Q.Briefly describe the structure of the brain.

The brain is the central information-processing organ and the command-and-control system of the body. It is protected inside the bony skull and wrapped in three membranes called the cranial meninges — the tough outer dura mater, the thin middle arachnoid, and the innermost pia mater in direct contact with brain tissue. For study, the brain is divided into three major parts:

• Forebrain — the largest part, made of the cerebrum (two hemispheres joined by the corpus callosum, covered by the folded cerebral cortex or grey matter), the thalamus (a relay centre for sensory and motor signals), and the hypothalamus (controls temperature, hunger and thirst, and secretes hypothalamic hormones). Deep structures form the limbic system, linked to smell, emotion and motivation.
• Midbrain — lies between forebrain and hindbrain, passed through by the cerebral aqueduct and bearing four rounded lobes, the corpora quadrigemina.
• Hindbrain — made of the pons (fibre tracts linking brain regions), the cerebellum (folded surface; coordinates posture, balance and smooth movement), and the medulla oblongata (controls respiration, cardiovascular reflexes and gastric secretions).

The midbrain, pons and medulla together form the brainstem, which connects the brain with the spinal cord.

The brain is the central information-processing organ of the body and acts as its command and control system. Almost every activity that keeps the body running — from voluntary movement and balance to the working of vital involuntary organs, thermoregulation, hunger and thirst, and even memory, intelligence, emotion and thought — is coordinated here.

Because it is so vital, the brain is very well protected. It sits inside the bony skull, which shields it from mechanical injury, and within the skull it is wrapped in three membranes called the cranial meninges. From the outside inward these are the dura mater (tough outer layer), the arachnoid (a very thin middle layer), and the pia mater (the innermost layer, lying directly on the brain tissue).

For the sake of study the brain is divided into three major parts.

• Forebrain — the largest and most elaborate division. It contains three structures:

  • The cerebrum, the major part of the brain, split lengthwise into left and right hemispheres joined by a thick band of fibres, the corpus callosum. Each hemisphere is covered by the folded cerebral cortex, called grey matter because it is packed with neuron cell bodies; the cortex has motor, sensory and large association areas. Beneath it lie myelinated fibre tracts that form the whitish white matter.
  • The thalamus, which the cerebrum wraps around, serving as a major relay and coordinating centre for sensory and motor signalling.
  • The hypothalamus, lying at the base of the thalamus, which regulates body temperature and the urges to eat and drink, and holds neurosecretory cells that secrete hypothalamic hormones. Deep structures such as the amygdala and hippocampus, with the inner cerebrum, form the limbic system, involved in smell, sexual behaviour, emotion and motivation.

• Midbrain — the short middle portion, lying between the thalamus–hypothalamus region above and the pons below. A narrow canal, the cerebral aqueduct, runs through it, and its dorsal surface bears four rounded swellings, the corpora quadrigemina.

• Hindbrain — the rearmost division, built from three parts:

  • The pons, made largely of fibre tracts that act as a bridge linking different brain regions.
  • The cerebellum, with a deeply folded surface that packs in a very large number of neurons; it coordinates posture, maintains balance and equilibrium using inputs from the inner ear and auditory system, and makes voluntary movements precise and smooth.
  • The medulla oblongata, the lowest part, directly connected to the spinal cord, housing vital centres that control respiration, cardiovascular reflexes and gastric secretions.