Final answer:
Complex reflexes can be shorter in length than simple reflexes because the CNS may not be involved. In a short reflex, the sensory neuron directly activates the ganglionic neuron, bypassing the CNS.
Step-by-step explanation:
The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS. However, in the autonomic system there is the possibility that the CNS is not involved. Because the efferent branch of a visceral reflex involves two neurons—the central neuron and the ganglionic neuron—a 'short circuit' can be possible. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved.
Reflexes are crucial in that they involve a response to stimuli without conscious thought, their mechanisms varying slightly between affecting skeletal muscle contraction in somatic reflexes or targeting cardiac and smooth muscles, as well as glandular tissues in autonomic reflexes.