PSL300 > 25 > Flashcards

25 Flashcards

(63 cards)

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1
Q

4 somatic senses

A

touch, temperature, proprioception, and nociception

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2
Q

define proprioception

A

awareness of the position of body parts relative to each other

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3
Q

define nociception

A

detects tissue damage or the threat of it, and is perceived as pain or itch

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4
Q

what are somatosensory receptor cells?

A

neurons

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5
Q

receptors for somatic sensation below the chin have their cell bodies in

A

the dorsal root ganglia

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6
Q

receptors for the head have their cell bodies in

A

the brain

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7
Q

where are the parts of neurons that transduce touch, pressure, etc into electrical signals?

A

in their nerve endings (ie in the tips of their fibres, in the skin and viscera)

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8
Q

three types of receptors in the skin

A

free nerve endings
merkel receptors (=merkel disks)
encapsulated receptors

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9
Q

free nerve endings

A

detect mechanical stimuli, temperature, and chemicals

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10
Q

merkel receptors (= merkel disks)

A

mechanoreceptor nerve endings in contact with specialised epithelial cells called Merkel cells

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11
Q

encapsulated receptors

A

mechanoreceptors sheathed in connective tissue (eg Meissner and Pacinian corpuscles)

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12
Q

where are Merkel disks located?

A

at the bottom of the epidermis

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13
Q

describe the structure/function of Merkel discs

A
  • very sensitive to deformation of the skin
  • signal contact
  • more tonic than phasic (ie send a sustained message as long as the deformation persists)
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14
Q

are most mechanoreceptors phasic or tonic? explain what this means.

A

phasic:
- give a sustained, constant stimulus, the nerve ending’s membrane depolarises but then returns to baseline in ~3ms (registers changes, not steady levels)
- thus, you don’t perceive much unless the stimulation is changing: if you run your hand along a surface, you get a vivid impression of its texture; after your hand stops, you sense far less

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15
Q

where are Meissner corpuscles located?

A
  • located at the top of the dermis
  • mainly in the tongue and hairless skin - erogenous zones, palms, and fingertips
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16
Q

describe the structure of Meissner corpuscles

A
  • egg-shaped
  • inside each corpuscle are many looping nerve endings. They detect sideways shearing, as when you stroke a surface or lift something with your fingertips
  • they are phasic, so they sense changes in shear
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17
Q

where are Pacinian corpuscles located?

A

located deep in the dermis

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18
Q

describe the structure of Pacinian corpuscles

A
  • onion-shaped
  • the nerve endings are sheathed in many layers, they can sense tiny displacements if the motion is quick (eg vibrations)
  • they are phasic, and so they respond strongly to vibration and other fast-changing stimuli
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19
Q

describe the distribution of receptors over the body surface

A
  • not uniformly distributed over the body surface
  • palms, fingertips, and lips are the ‘foveas’ of the somatosensory system; they have more densely packed receptors, and therefore higher acuity than other areas
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20
Q

what is the test of acuity?

A

2-point discrimination: If your skin is touched at 2 places simultaneously, can you tell whether there are one or 2 contact points

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21
Q

distinguish between acuity in lips/fingertips and calves

A

lips/fingertips: you can distinguish points 2-4mm apart, but on your calves you need 40mm

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22
Q

what do thermoreceptors consist of?

A

free nerve endings

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23
Q

describe the two main types of thermoreceptors

A
  • cold receptors respond maximally at ~30°C (well below body temperature)
  • warm receptors respond maximally at ~45°C
  • both are phasic-tonic, which is why we get used to hot bath or cold lake
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24
Q

what happens above 45°C?

A
  • pain receptors are activated
  • cold fibres also respond briefly to temperatures >45°C, causing paradoxical cold
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25
what is paradoxical cold?
a hot object, touched briefly, may feel cold
26
describe the numbers of thermoreceptors
- we have more cold receptors than warm - we have few thermoreceptors in total - as few as 1000 fibres may carry temperature information up to the spinal cord to the brain (precise localisation isn't crucial for temperature)
27
what do nociceptors consist of?
free nerve endings that respond to noxious stimuli
28
describe the two main types of nociceptors
- some respond to damaging mechanical stimuli - some respond to damaging heat or chemicals
29
give examples of nociceptors that respond to chemicals
- some respond to chemicals released from damaged cells (K+, histamine, prostaglandins) - some respond to platelets in response to injury
30
somatosensory afferents fall into 2 groups:
small and large
31
define a somatosensory afferent
a nerve fiber that transmits sensory information from the body to the central nervous system
32
small somatosensory afferent fibres
- C and Aδ (A-delta) - come mainly from free nerve endings - different small fibres respond to different adequate stimuli, such as mechanical stimuli, chemicals, or temperature
33
C fibres
unmyelinated, and conduct spikes at speeds up to 2m/s
34
Aδ fibres
thicker than C's, myelinated, conduct at up to 30m/s
35
large somatosensory afferent fibres
- Aβ (A-beta), come from Merkel disks or encapsulated mechanoreceptors such as Meissner or Pacinian corpuscles - myelinated, and conduct at 70m/s
36
how do large fibres project?
- they turn upward on reaching the spinal cord, and run ipsilaterally up to the medulla in tracts called the dorsal columns - in the medulla they synapse on cells whose axons cross the midline
37
how do small fibres project?
they synapse directly onto second order neurons or via interneurons on motoneurons (for reflex responses) or on dorsal-horn neurons whose axons cross the midline and run in the spinothalamic tracts, in the lateral part of the cord, between the dorsal and ventral horns
38
function of large fibres
provide feedback to the brain, especially to motor cortex, as it manipulates objects. the information has to travel a long way (up to the brain) quickly
39
function of small fibres
evoke simple responses to specific stimuli: withdrawing from pain, brushing away a bug, thermoregulatory and sexual responses. many of these tasks can be handled in the spinal cord, without immediate input from the brain
40
where do signals ultimately travel to and how?
signals travel via the thalamus to the cortex
41
signals from the spinal cord travel via ---------- ---- of the thalamus
the ventroposterolateral (VPL) nucleus of the thalamus
42
signals from the head travel via the ----- ------ of the thalamus
the ventroposteromedial (VPM) nucleus of the thalamus
43
what is the similarity in how signals from the spinal cord and from the head travel?
after converging, both pass to the primary somatosensory cortex, S1
44
describe the primary somatosensory cortex in one word
somatotopic
45
describe and explain the somatotopic arrangement of S1
- neighbouring areas of skin project to neighbouring cells in the cortex, so S1 is a map of the contralateral body surface - the map is distorted, as areas of high sensitivity and acuity (such as hands and lips) get a lot of cortical space
46
where is S1 located?
- in the parietal lobe - sits along the post central gyrus - strategic position: allows it to act as a hub
47
what is the role of lateral inhibition in somatic senses?
- there is lateral inhibition among somatosensory fibers - this enhances spatial differences - eg if you step into a very hot bath, you feel the most discomfort not in your foot but at the line formed by the water surface around your leg, because that is the temperature edge
48
describe the ion channels of nociceptors and thermoreceptors
many nociceptors and thermoreceptors have ion channels belonging to a family called transient receptor potential (TRP) channels
49
TRPV1 channels
- vanilloid receptors - respond to damaging heat and to chemicals, including the capsaicin in child peppers
50
TRPM8 channels
respond to cold and menthol
51
give a practical demonstration of the importance of nociceptive signals
people with congenital analgesia usually die before they are 20, because of injury and infection
52
what are the two types of pain?
fast and slow
53
compare fast and slow pain
fast: - eg when you stub your toe, you feel an immediate sharp pain - carried by Aδ fibres slow: - eg the pain following the stub ~1s later, a duller sensation - carried by C fibres
54
what is the reason for having two types of pain?
pain evokes two distinct responses: quick withdrawal (to get away from the painful thing) and prolonged immobilisation (to promote healing)
55
describe the two ways in which nociceptive signals evoke responses form the CNS
- trigger withdrawal (eg pulling your hand away from a hot stove). this is a spinal reflex, so doesn't need immediate brain input - reach the limbic system and hypothalamus, causing emotional distress, nausea, vomiting and sweating
56
what is an adaptation we have in response to emergencies where survival depends on ignoring pain?
descending pathways through the thalamus can block nociceptive cells in the spinal cord
57
define referred pain
pain in an internal organ is often felt on the body surface
58
explain referred pain
- nociceptors from different locations converge on a single ascending tract. so, when that tract sends signals to the brain, the brain doesn't know where the stimulus came from - as pain is more common in skin than in internal organs, the brain assumes the problem is on the body surface
59
slides 32-33
60
how can pain be gated?
- in the dorsal horn, C fibres contact secondary neurons. those secondaries are inhibited by Aβ fibres via interneurons - so Aβ's can block or dampen pain signals (eg if you rub a sore shoulder, it feels better)
61
acetylsalicylic acid (aspirin)
inhibits prostaglandins and inflammation, and slows transmission of pain signals
62
opioids (eg morphine and codeine)
decrease transmitter release from primary sensory neurons and postsynaptically inhibit secondary sensory neurons
63
describe the body's natural painkillers
endorphins, enkephalins, dynorphins
PSL300
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