SS Tracts: Tactile, Discrimination, and Position Sense (Dennis) Flashcards
What is the function of the somatosensory system and the associated pathways?
- function: transmits and analyzes touch or tactile info from external and internal locations on the body and head (i.e. discriminative touch, flutter-vibration, proprioception, crude touch, thermal sensation, nociception)
- pathways: posterior column medial lemniscal pathway, trigeminothalamic pathway, spinocerebellar pathway, anterolateral system
- ascending tract involved w/ perception of mechanical stimuli
- discriminative touch (two-point discrimination); pressure, stretch, and vibrations of skin; and proprioception
- has fast conduction velocities and precise somatotopic organization
posterior column-medial lemniscal system (PCMLS)
- area of skin innervated by somatic afferent fibers
- small fields have high receptor density while larger fields have lower receptor density
receptive field
What are the 1st order neurons within the PCMLS, where are they located, and what are their attributes?
- primary afferent fibers that are sensory axons located within the DRG
- large-diameter fibers that relay discriminative touch, flutter-vibration, and proprioception
- enter the SC via medial division of the posterior root (gray horn) and then branch
- largest set of branches ascend cranially and form posterior columns (white matter): fasiculus gracilis (below T6) and fasiculus cuneatus (above T6)
- some branches terminate on 2nd order neurons in, at, above, and below level of entry (reflexes)
What is the organization of FG and FC?
fibers are organized topographically, sacral fibers are medial and fibers from progressive rostral levels are added laterally
What would a lesion in the FG or FC area of the SC result in?
(fibers in FG and FC run ipsilateral, meaning they do not crossover)
- lesions would result in *ipsilateral reduction or loss of discriminative, positional, vibratory, tactile sensations at and below the segmental level of injury*
- sensory ataxia: loss of muscle stretch (tendon) reflexes, and proprioceptive losses from extremities due to lack of sensory input
- also possible is a wide-based stance and placing of feet to the floor w/ force, in effort to create missing proprioceptive input
Where are the 2nd order neurons located in the PCMLS and what is their function?
- 2nd order neurons are located in the nucleus gracilis and nucleus cuneatus in the posterior medulla
- receive input from 1st order neurons (primary afferents) from ipsilateral DRG
- each nucleus receives inputs from its corresponding fasciculus, maintaining somatotopic organization of projections
Where are the 3rd order neurons located in the PCMLS and how are these fibers projected elsewhere?
- ipsilateral NC and NG combine and loop contralaterally in the posterior medulla posterior to the pyramids, creating the internal arcuate fibers
- these fibers cross the midline (sensory decussation) and ascend as the nerve bundles of right and left medial lemniscus (ML)
- ML terminates in the ventral posterolateral nucleus (VPL) in the thalamus, where the 3rd order neurons are housed that are projected into the cortex
How does the ML fibers rotate through the brainsteam and what is its orientation throughout the rotation?
- in the medulla, the ML is oriented w/ as if the fibers are “standing up” on the pyramids w/ the UE fibers posterior and LE fibers anterior
- in rostral medulla and caudal pons, the ML flattens horizontally so that the UE fibers lie medially and LE fibers lie laterally
- in the midbrain, the ML turns vertically and shifts laterally so that the UE fibers lie anterior and LE fibers lie posterior
*somatotopic orientation shifts as the fiber tract rotates*
(figure image captions are incorrect)
What is the role of VPL in the PCMLS?
- the ventral posterolateral (VPL) nuclei are located in the caudal thalamus and contain 3rd order neurons of the PCMLS
- somatotopic organization maintained in VPL
- these neurons traverse posterior limb (1/3) of internal capsule, eventually moving in the primary (SI) and secondary (SII) somatosensory cortices
What would a lesion in the VPL or IC portion of the PCMLS result in?
- these regions are supplied by thalamogeniculate branches of posterior cerebral artery
- lesion or infarction could result in loss of all tactile sensation over the contralateral body (VPL) and head/face (VPM)
Where do fibers travel after the internal capulse in the PCMLS?
- fibers move to corona radiata and then the primary (SI) and secondary (II) somatosensory cortices
- primary (SI) cortex: comprises postcentral gyrus and posterior paracentral gyrus (bordered by central sulcus anteriorly and postcentral sulcus posteriorly); subdivided anterior to posterior into Brodmann areas 3a, 3b, 1, and 2
- secondary (SII) cortex: located in the inner face of the upper bank of lateral sulcus and lateral to SI, receives inputs from ipsilateral SI cortex and ventral posterior inferior nucleus (VPI) of the thalamus
- parietal cortical regions also receive tactile inputs apart of the PCMLS
What would a lesion in the primary SS cortex result in?
primary (SI) cortex lesion
- blood supply to SI cortical areas is provided by anterior and middle cerebral arteries
- MCA lesions produce tactile loss over contralateral upper body and face
- ACA lesions affect the contralateral lower limb
What would a lesion in the parietal cortical region result in?
parietal cortical region lesion
- lesions in this area can produce agnosia
- contralateral body regions lost from body map
- limb is not recognized as part of the patient’s own body
- sensation is not radically altered
- “foot to tongue” pattern along medial to lateral axis of SI and SII
- regions w/ increased receptor density (hands/lips), have large amount of dedicated cortical tissue
- regions w/ decreased receptor density (back) have small cortical representations
somatosensory homunculus
What is the general pathway of the PCMLS?
- axons enter spinal cord from spinal ganglia (1st order neurons) and pass directly to ipsilateral posterior column; caudal fibers (up to and including T6) enter fasiculus gracilis (medial) and rostral fibers (above T6) enter fasiculus cuneatus to ascend
- these fibers terminate in nucleus gracilis and nucleus cuneatus (2nd order neurons); from these nuclei, axons of secondary neurons cross the midline as internal arcuate fibers and form the medial lemniscus
- in the rostral medulla, the fibers travel as the medial lemniscus adjacent to the midline (“standing” on the pyramids)
- in the caudal pons, the medial lemniscus flattens horizontally
- as the medial lemniscus continues to ascend through the rostral pons and midbrain, it moves laterally and becomes more vertical
- the medial lemniscus terminates in the VPL of the thalamus; from the thalamus, fibers project through the internal capsule and corona radiata to terminate in the primary somatosensory cortex (postcentral gyrus)
What are the divisions of the trigeminothalamic pathway and what somatosensory info does it receive?
- afferent fibers are primarily from trigeminal ganglion (ophthalmic, maxillary, and mandibular nerves), the ganglion are where the 1st order neurons are located
- V1-V3 delineate facial dermatomes that are sharply defined (in contrast to spinal dermatomes)
- receives info regarding pain, temperature, and nondiscriminative touch
- also receives discriminative touch and proprioceptive input
*additional afferents arise from geniculate ganglion (CN VII) and superior ganglion (CN IX, CN X), however trigeminal ganglion is the majority*
What are the 4 trigeminal nuclei that form an elongated, continuous column in the brainstem?
(these nuclei are considered central pathways of CN V)
- mesencephalic nucleus (midbrain): proprioceptive afferents from TMJ, masticatory, and extraocular muscles
- principal sensory nucleus (midpons): discriminative touch and pressure
- trigeminal motor nucleus (midpons): not discussed within this lecture as it is motor and not sensory
- spinal nucleus (obex > C2/C3): pain, temp, nondiscriminative touch
Where are the 2nd order neurons located in the trigeminothalamic pathway?
- 2nd order neurons are located in the principal sensory nucleus
- primary afferents (1st order neurons) relay discriminative tacticle and proprioceptive sensations to the 2nd order neurons in the PSN
- V1 fibers are anterior, V2 fibers are in between, and V3 fibers are posterior (face represented upside down)
- 2nd order fibers project via anterior and posterior trigeminothalamic tracts: anterior is contralateral and posterior is ipsilateral (posterior receives input from V3 only)
Where are the 3rd order neurons located in the trigeminothalamic pathway?
- 2nd order fibers project via anterior and posterior trigeminothalamic tracts: anterior is contralateral and posterior is ipsilateral (posterior receives input from V3 only)
- these fibers travel to ventral posteromedial (VPM) nucleus where the 3rd order neurons are located (somatotopic arrangement is maintained; oral cavity fibers = medial, external face fibers = lateral)
- 3rd order neurons from VPM project via posterior limb of internal capsule to the primary (SI) somatosensory cortex
- nucleus comprised of pseudounipolar neurons located in the mesopontine junction
- only ‘nucleus’ within the CNS, can be considered as displaced trigeminal ganglion cells
- conveys unconscious proprioceptive and pressure information from muscles of the oral region and extraocular muscles
- prevents biting down hard enough to lose a tooth or injure the tongue
- mechanoreceptive nerves in the periodontal ligament sense tooth movement and project to this nucleus; afferent fibers from muscle spindles, the sensory organs of skeletal muscle, are stimulated by the stretch of hard contraction of jaw muscles
- this nucleus projects via the mesencephalic tract to the principal sensory nucleus and spinal nucleus
- some axons terminate in trigeminal motor nucleus which forms circuit for jaw-jerk reflex
mesencephalic nucleus
What is the pathway that leads to the jaw-jerk reflex?
What does the jaw-jerk reflex test?
What may enchance amplitude jaw-jerk reflex?
- pathway: afferent limb: neuron within cell body in mesencephalic nucleus projects to dendritic process within muscle spindle of masseter M. > these fibers travel to an axon that synapses in the trigeminal motor nucleus > efferent limb: activation of trigeminal motor neuron
- test: stretching masseter (downward tap on chin) causes it to contract bilaterally; assesses function of trigeminal brainstem nuclei and V3
- upper motor neuron may enchance amplitude after it is damaged
Vascular territories for:
- mesencephalic nucleus in the pons:
- trigeminal motor nucleus in the pons:
- principal sensory nucleus in the pons:
- mesencephalic nucleus and tract in the midbrain:
Vascular territories for:
- mesencephalic nucleus in the pons: mostly by the long circumferential branches of basilar A. and branches of superior cerebellar A., small portion by the anterior inferior cerebellar A.
- trigeminal motor nucleus in the pons: long circumferential branches of basilar A. and the anterior inferior cerebellar A.
- principal sensory nucleus in the pons: mostly by the long circumferential branches of basilar A. and the anterior inferior cerebellar A., small portion by the branches of superior cerebellar A.
- mesencephalic nucleus and tract in the midbrain: long circumferential branches of basilar A. and branches of superior cerebellar A.
In the below figure, name the pathways and determine where the tactile and proprioceptive info is coming from in the body:
- green is principal sensory nucleus trigeminothalamic pathway from head and neck
- red is PCMLS from upper extremity
- blue is PCMLS from lower extremity
