Primary Afferent Nerves and Gate Theory

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  1. what constitutes primary afferent fibers?
    how are the two pathways situated
    pathways carrying the stimuli of the a-delta and c fibers are segregated as they ascend through the central nervous system on the way to the cortex
  2. when providing predominant sensory info where do the a-delta and c fibers divide
    they convey info on the anterolateral pathways and divide upon entering the spinal cord
  3. dorsolateral tract of lissauer
    area where first order neurons are found prior to synapsing on the second order neurons
  4. collateral fibers are sent where
    axons send collateral fibers into sensory neurons in the spinal cord's dorsal horn
  5. second order sensory neurons
    the dorsal horn cells that receive afferent input from the primary sensory afferents
  6. what are the two pathways of the dorsal horn
    • both a-delta and c fibers make different synaptic connections in the dorsal horn which make up the:
    • spinothalamic enter from the spine but ends up on the thalamus
    • spinoreticular enter from the spine but ends in the reticulum
  7. how many layers are there of the dorsal horn
    6 and they are called laminae "nee"
  8. layers of the dorsal horn:
    lamina I
    • dorsal most part in the most back part
    • neurons respond to sharp pricking painand cold from a-delta nociceptors and a-delta thermoreceptors
  9. layers of the dorsal horn:
    lamina II
    • substantia gelatinosa
    • neurons primarily activated by:
    •    c fiber nociceptors
    •    thermoreceptors
    •    mechanoreceptors conveying noxious mechanical and thermal stimuli
  10. layers of the dorsal horn:
    laminae III AND IV
    • Neurons respond to cutaneous a-delta mechanoreceptors
    • on the skin twisting, turning, punching
    • also to large diameter a-beta primary afferent neurons conveying light touch
  11. layers of the dorsal horn:
    laminae V AND VI
    • lie in the ventral portion of the dorsal horn
    • cutaneous nociceptive a-delta primary afferent fibers synapse onto second order projection neurons in lamina V
    • some abeta fibers carrying touch info send collaterals that terminate in VI
  12. projection neurons I AND V
    Contribute the most to the anterolateral pahtways
  13. which lamina(e) function as integration centers for sensory info
  14. Explain the gate control theory
    • 1. If there is no stimulation, the large and small fibers are quiet and the inhibitory interneurons block the projection nerves from sending a warning signal to the brain. this means that the gate is closed and there is no pain perceived.
    • 2. In non painful stimulation (as in a light touch of the skin) the large nerve fibers are activated. This means that projection nerves are activated but so are the inhibitory interneurons.  The IIs work to prevent the projection neurons from sending warning signal to brain.  Again, no pain signal.
    • 3. With pain stimulation, small nerve fibers are activated and they in turn activate the projection neurons which BLOCK the IIs.  This means that the warning signal is sent to the brain notifying the body to get out of the situation. The gate is open meaning PAIN.
  15. What happens when you have some sort of harmful stimulation on your person?  Why might this sooth the pain?
    • you tend to rub or put pressure on the injured site.  
    • an injury would primarily activate small nerves and projection neurons but the rubbing would activate large fibers and IIs which block the projection neurons from continually sending the information to the brain.
  16. what are the two areas of the brain that are involved in reducing pain
    periacqueductal gray pag and nucleus raphe magnus
  17. what may also play a role in the perception of pain
    psychological mechanism
  18. sensory information for the face is primarily supplied by
    • trigeminal nerve, cranial nerve V
    • primary afferent axons cell bodies lie inthe trigeminal ganglia
  19. what are the two groups of primary afferent fibers of the trigeminal nerve
    • 1. large diameter, myelinated a-alpha and a-beta fiber afferents
    • 2. small diameter a-delta and c-fiber afferents
  20. pars caudalis
    where second order neurons for pain and temp impulses from the face are found and is in the trigeminal nucleus (spinal V) which is continuous with the cervical spinal cord
  21. thalamic pain syndrome
    • lesion located in the thalamus of the brain that can greatly effect pain thresholds.
    • depending on size and location increased or decreased thresholds for sensations of pain, touch, and temp on the contralateral side of the body
  22. discriminative touch
    dorsal column pathway
    • the most important sensory pathway carrying info about sensation of the body into the cns
    • the main fiber bundle travels w/o synapsing in the spinal cord and up to the dorsal column nuclei in the medulla where the dorsal column fibers synapse and decussate, and the pathway continues on the thalamus as the medial lemniscus
  23. where are receptors mediating discriminative touch located
    in the skin
  24. discriminative touch
    degree of touch
    touch which causes no harm or typically would not send out warning signal to brain will cause mechanoreceptors to activate (animal cut or knife cut)
  25. what are the major differences of mechanoreceptors
    • there are different types and major differences include where they are located, those closer to the skin have low threshold, corpuscles get larger as they go down (structure and function)
    • mechanoreceptors lie either superficially near the epidermis or deep in the dermis
    • types are similar for hairy and smooth skin, with the exception of hair follicles and their smooth skin counterparts: meissner's corpuscles
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Primary Afferent Nerves and Gate Theory
454 neuro
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