Chapter 2: Control and Coordination

Introduction

The life activities of an organism are controlled and coordinated by two systems in the body. They are the nervous system and the endocrine system. Nervous system exerts a rapid and more specific control over body activities. The endocrine system is involved in the regulation of slower processes such as metabolism, activity of smooth muscle etc. The action of endocrine system is through the production of hormones.

Various life processes are involved in the maintenance functions in living things. If we see something moving, it is alive. In plants, some of these movements are related to growth. Example, Germination and growth of a seed. In animals, some movements are not related to growth. E.g., Children playing. We can associate such visible movements with life. We think of movements as a response to a change in the environment. Movement by an organism is an attempt to use environmental changes to its advantage.

Animals—Nervous System

The nervous system is composed of nervous tissue. The basic unit of nervous tissue is the neuron or nerve cell.

A neuron has a cyton or cell body. The cell body encloses the nucleus and other cellular organelles. The processes that arise from the cell body are dendrites and axon. Dendrites carry impulses toward the cell body. The axon is otherwise called as a nerve fibre. It conducts impulses away from the cell body. Some axons are covered by a layer of fatty substance called the myelin sheath and a membrane called neurilemma. The axon ends in branches called neurolemma. The telodendria bear knob like nerve endings called as terminal buttons.

Synapse

All information from the environment is detected by specialized tips of nerve cells. These receptors are located in our sense organs. The gustatory receptors will detect taste and olfactory receptors will detect smell.

The information acquired at the tip of dendrite causes a chemical reaction. As a result of this reaction, an electrical impulse is created. This impulse travels from the dendrite to the cell body and then along the axon. It finally reaches the axonal end. At the end of the axon, the electrical impulse sets off the release of some chemicals. These chemicals are called as neurotransmitters. These chemicals cross the synapse. Synapse is the functional junction between two or more neurons. This neurotransmitter can generate an electrical impulse in the dendrite of the next neuron. In this way, the nerve impulses travel in the body.

The synapse finally allows the delivery of nerve impulses from neurons to muscle cells or glands. The junction between a neuron and muscle is called as neuro muscular junction.

Sensory Detection

Detection of Smell: The receptors for smell are situated in the mucous membrane of nostrils. The molecules of a substance are carried to the nasal cavity by the air we breathe in. These particles dissolve in the mucus secreted by the mucus membrane. These dissolved particles stimulate the smell receptors. These stimulations create nerve impulses. These impulses are carried by the olfactory nerve to brain. The brain interprets these impulses as smell.

Detection of Taste: Taste receptors are situated on the surface of the tongue. Numerous papillae protrude from the tongue surface. Each papilla contains many taste buds. The substance we taste dissolves in the saliva. It reaches the taste buds through the pores on the surface of the tongue. Taste buds are in contact with sensory nerve endings. If the particles of substances come in contact with taste buds, their nerve endings will be stimulated. This creates a nerve impulse. These impulses are carried to the taste centre of brain. Brain interprets these impulses as taste.

Reflex Actions

The body activities are controlled by the brain and spinal cord as a result of conscious responses. But during emergency conditions body needs quick and immediate action. Usually, these responses are unconscious in nature. Such involuntary and unconscious response to the stimulus is called as reflex action. For example, touching a flame is an urgent and dangerous situation. One simple way to respond to this situation is to think consciously about the pain and the possibility of getting burnt. Therefore we move our hand. The thinking process involves the creation of nerve impulses. Thinking is a complex activity. It involves a complicated interaction of many nerve impulses from many neurons.

Reflex Arc: The specific pathway of impulses in a reflex action is called reflex arc. It consists of:

1. A receptor organ which receives the stimulus.
2. A sensory neuron which receives impulses from the sense organ and transmits to the central nervous system.
3. An intermediate neuron which connects the sensory and motor neurons.
4. A motor neuron which transmits impulses from the CNS to the effector organs.
5. An effector organ which is a muscle or a gland which responds to the stimuli.

Human Brain

The brain and the spinal cord constitute the central nervous system. The brain is the main controlling centre of the body. The peripheral nervous system consists of cranial nerves and spinal nerves. The nerves arising from the brain are called as cranial nerves. The nerves arising from the spinal cord are called as spinal nerves.

In man, there are 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Autonomous Nervous System

It is made up of motor nerves and ganglia. The autonomous nervous system is divided into sympathetic nervous system and parasympathetic nervous system. It cannot work independently without the involvement of central nervous system.

Parts of the Brain

The brain is divided into three major parts:

• Fore-brain (Prosencephalon): Includes Cerebrum, Thalamus, Hypothalamus. It is the main thinking part of the brain.
• Mid-brain (Mesencephalon): Connects fore-brain and hind-brain.
• Hind-brain (Rhombencephalon): Includes Cerebellum, Medulla, Pons.

Protection of Nervous Tissue

• Bone: The brain is enclosed within a bony box called cranium. The spinal cord is located inside the vertebral column.
• Meninges: Three layered membrane covering the CNS. Outer duramater, middle arachnoidmater, inner piamater.
• Cerebrospinal Fluid (CSF): The fluid filling the space between meninges and ventricles. It gives protection against shock and mechanical injury.

Voluntary and Involuntary Muscles

Coordination in Plants

Plants lack a nervous system and muscular system. However, they show growth dependent and growth independent movements.

Immediate Response to Stimulus (Nastic Movement)

When we touch a sensitive plant (Mimosa pudica), the leaves fold up. This movement is not related to growth. The plants use electrical-chemical means to convey information from cell to cell. Plant cells change shape by changing the amount of water in them (swelling or shrinking).

Nastic movements are those that are not related to the direction of stimulus. E.g., Opening and closing of petals of flowers.

Movement Due to Growth (Tropic Movement)

Tropic movements take place in accordance with the direction of stimulus.

• Phototropism: Movement in response to light. Shoots are positively phototropic (grow towards light); roots are negatively phototropic (grow away from light).
• Geotropism: Movement in response to gravity. Roots are positively geotropic; shoots are negatively geotropic.
• Hydrotropism: Movement in response to water. Roots are positively hydrotropic.
• Chemotropism: Movement in response to chemicals. E.g., Growth of pollen tube towards ovules.
• Haptotropism: Movement induced by touch. E.g., Tendrils winding over a support.

Plant Hormones (Phytohormones)

Plants use chemical communication. Special molecules (hormones) are released by stimulated cells and diffuse to other cells.

• Auxins: Synthesized at the shoot tip. Help cells to grow longer. When light comes from one side, auxin diffuses to the shady side, stimulating cells there to grow longer, causing the plant to bend towards light.
• Gibberellins: Help in the growth of the stem.
• Cytokinins: Promote cell division (present in fruits and seeds).
• Abscisic Acid: Inhibits growth. Causes wilting of leaves.
• Ethylene: Flowering hormone (florigen) and fruit ripening.

Hormones in Animals

The nervous system controls various life activities, but some (like growth, metabolism) are regulated by the endocrine system.

Human Endocrine System

Endocrine glands are ductless glands; they release hormones directly into the blood.

Adrenaline (Emergency Hormone): Secreted by adrenal glands. It prepares the body for emergency situations. It acts on the heart (increases heartbeat), diverts blood to skeletal muscles, increases breathing rate, and prepares the body to fight or run.

Thyroxin: Secreted by Thyroid gland. Iodine is essential for its synthesis.

• Regulates carbohydrate, protein and fat metabolism.
• Regulates growth.
• Deficiency of iodine causes Goitre (swollen neck).

Growth Hormone: Secreted by Pituitary gland. Regulates growth and development. Deficiency in childhood leads to dwarfism; excess leads to gigantism.

Sex Hormones:

• Testosterone (Males): Secreted by testes. Controls secondary sexual characters.
• Oestrogen (Females): Secreted by ovaries. Controls secondary sexual characters.

Insulin: Secreted by Pancreas. It regulates blood sugar levels. Deficiency leads to diabetes (high blood sugar).

Feedback Mechanism: The timing and amount of hormones released are regulated by feedback mechanisms. For example, if blood sugar rises, pancreatic cells detect it and produce more insulin. As sugar falls, insulin production is reduced.

Points to Remember

• Two systems control body activities: Nervous system and Endocrine system.
• Reflex Action: Involuntary, unconscious response to stimulus.
• CNS: Brain + Spinal Cord.
• Pheromones: Chemical substances released into the environment by animals for communication (e.g., musk).
• Cerebrum: Largest part of the human brain.
• Corpus Callosum: Connects two cerebral hemispheres.
• Diabetes: Caused by insulin deficiency. Treated with insulin injections.