What are reflexes and how do they produce movement quickly?

A reflex occurs when movement is produced without conscious thought. They can therefore be described as autonomic and produce movement faster than voluntary movements (which do require conscious thought). Reflexes occur in response to an external stimulus.
A reflex utilises the following pathway, using a monosynaptic bicep tendon reflex as an example:1.A sensory input is detected. E.g as the hammer strikes the tendon, a muscle spindle in the tendon will release an action potential2.Nerve impulses will travel to the spinal cord via sensory neurones.3.Sensory neurones will then terminate directly onto a motor neurone. 4.The motor neurone will leave the spine and travel to the target muscle.5.The motor neurone terminates at this muscle, and excites muscles fibres in order to produce movement. E.g the bicep muscle will contract, producing movement at the elbow.
In voluntary movement, after entering the spine, the sensory neurone will send information up to higher centres (eg. the brain), where it will then connect to a motor neurone. By synapsing onto a motor neurone at the spine, reflexes bypass higher centres (eg. the cerebral cortex of the brain) and hence produce movement a lot faster.