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UNIT2STRUCTURE OF THE BRAIN AND ITS

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Divisions of the Brain

The brain, one of the largest organs in man, consists of the following major divisions, named in ascending order beginning with most inferior part:

I. Brain stem

A. Medulla oblongata

B. Pons

C. Midbrain II. Cerebellum III. Diencephalon

A. Hypothalamus B. Thalamus IV. Cerebrum

Figure 23: Major regions of the Central Nervous System. A, Sagital sections of the brain and spinal cord. B, Section of preserved brain (Source: Carola, R., Harley,J.P., Noback R.C., (1992)

Brain Stem

The lowest part of the brain stem is the medulla oblongata. Immediately superior to the medulla oblongata lies the pons and superior to that, the midbrain. Together these three structures are called the brain stem (Figure 23). The medulla oblongata is an enlarged, upward extension of the spinal cord. It lies just inside the cranial cavity superior to the large opening in the occipital bone called the foramen magnum. Like the spinal cord, the medulla consists of gray and white matter, but their arrangement differs in the two parts of the brain. In the medulla, bits of gray matter mix closely and intricately with white matter to form the

reticular formation (reticular means "net-like"). In the spinal cord, gray matter and white matter do not intermingle; gray matter forms the core of the cord, and white matter surrounds it. The pons and midbrain, like the medulla, consist of white matter and scattered bits of gray matter.

Diencephalon

The diencephalon is a small but important part of the brain located between the midbrain inferiorly and the cerebrum superiorly. It consists of two major structures: the hypothalamus and the thalamus. The ventricle of the diencephalons is the 3rd ventricle.

Hypothalamus: The hypothalamus, as its name suggests, is located inferior to the thalamus. The posterior pituitary gland, the stalk that attaches it to the undersurface of the brain, and areas of gray matter located in the sidewalls of a fluid-filled space called the third ventricle are extensions of the hypothalamus. The hypothalamus is a crucial part of the mechanism for maintaining body temperature. Therefore a marked elevation in body temperature in the absence of disease frequently characterizes injuries or other abnormalities of the hypothalamus. In addition, this important center is involved in functions such as the regulation of water balance; sleep cycles, and the control of appetite and many emotions involved in pleasure, fear, anger, sexual arousal, and pain.

Thalamus: Immediately superior to the hypothalamus is a dumb bell shaped section or largely gray matter called the thalamus. Each enlarged end of the dumbbell lies in a lateral wall of the third ventricle. The medial surfaces of the thalamus are connected across the third ventricle by a thin white matter called the inter-thalamic adhesion. The thalamus is composed chiefly of dendrites and cell bodies of neurons that have axons extending up to the sensory areas of the cerebrum. It performs the following functions:

1. It helps to produce sensations. Its neurons relay impulses to the cerebral cortex from the sense organs of the body.

2. It associates sensations with emotions. Almost all sensations are accompanied by a feeling of some degree of pleasantness or unpleasantness. The way that these pleasant and unpleasant feelings are produced is unknown except that they seem to be associated with the arrival of sensory impulses in the thalamus.

3. It plays a part in the so-called arousal or alerting mechanism.

4. It contains important nuclei such as medial geniculate body, which is responsible for auditory sense and lateral geniculate body, which is responsible for vision.

Cerebellum

The cerebellum is the second largest part of the human brain. It lies under the occipital lobe of the cerebrum. In the cerebellum, gray matter composes the outer layer, and white matter composes the bulk of the interior.

Function

The cerebellum plays an essential part in the production of normal movements. Perhaps a few examples will make this clear. A patient who has a tumor of the cerebellum frequently loses balance and may topple over and reel like a drunken person when walking. It may be impossible to coordinate muscles normally. Frequent complaints about being clumsy and unable to even drive a nail or draw a straight line are typical.

With the loss of normal cerebellar function, the ability to make precise movements is lost. The general functions of the cerebellum, therefore, are to produce smooth coordinated movements, maintain equilibrium, and sustain normal postures.

Cerebrum

The cerebrum is the largest and uppermost part of the brain. If you were to look at the outer surface of the cerebrum, the first features you would notice might be its many ridges and grooves. The ridges are called convolutions or gyri, and the grooves are called sulci. The deepest sulci are called fissures; the longitudinal fissure divides the cerebrum into right and left halves or hemispheres. These halves are almost separate structures except for their lower mid-portions, which are connected by a structure called the corpus callosum (Figure 22c). Two deep sulci subdivide each cerebral hemisphere into four major lobes and each lobe into numerous convolutions.

The lobes are named after the bones that lie over them: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. Identify these in Figure below.

a b

Figure 24: The cerebrum: a. Gyri, fissures and lobes; b. Functional areas

i. Functions of the cerebrum: The neurons of the cerebrum do not function alone. They function with many other neurons in many other parts of the brain and in the spinal cord. Neurons of these structures continually bring impulses to cerebral neurons and continually transmit impulses away from them. If all other neurons were functioning normally and only cerebral neurons were not functioning, here are some of the things that you could not do: You could not think or use your will.

ii. You could not remember anything that has ever happened to you.

iii. You could not decide to make the smallest movement, nor could you make it.

iv. You would not see or hear.

v. You could not experience any of the sensations that make life so rich and varied.

vi. Nothing would anger or frighten you, and nothing would bring you joy or sorrow.

vii. You would, in short, be unconscious.

These terms, then sum up cerebral functions: Consciousness, thinking, memory, sensations, emotions, and willed movements. Figure 23(b) above shows the areas of the cerebral cortex essential for willed movements, general sensations, vision, hearing, and normal speech.

Specific areas of the cortex have very specific functions. For example, the temporal lobe's auditory areas interpret incoming nervous signals from the ear as very specific sounds. The visual area of the cortex in the occipital lobe helps you identify and understand specific images.

Localized areas of the cortex are directly related to specific functions, as shown in Figure 23.