OpenRevise

Biology · Homeostasis and the Nervous System

Knowledge organiser

The whole chapter on two pages. Print it on A4 — they are already landscape — or cover a box and see how much of it you can say from memory.

Back to the chapter

Biology · AQA Paper 2

Homeostasis and the Nervous System

Homeostasis and Control Systems

Homeostasis keeps the internal environment constant using receptors, coordination centres and effectors, so that enzymes and cells work at their best.

  • Homeostasis The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function.
  • Internal environment The conditions inside the body, such as the blood and tissue fluid.
  • Receptor A cell or organ that detects a change.
A flow chart of a negative feedback loop from a change, to a receptor, to a coordination centre, to an effector, to a return to normal.
In negative feedback the response reverses the change.

Nervous and hormonal control

  • Nervous control Nerve impulses; Very fast response; Used for reflex actions and quick changes
  • Hormonal control Hormones in the blood; Slower response, but longer-lasting; Used for slower changes

Neurones and the Nervous System

The nervous system uses neurones to carry electrical impulses from receptors, through the central nervous system, to effectors.

  • Neurone A nerve cell that carries electrical impulses.
  • Stimulus A change in the surroundings that can be detected.
  • Receptor A cell that detects a stimulus and starts an impulse.
A flow chart from stimulus to receptor to central nervous system to effector to response.
Impulses travel from the receptor to the central nervous system, and then to an effector.

Receptors and effectors

  • Receptors Detect a stimulus; Change it into an electrical impulse; Found in sense organs such as the eye and skin
  • Effectors Carry out the response; Muscles contract and glands release chemicals; Receive impulses from motor neurones

Reflex Arcs

A reflex arc is a short pathway from a receptor, along a sensory neurone, a relay neurone and a motor neurone, to an effector.

  • Reflex action A rapid, automatic response that protects the body.
  • Reflex arc The pathway of neurones involved in a reflex action.
  • Relay neurone A neurone in the central nervous system that links a sensory and a motor neurone.
A reflex arc from a pin on a finger, along a sensory neurone to the spinal cord, through a relay neurone, and out along a motor neurone to a muscle.
In a reflex arc the impulse passes from receptor to sensory neurone, relay neurone, motor neurone and effector.

Reflex and voluntary actions

  • Reflex action Very fast; Automatic, without thinking; The pathway goes through the spinal cord
  • Voluntary action Slower; You choose to do it; The pathway goes through the brain

Required practical 7: reaction time

  • Set up A partner holds a ruler vertically so that the zero mark is…
  • Drop Your partner lets go without warning.
  • Measure Read the distance on the ruler at the top of your thumb.
  • Repeat Repeat at least three times and calculate a mean.
Page 1 of 2 openrevise.com

Biology · AQA Paper 2

Homeostasis and the Nervous System continued

The Brain (Separate only)

The brain is divided into regions with different functions.

  • Cerebral cortex The largest part of the brain, which controls thought, memory, language and voluntary movement.
  • Cerebellum The part of the brain that coordinates movement and balance.
  • Medulla The part of the brain that controls the heart rate and breathing.
A cross-section of the brain with the cerebral cortex, cerebellum and medulla labelled.
Each region of the brain has its own job.

What each region controls

  • Thinking and movement Cerebral cortex: conscious thought, memory, language, voluntary movement; Cerebellum: coordination of movement and balance
  • Keeping the body going Medulla: heart rate and breathing

The Eye (Separate only)

The cornea and the lens focus light on the retina, the iris controls how much light enters.

  • Cornea The transparent front of the eye that refracts light.
  • Sclera The tough, white outer layer of the eye.
  • Iris The ring of muscle that controls the size of the pupil.
The eye in cross-section with the cornea, sclera, iris, pupil, lens, ciliary muscles, suspensory ligaments, retina and optic nerve labelled.
Light is bent by the cornea and the lens, and forms an image on the retina.

Near and distant objects

  • Looking at a near object Ciliary muscles contract; Suspensory ligaments slacken; Lens becomes thicker and more curved
  • Looking at a distant object Ciliary muscles relax; Suspensory ligaments are pulled tight; Lens becomes thinner and less curved

Control of Body Temperature (Separate only)

The thermoregulatory centre monitors the temperature of the blood and receives impulses from the skin.

  • Thermoregulatory centre The part of the brain that monitors and controls body temperature.
  • Sweat gland A gland in the skin that produces sweat.
  • Vasodilation The widening of the blood vessels in the skin, so that more energy is transferred to the environment.
A flow chart from a change in body temperature to receptors, to the thermoregulatory centre, to the effectors, and back to about 37 degrees.
The thermoregulatory centre receives information from receptors and sends impulses to the effectors.

Too hot and too cold

  • Too hot Sweat is produced; Blood vessels dilate; No shivering
  • Too cold Sweating stops; Blood vessels constrict; Skeletal muscles shiver