Somatosensory Tracts 1

Question 1

What information is transmitted and analyzed by the somatosensory system?

Answer 1

Touch or tactile information from external and internal locations on the body and head
Discriminative touch, flutter-vibrations and propioception
Crude touch, thermal sensation and nociception

Question 2

The somatosensory system is transmitted via what?

Answer 2

Posterior column-medial lemniscal pathway
Trigeminothalamic pathway
Spinocerebellar pathway
Anterolateral system

Question 3

What is the posterior column-medial lemniscal system (PCMLS) involved with?

Answer 3

The perception of mechanical stimuli
Size, shape, texture discrimination and 3D shape
Conscious awareness of body position and limb movement in space
Fast conduction velocities and precise somatotopic organization

Question 4

What is two point discrimination?

Answer 4

Ability to discriminate between two stimuli simultaneously

Question 5

What is a receptive field?

Answer 5

Area of skin innervated by somatic afferent fibers
Small receptive fields have high receptor density
Large receptive fields have low receptor density

Question 6

Describe the primary afferent fibers of the PCMLS

Answer 6

Sensory axons within cell bodies in the DRG that enter the spinal cord and branch/terminate on neurons in the spinal gray matter
Large diameter fibers relay discriminative touch, flutter vibration and proprioception
Enter the spinal cord via medial division of the posterior root and then branch

Question 7

Describe the branches of the primary afferent fibers in the PCMLS

Answer 7

Some select branches terminate on 2nd order neurons in, at, above and below the level of entry (for reflexes)
Largest set of branches ascends cranially and form the posterior columns (fasciculus gracilis and cuneatus)

Question 8

The fasciculus gracilis carries information from what?

Answer 8

Sacral region to T6

Question 9

The fasciculus cuneatus carries information from what region?

Answer 9

T6 -> superior

Question 10

What are the fasciculi gracilis and cuneatus?

Answer 10

Fibers within the posterior columns that are organized topographically
Sacral level fibers (gracilis) are positioned medially
Fibers from progressively more rostral area levels (cuneatus) are added laterally

Question 11

SC lesions of the posterior column (PCMLS) result in what?

Answer 11

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

Question 12

What is sensory ataxia?

Answer 12

Loss of muscle stretch (tendon) reflexes and proprioceptive losses from the extremities due to lack of sensory input
Pt may also have a wide based stance and may place the feet to the floor with force in an effort to create the missing proprioceptive input
Can be a result from a SC lesion of the posterior column

Question 13

Describe the posterior column nuclei

Answer 13

Gracile nucleus (from fasciculus gracilis) and cuneate nucleus (from fasciculus cuneatus) are in posterior medulla
Contain 2nd order neurons of the PCMLS
Receive input from 1st order neurons (primary afferents) from ipsilateral DRG
Each nucleus receives inputs from its corresponding fasciculus
Somatotopic organization of projections is maintained

Question 14

Describe the medial lemniscus

Answer 14

2nd order neurons within nuclei send axons to thalamus on contralateral side
Internal arcuate fibers loop contralaterally in the medulla -> cross the midline (sensory decussation) -> ascend as medial lemniscus (ML)

Question 15

Where does the medial lemniscus terminate?

Answer 15

In the ventral posterolateral nucleus (VPL) of the thalamus which houses 3rd order neurons that projection to the cortex

Question 16

Describe shifting of the ML

Answer 16

Before reaching the VPL of the thalamus the ML will rotate
1. ML flattens horizontally in the rostral medulla/caudal pons -> UE fibers lie medially and LE fibers lie laterally
2. Turns vertically and shifts laterally in the midbrain -> UE fibers lie anteriorly and LE fibers posteriorly
Somatotopic orientation shifts as the fiber tract rotates

Question 17

Describe the ventral posterior nuclei

Answer 17

Wedge shaped cell group located in caudal thalamus -> ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei
Somatotopic arrangement maintained in VPL
Third order neurons traverse posterior limb of the internal capsule -> primary (SI) and secondary (SII) somatosensory cortices

Question 18

What supples the ventral posterior nuclei?

Answer 18

Thalamogeniculate branches of the posterior cerebral artery

Lesions result in loss of all tactile sensation over the contralateral body and head

Question 19

What is the primary somatosensory (SI) cortex?

Answer 19

Comprises the postcentral gyrus and posterior paracentral gyrus
Bordered by central sulcus (anteriorly) and postcentral sulcus (posteriorly)
Subdivided anterior -> posterior into Brodmann’s areas 3a, 3b, 1 and 2

Question 20

What provides blood supply to the SI cortical areas?

Answer 20

The 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 21

Describe the somatosensory homunculus

Answer 21

Foot to tongue pattern along medial -> lateral axis
Regions with increased receptor density (hand/lips) have large amount of dedicated cortical tissue
Regions with decreased receptor density (back) have small cortical representations

Question 22

What is the secondary somatosensory (SII) cortex?

Answer 22

Inner face of the upper back of lateral sulcus

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

Question 23

Describe parietal cortical regions

Answer 23

Also receive tactile inputs
Lesions in parietal association area can produce agnosia
Contralateral body region is lost from body map
Limb is not recognized as part of the pt’s own body
Sensation is radically altered

Question 24

Describe the trigeminal nerve

Answer 24

Afferent primarily from trigeminal ganglion
Opthalamic, maxillary and mandibular nerve
V1-3 delineate facial dermatomes that are sharply defined (in contrast to spinal dermatomes)
Pain, temperature and non-discriminative touch
Discriminative touch and proprioceptive touch

Question 25

What are the central pathways of the CN V?

Answer 25

Trigeminal nuclei (4 total) form an elongated but continuous column in the brainstem 
Nuclei include mesencephalic nucleus (midbrain), principal sensory nucleus (midpons), trigeminal motor nucleus, spinal nucelus (obex -> C2/3)

Question 26

Describe the mesencephalic nucleus

Answer 26

Conveys unconscious proprioceptive and pressure information from TMJ, masticatory and extraocular muscles (muscles of oral region and eye muscles)
Projects via the mesencephalic tract to the principal sensory nucleus and spinal nucleus
Some axons terminate in trigeminal motor nucleus -> forms circuit for the jaw jerk reflex

Question 27

Describe the principal sensory nucleus

Answer 27

Primary afferents relay discriminative tactile (touch and pressure) and proprioceptive sensations
Primary afferents target 2nd order neurons in principal sensory neurons

Question 28

What is the somatotopic arrangement within the principal sensory nucleus?

Answer 28

V1 (opthalamic) fibers are anterior
V3 (mandibular) fibers are posterior
V2 (maxillary) are in between

Question 29

Describe the path of neurons from the principal senosoy nucleus

Answer 29

Primary afferents target 2nd order neurons in principal sensory nucleus -> 2nd order fibers project via the anterior and posterior trigeminothalamic tracts (anterior = contralateral; posterior = ipsilateral) -> target ventral posteromedial (VPM) nucleus (somatotopic arrangement; oral cavity = medial; external face = lateral) -> 3rd order neurons from VPM project via posterior limb of internal capsule -> primary somatosensory cortex

Question 30

Describe the jaw jerk reflex

Answer 30

Stretching the masseter (downward tap on chin) causes it to contract bilaterally
Assess function of trigeminal brainstem and V3
Amplitude of the reflex is minor but is enhanced after UMN damage

Question 31

What are the afferent and efferent limbs of the jaw jerk reflex?

Answer 31

Afferent limb: neuron with cell body in mesencephalic nucleus -> dendritic process within muscle spindle of masseter muscle -> axon that synapses in the trigeminal motor nucleus -> efferent limb: activation of trigeminal motor neuron

Question 32

What are the trigeminal nuclei mesencephalic, motor and principal sensory supplied by?

Answer 32

Long circumferential branch of basilar artery and branches of anterior inferior cerebellar artery
Mesencephalic: mostly long circumferential branches of basilar A and branches of superior cerebellar artery

Question 33

What deficits are seen with damage to the primary afferents?

Answer 33

Ipsilateral side effected
No change in strength
Decrease/loss of function in reflexes and sensation

Question 34

What are the effects of damage to sensory pathways (brainstem/2nd order neurons)

Answer 34

Contralateral side effected
No change in strength or reflexes
Loss of function in sensation

Question 35

What are the effects of damage to the primary somatosensory cortex

Answer 35

Contralateral side affected
No change in strength or reflexes
Loss of function in sensation

Question 36

Vascular lesions involving the MCA produce what?

Answer 36

Tactile loss over the contra lateral upper body and face

Question 37

Vascular lesions involving the anterior cerebral artery affect what?

Answer 37

The contralateral lower limb

Question 38

Lesions of the primary somatosensory cortex result in what?

Answer 38

Loss of proprioception, position sense, vibratory sense and pain and thermal sensations on the contralateral side of the body

Question 39

A lesion of the medial lemniscus would result in deficits on which side?

Answer 39

If lesion is on the R side pt will experience loss of discriminative touch and proprioception on the L side