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Neuro > Somatosensation > Flashcards

Somatosensation Flashcards

1
Q

A-alpha afferents (Group 1a)

A

Axons associated with muscle spindles, sensory structures within voluntary muscles, important for reflex control of movement. Also gives us our sense of limb position.

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2
Q

A-beta afferents

A

Axons associated with cutaneous receptors, which give us our sense of fine touch, as well as pressure and vibration.

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3
Q

A-delta afferents

A

Axons associated with free nerve endings in the skin. May be mechanosensory in function (responsible for crude, poorly localised touch). May also be sensitive to temperature (painful and non-painful) and noxious (potentially damaging, painful) stimuli.

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4
Q

C-fibres

A

Thin, unmyelinated axons associated with free nerve endings in the skin. Slow conducting. Sensitive to a variety of noxious and non-noxious stimuli (chemosensory, mechanosensory, temperature, multimodal)

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5
Q

Dorsal root ganglia

A

Ganglia located along spinal nerves, just outside the spinal cord. Contain the cell bodies of primary afferents of the somatosensory system.

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6
Q

Which sensations do the somatosensory system convey?

A
  • Touch
  • Proprioception
  • Heat, cold
  • Pain, itch
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7
Q

What is proprioception?

A

Perception or awareness of the position and movement of the body

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8
Q

How are sensations conveyed to the CNS?

A

It is conveyed by the peripheral nerves that connect the body to the central nervous system.

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9
Q

How is the CNS connected to the body?

A

Connected to the body through spinal nerves (31 pairs) and cranial nerves (12 pairs)

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10
Q

How many spinal nerves and cranial nerves do we have?

A

31 pairs of spinal nerves

12 pairs of cranial nerves

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11
Q

What is the difference between a nerve and a neuron?

A

A nerve is a bundle of axons, and a neuron is processes of individual neurons. A nerve is a macroscopic structure and a neuron is a microscopic structure.

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12
Q

Describe the structure of the peripheral nerves

A

A nerve is made up of fascicles (bundles of axons) and connective tissue elements.

  • Endoneurium
  • Perineurium
  • Epineurium
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13
Q

What is the endoneurium of the peripheral nerves?

A

Connective tissue surrounding the individual axons

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14
Q

What is the perineurium of the peripheral nerves?

A

Binding axons into fascicles - separates the individual fascicles

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15
Q

What is the epineurium of the peripheral nerves?

A

Binding fascicles into a nerve - Outer connective tissue sheath

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16
Q

Path of a peripheral nerve

A

As the nerve goes out of the spinal cord to the periphery, it will divide multiple times to innervate different regions of the area. Peripheral nerves are almost always mixed so can contain incoming sensory axons and outgoing motor axons.

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17
Q

What are mixed peripheral nerves?

A

Contain incoming sensory axons and outgoing motor axons

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18
Q

Function of peripheral nerves

A

Control movement of hands and fingers and detect sensations

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19
Q

Where is the spinal cord and nerves located?

A

The spinal cord is within the vertebral column. The spinal nerves arise from the cord and become peripheral nerves. Each peripheral nerve entering the spinal cord divides into a dorsal and ventral root.

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20
Q

Structure of a typical peripheral nerve

A
  • Contains the dorsal ramus and the ventral ramus
  • Dorsal root ganglion
  • Ventral root ganglion
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21
Q

What does ventral rami become?

A

May join with each other from adjacent sections and form complex nerves of the arm.

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22
Q

What is the dorsal root ganglion?

A
  • Sensory root
  • Contains the cell bodies of sensory neurons
  • Sensory neurons enter here
  • Incoming from the periphery to the spinal cord at the dorsal root ganglion
  • Central portion of the sensory axon enters and will synapse locally and/or send the axon up the white matter of the spinal cord towards the brain
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23
Q

What is the ventral root ganglion?

A
  • Motor root
  • Motor neurons enter here
  • Cell bodies located in the grey matter of the spinal cord giving rise to axons that will emerge from the ventral root and travel along the peripheral nerve to the innervation muscle
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24
Q

Which roots are the sensory receptors of the somatosensory system?

A

Dorsal root ganglion cells

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25
Q

Length of Sensory axons

A
  • Sensory axons are very long axons
  • Have terminals in the fingertips via a peripheral nerve entering the thoracic/cervical region of the spinal cord
  • Cell body is located in the dorsal root ganglion
  • They are pseudo unipolar neurons
  • More than a metre in length from the fingertip to the cerivcal spinal cord
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26
Q

What are pseudo unipolar neurons?

A

Contain a cell body in the dorsal root and contains a peripheral and central portion.

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27
Q

What are the two anatomical and functional systems of nerves?

A
  • Large fibres: large in diameter, myelinated, fast condution used for tactile and proprioception
  • Small fibres: small diameter, thinly myelinated or unmyelinated, medium or slow conducting used for temperature, pain, itch, crude touch
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28
Q

What does the quality of sensation depend on?

A

Depends on afferent fibre type.

Afferent sensory fibres respond to a specific stimulus quality and are not indiscrimate.

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29
Q

What are mechanosensitive fibres?

A

Sensitive to light touch but insensitive to thermal stimulation

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30
Q

What are thermosensitive fibres?

A

Sensitive to warmingor cooling

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31
Q

What are the cutaneous receptors of the somatosensory system?

A

Located in the skin, either immediately beneath the epidermis in the dermal papillae or in the dermis

  • Meissner’s Corpuscles (in the dermal papillae)
  • Merkel’s Disks (dermal papillae)
  • Pacinian Corpuscles (dermis)
  • Ruffini’s corpuscles (dermis)
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32
Q

How do the cutaneous receptors detect sensations?

A

They contain distal or peripheral terminals of the axons in the receptors. The nerve endings are associated with the specific highly structure connective tissue structures. They all respond to tactile senses but it varies depending on class.

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33
Q

What are free-nerve endings?

A

Associated with smaller diameter axons, the distal portion isn’t associated with specialised structures in the skin and the nerve terminals just end (hence free nerve endings)

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34
Q

What is the most important class of proprioception receptors?

A

Muscle spindle

35
Q

What are muscle spindles?

A

They are specialised muscle fibre within the body of a muscle but don’t contribute to the actual contraction.
Stretch receptors in the body of muscle that will detect the changes in the length of the muscle.

36
Q

Function of muscle spindles

A
  • Associated with sensory axons that are sensitive to the length of a spindle - they are mechanosensory afferents.
  • Have sensory axons called group 1 and 2 afferent axons that change firing rate dependent on muscle length.
  • As the muscle lengthens, they increase their firing rate in proportion to muscle length. Hence, the signal from muscle spindles can be used to decode limb position.
  • Stimulation of muscle spindles will elicit contraction in the stretched muscle and inhibit APs in antagonistic muscles
37
Q

What are the proprioception receptors of the somatosensory system?

A
  • A-alpha afferents also called group 1a
  • Large diameter, myelinated, fastest conducting
  • Important in muscle spindles
38
Q

What are the tactile afferents for discrimative touch?

A
  • A-beta afferents
  • Large diameter
  • Myelinated
  • 2nd fastest conduction
  • Include:
    • > Cutaneous: Meissner’s and Merkel’s
    • > Deep: Ruffini’s and Pacinian
39
Q

What are the free nerve endings for the low-resolution tactile, temperature and pain in the somatosensory system?

A
  • A delta fibres: small diameter, thinly myelinated, moderate conduction velocity (<30 m/s)
  • C fibres: small diameter, unmyelinated, slow conducting (<1 m/s)
40
Q

Braile reading

A
  • Embossed letters
  • Detected by A-beta afferents to discriminate braille
  • Ruffini’s and Pacinian corpuscles have less well-defined receptor feels to braille so the response is longer.
41
Q

Receptive fields

A
  • Ability to localise depends on sensory receptive fields
  • Fine tactile discrimination means lots of small fine receptive fields close together
  • Weak fine tactile discrimination means fewer small receptive fields close together, larger receptive fields
  • If there are two different stimuli within the same receptive fields, then the nervous system has no way of resolving, and the tactile resolution depends on the receptive field size.
  • Higher tactile resolution, need to increase the small receptive fields
42
Q

What are the two major central pathways of the somatosensory system?

A
  • Dorsal column medial lemniscal system (DCML)

- Spinothalamic tract or the anterolateral system

43
Q

Dorsal column medial lemniscal system

A
  • Mediates discriminative touch, vibration and proprioception
  • Receives inputs form the A-beta and A-alpha afferent fibres
44
Q

Describe the DCML system

A
  1. A-beta and A-alpha axon sends out local branches within the grey matter of the spinal cord, but the main axon ascends in the dorsal columns (white matter) to the dorsal column nuclei (gracile and cuneate nuclei).
  2. Axons synapse at these nuclei, and the second order neurons originate from here. The axons of these neurons immediately cross the midline and ascend in a fibre tract called the medial lemniscus on the contralateral side.
  3. Goes through the midbrain and synapses in the ventral posterior nuclear complex of the thalamus
  4. The thalamic neurons are the third order neurons in the pathway, and their axons project to the cerebral cortex, specifically to the primary somatic sensory cortex. This is the first reception of tactile information in the cerebral cortex.
45
Q

Anterolateral system

A
  • Mediates coarse touch, temperature and pain

- Receives input from A-delta and C-fibres

46
Q

Describe the anterolateral system

A
  1. C-fibre and A-delta fibres enter the dorsal column of the spinal cord and synapses with a second order neuron.
  2. Second order neuron crosses to the other side of the spinal cord
  3. Ascends in the anterolateral tract
47
Q

Where is the primary somatic sensory cortex located?

A

The strip of cortical surface at the anterior end of the parietal lobe, partly buried in the sulcus.

48
Q

What is the somatotopic map?

A

The organisation of projects across the cortex from the sensations of the body.

49
Q

How are the different regions of the cortex identified?

A

Different areas of the cortex have the same cell types with same basic organisation.
Regional differences can be identified on the basis of relative thickness of different layers, cell size and density

50
Q

What is the Brodmann’s area?

A

Defined by its cytoarchitecture in the cerebral cortex. The strip of cortex containing the primary somatosensory cortex has 4 Brodmann areas.

51
Q

Why does the five digits of the hand have more somatotopic map?

A

Due to lots of small densely packed receptive fields covering the digits for high resolution, more cortical territory is needed to process the larger number of high resolution inputs from fingertips compared to less sensitive areas like neck, shoulders and arms.

52
Q

If there is a lesion in the right side of the spinal cord, what will happen?

A
  • Loss of tactile sensation on the right side (A-beta fibres) as the fibres enter on the same side as they ascend
  • Loss of pain and temperature sense (C-fibres) from the left side as the fibres enter one side and cross in the spinal cord then ascend. So if it enters from the left side of the body, it will cross to the right where the lesion is located.
53
Q

Where do the pain receptors project to in the brain?

A

To the ventral-posterior nuclear complex of the thalamus

  • Body: Ventro posterolateral nucleus (VPL)
  • Face: Ventro posteromedial nucleus (VPM)
54
Q

What is the dual-aspect model of pain?

A

This means that there are two ways of defining pain. These are sensory quality and affective-motivational.

55
Q

What is sensory-discriminative pain?

A 
Pain as a sensory aspect. 
- Location
- Intensity (mild or severe) 
- Duration 
- Quality 
These are mediated by C-fibres and A-delta fibres
56
Q

What is affective-motivational pain?

A

Unpleasantness - the painfulness of pain

Effects on arousal, mood and behaviour

57
Q

Nociceptors

A

Specialised for detection of painful stimuli and for potentially tissue-damaging stimuli

58
Q

Which fibres contribute to different aspects of pain sensation?

A

A-delta fibres (thinly myelinated and fast conduction) and C-fibres (unmyelinated and slow conducting)

59
Q

Describe the two wave pain sensation

A

The first wave of pain is immediate and sharp mediated by the A-delta fibres and the secondary wave of pain is initial slower, duller pain mediated by the C-fibres.

60
Q

Which pain can be felt on its own?

A

The first pain can be felt without the second pain.

61
Q

Which receptors are involved in transduction of noxious heat?

A

TRPV1 receptors or vanilloid receptors

62
Q

What are TRPV1 receptors?

A
  • They are tactile mechanoreceptors that have mechanically sensitive ion channels sensitive to stretch.
  • On certain classes of A-delta and C-fibres
63
Q

Describe the action of the Vanilloid receptors

A

Opened by heat as well as other chemicals like acid.

64
Q

What is capsaicin?

A

The active ingredient in chilli peppers that has a binding site on the ion channel by passing the normal thermal activation.
- It is a lipid soluble molecule that can get through the lipid bilayer and bind to the vanilloid receptor.

65
Q

What maintains the depolarisation and sensitivity of C-fibre terminals after the original stimulus?

A

The ‘inflammatory soup’ of cytokines, prostaglandins and small signalling molecules.

66
Q

What other substances is released in nociceptive neurons?

A

Substance P that is released by branches of the axon in the periphery releasing transmitter substances

67
Q

Hyperalgaesia

A

Hypersensitivity to pain

68
Q

Allodynia

A

Pain from non-painful stimuli

69
Q

Normal inoculus stimulus

A

Illicits pain

70
Q

What maintains the depolarisation caused by a burn?

A

Chemicals such as bradykinin acting on the transducer channels maintaining the sensitivity of the fibre

71
Q

Where is the synaptic terminal for C-fibres and A-delta fibres in the anterolateral system?

A

In the dorsal horn

72
Q

What is the dorsal horn?

A
  • Located in the superificial and deep layers of the dorsal horn -> contains the incoming sensory information at the cell bodies
  • Can be subdivided into different layers - dorsal horn interneurons receive input from C-fibres and A-delta fibres
  • Synaptic input from the fibres arrives from a particular origin that is sense specific however, there is a lot of convergent input.
  • Axons cross and ascend in the anterolateral white matter
73
Q

What are multi-model afferents?

A

Receive convergent nocicpetive and non-nociceptive inputs

74
Q

Referred pain

A

Perceiving pain from a different region than the region of origin

75
Q

Angina and referred pain

A

The visceral afferents from the heart will enter into the spinal cord via the sympathetic plexus. They will synapse onto the region of the dorsal horn interneurons that receive input from there. This means they may converge and activity is interpreted by the higher regions as receiving pain from a different region than the origin.
However, if the C-fibres are activated by convergent afferents coming from the viscera, then the pain originating from tissue damage is perceived as originating from some part of the body surface e.g. angina.

76
Q

What is the cortical representation of pain?

A
  • STT projects to S1 (the primary somatosensory cortex) via VP (ventroposterior) nuclei of the thalamus (like the DCML system).
  • However, the STT and DCML axons do not converge on the same thalamic neurons - even though the pathways are parallel.
  • S1 is necessary for the localisation of pain, but stimulation of S1 gives rise to referred tactile, not painful sensations.
77
Q

What are the two systems in central pain processing?

A
  • Lateral system
  • Medial system
    The two systems that diverge at the level of the thalamus.
78
Q

Lateral system

A

Ventroposterior nuclei of thalamus - in parallel with DCML system
Primary and secondary somatosensory cortex (SI and SII)

79
Q

Medial system

A

Midline nuclei of thalamus (intralaminar

Anterior cingulate and insular cortex

80
Q

Which aspects of pain are the lateral and medial systems involved in?

A

Lateral is involved in the sensory-discriminative pain and projects to the somatosensory cortex via the ventral posterior nucleus also via specific somatosensory thalamic nuclei.

Medial is involved in affective-motivational pain and projects to different cortical areas via (non-specific) midline thalamic nuclei

81
Q

How are the pain pathways modulated?

A
  • Analgaestic properties of opium
  • Endogenous opioids and opioid receptors
  • Opiate receptors are located in various positions of the brainstem
82
Q

Treatment of pain

A
  • NSAID and Opiate drugs successful at treating pain as neurophysiological response to tissue damage
  • Tricyclic antidepressants for chronic pain
83
Q

Chronic pain

A

More than 3 months
High prevalence
Due to nerve damage from prior injury: neuropathic pain but it is accepted that pain can be dissociated from tissue damage

84
Q

Phantom limb pain

A

People who have had an amputation can have the sensation that the limb or body part is still there this can be associated to central pain despite it being dissociated from tissue damage

Neuro (20 decks)

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