Nervous System

The functioning of the nervous system depends on detecting a stimulus in the internal or external environment and responding to it. A stimulus is an agent or an environmental change which can initiate a response in the body. The stimuli can be of several types. It could be physical (touch, prick, pressure), auditory, chemical, radiant (light), heat or cold, or electrical.

The nervous system has two main divisions: Central Nervous System (CNS) that includes brain and spinal cord and Peripheral Nervous System (PNS) which includes the nerves arising from the brain and spinal cord.

Neuron (Nerve Cell)

Neurons (individual cells of the nervous system) communicate with one another and other tissues to receive and transmit information throughout the body. It consists of three parts - Dendrites, Cell body and Axon.

Dendrites

Dendrites are branched cytoplasmic projections from the cell body. The dendritic tip of the nerve cells receive impulses and sets off a chemical reaction that creates an electrical impulse which is further transmitted to the cell body.

Cell body

The cell body contains a well defined nucleus, surrounded by cytoplasm. It has cell organelles like any other cells. The cell body further transmits the impulse to the axon.

Axon

One branch arising out of the cell body is very long in comparison to others. This branch is called axon or nerve fibre. In most neurons, it is covered by an insulating fatty sheath called neurilemma.

The fatty sheath is missing at intervals which are called Node of Ranvier. The absence of neurilemma helps Node of Ranvier to generate electrical activity and in transmission of nerve impulse. The end portions of the axon have swollen ends like "bulbs" which store chemicals called neurotransmitter.

Axon bulbs are closely placed near the dendrites of another neuron. This junction of two neurons in called synapse and the space at the synapse separating the two neurons called synaptic cleft. There are many synapses between the millions of nerve cells present in our body.

Through the synapse the impulse passes from one neuron to the next neuron. When an impulse reaches the end of first neuron, a neurotransmitter is released in the synaptic cleft of the synapse. These chemicals cross the gap or synapse and start a similar electrical impulse in the next neuron. Finally, the impulse is delivered from neurons to other cells, for example the muscle cells or glands to elicit the desired action.

Types of Neurons

There are three types of neurons:

  1. Sensory neurons convey the impulse from receptor (sense organ) to the main nervous system (brain or spinal cord).

  2. Motor neurons carry the impulse from the main nervous system to an effector (muscle or gland).

  3. Association (Connecting) neurons are located in the brain and spinal cord and interconnect the sensory and motor neurons.

Nerves

Nerves are thread-like structures which emerge from the brain and spinal cord and branch out to almost all parts of the body. A nerve is formed of a bundle of nerve fibres (axons) enclosed in a tubular sheath. There are three kinds of nerves:

  1. Sensory nerves: Sensory nerves that contain sensory fibres. These nerves bring impulse from the receptors (sense organs) to the brain or spinal cord. Example: Optic nerve arising from the eye and ending in the brain.

  2. Motor nerves: Motor nerves which contain motor fibres. These nerves carry impulse from the brain or spinal cord to the effector organ like muscles or glands. Example: a nerve arising from the brain and carrying impulse to the muscles of the eye.

  3. Mixed nerves: Mixed nerves are those that contain both sensory and motor fibres and perform a mixed function. Example: a spinal nerve.

Sense Organs

Receptor organ like nose, eyes and ears receive the stimulus. The stimulus than reaches the spinal cord and the brain through sensory nerves where it is integrated. The message is then send by the motor nerves to the required organ (muscles or gland) for suitable action. In this ways a response is generated.

Major Divisions of Nervous System

The nervous system has two main divisions: Central Nervous System (CNS) that includes brain and spinal cord and Peripheral Nervous System (PNS) which includes the nerves arising from the brain and spinal cord.