What is a reflex arch?

Reflex arch is a neural pathway that controls a fast, involuntary reflex. The pathway is as follows: 1)Receptors detect a stimuli such as fire or a neddle pricking your finger. 2) Nerve impulses from the receptor are carried by the sensory neurons to the relay neurons in the central nervous system (CNS). There is a synapse(junction) present between the sensory neurons and the relay neurons. Therefore, a chemical messenger (neurotransmitter ) is required to transmit the signal from the sensory neuron to the relay neuron in the CNS. 3)The relay neuron then communinates with the motor neuron via a chemical messenger that transmits signal across the junction. 4)The motor neurons carry the nerve impulses to the effector muscle to bring about a response. 5) Usually, an effector muscle is either a muscle or a gland. Muscles respond to stimuli by contracting whereas glands repsond by secreting fluids. In vertebreas, the sensory neurons don't transmit the nerve impulses from receptors to brain cells but the synapses in the spinal cord for reflex arch.