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c) The hypothalamus forms the floor and lower part of the brain formed by the thickening of the diencephalon. Part of the hypothalamus also extends downwards and contributes to the pituitary body. Here, the nervous and hormonal systems interact. It is concerned with different bodily activities, homeostasis, and is the center for the parasympathetic nervous system. Internally, the downgrowth from the diencephalon forms the pituitary body, while the thin and vascular roof (anterior choroid plexus) secretes cerebrospinal fluid into the third ventricle.

2) The midbrain is a short, constricted mass mostly of white matter surrounding a central cavity. Ventrally, it connects the pons and cerebellum with the cerebral hemispheres by tracts of nerve fibers. In mammals, the analysis of vision has moved out of the midbrain and has become a function of the forebrain. The dorsal part of the midbrain has four small rounded projections which contain visual reflex (superior colliculi) and auditory reflex centers (inferior colliculi).

3) The hindbrain or rhombencephalon consists of two subdivisions, anterior metencephalon and a posterior myelencephalon. The hindbrain has 3 main parts : (a) cerebellum (b) medulla and (c) pons.

(a) The cerebellum occupies the lower posterior part of the skull cavity. It is the second largest part of the brain and consists of the outer layer of grey matter (cortex) and an inner core of white matter (medullary body). Its surface is also grooved, but the convolutions are less prominent than cerebral hemispheres. The cerebellum has two large lateral masses, the cerebellar hemispheres, and a central portion, the vermis. The internal white matter has short and long nerve tracts, which connect various portions of the cerebellum with each other, and with other parts of the brain and spinal cord.

(b)The medulla (medulla oblongata), also called the "brainstem" is the anterior enlargement of the spinal cord, and consists of ascending and descending tracts of white matter. The vital nuclei in the medulla control vital reflex centers such an heart action, respiration, diameter of blood vessels, etc. The nonvital nuclei contain reflex centers for activities such as vomiting, coughing, swallowing, sneezing, etc.

(c) The pons or pons varioli lies just above the medulla and is composed of white matter. It contains transverse tracts that conduct impulses between the cerebellum, medulla and cerebrum. In pons, the nerve tracts between brain and the spinal cord cross from the left side to the right side. Because of this crossover, the left side of the brain controls activities of the right side of the body and vice versa.

Functions of different parts of the brain

1) The cerebral hemispheres (cerebrum) perform (a) all mental functions, reason, will, memory and intelligence, and (b) many essential motor sensory and visceral activities. Certain functional areas of cerebral cortex like sensory, motor and association areas are mapped out after experimentation; these control motor, sensory and other activities (Figure 23.3). The portion of cerebrum that governs muscular movements is known as the motor areas; those involved in analysis of sensations are called sensory areas and those concerned with the higher faculties such as reasoning, will, understanding, memory, personality, ethics, etc., are called association areas.

2) Basal ganglia or nuclei play a role in producing movement.

3) The thalamus is associated with pain, pleasure and emotions. It functions as an arousal or alert mechanism of the body. It also has a role in producing complex reflex movements.

4) The hypothalamus is the control center for homeostasis, partial control of sleep and wakefulness, regulation of body temperature and food intake, and controls of metabolism and water balance through synthesis of ADH secreted by pituitary,  regulation of autonomic activities, and control of various reproductive functions.

5) The pituitary body is the master endocrine gland.

6) The cerebellum coordinates the voluntary skeletal muscle activities and several other brain centers. It controls skeletal muscles related to maintenance of equilibrium, smooth, timed, precise and steady body movements.

7) The medulla is the automatic control center for the heart beat, breathing, swallowing, sneezing, etc. Sleep and loss of consciousness and cerebrum activities are controlled by tissues within the medulla.

8) The pons serves as a reflex center, regulates respiration, and serves as a conduction pathway between the spinal cord and the brain.

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Figure 23.3 Functional areas of the brain

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Table of Contents

23.0 Introduction
23.1 Central nervous system
23.2 The automatic nervous system
23.3 Receptors and effects
23.4 Reflex action - mechanism of nervous action

Chapter 24


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