2.2 Neurons and Synaptic Transmission

2.2 Neurons and Synaptic Transmission

1. The structure and function of neurons

   • Types of Neuron
     • Sensory Neuron
       • Carry messages from the peripheral nervous system to the central nervous system.
     • Relay Neuron
       • Found between the sensory input and motor output/response.
         • Mainly in the brain and spinal cord and allow sensory and motor neurons to communicate.
     • Motor Neuron
       • Found in the CNS
         • Connects the CNS to effectors such as muscles and glands in order to control muscle movement.
           • When stimulated they release neurotransmitters that bind to the receptors on muscles to trigger a response which lead to movement.
   • Structure of a Neuron
     • Cell Body
       • Includes the nucleus which contains the genetic material.
     • Axon
       • Carries impulses in the form of an electrical signal known as an action potential, AWAY from the cell body.
     • Myelin Sheath
       • Insulates the axon to protect it and speed up electrical transmission.
     • Dendrites
       • Branch like structures that receive nerve impulses from neighbouring neurons or sensory receptors
     • Node of Ranvier
       • Gaps between the myelin sheath that speed up transmission of impulses.
     • Terminal buttons
       • At the end of an axon, communicate with the next neuron.
   • Electrical transmission
     • When a neuron is at resting state the inside of a cell is negatively charged, compared to the outside.
       • When a neuron is activated by stimulus, inside becomes positively charged causing an action potential.
         • This creates an electrical impulse which travels down the axon.

2. Synaptic Transmission

   • Impulse arrives at the PRESYNAPTIC NEURON
     • Neurotransmitters released from vesicles:
       • - Diffuse across synapse -Bind to receptor sites on post synaptic neuron
         • Chemical message is converted back into an electrical impulse.
           • Neurotransmitters are broken down and reabsorbed into the presynaptic neuron.
   • Excitation and Inhibition
     • Neurotransmitters can have one of two effects on neighbouring neurons.
     • Some cause it to become positively charged and more likely to fire
       • E.G Adrenaline causes exhitation
     • Some cause an increased negative charge and less likely to fire
       • E.G Serotonin causes inhibition in the receiving neuron
   • Summation
     • Majority rules. The action potential will only be triggered if the sum of the excitatory and inhibitory signals when it reaches threshold.
   • Synaptic vesicles containing neurotransmitter are ONLY PRESENT on presynaptic membrane.
     • So can only move in ONE DIRECTION.

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