Week 3 - Spinal Reflexes and Electromyogram

Question 1

what are characteristics of sensory receptors acting on spinal motoneurons?

Answer 1

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

Question 2

what is a stretch receptor?

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

Answer 2

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

Question 3

how do gamma motoneurons compare with alpha motoneurons?

Answer 3

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

Question 4

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

Answer 4

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)

Question 5

what is perturbation in terms of the arm stretch reflex?

Answer 5

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)

Question 6

what is correction in terms of the arm stretch reflex?

Answer 6

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

Question 7

what happens when extrafusal (muscle) fibers contract?

Answer 7

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

Question 8

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

Answer 8

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

Question 9

what is a golgi tendon organ?

Answer 9

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

Question 10

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

Answer 10

Ia - steady activity after stretch, none after contract

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

Question 11

what is the golgi tendon organ action?

Answer 11

activity in Ib afferent (checks and balances)

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

Question 12

what is the flexion crossed extension reflex?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

what are implications of CPG training?

Answer 15

• spinal cord plasticity
• activity-dependent
• rehabilitation (?)

Question 16

what are electromyograms used for?

Answer 16

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

Question 17

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

Answer 17

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

Question 18

what does the electrode design influence?

Answer 18

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

Question 19

what are types of electrode designs?

Answer 19

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)

Question 20

why is standardization important?

Answer 20

allows comparison of findings between labs and across subjects

Question 21

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?

Answer 21

1. spinal cord injury/stroke
2. incomplete spinal cord injury
3. myasthenia gravis (unmyelinated)
4. denervated then partially innervated