The Brain

.. rious afferent and efferent tracts, when correlated with symptoms and signs, enables physicians to localise with considerable accuracy the level and extent of lesions in the nervous system. The other 10 cranial nerves, in descending order of location, are the oculomotor, trochlear, trigeminal, abducens, facial, acoustic, glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves. Cerebellum The cerebellum accounts for about 10 percent of the brain’s weight and is a centre for co-ordinating automatic (reflex) and voluntary movements of the body. It receives afferent impulses from the spinal cord as well as from various brain-stem nuclei.

The cerebellum is connected by fibres, both going and coming, to parts of the basal ganglia and the extrapyramidal system (various nuclei and tracts governing motor function that are not part of the corticospinal, or pyramidal, tract). Fibres also go to the cerebral cortex by way of thalamic nuclear relays. The cerebral cortex (principally the frontal lobe) is connected to the cerebellum by numerous fibres. Lesions of the cerebellum and basal ganglia cause incoordination and other disorders of movement, such as tremor or choreiform movements (sudden involuntary movements or muscle contractions). Reticular Formation Deep within the brain stem is a large group of cells known as the reticular formation.

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Ascending fibres from this area project via thalamic nuclei to large association areas in the cortex. The reticular formation is a regulator of the state of alertness; its destruction at the midbrain level results in a state of coma. In contrast, large areas of the cerebrum may be destroyed without a loss of consciousness. The mechanism of cerebral concussion is thought to be a temporary derangement in the reticular formation. Hypothalamus The hypothalamus, located below the thalamus and including the pituitary gland, is an important part of the diencephalon. It contains centres for regulating body temperature, blood pressure, pulse rate, perspiration, and other functions controlled by the autonomic nervous system.

The hypothalamus is the neural regulatory centre for the production and release Left and Right Brain Functions Although the cerebrum is symmetrical in structure, with two lobes emerging from the brain stem and matching motor and sensory areas in each, certain intellectual functions are restricted to one hemisphere. A persons dominant hemisphere is usually occupied with language and logical operations, while the other hemisphere controls emotion and artistic and spatial skills. In nearly all right-handed and many left-handed people, the left hemisphere is dominant. of hormones. Lesions of it that occur early in life may affect an individual’s growth and sexual development. In adults, a sudden, destructive lesion such as an intraventricular haemorrhage may result in diabetes insipidus, hypothermia (high body temperature), and frequently death.

Emerging from the rear of the diencephalon is a small organ, the Pineal Gland, which plays a role in controlling certain biorhythms such as the onset of puberty. Cerebral Cortex In terms of evolution, the cerebral cortex of humans has become increasingly complex. In present-day humans, it has a highly convoluted surface, greatly increasing the total area of the cortex. It comprises about 85 percent of the nerve cells in the brain. Because it is easily accessible, the cerebral cortex has been studied extensively, and some parts have been mapped in great detail.

In primates, the deep horizontal Sylvian fissure (groove) separates in each hemisphere the temporal lobe, below, from the frontal, parietal, and occipital lobes, above. Running vertically, the less constant Rolandic fissure separates the frontal from the parietal lobe. Other landmarks separate the parietal from the occipital lobe on each side. The motor cortex in the frontal lobe and the sensory cortex in the parietal lobe are involved in the integration of muscular action and sensations. An important area of the frontal lobe is Broca’s area, which lies in the left hemisphere of the brain of right-handed people, just forward of the lips-teeth-tongue areas of the sensorimotor strip.

A lesion here causes a motor aphasia, the inability to produce meaningful spoken language (in the absence of any weakness of the muscles used in speech and with understanding preserved, including the ability to read and follow commands). In right-handed people, language dominance is located in the left hemisphere. About 50 percent of left-handed persons have right cerebral dominance. A lesion in Wernicke’s area (the posterior, upper part of the temporal lobe on the dominant side) causes a receptive aphasia. The individual may verbalise extensively, talking in gibberish with occasional mispronounced words.

The person is also unable to understand spoken or written language and may show a lack of concern over his or her plight. Olfactory impulses enter the brain through the olfactory bulbs, and synapse occurs in the olfactory cortex of the limbic system. Visual impulses travel through the optic nerves to the optic chiasma, where half of the fibres from each eye cross to the opposite side. They then synapse in a part of the thalamus that projects to the primary visual cortex in the occipital lobe. A complete lesion in one optic tract behind the chiasma causes an inability to see objects on one side of the midline opposite the lesion. Auditory impulses travel up the brain stem in a series of complex crossings and uncrossings with several synapses, ending in the auditory primary cortex deep in the temporal lobe. The temporal lobe cortex and certain portions of the limbic system have important functions involving recall and memory.

This was first observed by Wilder Graves Penfield (1891-1976) during surgical operations on people having an unusual type of temporal lobe seizure during which they had vivid auditory and visual hallucinations of previously experienced events. Their experiential hallucinations could be reproduced by stimulating the surface of the temporal lobe–an activation of a specific complex memory, or engram. Bilateral removal of the hippocampal gyri causes a loss of recent memory or an inability to retain new information or experiences longer than a few minutes. Memory of information acquired before the lesions occurred is preserved, however, as is abstract thinking and general intelligence. L.D.S.


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