Somatosensory Tracts Part 1 (dennis)

Question 1

Which pathways do proprioception and Tactile sensation follow?

Answer 1

Posterior column-medial lemniscal pathway

Trigeminothalmic pathway

Question 2

PCMLS (posterior column-medial lemniscal system) perceives what type of stimuli?
Features of this pathway?

Answer 2

-Perception of mechanical stimulus (features of stuff), vibration and proprioception)
-fast conduction velocity and precise somatotopic organization
2 point discrimination

Question 3

What is two point discrimination?

Answer 3

Ability to discriminate between two stimuli when presented simultaneously
Related to density of peripheral nerve endings

Question 4

What is a receptor density gradient

Answer 4

Varying densities of peripheral receptors in various parts of the body
Digits and perioral region = more density of tactile receptors
Back = less density of tactile receptors

Question 5

What is a receptive field?

Answer 5

Area of skin innervated by somatic afferents.

Receptor fields are proportional to receptor density (high RF = high RD)

Question 6

DCML fibers relay what type of info?

They enter the spinal cord via?

Answer 6

Discriminative touch, flutter-vibration and proprioception

Medial divison of the posterior root, then branch out

Question 7

What forms the posterior columns?

Answer 7

Largest set of primary afferents that ascends cranially.
Fasciculus gracilis - sacral to T6
Fasciculus cuneatus - T6 upwards

Question 8

Where could primary afferent fibers terminate?

Answer 8

at the gracile or cuneate nuclei with 2nd order neurons in, above or below level of entry

Question 9

How are the fasciculi on the posterior columns organized?

Answer 9

Sacral fibers - medially

Rostral fibers - laterally

Question 10

Spinal cord lesions - DCML pathway:

Answer 10

Loss of discrimination, position or vibratory sensations at and below the lesion. Same side

Question 11

Sensory ataxia:

Answer 11

Loss of the tendon relfexes, no proprioception sense in the extremities due to lack of sensory input
Patients also have wide-based stance and force feet down the floor to try and get proprioception sense

Question 12

What nuclei are found on the posterior column?

Answer 12

Gracile nucleus and cuneate nucleus (found in posterior medulla)

Question 13

Gracile and cuneate nuclei contain which type of neurons?

Where do these neurons receive input from?

Answer 13

2nd order neurons
Primary afferents (1st order neurons) coming from the Dorsal root ganglion of the same side

Question 14

Where do the axons of the 2nd order neurons go?

Answer 14

Fibers are sent on the contralateral side (eventually to contralateral thalamus) via the Internal arcuate fibers. The IAF cross at the sensory decussation and form the Medial lemniscus, which ascends up to the brain

Question 15

What happens to the medial lemniscus topographically?

Answer 15

Lower medulla level - UL axons are dorsal and LL axons are ventral
Lower pons level - UL axons are medial and LL axons are lateral

Question 16

Where do the axons of the Medial lemniscus terminate?

Answer 16

Ventral posterolateral nucleus (VPL) of the thalamus

Question 17

Ventral posterior nuclei

Answer 17

2 nuclei: the lateral (VPL) and the medial (VPM)

Blood supply: Thalamogeniculate branches of the posterior cerebral

Question 18

*Lesions in the Ventral posterior nuclei results in …

Answer 18

Loss of all tactile sensation on contralateral body and head

Question 19

What happens at the VPL in the PCML pathway?

Answer 19

Fibers synapse on 3rd order neurons > travel to the inferior limb of the internal capsule > synapse to the primary and secondary somatosensory cortex on respective homunculus area

Question 20

Primary somatosensory cortex
Consists of?
Blood supply?

Answer 20

Consists of the postcentral gyrus and posterior paracentral gyrus
anterior and middle cerebral arteries

Question 21

*MCA lesions:

ACA lesions:

Answer 21

Tactile loss on contralateral upper body and face

Contralateral lower limb

Question 22

Label the brodmann areas of the primary somatosensory cortex

Answer 22

ok

Question 23

Homunculus and cortex relationship

Answer 23

More receptor density (hand/lips) = larger amount of cortical tissue
Less receptor density (back) = smaller cortical tissue

Question 24

Secondary somatosensory cortex:
Location:
Inputs:

Answer 24

Located on inner upper part of the lateral sulcus

Inputs come from the ipsilateral SI cortex and Ventral posterior inferior nucleus of the thalamus (VPI)

Question 25

*What other cortical region receive tactile inputs?

What happens when this is lesioned?

Answer 25

• Parietal cortical region

- Agnosia. Contralateral region lost. Limb is dissociated (not recognized as part of body).

Question 26

Trigeminal nerve:

Role in sensory pathways?

Answer 26

-Primary afferents arise from the trigeminal ganglion (where cutaneous nerves of the CN V synapse). Afferents also arise from geniculate (VII) and superior ganglion (IX, X)

Question 27

Trigeminal pathway conveys what type of sensory info

Answer 27

Discriminative, two point localization and vibratory sense from face

Question 28

Trigeminal nuclei and what types of info do they receive?

Answer 28

Mesencephalic nucleus: proprioception from TMJ, mastication and EOMs
Principal/chief sensory nucleus: discriminative touch and pressure
Spinal nucleus: pain, temp and nondiscriminative touch

Question 29

2nd order neurons on the Dorsomedial part of the principal sensory nucleus receive info from ….
Ventrolateral part …

Answer 29

• primary afferents from the oral cavity (via the posterior trigeminothalamic tract)
• V1, V2, V3 (via the anterior trigeminothalamic tract)

Question 30

Where do the 2nd order neurons travel?

Answer 30

VPM (Ventral posteromedial nucleus of the thalamus) where they synapse to the 3rd order neurons

Question 31

3rd order neurons go to …

Answer 31

Also called thalamocortical neurons. Travel to the Posterior limb of the internal capsule and terminate on the primary and secondary somatosensory cortex

Question 32

Trigeminal motor nucleus

Answer 32

Some axons on this pathway terminate here and cause the jaw jerk reflex

Question 33

What is the jaw jerk reflex?

Answer 33

Stretching the masseter causes it to contract bilaterally

Reflex is enhanced after Upper Motor Neuron damage

Question 34

Pathway of the jaw jerk reflex?

Answer 34

Mesencephalic trigeminal neuron has a peripheral process (serves as the afferent limb) that innervates the masseter and a central process that synapses on trigeminal motor neuron (efferent limb)

Question 35

So generally
Head and Neck normal sensation goes via the () pathway
Upper and Lower extremity normal sensation goes via the () pathway

Answer 35

Trigeminal

PCMLS