Nervous System

• What is a stimulus ?

  A change in the external or internal environment of an organism.
• What is a receptor ?

  A structure which detects the stimulus.
• What is an effector ? Give an example.

  A structure which produces the response e.g. a muscle
• What does the corodinating system do ?

  Receives information from the receptor and passes it on to the effector.
• What is a response ?

  An action that results from a stimulus.
• What is a reflex action ?

  a rapid, automatic, involuntary response to a stimulus
• What are the three neurones ?

  Sensory, relay and motor
• Give the sequence of events in the reflex ac if someone was to touch a hot surface.

  1. A receptor detects and responds to the stimulus
  2. Receptors send an impulse via the sensory neurone to the spinal cord
  3. The relay neurone links the sensory neurone to the motor neurone in the spinal cord
  4. Motor neurone carries impulse from spinal cord to the effector (muscle)
  5. The response of the muscle is to contract and pull away from the hot surface
• What is the order of a simple reflex arc ?

  1. Stimulus
  2. Receptor
  3. Sensory neurone
  4. Relay neurone
  5. Motor neurone
  6. Effector
  7. Response
• Explain the importance of a reflex arc.

  Fast, innate, automatic response to a dangerous stimulus that keeps us from harm.
• What does the sensory neurone do ?

  Transmits impulses from receptors towards the central nervous system.
• What is the central nervous system made up of ?

  Brain and spinal cord
• What does the motor neurone do ?

  Transmits impulses away from the central nervous system to effectors.
• What does the relay neurone do ?

  Transmits impulses between sensory and motor neurones and are located within the central nervous system.
• Name three parts of a neurone.

  cell body, dentrites and axons
• Describe the structure and the function of the cell body of a neurone.

  Structure - the control centre of the neurone

  Function- directs impulses from the dentrites to the axon
• Describe the structure and the function of the dentrites of a neurone.

  Structure - fine hair like extensions on the end of the neurone

  Function - receive incoming impulses
• Describe the structure and the function of the axon of a neurone.

  Structure - pathway for the nervous impulses

  Function - conducts impulses from the cell body to the opposite end of the neurone
• Describe the structure of the schwann cells of a neurone.

  Structure - cells wrapped around the axon - ther cytoplasm can be rich in myelin (an insulating lipid)
• Describe the structure and the function of the myelin sheath of a neurone.

  Structure - made up of schwann cells

  Function - insulates the impulses as it passes down the axon
• Describe the structure and the function of the nodes of ranvier of a neurone.

  Structure - gaps between schwann cells where membrane is unmyelinated

  Function - saltatory conduction (where the speed of an impulse is greatly inreased by the message jumping from node to node)
• What is the resting potential ?

  The potential difference across the neurone membrane when it is not transmitting an impulse.
• What is the charge of the inside and outside of the axon when resting ?

  Inside: negative Outside: positive
• What is the membrane said to be when resting ?

  Polarised
• What is a nerve impulse ?

  A temporary reversal of the electrical potential difference across the axon membrane.
• How is the resting potential maintained ?


  • Active transport of Na+ out and K+ in across the membrane by transport proteins in a ratio of 3:2 so the inside of the membrane loses positively charged ions faster than gaining them
  • The passive diffusion of Na+ and K+ across the membrane through channel proteins . Na+ diffuses more slowly than K+ diffuse out.
  • Large anions are inside the axon
• Why is it that Na+ ions cannot pass through the same ion channel as K+ ions ?

  The ion channels are made of proteins which have a specific tertiary structure which is complementary to only one ion.
• Why is the membrane of a neurone said to be polarised ?

  Because opposite sides of the membrane have a different charge (negatively charged inside and positively charged outside)
• What causes an action potential ?

  A stimulus
• What happens to the membrane of the axon when there is an action potential ?

  It becomes depolarised
• What does the charge of the inside of the axon membrane change to when there is an action potential ?

  +40mv becomes more positive