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Coordination and Responses

Chapter: Cordination and Responses

Human Nervous System

The human neural system is divided into two parts:

  1. The Central Neural System (CNS).
  2. The Peripheral Neural System (PNS).

✓ The central nervous system is composed of brain and spinal cord; it is the site of information processing and control.

✓ The peripheral nervous system comprises of all the nerves of the body associated with CNS

Reflex action and Reflex arc

Sudden action in response to something in the environment is called reflex action. A common example is pulling back of hand from the flame. The reflex action is the entire process of response to a peripheral nervous stimulation happens reluctantly, i.e., without conscious effort or thought and also requires the participation of a part of the central nervous system. The reflex pathway comprises of at least one afferent neuron (receptor) and one efferent neuron (exciter) properly prearranged in a series. The afferent neuron receives signal from sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of spinal cord). The efferent neuron carries signals from CNS to the effector. A reflex arc is formed by the stimulus and response.

Muscles and Glands as effectors

Muscles and glands work as effectors; they are the exercising organs for doing action. Effectors are the organs which bring about changes in the environment. We perform various activities by the action of hands, legs and muscles. Secretions from the glands provide energy for these activities. The effects which are brought by the effectors are called responses. Salivary glands, digestive glands and sweat glands are some of the examples of glands. All the receptors are connected with effectors through the nervous system. The nerve impulses are carried by efferent neurons to the muscles or glands in order to contract, relax or expand according to the responses made by the organisms.

Action of antagonistic muscles

Muscles can cause movement by contracting. They can only pull and not push. Muscles are arranged in pairs having opposite action. One muscle contracts to move bone in one direction and the other contracts to move it back. These pairs are called antagonistic muscles. Let us take a look at an example. A flexor muscle (biceps) contracts to bend the limb and extensor muscle (triceps) contracts to straighten the limb.

Sense organs

Senses are the ability of living organism to be aware of different aspects of the environment. For example, the sense of sight allows us to be aware of light stimuli, detected by photoreceptors. The receptors that provide us with senses do not work on their own. They need blood supply to deliver oxygen, nutrients and to remove waste material. Receptors need help in receiving stimulus therefore the receptor cells are grouped with other tissues to form a sense organ.

  1. Taste depends on chemical stimulation of tongue
  2. Temperature sensitivity depends on heat stimulation of the skin.
  3. Smell depends on chemical stimulation of the nose.
  4. Balance and hearing depend on mechanical simulation of the ear.
  5. Touch depends on mechanical stimulation of the skin.
  6. Sight depends on light stimulation of the eye.

The eye



  1. Pupil: The circular opening which lets light into the eye. It is black since choroid is visible through it.
  2. Iris: The coloured part of the eye which can expand and contract to control the amount of light that enters the eye.
  3. Ciliary muscles, suspensory ligament and lens together control light focusing on retina.
  4. Sclera: It is the tough outer coat that protects the eye against damage. The muscles which help in the movement of the eye are attached to sclera.
  5. Choroid: It is a darkly coloured layer which reduces reflections inside the eye and contains blood vessels which nourish the cells of retina.
  6. Retina:  The rods and cones which are the light sensitive cells are present in retina.
  7. Fovea: This area has the highest density of cone and therefore offers maximum sharpness but it works only at full efficiency of bright light.
  8. Optic nerve: Composed of sensory neurons which carry nerve impulses to the visual centre in the brain.
  9. Blind spot: It is present at the exit point of optic nerve. There are no light sensitive cells here so light falling on this region cannot be detected.
  10. Vitreous humour: A jelly like substance which helps to keep the shape of the eye ball, supports the lens and keeps the retina in place at the back of the eye.
  11. Aqueous humour:  Watery fluid which supports the cornea and the front chamber of the eye.
  12. Cornea: A transparent layer responsible for most of the refraction of light rays that enter the eye.


Hormones are the chemical substances secreted by the endocrine gland into the blood stream. They alter the activity of one or more specific target organs and then are destroyed by liver.


It is one of the widely studied hormone. It seems to bridge gap between nervous and endocrine control. It is a chemical messenger which is released into the blood but its actions are very rapid and may last only for short period of time. The major function of adrenaline is it increases hear rate and blood pressure. It promotes glycogenolysis in liver i.e. is conversion/breakdown of glycogen into glucose. Adrenaline is known as the fight/flight hormone. It is released when the body is given a shock.

Difference between nervous systen and endocrine system




Speed of action

Very rapid

Can be slow

Nature of message

Electrical impulses travelling along nerves

Chemical messengers travelling in bloodstream

Duration of response

Usually completed within seconds.

May take years before completed

Area of response

The response is localized- usually confined to one area of the body

Usually noticed in many organs- the response is widespread

Examples of processes controlled

Reflexes such as blinking, movement of limbs

Growth, development of reproductive system.


Tropic responses

Light and gravity are some of the environmental triggers which change the directions that plant parts grow in. These directional movements can be either towards the stimulus, or away from it.

  1. Phototropism:It is the response in which plant grows towards or away from the direction from which the light is coming shoots respond by bending towards light while roots respond by bending away from it.
  2. Geotropism: It is the response in which plants grow towards or away from the light. Roots respond by growing downwards, towards the gravitational pull and the shoots grow upward, away from the gravitational pull.


Homeostasis is the process that maintains stability of the human body’s internal environment in response to changes in the external environment. It is necessary to maintain constant body temperature since many biological and physical processes are affected by the temperature.  Many organs play a part in homeostasis. Cells change the composition of tissue fluid as they remove food and oxygen and add carbon dioxide and other wastes. Heart supplies a constant pressure needed to deliver blood to the tissues and to form tissue fluid. Skin is the main organ for the control of heat exchange. The epidermis of the skin protects against water loss by convention, radiation and evaporation.


Any substance taken into the body that modifies or affects chemical reactions in the body is called as drug.


Antibiotics or the antibacterials are class of drugs that are specifically used against bacteria. Antibiotics are mostly used in the treatment of bacterial infections. Antibiotics can kill or inhibit the growth of bacteria. Different antibiotics have different mode of action, depending nature of their structure and degree of affinity to certain target sites within bacterial cells.  Some antibiotics are the inhibitors of cell wall synthesis and some are the inhibitors of cell membrane function in bacteria. Some antibiotics inhibit the proteins synthesis in bacteria which finally leads to the death of bacteria. The nucleic acid synthesis, i.e. the synthesis of DNA and RNA is inhibited by some antibiotics which will ultimately compromises bacterial multiplication. Some of the examples of antibiotics are cephalosporins, polymixin B, macrolides, and quinones.

Effects of heroin

Narcotics such as heroin act like depressants but particularly target the brain. They work as pain-killers and bring about the feeling of drowsy well being. Usage of narcotics like heroin brings drug dependence in the user. The basic functions like breathing and heart beat become slow. After the effects of the drug have decreased the body craves for more of it. If he does not take the drug he will start to experience withdrawal symptoms and these include restlessness, aches and pains in the bone, diarrhea, vomiting and severe discomfort. Diseases like HIV can be transmitted from the infected persons if the needles used to inject the drug are shared.


Alcohol is also a type of a drug but is not considered as medicine. Excessive consumption of alcohol can lead to severe health problems.  The effects of alcohol on the body may extend beyond the nervous system. It has wide spread effects throughout the body.

  1. Cardiovascular effects include anaemia- alcohol poisons the bone marrow and deposits fat in the coronary arteries.
  2. Skin blood vessels dilate so warm blood flows to the skin and so the body feels warm and comfortable, but hypothermia may result at lower environmental temperature.
  3. Sex organs are stimulated but they do not work well. Sperm count reduces.
  4. Intestines are irritated causing indigestion, nausea, diarrhoea and ulcers.
  5. The consumption of alcohol in pregnant women affects the foetus. Alcohol crosses the placental barrier and affects the nervous system of the foetus and the development of child is very slow. This is called fetal alcohol syndrome.

Effects of tobacco

Tobacco contains a drug called nicotine which mimics the natural neurotransmitters in the part of the nervous system concerned with control of heart beat and blood pressure. Smoking/chewing tobacco is bad for health because they contain chemicals which affect the body. Carbon monoxide from the smoke enters the blood stream instead of oxygen and as a result cells get less amount of oxygen for respiration therefore we are more likely to get breathless when we exercise. The cilia and the bronchi are anesthetised as a result they cannot move mucus and pathogens away from the lungs so we have more chances to get infections in the breathing system. Tobacco smoke contains tar, which is a black sticky substance along with many other chemicals. This tar coats the lining of alveoli and increases the risk of emphysema. Some of the chemicals along with tar are said to be carcinogenic. They cause cancer.