Spinal reflex arcs are carried out by neurons in the spinal cord alone, without the immediate involvement of the brain. A reflex arc always starts with a sensory neuron and ends with a motor neuron. The simplest reflex arc is a monosynaptic reflex arc involving two neurons and one synapse. The sensory and motor neurons in a monosynaptic reflex arc synapse directly. Although most reflex arcs are more complex, a monosynaptic reflex arc demonstrates the essential components of a reflex arc.