ALS Pathway

Question 1

Characteristics shared by all 3 pathways in 1st order neuron

Answer 1

. Cell body located in DRG

. Pseudounipolar neuron w/ peripheral and central process

Question 2

Peripheral receptors assoc. w/ system

Answer 2

. Nociceptors
. Thermoceptors
. Thermal nociceptors

Question 3

Characteristics shared by all 3 pathways in peripheral axonal processes

Answer 3

. From peripheral receptor to DRG

Question 4

Characteristics shared by all 3 pathways in central axonal processes

Answer 4

. From cell body in DRG to dorsal horn of spinal cord
. Fibers enter through lateral dorsal root entry zone to enter Lissauer’s tract
. Collateral branches may ascend or descend in Lissauer’s tract and terminate in spinal gray matter to facilitate spinal reflexes

Question 5

Characteristics shared by all 3 pathways in second order neurons

Answer 5

. Located in either substantial gelatinosa (Lamina II) or lamina V of dorsal form
. Considered to be a tract cell
. Neurons will decussate in ant. White commissure of spinal cord and coalesce ventrolat. In spinal white matter as part of ALS
. Decussation dows not rake place at level at which info enters spinal cord on primary afferent, but occurs 1-2 spinal segments rostral to entry level

Question 6

Spinothalamic tract (STT)

Answer 6

. Concerned w/ sharp, fast pain sensations
. Considered to be direct spinothalamic tract
. Organized so axons from spinal cord are lat. and cervical are med.
. Ascends in ventrolat. Aspect of white matter over length of spinal cord
. Located in inf. Olivary complex in medulla and lat. to med. lemniscus in pons
. Dorsolat. To med. lemniscus in midbrain
. Enter and terminate in ventral posterolat. Tract (VPL) of thalamus to synapse onto 3rd order neuron

Question 7

3rd order neurons of STT

Answer 7

. Start in VPL of thalamus, then project lat. out of thalamus and course somatotopically through post. Limb of internal capsule to terminate in primary somatosensory cortex of the postcentral gyrus (3,1,2)

Question 8

Spinoreticulothalamic tract (SRTT)

Answer 8

. Concerned w/ slow, dull, aching pain and crude touch
. Indirect spinothalamic tract
. Axons ascend up spinal cord in ALS, but leave tract as it purses through brainstem to synapse on neurons in reticular formation (RF)
. 3rd order neurons in RF of brain stem and project axons rostrally to intralaminar nuclei of thalamus
. 4th order thalamus neurons starts in intralmainar nuclei and projects to widespread areas or cortex, not just primary somatosensory cortex

Question 9

Spinotectal tract (SpTT)

Answer 9

. Axons ascend up spinal cord in ALS but leave as it courses midbrain to synapse in sup. Colliculus and PAG
. 3rd order neuron in sup. Colliculus create descending tract to upper cervical spinal cord for reflex control of head, neck , and eye movement to noxious stimuli
. 3rd order neuron in PAG activate polysynaptic, endogenous-opiate descending pathway that inhibits incoming pain afferents before it can activate STT and SRTT

Question 10

Pain inhibition in spinotectal tract (SpTT)

Answer 10

. Primary neurons ending in substantia gelatinosa utilize substance P as a neurotransmitter
. Descending pathway from PAG will ultimately release enkephalins 9which inhibit release of substance P) into the dorsal horn of spinal cord
. Info carried in the SpTT does not reach levels of consciousness
. Descending pathways intended as control (feedback) from sensory cortices and subcortical areas to the 1st and 2nd order neurons