Hormones are very important biochemicals produced in the body by specialized group of cells called glands. Chemically hormones are composed of proteins. These regulate the various metabolic reactions in the body. These are required only in small amount. Their low or over production in the body may cause serious health problems.

Characteristics of Hormones

1. Hormones are produced within the body by endocrine glands.
2. Hormones do not act on the glands which produce them. They act on some other organs of the body called target organs.
3. Hormones are directly released into the blood by endocrine glands so that they may reach the target organ by blood circulatory system.
4. Hormones may accelerate or inhibit specific physiological processes.
5. Hormones are effective only in small amount.

Test Your Understanding and Answer These Questions:

1. What are hormones?
2. What are characteristics of hormones?

Endocrine system is a group of glands that work together to control and coordinate the activities of animals. Glands are groups of cells which produce specific chemicals called hormones. The first hormone was discovered by English scientists William Bayliss and Ernest Straling in 1903. Hormones do a variety of jobs in our body.

Types of Glands

In animal body two types of glands are present:

    1. Exocrine glands and
    2. Endocrine glands

Exocrine Glands
Those glands which secrete hormones via a duct directly into their target organ are called exocrine glands, e.g., live and salivary glands.

Endocrine Glands
Those glands which do not have a duct and secrete hormones directly into the blood are called endocrine glands, e.g., pituitary glands, thyroid glands, adrenal glands, testes, ovaries etc. these glands are also known as ductless glands as they do not have ducts for secretion of hormones.

We will now discuss various endocrine glands present in human body.

1. Pituitary Gland (Master Gland)
Pituitary gland is located just below the brain. Pituitary gland is also called master gland because the hormones secreted by pituitary gland regulate the working of all the other endocrine glands present in the body. In the body following hormones are secreted by pituitary gland.

    1. Growth Hormone (GH)
    2. Thyroid Stimulating Hormone (TSH)
    3. Follicle Stimulating Hormone (FSH)
    4. Anti-diuretic Hormone (ADH)
    5. Oxytocin

2. Thyroid Glands
Thyroid gland is located in the neck in our body. The hormone produced by thyroid gland is called thyroxine. The function of thyroxine hormone is to increase metabolic rate and control growth of body. Deficiency of thyroxine hormone in the body may cause a disease called goiter.

3. Parathyroid gland
These are four separate glands which are present on the backside of thyroid glands. These glands produce a hormone called parathormone which maintains the level of calcium and phosphate in blood.

4. Pancreas
The pancreas lies below the stomach in our body. It secretes two hormones: insulin and glucagon. Insulin decreases the level of sugar in blood. In case of deficiency of insulin in the body, the level of sugar rises in the blood above the normal level. The rise of blood sugar level in the body causes a disease called diabetic mellitus.

The function of glucagon is opposite to that of insulin. It increases the level of sugar in blood.

5. Adrenal Gland
The human body has two adrenal glands one above each kidney. The hormone secreted by adrenal glands is called adrenaline. It is also called emergency hormone, because it is produced under the conditions of fear, anger and stress. Adrenaline hormone prepares the body to face these situations by increasing body temperature, metabolic rate, heart beat and flow of blood to muscles and increase availability of glucose in blood to give us a lot of energy.

6. Testes
These are two glands which are present only in males. These glands secrete male sex hormone called testosterone. Testosterone controls the development of male sex organs and male sex characters like beard, moustache, low pitch voice and more body hair.

7. Ovaries
These are two glands which are present only in females. These glands secrete two female sex hormones called oestrogen and progesterone. The oestrogen controls the development of female sex organs and female sex characters such as high pitch voice, breasts and soft skin. The progesterone controls the formation of placenta and maintenance of pregnancy.

It should be noted that the hormone testosterone produced from the testes of males and oestrogen and progesterone produced from the ovaries of females are also called sex hormones.

Test Your Understanding and Answer These Questions:

1. What are sex hormones?
2. What is endocrine system?
3. What is diabetes? How is it caused?
4. What are characteristics of hormones?
5. Why pituitary gland is called master gland?
6. Name the scientists who discovered hormones.
7. List the functions of testosterone and oestrogen.
8. Why adrenaline hormone is called emergency hormone?
9. Give differences between endocrine and exocrine glands.
10. Write the names of three hormones secreted from pituitary gland.
11. What are endocrine glands? Why these are called ductless glands?
12. Name the hormones secreted by thyroid, parathyroid and pancreas.
13. How the hormones secreted by endocrine glands reach their target organs?
14. Give the names of some endocrine glands present in human body. Also write the names of hormones produced by them and their functions.
15. Write the name of a disease caused by deficiency of insulin and excess of thyroxine in the body.

Reflex action is a sudden and involuntary response of effectors (muscles and organs) to a stimulus. Withdrawl of hands when touched by hot things, watering of mouth on seeing favourite food, coughing, sneezing and blinking of eyes are examples of reflex actions. The reflex actions are controlled both by brain and spinal cord. The reflex actions which are controlled by brain are called cerebral reflex actions. And the reflex actions which are controlled by spinal cord are called spinal reflex actions.

The path traveled by a nerve impulse from the region of stimulation to the region of response is called reflex arc. It involves following five structures:

    1. Receptors
    2. Sensory nerves
    3. Brain or spinal cord
    4. Motor nerves and
    5. Effectors.

To understand reflex arc more clearly, let’s take an example. When we suddenly touch a hot plate, we pull our hands away from hot plate very quickly and without thinking. Here, following things happen: the skin of our hands (receptors) feels the hotness of plate and sends a nerve impulse to the spinal cord through sensory nerves. The spinal cord interprets the message and decides what the response should be. Then a nerve impulse is sent by spinal cord to the muscles (effectors) in arm through motor nerves. The muscles receive the message and contracts to pull hand away from the hot plate. This pathway of reflex action can be shown as given below:

Test Your Understanding and Answer These Questions:

1. Explain reflex action and reflex arc.
2. Define reflex action with suitable examples.
3. Arrange the following in correct order: Receptors, muscular activity, spinal cord, motor nerve, sensory nerve.

The nervous system can be divided into three main divisions:

1. The central nervous system (CNS)
2. The peripheral nervous system (PNS)
3. The autonomic nervous system (ANS)

Central Nervous System
The central nervous system consists of brain and spinal cord.

Peripheral Nervous System
The peripheral nervous system consists of nerves arising from the brain and spinal cord. The nerves that originate from the brain are called cranial nerves, and the nerves that originate from the spinal cord are called spinal nerves.

Autonomic Nervous System
Autonomic nervous system is a special system which controls and coordinates those activities of body that which are not under the control of will (involuntary organs). So, autonomic nervous system controls and coordinates those activities of body which do not require thinking such as beating of heart, secretions of glands, blinking of eyes and sneezing. It is further divided into sympathetic and parasympathetic nervous system.

The functions of sympathetic and parasympathetic nervous system are opposite to each other. For example, the sympathetic nervous system increases the heart beat whereas the parasympathetic nervous system decreases the heart beat. In the same way, the sympathetic nervous system constricts the arteries and raises blood pressure whereas the parasympathetic nervous system dilates arteries and lowers blood pressure.

Test Your Understanding and Answer These Questions:

1. Expand PNS and CNS.
2. What is autonomic nervous system?
3. Give differences between sympathetic and parasympathetic nervous system.

In an insect, a long nerve cord is present which runs along the whole body. Like hydra, insects also do not have a brain. Instead of brain, many ganglia are present at intervals on the nerve cords.

A ganglia is a mass of nerve cells. The largest ganglion is present in head of an insect. The ganglia receive messages from the sense organs and direct them to other parts of the body through nerves which are attached to ganglia.

Test Your Understanding and Answer These Questions:

1. Explain nervous system of hydra by drawing a neat and clean diagram.

Neuron – Unit of Nervous System

The nervous system of animals consists of a network of specialized cells called neurons or nerve cells. Function of neuron or nerve cells is to carry information from receptors to brain and then from brain to effectors in the form of electrical signals. The passing of information through neurons in the form of electrical signals is called nerve impulse. Thus, a neuron is a functional and structural unit of nervous system. Neuron is the largest cell present in the body. It consists of three parts:

1. Cell body
2. Dendrites and
3. Axon

Cell body
A cell body is an irregular structure having cytoplasm and nucleus.

Dendrites
Dendrites are short and branched fibres which stretch out from the cell body.

Axon
Axon is a long fibre which also extends from the cell body. It carries messages in the form of electrical signals to other cells. It is enclosed in a sheath of protein myelin, which insulates them and helps to increase the speed at which they carry signals.

Neurons do not connect with each other directly. There is always a small gap between two neurons. This gap is called synapse. Information is passed from one neuron to another across the gap by a chemical substance called neurotransmitter.

Types of Neurons

There are three types of neurons:

1. Sensory Neurons: These carry information from receptors to central nervous system (brain & spinal cord)
2. Motor Neurons: These carry information from central nervous system to effectors.
3. Association Neurons: These link sensory neurons to motor neurons.

Test Your Understanding and Answer These Questions:

1. What is a neuron?
2. What is synapse?
3. What is a nerve impulse?
4. What is a neurotransmitter?
5. Explain structure of a neuron.
6. Draw the diagram of a neuron.
7. Explain different types of neuron.
8. What is the function of neurons or nerve cells?

The nervous system of human beings consists of brain, spinal cord and nerves.

Brain

The brain is the anterior part of the central nervous system. It is situated within the hard bones of the skull. It is wrapped in three separate membranes called meninges. The space present between these layers is filled with cerebro-spinal fluid (CSF). This fluid serves as a cushion and protects the brain from injuries and shocks. Six pairs of cranial nerves originate from the brain.

The brain of human beings is divisible in three major parts:

1. Forebrain
2. Midbrain
3. Hindbrain

1. Forebrain

Fore brain of human beings consists of cerebrum and olfactory lobes.

Cerebrum

The cerebrum is the largest part of the brain which is dome shaped in structure. It is divided into two hemispheres called cerebral hemispheres by a deep fissure in the centre. The two cerebral hemispheres are connected to each other by a structure called corpus callosum. Each hemisphere of the cerebrum is further divided into four lobes.

1. Frontal lobe:- This part of the cerebrum controls muscular activities and voluntary movements.
2.  Parietal lobe:- This part of the cerebrum controls general sensations such as taste, smell, pain, touch and temperature.
3.  Temporal lobe:- This part of the cerebrum is concerned with sense of hearing.
4.  Occipital lobe:- This part of cerebrum is concerned with sense of sight.

Olfactory Lobes

These are club shaped structures which are present at the anterior part of the brain. Olfactory lobes are concerned with sense of smell.

Functions of Forebrain

1. It receives information from sense organs like eyes, ears, skin, nose and tongue.
2.  It interprets information and controls voluntary movements.
3.  It changes our behaviour on the basis of past experiences and memories.
4.  It maintains a balance between stimulus and response.
5.  It regulates the sense of sight, hearing, smell, touch, temperature and pain.

2. Midbrain

Mid brain of human beings is not further divided into parts. It is mainly concerned with sense of sight and hearing.

3. Hindbrain

Hind brain of human beings consists of cerebellum, pons and medulla oblongata.

Functions of Hindbrain

1. The cerebellum coordinates the movements and maintains posture.
2.  Pons is concerned with regulation of respiration.
3.  Medulla oblongata regulates heart beat, blood pressure, coughing, sneezing and rate of respiration.

Spinal Cord

Spinal cord is a cylindrical extension of the brain. It runs through a channel within the bony cage of vertebral column. Like brain, spinal cord is also wrapped in meninges. Thirty one pairs of spinal nerves originate from the spinal cord and reach every part of the body. The length of spinal cord varies from 37 to 45 cm. depending on the height of the person.

Functions of Spinal Cord

1. It conducts nerve impulses from every part of the body to brain and carries messages back again.
2.  It regulates the reflex actions like instant removal of hands from a hot object.

Test Your Understanding and Answer These Questions:

1. Write a note on meninges.
2. Mention the functions of fore brain.
3. Explain nervous system of human beings.
4. What are the functions of medulla oblongata?
5. Write the functions of any one part of hind brain.
6. What are the functions of cerebral hemispheres?
7. What is cerebrospinal fluid? What are functions of cerebrospinal fluid?
8. Name the structure which joins the two cerebral hemispheres with each other.

       Animals control and coordinate their activities with the help of nervous system and endocrine system. The nervous system consists of a group of organs which control and coordinate the activities of the various parts of the body in response to changes in environment. Nervous system receives information from the surrounding with the help of sense organs also called receptors.

Various types of receptors are present in body of animals which have different functions. For example, photoreceptors respond to light, phonoreceptors respond to sound, olfactoreceptors respond to smell and chemoreceptors respond to chemicals. The information received by receptors is then sent to brain by sensory nerves. Brain interprets the information and decides what the response should be. Then the information is sent back to concerned part of the body such as muscles or organs (also called effectors) to do the desired activity.

Test Your Understanding and Answer These Questions:

1. Define nervous system.
2. Mention the receptors for light and sound in animals.
3. What are receptors? Mention the various types of receptors present in animals.