Describe how a nerve impulses crosses a synapse in the nervous system.

A synapse is a junction or space between two neurons or a neuron and a receptor which allows the transmssions of an electrical or chemical signals to another cell. The first neuron is called the presynaptic neuron and the target cell is refered to as the post synaptic neuron. In order for a nerve impulse to continue it must cross the synaptic cleft, the fluid filled space between the neurons plasma membranes.This is done through the use of neurotransmitters. When an action potential reaches the end of the presynaptic neuron, the synaptic knob, depolarisation of the membrane occurs triggaring a cascade, leading to the opening of calcium channels. The influx of calcium causes vesicles containing neurotransmitters to fuse with the presynaptic plasma membrane and release the neurotransmitter into the synaptic cleft via exocytosis. The neurotransmitters diffuse across the synaptic cleft and bind to transmitted-gated ion channel receptors on the postsynaptic membrane. When the neurotransmitter binds these channels are opened allowing an influx of sodium into the ppostsynaptic neuron causing it's membranes depolarisation. this depolarisation allows the poropagation of an action potential down the postsynaptic neuron. The neurotransmitter that remains in the cleft is quickly degraded and the calcium ions from the presynaptic neuron are pumped back into the synaptic cleft.