Somatic Sensation and Pain Flashcards
Define asterognosia
Inability to identify objects by touch
What are the 4 types of cutaneous mechanoreceptor in glabrous skin?
Slowly adapting type 1 (SA1) afferents that end in Merkel cells
Rapidly adapting RAI afferents that end in Meissner’s corpuscles
Rapidly adapting RAII Pascinian corpuscles
Slowly adapting type 2 SAII afferents that end in Ruffini endings
Discuss pascinian corpuscles Stim type they respond to Depth in the skin Sensitivity Receptive field Afferent Density
Respond to object held in the hand, good at detecting successive quick events, not to detect ongoing stimulation (but does detect high freq stim)
Deepest
Very, respond to 10nm motion at 200Hz
Large, central zone of maximum sensitivity
RAII 1:1
350 in finger, 800 in palm
Discuss Meissner's corpuscles Stim type they respond to Depth in the skin Sensitivity Receptive field Afferent Density
Low frequency vibration, I.e. perception of slip used for feedback for grip
x
High: enhanced sensitivity, poor spatial resolution
Small 3-5mm
RAI
High, 150/cm2
Discuss Merkel cells Stim type they respond to Depth in the skin Sensitivity Receptive field Afferent Density Size
Indentation: points/edges/curvature
x
Lower - higher spatial resolution. 10x more sensitive to dynamic stimuli.
Small, highly localised RF (0.5mm resolution)
SAI 20:1
Densely innervate skin
10s of micrometers
Discuss Ruffini endings Stim type they respond to Depth in the skin Sensitivity Receptive field Afferent Density Size
Hand shape and finger position, respond to stretching skin in a certain direction Deep x x SAII 1:1 x Few 100 micrometers
What does the lamellae of the pascinian corpuscle contribute and how do we know?
Know: peel then away
Find: when removed, they act like slowly adapting fibres. With lamella, in response to a stimulus onset or offset, the RP rises then decays quickly. Direct from the nerve ending, the stimulus triggers RP rising and then decaying slowly.
Define accessory structures
Structural components of sense organs which may play an important role in protection, conduction, concentration, analysis, sensitisation or inhibition, but that are not directly involved in the transduction process.
Name an accessory structure in somatosensation, proprioception, vision, auditory
Lamellae in PCs
Intrafusal muscle fibres
Eye structures
Basilar membrane
How can you detect vibration?
RA afferents
What is the evidence for the existence of 2 RA channels?
Altering the sensitivity of detection of vibration
Normally peak sensitivity Meissner’s 30Hz ish, Pascinian 300Hz (i.e. area at lowest threshold for dep)
Add local anaesthetic, increases threshold for Meissner’s as less deep in the skin
Stim for a while at high freq (for PC) or low freq (Meissner’s) –> adaptation of the relative frequency,
Which afferent is best at reading Braille? How do we know?
Merkel
Show on spatial event plots: AP evoked by Braille
What is the test of tactile acuity? What sets tactile acuity?
Two point limen
Receptive field size - if two points contacting the skin both stim the same receptive field, we have no information that two points on the skin were stimulated
Are there more warm or cold spots?
Cold, but different body areas have different proportions of cold and warm spots
What is the structure of thermoreceptors?
Unencapsulated nerve endings
What is spatial summation?
Many more receptors exist than there are responses, and it normally requires the simultaneous activation of many receptors to get the response
What are the different thermoreceptor channels? What are their afferents?
Hot = capsaicin/above 43 degrees channel Trpv1. Sub population of C fibres
Cold = menthol/below 25 degrees = Trpm8. Adelta and C fibres
There are more, overlapping to create a range
Explain paradoxical cold spots. What does this tell us?
When a heat stim of >45 degrees is applied to a single cold spot, feels cold.
Cold receptor has a parabola shaped sensitivity curve
Tells us that activity in the cold fibre is labelled line
Define labelled line
Activity in a fibre is experienced in the same way, irrespective of the physical nature of the stimulus
Our sensory experience is determined by the central connections of the neuron, not the stimulus that evoked the AP
What are nociceptors?
Pain fibres
Free nerve endings
How do you separate the different aspects of pain? Which fibres mediate each aspect?
- Early first pain - sharp. Adelta
2. Second dull pain - burning. C fibres
What do Adelta fibres mediate in sensation?
Cold, first component of pain
What do C fibres mediate in sensation?
Warm, cold, second component of pain
Most C fibres are polymodal and respond to thermal, strong mechanical and chemical stimuli (chilli peppers, acid)
Describe the different types of peripheral nerve
AalphaAbeta: 40-80m/s, myelinated large. Touch, proprioception
Adelta: 5-30m/s, thinly myelinated. Cold, stabbing pain
C: 0.5-2m/s unmyelinated. Warmth, itch, burning pain
What is a compound action potential?
A sum of all fo the action potentials in a nerve
Which kinds of nerve fibres do anoxia and local anaesthetic affect?
Anoxia = large
Local anaesthesia = small unmyelinated shallow
Where are the spinal cord enlargements and what causes them?
Cervical and lumbar
Where the limbs join
Define dermatome
Area of skin innervated by a single dorsal root. They overlap as several roots can innervate a peripheral nerve.
What splits the spinal cord in half?
Dorsal median sulcus and ventral median fissure
What is the grey matter divided into?
Rexed's laminae I- VI dorsal horn VII intermediate zone VIII IX ventral horn X ventral commissure
What is the main ascending pathway for touch and proprioception?
DC-ML dorsal column medial lemniscal
Describe the DC-ML pathway
- Primary sensory axons enter spinal cord
- Primary afferent bifurcates
- Small branch enters dorsal horn, large branch enters dorsal columns
- Large branch below mid-thoracic = ascend in fasciculus gracilis gracile nucleus
- Large branch above mid-thoracic = ascend in fasciculus cuneatus cuneate nucleus
- Leave dorsal column nuclei (these in lower medulla) as second order neurons
- Sensory decussation: cross brainstem
- Ascend in fast-conducting medial lemniscus to thalamus
- In ventral posterior nucleus of the thalamus, synapse on third order neurons
- Internal capsule
- Primary somatosensory cortex in postcentral gyrus of the parietal lobe
How are the dorsal column nuclei organised
Dorsal column nuclei somatotopically organised with leg medially and arm laterally
What does the DCML carry?
Tactile, vibratory, proprioceptive
What are the major ascending nociceptive pathways?
Spinothalamic tract/Anterolateral system
Spinoreticular
Spinomesencephalic
Describe the spinothalamic pathway
- Primary sensory axons enter spinal cord
- Bifurcate into short ascending and descending branches that run for about a spinal segment in Lissauer’s tract
- Branches terminate in the superficial part of the spinal cord dorsal horn: substantia gelatinosa
- One local synaptic relay in superficial dorsal horn
- Second order neurons arise on IPSILATERAL SIDE
- Neurons in layers I and V-VII of dorsal horn
i. V = tactile system via wide dynamic range neurons too
ii. I = nociception specific - These second order neurons cross ventral commissure to contralateral anterolateral white matter
- Ascend in this lateral column
- Synapse in thalamus. VMPO post part ventromedial nucleus
- Third order neurons ascend via internal capsule
- Project to SI, also anterior cingulate cortex and insula
Where is the spinoreticular tract to and from?
From laminae VII and VIII
Terminates in reticular formation and thalamus
Some ipsilateral
Influences level of arousal through actions on sympathetic system and other ascending systems.
Where is the spinomesencephalic tract to and from?
Laminae I and V
Anterolateral quadrant of spinal cord
Mesencephalic reticular formation and periaqueductal gray
Periaqueductal gray modulates anterolateral ascending system (a descending pathway)
What are the two extra cell groups found in the thoracic cord?
Intermediolateral nucleus: sympathetic pre ganglionic neurons, with axons that run in the sympathetic chain
Clarke’s nucleus: relay cells for proprioception of the lower limbs
Describe the shape of the spinal cord at each level?
Cervical: oval, large, thick white matter
Thoracic and upper lumbar: circular, thin H profile of grey matter
Lumbosacral: Circular, expanded grey matter, white matter thinning out
Lower sacral: circular, little white matter
What is the substantia gelatinosa?
Rexed’s lamina: Layer II
Processing of noxious information - allows modulation and gating of pain processing e.g. through actions of endogenous opioid peptides.
Consists of unmyelinated fibres, fine cell processes, very small cell bodies
Pale, jelly-like appearance
What is Lissauer’s tract?
Contains incoming axons carrying pain or temperature information
Axons travel up or down the cord to an adjacent segment before entering the dorsal horn
What do layers IV and V do?
Contain relay neurons with many axons that cross the midline in the ventral commissure to ascend in the anterolateral column
What does lamina VII do?
Centre of spinal cord
Spinal interneurons concerned with local processing e.g. reflexes
How many neurons are involved in the dorsal column pathway?
3
Explain referred pain
Pain from internal organs being felt in a more superficial region
Signals from an inflamed visceral organ converge on projection neurons at the dorsal horn of the spinal cord.
Sensory input from a distant somatic structure converges on the same neurons
CNS cannot distinguish between superficial and deep pain, and failure results in incorrect assignment of pain to the healthy somatic area
Where is angina pectoris pain referred to?
Chest and left arm
Describe Brown-Sequard syndrome
Consequence of a spinal hemi-section
Affects both spinothalamic and DC-ML
As DC-ML ascends on ipsilateral, affects ipsilateral touch/proprioception
Spinothalamic ascends contralateral, so affects contralateral pain/temperature sensation
both BELOW the lesion
Describe infarction of the inferior cerebellar arteries
DCML and anterolateral system still separate through medulla, so a similar separation of tactile and nocicipetive sensation can follow damage here
Infarction ICA can damage laterally located anterolateral system: tactile system intact. Could sense contralateral pin prick as a gentle touch
Describe syringomelia
Central canal expands
Damages crossing anterolateral axons (pain and temp) crossing ventral commissure
Cape like distribution of loss of pain and temperature sensation in upper limbs and trunk but preservation of touch and pressure sensation
Describe tabes dorsalis
Posterior column syndrome
In late stages of syphilis
Causes a degeneration of central projections from the dorsal root ganglia esp in fasciculus gracilis and cuneatus.
Bilateral loss of touch below level of lesion, and also loss of proprioceptive feedback leading to a characteristic stamping gait.
What are spinal dural arteriovenous fistulae?
Relatively rare vascular malformations due to an abnormal connection between a meningeal branch of a segmental artery that normally supplies the dura, and the intradural radicomedullary vein that normally drains the spine. Lesion –> high pressure blood flows retrograde into the coronal venous plexus of the spinal cord, leading to venous congestion, oedema, and spinal cord injury.
Present with sensory disturbances due to dorsal location, but if left untreated, progress to cause motor weakness.
Treat by dividing abnormal connection.
What can vitamin B12 deficiency lead to?
Anaemia
Dorsal column demyelination
What is trigeminal neuralgia?
Neural condition where gentle stroking of the face or mouth provokes massive stabbing pain