Lecture 17 (Posture) Flashcards
Motor unit
A single motor neuron and all the muscle fibers it innervates
Slow-twitch muscle fibers
Oxidative - red (myoglobin)
- Aerobic
- Mitochondria
- Supplied with O2-carrying blood
- Contract weakly & slowly
- able to sustain contraction
Fast-twitch (IIX)
Glycolytic
1. White
2. Poorer blood supply
3. Anaerobic catabolism
4. Extensive sarcoplasmic reticula (store and reabsorb Ca2+)
5. ATPase is much faster
6. Contract rapidly but fatigue fast
Fast-twitch (IIA)
Oxidative
1. White or pink
2. Intermediate fibers
3. F-t speed combined with oxidative capacity
4. Fatigue resistant - compared to IIX
Fast vs. Slow-twitch
Vary in contraction speed
And ATP production (oxidative or glycolytic)
Most muscles contain all three
1b - GTO afferent fibers
Adjusts muscle contraction
1a - primary spindle endings
Afferent fibers
Monitor rate and degree of stretch of spindle
Gp II - Secondary spindle endings
- Medium size
- Myelinated
- Afferents from muscle spindle
- Monitor degree and stretch of muscle
Gp IV - Free nerve endings
- Unmyelinated
- Activated by noxious stimuli
Roles postural system when stationary
- Maintain balance
- Remodel stance in preparation for voluntary movement
- Alignment of body to maintain erect stance
Roles postural system in movement
- Maintains balance while constant shift in center of gravity
Postural UMNs
Originate in brain stem
1. Tectospinal
2. Reticulospinal
3. Medial & lateral vestibulospinal
4. Rubrospinal
(5. Corticospinal)
Medial UMN -> LMN -> posture and girdle muscles
Lateral UMN -> LMN -> distal muscles (fine movement)
Tectospinal
- Arises in superior colliculus
- Crosses in dorsal tegmental decussation
- Synapse in cervical spinal cord - head
- Coordinates head and eye movements
- Mediates reflex postural movements of the head, primarily in response to visual stimuli
Reticulospinal
Medial (MRST)
1. Originates in pontine RF
2. Descend ipsilaterally
3. Facilitates ipsilateral LMN innervating postural muscles and limb extensors
Lateral (LRST)
1. Originates in medullary RF
2. Descend bilaterally
3. Has opposite effect to MRST
Vestibulospinal
Support against gravity
Make postural adjustments for balance
Medial (MVST)
1. Bilateral projection (cervical and thoracic)
2. Excitatory and inhibitory
3. Terminate on neck flexor and extensor MN
Lateral (LVST)
1. Ipsilateral
2. Excitatory
3. Facilitates LMN to extensors and indirect relaxation of flexors
Vestibular nucleus (VN)
- receives direct proprioceptive input
- supports extensor activity in upper & lower body
- Motor Cortex tonically inhibits VN
Vestibular-Ocular Reflex (VOR)
- Cancels effect of head movements
- Eyes able to maintain fixation during head movement
Rubrospinal
- Originates in red nucleus
- Crosses to opposite side
- Descends and synapses on LMN innervating upper limb flexor muscles (exc)
- RN (facilitatory to flexors in upper body) and PRF (facilitatory to extensors in upper/lower body)
Decorticate rigidity
- Lesion occurs above brain stem
- All postural centers still influence LMN
- Loss of cortical influence
Decerebrate rigidity
- Lesion between mid brain and pons
- Loss of cortical and RN influence
Paresis
Loss of voluntary activation of muscle
