Nervous system

The human nervous system consists of:

  • the central nervous system (CNS) – the brain and spinal cord
  • the peripheral nervous system – nerve cells that carry information to or from the CNS
Human body showing the nervous system with the brain and spinal cord labelled.  The brain and spinal cord comprise the central nervous system.

Nerve cells

Nerve cells are also called neurones. They are adapted to carry electrical impulses from one place to another:

  • they have a long fibre (axon) which is insulated by a fatty sheath
  • they have tiny branches (dendrons) which branch further as dendrites at each end
Motor neurone. At one end there is a nucleus surrounded by cytoplasm and dendrite.  All enclosed by a cell membrane.  There is a long tail holding the Axon which connects with the nerve endings.
  1. diffuse When particles spread out from a region of higher concentration to a region of lower concentration.
  2. effector Organ which has an effect when stimulated (eg muscles or glands).
  3. hormone Chemical messengers produced in glands and carried by the blood to specific organs in the body.
  4. motor neurone The nerve cell that carries electrical impulses from the CNS to effectors such as muscles or glands.
  5. neuron Another term for a nerve cell that is specialised to transmit electrical impulses.
  6. neurotransmitter Chemical involved in passing nerve impulses from one nerve cell to the next across a synapse.
  7. reflex action Automatic and rapid response to a stimulus.
  8. relay neurone The nerve cell that transmits electrical impulses from sensory neurones to motor neurones.
  9. sensory neurone The nerve cell that transmits electrical impulses from receptors in the sense organs to the CNS.
  10. stimuli Things that set off a reaction in the nervous system, for example, light, heat, sound, gravity, smell, taste, or temperature. The singular is stimulus.
  11. stimulus Any change in the environment that can be detected by receptors in an organism.