CBSE Class 10 Science Control And Coordination Notes Set 09

These CBSE Class 10 Science notes for Chapter 6, Control and Coordination, cover the neuron and the nerve impulse, reflex action and reflex arc, the human brain and spinal cord, tropic and nastic movements in plants, plant hormones, the endocrine glands and hormones of animals, and the feedback mechanism, with comparison tables for quick revision.

Important Notes

Need for Control and Coordination

All living organisms respond to changes in their surroundings, called stimuli, such as light, heat, sound, smell, taste and touch. Each stimulus needs an appropriate response, and the working together of many organs in an organised way is called coordination. In multicellular organisms specialised tissues provide this control and coordination.

1. In animals there are two systems: the nervous system and the endocrine (hormonal) system. Together they form the neuro-endocrine system.
2. In plants there is no nervous system. Plants control and coordinate their activities only by chemical means, using plant hormones.

Receptors

Information from the environment is detected by the specialised tips of some nerve cells, called receptors. They are usually located in the sense organs.

The Neuron

The nervous system is made up of an organised network of nerve cells or neurons. A neuron is the structural and functional unit of the nervous system. It is the longest cell of the human body. A neuron has three parts:

1. Dendrites: short, branched fibres which receive the information from the receptor or from another neuron. The information acquired at the tip of a dendrite sets off a chemical reaction that creates an electrical impulse.
2. Cell body (cyton): it contains the nucleus and cytoplasm. The impulse travels from the dendrite to the cell body.
3. Axon: a single long fibre that carries the impulse away from the cell body. Each branch of its end has a swollen tip called the nerve ending.

Types of neurons: Sensory neurons carry impulses from the receptors to the brain or spinal cord. Motor neurons carry impulses from the brain or spinal cord to the muscles and glands. Relay neurons (interneurons) connect the sensory and motor neurons inside the spinal cord and brain.

Transmission of the Nerve Impulse

1. The impulse travels in the order: dendrite → cell body → axon → nerve ending.
2. At the end of the axon, the electrical impulse releases some chemicals (neurotransmitters).
3. These chemicals cross the small gap, called the synapse, between two neurons and start a similar electrical impulse in the dendrite of the next neuron.
4. The same kind of junction passes the impulse from a neuron to a muscle or a gland cell. In this way, information reaches the muscles or glands, which respond.

A synapse works as a one-way valve, since the chemical is released only from the axonal end.

Reflex Action and Reflex Arc

A reflex action is a sudden, automatic and involuntary response to a stimulus, without thinking. Examples are the withdrawal of the hand on touching a hot object, blinking, sneezing, coughing and the shrinking of the pupil in bright light. The pathway taken by the nerve impulses in a reflex action is called the reflex arc.

1. Receptor: detects the stimulus (for example, the skin).
2. Sensory neuron: carries the impulse to the spinal cord.
3. Relay neuron in the spinal cord: passes the impulse to the motor neuron.
4. Motor neuron: carries the impulse to the effector.
5. Effector: the muscle or gland, which gives the response.

Reflex arcs are formed in the spinal cord, so the response is quick and the body is protected from harm. The information about the reflex still reaches the brain, but the brain does not take part in the quick response. Reflex arcs have evolved because the thinking process of the brain is not fast enough for sudden dangers.

Voluntary actions (such as writing, walking and talking) are under our will and are controlled by the brain. Involuntary actions (such as the heartbeat, breathing and the movement of food in the gut) go on without our will and are controlled by the medulla in the brain. Reflex actions are involuntary actions that occur in response to a stimulus, and they are controlled by the spinal cord.

The Human Nervous System

The Human Brain

The brain is the main coordinating centre of the body. It is enclosed in the bony box called the cranium. Inside the box, the brain is contained in a fluid-filled balloon (cerebrospinal fluid) that acts as a shock absorber. The brain has three major regions.

The spinal cord is the continuation of the medulla. It is protected by the vertebral column. It conducts impulses between the brain and the body and acts as the centre for reflex actions. Damage to the spinal cord disturbs the reflex actions and the signals between the brain and the body parts.

Coordination in Plants

Plants do not have nerves or muscles. They respond to stimuli by movements of their parts and by hormones. Plants use electrical-chemical signals to pass information from cell to cell, and they change shape by changing the amount of water in their cells. Plant movements are of two kinds.

1. Tropic Movements (Tropism)

Tropic movements are directional growth movements of plant parts in response to a stimulus acting from one direction. If the part grows towards the stimulus it is a positive tropism, and if away from it, a negative tropism.

1. Phototropism (stimulus: light): shoots bend towards light (positive) and roots bend away from light (negative).
2. Geotropism (stimulus: gravity): roots grow downwards (positive) and shoots grow upwards (negative).
3. Hydrotropism (stimulus: water): roots grow towards water.
4. Chemotropism (stimulus: chemicals): the pollen tube grows towards the ovule.
5. Thigmotropism (stimulus: touch): tendrils coil round a support.

2. Nastic Movements

Nastic movements are non-directional and do not depend on growth. They occur due to changes in the amount of water (turgor) in the cells. Examples: the leaves of the touch-me-not plant (Mimosa pudica) fold and droop on touching; a dandelion flower opens in the morning and closes at night.

Phototropism and Auxin

When growing plants detect light, a hormone called auxin is synthesised at the shoot tip. When light falls from above, auxin spreads evenly and the shoot grows straight. When light falls from one side, auxin diffuses towards the shady side of the shoot. This higher concentration of auxin makes the cells on the shady side grow longer, so the shoot appears to bend towards the light.

Tendrils: When a tendril touches a support, auxin moves to the side away from the support, where the cells grow faster. The tendril therefore coils round the support.

Activity (hydrotropism): Take two troughs of soil with seedlings. Water trough A uniformly. In trough B, keep water only in a buried clay pot. The root in A grows straight, while the root in B bends towards the clay pot, showing that roots grow towards water.

Plant Hormones

Plant hormones (phytohormones) are chemical substances made in small quantities in one part of the plant. They diffuse to the area of action and coordinate growth, development and the responses to the environment.

Hormones in Animals

Nerves do not reach every cell of the body, and electrical impulses cannot be sent continuously. So multicellular animals also use chemical communication. Hormones are chemical messengers that are secreted by endocrine glands in small amounts directly into the blood. The blood carries them to the target organs, where they bring about a particular response. Endocrine glands have no ducts, whereas exocrine glands pass their secretions through ducts.

Characteristics of hormones:

1. They are secreted by endocrine glands directly into the blood.
2. They are required in trace amounts.
3. They act on specific target organs, away from the place where they are made.
4. Their effect is slower than that of nerves but lasts longer.

Endocrine Glands and Their Hormones

Adrenaline: When adrenaline is secreted into the blood, the heartbeat increases, so more oxygen is supplied to the muscles. The breathing rate also increases. The small arteries in the digestive system and the skin contract, so more blood goes to the skeletal muscles. All these responses prepare the body to deal with the emergency.

Feedback Mechanism

Hormones should be secreted in the right amount. The timing and amount of the hormone released are regulated by a feedback mechanism. For example, if the sugar level in the blood rises, it is detected by the cells of the pancreas, which respond by producing more insulin. As the blood sugar falls, insulin secretion is reduced.

Nervous and Hormonal Control Compared

Quick Revision

1. The neuron is the structural and functional unit of the nervous system, and the impulse travels dendrite → cell body → axon → nerve ending.
2. The synapse passes the signal from one neuron to the next by chemicals, in one direction only.
3. Reflex action is controlled by the spinal cord through the reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector.
4. Forebrain: thinking and hunger; midbrain: reflexes of the head and eyes; hindbrain: balance (cerebellum), breathing (pons) and involuntary actions (medulla).
5. Tropic movements are directional and growth-based; nastic movements are non-directional and fast.
6. Plant hormones: auxin, gibberellin, cytokinin, abscisic acid and ethylene.
7. Iodine is needed for thyroxine, insulin controls blood sugar, and adrenaline prepares the body for emergencies.
8. Feedback mechanisms regulate the secretion of hormones.