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Flashcards in Ascending Sensory Systems Deck (32)
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What are the 3 main mechanisms of Cerebral blood flow?

1) Autoregulation: vessels are stretch sensitive, so constrict when ↓ pressure; dilate when pressure ↑
2) Collaboration of brain & vessels, response to increased brain activity (glutamate sensing)
3) Cerebral vascular autonomics (may be more important at extremes of autoregulatory range)


What are Somatosensory Receptors?

All are pseudounipolar neurons with:
1) Cell body in DRG or cranial nerve ganglion
2) A central CNS process (spinal cord or brainstem)
3) Peripheral process with an ending in skin, muscle, or a joint


What do Somatosensory Receptors do?

Detect mechanical, chemical or thermal changes


The skin is richly innervated, with a variety of endings broadly divided into encapsulated and non encapsulated receptor. What are they called?

Cutaneous receptors


What Cutaneous receptors are encapsulated?

Pacinian (vibration) and Meissener (touch) corpuscles, & Ruffini (pressure) endings


What Cutaneous receptors are non encapsulated?

Endings around hairs (touch), Merkel (touch) endings, Free Nerve endings (pain, temperature, itch, touch)


What do receptors do in hairy skin?

- Receptor endings wrap around hairs

- Nerve ending (NT) at a Merkel cell (M) in the basal layer of skin


What receptor types are in glabrous (hairless skin)?

M, Meissner corpuscle
Me, Merkel cell
PC, Pacinian corpuscle
R, Ruffini ending


Meissner Corpuscle

- Discriminative touch (two point discrimination)
- Concentrated in finger tips
- A-beta fiber - fast conducting


Merkel Nerve Ending

- Discriminative touch (two point discrimination)
- Fine touch detail (edges of objects; texture)
- A-beta fiber - fast conducting

*Merkel Disk – sensitive to edges of objects held in the hand; are slow adapting. Are non-encapsulated; throughout skin and mucous membranes but concentrated in the fingertips
Can become cancerous (rare). Forms bluish-red spot that enlarges and bleeds easily. Can grow and spread rapidly.


What is discriminative touch?

- Posterior column – medial lemniscal pathway
a) Two point discrimination
b) Conscious proprioception
c) Vibratory sense
A-beta fiber
Collateral to Lamina II
d) Decussates in medulla
e) Relays in lateral thalamus
Ventral posterolateral nucleus (VPL)
f) Terminates in postcentral gyrus


Spatial resolution of stimulus

- Spatial resolution correlates with number of cutaneous receptors
- There are more Meissner corpuscles and Merkel endings/cm2 in the finger tip than in the hand so two-point discrimination is more sensitive in the finger tip


Pacinian Corpuscle

- Detection of vibration
a) Concentrated in fingers and in palm
b) A-beta fiber - fast conducting


Free Nerve Endings

Sensation of:
a) Pain
b) Crude touch
c) Temperature

* Nociceptors, thermoreceptors and some mechanoreceptors are Free nerve endings with myelinated or unmyelinated fibers.
- Temperature sensitivity is due to channels that open with a specific range of temperatures
- Two phases of pain
1) Sharp prick, well-localized, short duration – carried by rapidly conducting myelinated fibers, called fast or delta pain
2) Slow, poorly localized, aching pain that may follow – carried by unmyelinated fibers, called slow pain


Ascending & Descending Pathways have well defined locations in spinal cord _____.

white matter


Ascending & Descending Pathways consist of three general types:

1) Long, ascending fibers going to thalamus, cerebellum or various brainstem nuclei
2) Long, descending fibers going from cerebral cortex or various brainstem nuclei to spinal cord gray matter
3) Short, propriospinal fibers interconnecting different spinal cord levels
*These fibers help coordinate flexor reflexes


Fibers with similar connections (destinations) tend to travel ....

together and form tracts in the spinal cord


Descending tracts primarily located in the ...

anterior (AF) & lateral (LF) funiculi


Ascending tracts found in ...

all three funiculi (AF, LF, PF)


Propriospinal fibers surround the ...

spinal cord gray matter (propriospinal tract)


Posterior column – medial lemniscus system

1) Conveys touch and limb position information *Maintains information related to location and nature of a stimulus
*Carries info important for conscious appreciation of touch, pressure, vibration, joint position/ movement

2) Posterior columns- mostly ascending large myelinated primary afferents from various mechanoreceptors
3) Main way info from cutaneous, joint and muscle receptors reaches cortex


Posterior column – medial lemniscus system Spinal afferents have their cell bodies in ...

ipsilateral Dorsal Root Ganglions

*As DRG rootlets enter cord fibers divide into two divisions:
1) Medial – heavily myelinated, large diameter fibers; enter posterior column and ascend to brainstem
2) Lateral – finely myelinated and unmyelinated, small diameter fibers


Rostral to T6 a few fibers are added to ...

fasiculus gracilis and fasiculus cuneatus starts to form

-Fibers reach the brainstem and synapse in nucleus gracilis and cuneatus (posterior column nuclei)

-Second order fibers cross the midline in caudal medulla and form the medial lemniscus

-Third order fibers originate in thalamus (ventral posterolateral nucleus) ascend thru internal capsule and synapse in primary somatosensory cortex in postcentral gyrus

*Fibers entering posterior columns added laterally to those already present, so a pattern of lamination develops
*Sacral levels most medial and cervical levels most lateral, Somatic organization is preserved as fibers end in posterior column nuclei and in medial lemniscus


Injury of the Posterior column – medial lemniscus system result in what?

Impaired proprioception and discriminative tactile functions, especially complex tactile discrimination. It is Tested clinically by placing a vibrating tuning fork on body surface or have patient identify a pattern drawn on their skin.

- Proprioception can be lost completely after posterior column injury, can result in ataxia; uncoordinated movements as brain is unable to direct motor activity without feedback from the body

-Sensory info reaches brain in multiple pathways, so damage to a single pathway rarely leads to total loss of function

-If posterior columns are injured there is a large deficit at first but in time some recovery is possible

-The ability to discriminate complex shape of an object (stereognosis) or the direction or speed of the stimulus moving across the skin will likely remain impaired


What is the Spinothalamic tract/ Anterolateral pathway?

a) One of multiple pathways that convey pain and temperature info - Conscious awareness of pain & temperature; nature (e.g. burn, sting, etc) and location - Cortical areas involved are widespread (postcentral gyrus, insula, others) reflects our complex conscious awareness of pain
b) Involved in awareness and localization of painful stimuli
c) Ultimately ends in VPL of thalamus and some nearby thalamic nuclei
d) Other pain pathways end in reticular formation or limbic system to mediate other pain responses, these tracts and the spinothalamic tract together are known as the anterolateral pathway (in anterior half of lateral funiculus


Anterolateral Pain, temperature and some mechanoreceptor fibers enter cord via ...

lateral division of dorsal root, project branches to posterior horn many synapse in substantia gelatinosa


How is the Anterolateral pathway formed?

a) Neurons in other laminae form 2nd order neurons cross midline with rostral inclination, collect and form anterolateral pathway
b) Neurons in other laminae form 2nd order neurons cross midline with rostral inclination, collect and form anterolateral pathway

*Somatotopic organization: Caudal body parts in posterolateral portion, rostral parts in anteromedial portion


True or false: Anterolateral pathway has multiple origins, destinations and functions related to pain and temperature



Injury of Anterolateral pathway?

- Damage causes loss of
a) Pain and temperature sensations
b) Itch and tickle sensations
- Bowel and bladder pressure, sexual sensations – ascending bilaterally so unilateral injury does not leave a deficit
- Some tactile and pressure information is carried in this pathway, but most of this info is in posterior columns so no tactile deficit is noted


How are pathways organized in the sensory homunculus?