How many neurons between the central nervous system and visceral effector?

There are usually two neurons between the central nervous system (CNS) and the visceral effector in the autonomic nervous system. The first neuron, called the preganglionic neuron, has its cell body in the CNS and sends its axon to a ganglion, which is a cluster of neuron cell bodies outside the CNS. The second neuron, called the postganglionic neuron, has its cell body in the ganglion and sends its axon to the visceral effector, such as a muscle or gland.

In some cases, there may be only one neuron between the CNS and the visceral effector, as in the case of the parasympathetic innervation of the eye. In this case, the preganglionic neuron sends its axon directly to the ciliary ganglion, which is located near the eye, and the postganglionic neuron sends its axon to the sphincter pupillae muscle, which controls the size of the pupil.