Somatosensory tracts 1: Proprioception and tactile pathways

Question 1

What types of somatosensation travels in the Posterior Column-Medial Lemniscal system (PCMLS)

Answer 1

Two-point discrimination: abillity to discriminate between two stimuli simultaneously

Proprioception: conscious awareness of body position and limb movement in space

flutter vibration

Question 2

Receptor density gradient

Answer 2

Digits and perioral regions have high densities of tactile receptors

Other regions like the back has low density

Question 3

Receptive field

Answer 3

area of the skin innervated by a somatic afferent fiber

small receptive fields have high receptor density

large receptive fields have low receptor density

Question 4

In what position does discrimitive touch, flutter vibration and proprioception enter the spinal cord

Answer 4

the medial division and aspect of the posterior root

Question 5

Where does first order neurons synapse in the PCML

Answer 5

if it involves reflexes the 1st order neurons synapse on the second order neurons at, above, or below the level of entry.

if it doesent involve reflexes the 1st order neuron travels in the ipsilateral fasciculus gracilis or fasciculus cuneatus, and will synapse on their nuclei (these contain the cell bodies of the 2nd order neuron)

Question 6

What travels in the fasciculus gracilis

Answer 6

ipsilateral discrimitive touch, vibration, and proprioception from the spinal cord of the sacrum to T6 (contains info from the legs and feet)

Question 7

What travels in the fasciculus cuneatus

Answer 7

ipsilateral disrimitive touch, vibration, and proprioception

from the spinal cord of the T6 level and up, will contain information from the arms

Question 8

what is the topographical organization of the Fasciculi Gracile and Cuneatus

Answer 8

Sacral levels are positioned medially and fibers progressively more rostral levels are added laterally

(upper cervical is the farthest lateral region)
(regions that are just below T6 are the farthest laterally in the fasciculus Gracile

Question 9

What happens if their are lesions to the Fasciculi gracile and Cuneatus

Answer 9

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

Question 10

What is sensory ataxia

Answer 10

loss of muscle stretch reflexes and proprioceptive losses from the extremities due to lack of sensory input

Patient may also have a wide based stance and may place the feet to the floor with force

(Tabes dorsalis)

Question 11

What is the arterial blood supply to the Posterior column

Answer 11

Posterior Spinal A, therefore lesions here would cause issues to projections of ipsilateral Discriminative touch, proprioception and vibration

Question 12

what is the Pathway of the 2nd order neuron of the PCMLS

Answer 12

once the primary afferent synapses at the cuneatus and gracile nucleus or the bodies of the 2nd order neurons

• THe second order neurons will decussate as the the internal arcuate fibers to the contralateral Medial Leminiscus
• From their the Discrimitive touch, proprioception, and vibration will travel up the contralateral medial leminiscus until it reaches the Ventral Posterolateal nucleus of the thalamus

as the info travels up the the cuneate info will stay more posterior and more medial

Question 13

What somatototopic arrangement is carried in the Ventral Posterolateral nucleus of the thalamus

Answer 13

foot is the most lateral and the neck is the most medial in the Ventral posterolateral nucleus

Question 14

what is the pathway of the 3rd order neurons of the PCMLS

Answer 14

the 2nd order neurons will synapse on the Ventral Posterolateral nucleus (contain the third order neuron cell bodies) and then the 3rd order neuron will travel in the posterior limb of the internal capsule to the primary and secondary somatosensory cortices

Question 15

What is the blood supply of the Vental Posterolateral and the Ventral Posteromedial nuclei of the thalmus

Answer 15

THalamogeniculate arteries which is a branch off the Posterior Cerebral Artery

lesions here would cause contralateral deficits of all tactile sensation over the body and head

Question 16

What is the blood supply to the primary somatosensory cortex

Answer 16

the medial portion (Foot, LE, hip) by the longitudinal fissure is the Anterior cerebral artery

the lateral and inferior portion (face, hand, UE, chest) is the middle cerebral artery

Question 17

What is the broadmans area for the Postcentral Gyrus

Answer 17

3a, 3b,1,2

Lateral to medial (3a,3b are in the central sulcus)

Question 18

Secondary somatosensory cortex

Answer 18

SII

innerface of the upper bank of the lateral sulcus

Inputs from ipsilateal SI cortex and ventral posterior inferior nucleus of the thalamus

Question 19

Parietal cortical regions also receive tactile inputs therefore lesions here can lead to what

Answer 19

lesions in parietal association area can produce Agnosia

• Contralateral body regions lost from the map
• limb is not recognized as part of the patients own body
• sensation is not radically altered

Question 20

what are the primary afferents of the face and their associate nuclei

Answer 20

Primarily trigeminal: ophthalmic, maxillary and mandibular nerve
(does pain, temp, nondiscrimitive touch, discrimitive touch)

also have genticulare ganglion: CN VII
superior ganglion: CN IX, CNX

Proprioceptive info arise via the mesencephalic nucleus

Question 21

What modalities gives rise from the Mesencephalic nucleus

Answer 21

proprioceptive afferents from TMJ, masticatory, and the extraocular muscles

Question 22

What modalities gives rise from the Principal (chief) nucleus

Answer 22

Discriminative touch and pressure

Question 23

what modalities gives rise from the nucleus of the spinal tract of the trigeminal (spinal nucleus

Answer 23

pain, temperature, and nondiscriminative touch

Question 24

what is the pathway of the neurons in the Principal sensory nucleus

Answer 24

the primary afferents relaying discriminative tactile and vibration, synapse on the principle sensory nucleus

information relaying from the oral cavity and located dorsomedial on the nucleus will stay ipsilateral and travel via the Posterior trigeminothalamic tract to the ventral posteromedial nucleus in the pons

information relaying from V1, V2, V3 (located on the ventrolateral division of the nucleus) will crossover at the pons of the brainstem and will travel up the contralateral anterior trigeminothalamic tract

in the Ventral posteromedial nucleus of the thalamus, the 2nd order neurons will synapse and the third order neurons will carry the information to the lateral aspect of the primary sensory cortex via the posterior limb of the internal capsule

Question 25

what is the blood supply to the trigeminal motor nucleus, principle sensory nucleus, and the mesencephalic tract and nucleus

Answer 25

the superior cerebellar artery:

• Trigeminal motor nucleus
• Principle sensory nucleus
• Mesencephalic nucleus

the Long circumferential branches of the basilar artery and branches of the anterior inferior cerebellar artery:

• principlal sensory nucleus
• trigeminal motor nucleus
• anterolateral systerm (in that area)

Question 26

Somatotopic arrangement of the Ventral Posteromedial nucleus of the thalamus

Answer 26

Oral cavity is medial

External face is lateral

Question 27

WHat information synapses on the Mesencephalic nucleus

Answer 27

conveys unconscious proprioceptive and pressure information from the muscles of the jaw teeth and extraocular muscles

then this information will project to the principal and spinal nucleus

some information will go to the trigeminal motor nucleus to form the jaw jerk reflex

Question 28

Jaw jerk reflex

Answer 28

Stretching the masseter (downard tap on the chin will cause it to contract bilaterally

afferent limb: mesencephaplic trigeminal neuron whese peripheral process innervates the masseter muscle spindle and whose central process synapses on a trigeminal motor neuron (effrnt limb)

reflex is enhanced after UMN damage