Homeostasis and response

• Homeostasis is maintaining a constant environment inside your body in response to changes in the external (and internal) environment.
• You have nervous and hormonal communication systems.
• Hormonal system - slow, indirect, long-term responses
• Nervous system - fast, direct, short-term responses
• All automatic control systems are made of three main components: receptors, coordination centres (like the brain, spine or pancreas) and effectors.
• Receptors are the cells that detect stimuli.
• Effectors respond to the instruction by changing something.
• The coordination centre processes information from the receptor and sends out instructions to an effector
• The CNS is connected to the body by sensory and motor neurones.
• Sensory neurones carry information from the receptors to the CNS.
• Motor neurones carry impulses from the CNS to effectors.
• A reflex arc is a simple pathway between a receptor, coordinating centre and effector.
• The connection between two neurones is called a synapse. Chemicals diffuse across the gap and set off a new electrical signal in the next neurone.
• The brain is part of the central nervous system.
• The cerebral cortex controls consciousness, intelligence, memory and language.
• The medulla controls unconscious activities (like breathing and heartbeat)
• The cerebellum is responsible for muscle coordination.
• To study the brain, scientists:

  • study patients with brain damage
  • electrically stimulate the brain
  • MRI scans
• The sclera is the tough supporting wall of the eye.
• The cornea is the transparent layer at the front of the eye that refracts light.
• The iris is the coloured part of the eye that controls the size of the pupil.
• The lens is a transparent structure that changes shape to focus light onto the retina.
• The retina contains receptors that convert light into electrical signals.
• The shape of the lens is controlled by ciliary muscles and suspensory ligaments.
• The optic nerve is the nerve that carries impulses from the retina to the brain.
• When the receptors in the eye detect bright light, the circular muscles contract and the radial muscles relax.
• When the receptors in the eye detect low levels of light, the radial muscles contract and the circular muscles relax.
• Changing the shape of the lens to focus is called accommodation.
• To focus on near objects, the ciliary muscles contract and the suspensory ligaments relax. The lens becomes more curved (fat).
• To focus on far objects, the ciliary muscles relax and the suspensory ligaments tighten. The lens becomes less curved (thinner).
• Long sightedness (hyperopia) occurs when light isn't refracted enough or the eye is too short. Convex lenses are used to correct it.
• Short sightedness (myopia) occurs when light is refracted too much or the eye is too long. Concave lenses are used to fix it.
• There are different ways to treat vision defects:

  • contact lenses - convenient and invisible but can cause infection
  • laser eye surgery - can have complications
  • replacement lens surgery - can damage the retina
• The thermoregulatory centre in the brain keeps body temperature at a constant 37 degrees.
• When you're too hot:

  • sweat is produced and evaporates
  • the blood vessels supplying the skin dilate (vasodilation)
• When you're too cold:

  • hairs stand up to trap heat
  • blood vessels supplying the skin constrict (vasoconstriction)
  • you shiver and respiration transfers energy to warm the body
• The endocrine system is a network of glands that secrete hormones into the bloodstream.
• Hormones travel in the bloodstream.
• The pituitary gland is known as the 'master gland' and produces many different hormones that act on other glands.
• The ovaries produce oestrogen.