How does a simple reflex work?

A pain stimulus, for example, pressure from a pin, is sensed by a receptor, for example, pacinian corpuscle in the skin of the fingertip.This causes a change in shape of the receptor, so sodium ions diffuse down the potential gradient, into the sensory neurone. This causes the sensory neurone to depolarise (define if necessary/desired), and causes an impulse (action potential) to be sent along the sensory neurone.At the end of the sensory neurone, there is a synapse (a gap between this neurone and the next). When the end of the sensory neurone is depolarised, neurotransmitters within vesicles are released into the synapse. The neurotransmitters diffuse across the synapse to receptors on the relay neurone in the spinal cord. When the neurotransmitters reach the receptor, the receptor allows sodium ions to diffuse into the relay neurone, depolarising it.An impulse is sent through the relay neurone until the next synapse is reached, before the motor neurone. The same synaptic process then occurs (ask pupil to repeat vesicle/neurotransmitter piece). Neurotransmitters reach the receptors on the motorneurone and sodium ions depolarise the neurone. The impulse (use 'action potential' if understood) is sent along the motorneurone, to the effector organ. In the pin-fingertip example, this would be a muscle, for example, the biceps brachii. The muscle would contract, and move the hand (and fingertip) away from the painful stimulus.