Basic Neuroanatomy

Embryology

The human brain and spinal cord are ectodermal derivatives. The early embryologic portions of the developing brain are the prosencephalon, mesencephalon, and rhombencephalon. The prosencephalon gives rise to the telencephalon, which will become the two cerebral hemispheres, and the diencephalon, which is composed mainly of thalamus and hypothalamus. The mesencephalon becomes the midbrain. The rhombencephalon gives rise to the metencephalon, which is composed of pons and cerebellum, and the myelencephalon, which becomes the medulla oblongata.

Cerebral Hemispheres

The cerebral hemispheres in man are large. Histologically, the neocortex that forms the cerebral cortex has six cell layers. These layers form the outer grey matter, with axons that descend inward to form the white matter beneath the cortex, called the corona radiata. The amount of cerebral tissue packed into the skull is increased by complex folding, with resultant gyri, which are the folds of cerebral tissue, and the intervening sulci.

The cerebral hemispheres can be divided into four lobes. The frontal lobe includes prefrontal cortex (association cortex) used for thought, as well as the premotor and motor cortex with upper motor neurons that are involved with voluntary movement. The parietal lobe, just posterior to the frontal lobe, contains the general sensory, or somesthetic, area. The temporal lobes, located below the frontal and parietal lobes, contain areas involved with speech and hearing. The occipital lobes are located at the posterior of the hemispheres and contain the visual cortex.

A "stroke" from an infarction or hemorrhage within the cerebral cortex may result in a variety of deficits, depending upon the size and location of the lesion. Small lesions may not produce clinically identifiable problems. A larger lesion in frontal lobe may result in loss of movement (hemiparesis) on the side opposite the lesion. A larger lesion in occipital lobe may result in visual loss.

The bulk of the connections between the right and left cerebral hemispheres consist of fibers that form the corpus callosum, a large mass of white matter deep to the hemispheres and external to the lateral ventricles. The anterior bend of the corpus callosum is called the genu, and the posterior aspect is known as the splenium. A small bundle of fibers known as the anterior commissure also provides interhemispheric connections.

Cerebellum

The cerebellum functions to control movement, including balance and fine motor control. The cerebellum, like the cerebral cortex, has grey and white matter. The cerebellar grey matter consists of an outer molecular layer and inner granular layer, with large Purkinje cells sandwiched between. Axons travel inward to form the white matter. The folds of the cerebellum seen over the surface are called folia. The cerebellum has two lateral hemispheres and a midline vermis, which extends anteriorly. The cerebellum is connected to brain stem via superior, middle, and inferior cerebellar peduncles. Lesions of the cerebellum may be manifested clincally by ataxia and tremors.

Thalamus

The thalamus has many grey matter nuclei containing neurons with axons projecting to many areas of cerebral cortex. The medial and lateral geniculate nuclei are part of the thalamus. The lateral geniculate is part of the visual pathway from the eye to the calcarine cortex in occipital lobes. Some thalamic nuclei have motor control functions. Some thalamic nuclei receive sensory input from many sources. Lesions of the thalamus may result in abnormal perceptions of pain.

Basic Motor Functions

Voluntary movement begins in the cerebral cortex. Upper motor neurons including Betz cells and pyramidal cells in the motor cortex of frontal lobes have axons that descend downward, coalescing in the white matter called the corona radiata, and passing down through the posterior limb of the internal capsules. These axons further descend as the corticospinal and corticobulbar tracts (also known as pyramidal tracts) into the brain stem. Collaterals are given off to areas in the thalamus, brain stem, and reticular formation.

Corticospinal tracts descend into the pyramids of the medulla, where about 90% decussate (cross) and descend to the spinal cord as the lateral corticospinal tracts. About 10% do not cross and become the ventral corticospinal tracts. In the ventral (anterior) horns of the spinal cord these tracts synapse with lower motor neurons that have axons exiting via the ventral (anterior) roots of the spinal cord to form nerves that extend to skeletal muscles of the neck, trunk, and extremities.

Corticobulbar tracts descend to the cranial nerve motor nuclei in the brain stem. Some fibers are crossed, some are uncrossed. The cranial nerve nuclei have lower motor neurons sending axons to innervate skeletal muscles of the head.

The cerebellum functions in control of movement by adjusting movement via balance, coordination, eye movement, and muscle tone. The cerebellar connections to the nervous system occur through three peduncles. The inferior cerebellar penducle carries input from spinocerebellar tracts, vestibular nuclei, and olive, providing input about muscle tone, perception of balance, and perception of spatial relations. The middle cerebellar peduncle receives input from pontine nuclei which receive corticopontine fibers from cerebral cortex. The superior cerebellar peduncle is the output path, mainly to the thalamus. The Purkinje cells of the cerebellum have an inhibitory function, with axons extending to a variety of deep cerebellar nuclei, including the dentate nucleus.

The caudate, putamen, and globus pallidus constitute the corpus striatum, which are the major components of the basal ganglia. The caudate and putamen are similar in function and are separated by the anterior limb of the internal capsule. There are connections to globus pallidus, to substantia nigra of midbrain and to thalamus. The globus pallidus consists of an inner and outer portion, with connections to thalamus, including the subthalamic nuclei, and the putamen. The basal ganglia function to selectively enhance or inhibit movement.

Problems with movement can arise via a number of mechanisms. A "stroke" with infarction of cerebral cortex, particularly in the distribution of the middle cerebral artery which supplies the motor regions, can destroy upper motor neurons. Since almost all of the corticospinal tract axons decussate (cross) at the medullary pyramids, right cerebral infarction will result in left hemiparesis. Cerebellar disorders can result in gait and balance problems. Lesions of the basal ganglia, such as a hypertensive hemorrhage, can disrupt the corticospinal and corticobulbar fibers in internal capsule and result in findings similar to an infarction. More specific lesions such as Huntington's disease with atrophy of the caudate produce movement disorders (choreiform movements). A disease such as poliovirus infection that destroys bulbar nuclei and anterior horn cells may also cause paralysis.

Basic Sensory Functions

Sensory nerve fibers receive input from a variety of receptors throughout the body. Sensation to pain, touch, heat, cold, proprioception, movement, and pressure are distributed originates in specialized receptors. The dorsal root ganglia contain the neurons whose axons arrive from receptors in peripheral locations and extend upward in the spinal cord to the gracile and cuneate nuclei of the medulla where they synapse with neurons that send axons upward that decussate (cross) and form the medial lemniscus that extends to the thalamus. In the thalamus, these nerves synapse with neurons that send axons via the posterior limb of the internal capsule to the corona radiata and to the cerebral cortex, mainly the postcentral gyrus.

Cranial nerves provide sensation in the head. Some of the cranial nerves are involved with the senses: olfactory (smell), optic (sight), acoustic (sound), facial-glossopharyngeal-vagus (taste). Other cranial nerves such as the trigeminal conduct sensory fibers that function for the head like the peripheral sensory nerves from spinal cord for the body.

Limbic System

This is a part of the central nervous system composed of cortical structures that have three neuronal layers, and thus is more "primitive" (paleocortex) than the six-layered neocortex of the cerebral hemispheres. The limbic system includes a number of bilateral structures: hippocampus with amygdala anterior to it, mammillary bodies, columns of the fornix, and mammillothalamic tracts. The limbic system is functionally associated with emotional responses and with formation of memories.

The amygdala has connections to the olfactory system. It is involved with primitive emotions. The hippocampus is involved with formation of recent memories, and has important connections to the cingulate gyrus. The hippocampus gives rise to the fornix posteriorly. The fornices extend up and around and forward, then down to the mammillary bodies. Mammillothalamic tracts extend upward from the mammillary bodies to the anterior nuclei of the thalamus.

Reticular Formation

The reticular formation includes a variety of brainstem structures that receive sensory input and have output to many other areas of the brain, including the motor pathways. The all-important respiratory and cardiac centers that maintain breathing and heart rate are included. The general level of wakefulness and alertness is regulated by the reticular formation.

Autonomic Functions

The hypothalamus is involved with autonomic functions. The hypothalamus receives input from brainstem nuclei, from the limbic system, and other sources. It sends axons out to thalamus, to brain stem nuclei, to limbic system, and to spinal cord. Another important output of axons descends to the posterior pituitary gland. Sympathetic and parasympathetic preganglionic nuclei in the spinal cord send axons out to the sympathetic ganglia and to the parasympathetic ganglia.

Ventricular System

The choroid plexus is a highly vascularized outgrowth from the ependymal lining of the lateral ventricle in each cerebral hemisphere and the fourth ventricle. About 0.5 L of cerebrospinal fluid (CSF) is generated from choroid plexus each day. The CSF is an ultrafiltrate of blood plasma and is a clear, colorless, watery fluid. CSF contains about 2/3 the concentration of glucose of the blood plasma. A small amount of protein, mainly IgG, is found in the CSF.

The CSF fills the lateral ventricles, which are deep to the white matter of the cerebral hemispheres. The lateral ventricles have extensions, called horns. There are horns extending forward to frontal lobes, inferiorly and forward to temporal lobes, and posteriorly to occipital lobes. The lateral ventricles connect to the midline third ventricle via the foramina of Monro. The third ventricle drains into the narrow aqueduct of Sylvius that transverses through the dorsal portions of midbrain and pons. This aqueduct opens into the fourth ventricle located above the medulla and below the cerebellum. From the fourth ventricle, CSF flows out a lateral foramen of Luschka on each side of the medulla, as well as a midline foramen of Magendie. Then the CSF flows through the subarachnoid space, over the surface of the brain. Eventually, the CSF is reabsorbed into the arachnoid granulations at the vertex of the hemispheres.

Blockage of CSF flow leads to a condition called hydrocephalus. This can occur for several reasons. If there is intraventricular hemorrhage (IVH) or subarachnoid hemorrhage (SAH), then the flow of CSF can be retarded. A mass such as a tumor, particularly an ependymoma arising from the ependymal lining of the ventricular system, can obstruct CSF flow. Inflammation, as with meningitis, at the base of the brain can result in scarring of the foramina of Luschka or Magendie that blocks CSF flow. Inflammation at the vertex may disrupt the arachnoid granulations and reabsorption of fluid. Congenital problems can result in obstruction to CSF flow. Congenital forking or atresia of the aqueduct of Sylvius can do this, while an abnormal position of posterior fossa contents with the Arnold-Chiari malformation can cause hydrocephalus as well.

CSF can be sampled for diagnostic purposes via a procedure known as lumbar puncture (LP). An LP is performed by inserting a hollow needle between the lower lumbar vertebrae posteriorly, through the dura, and into the subarachnoid space around the cauda equina of the lower spinal cord. An LP is contraindicated if there is increased CSF pressure. The CSF can be assessed for protein, glucose, and cell count. If infection is suspected, then CSF samples may be sent for gram stain, cultures, and serology, depending upon which infectious agents are suspected.

Cerebral Arterial Circulation

The internal carotid artery extends upward after branching from the common carotid artery in the neck on right and left. The internal carotids enter through the skull on each side of the sella turcica and give rise to the anterior part of the circle of Willis at the base of the brain, including the major branches of anterior and middle cerebral arteries. A posterior communicating branch is given off on each side to anastomose with the posterior cerebral artery on each side, completing the anastomosis with the posterior part of the circle of Willis, which is fed by a vertebral artery on both right and left which enter the skull posteriorly. The vertebral arteries give off a posterior inferior cerebellar artery on each side, then join to form the basilar artery, which gives off a right and left anterior inferior cerebellar artery and a right and left superior cerebellar artery before finally bifurcating into the posterior cerebral arteries.

The circle of Willis provides collateral circulation to the cerebral arteries. Blockage of just one, or even two, major vessels from the neck will not disrupt cerebral blood flow, given that the cerebral arteries themselves are not diseased. Cerebral artery lesions at the base of the brain, such as atherosclerotic plaques, are likewise not devastating if collateral flow can be provided by the circle of Willis. Emboli, typically coming from the heart, can travel peripherally to cause ischemia and infarction. since the majority of the blood flow comes from the internal carotids, and the middle cerebral artery is the largest branch, embolic phenomena are most likely to be manifested in the middle cerebral artery circulation. Very small emboli, such as tumor metastases, are likely to travel peripherally to the grey-white junction in cerebral cortex, because that is where arteries branch and narrow significantly.