Control and Coordination

Living organisms must respond to changes in their surroundings. A change that an organism responds to is a stimulus, and the reaction is a response. In animals, control and coordination are carried out by the nervous system and the hormonal (endocrine) system working together.

The basic unit of the nervous system is the nerve cell or neuron. A neuron has three parts: the dendrites (receive the message), the cell body and a long axon (carries the message away). Information travels along a neuron as an electrical impulse. The tiny gap between two neurons, across which the message passes using chemicals, is called a synapse.

A reflex action is a sudden, automatic and quick response to a stimulus that does not involve conscious thought — for example, pulling the hand away from a hot object. The pathway taken by the impulse during a reflex is the reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector (muscle). Reflex actions are controlled mainly by the spinal cord, so the response is fast and the brain is informed only afterwards. This protects us from harm in dangerous situations.

The brain and spinal cord together form the central nervous system (CNS), while the nerves arising from them form the peripheral nervous system. The brain is protected by the bony skull and by fluid-filled membranes. It has three main parts:

• Fore-brain (cerebrum) — the main thinking part. It controls voluntary actions, stores memory, and is the seat of intelligence, reasoning and the senses (sight, hearing, smell, etc.).
• Mid-brain — connects the parts and controls some reflexes of the eyes.
• Hind-brain — includes the cerebellum (controls posture, balance and precise voluntary movements such as walking and writing), the medulla (controls involuntary actions like heartbeat, breathing, blood pressure, vomiting) and the pons.

The nervous system controls voluntary actions (under our will, e.g. talking, walking) through the cerebrum, and involuntary actions (not under our will, e.g. heartbeat, digestion) through the medulla. Nervous control is fast but its effect is usually short-lived; it works together with the slower, longer-lasting hormonal control to coordinate the body.

Plants have no nervous system, yet they respond to stimuli — they coordinate using chemical substances (plant hormones). Plant responses are of two kinds:

• Tropic movements (tropisms) — directional growth movements in response to a stimulus. In phototropism a shoot bends towards light; in geotropism (gravitropism) roots grow downward (towards gravity) and shoots upward; in hydrotropism roots grow towards water; in thigmotropism tendrils coil around a support.
• Nastic movements — non-directional and quick, e.g. the folding of the leaves of the touch-me-not (Mimosa) on touch.

Plant hormones include auxin (promotes cell growth and is responsible for phototropism), gibberellin (stem growth), cytokinin (cell division), and abscisic acid (inhibits growth; causes wilting and falling of leaves).

In animals, the endocrine glands secrete hormones directly into the blood, which carries them to target organs. Important examples: the thyroid gland makes thyroxine (needs iodine; controls metabolism — its deficiency causes goitre); the pancreas makes insulin (controls blood sugar — its deficiency causes diabetes); the adrenal gland makes adrenaline (the ‘fight-or-flight’ hormone for emergencies); the pituitary makes growth hormone; and testes/ovaries make the sex hormones (testosterone/oestrogen) that bring about changes at puberty. Hormonal action is slower than nervous action but its effects last longer.