HNS14 Somatic Sensation Flashcards
(27 cards)
Sensation physiology
Stimulus
—> Specific sensory receptor
—> Electrical signal (information that brain can interpret)
—> Transmit to the CNS (brain / spinal cord)
—> Sensation (recognise stimulus)
—> Perception (interpretation of sensation, can vary from person to person)
3 types of Sensation
- Special sense
- Visceral sense
- Somatic sense
- Mechanoception (Touch)
- Proprioception (Body position and movement)
- Thermoception (Temperature)
- Nociception (Pain)
Somatosensory receptors and properties
- Pseudounipolar
—> one neurite (1 single process)
—> two direction (to peripheral + to spinal cord) - Cell body in dorsal root ganglion
***Types of axons from somatosensory receptor
Type Aα:
- ***Proprioceptors of skeletal muscle
- highly myelinated
- largest diameter
Type Aβ:
- ***Mechanoreceptors of skin (Touch)
Type Aδ:
- ***Pain, temperature
Type C:
- ***Temperature, pain, itch
- unmyelinated (much slower conduction speed)
- smallest diameter
***Mechanoception in skin
- Activated by mechanical stimulus
- Each mechanoreceptor responds best to a specific submodality
—> Touch, Pressure, Vibration, Flutter - Receptor may have an encapsulated nerve ending
- Cell body of receptor in dorsal root ganglion (limb + trunk) / trigeminal ganglia (head + neck)
Encapsulated:
- Meissner’s corpuscles: Flutter, touch, movement (記: MIF)
- Pacinian corpuscles: Vibration (記: PV)
- Ruffini corpuscles: Skin stretch (記: RS)
Unencapsulated:
- Merkel disc: Pressure, touch, form (記: Uncapped Perk)
- Hair follicle receptor: Direction, velocity of movement
(Mechanisms of mechanoreceptors)
- Direct activation through lipid bilayer tension
- Direct activation through structural proteins
- Indirect action through membrane structural proteins
—> ALL allow influx of cation and depolarisation of neuron
—> action potential
***Properties of mechanoreceptors
The 4 types of mechanoreceptors (Flutter, Pressure, Vibration, Skin stretch) have different properties:
- Locations of terminals / nerve endings
—> Superficial skin: Meissner’s corpuscle, Merkel cells
—> Deeper skin: Pacinian corpuscle, Ruffini endings - Morphologies (e.g. in hairless skin, glabrous skin)
- Size of receptive field + Innervation density
—> Spatial acuity - Rate of adaptation
Receptive field
Area of skin in which a stimulus activates a particular mechanoreceptor
Meissner’s corpuscle, Merkel cells:
- small
- defined
- a lot of hot spots —> determine how much information can extract (e.g. texture)
Pacinian corpuscle, Ruffini endings:
- large
- uniform
- less hot spots
Spatial acuity
- Higher spatial acuity —> Better to resolve spatial details
- Measured by 2-point discrimination (distance in which 2 point sensation can be discriminated)
- Depends on:
- Size of receptive field
—> smaller receptive field: Higher acuity (e.g. tongue tip, finger tip)
—> larger receptive field: Lower acuity (e.g. back, neck) - Innervation density
Rate of adaptation
Decline in firing rate of afferent fibres with prolonged stimulus
- Slowly adapting
- measures depth of skin indentation
- intensity detector, perceives **pressure, **form, ***textures
- Merkel disk, Ruffini endings
—> continue to respond during course of stimulation - Moderately rapidly adapting
- Velocity detector
- perceives **flutter, **motion
- Meissner’s corpuscle, Hair follicle receptor - Rapidly adapting
- Acceleration (***Vibration) detector
- records rapid repetitive displacement of skin
- Pacinian corpuscle (in glabrous skin and hairy skin)
Proprioception
- Sense of body position and movement (Kinesthesia)
- important in maintaining posture and balance
- Perception of head/body position in space is derived from integrative inputs of:
1. Proprioceptors
2. Labyrinth receptor of inner ear
3. Visual input
3 sub-modalities:
- Position (static limb position + trunk orientation)
- Movement (dynamic movement, velocity and direction of joint movement)
- Forces generated by muscle contractions
***3 types of proprioceptors
- ***Muscle spindle
- Annulospiral (type 1a fibre) + Flower-spray endings (type 2 fibre)
- Sense
—> 1. Muscle length / stretch (static)
—> 2. Velocity of stretch during body movement (dynamic)
- Sensitivity range controlled by activity of γ motor neuron - ***Golgi tendon organ
- Sense muscle tension (force of muscle contraction) - ***Joint receptor
- Found in connect tissue, capsule, ligaments of joints
- Free nerve ending and corpuscular receptors
- Dynamic response (position of limbs), Finger position
Muscle spindle
Within intrafusal muscle fibres:
- Intrafusal muscle fibres
- Sensory nerve endings (Proprioceptors)
- Annulospiral receptor (type 1a fibre): muscle length during **movement
- Flower-spray receptor (type 2 fibre, near periphery of intrafusal muscle): muscle length during **static state
Romberg test
- require patient to maintain balance while standing with feet together and eyes closed
- tests whether proprioceptive components are working properly when visual cues are missing
Thermoception
2 types of thermoceptors:
- Cold receptor
- active when temperature around 20-30oC - Warm receptor
- active when temperature around 40-45oC
—> Express ***temperature-gated ion channels identified by mints and capsaicin (chemical stimulants)
Outside of temperature range (extreme temperature): below 10oC / above 45oC
—> Nociceptors are stimulated
—> Pain
Ascending pathways to the brain: Entry into spinal cord + Ascending to brain
Different afferent fibres mixed together in spinal nerve before entering spinal cord —>
- Segregation of sensory afferent fibres when different modalities when they enter spinal cord (specifically Dorsal horn):
- Mechanoception + Proprioception (Aβ, Aα): **Medial dorsal horn
- Nociception + Thermoception (Aδ, C): **Lateral dorsal horn - Different sensory afferent ascend to brain through different routes along spinal cord
- Mechanoception + Proprioception (Aβ, Aα):
—> Dorsal column-medial lemniscal pathway (Spinocerebellar pathway as well)
—> Dorsal column (white matter) —> Medulla oblongata —> synapse with 2nd order neuron - Nociception + Thermoception (Aδ, C):
—> Anterolateral system
—> Synapse with 2nd order neuron in Dorsal horn —> cross in spinal cord (minority ascend in ipsilateral side) —> ascend to brain
Somatosensory pathways to brain
From body to cerebral cortex:
- Dorsal column-medial lemniscal pathway (Aα, Aβ)
- **discriminative touch (recognition of shape, size, texture), **pressure, **vibration, **proprioception
- decussate in medulla oblongata (2nd order neuron in medulla oblongata)
- synapse with 3rd order neuron in Thalamus - Anterolateral system (Aδ, C)
—> Spinothalamic tract
—> Spinoreticular tract
—> Spinomesencephalic tract
- **crude touch, **temperature, **pain, **tickle, itch
- decussate in spinal cord (2nd order neuron in spinal cord)
- synapse with 3rd order neuron in Thalamus
From body to cerebellum:
- Spinocerebellar pathway (Aα, Aβ)
- mechanoception, proprioception
3 Somatosensory cortex
- Primary somatosensory cortex (S-I; Brodmann areas 3, 2, 1 (Postcentral gyrus))
- Secondary somatosensory cortex (S-II, Brodmann area 43)
- Posterior parietal cortex (Brodmann areas 5, 7, 39, 40)
Somatosensory cortex function
- Decode Localisation + Types/Qualities of sensory information
- highly organised —> unique group of neurons that are activated by specific stimuli modality/submodality + from a particular location - Integrate sensory information from different locations + different types/qualities
- receive multiple inputs (different modalities)
Somatotopical map
Axonal endings of specific somatic pathways are grouped according to location of sensory receptors
Sensory Homunculus:
- distorted map of the opposite side of body surface
- areas of high spatial acuity are magnified (more sensory information): greater region of somatosensory cortex
***Somatosensation inputs to Thalamus
- **Proprioceptive stimuli:
- VPS nucleus
- **Tactile (mechanoceptive: fine touch and vibration) stimuli:
- VPL nucleus (limbs: DC pathway)
- VPM nucleus (face: Trigeminothalamic pathway)
(Pain, Temperature stimuli:
- VPL nucleus)
***Projections from Thalamus to S-I cortex
Most inputs from Thalamus go to areas 3a (Proprioceptive input) and 3b (Mechanoceptive input) of S-I cortex
—> then project to areas 1 (Mechanoceptive) and 2 (Both Proprioceptive and Mechanoceptive)
Pathways:
Proprioceptive: VPS —> 3a, 2
Mechanoceptive: VPM, VPL —> 3b, 1 —> 2
—> ∴ specificity of sensory receptors to different submodalities is maintained after they project to the brain (Labeled line principle: different areas of cortex for different submodalities)
Columnar organisation of S-I
- 6 layers of neurons —> arranged into functional columns
- each column is very narrow (300-600μm wide)
—> all neurons within a column receive inputs from same body area with a specific sensory submodality
—> **Rapidly adapting (e.g. vibration) vs **Slowly adapting (mechanoceptive)
Higher-order processing of S-I neuron
- Receptive field of neuron in S-I is much larger than that of mechanoreceptor on skin (∵ each S-I neuron receives input from multiple receptor neurons)
- Area 1 and 2 deal with more **abstract features than simply location of tactile stimuli (i.e. receive input from larger no. of neurons compared to 3a/3b)
—> Area 1 (mechanoceptive): **Texture
—> Area 2 (both mechanoceptive + proprioceptive): ***Size and Shape - Some neurons in Area 2 of S-I are stimulated by specific **combination of stimuli e.g. respond to specific motion rather than touch at a single point
—> **Orientation selectivity
—> ***Direction selectivity
