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RB Y3 1.1 Neuroanatomy > The Somatosensory System > Flashcards

The Somatosensory System Flashcards

1
Q

define enteroceptive

A

reports upon internal state of the body, closely related to autonomic function

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2
Q
A
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3
Q

which sensations does DCML transmit

A

fine touch, vibration and proprioception

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4
Q

outline the DCML pathway

A

3 groups of neurons:

  1. Carry sensory to the medulla. Signals from upper limb (input above T6) travel in fasciculus cuneatus (part of the dorsal column) and then synapse in the nucleus cuneatus of the medulla oblongata. Signals from lower limb (T6 and below) travel in the fasciculus gracilis and then synapse in the nucleus gracilis of the medulla oblongata.
    * Leg is represented medially and the arm laterally in the posterior columns
  2. Begin in nucleus cuneate or gracilis. Deliver information from 1st order neurons to 3rd order neurons in the thalamus. The fibres decussate within the medulla.
  3. Transmit sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain (postcentral gyrus) via the posterior internal capsule
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5
Q

lateral inhibition in the DCML tract

A
  • the ability of an excited neuron to reduce the activity of tis neighbours via inhibitory interneurons
  • creates a greater contrast in stimulation which allows for increased sensory acuity
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6
Q

graded generator potential

A
  • stimulus of primary sensory neurons results in a graded generator (receptor) potential, if this depolarizes the trigger zone to threshold an all-or-none AP is produced
  • amplitude of receptor potential is graded and proportional to stimulus intensity
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7
Q

describe the relationship between graded potential and stimulus intensity

A

non - linear: greatest sensitivity to change at low stimulus strength

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8
Q

wht does threshold means in terms of primary sensory afferents

A

the intensity of stimulus required to excite a sensory neuron

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9
Q

what is the purpose of having primary sensory afferents of different thresholds

A
  • low threshold units respond to low intensity (non damaging, innocuous) stimuli
    • eg LT mechanoreceptors - mediate fine discriminatory touch
    • eg LT thermoreceptors - mediate cool, cold, warm
  • high threshold units respond to high (damaging) intense stimuli
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10
Q

what are the 3 types of adaptation mechanoreceptors show

A
  • slowly adapting
    • adapt slowly to stimulus and continue to produce AP over the duration of the stimulus
  • fast adapting (phasic)
    • show a substantial drop in discharge rate during a maintained stimulus
  • rapidly adapting
    • produce one or two APs at the beginning of skin displacement
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11
Q

what are slowly adapting mechanoreceptors good for

A

stretch receptors and temperature

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12
Q

whta are FA mechanoreceptors good for

A

texture and vibrations

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13
Q

out of Aa, Ab, A(sigma) and C fibres, which are the fastest conducting

A
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14
Q

what is a receptive field

A

the area of skin served by the receptor endings of a given axons, the area over which an appropriate stimulus will generate a response in the axon

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15
Q

describe the relationship between sensory acuity and receptive field

A
  • inverse correlation
  • greater density of receptors means smaller receptive field - greater sensory acuity
  • note that receptive fields overlap
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16
Q

where is the greatest density of receptors found

A

greatest density in finger tips, perioral region

less on the back

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17
Q

how can somatosensory function be measured

A

two point discrimination (spatial acuity)

  • clinically tested by simultaneously applying two sharp point stimuli at a set distance apart - subject reports where they are sensed/if they are both sensed
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18
Q

in two point discrimination, which areas would have the highest discriminative capacity

A

fingers and peri-oral region - highest density of receptors (and smallest receptive fields)

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19
Q

which group of sensory axons do mechanoreceptors have

A

alpha beta (moderate conduction velocity)

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20
Q

name 3 FA mechanoreceptors

A
  • Meissner corpuscle
    • light touch
  • Pacinian corpuscle
    • vibration and changes in pressure
  • hair follicle receptors
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21
Q

Meissner corpuscle

A
  • encapsulated, located in the basal epidermis and dermis of glabrous (not hairy) skin
  • FA1
  • sensitive to stroking, fluttering, low frequency vibration
  • discriminatory touch
  • eg Braille
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22
Q

Pacinian corpuscles

A
  • encapsulated, located in the dermis of teh skin and in fascia surrounding muscle and bone in the peridontium (deep)
  • FA
  • respond to sudden changes in vibration and pressure
  • act as event detectors during the manipulation of objects
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23
Q

Merkel cell nurite complexes

A
  • SA1 - slow adapting touch receptors
  • signal tonic events eg small indentations in skin
  • can encode stimulus intensity and duration
  • eg reading Braille
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24
Q

Ruffini endings

A
  • SA encapsulated nerve endings
  • found in the deep layer of the skin and in fascia surrounding bone and muscle
  • sensitive to sustained pressure and show v little adaptation - sensitive to drag and shearing forces
  • highest density around the fingernails where they are believed to be useful for monitoring slippage of objects against the surface of the skin
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25
Q

hair plexus

A
  • group of mechanoreceptors conveying information about hair movement
  • contain several of the previously mentioned mechanoreceptors, however not Meissner corpuscles
  • follicular nerve endings either wrap around the follicle or run parallel to it (palisade fibres)
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26
Q

describe the somatosensory homunculus

A
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27
Q

what sensations does ALS transmit

A
  • crude touch and pressure
  • pain and temperature
  • Classically the anterior spinothalamic tract was described as carrying the sensory modalities of crude touch and pressure, and the lateral spinothalamic tract of pain and temperature. Current thinking holds that all parts of the ALS carry all the modalities.
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28
Q

ALS: sensory receptors

A
  • Receptors for non-discriminative touch, innocuous thermal stimuli and nociceptive stimuli in the skin are all free (naked) nerve endings, meaning they lack specialized receptor cells or encapsulations
29
Q

outline the ALS tract

A

3 groups of neurons:

  1. 1st order neurons arise from sensory receptors in the periphery. Enter the spinal cord, ascent 1-2 vertebral levels and synapse at the tip of the dorsal (posterior) horn.
  2. Carry sensory information from here to the thalamus. Decussate within spinal cord, and form distinct anterior and lateral spinothalamic tracts
  3. Sensory signals from thalamus to ipsilateral primary sensory cortex of brain
30
Q

what do spinocerebellar tracts transmit

A

unconscious sensations

31
Q

define pain

A

Pain is an unpleasant sensory and emotional experience, associated with actual tissue damage or described in terms of such damag

32
Q

outline the 3 types of pain

A
  • Nociceptive pain – adaptive (an immediate protective response, short-lived)
  • Inflammatory pain – adaptive (assists in healing, persists over days, possibly weeks)
  • Pathological pain – maladaptive (no physiological purpose, persists over months, years, or even a lifetime)
33
Q

nociceptors

A
  • specific peripheral primary sensory afferent neurons - first ordder
  • have free (naked) nerve endings in peripheral locations (cutaneous skin and deep structures)
  • normally activated by intense stimuli that are noxious
34
Q

describe A(sigma) mechanical nociceptors

  • what type of sensory information do they respond to
  • what type of pain do they transmit
  • neurotransmitter
A
  • found at the end of thinly myelinated fibres
  • respond to mechanical injury accompanied by tissue damage and thermal injury
  • mediate first, fast, pain
  • their neurotransmitter in the dorsal horn is glutamate acting on fast ligand gated ion channels
35
Q

describe C polymodal nociceptors

  • what type of sensory information do they respond to
  • what type of pain do they transmit
  • neurotransmitter
A
  • respond to mechanical, thermal and chemical stimuli (all)
  • mediate second, slow, pain
  • their neurotransmitter in the dorsal horn is glutamate and other peptides (substance P)
  • have fast ion channel receptors and NMDA receptors
36
Q

NMDA receptors

A
  • Severe/prolonged tissue or nerve injury can induce hyperexcitability of dorsal horn neurons of the spinal cord resulting in persistent pain, hyperalgesia and allodynia
  • These changes are mediated by NMDA glutamate receptors in the spinal cord
  • NMDA receptors facilitate and prolong the transmission of nociceptive messages through the release of substance P and glutamate.
37
Q

which ion channels of C polymodal fibres are activated specifically by thermal stimuli

A

transient receptor potential family: TRPA1, TRPC3, TRPV1

38
Q

which specific transient receptor potential family ion channel increases its sensitivity with tissue inflammation to cause hyperalgesia and allodynia

A

TRPV1

39
Q

how are peptidergic polymodal nociceptors (subset of C fibres) defined

A

by the presence of one or both of substance P and CGRP

40
Q

what is unsual about peptidergic polymodal nociceptors

A

they have afferent and efferent functions

  • afferent: transmit nociceptive information to CNS via release of glutamate and peptides within the dorsal horn
  • fibres decussate via the central white commissure at all levels of the spinal cord
  • efferent: release pro-inflammatory mediators (CGRP, substance P) from peripheral terminals to induce hyperalgesia and allodynia
  • this contribute to neurogenic inflammation
41
Q

what is neurogenic inflammation

A

inflammation arising as a direct outcome of stimulation of primary sensory afferent neurons, with local release of inflammatory mediators from afferent neurons

42
Q

substance P actions

A
  • Vasodilation and extravasation (leakage) of plasma proteins (promotes formation of bradykinin and prostaglandins)
  • Release of histamine from mast cells (vasodilation and increased vascular permeability)
  • Sensitizes surrounding nociceptors
43
Q

CGRP actions

A

induces vasodilation

44
Q

what is hyperalgesia

A
  • pain receptors become more sensitive (lower pain threshold) and thus more responsive (manifested by an increased firing rate) to noxious stimuli within their receptive fields
45
Q

primary hyperalgesia

A

occurs in the region of damaged skin

46
Q

2y hyperalgesia

A
  • occurs in the skin bordering the damaged tissue
  • increase in receptive field size of the posterior horn neuron
  • decreased response to new inputs in that area
  • increased response to pain
47
Q

lamina of Rexed

A

layers in the grey matter of the spinal cord

48
Q

which lamina of Rexed do A(sigma) and C fibres terminate on in the dorsal horn of the spinal cord

A

Nociceptive primary afferent fibres mainly terminate in laminae I, II (and V for Aδ fibres)

49
Q

what do wide dynamic range neurons in the spinal cord recieve input from

A

receive input from C, Aß and A(sigma) fibres and so respond to a wide range of stimuli

50
Q

how does visceral pain usually present

A

poorly localised - dull, vague, fullness, nausea

51
Q

describe the path of visceral afferents from nociceptors before entering the dorsal horn

A
  • follow sympathetic efferent fibres
  • terminate in lamina I and V of the dorsal horn
  • ascend in ALS tract
52
Q

viscerosomatic pain

A
  • sharp, well localised pain
  • occurs when inflammatory exudate from a diseased organ contacts a somatic structure eg parietal peritoneum
  • may present with diffuse visceral pain that progresses to sharp viscerosomatic pain eg appendicitis
53
Q

regulation of pain by low threshold mechanoreceptors

A
  • Pain evoked by activity in nociceptors (C- and Aδ- fibres) can be reduced by simultaneous activity in low threshold mechanoreceptors (Aβ-fibres)
  • stimulation of touch sensors (mechanoreceptors - Aß) in the skin by rubbing it can disrupt the sense of pain from a nearby structure
  • gate control theory: Aß afferent fibres excite interneurons that in turn inhibit the neurons carrying pain information from dorsal horn to brain
54
Q

describe the 2 classes of non-nociceptive thermoreceptors

A
  • 2 classes: those activated by heat and those by cold
  • show a graded response to changes in ambient temperature - whereas burns produced by prolonged thermal stimulation evoke a high frequency reponse in thermonociceptors
55
Q

anterior and posterior borders of primary somatosensory cortex (SI)

A

anteriorly bordered by central sulcus and posteriorly by postcentral sulcus

56
Q

homunculus

A
  • body regions with a high density of receptors have a disproportionately large amount of cortical tissue dedicated to their central representation
  • vice versa
  • legs in central sulcus
57
Q

blood supply to SI

A

anterior and middle cerebral arteries

58
Q

what sensory problems do vascular lesions to the anterior and middle cerebral arteries produce

A
  • middle - tactile loss over contralateral upper body and face
  • anterior - tactile loss over contralateral lower limb
59
Q

outline the Broadmann areas of SI

A

3a, 3b, 1 and 2

60
Q

outline the submodality specific inputs to Broadmanns areas

A
61
Q

SI structure

A
  • has layers and columns
  • 6 layers
  • each column consists of neurons with similar inputs and responses
62
Q

which layer to thalamic inputs to SI terminate in

A
  • layer IV - like other sensory cortex eg visual and auditory
  • from here they project to cells toward the surface of teh cortex and also deeper layers
63
Q

how does the SI respond to eg losing a finger

A
  • the area of SI representing the lost finger would start to be taken over by adjacent digits
  • e.g. if the sensory input from a finger increases, the cortical representation of that digit show increases relative to that of input generating less activity
64
Q

where is SII located

A

deep in the upper side of the lateral sulcus

65
Q

where does SII receive inputs from

A
  • also contains a somatotopically organise representation of the body surface
  • receives inputs from ipsilateral SI cortex and other cortical and sub cortical areas
66
Q

innervation of dura mater

A

CNV

67
Q

outline the trigeminal system

A
  • Impulses originate at the terminals of the 2 trigeminal nerves (paired), each of which has 3 divisions
  • The soma of sensory neurons are located in the trigeminal sensory ganglion. The central processes of trigeminal ganglion cells form the large sensory root of the CNV as they enter the lateral aspect of the pons
  • Central terminals of the CNV synapse on 2nd order neurons in the chief sensory nucleus (general tactile stimuli) or spinal nucleus (pain, temperature information)
  • These decussate and project (via trigeminal lemniscus) to the ventroposteriomedial (VPM) nucleus of the thalamus
  • 3rd order neurons relay information to the cortex via the thalamocortical neurones
68
Q

which type of stimuli does the chief sensory, spinal and mesencephalic nuclei of the trigeminal system receive

A
  • chief sensory - general tactile
  • spinal - pain and temperature
  • mesencephalic - prorioception from muscles of mastication and teeth
69
Q

what is special about the mesencephalic nucleus

A

it is the only site in the CNS where the cell bodies of the primary afferent neurons live inside the CNS - a ‘retained’ sensory ganglion

RB Y3 1.1 Neuroanatomy (10 decks)

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