Thalamus Health Dictionary

That part of the fore-brain situated beneath and linked with the THALAMUS on each side and forming the ?oor of the third ventricle (see BRAIN). Also linked to the PITUITARY GLAND beneath it, the hypothalamus contains collections of nerve cells believed to form the controlling centres of (1) the sympathetic and (2) the parasympathetic nervous systems (see under NERVOUS SYSTEM). The hypothalamus is the nervous centre for primitive physical and emotional behaviour. It contains nerve centres for the regulation of certain vital processes: the metabolism of fat, carbohydrate and water; sleep; body temperature and sexual functions.... hypothalamus

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

The sense by which certain characteristics of objects, such as their size, shape, temperature, and surface texture, can be ascertained by physical contact.

The skin has many types of touch receptors, including Merkel’s discs and Meissner’s corpuscles to detect light touch, and Pacinian corpuscles to sense deep pressure and vibration. Signals from these receptors pass, via sensory nerves, to the spinal cord, from there to the thalamus in the brain, and on to the sensory cortex, where touch sensations are perceived and interpreted.

The various parts of the body differ in their sensitivity to painful stimuli and in touch discrimination. For example, the

’.

cornea is several hundred times more sensitive to painful stimuli than the soles of the feet. (See also sensation.)... touch

several large masses of grey matter embedded deep within the white matter of the *cerebrum (see illustration). They include the caudate and lenticular nuclei (together known as the corpus striatum) and the amygdaloid nucleus. The lenticular nucleus consists of the putamen and globus pallidus. The basal ganglia have complex neural connections with both the cerebral cortex and thalamus: they are involved with the regulation of voluntary movements at a subconscious level. Diseases of the basal ganglia cause a range of disorders predominantly affecting movement, the commonest being *parkinsonism.... basal ganglia

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

n. an anatomical division of the forebrain, consisting of the epithalamus, thalamus (dorsal thalamus), hypothalamus, and ventral thalamus (subthalamus). See brain.... diencephalon

the system of nerve tracts and pathways connecting the cerebral cortex, basal ganglia, thalamus, cerebellum, reticular formation, and spinal neurons in complex circuits not included in the *pyramidal system. The extrapyramidal system is mainly concerned with the regulation of stereotyped reflex muscular movements.... extrapyramidal system

n. the surgical operation of interrupting the pathways of white nerve fibres within the brain: it was formerly the most common procedure in *psychosurgery. In the original form, prefrontal leucotomy (lobotomy), the operation involved cutting through the nerve fibres connecting the *frontal lobe with the *thalamus and the association fibres of the frontal lobe. This was often successful in reducing severe emotional tension. However, prefrontal leucotomy had serious side-effects and the procedure has now been abandoned.

Modern procedures use *stereotaxy and make selective lesions in smaller areas of the brain. Side-effects are uncommon and the operation is very occasionally (only a few each year in the UK) used for intractable pain, severe depression, obsessive–compulsive disorder, and chronic anxiety, where very severe emotional tension has not been relieved by other treatments.... leucotomy

n. a part of the *thalamus consisting of two nuclei through which impulses pass from the eyes and ears to be distributed to the cerebral cortex.... metathalamus

n. the anterior and central part of the *thalamus, older in evolutionary terms than the lateral part, the neothalamus, which is well developed in apes and humans.... palaeothalamus

n. 1. (pons Varolii) the part of the *brainstem that links the medulla oblongata and the thalamus, bulging forwards in front of the cerebellum, from which it is separated by the fourth ventricle. It contains numerous nerve tracts between the cerebral cortex and the spinal cord and several nuclei of grey matter. From its front surface the *trigeminal nerves emerge. 2. any portion of tissue that joins two parts of an organ.... pons

n. the expanded posterior end of the *thalamus.... pulvinar

n. (pl. striae) (in anatomy) a streak, line, or thin band. The striae gravidarum (stretch marks) are the lines that appear on the skin of the abdomen of pregnant women, due to excessive stretching of the elastic fibres. Red or purple during pregnancy, they become white after delivery. The stria terminalis is a white band that separates the thalamus from the ventricular surface of the caudate nucleus in the brain.... stria

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (International Association for the Study of Pain, 1979). Pain is perceived in the cerebral cortex (see BRAIN) and is always subjective. Sometimes sensations that would usually be benign can be perceived as painful – for example, allodynia (extreme tenderness of the skin) or dysaesthesia (unpleasant skin sensations resulting from partial damage to sensory nerve ?bres, as in herpes zoster, or shingles).

Acute pain is caused by internal or external injury or disease. It warns the individual that harm or damage is occurring and stimulates them to take avoiding or protective action. With e?ective treatment of disease or injury and/or the natural healing process, the pain resolves – although some acute pain syndromes may develop into chronic pain (see below). Stimuli which are su?ciently intense potentially to damage tissue will cause the stimulation of speci?c receptors known as NOCICEPTORS. Damage to tissues releases substances which stimulate the nociceptors. On the surface of the body there is a high density of nociceptors, and each area of the body is supplied by nerves from a particular spinal segment or level: this allows the brain to localise the source of the pain accurately. Pain from internal structures and organs is more di?cult to localise and is often felt in some more super?cial structure. For example, irritation of the DIAPHRAGM is often felt as pain in the shoulder, as the nerves from both structures enter the SPINAL CORD at the same level (often the structures have developed from the same parts of the embryo). This is known as referred pain.

The impulses from nociceptors travel along nerves to the spinal cord. Within this there is modulation of the pain ‘messages’ by other incoming sensory modalities, as well as descending input from the brain (Melzack and Walls’ gate-control theory). This involves morphine-like molecules (the ENDORPHINS and ENKEPHALINS) amongst many other pain-transmitting and pain-modulating substances. The modi?ed input then passes up the spinal cord through the thalamus to the cerebral cortex. Thus the amount of pain ‘felt’ may be altered by the emotional state of the individual and by other incoming sensations. Once pain is perceived, then ‘action’ is taken; this involves withdrawal of the area being damaged, vocalisation, AUTONOMIC NERVOUS SYSTEM response and examination of the painful area. Analysis of the event using memory will occur and appropriate action be taken to reduce pain and treat the damage.

Chronic pain may be de?ned in several ways: for example, pain resistant to one month’s treatment, or pain persisting one month beyond the usual course of an acute illness or injury. Some doctors may also arbitrarily choose the ?gure of six months. Chronic pain di?ers from acute pain: the physiological response is di?erent and pain may either be caused by stimuli which do not usually cause the perception of pain, or may arise within nerves or the central nervous system with no apparent external stimulation. It seldom has a physiological protective function in the way acute pain has. Also, chronic pain may be self-perpetuating: if individuals gain a psychological advantage from having pain, they may continue to do so (e.g. gaining attention from family or health professionals, etc.). The nervous system itself alters when pain is long-standing in such a way that it becomes more sensitive to painful inputs and tends to perpetuate the pain.

Treatment The treatment of pain depends upon its nature and cause. Acute pain is generally treated by curing the underlying complaint and prescribing ANALGESICS or using local anaesthetic techniques (see ANAESTHESIA – Local anaesthetics). Many hospitals now have acute pain teams for the management of postoperative and other types of acute pain; chronic pain is often treated in pain clinics. Those involved may include doctors (in Britain, usually anaesthetists), nurses, psychologists and psychiatrists, physiotherapists and complementary therapists. Patients are usually referred from other hospital specialists (although some may be referred by GPs). They will usually have been given a diagnosis and exhausted the medical and surgical treatment of their underlying condition.

All the usual analgesics may be employed, and opioids are often used in the terminal treatment of cancer pain.

ANTICONVULSANTS and ANTIDEPRESSANT DRUGS are also used because they alter the transmission of pain within the central nervous system and may actually treat the chronic pain syndrome.

Many local anaesthetic techniques are used. Myofascial pain – pain affecting muscles and connective tissues – is treated by the injection of local anaesthetic into tender spots, and nerves may be blocked either as a diagnostic procedure or by way of treatment. Epidural anaesthetic injections are also used in the same way, and all these treatments may be repeated at intervals over many months in an attempt to cure or at least reduce the pain. For intractable pain, nerves are sometimes destroyed using injections of alcohol or PHENOL or by applying CRYOTHERAPY or radiofrequency waves. Intractable or terminal pain may be treated by destroying nerves surgically, and, rarely, the pain pathways within the spinal cord are severed by cordotomy (though this is generally only used in terminal care).

ACUPUNCTURE and TRANSCUTANEOUS ELECTRICAL NERVE STIMULATION (TENS) are used for a variety of pain syndromes, particularly myofascial or musculoskeletal pain. It is thought that they work by increasing the release of endorphins and enkephalins (see above). It is possible to implant electrodes within the epidural space to stimulate directly the nerves as they traverse this space before passing into the spinal cord.

Physiotherapy is often used, particularly in the treatment of chronic backache, where pain may be reduced by improving posture and strengthening muscles with careful exercises. Relaxation techniques and psychotherapy are also used both to treat chronic pain and to help patients cope better with their disability.

Some types of chronic pain are caused by injury to sympathetic nerves or may be relieved by interrupting conduction in sympathetic nerves. This may be done in several ways. The nerves may be blocked using local anaesthetic or permanently destroyed using alcohol, phenol or by surgery.

Many of these techniques may be used in the management of cancer pain. Opioid drugs are often used by a variety of routes and methods, and management of these patients concentrates on the control of symptoms and on providing a good quality of life.... pain

The sense that enables sound to be perceived. The ear transforms the sound waves it receives into nerve impulses that pass to the brain.

Each ear has 3 distinct regions: the outer, middle, and inner ear. Sound waves are channelled through the ear canal to the middle ear, from where a complex system of membranes and tiny bones conveys the vibrations to the inner ear. The vibrations are converted into nerve impulses in the cochlea. These impulses travel along the auditory nerve to the medulla of the brain. From there, they pass via the thalamus to the superior temporal gyrus, part of the cerebral cortex involved in perceiving sound. (See also deafness.)... hearing

a collection of grey matter, shaped like a biconvex lens, lying beneath the *thalamus and close to the *corpus striatum, to which it is connected by nerve tracts. It has connections with the cerebral cortex and several other nuclei nearby. Stimulation of this nucleus is now being used in the treatment of Parkinson’s disease.... subthalamic nucleus

(thalamo-) combining form denoting the thalamus. Example: thalamolenticular (relating to the thalamus and lenticular nucleus of the brain).... thalam

n. the structures, collectively, at the anterior end of the brainstem, comprising the *epithalamus, *thalamus, *hypothalamus, and subthalamus, all of which are concerned with the reception and processing of information that enters from sensory nerve pathways.... thalamencephalon

a condition resulting from damage to the thalamus, often by a stroke, that is characterized by severe intractable pain and hypersensitivity in the area of the body served by the damaged brain region. It is extremely resistant to treatment.... thalamic syndrome

n. an operation on the brain in which a lesion is made in a precise part of the *thalamus. It has been used to control psychiatric symptoms of severe anxiety and distress, in which cases the lesion is made in the dorsomedial nucleus of the thalamus, which connects with the frontal lobe. It is now extremely rarely used. See also psychosurgery.... thalamotomy

n. absence of the ability to recognize the sensations of heat and coldness. When occurring as an isolated sensory symptom it indicates damage to the spinothalamic tract in the spinal cord, which conveys the impulses of temperature to the thalamus.... thermoanaesthesia

the clinical condition of unawareness of the self or the environment. The patient breathes spontaneously and has a stable circulation and sleep/wake cycles. It results from extensive damage to the cerebral cortex and thalamus while the brainstem and hypothalamus remain intact. The commonest causes are traumatic brain injury (e.g. road-traffic accidents) and cardiopulmonary arrest. Conditions that mimic the vegetative state include the psychiatric state of *catatonia and the locked-in syndrome, resulting from damage to the brainstem, in which the patient is conscious but unable to speak or make any movements of the body except for blinking and upward eye movements. See also persistent vegetative state.... vegetative state