Neurones

• information is sent through the nervous system in the form of electrical impulses that pass along nerve cells known as neurones
• a bundle of neurones is known as a nerve
• there are 3 types of neurones: sensory neurones, relay neurones, and motor neurones
• nerve impulses follow this pathway;

  stimulus > receptor > sensory neurone > CNS > motor neurone > effector
• hormones are chemical substances (often referred to as chemical messengers) produced by the endocrine glands and carried by the blood
• hormones act slower than nerve impulses and are therefore used to control functions that do not need instant responses
• hormone action follows this pathway:

  stimulus > receptor > hormone > effector
• Features found in all neurones;

  • an axon (long fibre)
  • a cell body that contains the nucleus and other cellular structures
  • axon terminal (has many nerve endings which allow neurones to connect and receive impulses from other neurones - allows easy communication)
• Some neurones are myelinated, meaning that they have a fatty, insulating sheath around them. The myelin sheath is made up of specialised cells known as Schwann cells which wrap themselves around the axon. There are uninsulated gaps between the Schwann cells known as the nodes of Ranvier. Electrical impulses in myelinated cells do not travel down the whole axon, but jump from node to node (this speeds up impulse transmission)
• in non-myelinated neurones, the axon is not insulated by Schwann cells, and the impulse travels slower as it moves through the entire length of the axon
• sensory neurones carry impulses from receptors to the brain and spinal cord in the CNS
• Relay neurones are found entirely within the CNS and connect sensory and motor neurones
• Motor neurones carry impulses from the CNS to effector muscles or glands
• Structure of motor neurones;

  • a large cell body at one end (which lies within the spinal cord or brain)
  • many highly-branched dendrites extending from the cell body, providing many connections with the axon terminals of other neurones
• structure of relay neurones;

  • short neurones with axons and highly branched dendrites
• structure of sensory neurones;

  • cell body that branches off in the middle of the axon
  • the dendrites are attached to a receptor cell
  • a dendron (the section of neurone that links the axon terminal with the cell body) which delivers the electrical impulse to the cell body
  • axon (connects the cell body with the CNS)