6.2 - The Nervous System & Reflexes

The nervous system and how it coordinates a response

• The nervous system lets the body react to its surroundings and coordinate its response.
• It's built from nerve cells called neurones, which run to all parts of the body.
• The central nervous system (CNS) is made up of the brain and the spinal cord.
• The peripheral nervous system (PNS) is the nerves outside the brain and spinal cord, connecting the CNS to the rest of the body.

Stimuli, receptors and effectors

• Stimulus - A change in the environment inside or outside the body.
• Receptors - Cells that detect a particular kind of change. Different receptors detect different stimuli, and many are grouped together in sense organs:
  • Light receptors in the eyes
  • Sound receptors in the ears
  • Touch and temperature receptors in the skin
  • Chemical receptors in the tongue and nose
• Effectors - The parts that carry out the response. They are either:
  • Muscles - Respond by contracting
  • Glands - Respond by secreting a substance such as a hormone
• When a receptor detects a stimulus, the information is carried as an electrical impulse.

The coordination pathway

1. Information passes from the receptor along a sensory neurone to the CNS.
2. The CNS is the coordinator - it receives the information, decides the response, and links the right receptor to the right effector.
3. It sends an electrical impulse out along a motor neurone to an effector.
4. The effector produces the response.

The whole pathway runs: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.

Reflex actions and their properties

• A reflex action is an automatic, rapid response to a stimulus that happens without any conscious thought.
• Because the body doesn't spend time deciding what to do, a reflex is faster than an ordinary, considered response.
• Reflexes are often protective - they help prevent injury.

Examples of reflexes

• Pulling your hand away from a hot object before you've thought about it.
• Blinking when something comes near your eye.
• The pupil of the eye getting smaller in bright light to protect the eye.