neurons and synaptic transmission

• Neurons


  Cells that are specialised to carry neural information throughout the body
• Types of neurons


  • Sensory neurons
  • Relay neurons
  • Motor neurons
• Neuron structure

  • Cell body
  • Dendrites
  • Axon
• Dendrites

  Receive signals from other neurons or from sensory receptors
• Cell body


  Control centre of the neuron
• Axon

  Impulse is carried along, terminating at the axon terminal
• Myelin sheath


  Insulating layer around the axon that allows nerve impulses to transmit more rapidly
• The length of a neuron can vary from a few millimetres up to one metre
• Sensory neurons


  • Carry nerve impulses from sensory receptors to the spinal cord and the brain
  • Convert information from sensory receptors into neural impulses
  • Impulses are translated into sensations in the brain
• Some sensory information travels only to the spinal cord, allowing for reflex actions
• Relay neurons


  Allow sensory and motor neurons to communicate with each other, lying wholly within the brain and spinal cord
• Motor neurons


  • Conduct signals from the CNS to effector organs such as muscles
  • Form synapses with muscles and control their contractions
  • Muscle contraction depends on the rate of firing of the motor neuron axons
  • Muscle relaxation is caused by inhibition of the motor neuron
• Synapse

  The area between the presynaptic neuron and the postsynaptic neuron, including the end of the presynaptic neuron, the membrane of the postsynaptic neuron, and the gap in between
• Synaptic vesicles


  Sacs at the end of the axon that contain neurotransmitters
• Synaptic transmission


  1. Action potential reaches synaptic vesicles

  2. Vesicles release neurotransmitters through exocytosis

  3. Neurotransmitters diffuse across synaptic gap

  4. Neurotransmitters bind to receptors on postsynaptic neuron

  5. Receptors produce excitatory or inhibitory effects
• Re-uptake

  Process where the neurotransmitter is taken up again by the presynaptic neuron
• Antidepressant drugs prolong the action of neurotransmitters by inhibiting the re-uptake process
• Excitatory neurotransmitters


  Increase the likelihood that an excitatory signal is sent to the postsynaptic cell, making it more likely to fire
• Inhibitory neurotransmitters


  Decrease the likelihood of the neuron firing, generally responsible for calming the mind and body
• Excitatory postsynaptic potential (EPSP)


  Electrical change in the postsynaptic cell membrane caused by an excitatory neurotransmitter, making the cell more likely to fire
• Inhibitory postsynaptic potential (IPSP)


  Electrical change in the postsynaptic cell membrane caused by an inhibitory neurotransmitter, making the cell less likely to fire
• Summation


  Adding up the excitatory and inhibitory synaptic input to determine the likelihood of the cell firing
• Spatial summation


  Many EPSPs generated at different synapses on the same postsynaptic neuron at the same time
• Temporal summation


  Many EPSPs generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron
• The rate at which a cell fires is determined by the activity of its excitatory and inhibitory synapses