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HNNS > HNS14 Somatic Sensation > Flashcards

HNS14 Somatic Sensation Flashcards

1
Q

Sensation physiology

A

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)

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2
Q

3 types of Sensation

A
  1. Special sense
  2. Visceral sense
  3. Somatic sense
    - Mechanoception (Touch)
    - Proprioception (Body position and movement)
    - Thermoception (Temperature)
    - Nociception (Pain)
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3
Q

Somatosensory receptors and properties

A
  • Pseudounipolar
    —> one neurite (1 single process)
    —> two direction (to peripheral + to spinal cord)
  • Cell body in dorsal root ganglion
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4
Q

***Types of axons from somatosensory receptor

A

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
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5
Q

***Mechanoception in skin

A
  • 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:

  1. Meissner’s corpuscles: Flutter, touch, movement (記: MIF)
  2. Pacinian corpuscles: Vibration (記: PV)
  3. Ruffini corpuscles: Skin stretch (記: RS)

Unencapsulated:

  1. Merkel disc: Pressure, touch, form (記: Uncapped Perk)
  2. Hair follicle receptor: Direction, velocity of movement
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6
Q

(Mechanisms of mechanoreceptors)

A
  1. Direct activation through lipid bilayer tension
  2. Direct activation through structural proteins
  3. Indirect action through membrane structural proteins

—> ALL allow influx of cation and depolarisation of neuron
—> action potential

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7
Q

***Properties of mechanoreceptors

A

The 4 types of mechanoreceptors (Flutter, Pressure, Vibration, Skin stretch) have different properties:

  1. Locations of terminals / nerve endings
    —> Superficial skin: Meissner’s corpuscle, Merkel cells
    —> Deeper skin: Pacinian corpuscle, Ruffini endings
  2. Morphologies (e.g. in hairless skin, glabrous skin)
  3. Size of receptive field + Innervation density
    —> Spatial acuity
  4. Rate of adaptation
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8
Q

Receptive field

A

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
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9
Q

Spatial acuity

A
  • Higher spatial acuity —> Better to resolve spatial details
  • Measured by 2-point discrimination (distance in which 2 point sensation can be discriminated)
  • Depends on:
  1. Size of receptive field
    —> smaller receptive field: Higher acuity (e.g. tongue tip, finger tip)
    —> larger receptive field: Lower acuity (e.g. back, neck)
  2. Innervation density
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10
Q

Rate of adaptation

A

Decline in firing rate of afferent fibres with prolonged stimulus

  1. Slowly adapting
    - measures depth of skin indentation
    - intensity detector, perceives **pressure, **form, ***textures
    - Merkel disk, Ruffini endings
    —> continue to respond during course of stimulation
  2. Moderately rapidly adapting
    - Velocity detector
    - perceives **flutter, **motion
    - Meissner’s corpuscle, Hair follicle receptor
  3. Rapidly adapting
    - Acceleration (***Vibration) detector
    - records rapid repetitive displacement of skin
    - Pacinian corpuscle (in glabrous skin and hairy skin)
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11
Q

Proprioception

A
  • 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:

  1. Position (static limb position + trunk orientation)
  2. Movement (dynamic movement, velocity and direction of joint movement)
  3. Forces generated by muscle contractions
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12
Q

***3 types of proprioceptors

A
  1. ***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
  2. ***Golgi tendon organ
    - Sense muscle tension (force of muscle contraction)
  3. ***Joint receptor
    - Found in connect tissue, capsule, ligaments of joints
    - Free nerve ending and corpuscular receptors
    - Dynamic response (position of limbs), Finger position
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13
Q

Muscle spindle

A

Within intrafusal muscle fibres:

  1. Intrafusal muscle fibres
  2. 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
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14
Q

Romberg test

A
  • 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
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15
Q

Thermoception

A

2 types of thermoceptors:

  1. Cold receptor
    - active when temperature around 20-30oC
  2. 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

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16
Q

Ascending pathways to the brain: Entry into spinal cord + Ascending to brain

A

Different afferent fibres mixed together in spinal nerve before entering spinal cord —>

  1. 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
  2. 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
17
Q

Somatosensory pathways to brain

A

From body to cerebral cortex:

  1. 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
  2. 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:

  1. Spinocerebellar pathway (Aα, Aβ)
    - mechanoception, proprioception
18
Q

3 Somatosensory cortex

A
  1. Primary somatosensory cortex (S-I; Brodmann areas 3, 2, 1 (Postcentral gyrus))
  2. Secondary somatosensory cortex (S-II, Brodmann area 43)
  3. Posterior parietal cortex (Brodmann areas 5, 7, 39, 40)
19
Q

Somatosensory cortex function

A
  1. 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
  2. Integrate sensory information from different locations + different types/qualities
    - receive multiple inputs (different modalities)
20
Q

Somatotopical map

A

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
21
Q

***Somatosensation inputs to Thalamus

A
  • **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)

22
Q

***Projections from Thalamus to S-I cortex

A

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)

23
Q

Columnar organisation of S-I

A
  • 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)
24
Q

Higher-order processing of S-I neuron

A
  • 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
25
Q

Summary of primary somatosensory cortex S-I

A
  • Postcentral gyrus and in depth of central sulcus
  • Brodmann area 1, 2, 3a, 3b
  • Separate cortical zones for processing tactile (3b, 1, 2) + proprioceptive (3a, 2) inputs
  • 6 layers of neurons
  • columnar arrangement
  • for initial processing of information (esp. 3a, 3b) from ***contralateral side of body
  • some neurons (Area 2) have higher order processing: Orientation, Direction-sensitive
26
Q

S-II and Posterior parietal cortex

A

Even higher-order processing power
—> larger receptive fields than neurons in S-I
—> often ***bilateral (receive input from both sides of body)

Secondary somatosensory cortex (S-II)

  • receives inputs from S-I
  • recognise objects
  • input to **Amygdala and **Hippocampus (for **emotion and **memory, S-II believed to filter out important information)

Posterior parietal cortex

  • receives inputs from S-I + S-II
  • integrates different sensory modalities (MORE than just tactile/proprioceptive e.g. tactile and ***visual stimuli for eye-hand coordination) —> necessary for reaching objects
  • input to **Motor areas —> relay sensory information for **guidance of movement
27
Q

Lesion of somatosensory cortex

A

Lesion of S-I:

  • Difficulty with simple tactile tests
  • ***Contralateral loss of discriminative touch (difficulty to discriminate size and form of objects)
  • ***Positional sense impaired

Lesion of posterior parietal cortex
- Only mild difficulty with simple tactile tests
- **Tactile apraxia (difficulty with motor planning to perform tasks, disturbed hand movement with an object)
- Deficit in ability to relate to extrapersonal space
—> **Constructional apraxia (inability to construct configurations e.g. intersecting shapes)
—> ***Neglect syndrome (inability to perceive stimuli on one side of body even though there is sensation)

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