Somatosensory Physiology N19 Flashcards Preview

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Flashcards in Somatosensory Physiology N19 Deck (64)
1

Sensory Transduction

process by which a stimulus is transformed into an electrical response

2

Sensory unit

one afferent peripheral process with all of its receptor endings and receptors

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Receptors can be either

specialized endings of afferent neurons or separate receptor cells that signal afferent neurons via chemical messengers

4

Stimulus generates a

graded potential, or receptor potential, in the nerve generating a local current

5

Threshold in an afferent neuron

If threshold is reached, an AP travels to the CNS

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Increase in graded potential magnitude generates

an increase in AP frequency and an increase in neurotransmitters released at the CNS synapse

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Increase in graded potential magnitude does not change AP's _____

magnitude

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Factors that change the receptor potential magnitude

stimulus strength, rate of change of stimulus strength, temporal summation, and adaptation

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Adaptation refers to

a change in receptor sensitivity due to constant stimuli, resulting in decreased AP frequency despite stimuli

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Coding

conversion of stimulus energy into a signal that conveys relevant sensory information

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Stimulus characteristics

location, type of energy, intensity of energy

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Location of stimulus is determined by

which neuron is activated, each neuron terminates in a specific region of the CNS

13

Stimulus modality

type of sensory receptor a stimulus activates plays the primary role in coding

14

Mechanoreceptors

respond to mechanical stimuli (bending of hair, dee pressure, vibrations, stretch, and superficial touch)

15

Thermoreceptors

sensitive to temperature (cold: 10-38C, warm: 32-45C)

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Thermal nociceptors

sensitive to cold pain or hot pain

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Photoreceptors

respond to light of a particular wavelength

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Chemoreceptors

respond to the binding of particular chemicals to the receptive membrane (internal: gas levels in blood; external: tastebuds on tongue)

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Nociceptors

respond to stimuli that cause pain (excessive mechanical deformation, excessive temperatures, chemicals) free nerve endings

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Adequate Stimulus

type of energy to which a particular receptor responds (very sensitive to specific energy form)

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Sensory stimulation vs sensation

electrical stimulation of a "cold" primary afferent neuron may be perceived as a cold sensation, even though the "cold" thermoreceptor was not stimulated

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Frequency Coding

the intensity of the stimulus is coded by the frequency of APs (stronger stimulus = greater AP frequency)

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Population Coding

Stronger stimulus = larger area of receptors activated

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Threshold

lowest stimulus intensity that a subject can reliably detect (varies by context, experience, fatigue, BUT does not represent a change in the receptor)

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What would be a situation where the threshold is decreased?

Anticipation

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What would be a situation where the threshold is increased?

Childbirth, competitive sports

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Rapidly Adapting Receptors

Respond rapidly at onset/offset of stimulus, but fir slowly or stop during remainder of stimulus. (Important for transient or dynamic stimuli)

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Slowly Adapting Receptors

Maintain activation throughout stimulus, provide info on static qualities of a slow changing or prolonged event

29

Presynaptic Inhibition

descending inhibitory controls exerted on primary afferent neurons

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Central control of afferent information

as the AP travels to the cerebral cortex, signals may be modified at synapses

31

Sensory Information vs Sensation vs Perception

Sensory Information: any signal received by afferent sensory neurons
Sensation: sensory signals that reach CONSCIOUSNESS
Perception: our interpretation of the signal

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Sensation and Pereption occur after CNS modification

CNS may accentuate, modify, or filter sensory information, and perception is also based on previous experiences

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Algesia

sensitivity to pain

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analgesia

selective suppression of pain

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Pain

a sensory or emotional experience associated with real or potential injuries

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First (fast) pain

nociceptive pain, processing of an acute, well localized stimulus; PROTECTIVE SENSATION

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Second (slow) pain

prolonged noxious stimulus that outlasts the stimulus, associated with inflammation due to tissue damage (NSAIDS decrease 2nd pain)

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Third (Neuropathic) Pain

chronic, non-protective, and pathological pain due to damage of the nerves (electric, burning) that may invoke fear, depression, anxiety

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Phantom Limb pain

traumatic or surgical limb amputation may result in chronic pain due to nerve damage

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Causalgia

Complex regional pain syndrome may occur after damage to a peripheral nerve

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Sensory component of pain (what information is processed and where)

processed at the spinal cord and higher brain centers; type, location, intensity, duration of pain

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Nociception activation

not necessarily lead to pain sensation, perception is the product of the brain's processing of the sensory input

43

Responses to painful stimuli include:

arousal, attention, somatic or autonomic reflexes, endocrine response, emotional change

44

Nociceptor Anatomy

Free nerve ending in periphery --> release of glutamate and neuropeptide substance P in the spinal dorsal horn

45

Thermal nociceptors

respond to extreme temperatures; A-delta small-diamter, thinly myelinated fibers

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Mechanical nociceptors

respond to intense pressure; A-delta small-diamter, thinly myelinated fibers

47

Polymodal Nociceptors

respond to high intensity mechanical, chemical, or thermal stimuli; small diameter, non-myelinated slow-conductin C fibers

48

First and Second pain response to injury

Pain is first rapidly realizes by "sharp" first pain A-delta fibers, while 2nd pain is mediated by C fibers that cause a dull ache

49

Referred Pain

pain that is perceived to be coming from an area that is remote from the actual source of pain, due to convergence of somatic and visceral nociceptive input onto the same dorsal horn secondary neuron (indiscriminative of pain location)

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Sensitization of Nociceptors

after injury nociceptors may become sensitized (or activation of interneurons)

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Primary Hyperalgesia

noxious stimuli contacts injured site, the magnitude of pain increases due to sensitization and recruitment of additional nociceptors

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Secondary Hyperalgesia

nociceptors surrounding the injury site may become tender or sensitized and application of noxious stimulus may generate an excessive pain response

53

Allodynia

non-noxious stimulus now evokes pain due to sensitization

54

How does injury cause sensitization

injured tissues release bradykinin, serotonin, K+, and prostaglandins --> activate nociceptors--> local release of Substance P and Calcitonin gene related peptide (CRGP) --> causes mast cell degranulation --> histamine release --> directly exciting nociceptors

55

Bradykinin is one of the most active pain producing agents because

it activates A-delta and C fibers and increases the release of prostaglandins

56

How do NSAIDS block 2nd pain?

Block cyclooxygenase, which prevents the synthesis of prostaglandins

57

Which substances are activators of afferent fibers?

K+, Serotonin, Bradykinin, Histamine

58

Which substances cause sensitization of afferent fibers?

Prostaglandins, leukotrienes, and Substance P

59

Inflammation and Substance P

dilation of peripheral vessels (heat, redness) and plasma extravasation (swelling); Substance P can cause all 3 symptoms and Substance P antagonists can block all 3 symptoms

60

Hyperalgesia

caused by Substance P and CGRP mast cell activation --> histamine directly excites nearby nociceptors

61

A-delta fibers release glutamate and

activate AMPA glutamate receptor subtype in dorsal horn = 1st pain

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C-fibers release glutamate and Substance P and

activate NMDA glutamate receipts subtype and NK-1 receptors = 2nd pain

63

Central sensitization

chronic activation of C fibers and constant release of glutamate and Substance P = hyperexcitability and diffusion of neurotransmitters that activate nearby neurons

64

Central sensitization decreases the threshold and produces:

hyperalgesia, allodynia, and neurpathic pain