Coordination & response

Stimulus — a change in the environment (light, heat, pressure, chemicals) that is detected by a receptor.

• Receptors detect the stimulus. The CNS (central nervous system) = brain + spinal cord and acts as the coordinator. Effectors are muscles or glands that produce the response.
• The PNS (peripheral nervous system) = all the nerves outside the CNS. They carry electrical impulses between the CNS and the rest of the body.
• Impulses are electrical signals that travel along neurones (nerve cells).

• Sensory neurone: carries impulses from a receptor to the CNS.
• Relay neurone: inside the CNS, links the sensory neurone to the motor neurone.
• Motor neurone: carries impulses from the CNS to an effector (muscle or gland).
• Long neurones have a myelin sheath, a fatty covering that speeds up the impulse.

Reflex action — a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

• Reflex arc = the pathway: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector → response.
• Examples: pulling your hand from a hot object, blinking, the pupil narrowing in bright light, coughing, the knee jerk.
• Why are reflexes useful? They are fast (few neurones, no thinking), automatic (you cannot stop them) and protect the body from harm.

Synapse — a junction between two neurones, with a tiny gap between them.

• EXT What happens: (1) the impulse arrives at the end of the first neurone, (2) vesicles release neurotransmitter, (3) the neurotransmitter diffuses across the gap (diffusion), (4) it binds to receptors on the next neurone and a new impulse starts.
• EXT One way only: only the first neurone has vesicles of neurotransmitter and only the second has receptors, so impulses can only travel forwards.

Sense organs and receptors

• A sense organ is a group of receptor cells that respond to a specific stimulus and send impulses to the brain along sensory neurones.

Accommodation — EXT the change in shape of the lens so that light from objects at different distances is focused on the retina.

• EXT Distant object: the ciliary muscle relaxes, the suspensory ligaments pull tight, the lens is pulled thin and refracts light less.
• EXT Near object: the ciliary muscle contracts, the suspensory ligaments go slack, the lens becomes fat (more curved) and refracts light more.

• EXT The fovea has the most cones, so it gives the sharpest, colour vision. The blind spot has no rods or cones.

Hormone — a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

• Endocrine glands release hormones directly into the blood (not through a duct).

• Adrenaline is released by the adrenal glands when you are frightened, in danger or excited.
• Effects: heart rate increases, breathing rate increases, pupils widen and the liver releases glucose (blood glucose rises).
• Why? Muscles get more oxygen and glucose for aerobic respiration (respiration), releasing more energy to run away or fight.

Keeping conditions steady

Homeostasis — the maintenance of a constant internal environment.

• The internal environment includes blood glucose concentration, body temperature and water content. Cells work best when these stay steady.
• Insulin is a hormone made by the pancreas that lowers blood glucose.

• EXT Blood glucose too high (after a meal): the pancreas secretes insulin. The liver converts glucose into glycogen for storage and cells take up glucose, so blood glucose falls.
• EXT Blood glucose too low: the pancreas secretes glucagon. The liver converts glycogen back to glucose and releases it, so blood glucose rises.

Negative feedback — EXT a change in conditions causes a response that reverses the change and returns conditions to normal.

• EXT Type 1 diabetes: the pancreas does not make enough insulin, so blood glucose stays too high. It is treated by insulin injections (and managing diet) before meals.
• EXT Reading the graph: the healthy person's glucose peaks lower and returns to normal within 2 to 3 hours. In untreated diabetes it peaks much higher and stays high.

• EXT Too hot: sweat glands release sweat; the evaporation of sweat cools the skin. Blood vessels near the surface widen (vasodilation) so more blood flows near the surface and more heat is lost.
• EXT Too cold: less sweat; the vessels near the surface narrow (vasoconstriction), so less heat is lost. Shivering (rapid muscle contractions) releases heat.
• EXT The temperature is detected by receptors in the skin and brain, which is negative feedback again.

Gravitropism — a response in which parts of a plant grow in a particular direction in response to gravity.

Phototropism — a response in which parts of a plant grow in a particular direction in response to the direction of light.

• Shoots are positively phototropic (grow towards light) and negatively gravitropic (grow up). Roots are positively gravitropic (grow down).
• Investigating phototropism: grow seedlings in a box with a slit of light from one side. Shoots bend towards the light. Keep a control with light from all sides (or a rotating clinostat).
• Investigating gravitropism: lay seedlings on their side in darkness (so light cannot affect them). Roots turn down and shoots turn up.

Auxin — EXT a plant growth hormone made in the shoot tip that diffuses down the shoot and makes cells elongate.

• EXT In one-sided light, auxin collects on the shaded side. Cells there elongate more than on the lit side, so the shoot bends towards the light.
• EXT Gravity has a similar effect on a shoot lying sideways: auxin collects on the lower side, the shoot bends upwards.