Somatosensory Tracts I

Question 1

What stimuli activate the PCML pathway?

Answer 1

Discriminative touch, vibration, proprioception.

Question 2

Small receptive fields have ________ receptor density.

Answer 2

High receptor density

Question 3

Large receptive fields have __________ receptor density

Answer 3

Low receptor density

Question 4

3 components of a primary afferent fiber

Answer 4

Pseudopolar cell body
Peripheral process extending from the DRG.
Central process extending from DRG to the CNS.

Question 5

What structure do primary efferent fibers enter the spinal cord in?

Answer 5

Medial division of posterior root

Question 6

What do the fibers ascend within?

Answer 6

Fasciculus gracilis or fasciculus cuneatus (posterior columns).

Question 7

Fibers above T6 travel in:

Answer 7

Fasciculus cuneatus

Question 8

Fibers below T6 travel in:

Answer 8

Fasciculus gracilis

Question 9

What is the result of Sc lesions of the posterior columns?

Answer 9

Ipsilateral loss of tactile sensations (discriminative, positional, vibratory) AT and BELOW segmental level of injury.

Question 10

Where are the neurons from that synapse in the nucleus gracilus and nucleus cuneatus?

Answer 10

They are 1st order neurons (primary afferents) from ipsilateral DRG (have not decussated yet).

Question 11

Where do the 2nd order neurons from the nuclei go?

Answer 11

They decussate (via arcuate fibers) and ascend in the medial leminiscus (ML) on the contralateral side.

Question 12

What happens to the medial leminscus (ML) in the pons?

Answer 12

It laterally rotates, where the UE fibers lie medial and LE lie laterally.

Question 13

Where does the ML terminate?

Answer 13

VPL in the thalamus

Question 14

Damage at brainstem levels lead to:

Answer 14

Deficits in tactile sensations (discriminative, positional, vibratory) over the contralateral side of the body (because the fibers have decussated).

Question 15

Blood source to the VPL and VPM:

What happens when it is compromised?

Answer 15

Thalamogeniculate branches from posterior cerebral a.

Same deficits as damage to brainstem (sensory loss on CL side).

Question 16

2 types of neurons in the VPL

Answer 16

1. 3rd order neurons from PCMLS which will travel to somatosensory cortex.
2. Local circuit interneurons (inhibitory neurons that influence the firing rate of 3rd order neurons).

Question 17

Trigeminal nuclei:
Main sensory nucleus (msT)
Trigeminal motor nucleus (mT)
Spinal nucleus
Mesencephalis nucleus (mes)

Answer 17

Trigeminal nuclei:
Main sensory nucleus (msT) - touch and pressure.
Trigeminal motor nucleus (mT) - ms. of mastication.
Spinal nucleus - pain and temp.
Mesencephalis nucleus (mes) - proprioception from TMJ and ms. of mastication.

Question 18

Dorsomedial division of the main sensory nucleus of Trigeminal nucleus function:
Where do they project?
Via which tract?

Answer 18

Afferent (sensory) input from oral cavity.
Project ipsilaterally.
Posterior trigeminothalamic tract.

Question 19

Ventrolateral division of the main sensory nucleus of Trigeminal nucleus function:

Answer 19

Afferents (sensory) from V1, V2, V3.
Projects contralaterally.
Anterior trigeminothalamic tract.

Question 20

Explain how sensory info from the face gets processed via trigeminal nuclei

Answer 20

1. Sensory fibers (dorso/ventrolateral fibers) synapse in the VPM in the thalamus.
2. 3rd order axons from VPM project via the posterior limb of the internal capsule to the primary somatosensory cortex.

Question 21

Function of mesencephalic nucleus

Answer 21

Conveys unconscious proprioceptive and pressure info from muscles of the oral region.

Question 22

Jaw-jerk reflex (3)

Answer 22

1. Strecth of masseter (from downward tap on chin), causes contraction BL.
2. Afferent limb is mesencephalic trigeminal neuron that innervates muscle.
3. Efferent limb is from central process that synapses on trigeminal motor neuron.

Question 23

Mesencephalic nucleus via spinal nucleus (Pars interpolaris) conveys:

Answer 23

Proprioceptive input from jaw related to chewing/jaw placement.

Question 24

Function of main sensory nucleus

Answer 24

Moderates amount of secondary neurons that project to the cerebellum.

Question 25

Chewing utilizes which 2 nuclei?

Answer 25

Mesencephalic nucleus via spinal nucleus (Pars interpolaris)

Main sensory nucleus