The Brain

	*Marvels of the Brain*
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The brain is a relatively small organ, weighing about 1,500 grams and constituting about 2 percent of total body weight. Three major divisions of the brain are recognized:

1. the massive, paired cerebral hemispheres {brain_b} (the cerebrum), derived from the telencephalon;

2. the brain stem, from which all true {nervous_system_cranial} cranial nerves emerge, consisting of the thalamus and {hypothalamus} hypothalamus, the {brain_e} midbrain, the {brain_e} pons, and the {brain_e} medulla oblongata (also known as the diencephalon, the mesencephalon, the metencephalon, and the myelencephalon); and

3. the {brain_e} cerebellum (“little brain”), derived from the pons, or metencephalon. The terms telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon denote five distinct embryonic subdivisions.

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The paired cerebral hemispheres are mirror-image duplicates composed of a gray cellular mantle called the cerebral cortex, an underlying mass of white matter composed of myelinated nerve fibers, and collections of subcortical neuronal masses known as the basal ganglia. Each hemisphere receives impulses conveying the senses of touch and vision largely from the contralateral half (that is, the opposite side) of the body, while auditory input comes from both sides. Pathways conveying the senses of smell and taste to the cerebral cortex are ipsilateral (that is, they do not cross to the opposite hemisphere). In turn, each cerebral hemisphere supplies motor function to the opposite side of the body, the side from which it receives sensory input.

In spite of this arrangement, the cerebral hemispheres are not functionally equal. In each individual, one hemisphere is dominant, the dominant hemisphere being concerned with language, mathematical and analytical functions, and handedness. The nondominant hemisphere is concerned with simple spatial concepts, recognition of faces, some aspects of music, and emotion. (For further discussion of cerebral dominance, see {nervous_emotion} Functions of the Human Nervous System: Higher cerebral functions.)

The hemispheres are partially separated from each other by the longitudinal fissure, but in central regions this fissure extends only as deep as a broad interhemispheric commissure called the corpus callosum. It is through the corpus callosum that corresponding regions of the cerebral hemispheres are connected by various nerve projections.

Branching nerve fibers form a dense network in the brain. The large object (to the left) is a cell body with fibers leading out from it. All around are outgrowths from other brain cells.

Each nerve cell in the brain can fire a very small electrical pulse, and does so according to inputs received from other nerve cells. Individual pulses are weak -- the entire brain only creates about 20 watts of power -- but each firing has to be sufficient to energize the next cell, or group of cells. Signal transmission from one cell to another is achieved chemically rather than electrically, with a chemical known as a neurotransmitter released at each nerve ending. The transmitter crosses the synapse (gap) to the next neuron and becomes lodged in a specific receptor site. This then activates a chain of events in the second neuron which causes it to fire and thus pass the impulse along. The passing of an impulse enables us to remember faces, think consciously and behave with intelligence.

The cortical mantle is highly convoluted; the crest of a single convolution is known as a gyrus, while the fissure between two gyri is known as a sulcus. Sulci and gyri form a more or less constant pattern, on the basis of which each cerebral hemisphere is divided into six so-called lobes: (1) frontal, (2) parietal, (3) temporal, (4) occipital, (5) central (or insular), and (6) limbic. Two important sulci located on the lateral aspect (that is, the side surface) of each hemisphere help to distinguish these lobes. The central sulcus separates the frontal and parietal lobes, and the deeper lateral sulcus forms the boundary between the temporal lobe and the frontal and parietal lobes.

The frontal lobe, largest of all the lobes of the brain, lies rostral to the central sulcus (that is, toward the nose from the sulcus). The precentral gyrus, located rostral to the central sulcus, constitutes the primary motor region of the brain; when parts of this gyrus are given electrical stimulation in conscious patients (operated upon under local anesthesia), they produce localized movements on the opposite side of the body that are interpreted by the patients as voluntary. Injury to parts of this gyrus results in paralysis on the contralateral (opposite) half of the body.

Parts of the inferior frontal lobe (close to the lateral sulcus) constitute Broca's area, a region concerned with neural mechanisms that convert thoughts into speech (see {nervous_system_functions} Functions of the human nervous system).

The parietal lobe, posterior to the central sulcus, is divided into three parts: (1) a postcentral gyrus, (2) a superior parietal lobule, and (3) an inferior parietal lobule. The postcentral gyrus receives sensory input, both superficial and deep, from the contralateral half of the body. The sequential representation is the same as in the primary motor area, with sensations from the head area being represented in inferior parts of the gyrus and impulses from the lower extremities represented above. Lesions in the postcentral gyrus result in impaired sensation from cutaneous (surface) and deep parts of the contralateral half of the body. The superior parietal lobule, located caudal to the postcentral gyrus, lies superior to the interparietal sulcus. This lobule is regarded as an association cortex, part of which may be concerned with motor function. The inferior parietal lobule (composed of the angular and supramarginal gyri) is a cortical region concerned with the integration of multiple sensory signals. Lesions in this lobule produce various syndromes of a devastating nature.

In the parietal and frontal lobes, each primary sensory or motor area is close to, or surrounded by, a smaller secondary area (see figure). The primary sensory area receives input only from relay nuclei in the thalamus, while the secondary sensory area receives input from the thalamus, the primary sensory area, or both. The motor regions receive input from the thalamus as well as the sensory areas of the cerebral cortex.

The temporal lobe, inferior to the lateral sulcus, fills the middle fossa of the skull. Near the margin of the lateral sulcus, two transverse temporal gyri constitute the primary auditory area of the brain. Audition is represented here in a tonotopic fashion—that is, with different frequencies represented on different parts of the area. The transverse gyri are surrounded by a less finely tuned secondary auditory area.

A medial, or inner, protrusion near the ventral surface of the temporal lobe, known as the uncus, constitutes a large part of the primary olfactory area. The outer surface of this lobe is an association area made up of the superior, middle, and inferior temporal gyri.

The occipital lobe lies caudal to (that is, below and behind) the parieto-occipital sulcus. As seen on the medial aspect of the hemisphere, this sulcus joins the calcarine sulcus in a Y-shaped formation. Cortex on both banks of the calcarine sulcus constitutes the primary visual area, which receives input from the contralateral visual field via the optic radiation. The visual field is represented near the calcarine sulcus in a retinotopic fashion—that is, with upper quadrants of the visual field laid out along the inferior bank of the sulcus and lower quadrants of the visual field represented on the upper bank. Central vision is represented most caudally and peripheral vision rostrally. Lesions in the calcarine cortex—or in the optic radiation, which projects to it—produce blindness in the contralateral (on the opposite) visual field.

The insular, or central, lobe is an invaginated triangular area on the medial surface of the lateral sulcus; it can be seen in the intact brain only by separating the frontal and parietal lobes from the temporal lobe. Branches of the middle cerebral artery cover the surface of the insula.

The limbic lobe is a synthetic lobe on the medial margin (or limbus) of the hemisphere. Composed of adjacent portions of the frontal, parietal, and temporal lobes that surround the corpus callosum, it is concerned with visceral, autonomic, and related somatic behavioral activities. This region of the cerebral cortex receives inputs from thalamic nuclei that are connected with parts of the hypothalamus and with the hippocampal formation, a primitive cortical structure within the inferior horn of the lateral ventricle.

Beneath the cerebral cortex is a mass of white matter, which is composed of nerve fibers projecting to and from the cerebral cortex, commissural systems connecting the two hemispheres via the corpus callosum, and association fibers connecting different regions of a single hemisphere. Myelinated fibers projecting to and from the cerebral cortex form a concentrated fan-shaped band, known as the internal capsule. In horizontal sections of the brain, the internal capsule can be seen to consist of two parts: (1) an anterior limb, between the caudate nucleus and the putamen; and (2) a larger posterior limb, running between the thalamus and the globus pallidus and putamen. These two limbs form an obtuse angle with the apex directed toward the centre of the brain; the junction is called the genu.

Deep

within the white matter are fluid-filled cavities that form the ventricular system. These cavities include a pair of C-shaped lateral ventricles with anterior, inferior, and posterior “horns” protruding into the frontal, temporal, and occipital lobes, respectively. Most of the cerebrospinal fluid is produced in the ventricles. About 70 percent of the fluid produced by the central nervous system is secreted by the choroid plexus, a collection of blood vessels in the walls of the lateral ventricles. The fluid drains via interventricular foramina, or openings, into a slitlike third ventricle, which, situated along the midline of the brain, separates the symmetrical halves of the thalamus and {hypothalamus} hypothalamus. From there it passes through the cerebral aqueduct in the midbrain and into the fourth ventricle in the hindbrain. Openings in the fourth ventricle permit cerebrospinal fluid to enter so-called {nervous_system_meninges} subarachnoid spaces surrounding both brain and spinal cord.

Deep within the cerebral hemispheres, large gray masses or nerve cells, called nuclei, form components of the basal ganglia. Four nuclei can be distinguished:

(1) the caudate nucleus,
(2) the putamen,
(3) the globus pallidus, and
(4) the amygdala.

The amygdala is the oldest of the basal ganglia and is therefore often referred to as the archistriatum; the globus pallidus is known as the paleostriatum, and the caudate nucleus and putamen are together known as the neostriatum, or simply striatum. The putamen and the adjacent globus pallidus are referred to descriptively as the lentiform nucleus, while the caudate nucleus, putamen, and globus pallidus form the corpus striatum.

The putamen lies deep within the cortex of the insular lobe, while the caudate nucleus has a C-shaped configuration that parallels the lateral ventricle. The head of the caudate nucleus protrudes into the anterior horn of the lateral ventricle, the body lies above and lateral to the thalamus, and the tail is in the roof of the inferior horn of the lateral ventricle. The tail of the caudate nucleus ends in relationship to the amygdaloid nuclear complex, which lies in the temporal lobe beneath the cortex of the uncus.

There is an enormous number of neurons within the caudate nucleus and putamen; these are of two basic types, spiny and aspiny. Spiny striatal neurons are medium-size cells with radiating {nerves} dendrites that are studded with spines. Axons of these cells project beyond the boundaries of the neostriatum. All afferent systems entering the neostriatum terminate upon the dendritic spines of spiny striatal neurons, and all output is via axons of the same neurons. Chemically, spiny striatal neurons are heterogeneous—that is, most contain more than one neurotransmitter. Neurotransmitters identified in spiny striatal neurons are gamma-aminobutyric acid (GABA), substance P, and enkephalin, with overwhelming dominance by GABA.

Aspiny striatal neurons have smooth dendrites and short axons confined to the caudate nucleus or putamen. Small aspiny striatal neurons secrete GABA, neuropeptide Y, somatostatin, or some combination of these. The largest aspiny neurons are evenly distributed cholinergic neurons that play a crucial role in maintaining the balance of dopamine and GABA.

Because the caudate nucleus and putamen receive varied and diverse inputs from multiple sources that utilize different neurotransmitters, they are regarded as the receptive component of the corpus striatum. The most massive input originates from virtually all regions of the cerebral cortex, with the connecting corticostriate fibers containing the excitatory neurotransmitter glutamate. In addition, afferent fibers originating from the {nervous_system_functions_pg3} substantia nigra in the midbrain or from intralaminar thalamic nuclei in the diencephalon project to the caudate nucleus or the putamen. The neurotransmitter secreted by thalamostriate neurons has not been identified, while neurons from the substantia nigra synthesize dopamine. All striatal afferent systems terminate in patchy arrays referred to as strisomes; areas not receiving terminals are called the matrix. Striatal efferent systems—that is, spiny neurons containing GABA, substance P, and enkephalin—project in a specific pattern onto the globus pallidus and the substantia nigra; GABA, an inhibitory neurotransmitter, is dominant in all neostriatal projections.

The globus pallidus, consisting of two cytologically similar wedge-shaped segments, the lateral and the medial, lies between the putamen and the internal capsule. Fibers terminating on the pallidum arise mostly from the caudate nucleus and putamen; these so-called striatopallidal fibers converge on the globus pallidus like spokes of a wheel. Both segments of the pallidum receive GABAergic terminals, but in addition the medial segment receives substance P fibers, and the lateral segment receives enkephalinergic projections. The output of the entire corpus striatum (i.e., the caudate nucleus, putamen, and globus pallidus together) arises from GABAergic cells in the medial pallidal segment and in the substantia nigra, both of which receive fibers from the striatum. GABAergic cells in the medial pallidal segment and the substantia nigra project to different nuclei in the thalamus; these in turn influence distinct regions of the cortex concerned with motor function. The lateral segment of the globus pallidus, on the other hand, projects almost exclusively to the subthalamic nucleus (in the ventral thalamus), from which it receives a reciprocal input. No part of the corpus striatum projects fibers to spinal levels.

Pathological processes involving the corpus striatum and related nuclei are associated with a variety of specific syndromes characterized by abnormal involuntary movements (collectively referred to as dyskinesia) and significant alterations of muscle tone. {parkinson} Parkinson's disease and Huntington's disease are among the more prevalent syndromes; each appears related to deficiencies in the synthesis of particular neurotransmitters.

The thalamus has long been regarded as the key to understanding the organization of the central nervous system. It is involved in the relay and distribution of most, but not all, sensory and motor signals to specific regions of the cerebral cortex. Sensory signals generated in all types of receptors are projected by complex pathways to specific relay nuclei in the thalamus, where they are segregated and systematically organized. The relay nuclei in turn supply the primary and secondary sensory areas of the cerebral cortex. Sensory input to thalamic nuclei is crossed for the somesthetic and visual systems, bilateral (but mainly crossed) for the auditory system, and ipsilateral (on the same side) for gustatory (taste) and olfactory (smell) sense.

The somesthetic relay nuclei of the thalamus, collectively known as the ventrobasal complex, receive input from the medial lemniscus (originating in the medulla), from spinothalamic tracts, and from the trigeminal nerve. Fibers within these ascending tracts that terminate in the central core of the ventrobasal complex receive input from deep sensory receptors, while fibers projecting onto the outer shell receive input from cutaneous receptors. This segregation of deep and superficial sensation is preserved in projections of the ventrobasal complex to the primary somesthetic (i.e., sensory) area of the cerebral cortex.

The {anat_terminology} medial and lateral geniculate bodies form what is called the metathalamus. Fibers of the optic nerve end in the lateral geniculate body, which consists of six cellular laminae, or layers, folded into a horseshoe configuration. Each lamina represents a complete map of the contralateral visual hemifield, and all laminae are in perfect registration. Cells in all layers of the lateral geniculate body project via the optic radiation to the visual areas of the cerebral cortex. The medial geniculate body receives auditory impulses from the inferior colliculus of the midbrain and relays them to the auditory areas on the temporal lobe. Only the ventral nucleus of the medial geniculate body is laminated and tonotopically organized; this part projects to the primary auditory area and is finely tuned. Other subdivisions of the medial geniculate body project to the belt of secondary auditory cortex surrounding the primary area.

Major output from the cerebellum projects to specific thalamic relay nuclei in a pattern similar to that for somesthetic input. The thalamic relay nuclei in turn provide a major input to the {nervous_system_functions_pg2} primary motor area of the frontal lobe. This large system appears to provide coordinating and controlling influences that result in the appropriate force, sequence, and direction of voluntary motor activities. Output from the corpus striatum, on the other hand, is relayed by thalamic nuclei that have access to the supplementary and premotor areas. The {nervous_system_functions_pg2} supplementary motor area, located on the medial aspect of the hemisphere (see figure to left), exerts modifying influences upon the primary motor area and appears to be involved in programming skilled motor sequences. The premotor area, rostral to the primary motor area, plays a role in sensorially guided movements.

Other major thalamic nuclei, besides those involved in relaying sensory impulses or controlling influences from the cerebellum and corpus striatum, include the anterior nuclear group, the mediodorsal nucleus, and the pulvinar. The anterior nuclear group receives input from the hypothalamus and projects upon parts of the limbic lobe (i.e., the cingulate gyrus). The mediodorsal nucleus, part of the medial nuclear group, has reciprocal connections with large parts of the frontal lobe rostral to the motor areas. The pulvinar is a huge posterior nuclear complex that, along with the mediodorsal nucleus, has projections to association areas of the cortex.

Output ascending from the reticular formation of the brain stem is relayed to the cerebral cortex by intralaminar thalamic nuclei, which lie in laminae separating the medial and ventrolateral thalamic nuclei. This ascending system is concerned with arousal mechanisms, maintaining alertness, and directing attention to sensory events.

The amygdala, (which means “almond-shaped”), controls our aggression and emotions. Many autistic individuals are aggressive towards themselves or others, or conversely, extremely passive. Furthermore, autistic children and adults often appear emotionless or ‘flat’ (even though they obviously do have emotions). Experimenters have also shown that when the amygdala is removed or damaged, animals exhibit behaviors similar to autistic individuals, such as social withdrawal, compulsive behaviors, failure to learn about dangerous situations, difficulty retrieving information from memory, and difficulty adjusting to novel events or situations. In addition, the amygdala is responsive to a variety of sensory stimuli, such as sounds, sights, and smells; as well as emotionally or fear-related stimuli. We know that autistic individuals often have problems with each of these senses. Interestingly, Georgie, whose childhood was described in her mother’s book, The Sound of a Miracle, often mentioned being afraid of many sounds prior to receiving auditory integration training from Dr. Guy Berard.

The hippocampus, (shaped like a “sea horse”) appears to be primarily responsible for learning and memory. Damage or removal of the hippocampus will lead to an inability to store new information into memory. This sounds similar to Dr. Bernard Rimland's cognitive theory of autism. In his 1964 award-winning book Infantile Autism, Dr. Rimland theorized that autistic children had difficulty relating new information to previously stored information. In addition, when the hippocampus is damaged or removed, animals will display stereotypic, self-stimulatory behaviors and hyperactivity.

The diencephalon consists of a pair of egg-shaped nuclear masses that lie on each side of the third ventricle and medial to the posterior limb of the internal capsule. Four subdivisions are recognized: (1) the epithalamus, (2) the thalamus, (3) the {hypothalamus} hypothalamus, and (4) the ventral thalamus, or subthalamus.

The epithalamus is represented mainly by the {pineal_gland} pineal gland, which lies in the midline posterior and dorsal to the third ventricle. This gland synthesizes melatonin and enzymes sensitive to diurnal light. Rhythmic changes in its activity in response to cyclical photic input suggest that the gland serves as a biological clock. With age it tends to accumulate calcium deposits.

The ventral thalamus is represented mainly by the subthalamic nucleus, a lens-shaped structure lying behind and to the sides of the hypothalamus and on the dorsal surface of the internal capsule. The subthalamic region is traversed by fibers related to the globus pallidus. Discrete lesions in the subthalamic nucleus produce hemiballism, the most violent form of dyskinesia known.

Below the diencephalon are the midbrain (or mesencephalon) and the hindbrain (or rhombencephalon). The hindbrain consists of the pons (or metencephalon) and the medulla oblongata (or myelencephalon). These parts of the brain stem are characterized by three main features: (1) a roof plate superior to the cerebral aqueduct and fourth ventricle; (2) a central core of gray matter, known as the reticular formation; and (3) a massive basal collection of fibers descending from the cerebral cortex to the brain stem and spinal cord.

The roof plate of the midbrain is formed by two paired rounded eminences, the superior and inferior colliculi. The superior colliculus receives input from the retina and the visual cortex and participates in a variety of visual reflexes, particularly the tracking of objects in the contralateral visual field. The inferior colliculus receives both crossed and uncrossed auditory fibers and projects upon the medial geniculate body, the auditory relay nucleus of the thalamus (see above Diencephalon). In the hindbrain the roof plate is formed by the cerebellum and a membrane containing the choroid plexus of the fourth ventricle.

The reticular formation contains a core collection of cells of various sizes that project to the thalamus, the cerebellum, and the spinal cord. Surrounding this core are long ascending and descending tracts in which various cranial nerve nuclei are embedded.

Fibers derived from the cerebral cortex lie on or near the ventral surface of the midbrain, pons, and medulla. At the midbrain they gather into two bundles called the crura cerebri; from there they descend into the pons, where most terminate upon cell nuclei that project into the cerebellum. These constitute the corticopontine tract. The other major tract, called the corticospinal tract, forms the medullary pyramids before descending to the spinal cord.

The midbrain contains the nuclear complex of the {nervous_system_cranial_b} oculomotor nerve as well as the {nervous_system_cranial_b} trochlear nucleus; these cranial nerves innervate muscles that move the {eyes} eye and control the shape of the lens and the diameter of the pupil. In addition, between the midbrain reticular formation (known here as the tegmentum) and the crus cerebri is a large, pigmented structure called the substantia nigra. This nucleus consists of two parts, the pars reticulata and the pars compacta. Cells of the pars compacta contain the black pigment melanin; these synthesize dopamine and project to cells of either the caudate nucleus or the putamen but not to both. By exercising an inhibitory action on large aspiny cholinergic neurons in the neostriatum, the dopaminergic cells of the pars compacta influence the output of the neurotransmitter GABA from spiny striatal neurons. These spiny neurons in turn project to cells of the pars reticulata, which, by projecting fibers to the thalamus, are in effect part of the output system of the corpus striatum.

At the caudal midbrain, crossed fibers of the superior cerebellar peduncle (the major output system of the cerebellum) surround and partially terminate in a large, centrally located structure known as the red nucleus. Most crossed ascending fibers of this bundle project to thalamic nuclei, which have access to the primary motor cortex. A smaller number of fibers synapse on large cells in caudal regions of the red nucleus; these give rise to the crossed fibers of the rubrospinal tract (see below {nervous_spinal_cord} The spinal cord: Descending spinal tracts).

The Pons

The pons consists of two parts: the tegmentum, a phylogenetically older part that contains the reticular formation; and the pontine nuclei, a larger part composed of masses of neurons that lie among large bundles of longitudinal and transverse fibers.

Fibers originating from neurons in all major lobes of the cerebral cortex terminate upon the pontine nuclei, which in turn project to the opposite cerebellar hemisphere. These massive crossed fibers form the middle cerebellar peduncle—in effect serving as the bridge that connects each cerebral hemisphere with the opposite half of the cerebellum.

The reticular formation in the pontine tegmentum contains multiple cell groups that exert facilitating influences upon motor function. It also contains the nuclei of several cranial nerves. The facial nerve and the two components of the vestibulocochlear nerve, for example, emerge from and enter the brain stem at the junction of the pons, medulla, and cerebellum. Motor nuclei for the trigeminal nerve lie in the upper pons. Located on the periphery of the pons are long ascending and descending tracts that connect the brain to the spinal cord.

The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping.

It is the most caudal segment of the brain stem, and appears as a conical expansion of the spinal cord. Both the pons and the medulla are separated from the overlying cerebellum by the fourth ventricle, and cerebrospinal fluid entering the fourth ventricle from the cerebral aqueduct passes into the cisterna magna, a subarachnoid space surrounding the medulla and the cerebellum, via foramina in the lateral recesses and in the midline of the ventricle.

At the transition from the medulla to the spinal cord, there are two major decussations, or crossings, of nerve fibers. The corticospinal decussation is the site at which 90 percent of the fibers of the medullary pyramid cross and enter the dorsolateral funiculus of the spinal cord. Signals conveyed by this tract provide the basis for voluntary motor function on the opposite side of the body (see below {nervous_spinal_cord} The spinal cord: Descending spinal tracts). In the other decussation, sensory fibers ascending in the fasciculus gracilis and fasciculus cuneatus of the spinal cord terminate upon large nuclear masses on the dorsal surface of the medulla. Known as the nuclei gracilis and cuneatus, these masses give rise to fibers that decussate above the corticospinal tract and form a major ascending sensory pathway known as the medial lemniscus. Present at all brain-stem levels, the medial lemniscus projects upon the somesthetic relay nuclei of the thalamus.

The medulla contains nuclei associated with the hypoglossal, accessory, vagus, and glossopharyngeal cranial nerves. In addition, it contains portions of the vestibular nuclear complex, parts of the trigeminal nuclear complex concerned with pain and thermal sense, and solitary nuclei related to the vagus, glossopharyngeal, and facial nerves that subserve the sense of taste.

Of several medullary relay nuclei that project to the cerebellum via the inferior cerebellar peduncle, the largest is the inferior olive.

The cerebellum is the second largest part of the brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates balance.

It overlies the posterior aspect of the pons and medulla and fills the greater part of the back portion of the skull. It consists of two paired lobes, or hemispheres, and a midline portion known as the vermis. Cerebellar cortex appears very different from cerebral cortex in that it consists of small, leaflike laminae, referred to as folia. Structurally the cerebellum consists of a three-layered, gray cellular mantle called the cerebellar cortex and a core of white matter containing four paired intrinsic (i.e., deep) nuclei, the dentate, globose, emboliform, and fastigial. Three paired fiber bundles—the superior, middle, and inferior peduncles—connect the cerebellum with the midbrain, pons, and medulla, respectively.

On an embryological basis the cerebellum can be divided into three parts: (1) the archicerebellum, related primarily to the vestibular system; (2) the paleocerebellum, or anterior lobe, concerned with control of muscle tone; and (3) the neocerebellum, known as the posterior lobe. Receiving input from the cerebral hemispheres via the middle cerebellar peduncle, the neocerebellum is the part most concerned with coordination of voluntary motor function.

The three layers of the cerebellar cortex are an outer synaptic layer (also called the molecular layer), an intermediate discharge layer (the Purkinje layer), and an inner receptive layer (the granular layer). Sensory input from all sorts of receptors are conveyed to specific regions of the receptive layer, which consists of enormous numbers of small nerve cells (hence the name granular) that project axons into the synaptic layer. There they excite the dendrites of the Purkinje cells, which in turn project axons to portions of the four intrinsic nuclei and upon dorsal portions of the lateral vestibular nucleus. Because most Purkinje cells are GABAergic and therefore exert strong inhibitory influences upon the cells that receive their terminals, all sensory input into the cerebellum results in inhibitory impulses being exerted upon the deep cerebellar nuclei and parts of the vestibular nucleus. Cells of all deep cerebellar nuclei, on the other hand, are excitatory (secreting the neurotransmitter glutamate) and project upon parts of the thalamus, red nucleus, vestibular nuclei, and reticular formation.

The cerebellum thus functions as a kind of computer, providing a quick and clear response to any set of sensory signals. It plays no role in sensory perception, but it exerts profound influences upon equilibrium, muscle tone, and the coordination of voluntary motor function. Lesions of the cerebellum produce a constellation of disturbances, including intention tremor, ataxia, hypotonus, easy fatigability, and disturbances of speech.