Lect 6 Touch and Proprioception

Question 1

Posterior Column/medial lemniscus system

Answer 1

concerned with light touch and pressure (2 point discrimination, vibration)
Proprioception

periphery receptors

Question 2

Touch and pressure receptors

Answer 2

Meissner’s corpuscle

Pascinian Corpuscle

Question 3

Proprioception receptors

Answer 3

GTO’s

Muscle spindles

Question 4

Posterior column medial lemniscus system light touch/pressure pathway

Answer 4

1st order: A-beta fibers enter C1-T6
- rise ipsilaterally through fasciculus cuneatus to nucleus cuneatus (lateral)
2nd order: Nucleus cuneatus body, cross contralaterally and rise to the VPL
3rd order: arise in VPL and project to the primary somatosensory cortex (postcentral gyrus, S-1)

Question 5

Posterior column medial lemniscus system vibration/proprioception

Answer 5

1st order: A-beta fibers enter T7-below
- rise ipsilaterally through fasciculus gracilis to nucleus gracilis (lateral)
2nd order: Nucleus cuneatus body, cross contralaterally and rise to the VPL
3rd order: arise in VPL and project to the primary somatosensory cortex (postcentral gyrus, S-1) —(double check)

Question 6

A-beta fibers

Answer 6

excite inhibitory interneurons in the substantia gelatinosa which inhibit nociceptive afferent (C and Adelta fibers) input. This prevents C and A delta fibers from exciting the spinothalamic tract, which closes pain gate
shown to terminate on lamina V neurons

Question 7

fasciculus gracilis

Answer 7

more medial than cuneatus, posterior column medial lemniscus pathway of proprioception

Question 8

fasciculus cuneatus

Answer 8

lateral, deals with light touch and pressure

primary afferents pass thru it

Question 9

internal arcuate fibers

Answer 9

second order sensory neurons that compose the gracile and cuneate nuclei of the medulla

Question 10

3rd order neurons of posterior-column medial lemniscus system projecting from VPL of thalamus

Answer 10

synapse through the posterior limb of the internal capsule to reach primary somatosensory cortex

Question 11

postcentral gyrus & 3rd order neurons

Answer 11

3rd order neurons project from thalamus to postcentral gyrus
cortical areas of post central gyrus are located in the parietal lobe
fibers from lower extremities course medially
fibers from the upper extremities course laterally

Question 12

Organization of primary somatosensory cortex (S-1)

Answer 12

highly organized
communication between cortical areas within S-I and outside S-I
S-I sends projections to M-I (motor cortex)
S-I also sends projections to S-II. As the initial sensory input becomes synaptically removed from S-I, the response characteristics of the cortical neurons are more and more abstract
-primary somatosensory cortex will tell us that is is a coin but need to put more information together. Higher processing at S-II

Question 13

Overview of Column/medial lemniscus System

Answer 13

Primary afferent fibers (soma in DRG) - enter dorsal horn (no synapse here if it is going directly to the cortex) and ascend in the spional cord as the fasiculus gracilis/cuneatus

• synapse in nucleas gracilis/cuneatus in the caudal medulla
• Axons of secondary neurons course from dorsal to ventral as the internal arcuate fibers and then cross midline to form the contralateral medial lemniscus
• then ascend as in the contralateral medial lemniscus to the thalamus (VPL)
• 3rd order project from thalamus to primary somatosensory cortex (postcentral gyrus, S-I)