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NEUR 2000 > Lecture 8: Touch > Flashcards

Lecture 8: Touch Flashcards

1
Q

What are some examples (6) of the diverse range of sensations from periphery?

A
  • Touch
  • Pressure
  • Limb position
  • Vibration
  • Heat/cold
  • Pain
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2
Q

What are the 4 different types of peripheral ‘receptors’? What do they do?

A
  • Transduce information
  • Mechanoreceptors
  • Thermoreceptors
  • Nociceptors
  • Chemoceptors
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3
Q

What do somatosensory afferents do? What type of neuron are they?

A
  • Convey information from skin to CNS
  • Pseudounipolar (receptor endings in periphery, peripheral/central components are continuous, attached to cell body in ganglia by single process)
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4
Q

What is sensory transduction?

A
  • Convert stimulation into electrical signal that body/brain can process
  • Alters cation channel permeability (depolarizing event)
  • Receptor/generator potential (inward current)
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5
Q

Where do receptors end in somatic sensory system?

A
  • Dermis
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6
Q

How are mechanoreceptors activated?

A
  • Physical forces on cell membrane (pulls open receptor/ion channel)
  • Direct activation through structural protein (forces act on linking protein which opens channel)
  • Indirect activation through force sensor (activates 2nd messengers, inducing ion channel opening)
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7
Q

How is an action potential generated in mechanoreceptors?

A
  • More membrane stretch = more open ion channels

- Sufficient accumulation of receptor potentials leads to AP (influx of +ve ions)

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8
Q

What are tactile senses? What 4 sensory afferents perceive tactile senses?

A
  • Touch
  • Merkel Cell
  • Meissner Corpuscles
  • Ruffini Corpuscles
  • Pacinian Corpuscles
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9
Q

What are Merkel Cells?

A
  • 1/4 of all sensory afferents in hand/fingertip
  • High spatial resolution
  • Useful in discriminating form/texture
  • Likely manage reading of Braille
  • As little as 0.5mm 2 point discrimination
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10
Q

What are Meissner Corpuscles?

A
  • 40% of mechanosensory nerves in hand
  • Contain 2-6 afferent nerve fibres
  • 4X more sensitive to skin deformation than Merkel
  • Larger receptive fields (less resolution)
  • Efficient lo-frequency vibration detection (detecting slippage)
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11
Q

What are Ruffini Corpuscles?

A
  • Least well understood
  • 1/5 of hand mechanoreceptors
  • Tracking movement/position of hand
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12
Q

What are Pacinian Corpuscles?

A
  • Likely most well understood
  • 10-15% of hand mechanoreceptors
  • Detect pressure/vibrations (more sensitive than Meissner; skin deformations in nm range)
  • Useful in skilled tool usage
  • Onion-like appearance
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13
Q

What is the receptive field?

A
  • Area of skin surface over which stimulation results in significant change in AP rate
  • Body regions vary with density of afferent fibres
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14
Q

What is 2-point discrimination?

A
  • Minimum interstimulus distance required to perceive 2 simultaneously applied stimuli as distinct
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15
Q

What are rapidly-adapting afferents/what do they do?

A
  • Fire rapidly when stimulus first presented; fall silent with continued stimulation
  • Effective in conveying info about changes in ongoing stimulation
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16
Q

What are slowly-adapting afferents/what do they do?

A
  • Spatial attributes of a stimulus
  • More constant/ongoing info
  • More needed to change (e.g. hands)
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17
Q

What is the sensory function, spatial acuity, and response to sustained indentation in Merkel Cells?

A
  • Shape and texture perception
  • 0.5 mm
  • Sustained (slow adaptation)
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18
Q

What is the sensory function, spatial acuity, and response to sustained indentation in Meissner Corpuscles?

A
  • Motion detection; grip control
  • 3 mm
  • None (rapid adaptation)
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19
Q

What is the sensory function, spatial acuity and response to sustained indentation in Pacinian Corpuscles?

A
  • Perception of distant events through transmitted vibrations; tool use
  • 10+ mm
  • None (rapid adaptation)
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20
Q

What is the sensory function, spatial acuity and response to sustained indentation in Ruffini Corpuscles?

A
  • Tangential force; hand shape; motion direction
  • 7+ mm
  • Sustained (slow adaptation)
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21
Q

What is proprioception?

A
  • Brain needs to know where body is in space
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22
Q

What is muscle spindle?

A
  • Found in nearly all striated/skeletal muscles
  • Detects changes in muscle length
  • Info about position of body part in space
  • 4-8 intrafusal muscle fibres surrounded by a tissue capsule (interspersed within and parallel to extrafusal fibres)
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23
Q

What are group Ia afferents?

A
  • Rapidly adapting (velocity and direction)
24
Q

What are group II afferents?

A
  • Sustained response (static position)
25
Q

What detects stretch?

A

Type Ia and II afferent axons

26
Q

What is sensitivity modulated by?

A
  • Gamma-motor neurons from ventral horn of spinal cord
27
Q

Is the density of spindle fibres consistent or variable?

A
  • Variable
  • Relatively few in large muscles with course movements (not a lot of fine control)
  • Far more in small muscles with fine movements (hand, neck, surrounding eye)
28
Q

What is the Golgi tendon organ? Where is it located?

A
  • Located at junction of muscle fibres and tendon (transition)
  • Capsule of braided Ib axonal branches interwoven between collagen fibres
  • When muscle is stretched, collagen fibres are pulled together which compress Ib afferent
  • Efficient measure of tension (force) in muscle
  • Ex. knee-jerk reflex (pulls together collagen and compresses 1b afferent)
29
Q

What is dermatome?

A
  • Helps to define location of suspected spinal lesion/injury

- Innervation of spinal cord arising from a single dorsal root ganglion and its spinal nerve

30
Q

What is the medial lemniscal pathway?

A
  • Lower limb, upper limb, trunk and neck
31
Q

How do the first-order neurons travel in the medial lemniscal pathway?

A
  • Enter dorsal root and bifurcate into ascending/descending branches
  • Project into grey matter (cell bodies) of dorsal horn
  • Ascending branches ascend ipsilaterally through dorsal columns to lower medulla
  • Synapse onto neurons in dorsal column nuclei
32
Q

How are the dorsal columns of the spinal cord organized?

A
  • topographically
33
Q

What is the gracile tract?

A
  • Lower limb fibres are medial

- Track ends at gracile nucleus (in medulla)

34
Q

What is the cuneate tract?

A
  • Upper limbs, trunk, neck are more lateral

- Track ends at cuneate nucleus (in medulla)

35
Q

How do the second-order neurons travel in the medial lemniscal pathway?

A
  • Axons exiting dorsal column nuclei projecting to somatosensory thalamus are internal arcuate fibers
  • Cross midline and follow a tract called medial lemniscus to ventral posterior lateral (VPL) nucleus of thalamus
36
Q

How do third-order neurons travel in the medial lemniscal pathway?

A
  • Synapse in thalamus then project to appropriate region of primary somatic sensory cortex (SI)
37
Q

What is the trigeminothalamic system?

A
  • Face
38
Q

How do the first-order neurons travel in the trigeminothalamic system?

A
  • Trigeminal nerve mechanoreceptors (face)

- Trigeminal ganglion cells enter brainstem at level of the pons to terminate in the trigeminal brainstem complex

39
Q

How do the second-order neurons travel in the trigeminothalamic system?

A
  • Axons cross midline and ascend to ventral posterior medial (VPM) nucleus of the thalamus by way of trigeminal lemniscus
40
Q

How do the third-order neurons travel in the trigeminothalamic system?

A
  • Neurons in the VPM send axons ipsilaterally to cortical areas SI and SII
41
Q

What are the proprioceptive receptor afferents?

A
  • Muscle spindles

- Golgi tendon organs

42
Q

How do lower body first-order neurons in the spinocerebellar tract travel?

A
  • Enter dorsal horn of spinal cord
  • Bifurcate into ascending and descending branches
  • Some synapse within dorsal horn (e.g. knee-jerk)
  • Proprioceptive afferents that enter the spinal cord b/n mid-lumbar and thoracic levels synapse on neurons in Clarke’s nucleus in medial aspect of dorsal horn
  • Neurons entering below this level ascend through dorsal column then synapse
43
Q

How do lower body second-order neurons in the spinocerebellar tract travel?

A
  • From Clarke’s nucleus ascend ipsilateral posterior lateral column of spinal cord to dorsal spinocerebellar tract in medulla
  • Continue to cerebellum but have collateral connections with neurons just outside nucleus gracilis
44
Q

How do lower body third-order neurons in the spinocerebellar tract travel?

A
  • Join the medial lemniscus

- Combine with cutaneous mechanoreceptors terminating at the VPL nucleus of the thalamus

45
Q

What is the function of the cerebellum?

A
  • Proper proprioception

- Maintain order of body in space

46
Q

How do the upper body first-order neurons in the spinocerebellar tract travel?

A
  • Enter dorsal horn of spinal cord
  • Travel via dorsal column cuneate tract to level of medulla
  • Synapse on proprioceptive neurons in dorsal column nuclei, including external cuneate nucleus
47
Q

How do the upper body second-order neurons in the spinocerebellar tract travel?

A
  • Project axons into ipsilateral cerebellum

- Project collateral branches across midline to join medial lemniscus pathway to VPL nucleus of thalamus

48
Q

What are the proprioceptive projections to thalamus and how are they organized?

A
  • Segregated passage of fibres throughout ascent from mechanoreceptors through organized topographical connectivity with the cortex
49
Q

Where is the somatotopic map of?

A
  • Postcentral gyrus of parietal lobe
  • 4 distinct regions (Brodmann’s areas)
  • Based on cytoarchitectural organization of brain regions
  • Many areas are functionally correlated
50
Q

What does the primary somatosensory cortex consist of?

A
  • Brodmann areas 1, 2, 3a and 3b
51
Q

What do Brodmann areas 3b and 1 do?

A
  • Sensory representation of cutaneous stimuli (skin)
52
Q

What does Brodmann area 3a do?

A
  • Proprioception
53
Q

What does Brodmann area 2 do?

A
  • Both tactile and proprioceptive stimuli
54
Q

What is the homunculus?

A
  • Somatotopic map
  • Body represented
  • Size of cortical representation is proportional to density of sensory receptors (smaller 2-point discrimination) in corresponding body part
  • Each Brodmann area is nearly an entire map
55
Q

What Brodmann area receives the bulk of input from ventral posterior thalamus? Which area(s) is it then projected to?

A
  • Area 3b
  • All inputs are integrated here
  • First step in cortical processing of somatosensory info
  • Projects to areas 1 and 2
56
Q

Where do all Brodmann areas project to, and where do they project from there?

A
  • SII
  • Believed to play a role in learning/memory and emotional processing
  • Project to amygdala and hippocampus
57
Q

Where does Area 2 project to?

A
  • Parietal areas 5a and 7b
  • Input into motor and premotor areas of frontal lobe (motor outputs)
  • Primary point where somatosensory inputs inform and execute voluntary movement

NEUR 2000 (10 decks)

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