Neurons

• Neuron: The basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages through electrical and chemical signals.
• Sensory neuron: These carry messages from the PNS to the CNS. They have long dendrites and short axons.
• Relay neurons: Connect sensory neurons to the motor or other relay neurons. They have short dendrites and short axons.
• Motor neurons: These connect the CNS to effectors such as muscles. They have short dendrites and long axons.
• Structure of a neuron:

  • The cell body has a nucleus which contains genetic material
  • Dendrites protrude from the cell body which carry nerve impulses.
  • Axon carry the impulses away from the cell body.
  • The myelin sheath protects the axons and speeds up electrical transmission.
  • Nodes of Ranvier speed up transmission by forcing it to jump across gaps of the axon.
  • Terminal buttons communicate with the next neuron across a synapse.
• Locations:

  • Motor- CNS long axons may form part of the PNS.
  • Sensory- outside the CNS, in PNS in clusters.
  • Relay- Make up 97% of neurons most found in the brain and visual system.
• Electrical transmission:

  • When a neuron is in a resting state the inside of the cell is negatively charged compared to the outside.
  • When 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.
• Synaptic transmission: The process by which neighbouring neurons communicate with each other by sending chemical messages across the gap that separates them.
• Neurotransmitter: Brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to the other. They can be excitatory or inhibitory.
• Excitation: When a neurotransmitter, such as adrenaline, increase the positive charge of the postsynaptic neuron. This increases the likelihood that the postsynaptic neuron will pass on the electrical impulse.
• Inhibition: When a neurotransmitter, such as serotonin, increases the negative charge of the postsynaptic neuron. This decreases the likelihood that the postsynaptic neuron will pass on the electrical impulse.
• Chemical transmission:

  • Neurons communicate with each other in groups known as neural networks.
  • Each neuron is separated by a synapse.
  • Signals within neurons are transmitted electrically but signals between the neurons are transmitted chemically .
  • When the electrical impulse reaches the end of the neuron it triggers the release of neurotransmitter from a synaptic vesicle.
• Neurotransmitters:

  • These are chemicals that diffuse across the synapse to the next neuron.
  • Once it crosses the gap it is taken up by a postsynaptic receptor site on the dendrites of the next neuron.
  • The chemical message is then transferred back into an electrical impulse.
  • Each neurotransmitter has it's own specific molecular structure that fits perfectly into a postsynaptic receptor site.
• Summation:

  • Whether a postsynaptic neuron fires is decided by the process of summation.
  • E.G. If the net effect on the postsynaptic neuron is inhibitory then the postsynaptic neuron is less 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.