Excitatory and inhibitory signals
Receptors on the postsynaptic membrane determines whether the signal received is:
- excitatory - increases contraction of muscle fibres
- inhibitory - decreases contraction of muscle fibres
Stimuli threshold
A critical number of Chemical involved in passing nerve impulses from one nerve cell to the next across a synapse. molecules is needed in order to affect enough receptors on the postsynaptic membrane.
A nerve impulse is only transmitted across the A tiny gap at the junction between two nerve cells, which nerve signals must cross. if enough neurotransmitter is released. Weak stimuli don't reach this critical number. As a result, not enough neurotransmitter reaches the postsynaptic neuron to generate an impulse. This filters out weak stimuli.
However, a series of weak stimuli can combine, increasing the amount of neurotransmitter that is released. This results in enough neurotransmitter being diffused across the synaptic cleft to trigger the impulse in the postsynaptic neuron. This process is called summation.