somatosensation Flashcards
L8-11, sem 4&5, SQ4-6, reading 4
Case Study IW
- rare neurological illness after a flu-like viral infection
- no sensation of touch below the neck
- still had pain, heat, but no touch or sense of position
- appeared drunk, slurred speech
- autoimmune response attacked nerves
- had to relearn movement & control using other senses, mostly sight
- takes immense concentration, attention and imagery
- socially isolated himself, avoided crowds
criteria for a sensory system (4)
- Specialised to receive particular stimulus – i.e. has specific receptors for specific physical energy/chemical molecules
- Performs signal transduction (stimulus → neuronal potential)
- Relays the neural signal to the brain via certain pathway (synapse 1 → synapse 2 → synapse 3 …)
- Has its own cortical region for processing (sensory cortices + association cortices)
Cutaneous “touch” stimuli
- mechanical compression
- vibration
- thermal energy transfer
touch pathway (non-painful cutaneous sensation)
lemniscal tract
→ ascending info via peripheral nerve
→ dorsal root
→ lemniscal tract
→ ascend to brain stem and cross midline in medulla
→ medial lemiscus
→ thalamus
→ primary somatosensory cortex
Pacinian corpuscle
- dynamic pressure, deformation but not vibration
- low threshold, rapidly adapting, for sudden stimuli
- e.g. tickle or poke
Meissner’s corpuscle
- only in glabrous skin (hairless and smooth)
- shallow, vertical orientation
- mechanically deformed by light touch
- very low threshold, shallow position,
rapidly adapting, respond easily to
dynamic, moderate stimulation - e.g. different textures
Ruffini’s corpuscle
- intermediate depth, horizontal
orientation (moderate surface area) - mechanically deformed by stretch
- slow response rate (2-3 Hz) allows
them to respond to stable, low frequency stimulation (e.g. stretch) - possible role in non-tactile signalling/proprioception?
Merkel’s disk
- Shallow, moderate surface area
- Mechanically deformed by pressure
(but not so easily as PCs) - Slow response rate and slowly adapting
- Respond to static pressure/touch
- static discrimination of shapes,
edges - E.g. when holding an object
Reflex arc
Sensory neuron (afferent information arrives from skin)
↓
Interneuron (AKA relay neuron)
↓
Motor neuron (efferent information exits out to muscles)
Ramachandran: q-tip demonstration
- single limb amputation
- feels on cheek and ‘arm’
- somatosensory cortex plasticity: adjacent areas invade hand area (which is no longer receiving input)
kinaesthesia vs proprioception
= the ability to sense the movement (kinesthesis) and position (proprioception) of our own body
- Arises from inside the body - requires a physical stimulus from the internal environment
- e.g. mechanical stretch/tension in muscles
Muscle spindles (proprioceptors):
- bunch of 4-8 muscle fibers
- respond to change in muscle length;
- high density in hand, neck, ocular muscles;
- low density in large muscles that generate coarse movement
Golgi tendon organs:
- similar to proprioceptors but located in tendon
- respond to change in muscle tension
Joint receptor neurons:
free nerve endings in joints and respond to joint movement
differentiating factors of nerve fibres
Diameter, Conduction Velocity, Myelination State
A group fibres (generally)
large diameter, high conduction velocity, and are myelinated.
a-alpha fibres
- thickest of A group
- myelinated, fast conduction
- for proprioception
type of nerve fibre for proprioception
a-alpha fibres
Posterior Parietal Cortex
- receives info from S1
- also from visual and auditory cortices, thalamus, hippocampus
- = association cortex
- lots of info out as well
fibre type with mechnoreceptors
A-beta fibres
- Intermediate size (6–12 µm) myelinated, and fast conduction velocity (33–75 m/s)
Chemaesthesis (def and receptors)
the sensitivity of mucosal surfaces or skin to
environmental chemicals
- involves receptor activation on free nerve endings (Cranial Nerve V)
Innervation of Cranial Nerve V
- Via mechanoreceptors
- Via thermoreceptors
- Via chemoreceptors
All on or associated with free nerve endings
Chemaesthesis - stimuli + esponse
usually classed as irritants/poisons bc activate thermoreceptors and/or nociceptors on free nerve endings
Serves as a safety surveillance system that initiates protective mechanisms: tearing, mucus, salivation, coughing, sneezing, vasodilation/flushing
A-delta fibres
- Smallest diameter of the ‘A’ group of fibres (1-6 µm)
- Partially myelinated, slow conduction velocity (9-11 m/s)
- Carry noci + thermo signals
C-fibres
- Very small diameter (< 1 µm)
- Unmyelinated
- Slowest conduction velocity (<1 m/s)
- Mechano, noci and thermo signals
spinothalamic tract
- Thermoreceptors on sensory nerve free endings
- 1st order (afferent) neuron projects to spinal cord
- Decussate (cross over) in the spinal cord
- Synapses with second order neuron, which travels to thalamus
- Synapses with 3rd order neuron which travels to primary somatosensory cortex (SI)
lateral vs anterior spinothalamic tracts
- Lateral spinothalamic tract for thermo, noci
- Anterior spinothalamic tract for crude mechanoreception
Nociceptors
- on free nerve endings
- A-delta and C fibres
- lots of different types, including TRP channels (noxious cold or heat)
- other ones relates to inflammation, low pH, heat, mechanical damage
3 Types of Pain:
- abrupt/strong cutaneous sensation, tissue damage
- damage to neural structures, neural supersensitivity
- physical pain of psychological origin
Pain matrix
SI, SII, insula, ACC, PFC
Dorsal Lemniscal System - info/function
- touch sensation (good localisation, graduations of stimulus intensity)
- phasic sensations (vibrations)
- sensations of movement against skin
- fine positional and pressure sensations
Antlat Spinothalamic System - info/function
- thermal sensation
- pain sensations
- crude pressure and touch
- tickle and itch
- sexual sensations
DL system - receptors
- proprioceptors (muscle spindles, golgi tendon organs, joint receptor neurons)
- mechanoreceptors in skin (meissner’s, pascinian, ruffini’s, merkel’s)
ALST system - receptors
nociceptors
decussations
DL = medulla (2º neuron)
ALST = spinal cord (2º neuron)
first synapses
DL = dorsal column nuclei in medulla
ALST = spinal cord
second synapse
DL = contralateral thalamus (ventral posterior thalamic nucleus (VPN))
ALST = thalamus
brain projections
DL = VPN → primary somatosensory cortex (SI) → secondary somatosensory cortex (SII) of the posterior parietal lobe.
ALST =
* medial thalamus to frontal cortex
* lateral thalamus to SI and SII of somatosensory cortex
pain and sensation - spinal cord injuries - ipsi or contra?
- Non-painful info ascends immediately, ipsilaterally
- Painful info crosses over, then ascends contralaterally
McGill Pain Questionnaire
used to monitor pain over time and to determine the effectiveness of any intervention
- all sections associated with numbered scale
- section 1: what does your pain feel like?
- section 2: how does your pain change with time?
- section 3: how strong is your pain?
motor movement during sleep
- no motor movement during REM sleep
- The subcoeruleus nucleus (SubC) in the brainstem is activated during REM sleep, and it inhibits the motor neurons (MN) deep in the brainstem
REM sleep behaviour disorder
⇒ muscle movement during sleep
- normally SubC inhibits MNS
- disordered = abnormalities/insufficient inhibition ⇒ excessive activity of muscles during sleep
ACC (ant cingulate cortex) role in pain
- implicated in cognitive pain modulation
- also in processing pain related emotion
- receives and processes sensory info
- also modulatory role: increases sensitivity to sensory inputs
