Primary afferent neuron

Primary sensory neuron

Primary sensory(-afferent) neurons carry nerve impulses from the sensory organs - the receptors - into the CNS where the end branches of the axonform boutons/nerve endings and make synaptic contacts with different kinds of interneurons.

Primary-afferent/sensory neurons are partly located in the PNS (dendrite tree, cell body and parts of the axon) and partly in the CNS (axon, axon branches, boutongs, synapses). The number of primary sensory neurons in humans should not exceed 20 million (2x107), the majority of which are olfactory and optic neurons.

The typical primary sensory neuron (as it is usually presented, but which does not apply to smell and sight)
has its cell body in a sensory ganglion (spinal or caranial nerve ganglion)
has no associated synapses,
usually has a single protrusion that divides into a branch out to the registration area (sensory organ) and a branch into the CNS.