FINISH Somatic sensation and pain Flashcards
Meissner’s corpuscles
Lies close to the surface of glabrous skin
Small receptive field
Adapts rapidly
Sensitive to low-frequency mechanical stimuli (“flutter”; about 10-100 Hz),
Innervated by large (Aß) fibres classified as “RA I” fibres.
Pacinian corpuscles
Found in the skin, but also in other sites (e.g. the mesentery).
In the skin lies deep in the dermis
Large receptive field. Central zone of maximal sensitivity surrounded by a large continuous surface
Adapts very rapidly
Sensitive to high-frequency mechanical stimulation
‘Desheathed’ PC (with lamellae removed) show loss of rapid adaptation.
Merkel cells
Lie close to the surface of glabrous skin
Small receptive field
Adapts slowly
Sensitive to sustained pressure
Innervated by large (Aß) fibres classified as “SA I”
Each receptor is composed of a number of epithelial cells (receptor cells) which are in synaptic contact with the terminals of sensory nerves. The receptor cells are thought to be the site of the initial transduction of the mechanical stimulus
SAI fibres most accurate at reading braille
Ruffini endings
Lies deep in the dermis
Large receptive field,
Adapts slowly
Sensitive to lateral stretching of the skin
Innervated by large (Aß) fibres classified as “SA II”
Adaptation
Difference between rapidly adapting and slowly adapting?
The decline in the response of a sense organ to a steady stimulus
Rapidly adapting receptors respond only to the onset of the stimulus
Slowly adapting receptors give a tonic response to a steady stimulus
Phase locking
A rapidly adapting receptors responds to a low frequency sinusoidal mechanical stimuli with a single action potential for each phase of the stimulus- effectively treats each period of the waveform as a new stimulus.
NB intensity of simusoidal stimulus must therefore be encoded by the number of sensory fibres active, rather than the frequency of firing. Number of active fibres ∝ amplitude of vibration.
Accessory structures
Structural components of sense organs which may play an important part in protection, conduction, concentration, analysis, sensitization or inhibition; but are not directly involved in the transduction process whereby stimulus energy is encoded into electrical changes in the receptor cell or sensory neuron.
e.g lamellae of the Pacinian corpuscle, intrafusal fibres of the muscle spindle, all structures of the eye other than the rods, cones, and nerve cells of the retina.
Two point limen
‘Compass test’
The smallest discriminable distance between 2 points of contact. A measure of tactile acuity
Shoulder: ~40mm. Fingers: ~2mm.
Acuity appears to increase with increase in mobility of body parts (holds less for lower extremities)
RAI and SAI fibres have small receptive fields, and the highest density on the fingertips.
Warmth and cold spots
Concentration of cold spots far > warm spots.
Different body areas have different proportions of warmth & cold spots e.g lip has 6x as many cold spots as the sole.
It is thought (especially for warmth) that many more receptors exist than there are spots, & that is requires the simultaneous activation of many receptors to elicit the sensation of warmth- spatial summation.
‘Cold’ receptors connected to Aδ and C fibres
‘Warm’ receptors connected to a sub-population od C fibres.
Spatial summation.
-
Trpv1 channels
Temperature receptor
Respond to capsaicin, and to painful increases in temp >43 degrees
Trpm8 channels
Temperature receptor
Respond to menthol, non painful decreases in temp below 25 degrees.
Paradoxical cold
-
Labelled line coding
-
Nociceptors
-
Aβ vs Aδ vs C
Aβ: myelinated, large. Touch & proprioception. Not pain
Aδ: unmyelinated, smaller. Cold & stabbing pain
C: unmyelinated, smallest. Warmth, itch, burning pain.
C most numerous, Aβ and Aδ about equal.
Anoxia affects Aβ > Aδ > C
Local anaesthetic affects C > Aδ > Aβ
Cauda equina
Lies below approximately L2
Consists of elongated spinal roots from the lumbosacral spinal cord.
Lumbar puncture
Hollow needles can be inserted below L2 into the subarachnoid space to remove CSF for diagnostic purposes
Epidural
Anaesthetics may be introduced into epidural space below L2 in surgical procedures
Dermatone
Area of skin innervated by a single dorsal root.
Dermatonal boundaries overlap
Internal structure of the spinal cord
Divided into 2 symmetrical halves by the dorsal median sulcus & the ventral median fissure
‘H’ shaped grey matter (nerve cell bodies) surrounds the central canal. Divided into functionally distinct laminae- Rexed’s laminae
Afferent & efferent axons run in the white matter. White matter divided into dorsal, lateral & ventral, defined relative to the grey matter.
Main pathway for info about touch & proprioception
Dorsal column- medial lemniscal system (DC-ML)
- large diameter myelinated fibres
- tactile, vibratory & proprioceptive sensations.
Short branch enters dorsal horn
Long branch enters dorsal columns
- those that enter below mid-thoracic level ascend in fasciculus gracilis, terminate in gracile nucleus
- those that enter above mid-thoracic level enter fasciculus cuneatus, terminate in cuneate nucleus.
Dorsal column nuclei = the cuneate and gracile nuclei.
Cells in the dorsal column nuclei are organised somatotopically - with leg located medially & arm laterally. This somatotopic organisation is preserved at all levels of the pathway.
On leaving the dorsal column nuclei the axons cross the brainstem, ascend to the thalamus in the medial lemniscus. Terminate in the ventral posterior nucleus.
Major ascending nociceptive pathway
= Spinothalamic tract aka anterolateral system
Axons from neurones in layers I and V-VII of dorsal horn ascend in the contralateral, anterolateral white matter.
Projection neurons in lamina I receive input from myelinated Aδ nociceptive fibres, and direct & indirect input from C fibres.
Lamina V neurons receive input from Aβ (mechanoreceptors) & info from nociceptors. Therefore respond to innocuous stimuli at low intensity & noxious stimuli at high intensities- wide dynamic range neurones.
Lesions reduce pain sensations in the contralateral half of the body. Unfortunately pain relief often only temporary.
Spinoreticular tract
Projects from laminae VII and VIII.
Terminates in the reticular formation & thalamus.
Some axons do not cross the mid-line, travel ipsilaterally.
Spinomesencephalic tract
Projects from laminae I and V.
Via anterolateral quadrant of the spinal cord
To the mesencephalic reticular formation & periaqueductal gray.
Referred pain
= pain felt in a part of the body other than its actual source
e.g angina pectoris.
Due to signals from viscera & signals from distant somatic structures converging on same neurones in spinal cord. CNS cannot distinguish between superficial & deep pain.
Brown-Sequard syndrome
Caused by hemi-section through spinal column.
Affects both DC-ML & spinothalamic tract on same side of the cord.
Loss of pain & temp sensation below & contralateral to site of the lesion.
Loss of fine touch & proprioception below & ipsilateral to site of lesion.
PACT TIP.
Syringomyelia
Caused by fluid filled cavity within the spinal (usually cervical) cord.
Usually disrupts decussating fibres of the anterolateral system but not ascending fibres of DC-ML.
‘Cape-like’ distribution of loss of pain and temp sensation in upper limbs & trunk, but preservation of touch & pressure sensation.
Posterior column syndrome
(Tabes dorsalis- 3’ syphilis). Affects dorsal columns bilaterally.
Bilateral absence of touch below level of lesion.
Loss of proprioceptive feedback below site of lesion -> characteristic stamping gait.
Complete transection of the spinal cord
Impairment of all sensory modalities below the level of the transection.
Trigeminal neuralgia
aka Tic douloureux.
Syndrome whereby gentle stroking of face/ mouth provokes massive stabbing pain.
NB opthalmic & maxillary branches of trigeminal N are purely sensory, mandibular is a mixed sensory & motor branch.
Allodynia
A condition in which a gentle stimulus which does not normally provoke pain, such as a breeze or
the touch of clothes, causes intense pain
Hyperalgesia
The evocation of pain by a either weak and otherwise innocuous stimulus to the skin or stimuli
which would normally produce only mild pain (published definitions differ). It is a frequent consequence of
nerve injury
Both crosstalk (between high- and low-threshold sensory channels in the peripheral or CNS) & sensitisation (decreased threshold) have been implicated.
Lateral inhibition
Reciprocal suppression of excitation by neighbouring neurons in a sensory network; the
effect is enhancement of contrast sensitivity, and an increase in dynamic range - at the cost of the ability to
make judgements about absolute levels of stimulus intensity.
Thalamus- role in pain & temp
DC-ML fibres terminate in ventral posterior nucleus.
Posterior part of ventral medial nucleus (VMPO) is involved in pain & temp sensation.
Study has shown that cluster of neurones in VMPO are inhibited by radiant warming of the contralateral tongue, but excited by cooling.
A single nociceptive neurone shows increases in discharge rate with increases in temperature of noxious heat pulses. Receptive field was located on the contralateral ulnar hand.
Gate control
- Failure of one input may unmask the presence of inputs normally suppressed by ‘inhibitory’ mechanisms
Higher touch pathways: Thalamus and cortex
Primary somatosensory cortex (S-I):
- located in post central gyrus
- Brodman’s area 3, 1 and 2.
Thalamic afferents terminate in layer IV.
Layer VI projects back to the thalamus (carrying more info than originally came from thalamus)
Layers II and III project to other cortical regions.
Layer V projects to subcortical structures.
Post central gyrus
Located immediately posterior to the central sulcus (fissure).
Primary somatosensory cortex located here.
Organisation of cortex
Organised into a series of vertical columns, 300-600μm wide, spanning all 6 layers from cortical surface to white matter.
All neurons within a column:
- receive input from the same area of skin (thus preserving stimulus location)
- only respond to a single modality e.g touch, temp (preserving stimulus modality)
Homunculus
= map of somatosensory inputs to the cortex
In the human much area devoted to the hands & face, in rodents whiskers, in star nosed mole huge nose.
Feature detection & attention in somatosensory cortex
Neuron in area 2 of S-I has been found to be direction-sensitive: responds strongly to movement from ulnar side of wrist (UW) -> radial fingers (RF). Mocving RF -> UW produced the smallest response
Attention can alter responses of cortical neurons in S-II.
Response of a single neuron to a tactile stimulus greatly diminished when simultaneously performing a visual task.
Plasticity in somatosensory cortex
-
Phantom limbs
-
Tinnitus
A phantom auditory sensation with many similarities to phantom limbs
Cortical areas involved in pain
S-I
Anterior cingulate cortex
Insula
Anterior cingulate cortex and pain
Part of the limbic system
Emotional/ motivational element of pain
Single neuron recordings & microstimulation were carried out in ACC of human patients undergoing cingulotomy.
- Responds to heat stimuli in the noxious range (above 48 degrees. 44 and 46 reported as warm, not painful)
- responded to experiencing pain > watching delivery of painful stimuli to the examiner.
Insular cortex and pain
Processes info on the internal state of the body
Contributes to autonomic component of overall pain response.
Pain- definition
‘A percept, an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage’
Gate control theory of pain
-
Descending systems that modulate pain
Periaqueductal gray (PAG) matter of the mid-brain
Raphe nuclei
Other nuclei of the rostral medulla.
PAG
Electrical stimulation can produce sufficient analgesia to perform abdominal surgery without anaesthesia. Other, non-painful sensations left intact.
Thought to control ‘nociceptive gate’ in the dorsal horn by integrating inputs from cortex, thalamus & hypothalamus.
Morphine- induced analgesia is blocked by injection of naloxone (an opiate antagonist) into PAG.
Bilateral transection of the dorsolateral funiculus blocks both stimulation- and morphine- induced analgesia.
