Midbrain Health Dictionary

A stalk of nerve tissue that forms the lowest part of the brain and links with the spinal cord. The brainstem acts partly as a highway for messages travelling between other parts of the brain and spinal cord. It also connects with 10 of the 12 pairs of cranial nerves (which emerge directly from the underside of the brain) and controls basic functions such as breathing, vomiting, and eye reflexes. Brainstem activities are below the level of consciousness, and they operate mainly on an automatic basis.

The brainstem is composed of 3 main parts: the midbrain, pons, and medulla. The midbrain contains the nuclei (nervecell centres) of the 3rd and 4th cranial nerves. It also contains cell groups involved in smooth coordination of limb movements. The pons contains nerve fibres that connect with the cerebellum. It also houses the nuclei for the 5th–8th cranial nerves. The medulla contains the nuclei of the 9th–12th cranial nerves. It also contains the “vital centres” (groups of nerve cells that regulate the heartbeat, breathing, blood pressure, and digestion (information on which is relayed via the 10th cranial nerve (see vagus nerve). Nerve-cell groups in the brainstem, known collectively as the reticular formation, alert the higher brain centres to sensory stimuli that may require a conscious response. Our sleep/wake cycle is controlled by the reticular formation.

The brainstem is susceptible to the same disorders that afflict the rest of the central nervous system (see brain, disorders of). Damage to the medulla’s vital centres is rapidly fatal; damage to the reticular formation may cause coma. Damage to specific cranial nerve nuclei can sometimes lead to specific effects. For example, damage to the 7th cranial nerve (the facial nerve) leads to facial palsy. Degeneration of the substantia nigra in the midbrain is thought to be a cause of Parkinson’s disease.... brainstem

CEREBRUM This forms nearly 70 per cent of the brain and consists of two cerebral hemispheres which occupy the entire vault of the cranium and are incompletely separated from one another by a deep mid-line cleft, the longitudinal cerebral ?ssure. At the bottom of this cleft the two hemispheres are united by a thick band of some 200 million crossing nerve ?bres

– the corpus callosum. Other clefts or ?ssures (sulci) make deep impressions, dividing the cerebrum into lobes. The lobes of the cerebrum are the frontal lobe in the forehead region, the parietal lobe on the side and upper part of the brain, the occipital lobe to the back, and the temporal lobe lying just above the region of the ear. The outer 3 mm of the cerebrum is called the cortex, which consists of grey matter with the nerve cells arranged in six layers. This region is concerned with conscious thought, sensation and movement, operating in a similar manner to the more primitive areas of the brain except that incoming information is subject to much greater analysis.

Numbers of shallower infoldings of the surface, called furrows or sulci, separate raised areas called convolutions or gyri. In the deeper part, the white matter consists of nerve ?bres connecting di?erent parts of the surface and passing down to the lower parts of the brain. Among the white matter lie several rounded masses of grey matter, the lentiform and caudate nuclei. In the centre of each cerebral hemisphere is an irregular cavity, the lateral ventricle, each of which communicates with that on the other side and behind with the third ventricle through a small opening, the inter-ventricular foramen, or foramen of Monro.

BASAL NUCLEI Two large masses of grey matter embedded in the base of the cerebral hemispheres in humans, but forming the chief part of the brain in many animals. Between these masses lies the third ventricle, from which the infundibulum, a funnel-shaped process, projects downwards into the pituitary body, and above lies the PINEAL GLAND. This region includes the important HYPOTHALAMUS.

MID-BRAIN or mesencephalon: a stalk about 20 mm long connecting the cerebrum with the hind-brain. Down its centre lies a tube, the cerebral aqueduct, or aqueduct of Sylvius, connecting the third and fourth ventricles. Above this aqueduct lie the corpora quadrigemina, and beneath it are the crura cerebri, strong bands of white matter in which important nerve ?bres pass downwards from the cerebrum. The pineal gland is sited on the upper part of the midbrain.

PONS A mass of nerve ?bres, some of which run crosswise and others are the continuation of the crura cerebri downwards.

CEREBELLUM This lies towards the back, underneath the occipital lobes of the cerebrum.

MEDULLA OBLONGATA The lowest part of the brain, in structure resembling the spinal cord, with white matter on the surface and grey matter in its interior. This is continuous through the large opening in the skull, the foramen magnum, with the spinal cord. Between the medulla, pons, and cerebellum lies the fourth ventricle of the brain.

Structure The grey matter consists mainly of billions of neurones (see NEURON(E)) in which all the activities of the brain begin. These cells vary considerably in size and shape in di?erent parts of the brain, though all give o? a number of processes, some of which form nerve ?bres. The cells in the cortex of the cerebral hemispheres, for example, are very numerous, being set in layers ?ve or six deep. In shape these cells are pyramidal, giving o? processes from the apex, from the centre of the base, and from various projections elsewhere on the cell. The grey matter is everywhere penetrated by a rich supply of blood vessels, and the nerve cells and blood vessels are supported in a ?ne network of ?bres known as neuroglia.

The white matter consists of nerve ?bres, each of which is attached, at one end, to a cell in the grey matter, while at the other end it splits up into a tree-like structure around another cell in another part of the grey matter in the brain or spinal cord. The ?bres have insulating sheaths of a fatty material which, in the mass, gives the white matter its colour; they convey messages from one part of the brain to the other (association ?bres), or, grouped into bundles, leave the brain as nerves, or pass down into the spinal cord where they end near, and exert a control upon, cells from which in turn spring the nerves to the body.

Both grey and white matter are bound together by a network of cells called GLIA which make up 60 per cent of the brain’s weight. These have traditionally been seen as simple structures whose main function was to glue the constituents of the brain together. Recent research, however, suggests that glia are vital for growing synapses between the neurons as they trigger these cells to communicate with each other. So they probably participate in the task of laying down memories, for which synapses are an essential key. The research points to the likelihood that glial cells are as complex as neurons, functioning biochemically in a similar way. Glial cells also absorb potassium pumped out by active neurons and prevent levels of GLUTAMATE – the most common chemical messenger in the brain – from becoming too high.

The general arrangement of ?bres can be best understood by describing the course of a motor nerve-?bre. Arising in a cell on the surface in front of the central sulcus, such a ?bre passes inwards towards the centre of the cerebral hemisphere, the collected mass of ?bres as they lie between the lentiform nucleus and optic thalamus being known as the internal capsule. Hence the ?bre passes down through the crus cerebri, giving o? various small connecting ?bres as it passes downwards. After passing through the pons it reaches the medulla, and at this point crosses to the opposite side (decussation of the pyramids). Entering the spinal cord, it passes downwards to end ?nally in a series of branches (arborisation) which meet and touch (synapse) similar branches from one or more of the cells in the grey matter of the cord (see SPINAL CORD).

BLOOD VESSELS Four vessels carry blood to the brain: two internal carotid arteries in front, and two vertebral arteries behind. These communicate to form a circle (circle of Willis) inside the skull, so that if one is blocked, the others, by dilating, take its place. The chief branch of the internal carotid artery on each side is the middle cerebral, and this gives o? a small but very important branch which pierces the base of the brain and supplies the region of the internal capsule with blood. The chief importance of this vessel lies in the fact that the blood in it is under especially high pressure, owing to its close connection with the carotid artery, so that haemorrhage from it is liable to occur and thus give rise to stroke. Two veins, the internal cerebral veins, bring the blood away from the interior of the brain, but most of the small veins come to the surface and open into large venous sinuses, which run in grooves in the skull, and ?nally pass their blood into the internal jugular vein that accompanies the carotid artery on each side of the neck.

MEMBRANES The brain is separated from the skull by three membranes: the dura mater, a thick ?brous membrane; the arachnoid mater, a more delicate structure; and the pia mater, adhering to the surface of the brain and containing the blood vessels which nourish it. Between each pair is a space containing ?uid on which the brain ?oats as on a water-bed. The ?uid beneath the arachnoid membrane mixes with that inside the ventricles through a small opening in the fourth ventricle, called the median aperture, or foramen of Magendie.

These ?uid arrangements have a great in?uence in preserving the brain from injury.... divisions

That part of the AUTONOMIC NERVOUS SYSTEM which is connected with the BRAIN and SPINAL CORD through certain nerve centres in the midbrain, medulla, and lower end of the cord. The nerves from these centres are carried in the third, seventh, ninth and tenth cranial nerves and the second, third and fourth sacral nerves. The action of the parasympathetic system is usually antagonistic to that of the sympathetic system. Thus it inhibits the action of the HEART and augments the action of the INTESTINE; whereas the sympathetic augments the action of the heart and inhibits that of the intestine. (See diagram of sympathetic and parasympathetic nervous systems under NERVOUS SYSTEM.)... parasympathetic nervous system

A small reddish structure, 10 mm in length and shaped somewhat like a pine cone (hence its name), situated on the upper part of the midbrain (see BRAIN). Many theories have been expounded as to its function, but there is increasing evidence that, in some animals at least, it is affected by light and plays a part in hibernation and in controlling sexual activity and the colour of the skin. This it seems to do by means of a substance it produces known as MELATONIN. There is also growing evidence that it may play a part in controlling the circadian rhythms of the body – the natural variations in physiological activities throughout the 24-hour day.... pineal gland

The 4th cranial nerve.

The 2 trochlear nerves arise in the brainstem, 1 on each side of the midbrain, and enter the eye sockets through gaps in the skull bones.

Each trochlear nerve controls 1 of the 2 superior oblique muscles, which rotate the eyes downwards and outwards.... trochlear nerve

one of the two swellings that develop in the roof of the midbrain during its development in the embryo.... bigeminal body

n. the enlarged and highly developed mass of nervous tissue that forms the upper end of the *central nervous system (see illustration). The average adult human brain weighs about 1400 g (approximately 2% of total body weight) and is continuous below with the spinal cord. It is invested by three connective tissue membranes, the *meninges, and floats in *cerebrospinal fluid within the rigid casing formed by the bones of the skull. The brain is divided into the hindbrain (rhombencephalon), consisting of the *medulla oblongata, *pons, and *cerebellum; the *midbrain (mesencephalon); and the forebrain (prosencephalon), subdivided into the *cerebrum and the *diencephalon (including the *thalamus and *hypothalamus). The brain is usually considered to be the site of the working of the mind, but to what extent the concepts of ‘brain’ and ‘mind’ are interchangeable is a matter of debate and of concern to anyone facing brain surgery. Anatomical name: encephalon.... brain

adj. (in electroencephalography) describing discharges that can be recorded synchronously from all parts of the brain. The source of this activity is in the *reticular formation of the midbrain. Centrencephalic epilepsy is associated with a congenital predisposition to seizures.... centrencephalic

n. (pl. colliculi) a small protuberance or swelling. Two pairs of colliculi, the superior and inferior colliculi, protrude from the roof of the midbrain (see tectum).... colliculus

n. a ribbon-like tract of nerve tissue conveying information from the spinal cord and brainstem upwards through the midbrain to the higher centres. On each side a medial lemniscus acts as a pathway from the spinal cord, while an outer lateral lemniscus commences higher up and is mainly concerned with hearing.... lemniscus

n. see midbrain.... mesencephalon

a tract of *motor neurons that extends from the midbrain down to different levels in the spinal cord, carrying impulses that have travelled from the cerebral and cerebellar cortex via the nucleus ruber (red nucleus). The tract plays an important part in the control of skilled and dextrous movements.... rubrospinal tract

n. the largest part of the hindbrain, bulging back behind the pons and the medulla oblongata and overhung by the occipital lobes of the cerebrum. Like the cerebrum, it has an outer grey cortex and a core of white matter. Three broad bands of nerve fibres – the inferior, middle, and superior cerebellar peduncles – connect it to the medulla, the pons, and the midbrain respectively. It has two hemispheres, one on each side of the central region (the vermis), and its surface is thrown into thin folds called folia (see illustration). Within lie four pairs of nuclei.

The cerebellum is essential for the maintenance of muscle tone, balance, and the synchronization of activity in groups of muscles under voluntary control, converting muscular contractions into smooth coordinated movement. It does not, however, initiate movement and plays no part in the perception of conscious sensations or in intelligence. —cerebellar adj.... cerebellum

n. the thickest part of the *corpus callosum, rounded and protruding backwards over the thalami, the pineal gland, and the midbrain.... splenium

a tract that conveys nerve impulses from the midbrain, across the midline as it descends, to the spinal cord in the cervical (neck) region. It contains important *motor neurons.... tectospinal tract

n. the roof of the *midbrain, behind and above the *cerebral aqueduct. From the nerve tissue protrude two pairs of rounded swellings called the superior and inferior colliculi, which contain cells concerned with reflexes involving vision and hearing, respectively.

teeth pl. n. see tooth.... tectum

n. the region of the *midbrain below and in front of the *cerebral aqueduct. It contains the nuclei of several cranial nerves, the *reticular formation, and other ascending and descending nerve pathways linking the forebrain and the spinal cord.... tegmentum