Neurons and synaptic transmission

• What is a neuron? What is the structure of a neuron?

  • A specialised cell designed to transmit information through electrical or chemical signals
  • Dendrites: branch-like structures that carry electrical impulses towards the cell body
  • Axon: carries electrical impulses away from the cell body
  • Myelin sheath: fatty later that protects the axon and speeds up electrical transmission by forcing impulses to jump across small gaps called nodes of Ranvier
  • Terminal buttons: end of an axon that communicates with the next neuron across a synapse
• What are the roles of the 3 types of neurons?

  • Sensory neurons carry messages from the PNS to the CNS with long dendrites and short axons - located in clusters ganglias in the PNS
  • Relay neurons connect sensory neurons to motor and relay neurons with short dendrites and long axons - located in the brain and visual system
  • Motor neurons connect the CNS to muscles and glands with short dendrites and long axons - located in the CNS but long axons form part of the PNS
• What is synaptic transmission? (1)

  • Neurons communicate with each other in groups called neural networks, and each one is separated by a small gap called the synapse
  • Signals WITHIN neurons are electrically transmitted, signals BETWEEN neurons are chemically transmitted across the synapse
  • When an electrical impulse reaches the end of the neuron (the presynaptic terminal) it triggers tiny sacs called synaptic vesicles to release neurotransmitters
• What is synaptic transmission? (2) What are neurotransmitters?

  • Neurotransmitters are chemicals that diffuse across the synapse - once it crosses the gap it is taken up by a postsynaptic receptor site on the next neuron's dendrites
  • The neurotransmitter is converted back into an electrical impulse and the process of transmission begins in the next neuron
  • Each neurotransmitter has a specific molecular structure that fits perfectly into a postsynaptic receptor site, like a lock and key
  • They also have specialised functions e.g. dopamine is released when experiencing pleasurable activities and leads to feelings of satisfaction
• What is excitation and inhibition?

  • Excitation: excitatory neurotransmitters like adrenaline create a positive charge in the postsynaptic neuron, creating an excitatory postsynaptic neuron (EPSP), meaning it is more likely to fire and continue the message
  • Inhibition: inhibitory neurotransmitters quieten down signals by having a negative charge, creating an inhibitory postsynaptic potential (IPSP) making it less likely that the cell will fire
• What is summation?

  • Nerve cells can receive both EPSPs and IPSPs at the same time - summation is the net result of adding up excitatory and inhibitory input which determines whether the cell will fire or not
  • For the cell to fire the net effect has to be excitatory more than inhibitory
  • Spatial summation: when a large number of EPSPs are generated at different synapses at the same postsynaptic neuron at the same time
  • Temporal summation: when a large number of EPSPs are generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron -> both increase the strength of EPSPs making the cell more likely to fire