General nerves

• The nervous system has 2 parts, the Central Nervous System and the Peripheral nervous system. The CNS is the brain and spinal cord, and the PNS is the sensory and motor neurones throughout the body
• The nervous system detects stimuli inside the body and in the environment. It processes and stores information and initiates responses
• Dendrite - thin extensions which carry/conduct the impulses towards the cell body
• Axon - long cytoplasmic extension which conducts impulses away from the cell body
• Schwann cells - cells which surround neurones and insulate them
• Myelin sheath - Schwann cells grow/wrap around the axons to form this multi-layered fatty sheath made of cell membrane/phospholipid bilayer. It acts as an electrical insulator that speeds up the transmission of nerve impulses along the axon and protects it
• Nodes of Ranvier - areas along the axon where the myelin sheath is missing
• Cell body - part of the neurone which contains the nucleus, RER, numerous mitochondria and other cell organelles
• Relay neuron:

  • also known as connector/intermediate neurones
  • these lie in the central nervous system - brain or spinal cord
  • They receive impulses from the sensory neurones (or other relay neurones) and send impulses to motor neurones (or other relay neurones)

  A) dendrites

  B) cell body

  C) axon

  D) synaptic knobs

  
• Motor neurone:

  • Carries impulses from the co-ordinator (CNS) to the effector (muscles or glands)
  • The effector brings about a response
  • If the effector is a muscle the response will be contraction
  • If the effector is a gland the response will be secretion of hormones

  A) cell body

  B) dendrites

  C) axon

  D) myelin sheath

  E) Schwann cell

  F) node of Ranvier

  G) axon terminal

  H) synaptic knobs

  
• Sensory neurone:

  • carries nerve impulses from receptor cells (in sense organs) to the co-ordinator
  • found in the central nervous system

  A) dendrites

  B) schwann cell

  C) cell body

  D) myelin sheath

  E) axon

  F) axon terminals

  G) synaptic knobs

  
• Why are starfish (with a co-ordinator, impulses in one direction, and a nerve net) have more co-ordinated movements than Hydra but they only move slowly?

  More co-ordinated:

  • co-ordinator in starfish but not in hydra
  • impulses travel in one direction only

  Slow:

  • nerve net slows down transmission
  • impulses in both directions slows down transmission