Human Brain

Human brain is a highly complex organ, which is mainly composed of the nervous tissue. The tissues are highly folded to accommodate a larger surface area in less space. This is similar to the way we fold clothes in order to pack more cloths in a suitcase.

The brain is covered by a three layered system of membranes. These membranes are called meninges. Cerebrospinal fluid is filled between the meninges. The CSF provides cushion to the brain against mechanical shocks. Furthermore, the brain is housed inside the skull for optimum protection. The human brain can be divided into three regions, viz. forebrain, midbrain and hindbrain.

Parts of Human Brain

1. Forebrain: It is composed of the cerebrum.
2. Midbrain: It is composed of the hypothalamus.
3. Hindbrain: It is composed of the cerebellum, pons and medulla oblongata.

Some main structures of the human brain are explained below.

Cerebrum

The cerebrum is the largest part in the human brain. It is divided into two hemispheres, called cerebral hemispheres.

Functions of cerebrum

• The cerebrum controls the voluntary motor actions. Actions which facilitate mobility are called motor actions. For example, movement of hands and feet are motor actions.
• It is the site of sensory perceptions, like tactile and auditory perceptions. Sense of touch is called tactile percetption. Sense of hearing is called auditory perception.
• It is the seat of learning and memory.

Hypothalamus

The hypothalamus lies at the base of the cerebrum. It controls sleep and wake cycle (circadian rhythm) of the body. It also controls the urges for eating and drinking.

Cerebellum

Cerebellum lies below the cerebrum and at the back of the whole structure. It coordinates the motor functions. When you are riding your bicycle, the perfect coordination between your pedaling and steering control is achieved by the cerebellum.

Medulla

Medulla forms the brain stem, along with the pons. It lies at the base of the brain and continues into the spinal cord. Medulla controls various involuntary functions, like heart beat, respiration, etc.

Reflex Action

Reflex action is a special case of involuntary movement. When a voluntary organ is in the vicinity of a sudden danger, it is immediately pulled away from the danger to save itself.

For example, when your hand touches a very hot electric iron, you move away your hand in a jerk. All of this happens in flash and your hand is saved from the imminent injury. This is an example of reflex action.

There are many other examples of reflex action, like dilation of pupil on excitement, jaw dropping in surprise, etc.

Reflex Arc: The path through which nerves signals (involved in a reflex action) travel is called the reflex arc. The following flow chart shows the flow of signal in a reflex arc.

Receptor ⇨ Sensory Neuron ⇨ Relay neuron ⇨ Motor neuron ⇨ Effector (muscle)

• The receptor is the organ which comes in the danger zone.
• The sensory neurons pick signals from the receptor and send them to the relay neuron.
• The relay neuron is present in the spinal cord.
• The spinal cord sends signals to the effector via the motor neuron.
• The effector comes in action and moves the receptor away from the danger.

The reflex arc passes at the level of the spinal cord and the signals involved in reflex action do not travel up to the brain. This is important because sending signals to the brain would involve more time. Although every action is ultimately controlled by the brain, the reflex action is mainly controlled at the level of spinal cord.

Muscular Movements and Nervous Control

• Muscle tissues have special filaments, called actin and myosin.
• When a muscle receives a nerve signal, a series of events is triggered in the muscle.
• Calcium ions enter the muscle cells. It results in actin and myosin filaments sliding towards each other and that is how a muscle contracts.
• Contraction in a muscle brings movement in the related organ.
• When calcium ions exit from calcium channels, actin and myosin slide away from each other. It results in relaxation of muscles.