Somatosensory ALS Flashcards
What is the first order neuron of the ALS pathway?
Unencapsulated free neuron endings which are sensory have cell bodies in DRG, enter the spinal cord via the lateral division of the dorsal root, synapsing in dorsal horn of the spinal cord
What is the second order neuron of the ALS pathway?
Dorsal horn neurons project across the anterior white commissure to form the contralateral ascending ALS pathway spanning anterior / lateral funiculi. This is called the lateral spinothalamic tract.
What special things do the second order neurons of the ALS pathway do and where do they ultimately synapse on?
Give off collaterals to reticular formation of the brainstem involved in arousal and emotions. Synapse in neurons of the VPL thalamus
What is the third order neuron of the ALS pathway?
Cell body in VPL thalamus projects axons to the ipsilateral somatosensory cortex in Brodmann’s area 3,1,2 postcentral gyrus. (Different axons as compared to DC-ML pathway). Also to insular + ACC.
What is the function of the ALS pathway?
Pathway of organ homeostasis - detects innocuous and damaging stimuli from all tissues but the brain.
What type of innocuous stimuli is the ALS system responsible for? What receptors do this?
- Temperature (warm / cold) - Thermoreceptors. Warm = 30-45 C. Turn off above 45 C when nociceptors are active. Cold = 10-35 C. Turn off below 10 causing numbness.
- Crude touch (diffuse or sensual) but not fine discriminatory
What type of damaging stimuli is the ALS system responsible for?
Mechanical (puncture / pressure)
Temperature (scalding / freezing)
Chemical (cell rupture, inflammation)
What are the two neural fiber types of the ALS pathway and what type of information do they convey?
- A-delta fibers: myelinated but very small diameter, convey initial information above tissue damage for sharp / fast pain. Important for reflexes. Same type of fibers are use for mechanoreceptors in muscle which detect too much stretch.
- C fibers - very slow, cause burning, aching sensations that are prolonged. Also transmit some crude touch like itch or sensual touch.
What are the types of nociceptors and what fiber types transmit it?
- Polymodal - 70% - nonselective (chemi, thermal, mech) using C fibers
- Mechanical - Detect strong pressure which may cause tissue damage - A-delta
- Thermal - greater than 45 C or extreme cold, A-delta.
- Silent - Detect histamine or prostaglandins in inflammatory response - C fibers.
2-4 are all UNIMODAL
What are hyperalgesia and allodynia and what causes them?
Tissue which has sustained damage is hypersensitive to stimulation. Mediated by silent nociceptors responding to prostaglandins
Hyperalgesia - increased pain around site of initial damage
Allodynia - easy pain in damage region (i.e. pain from sunburn touch)
What is the function of the posterolateral fasciculus?
Lissauer’s tract - 1st order axons send contributions up and down this tract before synapsing in dorsal horn. Helps integrate pain / temp information across several spinal levels for CNS to locate the correct dermatome
What is contained within the substantia gelatinosa?
Inhibitory interneurons which dampen the incoming signals from the ALS second order neuron
What explains why deep rubbing of the tissue reduces pain?
The gate theory of pain -> Abeta fibers from the DC-ML pathway will activate the inhibitory interneuron in the substantia gelatinosa to dampen the signals to the ALS secondary neuron
What explains referred pain?
Visceral afferents enter the spinal cord in the same place as the cutaneous nociceptive input, causing “neural confusion”
What is the strange thing about the spinal levels of the ALS pathway?
The ALS axons cross the anterior white commissure obliquely and take one spinal level to go up. Thus, the highest spinal level you would see axons for in the spinothalamic tract of T3 would be T4.
