Ch-7 Control and Coodination Notes

Stimulus The changes in the environment to which the organisms respond and react are called stimuli (singular of stimuli is stimulus). The living organisms show response to stimuli such as light, heat, cold, sound, smell, taste, touch, pressure, pain, water, and force of gravity, etc.

The response of organisms to a stimulus is usually in the form of some movement of their body part. For example, if a man touches a very hot utensil accidentally, he quickly pulls his hand away from the hot utensil. Here, heat is the stimulus and the man reacts by moving his hand away from the hot utensil.

Both, plants and animals react (or respond) to various stimuli around them but in different ways. The animals can react to stimuli in many different ways because they have a nervous system and an endocrine system involving hormones.

The plants, however, react to stimuli in a very limited way because the plants do not have a nervous system like the animals have. The plants use only the hormones for producing reaction to external stimuli.

The working together of the various organs of an organism in a systematic manner so as to produce a proper response to the stimulus, is called coordination.

The function of control and coordination in plants is performed by the chemical substances called plant hormones. There are four major types of plant hormones (or phytohormones) which are involved in the control and coordination in plants. These are :

1. Auxins, 2. Gibberellins, 3. Cytokinins, and 4. Abscisic acid (ABA).

• Auxin: This hormones synthesized at shoot tip. It helps the cells to grow longer and involved in phototropism (response towards light).

• Gibberellin : It helps in the growth of the stem.

• Cytokinins: It promotes cell division. This is present in greater concentration in fruits and seeds

• Abscisic Acid: It inhibits growth. It also cause wilting of leaves.

Auxins, Gibberellin and Cytokinins are the plant hormones which promote growth of plants. Abscisic acid is a plant hormone which inhibits (or prevents) the growth.

A growth movement of a plant part in response to an external stimulus in which the direction of stimulus determines the direction of response is called tropism. Thus, tropism is a directional movement of the part of a plant caused by its growth.

1. If the growth (or movement) of a plant part is towards the stimulus, it is called positive tropism, and 2. If the growth (or movement) of a plant part is away from the stimulus, then it is called negative tropism.

(1) Phototropism: The growth in a plant part in response to light is called phototropic movement or phototropism. Stems usually show positive phototropic movement, while roots usually show negative phototropic movement. If a plant is kept in a container in which no sunlight reaches and a hole in the container allows some sunlight; the stem finally grows in the direction of the sunlight.

These movements of the plant part are usually caused by an unequal growth in its two regions by the action of plant hormones, under the influence of the stimulus. For example, the auxin hormone is made and secreted by the meristematic tissue at the tip of stem (or tip of shoot). The auxin hormone speeds up the growth in stems. So, if one side of a stem has more auxin than the other side, then the side of stem having more auxin hormone will grow faster than the other side (having less auxin hormone). This will cause the stem to bend.

And when the stem bends to one side, we say that the stem is showing movement. This movement (or bending) of the stem has been caused by its growth. So, we can say that the bending of a stem (or shoot) (when exposed to light from one side) is a growth movement.

(2) Geotropism: The growth in a plant part in response to the gravity is called Geotropism or geotropic movement. Roots usually show positive geotropic movement, i.e. they grow in the direction of the gravity. Stems usually show negative geotropic movement.

(3) Chemotropism: The movement of a plant part in response to a chemical stimulus is called chemotropism. The growth of pollen tube towards the ovule during the process of fertilisation in a flower is an example of chemotropism (It is actually positive chemotropism).

(4) Hydrotropism: When roots grow in the soil, they usually grow towards the nearest source of water. This shows a positive hydrotropic movement.

(5) Thigmotropism Movement: The growth in a plant part in response to touch is called thigmotropism movement. Such movements are seen in tendrils of climbers. The tendril grows in a way so as it can coil around a support. The differential rate of cell division in different parts of the tendril happens due to action of auxin.

The movement which do not depend on the direction from the stimulus acts are called nastic movement. For example, when someone touches the leaves of mimosa, the leaves droop (bend or hang downwards). The drooping is independent of the direction from which the leaves are touched. Such movements usually happen because of changing water balance in the cells. When leaves of mimosa are touched, the cells in the leaves lose- water and become flaccid (soft and hanging loosely), resulting in drooping of leaves.

Sample Problem 1. Which of the following is a plant hormone ? (a) Insulin (b) Thyroxine © Oestrogen (d) Cytokinin

Answer. (d) Cytokinin.

Sample Problem 2. How do auxins promote the growth of a tendril around a support ?

Answer. When the tip of a tendril touches a support, then the auxins (plant hormones) present in its tip move to that side of tip which is away from the support. Auxins promote growth. So, due to more auxins in it, the side of tendril away from the support grows faster (and becomes longer) than the side which is in contact with the support, and makes the tendril curve (or bend) towards the support. This ‘curving’ tendril can then encircle the support and wind around it.

Sample Problem 3. How is the movement of the leaves of the sensitive plant different from the movement of a shoot towards light ?

Answer. The main differences between the movement of the leaves of a sensitive plant and the movement of a shoot towards light are as follows :

Sample Problem 4. What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs ?

Answer. The movement in the leaves of a sensitive plant takes place due to the sudden loss of water in the pad-like swellings (called pulvini) at the base of all the leaves. The loss of water makes the pulvini limp leading to drooping and folding of leaves. On the other hand, the movement in our legs takes place when the leg muscles pull on the leg bones.

Control and Coordination in animals is done with the help of two main systems:\ (i) Nervous system\ (ii) Endocrine system

Nervous System: A system made up of nerve cells is called nervous system. The coordination in simple multicellular animals takes place through nervous system only. It consists of Brain, Spinal Cord and a huge network of nerves.

Functions of Nervous System

(i) To receive the information from environment\ (ii) To receive the information from various body parts. (Stimuli ? Response) (iii) To act accordingly through muscles and glands.

Stimulus : Any change in environment to which the organisms respond is called stimulus. E.g., touching a hot plate. Here heat is stimulus.

Response : The reaction of our body to a stimulus. E.g. withdrawal of our hand on touching hot plate.

Coordination–The working together of various organs of the body of an organism in a proper manner to produce appropriate reaction to a stimulus is called coordination.

Receptors : Are specialized tips of some nerve cells that detect the information from the environment.

A neuron is the structural and functional unit of the nervous system.

Neuron: Neuron is a highly specialized cell which is responsible for the transmission of nerve impulses.

This impulse travels from the dendrite to the cell body and then at the end of the axon.

Chemicals are released at the end of the axon by the effect of electrical impulse.

These chemicals cross the gap (synapse) and start a similar electrical impulse in a dendrite of the next neuron.

The similar synapse finally allows delivery of such impulses from neurons to other cells, such as muscles cells or gland.

1. Sensory neuron: These neurons receive signals from a sense organ.

2. Motor neuron: These neurons send signals to a muscle or a gland.

3. Relay neuron: These neurons relay the signals between sensory neuron and motor neuron.

Synapse: It is the gap between the nerve ending of one neuron and dendrite of the other neuron. Here, electrical signal is converted into chemical signal for onward transmission.

Neuromuscular Junction (NMJ): NMJ is the point where a muscle fibre comes in contact with a motor neuron carrying nerve impulse from the control nervous system.

The nervous system can be divided into two main parts :

1. Central nervous system (consisting of brain and spinal cord), and

2. Peripheral nervous system (consists of Cranial Nerves which arise from the brain and Spinal Nerves which arise from the Spinal cord.)

Brain

Brain is the main coordinating centre of the body. It is enclosed in cranium (brain box) and is protected by cerebrospinal fluid which acts as a shock absorber. It has three major parts:

(a) Fore-brain\ (b) Mid-brain\ © Hind-brain

a) Fore-brain

It is the most complex or specialised part of the brain. It consists of cerebrum.

Functions of Fore-brain:

(i) Thinking part of the brain.

(ii) Control the voluntary actions.

(iii)Store information (Memory).

(iv) Receives sensory impulses from various parts of the body and integrate it.

(v) Centre associated with hunger.

b) Mid-brain

Controls involuntary actions such as change in pupil size and reflex movements of head, neck and trunk.

c) Hind-brain

It has three parts:

(i) Cerebellum : Controls posture and balance. Precision of voluntary actions. Example: picking pen.

(ii) Medulla : Controls involuntary actions. Example: blood pressure, salivation, vomiting.

(iii) Pons : Involuntary actions, regulation of respiration.

1. The brain receives information-carrying nerve impulses from all the sensory organs of the body.

2. The brain responds to the impulses brought in by sensory organs by sending its own instructions (through motor nerves) to the muscles and glands causing them to function accordingly.

3. The brain correlates the various stimuli from different sense organs and produces the most appropriate and intelligent response.

4. The brain coordinates the body activities so that the mechanisms and chemical reactions of the body work together efficiently.

5. The brain stores ‘information’ so that behaviour can be modified according to the past experience. This function makes the brain the organ of thought and intelligence.

Protection of Brain: Brain is protected by a fluid filled balloon which acts as shock absorber and is enclosed in cranium (skull or brain box).

Protection of Spinal Cord: Spinal cord is enclosed in vertebral column.

Spinal cord is a cylindrical structure. The spinal cord begins in continuation with medulla and extends downwards. It is enclosed in a bony cage called vertebral column.

The spinal cord is concerned with spinal reflex actions and the conduction of nerve impulses to and from the brain.

The peripheral nervous system is composed of the cranial nerves and spinal nerves.

Reflex Action: Reflex action is a special case of involuntary movement involuntary organs. When a voluntary organ is in the vicinity of sudden danger, it is immediately pulled away from the danger to save itself. For example, when your hand touches a very hot electric iron, you move away your hand in a jerk. All of this happens in flash and your hand is saved from the imminent injury. This is an example of reflex action.

Reflex Arc: The path through which nerves signals, involved in a reflex action, travel is called the reflex arc. The following flow chart shows the flow of signal in a reflex arc.

Receptor → Sensory neuron → Relay neuron → Motor neuron → Effector (muscle)

The receptor is the organ which comes in the danger zone. The sensory neurons pick signals from the receptor and send them to the relay neuron. The relay neuron is present in the spinal cord. The spinal cord sends signals to the effector via the motor neuron. The effector comes in action, moves the receptor away from the danger.

The reflex arc passes at the level of the spinal cord and the signals involved in reflex action do not travel up to the brain. This is important because sending signals to the brain would involve more time. Although every action is ultimately controlled by the brain, the reflex action is mainly controlled at the level of spinal cord.

Response: It is the final reaction after the reflex action.

Three types of responses:

(i) Voluntary: Controlled by fore brain. Example: talking, writing.

(ii) Involuntary: Controlled by mid and hind brain. Example: heart beat, vomiting, respiration.

(iii) Reflex action: Controlled by spinal cord. Example: withdrawal of hand on touching a hot object.

Need for Reflex Actions

In some situations such as touching a hot object, pinching etc. we need to act quickly, otherwise our body would be harmed. Here response is generated from spinal cord instead of brain. In this way, time for taking action is reduced which save us from injury.

Hormones are the chemical substances which coordinate the activities of living organisms and also their growth.

Glands: It is a structure which secretes a specific substances in body.