The structure and function of neurons

• There are three types of neuron:

  Motor neuron - connect the CNS to effectors such as muscles and glands. They have short dendrites and long axons.

  Sensory neuron - carry messages from the PNS to the CNS. They have long dendrites and short axons.

  Relay neurons - connect sensory neurons to motor or other neurons. They have short dendrites and axons.
• • Soma - includes a nucleus which contains the genetic material of the cell.
  • Dendrites - branch like structures that protrude from the cell body. These carry nerve impulses from neighbouring neurons towards the cell body.
  • Axon - carries the electrical impulse away from the cell body down the length of the neuron. It is covered in a fatty layer called the myelin sheath which protects it. Gaps in the axons called nodes of ranvier speed up the transmission of the impulse.
  • Terminal buttons - at the end of the axon and communicate with the next neuron in the chain across a gap in the synapse.
• Electric transmission - the firing of a neuron


  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, the inside of the cell becomes positively charged for a split second causing action potential to occur.

  This creates an electrical impulse that travels down the axon towards the end of the neuron.
• There are 100 billion nerve cells in the human nervous system, 80% of which are located in the brain.

  By transmitting signals electrically and chemically, these provide the nervous system with its primary means of communication.
• A synapse

  Each neuron is separated from the next by a tiny gap called the synapse.
• Chemical transmission - the events that occur at the synapse. 


  Signals within neurons are transmitted electrically; however, signals between neurons are transmitted chemically across the synapse.

  When the impulse reaches the pre-synaptic terminal, it triggers the release of neurotransmitter from the synaptic vesicles.

  Once the neurotransmitter has crossed the gap, it's received by the post synaptic receptor site on the next neuron.

  The chemical message is then converted back into an electrical impulse and electric transmission begins.
• Neurotransmitters


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

  Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, like a lock and key.

  Each has specific functions:

  • ACh found where a motor neuron meets a muscle, causing them to contract.
  • Serotonin affects mood and behaviour, which is why it has been implicated as a cause of depression.
• Excitation and inhibition - neurotransmitters generally have an excitatory or inhibitory effect on the neighbouring neuron.

  • Adrenaline - generally excitatory, increasing the positive charge of the postsynaptic neuron, making it more likely the neuron will fire.
  • Serotonin - generally inhibitory, increasing the negative charge of the postsynaptic neuron, making it less likely the neuron will fire.
  • Dopamine is an unusual neurotransmitter as it is equally likely to have excitatory or inhibitory effects on the next neuron in the chain.
• Summation


  Excitatory and inhibitory influences are summed and must reach a certain threshold in order for the action potential of the postsynaptic neuron to be triggered.

  If the net effect of the neurotransmitters is inhibitory, then the postsynaptic neuron is less likely to fire. It is more likely to fire if the net effect is excitatory.