Neuroscience Online (ii,4,4)

Section II: Sensory Systems
4. Somatosensory Pathways
Part 4 of 4

Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.

navigation - see bottom of page for text navigation

Spinal Trigeminal Pathway: Face Pain, Temperature and Crude Touch

The spinal trigeminal pathway carries and processes crude touch, pain and temperature information from the face (Figure 4.11) Consequently, it is the cranial homologue of the spinothalamic pathways i.e., homologous to all the spinothalamic pathways, the archi-, paleo- and neo-spinothalamic pathways. As in the spinothalamic pathways, the afferents carrying crude touch information are kept separate from those carrying temperature information and from others carrying pain information. Also the trigeminal afferents carrying sharp, cutting pain information are segregated from those carrying dull, burning pain and deep aching pain information.

Figure 4.11

The spinal trigeminal pathway.

Press to view the course of the pathway. Click on the structure labels to view their locations in the sections. Click on the label "Spinothalamic tract" to view the vascular supply to the spinal trigeminal tract and nucleus, which is provided by the posterior inferior cerebellar artery.

The 1° spinal trigeminal afferents

are located in the same nerves and ganglia as those of the main sensory trigeminal pathway (Table II).
have Ad and C peripheral axons that form free nerve endings in the dura and face.
on entering the brain stem, form the spinal trigeminal tract.

The spinal trigeminal tract

extends from mid pontine levels (the level of entry of trigeminal nerve) down to C1 of the spinal cord.
consists of spinal trigeminal 1° afferent axons (predominantly of the trigeminal nerve).
1° afferents synapse in the spinal trigeminal nucleus (2° spinal trigeminal afferents).

The 2° spinal trigeminal afferent axons

decussate and form the ventral trigeminal lemniscus contralateral to their cells of origin.
ascend in the ventral trigeminal lemniscus as crossed 2° spinal trigeminal afferents.
travel with afferents that leave the ventral trigeminal lemniscus as trigeminoreticular fibers, which terminate in the brain stem reticular formation.
are joined by the crossed 2° main sensory trigeminal afferents at mid-pons.
travel with afferents that leave the ventral trigeminal lemniscus as trigeminomesencephalic fibers, which terminate near the midbrain periaqueductal gray.
terminate in the VPM and in the intralaminar nuclei of the thalamus.

Multiple thalamic nuclei process information in this pathway

the VPM processes sharp pricking pain.
the intralaminar nuclei processes other poorly localized sensations of dull, burning pain, deep, aching pain, temperature and crude touch.

The 3° spinal trigeminal afferent axons from the thalamus:

travel in the posterior limb of the internal capsule.
end in multiple areas of the cerebral cortex.

The spinal trigeminal pathway terminates in multiple cortical areas:

the 3° VPM axons end in the primary somatosensory cortex (See Figure 4.4), which provides for accurate localization in the face area of the source of the sharp, pricking pain.
the intralaminar nuclei axons terminate in the cingulate gyrus and insula of the cerebral cortex, which provide for poorly localized sensations of dull and aching pain, temperature and crude touch.

Figure 4.12

Afferent neurons in the spinal trigeminal pathway activated by a pin prick applied to the left cheek.

Press to animate. The flash of light at each synapse represents the release of neurotransmitter by the presynaptic axon terminal.

Figure 4.12 illustrates the course of action potential generated in response to a pin prick to the left cheek. Free nerve endings in the left cheek are stimulated by the pin prick. Action potentials are generated and conducted by the 1° afferent Ad axon, past the pseudounipolar soma, and into the brain stem

The 1° afferent central process bypasses the main sensory trigeminal nucleus and descends the brain stem in the spinal trigeminal tract. The action potentials are conducted in this descending tract to the spinal trigeminal nucleus, where they initiate the release neurotransmitter from the 1° afferent axon terminals. The neurotransmitter is released onto 2° afferents within the spinal trigeminal nucleus. The 2° afferent generates action potentials that are conducted along its axon, which decussates to form the ventral trigeminal lemniscus. These action potentials are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the cheek where the stimulus was applied. The action potentials ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals. They release neurotransmitters onto the 3° afferents in the VPM. The action potentials generated by the 3° VPM afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the postcentral gyrus of the parietal cortex. These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the free nerve ending. The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus and for modality specific information used to identify the stimulus as a sharp pinprick.

Concluding Remarks

Clinically, it is important to remember what information is carried by a particular pathway and the level of the pathway at which decussation occurs (Figure 4.13).

Figure 4.13

The pathways involved with processing discriminative touch and proprioception from the body and face and the pathways involved with processing sharp pain and temperature from the body and face.

For example, if the posterior funiculus were sectioned, sparing the rest of the spinal cord, discriminative touch and proprioception would be affected but not pain, temperature, or crude touch. Also, the loss of discriminative touch and proprioception would be ipsilesional (e.g., on the right side of the body with section of the right posterior column) because the 1° afferent axons in the posterior columns do not decussate. Consequently, damage to the posterior column in the spinal cord would be suspected if a patient presented with a loss of discriminative touch and proprioception in the right leg and foot, with no change in pain or temperature sense in the body or face. This area of loss resulted because the ascending medial lemniscal 1° afferent axons from coccygeal to lower thoracic levels were cut off from the brain stem, and the information they carried could not be sent on to the thalamus and cortex. In contrast, a stroke affecting the posterior paracentral lobule would produce sensory deficits in discriminative touch, proprioception and sharp pricking pain contralateral to the site of stroke. However such a stroke would affect other pain, temperature and crude touch sensations less than a large spinal cord lesion because these somatic sensations are represented in diffuse areas of the cortex.

Table II The Nerve Roots and Ganglia Associated with the Somatic and Visceral Afferent Pathways
Nerve Root	Ganglia	Somatic Innervation	Visceral Innervation
Spinal Cord: Sacral	posterior root: S5 to S1	buttocks, back of leg and foot, genitals	lower pelvic region, e.g., Rectum
Spinal Cord: Lumbar	posterior root: L5 to L1	lower back, hip, pelvic area, side and front of leg and foot	leg and pelvic region, e.g., bladder
Spinal Cord: Thoracic	posterior root: T12 to T1	trunk (abdomen, back, and chest), part of arm	lower roots: Lower abdomen (e.g., kidney, colon, appendix) middle roots: Upper abdomen (e.g., stomach, liver, gall bladder) upper roots: Chest (e.g., diaphragm, esophagus, lung, heart)
Spinal Cord: Cervical	posterior root: C8 to C2	shoulder, arm, hand, fingers, neck and back of head	minor to blood vessels and sweat glands of upper body and extremities
Cranial Nerve: Vagus Nerve	jugular (superior) nodose (inferior)	back of ear, external auditory canal and dura	none throat, thoracic and abdominal viscera
Cranial Nerve: Glossopharyngeal	superior (jugular) petrosal (inferior)	back of ear (minor), ear drum, middle ear ear drum, middle ear, Eustachian tube, tonsil, pharynx, soft palate and posterior tongue	none carotid body and sinus
Cranial Nerve: Facial	geniculate	skin of ear	minor - Parotid gland
Cranial Nerve: Trigeminal	semilunar	face, eye, oral and nasal cavities, and meninges	none

Test Your Knowledge

Make the best match between the 1° somatosensory axon type and the sensations carried by the axon.

C fibers
A
B
C
D
E
F

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration This is an INCORRECT match.

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch This is an INCORRECT match.

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain This is the CORRECT match!

The C fibers carry information about dull and deep pain and warm/hot from somatic structures. They do not carry sharp "fast" pain information.

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction This is an INCORRECT match.

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain This is an INCORRECT match.

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch This is an INCORRECT match.

A delta fibers
A
B
C
D
E
F

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration This is an INCORRECT match.

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch This is an INCORRECT match.

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain This is an INCORRECT match.

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction This is an INCORRECT match.

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain This is the CORRECT match!

The A delta fibers carry information about sharp "fast" pain, cold/cool. The C fibers carry dull and deep pain and warm/hot information from somatic structures.

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch This is an INCORRECT match.

A beta fibers
A
B
C
D
E
F

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration This is the CORRECT match!

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch This is an INCORRECT match.

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain This is an INCORRECT match.

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction This is an INCORRECT match.

E. Sharp "fast" pain

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain This is an INCORRECT match.

F. Dynamic muscle stretch

A. Vibration

B. Static muscle stretch

C. Dull, burning pain

D. Isotonic muscle contraction

E. Sharp "fast" pain

F. Dynamic muscle stretch This is an INCORRECT match.

Question 1
A
B
C
D
E

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure

B. Internal arcuate fibers

C. Spinal trigeminal tract

D. Dorsal columns

E. Medial lemniscus

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure This answer is CORRECT!

B. Internal arcuate fibers

C. Spinal trigeminal tract

D. Dorsal columns

E. Medial lemniscus

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure

B. Internal arcuate fibers This answer is INCORRECT.

C. Spinal trigeminal tract

D. Dorsal columns

E. Medial lemniscus

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure

B. Internal arcuate fibers

C. Spinal trigeminal tract This answer is INCORRECT.

D. Dorsal columns

E. Medial lemniscus

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure

B. Internal arcuate fibers

C. Spinal trigeminal tract

D. Dorsal columns This answer is INCORRECT.

E. Medial lemniscus

The neospinothalamic tract crosses the midline in which of the following structures?

A. Anterior white commissure

B. Internal arcuate fibers

C. Spinal trigeminal tract

D. Dorsal columns

E. Medial lemniscus This answer is INCORRECT.

Question 2
A
B
C
D
E