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
Reticular formation fxn
regulating locomotor speed (due to input from mesencephalic locomotor region)
Anticipatory response to voluntary movement that helps maintain posture and balance (lifing weights)
How does the superior colliculus orient gaze and body position as well as merge auditory and visual space into body coordinates?
descending projections of the colliculospinal (tectospinal) tract target motor neurons that control the axial musculature of the neck to generate coordinated orienting responses
Brodmans area 4 homunculus representation
primary motor cortex (M1)
- disproportionate M1 surface area is devoted to controlling musculature used in fine motor tasks
- movements (not muscles) are mapped
- a true homunculus is not represented in motor cortex
(ie: diff parts of Motor cortex can activate identical muscles in hand)
Premotor cortex will be initiated when?
(25% of corticospinal tracts)
when movement is initiated by external cue (asked to move hand in response to light stimulus)
or
watching an intended consequence (intention)
Supplementary motor cortex will be initiated when?
self cued movements ( ie during mental rehearsal of movement)
Upper or lower motor neuron
- babinski sign
Upper
Upper or lower motor neuron
- lesion of premotor neurons
upper
Upper or lower motor neuron
- contralateral muscle flaccidity
upper
Upper or lower motor neuron
- loss or degeneration of motor neurons in spinal cord
lower
