L14 - Somatic sensation Flashcards
Define the 4 modalities of somatic senses.
- Touch (Mechanoception)
- Body position & movement (Proprioception)
- Temperature (Thermoception)
- Pain (Nociception)
3 types of sensations?
special sense, visceral sense and somatic sense
Location of soma and 5 types of mechanoreceptors?
Located in dorsal root ganglion or trigeminal ganglia
a) Encapsulated:
Meissner’s corpuscles
Pacinian corpuscles
Ruffini corpuscles
b) Unencapsulated:
Merkel disc
( Hair follicle receptor)
List the submodalities of mechanoreception?
1.Touch 2.Pressure 3.Vibration and flutter
Define receptive field of mechanoreceptors?
area of the skin in which a stimulus activates a particular mechanoreceptor
Submodality, location, receptive field size and rate of adaptation of Meissner’s corpuscles?
- Touch, flutter, movement
- Superficial
- Small, discrete hot spots of receptive field (more sensitive)
- Moderately rapid adapting
Submodality, location, receptive field size and rate of adaptation of Pacinian corpuscles?
- Vibration/ acceleration + rapid repetitive displacement of skin
- Deeper layer
- No hotspots of receptive field, big area
- Rapidly adapting
Submodality, location, receptive field size and rate of adaptation of Ruffini corpuscles ?
- Skin stretch, depth of skin indentation, intensity and texture
- Deep layer
- No hotspot of receptive field, big area
- Slowly adapting
Submodality, location, receptive field size and rate of adaptation of Merkel disc?
- Touch, pressure, form, depth of skin indentation
- Superficial layer
- Small hotspots (more sensitive)
- Slowly adapting
Define rate of adaptation in mechanoreceptors?
decline in firing rate of afferent fibers with prolonged stimulus
Slow = continue to respond through duration of applied stimulus
Rapid = Respond to constantly applied stimulus only at the first instance
Define the 3 rates of adaptation and give examples of mechanoreceptors.
- Slowly adapting: Merkel disk, Ruffini’s corpuscle
- Moderately Rapid adapting: Meissner’s corpuscle, hair follicle receptor
- Rapidly adapting: Pacinian corpuscle (in glabrous and hairy skin)
Compare what submodalities of stimuli are measured in mechanoreceptors of different rate of adaptation?
Slowly: Measures depth of skin indentation + Intensity detector + perceives form & texture
Moderately rapid = Velocity detector + Perceives flutter & motion
Rapidly = Acceleration, vibration +rapid repetitive displacement of skin
3 qualities of proprioception? Definition?
Sense of body position and sense of movement (kinesthesia)
- Position(static limb position & trunk orientation)
- Movement(dynamic movement; velocity & direction of joint movement)
- Forces generated by muscle contractions
Perception of head / body position in space is derived from integrative inputs of?
Proprioceptors
Labyrinth receptors of the inner ear
Visual input
Test for proprioception?
Romberg test:
Requires patient to maintain balance while standing with feet together, eyes closed
Tests whether the isolated proprioceptive components are working properly when visual cues are missing
3 types of proprioceptors? Respective function?
- Muscle spindle = Sense muscle length or stretch (static) & velocity of stretch during body movement (dynamic)
- Golgi tendon organ = muscle tension
- Joint receptor = dynamic response, finger position
(4. cutaneous mechanoreceptors)
Compare the nerve endings between muscle spindle and joint receptors?
Muscle spindle = intrafusal muscle fibers = Annulospiral and flower-spray endings (sensitivity controlled by activity of γ motor neuron)
Joint receptor = Free nerve ending & corpuscular receptors @ connective tissues, capsule and ligaments of joints
Muscle spindle sensitivity is controlled by which type of neuron?
Activity of γ motor neuron
Types of thermoreceptors and mechanism?
Cold, warm receptors
Express temperature-gated ion channels (identified by mints, capsaicin)
Different mechanisms of mechanoreceptors? (3)
Activated by mechanical stimulus (force changes conformation of ion channel) = action potential:
1) Direct activation through lipid tension
2) Direct activation through structural proteins
3) Indirect action through membrane structural proteins
Describe how temeprature detection changes in: constant temp, changing temp, extreme temp?
Static temperature detection: detect constant skin temperature
Dynamic temperature detection / sensation: detect transient changes in skin temperature
Below 10oC / above 45oC (extreme temperatures) = stimulate nociceptors
Different modalities of somatosensation are transmitted by the same type of fibers. True or false?
False
Different modalities of somatosensation have different:
Thickness of afferent axons
Myelination
Speed of conduction of action potential
Describe the segregation of sensory afferent fibers with different modalities when they enter the spinal cord?
Segregate into different laminae of the grey matter in dorsal horn of spinal cord
Pain, temperature (Aδ, C fibers) = I, II, V laminae (enters laterally into dorsal horn)
Mechanoreceptors of skin(Aβ fibers) + Proprioceptor (Aα fibers) = IV (enters medially into dorsal horn)
Compare the ascending pathways taken by different sensory afferents?
Proprioception, Mechanoception = DCML
Thermo, Nociception = spinothalamic
List the sensations relayed by DCML?
Mechanoception, proprioception
For discriminative touch (recognition of shape, size & texture), pressure, vibration, proprioception
List the sensations relayed by Spinothalamic tract?
Thermoception, Nociception
Crude touch, temperature and pain, tickle, itch
List the sensations relayed by Spinocerebellar tract?
mechanoception and proprioception
Divide the somatosensory cortex into 3 groups.
- Primary somatosensory cortex (S-I; areas 3, 2, 1)
- Secondary somatosensory cortex (S-II; area 43)
- Posterior parietal cortex (areas 5, 7, 39, 40)
Function of the somatosensory cortex?
1) DECODE the localization and types/qualities of the sensory information from contralteral side of body
2) INTEGRATE multiple input sensory information from different locations and of different types/qualities»_space; Higher order processing
Explain how the somatosensory cortex can decode sensory info.
Highly organized according to the location of the sensory receptors
Unique group/patterns of neurons activated by stimuli of specific modality/submodality from a particular location
Relate the areas of thalamus corresponding to Brodmann areas of primary somatosensory cortex after sensory activation.
1) Proprioceptive stimuli:
•VPS (ventral posterior superior nucleus)»_space; 3a, 2
2) Tactile (mechanoceptive) stimuli:
•VPL (ventral posterior lateral nucleus)»_space; 3b, 1, 2
•VPM (ventral posterior medial nucleus)»_space; 3b, 1, 2
3) Pain and temp:
•VPI»_space; S2 (secondary somatosensory cortex)
Most inputs from the thalamus go to areas 3a and 3b, which then project to areas 1 and 2
What sensory info can Brodman area 2 of Primary somatosensory cortex process?
Area 2 process both tactile and proprioceptive information
Describe the organization of primary somatosensory cortex (S-1)
- 6 layers of neurons (molecular, external granular, external pyramidal…etc); arranged into functional columns (1,2,3a,3b,4,5)
- Each column is very narrow and all neurons within the column receive inputs from the same body area with a specific sensory submodality
Receptive field of neuron in SI is much larger than that of the mechanoreceptor on the skin. True or False?
True
SI neuron receives inputs from multiple receptor neurons
Define what brodmann area 1, 2, 3a and 3b in primary somatosensory cortex process?
1: texture of objects (rapidly adapting receptors on skin)
2: size, shape of objects (pressure, joint position in deep tissue)
3a: muscle stretch receptors in deep tissue
3b: slowly, rapidly adapting receptors in skin
Describe preliminary higher order processing of the somatosensory cortex?
neurons in S1 stimulated by specific combination of stimuli, e.g. respond to specific motion
> > Processing to deduce Direction and Orientation
Location of Primary somatosensory cortex S-I?
Postcentral gyrus & in depth of central sulcus
Brodmann area 1, 2, 3a, 3b
Function of secondary somatosensory cortex?
- Receives inputs from S-I
- Recognize objects
- Input to amygdala and hippocampus (for emotion and memory)
Function of posterior somatosensory cortex?
- Receives inputs from S-I and S-II
- Integrates different sensory modalities (e.g. tactile and visual stimuli for eye-hand coordination)
- Output to motor areas; for guidance of movement
Clinical presentation of lesion of S-I?
Impaired mechanoreceptive, proprioceptive senses:
- Difficulty with simple tactile tests
- contralateral loss of discriminative touch (difficulty to discriminate size and form of objects)
- Impaired positional sense
Clinical presentation of lesion of Posterior parietal cortex?
- Only mild difficulty with simple tactile tests
- Tactile apraxia (disturbed hand movement with an object)
- Deficit in the ability to relate to extrapersonal space; e.g. constructional apraxia, neglect syndrome
