What is meant by stimulus and reaction?

The nervous system is a network of organs and tissues that controls and co-ordinates all the activities of the body with the help of nerves.

Nerves are the communication lines for transmitting impulses from one part of the body to another.
The difference between afferent and efferent nerves are:

A neuron or nerve cell is a highly specialized cell which acts as the structural and functional unit of nervous system.
A neuron basically consists of three parts, viz. cell body, axon and dendrites. The cell body contains large central nucleus surrounded by cytoplasm. The cytoplasm contains granules and neurofibrils along with mitochondria, endoplasmic  reticulum, golgi complex, etc. Numerous short, thread like structure arises from the cell...

a.    Cerebrum:
    - It governs the mental abilities (thinking, learning).
    - It enables sense organs to observe things.
b.    Corpus callosum:
    - It transmit info from one to other hemisphere of cerebrum.
c.    Cerebellum:
    - It maintain equilibrium and maintains posture.
    - It regulates and coordinates the contraction of voluntary muscles.
d.    Pons...

The main part of hind brain are:
a.    Cerebellum:
    - It maintain equibirium and controls posture.
    - It makes body movt. smooth, steady and coordinated.
    - It regulates and coordinates voluntary muscles.
b.    Pons varolii:
    - It serves as relay station carrying signal from the cerebrum to cerebellum.
    - It regulates respiration.
...

Spinal cord is a long, cylindrical structure that arises from the medulla oblongata of the brain and passes through the neural cannel of the vertebral column.
Spinal cord a found inside the boney cage of the vertebral column. It is protected by the spinal menings and cerebro spinal fluid, which cushions, the chord by acting as a shock absorber. Each vertebra has an opening on its left and right side to let spinal neve pass through. These...

A spontaneous, mechanical and automatic response to a stimulus controlled by the spinal cord without the involvement of the brain is called reflex action.
For eg.     Watering of mouth while seeing tasty food.

A reflex arc is a pathway followed by the sensory and motor nerves in a reflex action.

The function of sensory neurons is to carry message from receptor organ to the spinal cord.
The function of motor neuron is to carry message from the spinal cord to the effector organs.

The system formed by a group of endocrine gland is called the endocrine system.
The differences are:

Hormone are the chemical messengers which control, co-ordinate and integrate the function of various organs of the body.
Its properties are:
    - It is produced by endocrine glands.
    - They are required in small amount and their effect are long lasting.
    - They are chemically proteins, steriod and amino acid.
    - They act as chemical messengers.
Its functions are:
    - It...

Pituitary gland :
        Location: Below the mid brain attached to the lower surface of the brain.
        Hormone: (262)
Thyroid gland:
        Location: In front of trachea just below the larynx.
        Hormone: Thyroxine
Pancreas:
        Location: Attached to duodenum in...

The name of hormones produced by the ovaries are:
Oestrogen:
        - It controls the devt. of female sex organ.
        - It controls the secondary sex characteristics of female.
Progesterone:
        - It plays a role in ovulation.
        - It helps in the maintenance of pregnancy.

Hyposecretion of the thyroxine hormone cause simple goiters, the thyroid gland enlarges and become visible as a swelling of neck.
Hypersecretion of thyroxine hormone cause roughness on puffiness of the skin, falling of hairs, obesity, imbalance, digestive disorders, loss of weight and so on.

We all know, hormones are the chemicals carried to all parts of the body through blood circulation to bring about the hormonious working of the body. So, hormones are called chemical messengers.

We all know, the pituitary gland controls and stimulates many activities of the body and the secretion of other endocrine glands too. So, it is called master gland.

We all know, adrenal gland secretes adrenalin hormone at the time of emergency which prepares the body to face an emergency situation. So, it is called emergency gland.

We all know, animals have well developed nervous system that coordinates with the stimulus but plants don't have such system. So, animals can respond quickly to a stimulus but plants cannot.

The  groups of small nerves tissues that is located near the spinal cord made up of grey matter is ganglia.
It is located near the spinal cord. They establish connection between the spinal cord and the brain.

The nervous system consists of nervous tissues that communicate between the brain and different parts of the body is peripheral nervous system.
Its functions are:
        - It communicate between brain and different body parts.
        - It hold reflex action within it.

The nervous system that controls certain muscles and glands while sleeping or waking up is called the autonomic nervous system.
Its types are:
        - Sympathetic nervous system.
        - Para sympathetic nervous system.

The function of sympathetic nervous system is it increase the activities of the heart, stomach lungs, intestine, etc.
The function of para sympathetic nervous system is to bring activities of the heart, stomach, Intestine, etc. to normal condition.

When the adrenal cortex increases in male, enlargement of the breasts can be seen.

Masculine characters like horse sound, development of beard, etc. are seen when the adrenal cortex increases in the adult females.

Diabetes is a group of metabolic disease in which a person has high blood sugar.
Its causes are:
        - Too much carbohydrate in diet.
        - Lack of physical exercise.
        - Obesity.
        - Lack of secretion of insulin by pancreas.
Its symptoms are:
        - Excessive thirst
        - Dehydration.
        - Presence of sugar.
     ...

a.    Axon and dentrite.

b.    Cerebrum and cerebellum.

The human heart lies near the middle of the thoric cavity between two lungs. It is the hollow, conical organ made up of cardiac muscles. It is about the size of a fist of the owner. It measures about 12cm in length and 9 cm in breadth. The average weight of the heart is about 300 gm in adults.
The human heart contains four complete chambers. It E is divided into two halves (right and left) by septum. Each half consists of two chamber...