by the union of the deep middle cerebral vein, which lies in the depth of the lateral sulcus, and the anterior cerebral vein, which accompanies the anterior cerebral artery. The basal vein winds round the cerebral peduncle and ends in the great cerebral vein.


The great cerebral vein is formed by the union of the two internal cerebral veins which drain the interior of the cerebral hemisphere. It receives the basal veins and it drains into the straight sinus.



Cranial (dural) venous sinuses


The cranial venous sinuses (Fig. 8.15) are situated within the dura mater. They are devoid of valves and drain eventually into the internal jugular vein.



The cranial venous sinuses are:










The superior sagittal sinus begins in front of the crista galli, courses backwards along the attached border of the falx cerebri, and usually becomes continuous with the right transverse sinus near the internal occipital protuberance. At its commencement it may communicate with the nasal veins. A number of venous lacunae lie along its course and open into the sinus. The sinus and the lacunae are invaginated by arachnoid granulations. The superior cerebral veins drain into the superior sagittal sinus (Fig. 8.16).



The inferior sagittal sinus lies along the inferior border of the falx cerebri and is much smaller than the superior sagittal sinus. It receives the cerebral veins from the medial surface of the hemisphere and joins the great cerebral vein to form the straight sinus.


The straight sinus, formed by the union of the inferior sagittal sinus and the great cerebral vein, lies in the attachment of the falx cerebri to the tentorium cerebelli. It usually becomes continuous with the left transverse sinus near the internal occipital protuberance.


The transverse sinus lies in the groove on the inner surface of the occipital bone along the posterior attachment of the tentorium cerebelli. On reaching the petrous temporal bone, it curves downwards into the posterior cranial fossa to follow a curved course as the sigmoid sinus.


The sigmoid sinus passes through the jugular foramen and becomes continuous with the internal jugular vein.


The confluence of sinuses is formed by two transverse sinuses connected by small venous channels near the internal occipital protuberance.


The occipital sinus, a small venous sinus extending from the foramen magnum, drains into the confluence of sinuses. It lies along the falx cerebelli and connects the vertebral venous plexuses to the transverse sinus.



Cavernous sinus


The cavernous sinus (Fig. 8.17), one on each side, situated on the body of the sphenoid bone, extends from the superior orbital fissure to the apex of the petrous temporal bone. Medially, the cavernous sinus is related to the pituitary gland and the sphenoid sinus. Laterally it is related to the temporal lobe of the brain. The internal carotid artery and the abducens nerve pass through the cavernous sinus. On its lateral wall from above downwards lie the oculomotor, trochlear and ophthalmic nerves. The maxillary divisions of the trigeminal go through the lower part of the lateral wall or just outside the sinus. The endothelial lining separates these structures from the cavity of the sinus.



The connections of the sinus are illustrated in Fig. 8.15. Posteriorly, the sinus drains into the transverse/sigmoid sinus through superior petrosal sinus and via the inferior petrosal sinus, passing through the jugular foramen, into the internal jugular vein. The ophthalmic veins drain into the anterior part of the sinus.


Emissary veins passing through the foramina in the middle cranial fossa connect the cavernous sinus to the pterygoid plexus of veins and to the facial veins. The superficial middle cerebral vein drains into the cavernous sinus from above. The two cavernous sinuses are connected to each other by anterior and posterior cavernous sinuses lying in front of and behind the pituitary.


Cavernous sinus thrombosis is rare. If it occurs it affects oculomotor, trochlear and abducent nerves which are necessary for eye movement, and the ophthalmic division of the trigeminal nerve, which gives sensation to the top and middle portion of the head and face. Infections of the air sinuses (specifically the sphenoid sinus), eyes, eyelids, ears, or skin of the face can all lead to cavernous sinus thrombosis. The most common scenario is an infection of the sphenoid sinus that lies just below the cavernous sinus, allowing for easy spread of the bacteria.



CRANIAL NERVES





III: Oculomotor nerve


The oculomotor nerve contains two major components:




The somatic efferent nucleus (having five groups of cells, one for each muscle) and the Edinger–Westphal nucleus (parasympathetic) lie in the midbrain at the level of the superior colliculus. The oculomotor nerves emerge between the two cerebral peduncles, pass between the posterior cerebral and superior cerebellar arteries and run forward in the interpeduncular cistern on the lateral side of the posterior communicating artery. Each nerve pierces the dura mater lateral to the posterior clinoid process to lie on the lateral wall of the cavernous sinus. It then divides into a small superior and a large inferior division which enter the orbit through the superior orbital fissure. The superior division supplies the superior rectus and the levator palpebrae superioris, and the inferior division supplies the medial rectus, the inferior rectus, and the inferior oblique.


The parasympathetic fibres from the Edinger–Westphal nucleus leave the branch to the inferior oblique to synapse in the ciliary ganglion. Postganglionic fibres supply the ciliary muscles and sphincter (constrictor) pupillae via the short ciliary nerves.


Complete division of the third nerve results in:







The oculomotor nerve can be paralysed by:





A third nerve palsy with pupillary sparing often has an ischaemic or diabetic aetiology.





Nuclei of the trigeminal nerve







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Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Nervous system
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