Somatosensory Flashcards
Identify the spinal pathways mediating touch and proprioception?
[Peripheral receptor cells to spinal cord (1st order neuron cell body in dorsal root ganglion), with one collateral synapsing on gray matter at that level, another collateral ascending with the dorsal columns, terminating in nucleus gracilis (lower body) nucleus cuneatus (upper body). 2nd order neurons in gracilis/cuneatus cross to contralateral side of bran as arcuate fibers and form medial lemniscus tract which terminates in ventral posterolateral n (core, VPL). Axons of 3rd order neurons in VPL reach primary somatosensory cortex (Brodmann’s 3,1,2), terminate in layer IV]
Identify the spinal pathways mediating pain and temperature?
[Peripheral receptors to layer II (substantia gelatinosa, cell bodies in dorsal root ganglion). Dendrites of 2nd order neurons in layers I (C-fiber), III and V (A-delta) then cross spinal cord (anterior white commissure) to ascend to VPL of thalamus via spinothalamic tract. Some of these axons will have collaterals which terminate in reticular formation and periaqueductal gray (pathway of ‘suffering’). Axons of these reticular neurons (3rd order) then ascend to terminate in interlaminar n of thalamus. These (4th order) thalamic neurons then target insular cortex (pain affect or ‘suffering’). Neurons in VPL (3rd order) shell then ascend to somatosensory cortex.
What is the commonly accepted mechanism associated with endogenous analgesia?
[Ascending pain fiber collaterals (anterolateral or spinal trigeminal) synapse onto periaqueductal gray neurons whose axons project to raphe (serotonergic) neurons which descend to spinal or trigeminal levels to activate enkephalin neurons. These enkephalin neurons then cause pre-synaptic inhibition of 1st order pain fibers, and therefore, analgesia. Enkephalins are peptides that act on opioid receptors. They are extremely potent analgesics.
What kinds of receptors are found on skin?
[mechanical, chemical, thermal]
*What is a generator potential?
[the local potential associated with stimulating a sensory receptor. It can be associated with an accessory receptor structure or simply a free-nerve ending. If sufficiently large and activates enough voltage-gated Na+ channels, it will generate an action potential. If the potential is particularly large and long lasting, it can generate multiple action potentials].
How are touch receptors commonly differentiated?
[slow adapting and fast adapting]
What appears to be the best receptor for reading Braille? Are they rapid or slow-adapting?
[Merkel receptors, slow-adapting]
Which sensory receptor cells have the smallest receptive fields and where are they located?
[Merkel receptors, located near the skin surface on the tips of the fingers]
Where on the body do we exhibit our best two-point discrimination, How good is it, Which receptor is it?
[finger tips (anterior tips of middle finger is best), Can detect 2.5 mm separation (Merkel receptors)]
Identify the 1st and 2nd most sensitive places on our body (i.e., lowest threshold to touch)?
[cornea is by far the most sensitive, dental pulp is 2nd]
Which receptor appears best at detecting and discriminating skin vibration; Slow-adapting or fast-adapting?
[Paccinian corpuscles, > 300 Hz, rapid-adapting]
Do cold receptors increase or decrease their output with decreasing temperature?
[Increase]
What peptide is released by C-fibers when stimulated by inflammation?
[C-fibers can release substance P]
Is perception of ‘wet’ a labelled line?
[ No, it is derived from activation of touch and cold fibers, and ‘wet’ is better perceived with cold fluid]
*Is ‘itch’ a labeled line? Can it be suppressed?
[it seems to be, a sub-group of C fibers (which is processed via anterolateral system and whose parent receptors/fibers are pruriceptors)… it can be suppressed by activation of C-pain fibers]
