Pain Pathways

Spinothalamic Tract

Dorsal Root & Spinal Cord

The dorsal root ganglion houses the primary sensory neuron.
It is pseudounipolar, meaning it sends fibers both towards the spinal cord and also toward the periphery.

Indicate that where the roots merge, they form the mixed spinal nerve.
The sensory nerve activates a neuron in the dorsal horn (nociception most notably involves neurons of lamina I).
Spinothalamic tract (STT) fibers cross within the spinal cord to the contralateral side.

Spinothalamic Projection

Leg fibers synapse within the lumbar spinal cord and then decussate and ascend the anterolateral cord and brainstem to the thalamus.
Arm fibers synapse within the cervical spinal cord, and, also decussate and ascend the anterolateral cord and brainstem to the thalamus.
Trigeminothalamic fibers decussate within the pons and ascend the brainstem, medially, to the thalamus.

Thalamocortical Projection

Leg fibers project to the midline region; they terminate in the posterior paracentral gyrus.
Arm fibers project from the lateral thalamus to the upper convexity of the postcentral gyrus, lateral to the leg fibers.
Facial fibers project from the medial thalamus to the lateral cerebral hemisphere.

Medial-lateral orientation

Note that medial-lateral orientation of these fibers in the cerebral cortex is opposite of that in the thalamus.

Additional Pain Projection Fibers

Pathways, such as the spinoreticular, spinomesencephalic, and spinohypothalamic tracts, and the cervicothalamic tracts all also carry noxious stimuli from their self-named origins and termination sites and synapse in many regions, including the nuclei we addressed with the descending pain modulatory system. Pain is more than simply the detection of a direct noxious stimulus, it is the emotional integration of many different influencing stimuli.
For a detailed understanding of these pathways, as well as the trigeminothalamic tracts, and the correlating neurotransmitters that play a role in pain, review our tutorials on the: Midbrain, Pons, & Medulla.

descending pain modulatory system

Origins

Originates from numerous regions, including the anterior cingulate gyrus, amygdala, insula, etc... And along its descent innervates the:

Periaqueductal gray area

Parabrachial complex

The parabrachial complex in the dorsal pons at the superior cerebellar peduncle, which comprises the "brachium conjunctivum".

Rostral ventromedial medulla

Dorsal horn

Clinical Terminology

As a final part of this tutorial, let's learn some key clinical terminology, so we can get a sense of all of the ways that pain can manifest.

Pain types:

Neuropathic pain is a broad term, which can refer to pain that stems from any somatosensory nervous system lesion or pathologic process.
Whereas neuralgia is a more specific term, which refers to pain in a nerve distribution (or in the distribution of multiple nerves).
And neuritis specifically refers to inflammatory nerve pain.

Abnormal pain responses:

Hyper/hypo-esthesia refers to an increased (hyper) or decreased (hypo) sensitivity to a any sensory stimulus – it's a broad term.
Whereas hyper/hypo-algesia specifically refers to an increased (hyper) or decreased (hypo) pain response to a noxious stimulus.

Unexpected pain:

Allodynia refers to pain to a non-noxious stimulus, such as the bed-sheets brushing over your feet. This is especially common in complex regional pain syndrome (aka reflex sympathetic dystrophy).

Abnormal sensations:

Dysesthesia, which refers to an unpleasant response and can encompass allodynia or hyperalgesia.
Whereas, paresthesia is a similar term that describes an abnormal sensation but the sensation isn't unpleasant.

Absence of pain: