Tracts of the epicritic sensibility (posterior funiculus/medial lemniscus)

Question 1

I. Basics
1. What does Epicritic and proprioceptive sensibility mean?

Answer 1

Epicritic and proprioceptive sensibility includes light touch, vibration and sensation of joints, muscles, their positions and movements.

Question 2

I. Basics
2. What is the mechanism of Epicritic and proprioceptive sensibility?

Answer 2

1. Epicritic + proprioceptive stimulus in the trunk and limbs goes from receptor
   -> spinal dorsal funiculus -> gracile and cuneate nuclei -> medial lemniscus -> thalamus VPL -> cortex
2. Epicritic + proprioceptive stimulus in the head goes from receptor -> trigeminal
   ganglion -> chief sensory trigeminal nucleus -> dorsal trigeminothalamic tract -> thalamus VPM -> cortex

Question 3

II. Dorsal column medial lemniscus
1. Explain the pathway of dorsal column medial lemniscus

Answer 3

1. if stimulus comes from the trunk or limbs, the 1st order neuron will be found in the dorsal root ganglion
2. The central branch will enter the spinal cord via the dorsal root and ascend in the dorsal funiculus until it reaches the gracile and cuneate nuclei. -> Here we have the 1st synapse
3. From the gracile and cuneate nuclei, the medial lemniscus emerges and ascends into the thalamus. This medial lemniscus is found medially in the brainstem and will terminate on the VPL. Here we have the 2nd synaptic site
4. From here we have a thalamocortical projection terminating in the primary somatosensory cortex (postcentral gyrus). On the medial and superior part of the cortex
   => The fasciculus gracilis carries fibers only from the lower limbs, while the fasciculus cuneatum carries fibers only from the upper limbs -> no cuneate below T3-T6 level

Question 4

III. dorsal trigeminal lemniscus (EPICRITIC)

Answer 4

• In the case of the head, pseudounipolar neurons will be found in the trigeminal ganglion. Central branch will terminate in the principal (pontine) nucleus of trigeminal nerve. This is the 1st synapse
• From the principal trigeminal nucleus the dorsal trigeminal lemniscus emerges and terminate in the VPM. This is the 2nd synapse
• From the thalamus information goes to the cortex (primary somatosensory cortex) to the lateral part

Question 5

IV. Epicritic and mainly proprioceptive information is provided by the ___ (WHICH TRACTS)

Answer 5

spinocerebellar pathways.

Question 6

V. Dorsal spinocerebellar tract (of Flechsig)

Answer 6

• At the T9-L3 segments we can find the dorsal nucleus (Clark-Stilling nucleus), which receives epicritic and proprioceptive input from the periphery.
• From this Clark-Stilling nucleus the dorsal spinocerebellar tract arises and ascends on the ipsilateral side. Via the inferior cerebellar peduncle it terminates on the spinocerebellum (paleocerebellum)

Question 7

V. Ventral spinocerebellar tract (of Gowers)

Answer 7

• Emerges from the L3-S4 spinal segments. The axons will immediately decussate and ascend on the contralateral side until they reach the superior cerebellar peduncle
• From the superior peduncle the fibers will decussate again and terminate within the spinocerebellum (paleocerebellum)

Question 8

VI. Cuneocerebellar tract:

Answer 8

• It is common in the dorsal and ventral spinocerebellar tract that they both convey information from the lower limbs. In contrast to the former 2 pathways, the cuneocerebellar tract takes proprioceptive and epicritic stimuli from the upper half of the body through the inferior cerebellar peduncle to the spinocerebellum (paleocerebellum).
• It is an uncrossed tract – runs ipsilateral

Question 9

VII. What happen if we have non-noxious input?

Answer 9

If we have non-noxious input, spinal and cranial nerves will take this input into the CNS, to the thalamus and cerebellum.
- In the thalamus it will take it mainly to VPL (if source: limbs/trunk) and VPM( if source: head). VPL and VPM projects to primary somatosensory cortex (#3,1,2) to localize the stimuli.
- Goes to cerebellum (paleocerebellum/spinocerebellum). Important for sensory feed-back to organize fine movement.
- Non noxious stimulus related Aβ fibers with thick myelinated sheet will terminate in lamina 3 and 4

Question 10

VIII. Gate control theory

Answer 10

Gate control theory is based on the fact that projection neurons in lamina 3 receive input from both protopathic and epicritic fibers. So by activating the epicritic fibers with gentle touch, we can inhibit the pain stimulus from the Aδ fibers, because these fibers are faster and will activate a inhibitory interneuron. So by this we can reduce the pain stimulus.

Activate Aβ fibers –> inhibit Aδ fibers