2) Proprioception & Tactile Pathways Flashcards
What is the primary pathway that transmits discriminative touch, flutter-vibration & proprioceptive information?
Posterior column – Medial lemniscal pathway
Why type of signal coding registers a cell’s firing rate or intensity?
What is the other type of coding that registers distribution in time/space as well as motion/direction?
Frequency coding
Population coding
PCMLS provides two-point discrimation, what is two-point discrimination related to?
It is related to the density of peripheral nerve endings… more endings = better two-point discrimination
What is responsible for converting mechanical pressure into electrical signal?
The primary afferent neuron turns mechanical signals into action potentials, which is conveyed by the PCMLS
Receptive fields are areeas of skin innvervated by branches of a somatic afferent fiber.
Small receptive fields are found in areas such as _______.
Large receptive fields are present in areas such as ______.
Small = fingertips, where receptor density is high
Large = back, where receptor density is low
** small receptive fields provide increased discrimination
Denser areas of neurons take up ____ of the somatosensory cortex.
alot, a disproportionately large part
The trunk (large receptive fields) = small representation
Fingers (small receptive fields) = large representation
Peripheral distribution of the afferent nerves arising from each spinal level delineates what?
What is being relayed?
The segmental pattern of dermatomes
Pain/thermal information
What kind of primary fibers relay dscriminative touch, flutter-vibration & proprioception?
Where do these fibers enter the spinal cord?
Large-diameter primary fibers
Enter the SC via the medial division of the posterior root
One set of branches terminates on second-order neurons in the spinal cord gray matter at, above, & below the level of entry, while the largest set of branches does what?
Largest set of branches ascends cranially & contributes to the formation of the fasciculus gracilus & fasciculus cuneatus→ collectively termed the posterior columns
Fibers w/in the posterior columns are organized topographically, how are the fibers organized in both the fasciculus gracilus and fasciculus cuneatus?
Sacral level fibers are positioned medially & fibers from progressively more rostral levels (up to thoracic level T6) are added laterally to form the fasciculus gracilus
Thoracic fibers above T6 & cervical fibers form the laterally placed fasciculus cuneatus in the same manner
SC lesions at the fasciculi gracili & cuneatus result in what kind of deficit?
Ipsilateral reduction or loss of discriminative, positional, & vibratory tactile sensations at & below the segmental level of injury
If a patient has a wide-based stance and places feet to the floor with force, what might they be suffering from?
Sensory ataxia
Loss of muscle stretch (tendon) reflexes, & proprioceptive losses from the extremities due to lack of sensory input
Possibly at fasciculi gracili & cuneatus
After the primary neuron at the fasciculi where do the neurons go?
Nucleus gracilus & nucleus cuneatus are in posterior medulla. Contain second-order neurons of the PCMLS. Receive input from first-order neurons (primary afferents) from the ipsilateral DRG
Where do the second-order neurons send their axons to?
What are these fibers called?
Where do they cross?
What do they become?
To the contralateral thalamus
Internal arcuate fibers: loop anteromedially in medulla
Sensory decussation at the midline
Ascend as the medial lemniscus on the opposite side
As the medial lemniscus ascends, it ends up terminating, where is this at?
In the ventral posterolateral nucleus (VPL)
Damage to the brainstem levels of the posterior column (medial lemniscus) result it what?
Deficits in discriminative touch, vibratory, & positional sensibilities over the contralateral side of the body
What is the ventral posterior nuclei?
What are the two parts? What are they separated by?
Wedge-shaped cell group located in caudal thalamus
Ventral posterolater nucleus (VPL), and Ventral posteromedial nucleus (VPM)
Separated by arcuate lamina
What is the purpose of both the VPL and VPM respectively?
What artery supplies them?
VPL = maintains somatotopic arrangement of the body
VPM = proprioceptive information of the head
Posterior cerebral artery
What does the posterior cerebral artery supply?
What if it is compromised?
It supplies the VPL and VPM
Compromise can result in loss of all tactile sensation over the contralateral body & head
The VPL sends out third order neurons, what do these do?
They are large-diameter axons that terminate in the primary (SI) & secondary (SII) somatosensory cortices. (form homunculus)
Third-order axons from the thalamus (VPL) terminate where?
In the primary somatosensory (SI) cortex (homunculus)
What two gyri make up the homunculus?
What is the homunculus?
Postcentral and posterior paracentral gyri.
“Foot to tongue” pattern along medial → lateral axis. Regions with ↑↑↑ receptor density (hand/lips), have large amount of dedicated cortical tissue. Regions with ↓↓ receptor density (back) have small cortical representations
What provides the blood supply to the SI cortical areas?
Anterior & medial cerebral arteries
The anterior & middle cerebral artery supply _____?
What happens with compromise?
It supplies the SI cortical areas (primary somatosensory cortex)
MCA lesions produce tactile loss over the contralateral upper body & face
ACA lesions affect the contralateral lower limb
