Control and Coordination Notes

A change in the environment to which an organism reacts is a stimulus, and the reaction is a response. Detecting a stimulus is the job of receptors — for example, the gustatory receptors in the tongue detect taste, and olfactory receptors in the nose detect smell.

The neuron (nerve cell) is the functional unit of the nervous system. Information is picked up by the dendrites, travels as an electrical impulse along the axon to the nerve ending, and there it releases chemicals (neurotransmitters) that cross the synapse — the tiny gap between two neurons — to start an impulse in the next neuron.

A synapse makes the impulse travel in one direction only, from axon ending of one neuron to the dendrite of the next. The junction between a nerve ending and a muscle is similarly crossed by chemicals that make the muscle contract.

A reflex action is a rapid, automatic response to a stimulus that is not under conscious control — such as pulling the hand away from a hot object or blinking when something approaches the eye.

The pathway is the reflex arc: receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle). The signal is processed in the spinal cord rather than the brain, which saves time. Meanwhile the information also reaches the brain, so we become aware of the event just after we have reacted.

Reflex arcs evolved because the connection for thinking is not fast enough, and many animals do not have a well-developed brain. Even in animals with a good brain, reflexes are an efficient way of dealing with sudden danger.

The brain has three major regions. The forebrain is the main thinking part; it has areas for hearing, smell, sight, and other senses, and regions for reasoning, decisions, and voluntary actions. It also has centres that control hunger and other urges.

The midbrain and hindbrain control many involuntary actions. The medulla (in the hindbrain) controls blood pressure, salivation, and vomiting. The cerebellum (also hindbrain) is responsible for precision of voluntary actions and for maintaining posture and balance — for example, walking in a straight line or riding a bicycle.

The brain and spinal cord are delicate, so they are protected: the brain sits in a bony box (the skull), and the spinal cord is enclosed in the vertebral column. A fluid-filled balloon-like cushion (cerebrospinal fluid) further absorbs shocks.

Plants respond to stimuli slowly, by growth, or in some cases by rapid changes in the water content of cells. The folding up of the leaves of the sensitive plant (Mimosa / touch-me-not) when touched is caused by cells changing shape as they lose and gain water — no growth is involved and the movement is quick.

Directional growth responses are called tropisms. In phototropism a shoot grows towards light and a root grows away from it. In geotropism (gravitropism) roots grow towards gravity and shoots grow away from it. Hydrotropism is growth towards moisture, and chemotropism is growth towards a chemical, as when a pollen tube grows towards the ovule.

These growth movements are controlled by plant hormones. Auxin, made at the shoot tip, moves to the shaded side of the shoot and makes those cells grow longer, so the shoot bends towards light. Gibberellins promote stem growth, cytokinins promote cell division, abscisic acid inhibits growth and causes wilting and leaf fall.

The endocrine glands release hormones directly into the blood, which carries them to target organs. Hormones act in tiny amounts and their secretion is controlled by feedback so that the right amount is released.

Key human hormones: adrenaline from the adrenal glands prepares the body for emergency ('fight or flight') by raising heartbeat and breathing; thyroxine from the thyroid gland (needs iodine) regulates the metabolism of carbohydrates, proteins, and fats; growth hormone from the pituitary controls growth; insulin from the pancreas lowers blood glucose; testosterone and oestrogen control male and female secondary sexual characters.

Feedback example: when blood glucose rises after a meal, the pancreas releases more insulin, which brings the glucose level down; as it falls, less insulin is released. If insulin is not produced in sufficient quantity, blood sugar rises — this condition is diabetes, sometimes treated with insulin injections.