Introduction:

In animals, there are well-articulated systems for transmission and perception of stimuli. A stimulus is any change in the external or internal environment which can bring about a transform in either a part of or the entire of an organism. Reaction is the change in activity of an organism as an outcome of the stimulus.

In animals, activities like locomotion, digestion, respiration, excretion and response to external and internal environments are coordinated through the endocrine system and nervous system. The hypothalamus is the significant link among such nervous and endocrine system.

Difference between the Nervous system and Endocrine system:

The major difference among these two systems is that the nervous system co-ordinates the body parts via electrical impulses while the endocrine system too co-ordinates bodily functions though is composed of glands which excrete hormones. Nervous system controls the muscle movement, senses, breathing, heart-beat, digestion, memory and speech while the endocrine system controls the levels of glucose in blood, heat productivity, hydration levels, sexual maturity, sperm and egg production and development of cells and tissues.

Components and Organization of Nervous System:

1) The nerve cell or neuron: 

A neuron is a nerve cell which is the fundamental building block of the nervous system. Neurons are identical to other cells in the human body in a numerous ways; however there is one main difference between the neurons and other cells. Neurons are specialized to broadcast information all through the body.

These extremely specialized nerve cells are accountable for communicating information in both electrical and chemical forms. Sensory neurons transmit information from the sensory receptor cells all through the body to the brain. Motor neurons convey information from the brain to muscles of the body.

2) Functions of the nervous system:

a) It gathers information on changes in the atmosphere of the organism (internal and external)

b) It processes the received information

c) It takes steps on the information.

3) Organization of the nervous system:

The nervous system of mammals is comprised of two parts:

1) The central nervous system

2) The peripheral nervous system

In humans, the nervous system consists of two main divisions:

• The central nervous system (CNS)
• The peripheral nervous system (PNS)

The Central Nervous System (CNS):

The central nervous system comprises of brain and spinal cord. It is considered as the 'thinker' or 'information processor' of the body. In mammals the central nervous system (CNS) includes of the brain and the spinal cord.

Brain:

The human brain is extremely developed organ located in the skull. It weighs around 1200 to 1400 g in an adult. It consists of three main parts: Cerebrum, Cerebellum and Medulla oblongata.

a) Cerebrum: The cerebrum is the biggest and most important portion of the brain. Among all vertebrates cerebrum of humans is most extremely developed. It is splitted into left and right hemispheres having a deep median longitudinal groove.

b) Cerebellum: It is a small region of brain lying beneath the mid-brain which is beneath the big cerebrum. Similar to cerebrum, it as well consists of grey matter in its outer area and white matter in the inner area.

c) Medulla oblongata: This is the lowermost portion of the brain situated at the base of the skull. The medulla oblongata performs the given functions:

• It controls the internal organs such as movement of lungs, heart and so on by regulating the breathing and heart-beat.
• It mainly controls very important reflex centers like cardiac centre, respiratory centre and centers for swallowing, coughing, sneezing and vomiting.

Spinal cord:

The spinal cord is a long cord which extends from the medulla oblongata and continues downward within the vertebral column. Spinal cord consists of a narrow canal and this central canal of the spinal cord is filled by cerebrospinal fluid.

The arrangement of the white and grey and matter is simply reversed in the spinal cord. The grey matter lies on the internal side whereas the white matter on the external side. The spinal cord performs the given functions:

• It controls the reflexes beneath the neck area.
• It conducts sensory impulses from the muscles and skin to the brain.
• It conducts motor response from brain to the muscles of limbs and trunk.

Peripheral nervous system:

The peripheral nervous system gives the bridge among the brain, spinal cord and the whole body. It comprises of 12 parts of cranial nerves that occur from the brain and 31 pairs of spinal nerves that occur from the spinal cord. They are mostly of motor system and sensory system. The peripheral nervous system comprises of somatic nervous system and the autonomic nervous system.

1) Somatic nervous system:

It obtains and processes information from receptors in the skin, tendons, voluntary muscles, joints, tongue, eyes, nose and ears and therefore provides an organism the sensation of touch, heat, cold, pain, balance, sight, taste, smell and sound. It as well controls the voluntary actions such as movement of arms and legs.

2) Autonomic nervous system:

It comprises of a pair of chains of ganglion and nerves found on either side of the back-bone. It is classified into sympathetic and parasympathetic nervous systems. It mainly controls the involuntary actions of the internal organs of the body such as heart and so on.

Sense Organ:

We all taste, smell, touch, hear and see because of our sense organs. If a sense organ identifies a stimulus it sends messages all along the nerves to brain. The brain provides us feelings or sensations. Our sense organs and their functions are described below:

Skin:

The sense of touch is generated by the ends of nerve cells termed as nerve endings or receptors as they receive stimulation from the exterior world. Such nerve endings are of various types for dissimilar stimulations.

In a human adult the surface area of the skin is around 1.5 to 1.7 square meters. The thickness of the skin differs from around 0.5 to 3 mm. The skin is composed of two distinct layers: epidermis and dermis.

Functions of skin:

a) Skin mainly protects the body from bacterial infections, heat, cold and mechanical injuries.

b) Skin is sensory to pain, touch and temperature.

c) Skin regulates the body temperature. Extreme heat is lost via evaporation of sweat or else it is preserved by fat and hair in the skin.

d) Oily stuffs are freely absorbed by skin.

e) The surplus of water, salts and waste products are excreted via the sweat.

f) Vitamin-D is produced in the skin.

Eyes:

Eyes are highly protected organs of our body via which we see. The eyes are lodged in the skull. If we close our eyes a layer of skin including hair (eye lashes) in its margin wraps each eye. This is an eyelid. This protects our eye from an exterior discomfort. Various parts of the eye are cornea, pupil, iris, retina and so on.

Eye Defects:

a) Short sightedness or myopia: It takes place if the eye-ball is longer than normal. In this fault objects close to the eye can be focused correctly, however the point of focus for far-away objects is in front of the retina.

b) Long sightedness or Hypermetropia: It takes place if the eyeball is shorter than normal. In this fault distant objects can be focused correctly; however the point of focus for an object close to the eye is at the back of retina.

Ears:

Ears aid us in hearing various sounds and balancing our body. The air around us is occupied of vibrations termed sound waves. We contain one ear on each side of the head. Ears modify vibrations in the air into the nerve impulses that travel to brain where they are interpreted as sound.

The portion, which we name our ear, is a flap of skin in the shape of the funnel. This is the exterior ear. This leads to a tunnel - the ear canal, at the end of which is a thin sheet of skin termed as the eardrum. Sound waves are collected through the outer ear and directed within the ear canal, where they set the eardrum vibrating. The eardrum is joined to the internal ear by three small bones or semicircular canals termed as ear ossicles. These are hammer (or malleus), anvil (incus) and stirrup (stapes). Such bones transmit and intensify the vibrations raising their force by around 20 times.

The ossicles are joined to an oval window. Due to the vibrations, oval window moves in and out causing vibrations via the cochlea. The cochlea includes a carpet of tiny hair like structures, which are joined to nerves. They are in reality sensory cells that assist us to listens the sound.

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