5.0 Somatic Sensation Flashcards
What skin layers are mechanoreceptors found?
Epidermis + Dermis
Slowly adapting vs rapidly adapting receptors?
Rapidly adapting receptors respond only to onset of stimulus<br></br><br></br>Slowly adapting fibres respond to steady stimulus
Define accessory structure:
A structural component that is not involved in transduction directly however plays a role in other aspects such as protection, sensitivity, conduction etc
What are the four different mechanoreceptors?
1) Merkel cell (Slow adapting type I)<br></br>2) Ruffini endings (Slow adapting type II)<br></br>3) Meissner’s corpuscles (Rapid adapting type I)<br></br>4) Pacinian corpuscules (Rapid adapting type II)
What is tactile acuity?
Being able to differentiate 2 points (hands + feet > back)<br></br><br></br>Areas with high tactile acuity have small receptive fields
Define two-point limen:
Smallest discriminable distance between two points of contact (different in different areas smaller in more mobile parts of body - e.g. hands)
What thermoreceptors respond to warm?
1) TRPV1<br></br><br></br>2) TRPV2
What thermoreceptors respond to cold?
1) TRPM8<br></br><br></br>2) Anktm1
What does the TRPV1 receptor respond to?
1) Capsaicin (in chillis)<br></br>2) Painfully hot temp (>45 degrees)
What does the TRPM8 receptor respond to?
1) Menthol<br></br>2) Non-painful temp <25 degrees
What receptor responds to Horseradish (Isothiocyanates)?
TRPA1
Define labelled line coding:
Activity in a particular fibre will always be experienced as the modality it encodes irrespective of actual stimulus.<br></br><br></br>E.g. Activity in cold fibre will always be experienced as cold (even if activity is due to hot stimulus)
What are the nociceptive fibres?
<b>1) Aδ fibre</b><br></br>Thinly myelinated<br></br>‘First’ pain<br></br><br></br><b>2) C fibre</b><br></br>Unmyelinated<br></br>‘Second’ pain<br></br>Polymodal (respond to temp, mechanical and chemical)
Arrange axons in order of their diameter:
Aα > Aβ > Aδ > C
What fibres are most affected by anoxia?
Aβ
What fibres are most affected by anaesthetic?
C + Aδ
What conditions can show myelin sheath degeneration?
Diabetes<br></br>Multiple sclerosis
Define dermatome:
Area of skin supplied by a single dorsal root
What makes up the grey matter in the spinal cord?
Nerve cell bodies<br></br>Divided functionally into distinct laminae (Rexed’s laminae)
What makes up the white matter in the spinal cord?
Afferent and efferent axons<br></br>Divided into dorsal, lateral and ventral regions (relative to grey matter)
Which pathway transmits touch, vibration and proprioception?
Dorsal column - medial lemniscal
Which pathway transmits pain and temperature?
Spinothalamic pathway (anterolateral pathway)
What is referred pain?
<b>Referred pain is the incorrect assignment of pain in a healthy somatic area.</b><br></br><br></br>Due to the inability of the CNS to distinguish between superficial and deep pain due to the convergence of fibres (on the dorsal horn) from inflammed viscera and distant somatic structures
Symptoms of Brown-Sequard syndrome:
“Brown-Sequard syndrome = hemisection<br></br><br></br>1) Loss of <b>pain and temperature</b> below and <b>contralateral</b> to site of lesion<br></br>2) Loss of <b>fine touch and proprioception</b> below and <b>ipsilateral</b> to lesion<div><br></br></div><div><img></img></div>”
Symptoms of syringomyelia:
“Syringomyeli = fluid filled cavity in spinal cord (usually cervical region)<br></br><br></br>Disrupts the cross over in the spinothalamic tract<br></br>Results in <b>loss of pain and temperature</b> in a cape like lesion of upper limb and trunk<div><br></br></div><div><img></img></div>”
Symptoms of posterior column syndrome:
“Posterior column syndrome = <b>Tabes dorsalis</b> (tertiary syphilis)<br></br><br></br>Causes <b>bilateral absence of touch and proprioception below lesion</b><br></br>→ stamping gait<div><br></br></div><div><img></img></div>”
Symptoms of complete transection:
Loss of all sensory modalities below level of transection
What is the role of lateral inhibition in sensation?
Stimulation of a single point on skin → excitatory response surrounded by small inhibitory response.<br></br><br></br>Thus when stimulating two adjacent points of skin, lateral inhibition supresses the excitation of neurons between the points, thus ↑ contrast.<br></br><br></br>Important for reading Braille
Where do the fibres of the DC-ML system terminate?
<b>Ventral posterior nucleus</b> of thalamus
Where do the fibres of the spinothalamic system terminate?
”"”Posterior part of ventral medial nucleus (VMPO)* of thalamus”
What is immediate unmasking?
“Removal of a CNS cell’s normal input can the reveal the presence of other inputs. These other inputs are normally ““masked”” by lateral inhibition”
What are the layers in the S1 cortex?
“<div><img></img></div>”
What is a column in the S1 cortex?
Spans all 6 layers. <br></br><br></br>-Receives input from same area of skin <b>(preserves location)</b>
What is the homonculus?
Map of somatosensory inputs to the cortex
What is a phantom limb?
When amputees have a sense of the missing limb.<br></br><br></br>The theory is that another region in the body takes over and innervates the somatosensory resion in the cortex that used to be innervated by the limb<br></br><br></br>This is possible because the cortex is plastic
What cortical areas are involved with pain?
1) S1<br></br>2) Anterior cingulate cortex (ACC)<br></br>3) Insula
Define pain:
An unpleasant sensory + emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What layers of Rexed Laminae receive nociceptive fibres?
<b>Layer 1</b><br></br>From Aδ and C fibres<br></br>Narrow dynamic range (NDR)<br></br><br></br><b>Layer 2</b><br></br>From Aβ and other nociceptive fibres<br></br>Wide dynamic range (WDR) - can respond to normal and noxious
What descending systems modulate pain?
<b>1) Periaqueductal gray</b><br></br>Stimulation → analgesia<br></br><br></br><b>2) Raphe nuclei</b><br></br><br></br><b>3) Other nuclei of rostral medulla</b>
Define placebo:
Analgesic response is achieved after administering a substance known to be a non-analgesic
