6. Control and Coordination

A reflex action is a sudden, automatic response to a stimulus. It is controlled by the spinal cord. We do not think before it.

Walking is a voluntary action. It is under our control. It is controlled by the brain (cerebrum and cerebellum).

At the end of the axon, the electrical impulse releases some chemicals. These chemicals cross the gap called synapse. They start a similar electrical impulse in the dendrite of the next neuron. In this way, the message passes from one neuron to the next.

Answer: Cerebellum

It is a part of the hind-brain.

The smell of the agarbatti reaches our nose. The olfactory receptors in the nose detect it. They send the message to the fore-brain. The fore-brain interprets it as the smell of agarbatti.

Reflex action is controlled by the spinal cord, not the brain. But the message also goes to the brain. So the brain becomes aware of what has happened. It may also plan further action.

Plant hormones are chemical compounds that coordinate growth, development and responses to the environment in plants. Examples: auxins, gibberellins, cytokinins and abscisic acid.

Answer: Auxin

Gibberellins and cytokinins also promote growth.

When a tendril touches a support, auxin moves to the side away from the support. Auxin makes the cells on that side grow faster. So the tendril bends towards the support and winds around it.

• Take a glass trough and fill it with moist soil.
• Sow a seed on one side of the trough.
• Place a small porous pot filled with water on the other side, buried in the soil.
• Keep the set-up for a few days and observe the roots.

Observation: The roots of the seedling bend and grow towards the pot of water.

Conclusion: Roots grow towards water. This is called hydrotropism.

In animals, chemical coordination is done by hormones. Hormones are made by endocrine glands. They are released directly into the blood. The blood carries them to the target organs, where they act. A feedback mechanism controls the amount and timing of hormone release.

Iodine is needed by the thyroid gland to make the hormone thyroxin. Thyroxin controls the metabolism of carbohydrates, proteins and fats. Lack of iodine causes goitre (swollen neck). Iodised salt gives us enough iodine. So its use is advisable.

• The heart beats faster. So more oxygen is supplied to the muscles.
• Blood supply to the digestive system and skin is reduced. More blood goes to the skeletal muscles.
• The breathing rate increases.

All these prepare the body to deal with an emergency.

Insulin is a hormone made by the pancreas. It controls the blood sugar level. In some diabetic patients, the pancreas does not make enough insulin. So their blood sugar level rises. Insulin injections help to bring the sugar level to normal.

Answer: (d) Cytokinin.

Answer: (b) synapse.

Answer: (d) all of the above.

Receptors are specialised tips of nerve cells in our sense organs. They detect information from the environment, like light, sound, smell, taste, heat and touch. Then they send this message to the brain or spinal cord.

If receptors do not work properly, we cannot sense the stimulus. For example, if the heat receptors of the skin do not work, we may not feel a hot object and get burnt. If taste receptors do not work, we cannot taste food. So we cannot respond correctly and may get hurt.

Function: A neuron carries messages in the form of electrical impulses.

• Dendrites pick up information from receptors or other neurons.
• The information travels as an electrical impulse from the dendrite to the cell body and then along the axon.
• At the nerve ending, chemicals are released. They pass the message across the synapse to the next neuron or a muscle.

When light falls on a shoot from one side, auxin moves to the shady side of the shoot. Auxin makes the cells on the shady side grow longer. So the shoot bends towards light. This is called phototropism. Roots bend away from light.

The spinal cord carries messages between the brain and the body. If it is injured:

• Messages from the body (like touch and pain) cannot reach the brain.
• Messages from the brain cannot reach the muscles. So voluntary actions are affected.
• Reflex actions controlled by the spinal cord will also be disturbed.

In plants, chemical coordination is done by plant hormones.

• Auxin helps cells to grow longer. It is responsible for bending towards light.
• Gibberellins help in the growth of the stem.
• Cytokinins promote cell division. They are found in fruits and seeds.
• Abscisic acid stops growth. It causes wilting of leaves.

An organism has many organs. They must work together in a proper way. The organism must also respond to changes in the environment. A system of control and coordination makes all parts work together. It helps the organism respond correctly and survive.

Similarity: Both help the body to control and coordinate its activities.