2. Synaptic transmission

• Neurons and synaptic transmission

  • 100 billion neurons in the NS - 80% in the brain
  • Transmit signals electrically and chemically and are a primary mean of communication
• Types of neurons and structure

  • Sensory - relay - motor
  • Cell body contains a nucleus which contains genetic material
  • Dendrites protrude from the cell body and carry nerve impulses from neighbouring neurons to the cell body
  • The axon carries impulses away from the cell body down the length of the neuron and is covered in myelin sheath. This protects the axon and increases electrical transmission
  • Nodes of Ranvier increase transmission of the impulse
  • Terminal buttons at the end of the axon communicate with the next neuron in the chain across the synapse
• Synaptic transmission

  1. Electrical impulses (action potentials) travel down the axon and reach the presynaptic terminal and trigger the release of neurotransmitters from the synaptic vesicle
  2. Neurotransmitters cross the synaptic gap from vesicles to the next neuron
  3. Neurotransmitters then combine with the postsynaptic receptor site on the dendrites of the next neuron
  4. The chemical message is converted back into an electrical impulse and the process of synaptic transmission is repeated
  5. The stimulation of postsynaptic receptors by neurotransmitters results in excitation or inhibition of PSN
• Sensory neurons

  • Found in eyes, tongue and skin
  • Long dendrites and short axons
  • Carry messages from the PNS to CNS by converting information from these sensory receptors into neural impulses
  • When impulses reach the brain they are translated into sensations such as heat and pain so the organism can respond appropriately
  • Some neurons terminate in the spinal cord. This allows reflex actions to occur quickly without the delay of sending impulses to the brain
• Relay neurons

  • Short dendrites and short axons
  • Receive impulses from sensory neurons and carry these messages to other relay neurons or to motor neurons
  • Allows for communication
  • In the brain and spinal cord only
• Motor neurons

  • Short dendrites and long axons
  • Carry impulses away from the CNS to organs and muscles in the body
  • The cell body on a motor neuron is located in the spinal cord and the axon projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands
  • Axons are nerve fibres that carry signals from the spinal cord
• Structure of neurons

  • When the neuron is in a resting state the inside of the cell is negatively charged compared to the outside
  • However, when a neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second causing an action potential
  • This creates an electrical impulse that travels down the axon towards the end of the neuron
• Excitation and inhibition

  • Neurotransmitters can have either one of these effects on the neuron they communicate with
  • Excitatory - eg. adrenaline, it will cause the neuron it communicates with to fire an impulse - it excites it; this leads to a positive charge
  • Inhibitory - eg. serotonin, it will stop the neuron it communicates with from firing an impulse - it inhibits it; this leads to a negative charge
• Summation

  • Whether or not a postsynaptic neuron fires is decided by the process of summation
  • If the total influence is more excitatory, the next neuron will fire. If the total influence is more inhibitory, the next neuron will not fire