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The receptors, first, second and third order neurons and their terminal fields of the
spinothalamic system
Overview
The spinothalamic pathway transmits information from different parts of the body to the brain. So it is
an ascending tract, which actually travels sensory information such as temperature, itch, touch and
pain.
Orders of neurons
First order neurons: carry the sensory impulse from the sensory receptor to the spinal cord.
Second order neurons: carry the information from the spinal cord to the thalamus.
Third order neurons: carry sensory information from the thalamus to the cerebral cortex.
Spinothalamic pathway
Receptors
The transduction of painful stimuli occurs in the free nerve endings of unmyelinated and lightly
myelinated fibers. These fibers are the axons of to the first order neurons.
The majority of nociceptors respond to mechanical, thermal, and chemical stimuli, and are therefore
called polymodal nociceptors. However, many nociceptors show selectivity in their responses to
different stimuli. Thus, there are also mechanical nociceptors, showing selective responses to strong
pressure; thermal nociceptors, showing selective responses to burning heat or extreme cold; and
chemical nociceptors, showing selective responses to histamine and other chemicals. Nociceptors are
present in most body tissues, including skin, bone, muscle, most internal organs, blood vessels, and the
heart. They are notably absent in the brain itself, except for the meninges.
Level1
Information about pain (as well as temperature) in the body is conveyed from the spinal cord to the
brain via the spinothalamic pathway.
The free nerve endings of first order neurons will translate pain, temperature, itch, or touch sensations
into an electrical signal. These primary neurons can be pretty long and travel all the way from the
bottom of the foot to the spinal cord.
The cell bodies of these primary neurons are located in dorsal root ganglions. Their axons enter the
spinal cord through Lissauer’s fascicles. They will synapse in the posterior horn on the same side
(ipsilateral side).
Level2
The axons of the second-order neurons immediately decussate and ascend through the spinothalamic
tract running along the ventral surface of the spinal cord.
As the name implies, the spinothalamic fibers project up the spinal cord and through the medulla,
pons, and midbrain without synapsing, until they reach the thalamus.
The second order neurons synapse over a wide region of the thalamus. Some of the axons terminate in
the VP(ventral posterolateral) nucleus. Other spinothalamic axons end in the small intralaminar nuclei
of the thalamus.
Level3
From the thalamus, by the third order neurons pain and temperature information is projected to various
areas of the cerebral cortex. Actually to the primary somatosensory cortex.
The spinothalamic tract consists of two adjacent pathways: anterior and lateral. The anterior
spinothalamic tract carries information about crude touch. The lateral spinothalamic tract
conveys pain and temperature.