Proprioception and Tactile Pathways Flashcards
What does the somatosensory system do?
Transmits and analyzes touch or tactile information from external and internal locations on the body and head
What does the somatosensory system use to transmit information?
Posterior column-medial lemniscal pathway, trigeminothalamic pathway, spinocerebellar pathway, and anterolateral system
What does the posterior column-medial lemniscal system pathway do (PCMLS)?
Involved with the perception of mechanical stimuli, including conscious awareness of body position and limb movement in space; includes 2-point discrimination
What are the characteristic features of the PCMLS pathway?
Fast conduction velocities and precise somatotopic organization
What is a receptive field?
Area of skin innervated by a somatic afferent fibers; small receptive fields have a high receptor density, large receptive fields have low receptor density
What are dermatomes associated with clinically? What kind of fibers are these?
Fibers/pathways that convey pain/thermal information; these are primary afferent fibers
What do large diameter fibers do?
Relay discriminative touch, flutter-vibration, and proprioception; enter spinal cord via medial division of posterior root and then branch
The largest branches in the medial division of the posterior root are what?
Fasciculus gracilis (sacral to T6) and fasciculus cuneatus (T6 and up); ascend cranially and form posterior columns
Sacral level fibers in the posterior columns are positioned _________; fibers from progressively more rostral levels are added ___________.
Medially; laterally
What do spinal cord lesions in the posterior columns result in?
Ipsilateral reduction or loss of discriminative, positional, and vibratory tactile sensations at and below the segmental level of injury
What is sensory ataxia?
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
Where are the gracile nucleus and cuneate nucleus located?
Posterior medulla; contain 2nd order neurons of PCMLS and receive input from 1st order neurons (primary afferents) from ipsilateral dorsal root ganglion (DRG)
What do 2nd order neurons within nuclei do?
Send axons to contralateral thalamus
What do internal arcuate fibers do?
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
Where does the ML terminate?
Ventral posterolateral nucleus of the thalamus (VPL)
What do ventral posterolateral and ventral posteromedial nuclei do?
VPL: body
VPM: face
What is the blood supply for the ventral posterior nuclei? What do lesions result in?
Thalamogeniculate branches of posterior cerebral artery; lesions result in loss of all tactile sensation over the contralateral body and head
What do 3rd order neurons do in the ventral posterior nuclei?
Traverse posterior limb of internal capsule into the primary and secondary somatosensory cortices
What does the primary somatosensory cortex comprise?
Postcentral gyrus and posterior paracentral gyrus; bordered by central sulcus anteriorly and postcentral sulcus posteriorly
What does the primary somatosensory cortex subdivide into?
Anterior to posterior it is Brodmann areas 3a, 3b, 1, and 2
What provides blood to the primary somatosensory cortex? What do lesions result in?
Anterior and middle cerebral arteries; MCA lesions produce tactile loss over the contralateral upper body and face; ACA lesions affect the contralateral lower limb
Where is the secondary somatosensory cortex found and what does it do?
Inner face of the upper bank of lateral sulcus; inputs from ipsilateral SI cortex and ventral posterior inferior nucleus (VPI) of the thalamus
What do the parietal cortical regions do? What do lesions result in?
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
Where do primary afferents largely distribute from?
Trigeminal ganglion; opthalmic, maxillary, mandibular
What information do the trigeminal ganglion transmit?
Pain, temperature, and nondiscriminative touch**; also discriminative touch
**: additional primary afferents arise from the geniculate ganglion (VII) and superior ganglion (IX, X)
What arises from the mesencephalic nucleus?
Primary afferents conveying proprioceptive information (masticatory muscles, extraocular muscles, and periodontal ligament)
What does the principal/chief sensory nucleus do?
Discriminative tactile and proprioceptive sensations and pressure
What does the spinal nucleus do?
Pain, temperature, nondiscriminative touch
What does the dorsomedial division of the primary sensory nucleus do?
Oral cavity; target 2nd order neurons in the principal sensory nucleus
What does the ventrolateral division of the primary sensory nucleus do?
V1-3; target 2nd order neurons in the principal sensory nucleus
What do 2nd order fibers of the primary sensory nucleus do?
Ventrolateral division project contralaterally via anterior trigeminothalamic tract
Dorsomedial divison project ipsillaterally via posterior trigeminothalamic tract
What do 2nd order fibers in the primary sensory nucleus target?
Ventral posteromedial (VPM) nucleus; oral cavity is medial, external face is lateral
What do 3rd order neurons from the ventral posteromedial nucleus do within the primary sensory nucleus pathway?
Project via posterior limb of internal capsule to the primary somatosensory cortex
What is the mesencephalic nucleus? Where is it derived from?
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
Where does the mesencephalic nucleus project to?
Projects to principal sensory nucleus and spinal nucleus; some axons terminate in trigeminal motor nucleus and forms circuit for the jaw-jerk reflex
What is the jaw-jerk reflex?
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
What is the afferent limb in the jaw-jerk reflex?
Mesencephalic trigeminal neuron whose peripheral process innervates a masseter muscle spindle and whose central process synapses on a trigeminal motor neuron (efferent limb)
