Nervous System in Humans

A neuron is the longest cell of our body. It has three parts — cell body, axon and dendrites.

1. The cell body called cyton contains the nucleus.
2. Many short fibres called dendrons arise from the cell. They further divide to form dendrites. Dendrites receive messages from other neurons.
3. A single, long fibre called axon extends from the other side of the cell body. It is covered by a sheath called myelin sheath.

Below diagram shows the structure of a neuron:

The central nervous system consists of the brain and the spinal cord. It is the control centre for all body functions. It is the site for all information processing in the body. It is responsible for processing every sensation and thought a human being experiences. It has two parts — Brain and Spinal Cord.

1. Brain — The Brain is the main control centre of the nervous system situated in the cranium of the skull. It controls our thinking, feelings, movement and senses. It has three parts — Cerebrum, Cerebellum and Medulla oblongata.
   1. Cerebrum — The cerebrum is the uppermost, largest and the most prominent part of the brain. It is responsible for the sensation of pain, temperature, touch, sight, sound, taste and smell. It also controls mental activities such as memory, intelligence, thinking and reasoning.
   2. Cerebellum — The cerebellum is a small, egg-shaped lobe located at the base under the cerebrum. It helps in the muscle coordination and balance of the body.
   3. Medulla oblongata — The Medulla oblongata is the lowermost part of the brain located at the base of the skull. It controls breathing, heartbeat, coughing, sneezing and other involuntary functions.
2. Spinal cord — The spinal cord is the continuation of the medulla oblongata of the brain. It is like a cord that emerges from a hole at the base of the skull and runs through the vertebral column. The spinal cord conducts reflexes below the neck. It conducts sensory impulses from the skin and muscles to the brain. It conducts motor responses from the brain to the muscles of the trunk and limbs.

An automatic, spontaneous and mechanical response to a stimulus controlled by the spinal cord without the involvement of the brain is called a reflex action.

The path that an impulse takes in a reflex action is called a reflex arc. The reflex arc involves three neurons — sensory neurons, association neurons and motor neurons.

• If we touch a hot object, a stimulus triggers an impulse in a receptor on the skin.
• The impulse then travels from the receptor through a sensory neuron to the spinal cord.
• In the spinal cord, it passes through an association neuron to a motor neuron and finally reaches the muscles.
• The muscles of the effector organ contract moving it away from the hot object.

Below flow chart shows a simple reflex action of immediate withdrawal of hand on touching a hot object:

The organ system in our body that brings about coordination and integration of different body activities is called the nervous system.

The parts of a nerve cell are cell body, axon and dendrites.

The nerves that carry messages in the form of responses from the brain or spinal cord to other parts of the body such as muscles and glands are called motor nerves. They contain only motor fibres. For example, nerves arising from the brain leading to muscles of the eye balls are motor nerves.

There are 12 pairs of cranial nerves.

A synapse is a junction where communication between two neurons occurs.

The functions of the cerebrospinal fluid are:

1. It nourishes the brain.
2. It protects the brain from shocks by absorbing them.

The three important parts of the brain are:

1. Cerebrum
2. Cerebellum
3. Medulla oblongata

Two examples of reflex actions are:

1. Blinking of eyes when a foreign particle enters the eye.
2. Immediate withdrawal of hand if you unknowingly touch a hot object.

The basic structural and functional unit of the nervous system is a nerve cell or neuron.

Axon is covered by a sheath called myelin sheath.

Cerebrum is the largest part of the brain.

The Cerebrospinal fluid nourishes the brain and absorbs shocks.

The Cerebellum coordinates the movement of the voluntary muscles.

A neuron is the basic structural and functional unit of the nervous system.

A single long fibre extending from the cell body of a neuron is called axon.

The central nervous system consists of brain and spinal cord.

The spinal cord is the continuation of the medulla oblongata of the brain.

The nerve pathway of a reflex action is called a reflex arc.

The peripheral nervous system consists of cranial and spinal nerves.

cerebrum

Reason — cerebrum is part of brain while dendron, cyton, dendrite and axon are parts of neuron cell.

dendrite

Reason — cerebrum, cerebellum and medulla oblongata are parts of brain but dendrite is part of neuron.

1. The parts numbered a to d are:
   a → Cerebrum
   b → Cerebellum
   c → Meninges
   d → Medulla oblongata

2. The main function of the parts numbered c and d are:

   1. Function of Meninges (Part C) — They act as protective covering of brain with cerebrospinal fluid between them. They protect the brain from shock.
   2. Function of Medulla oblongata (Part D) — It controls the heartbeat, breathing and other involuntary movements.

1. The parts numbered a to d are:
   a → sensory neurons
   b → motor neurons
   c → association neurons
   d → cell body of motor neurons
2. The impulse enters through X and leaves through Y.
3. An automatic, spontaneous and mechanical response to a stimulus; controlled by the spinal cord without the involvement of the brain is called a reflex action.
4. Below flow chart shows a simple reflex action of immediate withdrawal of hand on touching a hot object:

Cerebellum is the part of brain which is affected by alcohol as it is the part which maintains the balance of the body and controls the coordination of voluntary muscles.

The watering of mouth with smell of tasty food is a conditioned reflex. Thus, Spinal cord is involved in it.

Below flowchart shows the reflex arc pathway involved in the action. The organ that controls such action is spinal cord.

dendrons

Reason — Many short fibres called dendrons arise from the cell body. They further divide to form dendrites.

muscles and glands

Reason — Motor nerves carry messages from brain or spinal cord to the muscles and glands of the body.

cranial

Reason — Cranial nerves emerge from the brain and reach organs in the head region.

impulse

Reason — Messages travel along the nerve cells in the form of a wave of chemical disturbance called an impulse. An impulse is like an electrical signal.

synapse

Reason — A synapse is a junction where communication between two neurons occurs.

meninges

Reason — The human brain is enveloped by three membranes called meninges.

medulla

Reason — The medulla oblongata controls involuntary actions of the body like breathing, heartbeat, coughing, sneezing, etc.

grey matter

Reason — The central canal of the spinal cord is surrounded by an H-shaped area called grey matter.

spinal cord

Reason — Reflex actions are the actions controlled by spinal cord.

voluntary actions

Reason — Peripheral nervous system is responsible for voluntary actions such as cycling, swimming, etc.

True

False

Corrected statement — Motor nerves carry impulses from the spinal cord to the sense organs.

False

Corrected statement — The largest portion of the brain is the cerebrum.

False

Corrected statement — Thinking, reasoning and memory are controlled by the cerebrum.

True

False

Corrected statement — The central nervous system consists of brain and spinal cord.