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The senses that evoke responses in organisms are called stimuli.

These are two types of stimuli

External stimuli are stimuli formed outside the body   eg; Sound, touch, heat, chemicals, pressure, cold, radiations.

Internal stimuli, are stimuli formed inside the body. Eg; Hunger, thirst,

Receptors; specialized cells in the sense organs and other parts of the body to receive stimuli. These cells are known as receptors.

Function; They receive stimuli and generate suitable impulses

Neuron or nerve cell ; it is the basic structural and functional unit of the nervous system

馃敇Parts of neuron;


鈥 Branches of Dendron.

FUNCTION;Part that receives impulses from adjacent neuron.


鈥 Short filament from the cell body.

FUNCTION;Carries impulses from dendrites to the cell body.


鈥 Longest filament from the cell body.

FUNCTION;Carries impulses from the cell body to outside


鈥 Branches of axon.

FUNCTION; Carries impulses to the synaptic knob

Synaptic knob

鈥 Tip of axonite.

FUNCTION;Secretes neurotransmitter.

馃敇Myelin sheath;

Axons of most of the neurons are repeatedly encircled by myelin, a membrane containing lipid. This is called myelin sheath.

functions of myelin sheath

鈥 Provide nutrients and oxygen to the axon.

鈥 Accelerate impulses.

鈥 Act as an electric insulator.

鈥 Protects the axon from external shocks.

The part of neuron, where myelinated neurons are present in abundance,is called as the white matter.

 The part of neuron where the cell bodies and non myelinated neurons are present, is called as the grey matter.

馃敇How generation and transmission of impulses occures

The outer surface of the plasma membrane of the neuron is positively charged and the inner surface is negatively charged. This is due to the difference in the distribution of certain ions.

When stimulated, the distribution of ions in that particular part changes and hence the inner surface becomes positively charged and the outer surface becomes negatively charged .This momentary charge difference stimulates its adjacent parts and similar changes occur there too. As this process proceeds, impulses get transmitted as electric charges. Nerve impulses are messages transmitted through the neurons


It is the junction between neurons or between neurons and muscles or glands is known as the synapse. It helps to regulate the speed and direction of impulses. The impulses are transmitted across the synaptic cleft only through a chemical (neurotransmitter), secreted from the synaptic knobs.

鉂擭eurotransmitters; Acetyl choline and Dopamine

鉁旓笍Function; Synapse helps to regulate the speed and direction of impulses

馃敇Nerves and functions

鉁旓笍Sensory nerves; Carry impulses from sense organs to the brain and spinal cord

鉁旓笍Motor nerves; Carry impulses from brain and spinal cord to different organs

鉁旓笍Mixed nerves; Carry impulses to and from brain and spinal cord to different organs

The nervous system consists of two parts

Central Nervous System; brain and spinal cord

Peripheral Nervous System; Cranial nerves (12 pairs)

                 – Spinal cord – Spinal nerves (31 pairs)

The brain is protected inside a hard skull and is covered by a three layered membrane, called the meninges. Cerebrospinal fluid,a fluid formed inside the meninges


Cerebrospinal fluid is a fluid formed from the blood inside the meninges and reabsorbed in to the blood.

鉁匜unctions of CSF

 It provides nutrients and oxygen to brain tissues.

鈥 Regulates the pressure inside the brain.

鈥 Protects brain from injuries


The largest part of the brain with many fissures and folds are seen


  • centre of thought, intelligence, memory and imagination.

鈥 evokes sensations.

鈥 controls voluntary movements


The second largest part. seen behind the cerebrum as two flaps

Function;Coordinates muscular activities and maintains equilibrium of the body.

鉁匨edulla oblongata

the rod shaped medulla oblongata is seen below the cerebrum, located near the cerebellum.


Controls involuntary actions like heart beat and breathing


It is situated below the cerebrum.

鈥 acts as relay station of impulses to and from the cerebrum.

鈥 analyses impulses from various parts of the body and sends the important ones to the cerebrum


Plays a major role in the maintenance of homeostasis

Spinal cord is protected inside the vertebral column and is covered by the meninges.


Spinal cord, which is  the continuation of medulla oblongata, is situated with in the vertebral column and is covered by a three layered membrane, called meninges. The outer part of spinal cord is white matter and  inner is grey matter. The central canal at its centre is filled with CSF. Spinal nerves originate from the spinal cord as dorsal root (sensory) and ventral root(motor).

鉂攆unctions of spinal cord.

鉁旓笍鈥 Transmitting impulses from different parts of our body  to and fro the brain.

鉁旓笍鈥 Coordinates the rapid and repeated movements during walking, running etc.

鉁旓笍鈥 Effects certain reflex actions

reflex actions;The accidental and involuntary responses towards stimuli are

called reflex actions

Reflex arc is the pathway of impulses in the reflex action. Mainly spinal cord acts as the centre of reflex action and such reflexes are called spinal reflexes


Activities that take place beyond the conscious level are controlled by the autonomous nervous system, a part of the peripheral nervous system. The sympathetic system and the parasympathetic system together form the autonomous nervous system

鉁匒lzheimer’s disease

鉂攃ause;Accumulation of an insoluble protein in the neural tissues of the
brain. Neurons get destroyed.

鉁旓笍symptoms;Loss of memory, inability to recognize friends and relatives, inability to do routine works.


鉁匬arkinsons disease


鉂攃ause;Destruction of specialised ganglions in the brain. Production of dopamine, a neurotransmitter in the brain gets reduced

鉁旓笍symptoms;Loss of body balance, irregular movement of muscles, shivering of the body, profuse salivation


鉂攃ause;Continuous and irregular flow of electric charges in the brain.

鉁旓笍symptoms;Epilepsy due to continuous muscular contraction, frothy discharge from the mouth,clenching of the teeth following which the patient falls unconscious.


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