Motor systems Flashcards
Degeneration of α motor neurons are the hallmarks of what
Amyotrophic lateral sclerosis (ALS) -
progressive weakness and wasting of skeletal muscles
describe generation of graded forces
Size principle
small α motor neurons will innervate small number of muscle fibers
- generate small forces
large α motor neurons will innervate a large number of muscle fibers =
- generate large forces
NS will recruit small motor units before it recruites large motor units (α MN + fibers) = SIZE PRINCIPLE
Motor units are plastic. Exercise and chronic stim. can shift motor unit phenotype from ___ to ___
fast to slow
- slowing fatigability and increasing endurance capacity
Intrafusal muscle fibers
- Innervated by ___ motor neurons
- Sensory afferents that convey stretch of muscle spindle and their purpose
- γ
- group Ia and II sensory afferent (parallel arrangement)
- these fire AP in response to stretch and contact α MN in spinal cord, which trigger muscle contraction of the homonymous muscle fiber .
(passive stretch lengthens muscle before straining the tendon - GTo for this later)
During voluntary contraction , which fires first? α or γ?
Both fire together
= shortens both intra and extrafusal muscle fibers together, and maintains sensitivity to stretch
(you can detect stretch of a contracted or relaxed muscle)
Golgi tendon organs (GTO) (at jxn of a muscle + tendon)
Sensory afferents
GTO: collagen structures
type Ib sensory afferents (in series)
coil around and within collagen strands.
Particularly sensitive to muscle tension, (since passive stretch (intrafusa) lengthens the muscle b4 straining the tendon.)
- During muscle contraction, F increases the tension on collagen strands and pinches the intertwined afferent fibers
(chinese finger trap)
Hammer tap: stimulates activity of which sensory afferents?
Ia sensory axons - fattest and fastest axons in body.
- reports stretch of muscle spindle (not tension)
→
relayed to and activates α MN in the spinal cord →
contracts the stretched muscle
1a sensory afferents (found in stretch reflex) makes contact with what?
- α MN that contact the homonymous muscle
- inhibitory interneurons that reside in spinal cord that then inhibit motor neurons controlling opposing antagonist muscle.
- synergist and antagonists are coordinated to contract the homonymous muscle and relax the antagonist simultaneously. = JERKING motion
Flexor-extensor coupling
relax synergist, contracts antagonist (opp of stretch reflex with 1a aff)
synergist and antagonist muscular is coordinated
instead of jerking = you see musculature protection from overexertion
- helpful to assessing lesions
1b sensory afferents (found in tension reflex) makes contact with what?
- inhibitory interneurons
- excitatory interneurons
- golgi tendon organs
- relax synergist, contracts antagonist (opp of stretch reflex with 1a aff)
If you come in contact with a lighter load than expected, what response will your motor neuron drive receive?
α + γ fire together Mismatch → 1a afferents will ↓ their firing rate → reduces α motor neuron drive → reduces muscle contraction
(the coactivation of both α + γ motor neurons serve as mech for rapid error correction, but the γ motor neurons are not engaged during reflexive contractions)
Crossed extension reflex (step on tack, swimming)
cutaneous sensory receptors (ie: nociceptors) innervate spinal inter neuronal motor networks:
- inhibitory interneurons
- excitatory interneurons
These coordinate IPS extensor relaxation, flexor contraction
Also coordinate contralateral converse extensor contraction and flexor relaxation
= CENTRAL PATTERN GENERATORS (CPG)
CPGs responsible for
coordinated extension-flexion alternation during locomotion
How does sensory info reach the vestibular nuclei? Then where do they project to?
sensory info detected by semicircular canals in inner ear → sent via 8th CN to vestibular nuclei →
- medial vestibulospinal tract → medial spinal cord
- (regulates head orientation and neck m. activation) - lateral vestibulospinal tract → lateral motor pools
- (controls proximal limb musculature)
from vestibular nuclei: project bilaterally to the abducens nuclei → occulomotor nucleus medially (or straight to lateral rectus) → medial rectus (
Vestibulospinal reflex vs vestibular ocular reflex
Vestibulospinal:
- protective reflex (arms out when falling)
VOR
- keep gaze fixed
