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Nervous system

Definition:  It is the system that maintains general co-ordination in multicellular animals.

Function of nervous system:

  1. To maintain a general co-ordination in the animal body.
  2. To respond to external stimuli and to adjust the body with the changing condition of environment.
  3. To maintain different equilibria in the body.


Unit of the system:

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

A neuron is a spider shaped cell consisting of two parts

  1. Cell body and the         ii) the cell processes

Cell body:  It is a main part of the cell and consists of a large amount of cytoplasm and a large conspicuous nucleus that remain externally guarded by cell membrane.  The cytoplasm carries the normal cell organelles except the centrosome and that is why a neuron can not achieve mitosis.  The cytoplasm carries a huge number of fine nissl’s body.

The cell processes:  The cytoplasm from the cell body extends in the form of threads such outgrowth coming out of the cell body are called cell processes they are of two types.

  1. Dendrite/dendron: These are shorter and highly branched cytoplasmic extensions that radiate from the cell body.

Function-  Dendrite carries nerve impulses towards the cell body.

2.  Axon: It is a longer extension, which normally does not get branched.  It ends into a few fibres each of, which carries a swollen tip called axon button.  The axon remains externally covered by a coat neurilemma, which is composed by special types of cell called Schwann cell.

Function-  The axon carries nerve impulse away from the cell body.

In some axons a special layer of fat gets sandwiched between the axon and neurilemma this fat is called myelin sheath or medullary sheath.  The medullary sheath is interrupted at some intervals and such a region is called the node of Ranvier.  This type of axon is called the medullated axon and they form a medullated nerve.  The axons, which do not carry any medullated sheath, are called non-medullated axon, which from non-medullary nerves.

Synapse:  Synapse is the junctional area between a pre-synaptic and post-synaptic neuron where they tend to join each other.  In a synapse the two neurons do not have any organic connection between them.  A microscopic gap called the synaptic cleft is left between the axon of the pre-synaptic neuron and dendrite of the post-synaptic neuron.

When a nerve impulse reaches the axon buttons of the pre-synaptic region a conducting liquid called the neurohumour rich in chemical substance called acetylcholine is secreted that bridges the gap between the two neuron thus the nerve impulses is relayed from the pre and post synaptic neuron.


Formation of nerve:  A few axons get bundled and covered by a common coat the endoneurium to form a nerve fibre.  A few nerve fibres aggregates and gets covered by a common coat the perineurium to form a nerve.  A nerve is thus the collection of some nerve fibres each of, which is a collection of a few axon fibres.  Some nerves are again collected and get covered by epineurium to form a nerve trunk.


Types of nerve:  According to function

  1. Sensory/afferent nerve- These  are the nerves that carry the impulses from the sensory organs to CNS (central nervous system) eg. optic nerve.
  2. Motor/efferent nerves- These are the nerves that carry stimulus from the CNS to the effector organs (muscles or glands) eg. oculomotor.
  3. Mixed nerve- These are the nerves that perform both the sensory and motor action eg. vagus.


Ganglion (pl. ganglia):  A ganglion is a swelling on the nerve formed as a result of aggregation of a number of cell bodies of neuron at a place.  From the ganglion the branching of the nerve is produced.


Types of Nervous system:

  1. CNS- It is that part of the nervous system which remains located on the main axis of the body.  In man it is composed of brain and spinal cord.
  2. Peripheral nervous system- This system is composed of the branches coming out of CNS in man it consists of twelve pairs of cranial nerves (ten pairs in toad) coming out of brain and 31 pairs of spinal nerves coming out from the spinal cord.
  3. Autonomic nervous system- It is composed of those nerves that function independently of any direct control by CNS. Visceral organs are generally supplied by these nerves and hence their movements can not be controlled by an individual.  It is composed of two types of nerves namely sympathetic and parasympathetic nerves, which act in an antagonistic manner for example on controls the dilation of the pupil while the other controls its constriction, one controls, the systole and other controls diastole of the heart.


The brain remains encased with in the cranium. The average adult brain weighs about 1,400 gms or approximately 2% of total body weight.  It is semisolid pinkish gray organ covered by a succession of three membranes called meninges.  The meninges from outside inwards are in the following.

  1. The durameter is the toughest layer and protects the inner parts.
  2. The arachnoid layer contains blood vessels and cerebrospinal fluids.
  3. Pia mater nourishes the brain.

The brain divided into three parts namely the forebrain, the mid brain and the hindbrain.

  1. The fore brain: It is the largest part of the brain and it is composed of the following structures
  2. a) Cerebrum: It is composed of two cerebral hemispheres. It is the largest parts of the brain and occupies about 9/10 part of the brain.  The cerebral cortex is highly enlarged and to accommodate such an enlarged area, it remains highly folded or wrinkled.

Functions:  It acts as the seat of intelligence higher mental abilities, emotion, memory etc.

  1. b) Thalamus and hypothalamus: These are two shortened areas that lies hidden by cerebrum.

Functions:  Various sensations like vision, smelling, tasting, hearing etc are practically persist in this region.

  1. The mid brain. It is a very short area composed of two optic lobes. Function. It regulates the various movement of eye balls.
  2. The hind brain. It is the posterior most part and it is composed of following structures.
  3. Cerebellum: A large globular structure situated behind the cerebrum. It controls the movements of the voluntary muscles.

Function:  It controls the movements of the voluntary muscles and regulates the balance of the body.

  1. Pons: It is swelled over structure situated below the cerebrum.

Function: It controls the mechanism of sleep and the process of engulfing the food.

  1. Medulla oblongata: It is the terminal part of the brain that gradually tapers to form the spinal cord. Function:  It controls the heartbeat, respiratory movements, gastric secretion, and production of sound.


In human brain there are two glands that lie inside the brain they are the pitutary gland and pineal gland.

Pitutary gland:  It secretes various hormones and controls the hormone secretion of other glands.

Pineal gland:  It is located at the roof of the third ventricle.  It secretes pineal gland secretes melatonin hormone.


Spinal cord-  The medulla oblongata narrows down to form the spinal cord.  The spinal cord is a cord like bundle of a huge number of nerves in man.  It is placed along the mid dorsal line of the body.  It remains protected by the vertebral column through, which it runs posteriorly.

Along the central part of spinal cord a fine canal runs and is known as central canal.  The spinal cord is composed of two areas.

  1. Grey matter: It occupies the central part of the spinal cord and is composed of cell bodies and dendrites of neuron.
  2. White matter: It is placed towards the peripheral part of the spinal cord and lies around the grey matter. It is composed of axons of the neuron.


N system in vertebrates

N system invertebrates

1.      There is a single spinal cord running along the mid dorsal line of the body.

2.      The spinal cord remains guarded by vertebral column.

3.      The brain is formed by the bulging out of the spinal cord.

1.      There is a double nerve cord passing along the mid ventral line of the body.

2.      The nerve cord remains naked.

3.      The brain is solid, formed by the fusion of ganglia.


Reflex action

Any involuntary and spontaneous response to a sensory stimulus through the mediation of the spinal cord is called the reflex action.

Reflex actions are of two types.

  • Simple or unconditioned reflex: They are the reflex acts, which are acquired by an living organism right from the time of birth, no amount of training is needed to achieve it.  sneezing, vomiting, evacuation.
  • Conditioned reflex actions: These are the reflex acts, which are learned by an individual and training is needed to achieve them.  These can be deconditioned through willful activities.  cycling, swimming.


Reflex arc- The neural path through which a reflex action is achieved is called a reflex arc.

It is composed of the following parts.

  1. Receptors: The receptor cells present in the sensory organs receive a particular stimulus, which sets in the reflex action.
  2. Sensory path: The stimulus received by receptor is carried by this path from the sense organ concern to the grey matter of the spinal cord, it is composed of sensory neurons.
  3. Inter neuron or associative neuron: These are the neurons that are present inside the grey matter of the spinal cord.  They bridge between the sensory and the motor paths.
  4. Motor path: The necessary responds to the stimulus is carried by this path from the spinal cord to the effectors organ.  It is composed of motor neurons.
  5. Effector organ: It is the organ that ultimately manifests the reflex action.  The order carried by the motor path is actually translated into activity by this organ it may be voluntary muscles or glands.  When our hand touches a hot body without knowing it the arm jerks off the hot substance.  This reflex action is achieved by reflex arc.


Sensory organs:  The organs that receive the external stimuli with the help of the receptor cell and transmit the same to the CNS the organs are called sensory organ.



Eye is the organ of vision in man the eye is composed of an eyeball situated inside the orbit of the skull.  Each eye remains protected by an immovable lower eyelid and movable upper eyelid both carry eyelashes.  Behind the upper eyelid there are some lacrimal glands, which secrete tears that keeps the eye wet.

The eyeball is unequally divided into two chambers by the insertion of the eye lens.  The anterior chamber is small and carries a liquid – aqueous humour.  The posterior chamber is larger and carries denser liquid vitreous humour.  The eyeball remains covered by three coats.

1)  Sclera-  It is the outer most coat of the eye ball and is opaque in nature coming in front of the lens it forms a transparent coat called cornea.  The cornea remains externally covered by a very thin delicate transparent coat called conjunctiva.

Function:  1.  To protect the eyeball from the external injury.  2.  To render the eyeball in light tight condition.

2)  Choroid-  It is middle coat of the eyeball.  It is generally black in colour and carries the major blood vessel of the eye.  The choroid coming in front of the lens almost encircles it, leaving a round gap in the centre.  This extended part of the choroid is called iris and the central gap is called the pupil.  The iris is contractile in nature.  Its contraction and relaxation lead to the dilation and constriction of the pupil respectively.


  1. It renders the eyeball light tight condition.
  2. It is the vascular layer as it carries the main blood vessel of the eyeball.

3)  Retina- It is the inner most coat of the eyeball and is restricted only to the posterior chamber.  It carries the special light sensitive cells the photoreceptors.  These are two types cone cells-  These are cone shaped cells, which lie more concentrated towards the periphery on the retina.  These are responsible for the vision in dim light.  The principal axis of eye lens when extended fall on particular area of retina this area of retina is called the yellow spot or fovea.  It carries the maximum concentration of cone cells and hence is responsible for the bright vision.

That part of the retina which remains connected to the optic nerve is devoid of any photoreceptors.  Thus vision is not possible along this part that is why called blind spot.  The eyeball carries a biconvex crystalline lens, which is kept in position by a special muscle called suspensory ligaments.  These muscles can contract and relax and thereby the curvature of the lens in changed.


Mechanism of process of vision:   Light rays reflected form an object enter the eye lens through the pupil.  The rays are then refracted through the lens and are focused on the retina to form an inverted image of the object.  This image formation illuminates the photoreceptors, which receive the stimulus.  The stimulus is then transmitted by the optic nerve to the brain where it is translated into a definite form of vision.


Accommodation of eye:  It is the mechanism by which eye lens gets so adjusted that the image is always formed on the retina irrespective of the distance of the object.  In man it is achieved by changing the curvature of the eye lens as the object moves closer to the eye the lens become more spherical and as the object goes away from the eye the lens becomes pointed.


Monocular vision:  When the two eyes cannot focus the same object simultaneously, the vision is called monocular vision. Here the distance of the object can not correctly be determined. Eg.  bird, fish and toad etc.

Binocular vision:  When the same object can simultaneously be focused by both the eye the vision is called binocular vision.  Here the distance of the object can correctly be determined. Eg. owl, man.


Compound eye:  A compound eye is an aggregation of so many eye elements each of which is called an omatidium. As each eye element has a lens in it.  So many images are formed at a time of the same object.  Hence it is called mosaic vision.  Eg. Prawn.


Simple eye:  A simple eye is that which carries a single lens.  Thus a single image of the object is formed.  That results brighter vision is man.



Ear is the organ of hearing.  Human ear is composed of three parts- external, middle and internal ear.

  1. External ear- It consists of the following structure.
  2. Pinna- Cartilaginous flattened part that helps in the conduction of the sound waves.
  3. Auditory meatus- A tunnel shaped tube leading from the auditory opening.
  4. Tympanum- A highly stretched membrane made up of muscle, it vibrates when sound waves strike it.

Function:  Collection of the sound waves, from the exterior and leading them into the middle ear.

  1. Middle ear- It is an air filled sac that remains connected to the pharynx by the eustachian tube.  Ear ossicles made up of three pieces of small bones are present in the middle ear.  They are malleus, incus, stapes, which act together as a liver system to amplify the sound waves.

Function:  Amplification of the sound waves and the transmission of the same to the internal ear.

  1. Internal ear: It is liquid filled sac, lying suspended in this liquid called perilymph are the following two structure.
  2. Cochlea- It is a tubular structure that remains coiled like the shell of a snail.  It also carries a liquid inside it called endolymph.  The sound receptor cells, called the organ of corti are present inner wall of the cochlea.

Function:  to receive the sound stimulus and to transmit the same to the brain.

  1. Semicircular canals- These are three semicircular canals that are responsible for the maintenance of balance.

Mechanism of hearing-  Sound waves enters the ear through the auditory opening pass through the meatus and strike the tympanum, which vibrates as a result the vibration are amplified.  The organ of corti are present inside the cochlea receive the sound stimulus and transmits the same via the auditory nerve to the brain where it is translated into definite form of hearing.




Nose is the organ of olfaction.  The inner wall of the nasal cavity is made up of nasal epithelium.  A definite area of the nasal epithelium acts as the olfactory epithelium.  This region carries the smell receptor cells, the chemoreceptor.  Each such cell is bipolar neuron.  The dendrite of such a cell collects the smell stimulus, which is passed to the nerve by its axon.

Mechanism of smelling:  The smelling particles radiated form the substance enter the nasal cavity and gets partially dissolved there in.  this chemical change inside the nasal cavity creates the smell stimulus, which is received by the chemoreceptors and sent to the brain where it is translated into definite form of smell.



Tongue is the organ of gustation.  Numerous small grainy structures lie scattered on the dorsum of the tongue, these are called papillae.  Each papillae carries a minute opening at a centre called taste pore.  Inside each papillae there is a taste bud lying just behind the taste pore.  Each taste bud is composed of a few chemoreceptor cells that remain in a bundle to form the taste bud.  Minute hair originates from each chemoreceptors of taste bud and they are most sensitive to taste stimulus.

Mechanism of taste:  Food particles coming in contact with tongue gets partially dissolved in saliva ni mucus.  This chemical change creates the stimulus, which is then received by the chemoreceptor of the taste bud and is transmitted to brain where it is translated into definite form a taste.  There are specific area o the tongue for the taste of each kind.


                                Saline             Saline

                               Sour                 Sour




Skin is the largest sensory organ in man.  It is responsible for various sensations like heat, cold, and pressure, pain, itching, tickling.  Numerous different receptors lie scattered on the skin for the purpose some of them are Meissner corpuscle, and bulb of Krause and end organ of Ruffini.

Plant has no nervous system.  The general co-ordination in maintained by plasmodesmata.

Plasmodesmata:  They are very minute opening present o the cell wall cytoplasm communicates between adjacent cell through these pores thus co-ordination is possible.

Hormones:  Plants secrete a wide range of hormones, which maintain the general co-ordination of the body.  


On (olfactory), old (optic), Olympus (oculomotor), towering (trochlear), top (trigeminal), a (abducen), fin (facial), and (auditory), German (glossopharyngeal), viewed (vagus), some (spinal accessory), Hop (hypoglossal)

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Hi viewers My name is Partha Pratim Goswami, (PpG) I am a teacher by profession. I have been teaching biology, chemistry and physics, sometimes mathematics also for last sixteen years. I would like to share my entire experience with all the viewers across the world through YouTube, which I think is an amazing platform to share all my experiences.

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