Tracts of the protopathic sensibility (anterolateral system)

Question 1

I. Basics
1. What is Protopathic sensibility?

Answer 1

Protopathic sensibility includes crude touch, pressure, heat and pain. All these stimuli are quite unpleasant, but are essential for survival.

Question 2

I. Basics
2. What is the mechanism of Protopathic sensibility?

Answer 2

1. Protopathic stimulus in the trunk and limbs goes from receptor -> DRG -> spinal dorsal horn -> spinothalamic tract -> thalamus VPL (and intralaminar and midline nuclei) -> cortex
2. Protopathic stimulus in the head goes from receptor -> trigeminal ganglion -> spinal trigeminal nucleus -> ventral trigeminothalamic tract -> thalamus VPM (and intralaminar and midline nuclei) -> cortex

Question 3

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
1. What is the general mechanism of Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)

Answer 3

• A-delta and C-fibers collect nociceptive signals from nociceptors
• Information enters the spinal dorsal horn and a multisynaptic nociceptive reflex arc is created
• Nociceptive signals will be processed in the spinal dorsal horn and projection neurons will give rise to ascending pathways
• Nociceptive signal will ascend in the lateral and ventral column of the spinal cord white matter
• Tract emerges in the spinal cord and ascends to the thalamus = spinothalamic tract -
  => Crude touch and pressure related information is found anteriorly (anterior
  spinothalamic tract), heat and pain related information is found laterally (lateral spinothalamic tract)

Question 4

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
2. What the tracts involving in Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)

Answer 4

1. Spinothalamic tract (anterolateral system)
2. Spinoreticulothalamic tract
3. Spinohypothalamic connections

Question 5

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
2. What is the pathway of Spinothalamic tract (anterolateral system)?

Answer 5

Receptors: free nerve endings in the skin for pain and temperature sensation

1st neuron: the cell bodies of these nerve endings are located in the dorsal root ganglion

2nd neuron:
- The central processes of the 1st neurons pass through the dorsal root into the white matter of the spinal cord and divide into ascending branches (Lissauer’s tract) and descending, then they terminate on secondary neurons in the posterior horn (including cells of the substantia gelatinosa)
- The axons of 2nd neurons cross in the anterior commissure in the spinal segment and ascend in the anterolateral funiculus on the opposite (contralateral) side as the lateral spinothalamic tract ascends through the medulla blongata, it is accompanied by the anterior spinothalamic tract and spinotectal tract
-> together they form the spinal lemniscus in the midbrain, the spinal lemniscus lies in the tegmentum, lateral
to the medial lemniscus

3rd neuron:
- Many of the fibers of the spinothalamic tract terminate in the VPL nucleus of the thalamus the axons of the 3rd neurons radiate to the primary sensory cortex in the postcentral gyrus

Question 6

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
2A. Describe 2nd neuron of Spinothalamic tract (anterolateral system)?

Answer 6

Receptors: free nerve endings in the skin for pain and temperature sensation

1st neuron: the cell bodies of these nerve endings are located in the dorsal root ganglion

2nd neuron:
- The central processes of the 1st neurons pass through the dorsal root into the white matter of the spinal cord and divide into ascending branches (Lissauer’s tract) and descending, then they terminate on secondary neurons in the posterior horn (including cells of the substantia gelatinosa)
- The axons of 2nd neurons cross in the anterior commissure in the spinal segment and ascend in the anterolateral funiculus on the opposite (contralateral) side as the lateral spinothalamic tract ascends through the medulla blongata, it is accompanied by the anterior spinothalamic tract and spinotectal tract
-> together they form the spinal lemniscus in the midbrain, the spinal lemniscus lies in the tegmentum, lateral
to the medial lemniscus

Question 7

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
2B. Describe 3rd neuron of Spinothalamic tract (anterolateral system)?

Answer 7

3rd neuron:
- Many of the fibers of the spinothalamic tract terminate in the VPL nucleus of the thalamus the axons of the 3rd neurons radiate to the primary sensory cortex in the postcentral gyrus

Question 8

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
3. What is the pathway of Spinoreticulothalamic tract

Answer 8

• Fibers from spinal cord terminate first in the reticular formation before reaching the thalamus.
• In the reticular formation they can form multisynaptic networks.
• Can be divided into the spinoreticular tract and reticulothalamic tract.
• Fibers terminate in the intralaminar and midline nuclei. These are considered as non-specific thalamic nuclei.

Question 9

II. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the limbs and trunk)
4. What is the pathway of Spinohypothalamic connections

Answer 9

• Projection neurons in the spinal dorsal horn directly reach the hypothalamus (spinohypothalamic tract), which is a direct pathway.
• Another possibility is that the information reaches the spinal dorsal horn, then project into the brainstem reticular formation (spinoreticular tract). From the reticular formation there is a projection to the hypothalamus (reticulohypothalamic tract)
• Similarly autonomic information (viscerosensory info) can terminate in the spinal dorsal horn and then ascend to the solitary nucleus (viscerosensory brainstem nucleus). From the solitary nucleus it goes to the hypothalamus

Question 10

III. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the HEAD)

Answer 10

The difference is actually negligible
- Instead of DRG in the trunks/limbs,
we now have the trigeminal
ganglion which similarly
accommodates the 1st order
neurons (pseudounipolar)
- The upward continuation of the
spinal dorsal horn in the brainstem
is the spinal trigeminal nucleus. So
that will be termination site of the
central branch of the pseudounipolar neuron
- From the spinal dorsal horn, the
spinothalamic tract emerges. Now,
in this area the trigeminothalamic
tract (ventral trigeminal lemniscus)
emerges from the spinal trigeminal
nucleus and terminates in the
thalamus (VPM)

Question 11

III. Protopathic sensibility – spinal nerves (sensing heat, pain and crude touch on the HEAD)
2. What happen when noxious stimulus reach the body? (a stimulus that is strong enough that it can threat the body)

Answer 11

Noxious stimulus reaches the body. Spinal and cranial nerves takes this stimulation into the CNS, and ascending pathways will either terminate in the thalamus or hypothalamus.
- It can terminate in intralaminar and midline nuclei (also anterior nuclei). These project to the prefrontal cortex, cingulate
gyrus and the limbic system. This will
determine the motivation, behavior and
the mood
- It also terminate in VPL and VPM which
project to the primary somatosensory cortex (#3,1,2). This area is essential for the localization of the stimulus
- Hypothalamus is considered as the highest center of endocrine and autonomic functions. Noxious information will modulate the cortisol secretion, blood pressure and the heart rate through the hypothalamus