NEURONS AND SYNAPTIC TRANSMISSION

• Definition of neurons


  Basic building blocks of the nervous system. Nerve cells that process and transmit messages through electrical and chemical signals.
• Sensory neuron


  Carries messages from the PNS (different organs) to the CNS (brain) - long dendrites and short axons
• Relay neuron


  Connects sensory neurons to motor neurons or other relay neurons (intermediary) - short dendrites and short axons
• Motor neuron


  Connects the CNS to effectors such as muscles and glands – short dendrites and long axons
• Nucleus

  Contains the genetic information of the cell, located in the cell body of the neuron
• Dendrites

  Branch-like structures that protrude from the cell body, carrying nerve impulses towards the cell body from neighbouring neurons
• Axon

  Carries the nerve impulse away from the cell body down the length of the neuron
• Myelin sheath

  Fatty layer that covers the axon and speeds up electrical transmission
• Nodes of Ranvier

  Segments the myelin sheath, otherwise the myelin sheath would slow transmission down
• Axon terminals

  Located at the end of the axon, connect with the next neuron across a gap called the synapse
• Electrical transmission: the firing of a neuron

  When the neuron is at rest, the inside of the cell is negatively charged in comparison to the outside of the cell which is positively charged. When the neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second, creating an action potential, which fires an electrical impulse down the axon to the end of the neuron.  
• Definition of synaptic transmission


  The process by which neighbouring neurons communicate with each other by sending chemical messages across the gap (synaptic cleft) that separates them. 
• Neurotransmitters - definition

  Chemicals that diffuse across the synapse to the next neuron. 
• Process of synaptic transmission

  An electrical impulse reaches the end of a neuron, triggering the release of neurotransmitters from the synaptic vesicles. The neurotransmitters chemically diffuse across the synapse, before being picked up by the post synaptic receptor site and transferred back into electrical transmission in the next neuron. 
• Excitation


  If the net effect is excitatory, it is more likely to fire i.e. the inside of the postsynaptic neuron becomes positively charged momentarily.
• Inhibition


  If the net effect on the postsynaptic neuron is inhibitory, then the postsynaptic neuron is less likely to fire.
• Serotonin


  The neurotransmitter serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire. 
• Adrenaline


  Adrenaline is both a hormone and a neurotransmitter and is involved in the stress response. It causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire. 
• Why can neurotransmitters only travel in one direction?

  The synaptic vesicles containing the neurotransmitters are only released from the presynaptic membrane. The receptors for the neurotransmitters are only present on the post synaptic membrane. It is the binding of the neurotransmitter to the receptor that allows information to be passed on to the next neuron.