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Additional Flashcards > Neurology - Somatosensory > Flashcards

Neurology - Somatosensory Flashcards

1
Q

What is a sensory modality

A

A modality is a type of stimulus that has specialised receptors which transmit information through specific anatomical pathways to the brain

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2
Q

What are the different sensory modalities and what receptors detect them

A 
Mechanoreceptors
     - Touch
     - Pressure
     - Vibration
     - Proprioception (joint position, muscle length, muscle tension)
Thermoreceptor
     - Temperature
Nociceptor
     - Nociception
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3
Q

What are the three types of sensory fibres

A

AB - thickest, myelinated, fastest transmission - mechanoreceptors of the skin
Ad - thick, myelinated, fast transmission - pain temperature
C - thin, slow, unmyelinated - temperature, pain and itch
A peripheral nerve will have all of these different fibres in it

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4
Q

What are the different types of nerve endings

A

C fibres have free nerve endings which are very close to the skin and are responsive to heat
Mechanoreceptor ending are encapuslated

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5
Q

What is the absolute threshold

A

The level of stimulus that produces a positive result 50% of the time
This will create a generator potential which will depolarise the nerve
The stronger the stimulus the more neurotransmitter is released

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6
Q

Explain the functionality of thermoreceptors

A
  • Free nerve ending with high thermal sensitivity
  • Change in temperature activates a family of transient receptor potential ion channels
  • There are 4 heat activated TRP channels (TRPV1-4) ranging from low heat to high heat
  • There are 2 cold activated TRP channels (TRPM8, TRPA1)
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7
Q

What are the different types of mechanoreceptors and what do they sense for

A
  • Meissner’s corpuscle - fine discriminative touch
  • Merkel cells - light touch and superficial pressure
  • Pacinian corpuscle - detects deep pressure, vibration and tickling
  • Ruffini endings - continuous pressure or touch and stretch
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8
Q

What are the two types of adaptation receptors

A

Tonic receptors
- Detect continous signal strength
- Continue to transmit impulses as long as the stimulus is present
- keeps the brain informed of the status of the body
- e.g. merkel cells - slowly adapt allowing for fine touch to be perceived
Phasic receptors
- Detect changes in stimulus strength
- Transmit an impulse at the start and at the end (when a chane is taking place)
- Also called movement receptors or rate receptors
- e.g. Pacinian corpuscle - sudden pressure excites receptor, transmits a signal again when pressure is released

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9
Q

What are somatosensory dermatomes

A
  • Each spinal nerve has a specific dermatome on the skin

- Each spinal nerve innervates a certain level in the spinal cord

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10
Q

What are receptive fields

A
  • The receptive field is the region of skin which causes activation of a single sensory neuron when activated
  • There are different size receptive fields on the body e.g. smaller on the fingers to allow fine touch but much larger on the back
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11
Q

Define two point discrimination

A

The minimum distance at which two points are perceived as separate (relative to the size of the receptive field)

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12
Q

What are the different fibre types used for in nociception

A

Ad fibres mediate sharp, intense or first pain
- Myelinated
- Type 1 : ad-mechanoheat receptors (noxious mechanical and thermal stimuli)
- Type 2 : mechanoreceptors (noxious mechanical stimuli)
C fibres mediate dull, persistent or second pain
- Unmyelinated
- Respond to thermal, mechanical and chemical stimuli (polymodal)

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13
Q

Where are sensory cell bodies found within the body and face

A

In the body they are in the dorsal root ganglia

In the face they are in the trigeminal ganglia

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14
Q

How are the sensory pathways organised within the dorsal horn

A

Organised into layers - the rexed laminae (1-7)

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15
Q

What terminates into each of the lamina

A

Pain and temperature (Ad and C fibres) terminate in laminae 1-2 (superficial)
Innocuous mechanical stimuli (AB fibres (and Aa)) termiante in laminae 3-6 (deep)

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16
Q

What connects between the different laminae and adjacent peripheral inputs

A

Inter neurons

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17
Q

What is lateral inhibition and why does it occur

A
  • Receptive fields can overlap and make it difficult to establish between two stimuli locations
  • Lateral inhibition prevents overlap and facilitates pinpoint localisation of the stimulus
  • It is mediated by interneurons within the dorsal horn of the spinal cord
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18
Q

What are the central sensory structures and where are they found

A 
Primary somatosensory cortex
     - In the postcentral gyrus
Secondary somatosensory cortex
     - In the parietal operculum
Posterior parietal complex
     - Spatial awareness of the body
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19
Q

What is the function of the dorsal column system

A
  • Transmits innocuous mechanical stimuli (fine discriminative touch, vibration)
  • AB fibres enter via the dorsal horn and enter the ascending dorsal column pathways
  • Information from the lower limbs and body (below T6) travel ipsilaterally along the gracile tract
  • Information conveyed from the upper limbs and body (above T6) travel ipsilaterally along the cuneate tract
20
Q

Where do first order neurons of the dorsal column system terminate

A

First order neruons terminate in the medulla

  • Fibres in the gracile tract have their first synapse in the Gracile nucleus
  • Fibres in the Cuneate tract have their first synapse in the Cuneate nucleus
21
Q

Where do second order neurons of the dorsal column system cross and terminate

A

Second order neurons cross in the medulla
- Second order axons decussate in the caudal medulla
- Forms the contralateral medial lensicus tract
Second order neurons terminate in the thalamus
- Axons of second order neurons terminate in the ventral posterior lateral nucleus of the thalamus
- There is a topographical representation of the body in this nucleus as the limbs terminate more laterally

22
Q

Where do third order neurons of the dorsal column system terminate

A

Third order neurons terminate in the somatosensory cortex

  • Third order neurons from the VPL project into the somatosensory cortex
  • Size of the somatotopic areas is proprtional to the density of sensory receptors in that body region
23
Q

What is the function of the spinothalamic (anterolateral) pathway

A
  • Pain and temperature sensations ascend within the lateral spinothalamic tract
  • Crude touch ascends within the anterior spinothalamic tract
24
Q

Where do first order neurons of the spinothalamic pathway terminate

A

First order neurons of the spinothalamic pathway terminate in the dorsal horn

  • Primary afferent axons terminate upon entering the spinal cord
  • Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract
25
Q

Where do second order neurons of the spinothalamic tract terminate

A

Second order neurons of the spinothalamic pathway terminate in the thalamus

  • Second order neurons terminate in the ventral posterior lateral nucleus of the thalamus
  • There is a topographic representation of the body (lower extremities are lateral)
26
Q

What are the key differences between the spinothalamic tract and the dorsal column

A

The spinothalamic tract involves pain, temperature, and course touch and crosses in the spinal cord whereas the dorsal column is involved with light touch, vibration and 2-point discrimination and crosses over in the brainstem

27
Q

Describe the pathway of pain

A

Pain is transmitted though the spinal cord to the parabrachial area in the brain stem and then to the limbic system

28
Q

What cortexes are involved in pain

A

Primary somatosensory cortex, secondary somatosensory cortex, insula cortex, anterior cingulate cortex and prefrontal cortex
Also the amygdala, cerebellum, brainstem

29
Q

How would you test the integrity of the ascending somatosensory pathways

A
  • Use a set of tools to look into different sensory modalities
  • This allows to detect what parts of the spinal cord are damaged, and from what level
30
Q

What are electrical perceptual thresholds

A

Electric currents are sent through the skin and the patient can either feel it or not feel it, this can then be mapped for the whole body

31
Q

Define pain

A

An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage

32
Q

What are the different types of pain

A
  • Nociceptive - tissue damage, typically acute (e.g. skin cut)
  • Muscle - lactic acidosis, ischaemia (e.g. stretching, fibromyalgia)
  • Somatic - well-localised (e.g. inflammation, infection)
  • Visceral - deep, poorly localised (e.g. stomach, colon, IBS)
  • Referred - from an internal structure/organ (e.g. angina)
  • Neuropathic - dysfunction of the nervous system
  • Chronic - pain lasting at least 6 months
33
Q

How does chronic pain manifest

A

Chronic pain manifests as hyperalgesia (higher levels of pain) and allodynia (pain from stimuli that would not normally cause pain)
Also has an emotional aspect of anxiety and depression

34
Q

What causes neuropathic pain

A

Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system

  • Pain in area of neurological dysfunction
  • Sharp, burning, electric shocks
  • Poor response to usual analgesic drugs (e.g. opiates)
35
Q

Give some examples of neuropathic pain

A
  • Radicular lower back pain (sciatica)
  • Diabetic neuropathy
  • Post herpetic neuralgia
  • post-surgical pain
  • HIV-induced neuropathy
  • Chemotherapy induced neuropathy
  • Complex regional pain syndrome (CRPS)
36
Q

How is neuropathic pain diagnosed and assessed

A

Using a variety of screening tools

  • Questionnaires
  • Simple sensory testing devices (pin prick, brush, heat)
37
Q

How can chronic pain be developed as a result of synaptic plasticity

A
  • Initiated by NMDA receptor activation
  • Ca2+ mediated synaptic plasticity in dorsal horn neurons
  • Increased synaptic strength (efficacy)
  • Reduced inhibitory influences on dorsal horn neurons
  • Persistent activation of NMDA receptors can result in the development of chronic pain (e.g. arthritis)
38
Q

What causes hyperalgesia and allodynia

A

The loss of inhibition on the pathway responsible for pain causes increased pain sensation

39
Q

What is the neuropathic phenotype

A

The neuropathic phenotype is the type of pain and how it has changed

40
Q

How can chronic pain be reduced

A

Chronic pain can be reduced by descending modulation using monoamines in the brainstem which inhibit spinal cord excitability

41
Q

What are the two ways in which monoamines are released into the brainstem

A 
PAG-RVM axis
- 5-HT release
Locus cereleus
- In the pons
- Uses noradrenaline as well as serotonin
42
Q

How do opioids increase descending inhibition

A
  • The PAG and RVM contain large amounts of u opioid receptors
  • Endogenous opioids enhance the descending inhibition from the PAG-RVM axis
  • This reduces pain transmission in the dorsal horn by inhibiting glutamate release
  • This forms part of the endogenous analgesic system
43
Q

How are descending control systems targeted for pain relief

A
  • Opioids are used tp target the PAG-RVM axis
  • SNRIs enhance descending noradrenergic inhibition which increases noradrenaline which activates more alpha 2 receptors which have an inhibitory effect
44
Q

How is conditioned pain modulation used to measure the level of descending control in patients

A

The better the descending control the better the efficacy of duloxetine (SNRI)

45
Q

How is transcranial direct current stimulation used to reduce chronic pain

A

This is non-invasive brain stimulation that causes changes in cortical excitability in the primary motor cortex that has been shown to reduce chronic pain in fibromyalgia and migraine patients

46
Q

What are the three clusters of neuropathic pain

A
  • Sensory loss
  • Thermal hyperalgesia
  • Mechanical hyperalgesia

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