Coordination, Response & Homeostasis

Why organisms need coordination

Living things must detect changes in their surroundings and respond to them to survive. A stimulus (a detectable change, such as light, heat or touch) triggers a response carried out by an effector (a muscle or gland). Between the two sits a system of communication that links information to action.

In animals there are two coordinating systems: the nervous system, which uses fast electrical impulses, and the endocrine (hormonal) system, which uses slower chemical messengers carried in the blood. Both keep the body working together as one unit.

Key terms

Stimulus — a change in the environment that is detected.

Receptor — a cell or organ that detects a stimulus.

Effector — a muscle or gland that produces a response.

The nervous system

The mammalian nervous system has two parts:

The central nervous system (CNS) — the brain and spinal cord. This processes information and coordinates a response.
The peripheral nervous system — the nerves carrying impulses to and from the CNS.

Information travels as electrical impulses along neurones (nerve cells). There are three types:

1. Sensory neurones carry impulses from receptors to the CNS.
2. Relay neurones lie within the CNS and connect sensory to motor neurones.
3. Motor neurones carry impulses from the CNS to effectors.

Neurones are long, thin cells. A fatty myelin sheath insulates the fibre and speeds up the impulse.

The reflex arc

A reflex action is a rapid, automatic response that does not involve conscious thought by the brain. Many reflexes are protective, for example pulling your hand away from a hot object or blinking when something approaches your eye.

The pathway of a reflex is the reflex arc:

stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → response

Reflexes are fast because the impulse takes a short, fixed route through the spinal cord without waiting for the brain to make a decision. This speed is what makes them protective — you withdraw your hand before you even feel the pain.

Synapses

Neurones do not touch. The tiny gap between two neurones is a synapse. When an impulse reaches the end of one neurone, it triggers the release of a chemical called a neurotransmitter. This diffuses across the gap and binds to receptors on the next neurone, starting a new impulse.

Watch out

Across a synapse the signal is chemical, not electrical. The neurotransmitter only travels in one direction, which is why impulses cross a synapse one way.

Receptors and sense organs

Receptors are specialised to detect particular stimuli. They are often grouped into sense organs:

The eye detects light. Light-sensitive cells in the retina respond and send impulses along the optic nerve to the brain. (The eye also adjusts to brightness by reflex — in bright light the pupil narrows.)
The ear detects sound and balance, the skin detects touch, temperature and pressure, and the tongue and nose detect chemicals.

Hormones and the endocrine system

The endocrine system is made of glands that release hormones directly into the blood. A hormone is a chemical messenger that travels in the blood and affects specific target organs.

Adrenaline is released by the adrenal glands in moments of fear, stress or excitement. It prepares the body for "fight or flight": heart rate and breathing rate increase, and more glucose is released into the blood, so muscles get more oxygen and energy.

Blood glucose is controlled by two hormones from the pancreas:

Insulin is released when blood glucose is too high. It causes the liver to convert excess glucose into glycogen for storage, lowering blood glucose.
Glucagon is released when blood glucose is too low. It causes the liver to convert glycogen back into glucose, raising blood glucose.

Real world

In type 1 diabetes the pancreas cannot make enough insulin, so blood glucose rises dangerously after eating. It is often treated with insulin injections.

Nervous versus hormonal control

Homeostasis

Homeostasis is the maintenance of a constant internal environment in the body, despite changes outside. Conditions kept steady include body temperature, blood glucose and water content. Enzymes work best within narrow ranges, so this stability keeps the body functioning.

Homeostasis usually works by negative feedback: when a factor moves away from its normal level, the body responds to bring it back. The response reverses the change, then switches off once normal levels return.

Key terms

Negative feedback — a control mechanism in which a change in a factor triggers a response that reverses the change, returning it to normal.

Controlling body temperature

Mammals keep a steady core temperature of about $37\,^{\circ}\text{C}$. The skin plays a central role, controlled by the brain.

When the body is too hot:

Sweating increases; as sweat evaporates it takes heat from the skin.
Vasodilation — blood vessels supplying the skin capillaries widen, so more warm blood flows near the surface and more heat is lost.
Hair erector muscles relax, so hairs lie flat and trap less insulating air.

When the body is too cold:

Shivering — muscles contract rapidly, releasing heat from respiration.
Vasoconstriction — skin blood vessels narrow, so less blood reaches the surface and less heat is lost.
Hair erector muscles contract, raising the hairs to trap a layer of warm air.

Watch out

Blood vessels do not move up and down in the skin. In vasodilation and vasoconstriction it is the small vessels (arterioles) supplying the skin capillaries that widen or narrow.

Controlling blood glucose

Blood glucose is a clear example of negative feedback using hormones. After a meal glucose rises, insulin is released, and the liver stores glucose as glycogen, bringing the level down. During exercise or fasting glucose falls, glucagon is released, glycogen is broken down, and the level rises again. The two hormones work as opposites to hold blood glucose steady.

Exam tip

Don't confuse the words: glucose (the sugar in blood), glycogen (the storage carbohydrate in the liver) and glucagon (the hormone). Insulin lowers, glucagon raises.

Responses in plants

Plants cannot move, but they respond to stimuli by growing in a particular direction. A growth response towards or away from a stimulus is a tropism:

Phototropism — a response to light. Shoots are positively phototropic: they grow towards light, gaining more light for photosynthesis.
Gravitropism (geotropism) — a response to gravity. Roots are positively gravitropic: they grow downwards towards gravity, anchoring the plant and reaching water.

These responses are controlled by a plant hormone called auxin, made at the tips of shoots and roots. Auxin causes cells to elongate (grow longer).

In a shoot, auxin moves to the shaded side. The extra auxin makes those cells grow longer, so the shoot bends towards the light. Auxin therefore explains how a plant tracks the light it needs.

Key terms

Tropism — a directional growth response of a plant to a stimulus.

Auxin — a plant hormone that controls cell elongation and causes tropisms.