neurons and synaptic transmission

• neuron - 


  nerve cells that process and transmit messages through electrical and chemical signals
• sensory neurons -

  • carry messages from the PNS to the CNS
  • long dendrites and short axons
• relay neurons -

  • connect the sensory neurons to the motor or other relay neurons
  • short dendrites and short axons
• motor neurons -

  • connect the CNS to effectors such as muscles and glands
  • short dendrites and long axons
• structure of a neuron -

  • cell body = includes a nucleus - contains the genetic material of the cell
  • dendrites = protrude from cell body - carry nerve impulses from neighbouring neurones towards cell body
  • axon = carries impulses away from cell body down the length of the neurone - covered in a fatty layer of myelin sheath that protects axon and speeds up electrical transmission of impulse
  • nodes of ranvier = segment the myelin sheath - speed up transmission by forcing it to 'jump' across gaps along the axon
  • terminal buttons = communicate with next neurone in chain across synapse
• location of neurones -

  • cell bodies of motor neurones may be in CNS but have long axons which form part of the PNS
  • sensory neurones are located outside of CNS, in the PNS in clusters known as ganglia
  • relay neurones make up 97% of all neurones and most are found within the brain and visual system
• electrical transmission -

  • when neurone 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 neurone
• synaptic transmission - 


  process by which neighbouring neurones communicate which each other by sending chemical menages across the synapse that seperates them
• chemical transmission -

  • neurons communicate with each other within groups known as neural networks
  • each neurone separated by synapse
  • signals within neurons are transmitted electrically
  • signals between neurons are transmitted chemically across synapse
  • when the electrical impulse reaches the end of the neuron (presynaptic terminal) it triggers the release of neurotransmitter from synaptic vesicles
• neurotransmitters info -

  • direction of travel is one way - neurotransmitters are released from presynaptic neuron terminal and received by postsynaptic neurone (at the receptor sites)
  • several dozen types of neurotransmitter identified in the brain, spinal cord and some glands
  • each neurotransmitter has own molecular structure - fits perfectly into a postsynaptic receptor site
  • neurotransmitters have specialist functions - eg acetylcholine is found at each point where a motor neuron meets a muscle - upon its release will cause muscles to contract
• synaptic transmission -

  1. when the electrical impulse reaches the end of the neuron (presynaptic terminal) it triggers the release of chemical neurotransmitter from synaptic vesicles
  2. neurotransmitter diffuses across the synapse to the next neuron
  3. once it crosses the gap it is taken up by a postsynaptic receptor site on the dendrites of the next neuron
  4. here the chemical message is converted back into an electrical impulse and process begins again

  
• neurotransmitter - 


  brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to another

  neurotransmitters can be broadly divided into those that perform an excitatory function and those that perform an inhibitory function
• excitation - 


  when a neurotransmitter increases the positive charge of the postsynaptic neuron

  increases likelihood that the postsynaptic neuron will pass on the electrical impulse

  eg adrenaline
• inhibition - 


  when a neurotransmitter increases the negative charge of the postsynaptic neuron

  decreases the likelihood that the post synaptic neuron will pass on the electrical impulse

  eg serotonin
• summation -

  • whether a postsynaptic neuron fires is decided by the process of summation
  • excitatory and inhibitory influences are summed:
  • if the net effect on the postsynaptic neuron is inhibitory the postsynaptic neuron is less likely to fire
  • if the net effect is excitatory it is more likely to fire
  • action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any one time reaches the threshold