Proprioception and Tactile Pathways

Question 1

What does the somatosensory system do?

Answer 1

Transmits and analyzes touch or tactile information from external and internal locations on the body and head

Question 2

What does the somatosensory system use to transmit information?

Answer 2

Posterior column-medial lemniscal pathway, trigeminothalamic pathway, spinocerebellar pathway, and anterolateral system

Question 3

What does the posterior column-medial lemniscal system pathway do (PCMLS)?

Answer 3

Involved with the perception of mechanical stimuli, including conscious awareness of body position and limb movement in space; includes 2-point discrimination

Question 4

What are the characteristic features of the PCMLS pathway?

Answer 4

Fast conduction velocities and precise somatotopic organization

Question 5

What is a receptive field?

Answer 5

Area of skin innervated by a somatic afferent fibers; small receptive fields have a high receptor density, large receptive fields have low receptor density

Question 6

What are dermatomes associated with clinically? What kind of fibers are these?

Answer 6

Fibers/pathways that convey pain/thermal information; these are primary afferent fibers

Question 7

What do large diameter fibers do?

Answer 7

Relay discriminative touch, flutter-vibration, and proprioception; enter spinal cord via medial division of posterior root and then branch

Question 8

The largest branches in the medial division of the posterior root are what?

Answer 8

Fasciculus gracilis (sacral to T6) and fasciculus cuneatus (T6 and up); ascend cranially and form posterior columns

Question 9

Sacral level fibers in the posterior columns are positioned _________; fibers from progressively more rostral levels are added ___________.

Answer 9

Medially; laterally

Question 10

What do spinal cord lesions in the posterior columns result in?

Answer 10

Ipsilateral reduction or loss of discriminative, positional, and vibratory tactile sensations at and below the segmental level of injury

Question 11

What is sensory ataxia?

Answer 11

Loss of muscle stretch (tendon) reflexes, and proprioceptive losses from the extremities due to lack of sensory input; patient may also have wide-based stance and may place feet to the floor with force, in an effort to create the missing proprioceptive input

Question 12

Where are the gracile nucleus and cuneate nucleus located?

Answer 12

Posterior medulla; contain 2nd order neurons of PCMLS and receive input from 1st order neurons (primary afferents) from ipsilateral dorsal root ganglion (DRG)

Question 13

What do 2nd order neurons within nuclei do?

Answer 13

Send axons to contralateral thalamus

Question 14

What do internal arcuate fibers do?

Answer 14

Loop anteromedially in medulla, cross the midline as sensory decussation, and ascend as medial lemniscus on opposite side; the ML rotates laterally in the pons and UE fibers lie medially and LE fibers laterally

Question 15

Where does the ML terminate?

Answer 15

Ventral posterolateral nucleus of the thalamus (VPL)

Question 16

What do ventral posterolateral and ventral posteromedial nuclei do?

Answer 16

VPL: body

VPM: face

Question 17

What is the blood supply for the ventral posterior nuclei? What do lesions result in?

Answer 17

Thalamogeniculate branches of posterior cerebral artery; lesions result in loss of all tactile sensation over the contralateral body and head

Question 18

What do 3rd order neurons do in the ventral posterior nuclei?

Answer 18

Traverse posterior limb of internal capsule into the primary and secondary somatosensory cortices

Question 19

What does the primary somatosensory cortex comprise?

Answer 19

Postcentral gyrus and posterior paracentral gyrus; bordered by central sulcus anteriorly and postcentral sulcus posteriorly

Question 20

What does the primary somatosensory cortex subdivide into?

Answer 20

Anterior to posterior it is Brodmann areas 3a, 3b, 1, and 2

Question 21

What provides blood to the primary somatosensory cortex? What do lesions result in?

Answer 21

Anterior and middle cerebral arteries; MCA lesions produce tactile loss over the contralateral upper body and face; ACA lesions affect the contralateral lower limb

Question 22

Where is the secondary somatosensory cortex found and what does it do?

Answer 22

Inner face of the upper bank of lateral sulcus; inputs from ipsilateral SI cortex and ventral posterior inferior nucleus (VPI) of the thalamus

Question 23

What do the parietal cortical regions do? What do lesions result in?

Answer 23

Receive tactile inputs; lesions in parietal association area produce agnosia; contralateral body regions lost from body map, limb is not recognized as part of patient’s own body, and sensation is not radically altered

Question 24

Where do primary afferents largely distribute from?

Answer 24

Trigeminal ganglion; opthalmic, maxillary, mandibular

Question 25

What information do the trigeminal ganglion transmit?

Answer 25

Pain, temperature, and nondiscriminative touch**; also discriminative touch

**: additional primary afferents arise from the geniculate ganglion (VII) and superior ganglion (IX, X)

Question 26

What arises from the mesencephalic nucleus?

Answer 26

Primary afferents conveying proprioceptive information (masticatory muscles, extraocular muscles, and periodontal ligament)

Question 27

What does the principal/chief sensory nucleus do?

Answer 27

Discriminative tactile and proprioceptive sensations and pressure

Question 28

What does the spinal nucleus do?

Answer 28

Pain, temperature, nondiscriminative touch

Question 29

What does the dorsomedial division of the primary sensory nucleus do?

Answer 29

Oral cavity; target 2nd order neurons in the principal sensory nucleus

Question 30

What does the ventrolateral division of the primary sensory nucleus do?

Answer 30

V1-3; target 2nd order neurons in the principal sensory nucleus

Question 31

What do 2nd order fibers of the primary sensory nucleus do?

Answer 31

Ventrolateral division project contralaterally via anterior trigeminothalamic tract

Dorsomedial divison project ipsillaterally via posterior trigeminothalamic tract

Question 32

What do 2nd order fibers in the primary sensory nucleus target?

Answer 32

Ventral posteromedial (VPM) nucleus; oral cavity is medial, external face is lateral

Question 33

What do 3rd order neurons from the ventral posteromedial nucleus do within the primary sensory nucleus pathway?

Answer 33

Project via posterior limb of internal capsule to the primary somatosensory cortex

Question 34

What is the mesencephalic nucleus? Where is it derived from?

Answer 34

Comprised of pseudounipolar neurons and can be considered as displaced trigeminal ganglion cells; conveys unconscious proprioceptive and pressure information from muscles of the oral region and extraocular muscles; origin is from neural crest but is unclear

Question 35

Where does the mesencephalic nucleus project to?

Answer 35

Projects to principal sensory nucleus and spinal nucleus; some axons terminate in trigeminal motor nucleus and forms circuit for the jaw-jerk reflex

Question 36

What is the jaw-jerk reflex?

Answer 36

Stretching the masseter (downward tap on chin), causes it to contract bilaterally; amplitude of the reflex is typically minor, but it is enhanced after UMN damage

Question 37

What is the afferent limb in the jaw-jerk reflex?

Answer 37

Mesencephalic trigeminal neuron whose peripheral process innervates a masseter muscle spindle and whose central process synapses on a trigeminal motor neuron (efferent limb)