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
