Homeostasis & The Nervous System

Homeostasis

• Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes
• It keeps conditions optimal for enzyme action and all cell functions
• In humans it controls blood glucose concentration, body temperature and water levels
• The automatic control systems may be nervous or chemical (hormonal). Every control system has:
  • Receptors: cells that detect stimuli (changes in the environment)
  • Coordination centres (brain, spinal cord, pancreas) that receive and process information from receptors
  • Effectors: muscles or glands that bring about responses which restore optimum levels

The nervous system

• The nervous system lets humans react to their surroundings and coordinate behaviour
• Information from receptors passes along neurones as electrical impulses to the central nervous system (CNS): the brain and spinal cord
• The CNS coordinates the response of effectors: muscles contracting or glands secreting hormones

$$\text{stimulus} \rightarrow \text{receptor} \rightarrow \text{coordinator} \rightarrow \text{effector} \rightarrow \text{response}$$

The reflex arc

• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain, so they protect the body from harm faster than a decision could
• The pathway:
  • A stimulus (touching a hot object) is detected by a receptor in the skin
  • An impulse travels along the sensory neurone to the CNS
  • At a synapse (a gap between neurones) chemicals diffuse across and trigger an impulse in the relay neurone in the spinal cord
  • Across another synapse, the impulse passes to the motor neurone
  • The motor neurone carries the impulse to the effector, a muscle, which contracts and pulls the hand away: the response

Required Practical 7: reaction time

• Plan and carry out an investigation into a factor (practice, caffeine, distraction, time of day) affecting human reaction time
• The ruler drop method: the subject rests their forearm on a table with their hand over the edge; the tester holds a ruler with the 0 mark level with the subject's thumb, and drops it without warning; the subject catches it as fast as they can and the distance on the ruler at the thumb is recorded. A conversion table gives the reaction time
• Repeat several times and take a mean; test several people
• Control: same hand, same ruler and starting position, same distance between finger and thumb, no practice between tests (or a fixed amount), and the same conditions for everyone
• Computer tests (clicking when the screen changes colour) are more accurate because they time to the millisecond and remove the tester's variation