The Structure & Function of Neurons

What is a neuron?

  • Neurons are nerve cells that process and transmit messages through electrical and chemical signals 

Types of Neurons 

  • Three types:
    • Motor neurons = connects the CNS to effectors such as muscles and glands = they have short dendrites and long axons
    • Sensory neurons = carry messages from the PNS to the CNS = they have long dendrites and short axons
    • Relay neurons = connect the sensory neurons to the motor neurons = they have short dendrites and long axons 
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The Structure & Function of Neurons

Structure of a neuron

  • The cell body of a neuron contains a nucleus which has genetic material of the cell 
  • Branch-like structures called dendrites extend beyond from the cell body = these carries nerve impulses from neighbouring neurons towards the cell body 
  • Axons carries the impulses away from the cell body down the neuron = axons are covered in a fatty layer of myelin sheath that protects the axon and speeds up electrical transmission of the impulse 
  • Myelin sheath are seperated by gaps called nodes of Ranvier to speed up the electrical transmission 
  • At the end of the axon are terminal buttons that communicate with the next nueron
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The Structure & Function of Neurons

Electric Transmission = the firing of a neuron 

  • When a neuron is in a resting state the inside of the cell is negatively charged 
  • The outside of the cell is positively charged 
  • When a neuron is activated by a stimulus
  • Inside the cell becomes positively charged for a split second causing an action potential 
  • This creates electrical impulse that travels down the axon
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Synaptic Transmission

What is synaptic transmission?

  • when neighbouring neurons communiate with each other by sending chemical messages across the gap that seperates them 

Chemical transmission - synapses 

  • Neurons are seperated from the next neuron by a synapse 
  • Synapse includes the space between them which is called the synaptic cleft 
  • It also includes the presynaptic terminal and a postsynaptic receptor site  
  • Signals within a neuron are transmitted electrically 
  • Signals between neurons, so through the synapse are transmitted chemically by synaptic transmission 
  • When the electrical impulse reaches the end of the neuron which is called the presynaptic terminal, it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles
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Synaptic Transmission


What is a neurotransmitter?

  • Neurotransmitters are chemicals that diffuse across the synapse to the next neuron
  • Once the neurotransmitter crosses the synapse, it is taken up by the postsynaptic receptor sites (the dendrites of the next neuron) 
  • At the postsynaptic receptor site, the chemical impulse is converted back to a electrical impulse 
  • Each neurotransmitter has it own specific molecular structure that fits perfectly into the post-synaptic receptor site 
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Synaptic Transmission

Excitation and Inhibition

  • Neurotransmitters have either an excitatory or inhibition effect on the neighbouring neuron 


  • What is it? = this is when neurotransmitter makes the charge of the postsynaptic neuron more negative = this decreases the likelihood that the neuron will fire and pass on the electrical impulse 
  • Serotonin makes the neuron more negatively charged and therefore is less liekly to fire 
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Synaptic Transmission


  • What is it? = when neurotransmitters inceases the postive charge of the postsynaptic neuron = which increases the likelihood that the neuron will fire and pass on the electical impulse 
  • A neurotransmitter called adrenaline increases the postive charge of the postsynaptic neuron and making it more likely to fire 
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Synaptic Transmission


  • Whether a postsynaptic neuron does fire is decided by the process of summation 
  • If the net effect on the postsynaptic neuron is inhibitory then the postsynaptic neuron is less likely to fire 
  • If the net effect on the postsynaptic neuron is excitatory, it is more likely to fire
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Synaptic Transmission


1. Electrical impulse reaches the pre-synaptic nerve terminal 

2. The electical impulse triggers the release of a neurotransmitter from tiny sacs called synaptic vesicles 

3. The chemical message diffuses across the synaptic cleft and to the post-synaptic receptor sites (the dendrites

4. The chemical message is then converted back into a electrical impulse and then travels along the axon 

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