Somatosensory Tracts Part 1 (dennis) Flashcards
Which pathways do proprioception and Tactile sensation follow?
Posterior column-medial lemniscal pathway
Trigeminothalmic pathway
PCMLS (posterior column-medial lemniscal system) perceives what type of stimuli?
Features of this pathway?
-Perception of mechanical stimulus (features of stuff), vibration and proprioception)
-fast conduction velocity and precise somatotopic organization
2 point discrimination
What is two point discrimination?
Ability to discriminate between two stimuli when presented simultaneously
Related to density of peripheral nerve endings
What is a receptor density gradient
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
What is a receptive field?
Area of skin innervated by somatic afferents.
Receptor fields are proportional to receptor density (high RF = high RD)
DCML fibers relay what type of info?
They enter the spinal cord via?
Discriminative touch, flutter-vibration and proprioception
Medial divison of the posterior root, then branch out
What forms the posterior columns?
Largest set of primary afferents that ascends cranially.
Fasciculus gracilis - sacral to T6
Fasciculus cuneatus - T6 upwards
Where could primary afferent fibers terminate?
at the gracile or cuneate nuclei with 2nd order neurons in, above or below level of entry
How are the fasciculi on the posterior columns organized?
Sacral fibers - medially
Rostral fibers - laterally
Spinal cord lesions - DCML pathway:
Loss of discrimination, position or vibratory sensations at and below the lesion. Same side
Sensory ataxia:
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
What nuclei are found on the posterior column?
Gracile nucleus and cuneate nucleus (found in posterior medulla)
Gracile and cuneate nuclei contain which type of neurons?
Where do these neurons receive input from?
2nd order neurons Primary afferents (1st order neurons) coming from the Dorsal root ganglion of the same side
Where do the axons of the 2nd order neurons go?
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
What happens to the medial lemniscus topographically?
Lower medulla level - UL axons are dorsal and LL axons are ventral
Lower pons level - UL axons are medial and LL axons are lateral
Where do the axons of the Medial lemniscus terminate?
Ventral posterolateral nucleus (VPL) of the thalamus
Ventral posterior nuclei
2 nuclei: the lateral (VPL) and the medial (VPM)
Blood supply: Thalamogeniculate branches of the posterior cerebral
*Lesions in the Ventral posterior nuclei results in …
Loss of all tactile sensation on contralateral body and head
What happens at the VPL in the PCML pathway?
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
Primary somatosensory cortex
Consists of?
Blood supply?
Consists of the postcentral gyrus and posterior paracentral gyrus
anterior and middle cerebral arteries
*MCA lesions:
ACA lesions:
Tactile loss on contralateral upper body and face
Contralateral lower limb
Label the brodmann areas of the primary somatosensory cortex
ok
Homunculus and cortex relationship
More receptor density (hand/lips) = larger amount of cortical tissue
Less receptor density (back) = smaller cortical tissue
Secondary somatosensory cortex:
Location:
Inputs:
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)
*What other cortical region receive tactile inputs?
What happens when this is lesioned?
- Parietal cortical region
- Agnosia. Contralateral region lost. Limb is dissociated (not recognized as part of body).
Trigeminal nerve:
Role in sensory pathways?
-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)
Trigeminal pathway conveys what type of sensory info
Discriminative, two point localization and vibratory sense from face
Trigeminal nuclei and what types of info do they receive?
Mesencephalic nucleus: proprioception from TMJ, mastication and EOMs
Principal/chief sensory nucleus: discriminative touch and pressure
Spinal nucleus: pain, temp and nondiscriminative touch
2nd order neurons on the Dorsomedial part of the principal sensory nucleus receive info from ….
Ventrolateral part …
- primary afferents from the oral cavity (via the posterior trigeminothalamic tract)
- V1, V2, V3 (via the anterior trigeminothalamic tract)
Where do the 2nd order neurons travel?
VPM (Ventral posteromedial nucleus of the thalamus) where they synapse to the 3rd order neurons
3rd order neurons go to …
Also called thalamocortical neurons. Travel to the Posterior limb of the internal capsule and terminate on the primary and secondary somatosensory cortex
Trigeminal motor nucleus
Some axons on this pathway terminate here and cause the jaw jerk reflex
What is the jaw jerk reflex?
Stretching the masseter causes it to contract bilaterally
Reflex is enhanced after Upper Motor Neuron damage
Pathway of the jaw jerk reflex?
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)
So generally
Head and Neck normal sensation goes via the () pathway
Upper and Lower extremity normal sensation goes via the () pathway
Trigeminal
PCMLS
