- The nervous system is concerned with receiving of stimuli from the external and internal environment of the body.
- The action of nervous system are performed by highly specialized cells called neurons which can receive the messages and conduct them to the brain where the they are interpreted and then returned to the concerned parts of the body.
- The nervous tissue is made up of an organized network of nerve cells which are specialized for conducting electrical impulse form one part of the body to another.
- Impulse is the signal transmitted along a nerve fibre.
- Neurons or nerve cells are the structural ad functional units of nervous system.
Structure of Neuron
- Cell body- it is rich in cytoplasm and has a spherical nucleus.
- Dendrites- short and branched tips of nerve cell stretching out from the cell body and detect the information of the environment.
- Axon- long cylindrical structure arising from the cell body. The information from the environment travels through the axon and reaches the next nerve to reach the brain.
- Nerve ending- branched ending of axon which helps in neurotransmission.
- Neurotransmission is the travelling of impulse to the brain through the nerve cells.
- Impulse is the signal transmitted along a nerve fibre.
- Any change in the environment is detected by the specialized tips of nerve cells called dendrites in the form of message.
- Dendrites are present in the sense organs.
- The message acquired sets off a chemical reaction which creates an electrical impulse.
- Electrical impulse travels from the dendrite to the cell body and then along the axon to the nerve endings.
- At the nerve endings, the electrical impulse sets off the release of chemicals.
- Synapse is the gap between two nerve cells, across which impulses pass to reach the next nerve cell.
- The chemicals released at the nerve endings, cross the synapse and start a similar electrical impulse in a dendrite of the next neuron and the process goes on.
- A similar synapse allows delivery of impulses from neurons to other types of cells such as muscle cells or glands.
Reflex action is the extremely quick, automatic, sudden action in response to something in the environment.
Example- i) Immediately pulling back of fingers after touching a hot plate.
ii) Closing of eyes when flashed with high intensity light.
iii) Salivation when hungry.
- Reflex arc is the pathway of nerve involved in reflex action.
- Reflex arc involves-
- Receptors- receive the message from external environment.
- Sensory neuron- carries the message from the receptor to the central nervous system.
- Central nervous system- processes the message and sends impulses to the concerned part of the body in the form of response.
- Motor neuron- carries the message from the central nervous system to the effector.
- Effector- responses against the stimulus.
Central Nervous System
- The brain and spinal cord together constitutes the central nervous system.
- Central nervous system receives the message from all the parts of the body and integrates it.
- The spinal cord is a long, tubular bundle of neurons which carry information between the brain and the rest of the body.
- Human brain is covered by a bony box inside which the brain is contained in a fluid-filled balloon which provides shock absorption.
- Spinal cord is enclosed in the vertebral column.
- Human brain consists of three major parts- fore-brain, mid-brain and hind-brain.
- Fore brain is the main thinking part of the brain.
- Separate areas of the forebrain are specialized for hearing, smell, sight etc.
- Involuntary actions of the body which are not under our control are controlled by the mid brain and hind brain.
- Hind brain is divided into two parts cerebellum and medulla.
- Cerebellum of the hind brain controls balance, posture and smooth movement of muscles.
- Medulla of the hind brain controls heart rate, breathing rate and reflexes such as sneezing, swallowing, vomiting etc.
Hormonal System in Animals
Electrical impulses are excellent means for transmission of information but there are few limitations of using electrical impulses-
Impulses reach only those cells that are connected with nervous tissue.
Cells cannot continuously create and transmit impulse, the cells need some time to reset the mechanism before these can create and generate a new impulse.
Due the limitations, instead of generating electrical impulse, the stimulated cells release a chemical compound called hormones.
Hormones will diffuse to all the cells of the body and the cells have receptors to detect the hormones and to receive the information and transmit it.
Hormones are the chemical messengers which are secreted by the endocrine gland into the blood.
Different animal hormones are-
- Secreted by thyroid gland with the use of iodine.
- Helps in the regulation of carbohydrates, proteins and fat metabolism in the body.
- Deficiency of the hormone causes goitre.
- Enlargedneck is the characteristic symptom of goitre.
- Secreted by pituitary gland.
- Regulates growth and development of the body.
- Deficiency of the hormone leads to dwarfism.
- Reproductive glands secrete testosterone in males and estrogen in females.
- Development of sexual characters.
- Secreted by pancreas.
- Regulates blood sugar level.
- Deficiency of the hormone causes diabetes mellitus, increase in blood sugar level.
- Hormones should be secreted in precise quantity.
- Excess amount as well as deficiency of hormones causes disorder.
- A mechanism is used by the body to regulate the secretion of hormone which is called as feedback mechanism.
For eg. If sugar level increases in blood, cells of the pancreas can detect this and produce more amount of insulin to control blood sugar and as the blood sugar level falls, secretion of insulin reduces.
Coordination in Plants
- Plants do not have nervous system or muscles to respond to the stimulus.
- Plants show two different types of movement in respond to the stimulus i) one dependent on growth ii) another independent of growth.
Movement Dependent on Growth
Plants respond to stimuli by growing in a particular direction and because of the directional growth it appears as if the plant is moving.
Tropic movement or tropism is the plant movement that is determined by the direction of an environmental stimulus.
Tropic movements can be towards the stimulus or away from it.
The tropic movements are of various types depending on the nature of stimulus-
- Phototropism – Here, the stimulus is light. Shoots respond by bending towards light while roots respond by bending away from it.
- Geotropism– Here, the stimulus is gravity. Roots of plants grow downward towards gravity whereas shoots grow upward against gravity.
- Hydrotropism– In hydrotropism, stimulus is water. Roots always grow towards water.
- Chemotropism– the movement in response to chemical stimulus is called as chemotropism. Growth of pollen tubes towards ovules, where ovules produce chemicals for the growth and passage of pollen tubes.
Movement Independent of Growth
- As no growth is involved, so plants move by changing the shape of the cells.
For eg. i) In sensitive plants, the plants move its leaves in response to touch
ii) The movement takes place at a point different from the point of touch.
iii) Some cells change shape by changing the amount of water in them and as a result the cells either shrink or swell and therefore change shapes.
Hormonal System on Plants
Different kinds of hormones are responsible for growth, development and response to the stimulus:
- Auxin– i) A plant hormone, synthesized at the shoot tips and helps the cells to grow longer
ii)When plants detect light auxins are synthesized and when light comes from one side of the shoot auxins diffuse towards the shaded side of the shoot.
iii) Concentration of auxin in the shaded side stimulates the cells to grow longer on the shaded side of the shoot and thus plants appear to bend towards light.
- Gibberellins– Plant hormone gibberellins help in the growth of stem.
- Cytokinins– This hormone promotes cell division and concentration is high in fruits and seeds.
- Abscisic acid– Unlikeauxin, gibberellins, cytokinis which promote growth, abscisic acid stops growth and causes wilting of leaves.