Neurons and synaptic transmission

• Neuron

  = basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages through electrical and chemical signals.
• Neurons 


  Sensory

  relay

  motor

  
• structure of a neuron

  The cell body includes a nucleus- contains genetic information.

  Branchlike structures called dendrites protrude from the cell body- carry nerve impulses from neighbouring neurones towards the body.

  The axon carries impulses away from the cell body down the length of the neuron. Axon covered in a fatty layer of myelin sheath that protects the axon and speeds up electrical transmission of the impulse.
• continued:

  • The myelin sheath is segmented by gaps called nodes of Ranvier.= speeds up transmission of impulse by forcing it to 'jump' across the gaps along the axon.
  • At the end of the axon are terminal buttons that communicate with the next neuron across the synapse.
• locations of neurons

  The cell bodies of motor neurones may be in the central nervous system but they have long axons which form part of the peripheral nervous system.

  • sensory neurones are located outside of the CNS, in the PNS in clusters known as ganglia.
• Electrical transmission

  When a neuron is in a resting state the inside of the cell is negatively charged compared to the outside.

  • when a neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second causing an action potential to occur.
  • creates an electrical impulse that travels down the axon towards the end of the neuron.
• Synaptic transmission

  = process by which neighbouring neurons communicate with each other by sending chemical messages across the gap that separates them.
• chemical transmission

  = neurons communicate with each other within groups known as neural networks. Each neuron is separated from the next by a tiny gap called the synapse.

  • signals within neurons are transmitted electrically.
  • signals between neurons are transmitted chemically.
  • when the electrical impulse reaches the end of the neuron it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles.
• Neurotransmitters

  = chemicals that diffuse across the synapse to the next neuron in the chain.

  • once the neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the dendrites of the next neuron.
  • the chemical message is converted back into an electrical impulse and the process of transmission begins again in this other neuron.
  • can only travel one way.
  • each neurotransmitter has it's own specific molecular structure that fits perfectly into a postsynaptic receptor site.
  • have specialist functions.
• excitation 


  = when a neurotransmitter increases the positive charge of the postsynaptic neuron- increases the likelihood that the postsynaptic neuron will pass on the electrical impulse.
• inhibition 


  = when a neurotransmitter increases the negative charge of the postsynaptic neuron. This decreases the likelihood that the postsynaptic neuron will pass on the electrical impulse.
• summation

  = whether a postsynaptic neuron fires is decided by summation.

  • the excitatory and inhibitory influences are summed: if the net effects on the postsynaptic neuron is inhibitory then the postsynaptic neuron is less likely to fire.
  • if it's more likely to fire then the inside of the postsynaptic neuron momentarily becomes positively charged.