Neurons

• Types

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• Sensory neurons

  • found in receptors such as the eyes, ears, tongue and skin, and carry nerve impulses to the spinal cord and brain. When these nerve impulses reach the brain, they are translated into 'sensations', such as vision, hearing, taste and touch. ( PNS )
  • However, not all sensory neurons reach the brain, as some neurons stop at the spinal cord, allowing for quick reflex actions.
• Relay neurons

  • found between sensory input and motor output/response.
  • Relay neurons are found in the brain and spinal cord and allow sensory and motor neurons to communicate. ( CNS )
• Motor neurons

  • found in the central nervous system (CNS) and control muscle movements.
  • When motor neurons are stimulated they release neurotransmitters that bind to the receptors on muscles to trigger a response, which lead to movement.
• The dendrites receive signals from other neurons or from sensory receptor cells. The dendrites are typically connected to the cell body, which is often referred to as the 'control centre' of the neuron, as it's contains the nucleus.
• The axon is a long slender fibre that carries nerve impulses, in the form of an electrical signal known as action potential, away from the cell body towards the axon terminals, where the neuron ends.
• Most axons are surrounded by a myelin sheath (except for relay neurons) which insulates the axon so that the electrical impulses travel faster along the axon.

  The axon-terminal connects the neuron to other neurons (or directly to organs), using a process called synaptic transmission.
• Synaptic Transmission

  • Refers to process by which nerve impulse passes across synaptic gap from one neuron ( presynaptic neuron ) to the other ( postsynaptic neuron )
  • Neurotransmitters are chemicals released from vesicles on the pre synaptic neuron.
  • Travel/diffuse across synapse and lock onto receptor sites on receiving/post synaptic neuron
  • Some neurotransmitters increase the rate of firing in the receiving neurons and others decease the rate of firing.
• Synaptic transmission


  1. Electrical impulses (action potential) reach the presynaptic terminal

  2. Action potentials then trigger the release of neurotransmitters such as serotonin

  3. Neurotransmitters cross the synapse from vesicles

  4. Neurotransmitters combine with receptors on the postsynaptic neuron
• Excitatory neurotransmitter


  Binding with a postsynaptic receptor causes an electrical change in the membrane of that cell resulting in an excitatory post-synaptic potential (EPSP) meaning that the postsynaptic cell is more likely to fire
• Inhibitory neurotransmitter


  Binding with a postsynaptic receptor results in an inhibitory postsynaptic potential (IPSP) making it less likely to fire
• Summation: nerve cell can receive both EPSP and IPSP at the same time, the likelihood of firing is determined by calculating the synaptic output of EPSP and IPSP, net result of this calculation - called summation will depend on whether the nerve cell will fire or not.