November 26, 2015

Senior Intermediate Zoology Important Questions - 8 Marks

Q: Give a brief account of the structure and functions of human brain. (8 Marks)

A: Human brain is lodged and protected in a bony box in the skull called cranium. Brian is covered by three membranes
called meninges, namely, outer duramater (double layered), middle arachnoid mater and inner piamater (attached to
outer surface of the brain). Duramater and arachnoid mater are separated by subdural space and arachnoid mater and piamater are separated by sub arachnoid space. Subdural space is filled with cerebro spinal fluid (colourless and alkaline). It acts as a shock absorbing medium.

Brain is divisible into three parts, namely, fore brain (procencephalon), mid brain (mesencephalon) and hind brain
(rhombencephalon).

A. Procencephalon: It is the largest and the anterior part of the brain and it consists of olfactory bulbs, cerebrum and diencephalon.

Olfactory bulbs: In the anterior part of the brain, is a pair of olfactory bulbs. They receive the sensory impulses of the smell from Olfactory epithelium.

Cerebrum: It is the largest part of the brain and is divided into two cerebral hemispheres (left and right) by a deep and longitudinal fissure. Beneath the cortex, left and right cerebral hemispheres are connected by transversely arranged bundle of myelenated nerve fibres called corpus callosum. The outer part of the cerebrum is formed by grey matter called cortex and inner part by white matter. Cortex contains nerve cell bodies. Cerebral cortex contains
many folds (gyri) among which, fissures (deep grooves) and sulci (shallow grooves) are present.

Cortex has three functional areas:
i. Sensory areas: receive and interpret sensory impulses.
ii. Motor areas: control voluntary muscular movements.
iii. Association areas: deals with integrative functions (such as memory and communications).

Each cerebral hemisphere is divided into four lobes, namely, frontal lobe, parietal lobe, temporal lobe and occipital lobe.

Diencephalon or thalamencephalon: It has an epithalamus, thalamus and hypothalamus.
i. Epithalamus is non nervous. It is fused with the piamater and forms the anterior choroid plexus. Behind the anterior choroid plexus, is a pineal stalk, which bears a pineal body at the tip.

ii. Thalamus acts as a coordinating centre for sensory and motor signaling.
iii. Hypothalamus ventrally has a hollow outgrowth called infundibulum. It bears the pituitary gland. Infundibulum contains several neurosecretary cells. Hypothalamus acts as and osmoregulatory, thermo regulatory, hunger, thirst and satiety centre.

The amygdala, hippocampus (deep structure of cerebral hemispheres) and inner parts of cerebral hemispheres form the limbic system. Along with hypothalamus, the limbic system is involved in sexual behaviour and emotions.

B. Mesencephalon: It lies beneath the thalamus and pons varoli. Ventrally, it contains pair of longitudinal bands of nervous tissue called crura cerebri. Connect the fore brain and the hind brain. Dorsally, mid brain has two pairs of spherical lobes called corpora quadrigemina (four optic lobes). The anterior pair is called superior colliculi (concerned with vision) and the posterior pair is called inferior colliculi (concerned with hearing).

C. Rhombencephalon: It is formed by cerebellum, pons and medulla oblongata.
i) Cerebellum: It is the second largest part of the brain and is composed of a median vermis and two lateral cerebellar lobes. Each cerebellar lobe contains an anterior and a posterior floccular lobe. The inner white
matter of cerebellum spreads like branching tree. Hence it is called arbor vitae. Cerebellum (little brain or gyroscope of the body) control and coordinate locomotor movements and maintains equilibrium. (damage to cerebellum results in ataxia).

ii) Pons varoli: Beneath the mid brain, just in front of cerebellum and above the medulla oblongata, pons varoli is present. It is in the form of a transverse bridge connecting the two cerebellar lobes. It acts as a relay centre
between cerebellum, spinal cord and the remaining part of brain. It contains a pneumotaxic centre (controls the respiratory muscles).

iii) Medulla oblongata: It is the posterior part of the brain and is continued as spinal cord. Its roof is thin, vascular and is known as the posterior choroid plexus. Medulla oblongata contains cardio vascular and respiratory centres and the centres of swallowing, cough, vomiting, sneezing, hiccupping etc.

Mid brain, pons varoli and medulla oblongata together form the brain stem.
Cavities of brain: Cavities of brain are known as ventricles. The cavities of right and left cerebral hemispheres are known as 1st and 2nd ventricles respectively. They are also known as lateral ventricles or paracoels. Diencephalon third ventricle or diacoel and medulla oblongata contains the fourth ventricle or myelocoel. The two lateral ventricles open into the diacoel through separate openings called foramina of Manro. Diacoel and myelocoel are connected by a narrow tube called iter or ductur Sylvius. Fourth ventricle is continuous with central canal of the spinal cord. All the ventricles are filled with cerebrospinal fluid. It was flushed four times in a day to
remove metabolites and toxins.

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