Synaptic transmission

• Synaptic transmission

  Chemical transmissions- synapses.

  Each neuron is separated by a synapse- includes space between them (synaptic cleft) as well as the presynaptic terminal and postsynaptic receptor site. Signals within neurons are transmitted electrically but across neurons are chemically by synaptic transmission. When the electrical impulse reaches the end of the neuron (presynaptic terminal) it triggers release of neurotransmitter from synaptic vesicles.
• Synaptic transmission

  Neurotransmitter

  Chemicals that diffuse across synapse to next neuron. Once crosses of gap, taken up by postsynaptic receptor sites. Here, chemical message is converted back into electrical impulse and process of transmission begins in other neuron. Each neurotransmitter has its own specific molecular structure that fits perfectly into postsynaptic receptor site (lock and key). Also have specialist functions - e.g., acetylcholine (ACh) is found at each point where motor neuron meets muscle and upon release causes the muscle to contract.
• Synaptic transmission

  Excitation and inhibition

  Neurotransmitters have either an excitatory or inhibitory effect on the neighbouring neuron. E.g., serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire. In contrast, adrenaline causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire.
• Synaptic transmission

  Summation

  Process that decides whether a postsynaptic neuron does fire. Excitatory and inhibitory influences are summed: if net effect on postsynaptic neuron is inhibitory then less likely to fire, excitatory, more likely. Once electrical impulse is created, it travels down the neuron. Therefore, action potential of postsynaptic neuron is only triggered if sum of excitatory and inhibitory signals at any one time reaches the threshold.