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NSCI201 > Chapter 9 - The Somatosensory System > Flashcards

Chapter 9 - The Somatosensory System Flashcards

1
Q

The sensations mediated by the somatosensory system are…

A

touch, pressure, vibration, limb position, heat, cold, itch, pain

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2
Q

Somatosensory subsystems

A

Tactile (cutaneous: fine touch/vibration/pressure), proprioceptive (position in space), pain/T°/sensual

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3
Q

Where are the somatosensory receptors located?

A

Skin, muscles, joints

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4
Q

What is the general pathway for somatosensory information?

A

Periphery —> spinal cord & brainstem —> CNS

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5
Q

What is a pseudounipolar neuron?

A

Neuron that has 1 extension from its soma: an axon that splits into 2 branches (to PNS and CNS)

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6
Q

What is sensory transduction?

A

Conversion of stimulus energy into an electrical signal

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7
Q

How does sensory transduction occur in somatosensory afferents?

A

Somatosensory timulus opens cation channels in afferent nerve endings —> receptor/generator potential —> APs in afferent fiber if reaches threshold

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8
Q

What is the name of the family of mechanotransduction channels?

A

Piezo1 and Piezo2

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9
Q

What is a mechanoreceptor?

A

Afferent fiber terminal that detects & transmits touch sensory stimuli (more sensitive ie lower threshold)

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10
Q

What is a free nerve ending?

A

Afferent fiber that lack specialized receptor cells (less sensitive ie higher threshold: for pain)

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11
Q

What are the characteristics that differentiate afferent types?

A
  • Axon diameter (determines conduction speed)
  • Receptive field size determined by arborization size & density of afferent innervation (determines 2-point discrimination, spatial accuracy)
  • Speed of adaptation ie temporal dynamics (info about spatial VS temporal attributes)
  • Channel & filter properties (determines response to qualities of somatosensory stimulation)
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12
Q

What are Pacinian corpuscles?

A

Receptor cells for rapidly adapting afferents

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13
Q

What are Nociceptors?

A

Receptors specialized in pain stimuli

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14
Q

What does Glabrous mean?

A

Hairless

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15
Q

What does Haptics mean?

A

Active touching

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16
Q

What is Stereognosis?

A

Capacity to identify an object by manipulating it with the hand

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17
Q

What are the 4 classes of mechanoreceptive afferents that innervate glabrous skin of hand?

A
  • Meissner corpuscule
  • Merkel cell
  • Ruffini corpuscule
  • Pacinian corpuscule
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18
Q

What are the 3 classes of mechanoreceptive afferents innervating the hair follicles in hairy skin?

A
  • Merkel cell afferents innervating touch domes
  • Circumferential endings
  • Longitudinal lanceolate endings (rapidly adapting, low-threshold)
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19
Q

What does Dermatome mean?

A

Geographically constrained zones that may present sensory loss in patients with nerve/spinal cord injury

20
Q

What characterizes Merkel cell afferents?

A
  • Slowly adapting fibers
  • 25% hand mechanosensory afferents
  • Info from epidermis
  • Merkel cells & afferents express Piezo2 (respectively for static & dynamic aspects of stimuli)
  • Highest spatial resolution, precise info about shape & texture
21
Q

What characterizes Meissner afferents?

A
  • Rapidly adapting fibers
  • 40% of hand mechanosensory afferents
  • In tips of dermal papillae, closest to skin surface
  • Elongated set of flattened lamellar cells
  • More sensitive to skin deformation, large receptive fields, less spatial resolution
  • Info about textured objects moving across skin (grip control)
22
Q

What characterizes Pacinian afferents?

A
  • Rapidly adapting
  • 10-15% hand mechanosensory afferents
  • Deep in dermis/subcutaneous tissue
  • Concentric layers of membranes surrounding afferent fiber –> filter
  • Low response threshold, but large receptive fields
  • Info about vibrations when making & breaking contact (tool use)
23
Q

What characterizes Ruffini afferents?

A
  • Slowly adapting
  • 20% skin mechanosensory afferents
  • Elongated, spindle-shaped, capsular specializations
  • Deep in the skin & in ligaments/tendons
  • Info about digit or limb movements causing skin stretches? (finger position & hand conformation)
24
Q

What is a Proprioceptor?

A

Receptor providing information about the position of limbs & body parts in space

25
Q

What are the 3 classes of proprioceptors?

A
  • Muscle spindles
  • Golgi tendon organs
  • Joint receptors
26
Q

What are the characteristics of muscle spindles?

A
  • In skeletal muscles
  • 4-8 intrafusal muscle fibers (parallel to extrafusal fibers of muscle) surrounded by capsule of connective tissue
  • Works with mechanically-gated ion channels
  • Innervated by primary (group Ia afferents: rapidly adapting) & secondary (group II afferents: slowly adapting) endings
  • Piezo2 expressed
  • Intrafusal fibers also controlled by separate set of motor neurons, which changes sensitivity of spindle afferents
  • Density varies in different muscles
27
Q

What are the characteristics of Golgi tendons?

A
  • Formed by branches of group lb afferents arranged with extrafusal muscle fibers
  • Info about tension in muscle
28
Q

What are the characteristics of Joint receptors

A
  • Contribute less to limb proprioception, but important for judging position of fingers
  • Protective role
29
Q

What is the central pathway for tactile information from body (aka PNS)?

A

It’s called the Dorsal Column-Medial Lemniscal System

  • Cutaneous mechanosensory afferents enter spinal cord through dorsal roots, terminate in dorsal horn
  • Ascending branches extend ipsilaterally through dorsal columns aka posterior funiculli
  • Both these first-order neurons & projection neurons from dorsal horn laminae project to the dorsal column nuclei (brainstem)
  • Second-order neurons in dorsal column nuclei project contralaterally to the VPL in the thalamus via medial lemniscus
  • Third-order neurons in VPL project ipsilaterally to the SI & SII in cortex via internal capsule
30
Q

What are First-order neurons?

A

Primary sensory neurons that detect a stimulus and transmit a signal to the spinal cord

31
Q

What are Second-order neurons?

A

Projection neurons that transmit a received sensory signal to the gateway (thalamus)

32
Q

What are Third-order neurons?

A

Neurons that carry a received sensory signal the rest of the way to the sensory region of the cerebral cortex

33
Q

What is Postsynaptic dorsal column projection?

A

Indirect projection of somatosensory information to the dorsal column nuclei of the brainstem by second-order neurons from the laminae III, IV, V of the dorsal horn

34
Q

What is Postsynaptic dorsal column projection?

A

Indirect projection of somatosensory information to the dorsal column nuclei of the brainstem by second-order neurons from the laminae III, IV, V of the dorsal horn

35
Q

What is the Gracile tract?

A

Aka fasciculus gracilis, medial bundle of fibers that conveys ascending information from lower limbs in the dorsal columns of the spinal cord

36
Q

What is the Cuneate tract?

A

Aka fasciculus cuneatus, lateral bundle of fibers that conveys descending info from upper limbs, trunk, neck

37
Q

What is the Gracile nucleus?

A

Aka nucleus gracilis, subdivision of the dorsal column nuclei where fibers carrying ascending info from the lower limbs synapses

38
Q

What is the Cuneate nucleus?

A

Aka nucleus cuneatus, subdivision of the dorsal column nuclei where fibers carrying descending info from the upper limbs, trunk, and neck synapses

39
Q

What are Internal arcuate fibers?

A

Axons exiting the forsal column nuclei, towards the somatosensory portion of the thalamus (contralateral)

40
Q

?What is the Medial lemniscus

A

Tract formed by the internal arcuate fibers that have crossed the midline

  • Ventrally: axons carrying info from lower limbs
  • Dorsally: axons carrying info from upper limbs
41
Q

What does Decussation mean

A

When fibers cross the midline

42
Q

What is the Ventral posterior lateral nucleus (VPL)

A

Portion of the thalamus responsible for somatosensory info

43
Q

What is the Internal capsule

A

Axon bundle via which sensory info from the thalamus VPL is projected to SI (postcentral gyrus of the cerebral cortex)

44
Q

What is the Primary somatosensory cortex

A

SI, aka postcentral gyrus, processing region in cerebal cortex

45
Q

What is the Secondary somatosensory cortex

A

SII, smaller processing region in cerebral cortex (upper bank of lateral sulcus)

46
Q

Central pathway for tactile information from face

A

NSCI201 (2 decks)

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