The structure and function of sensory, relay and motor neurons. The process of synaptic transmission, including reference to neurotransmitters, excitation and inhibition.

What this dot point is asking

AQA wants you to describe the three types of neuron and the process of synaptic transmission, including excitation and inhibition. The exam skill is to describe the electrical-then-chemical sequence accurately and to explain how excitation and inhibition combine through summation to determine firing.

Types of neuron

The three neuron types form a pathway that allows the body to detect and respond to the world. Sensory neurons carry information from receptors (in the skin, eyes, tongue and elsewhere) towards the central nervous system, and they typically have long dendrites and short axons. Relay neurons sit entirely within the central nervous system and act as connectors, linking sensory input to motor output, with short dendrites and short axons; they are by far the most numerous type. Motor neurons carry the instruction from the central nervous system out to effectors (muscles and glands), and have short dendrites and a long axon that reaches the muscle. A common worked example is the reflex arc: a painful stimulus is detected by a sensory neuron, passed to a relay neuron in the spinal cord, and relayed to a motor neuron that contracts the muscle to pull away, all without conscious involvement of the brain. The neuron itself carries the signal as an electrical impulse, the action potential, which travels down the axon when the cell is sufficiently stimulated.

Synaptic transmission

The crucial distinction is that transmission within a neuron is electrical but transmission between neurons is chemical. When the action potential reaches the end of the presynaptic neuron (the axon terminal), it causes synaptic vesicles to fuse with the membrane and release a neurotransmitter into the synaptic gap (the synaptic cleft). The neurotransmitter molecules diffuse across this tiny gap and bind to specific receptor sites on the postsynaptic neuron, where the chemical signal is converted back into an electrical one. Because the receptors are specific (a lock-and-key system) and only the presynaptic neuron releases the neurotransmitter, transmission is unidirectional, travelling one way only. Neurotransmitters have either an excitatory effect (for example adrenaline, which makes the postsynaptic neuron more positively charged and more likely to fire) or an inhibitory effect (for example GABA and serotonin, which make it more negatively charged and less likely to fire). A single neuron receives many inputs at once, and whether it fires depends on summation, the net balance of all the excitatory and inhibitory signals it receives, which only triggers an action potential if the excitatory signals are sufficiently dominant.