Synaptic transmission

• Neurons communicate with each other within groups known as neural networks. Each neuron is separated from the next by a gap called a synapse.
• Neurotransmitters are chemicals that diffuse across a synapse to the next neuron. Once a neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the dendrites of the next neuron.
• The direction of travel can only be one way. This is because neurotransmitters are released from the presynaptic neuron terminal and are received by the postsynaptic neuron.
• Signals within neurons are transmitted electrically. However, signals between neurons are transmitted chemically across the synapse.
• When the electrical impulse reaches the end of the neuron it triggers the release of neurotransmitter from tiny sacs called vesicles.
• Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, similar to a lock and key.
• Neurotransmitters have specialist functions. For example, acetylcholine is found at each point where a motor neurone meets a muscle, and upon its release, it will cause muscles to contract.
• Neurotransmitters either have an excitatory or inhibitory effect on the neighbouring neuron.
• Serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire.
• Adrenaline causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire.
• Whether a postsynaptic neuron fires is decided by the process of summation. The excitatory and inhibitory influences are summed: if the net effect is inhibitory then the postsynaptic neuron is less likely to fire. If the net effect is excitatory it is more likely to fire. Therefore, the action potential of the postsynaptic neuron is only triggered if the sum of the excitatory or inhibitory signals at any one time reaches the threshold.