The Brain

There are more than a thousand million neurons in the adult human
brain. An estimate shows that the cerebral cortex alone has about 102783000
synapses. Thus the brain is a complex organ.

On structural and functional basis the brain can be divided into 3
regions. They are (1). Fore brain, (2). Midbrain, (3). Hind brain.

Fore Brain (Prosencephalon)

This region of the brain comprises Diencephalon and the cerebrum.
The diencephalon is formed of thalamus and hypothalamus.

Thalamus (Fore brain)

It is the largest part of the diencephalon. This region contains a
cluster of nuclei. Most of the sensory inputs are conducted to the cerebral
cortex through the thalamus. Axons carrying auditory, visual and other
sensory informations synapse with specific nuclei of this region. This
region may also influence mood and general body movements due to strong
emotions such as fear or anger.

Hypothalamus (Fore brain)

This region contains small nuclei and nerve tracts. The nuclei called
mamillary bodies are involved in olfactory reflexes and emotional responses
to odours. The funnel shaped infundibulum from the hypothalamus
connects it to the posterior pituitary or neurohypophysis. This region
controls the secretions of the pituitary gland.

The hypothalamus receives inputs from several sensory systems such
as tongue, nose and external genitalia. It is associated with emotional and
mood relationships. It provides a relaxed feeling. Feeling good after a meal,
rage and fear are also due to this region. It also coordinates responses to the
sleep-wake cycle with other areas.

Cerebrum (Fore brain)

It is the largest part of the brain. It weighs about 1400g in males and
1200g in females. Larger brains are normally associated with larger bodies
and not with greater intelligence.

The grey matter on the outer surface of the cerebrum is the cortex. It
forms clusters deep inside the brain called nuclei. The inner part of the brain,
in between the cortex and the nuclei has white matter named as
cerebral medulla.

Cerebral cortex (Fore brain)

The cortex contains several primary sensory areas. These areas
include taste area, primary auditory cortex for processing auditory stimuli,
visual cortex for perceiving visual images and areas for other cutaneous
sensations.

The cortical areas adjacent to the primary sensory centers are called
the association areas. These areas are involved in the process of
recognition. For example the sensory stimulus from the retina of the eye
reaches the visual association area of the cortex. Here the visual
information is compared with past experiences. Further this area has
connections with other parts of the cortex, which influence decisions. Thus
visual information is judged several times. This may be one of the reasons
why two people who witness the same event can present somewhat
different versions of what happened.

The primary motor area of the cortex controls many voluntary
movements, especially the finer motor movements of the hands. Muscle groups
such as facial muscles, that have many motor units have greater innervation. They are represented by a large area of the motor cortex.

Anterior to the primary motor area are the premotor area. It is the
staging area in which motor functions are organized before they are
initiated in the motor cortex. For example, if a person decides to lift a hand,
the neurons of the premotor area are stimulated first. This area determines
the order and the degree to which the muscles must contract.

The prefrontal area provides motivation and foresight to plan and
initiate movements. This area is well developed only in primates and
especially in humans. Our emotional behaviour and mood are controlled by
this area.

The midbrain or Mesencephalon

The roof of this region contains four nuclei. The nuclei form mounds.
They are collectively called corpora quadrigemina. It is formed of 2
superiour colliculi or mounds and 2 inferior colliculi or mounds. The
superior colliculi are involved in visual reflexes. They control eye and head
movements. They aid in visual tracking of moving objects. The inferior colliculi
are involved in hearing.

The hindbrain or Rhombencephalon

This part of the brain comprises Cerebellum, Pons and Medulla
oblongata.

Cerebellum

This region communicates with other region of the CNS through three
large nerve tracts called the cerebellar peduncles.
The cerebellum consists of following three parts Parts Control

1. flocculonodular balance and maintenance
of muscle tone.

2. vermis – anterior part motor coordination and muscle tone.
3. vermis – posterior part and fine motor coordination and
lateral hemispheres muscle tone.

Cerebellar disfunction may cause decreased muscle tone, imbalance and lack
of co-ordination.

Pons

This region relays information from the cerebrum to the
cerebellum. It also contains sleep center and respiratory centers. These
centers along with medulla help to control respiratory movements.

Medulla oblongata

It is the most inferior part of the brain stem. It acts as a conduction
pathway for both ascending and descending nerve tracts. The nuclei inside
medulla oblongata function as centers of several reflexes involved in the
regulation of heart rate, blood vessel contractions, breathing, swallowing,
vomiting, coughing and sneezing.