What are the parts of a reflex arc?

Receptor, afferent neuron, interneuron, efferent neuron and effector.

A reflex arc can be considered a 'short cut' in the nervous system. It bypasses the brain for a quick response. The components are (see also image):

• Receptor: a specialized part of a neuron that receives a signal.
• Afferent neuron: a neuron that transmits the signal to the spinal cord.
• Interneuron: a neuron in the spinal cord that connects input to output.
• Efferent neuron: a neuron that transmits the signal to the effector.
• Effector: the tissue that turns the signal into action, usually a muscle.

Examples are withdrawing your hand when you touch something hot, or the knee jerk reflex.