A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell, e.g. a muscle or gland cell. The tiny gap between the cells at a synapse is called the synaptic cleft. The presynaptic neurone has a swelling called a synaptic knob. This contains synaptic vesicles filled with neurotransmitters. When an electrical impulse reaches the end of a neurone it causes neurotransmitters to be releases into the synaptic cleft. They diffuse across to the postsynaptic membrane and bind to specific receptors. When neurotransmitters bind to receptors they might trigger an electrical impulse, cause muscle contraction, or cause a hormone to be secreted. Because the receptors are only on the postsynaptic membranes, synapses make sure impulse are unidirectional - the impulse can only travel in one direction. Neurotransmitters are removed from the cleft so the response doesn't keep happening, e.g. they're taken back into the presynaptic neurone or they're broken down by enzymes and the products are taken into the neurone.