Week 3 - Spinal Reflexes and Electromyogram Flashcards

1
Q

what are characteristics of sensory receptors acting on spinal motoneurons?

A
  • excitatory or inhibitory
  • movement quality/efficiency
  • safety and protection
  • monosynaptic (direct synaptic contacts) or polysynaptic (indirect)
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2
Q

what is a stretch receptor?

  • how are they arranged?
  • what are they targets for?
A

component of a muscle spindle that sense changes in muscle length

  • only afferent in CNS that receives a nerve supply
  • arranged in parallel with extrafusal (regular) muscle fibers
  • target for gamma motoneurons
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3
Q

how do gamma motoneurons compare with alpha motoneurons?

A

gamma have smaller soma and lower conduction velocities

  • innervate intrafusal muscle fibers
  • smaller diameter
  • no force
  • affect stiffness of sensory regions
  • nuclear bag and nuclear chain types
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4
Q

what do intrafusal fibers include? what causes afferent firing, and what are the characteristics?

A

2 types of sensory zones that fire from spinal stretch

  • central (bag) region: Ia receptors
  • -dynamic, intense firing with stretch onset, then slows down (non-linear)
  • distal regions: II receptors
  • -steady firing, increasing with increasing stretch (linear)
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5
Q

what is perturbation in terms of the arm stretch reflex?

A

increased load (torque) about elbow causes rotation

  • spindle (intrafusal fibers) in biceps are stretched
  • afferent signals sent to spinal cord, activating (recruiting) biceps alpha motoneurons (stretch reflex)
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6
Q

what is correction in terms of the arm stretch reflex?

A

restoration of equilibrium

  • original (homonymous) and synergist (heteronymous) muscles excited
  • antagonist inhibited via Ia inhibitory interneuron and reciprocal inhibition
  • biceps contracts and shortens, elbow flexes, resisting perturbation
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7
Q

what happens when extrafusal (muscle) fibers contract?

A

shortening unloads spindle, eliminating ability to signal further changes in muscle length

  • a silent (unloaded) stretch receptor is no use to CNS
  • solve by firing gamma-motoneurons to shorten intrafusal fibers, so afferent endings remain sensitive to further changes in muscle length
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8
Q

how does alpha motoneuron activation differ with and without gamma motoneuron?

A

w/o: stimulating extrafusal muscle fibers (via alpha) will cause afferent activity with “silence” where the muscle contracted
w/: gamma stimulated causes Ia response to be “filled in”, and remains sensitive to further stretch

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9
Q

what is a golgi tendon organ?

A

type Ib receptor in series with muscle extrafusal fibers (VS spindle in parallel)

  • mechanical deformation of receptor endings opens channels, causing APs
  • active muscle contraction causes strong GTO firing, sensing force generation
  • passive force has little effect on GTO activity
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10
Q

how do muscle spindle (intrafusal muscle fibers, Ia) and golgi muscle tendon (Ib) differ when extrafusal muscle fibers stretch?

A

Ia - steady activity after stretch, none after contract

Ib - activity before/after stretch/contraction, but faster after the event

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11
Q

what is the golgi tendon organ action?

A

activity in Ib afferent (checks and balances)

  • fires Ib inhibitory interneuron
  • inhibits homonymous muscle
  • excites antagonist muscle
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12
Q

what is the flexion crossed extension reflex?

A

protective role - faster limb withdrawal than voluntary RXN time (if step on tack with one leg, and shift weight to another)

  • muscles working in agonist/antagonist pairs (reciprocal organization)
  • opposite activity ACROSS midline
  • reflexes are layered (growing complexity)
  • interneurons serve multiple roles:
  • -amplify/distribute excitatory signal
  • -change signal polarity
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13
Q

how would the muscles work if you stepped on a tack with your right foot?

A
  1. cutaneous afferent fiber from nociceptor (A-delta) sends info to 4 different receptors
  2. R extensor inhibited, R flexor stimulated so R leg flexes to withdraw
  3. L extensor stimulated, L flexor inhibted so L leg extends to support
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14
Q

what is the central pattern generator? how can it be activated?

A

CPG - distributed network of neurons in spinal cord that can produce coordinated movements in the absence of higher input

  • activated by e-stimulation in brainstem (mesencephalic lomotor region) or via pharmacologic input (noradrenaline)
  • can be modified by training
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15
Q

what are implications of CPG training?

A
  • spinal cord plasticity
  • activity-dependent
  • rehabilitation (?)
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16
Q

what are electromyograms used for?

A

to measure muscle units of a single motor unit, as they serve as natural biologic amplifiers of a nerve AP

17
Q

how easy/hard is it to activate and monitor/record muscle fibers?

A
hard to activate with direct electrical stimulation
-long pulse duration needed
-burns are common
easy to monitor/record activity
-commonly used for diagnosis if NMD
18
Q

what does the electrode design influence?

A
  1. volume of muscle recorded from
  2. signal characteristics
  3. frequency response
  4. sensitivity to noise (more selective = more noise)
19
Q

what are types of electrode designs?

A
  1. surface (non-selective; have conductive paste since skin is an insulator; can also use for stimulation)
  2. needle (non-selective sub-dermal; used in operating room)
  3. concentric bipolar (selective; standard for EMG exams)
  4. hollow cannula (selective; insulated along length, except at tip; used for Botox injections)
20
Q

why is standardization important?

A

allows comparison of findings between labs and across subjects

21
Q

what does it mean if the patient…

  1. is unable to recruit any motor units?
  2. can only recruit a few motor units with very slow firing rates?
  3. has APs that are unusually low amplitude?
  4. has APs that are unusually large amplitude?
A
  1. spinal cord injury/stroke
  2. incomplete spinal cord injury
  3. myasthenia gravis (unmyelinated)
  4. denervated then partially innervated