Motor units, Spinal relexes, SCI Flashcards
1
Q
Alternative names for lower motor neurons
A
- Lower motor neuron
- Alpha motoneuron
- Spinal motoneuron
2
Q
Characteristics of lower motor neurons
A
- Large cell body
- Extensive dendritic tree
- Large axon
- Myelinated with schwann cells
- Rapid conduction velocity (up to 60 m/s)
3
Q
Spinal somatotopy of motoneurons
A
- Medial ventral horn: proximal muscles
- Lateral ventral horn: distal muscles
4
Q
Motor neuron pool
A
- Total of all lower motor neurons innervating a given muscle
- Typically distributed over 2-3 neurologic segments
5
Q
Motor unit components
A
- Cell body
- Axon
- All of the muscle fibers that axon innervates
- Axon only innervates one muscle
6
Q
Muscle unit
A
- Collection of muscle fibers innervated by one axon
- Properties of muscle fibers within a muscle are about the same
- Normally simultaneous contraction of muscle unit
- Great variety of sizes
7
Q
Neuromuscular junctions transmitter
A
ACh
8
Q
Force generation in muscle
A
- Membrane conducts action potential
- Thick and thin filaments slide past one another–generates force
- Ca2+ binding leads to rotation of the myosin cross-bridge
- ATP consumed for cross bridge release
9
Q
Rigor mortis
A
- Occurs because there’s not enough ATP present
- Cross bridges don’t release and muscle remains contracted
10
Q
Slow motor units
A
- Slow rate of force increase during twitch
- Small peak force
- Little or no force loss with repeated twitches
- Slow to fatigue
11
Q
Fast fatigue resistant motor units
A
- Relatively fast rate of force increase during twitch
- Moderate peak force
- Moderate force loss with repeated twitches
- Moderate to fatigue
12
Q
Fast fatiguable motor units
A
- Fastest rate of force increase during twitch
- Large peak force
- Rapid force loss with repeated twitches
- Fatigues quickly
13
Q
Two ways to modulate force generation
A
- Recruitment: of other motor units
- Rate-coding: of an already firing motor unit
14
Q
Size principle
A
-Smaller motor units are the first to be activated
15
Q
Stretch receptor
A
- Senses changes in muscle length
- Only afferent in CNS that receives a nerve supply
- Arranged in parallel with extrafusal (regular) muscle fibers
- Target for gamma motor neurons
16
Q
Gamma motor neurons
A
- Target stretch receptors
- Smaller soma than A motor neurons
- Lower conduction velocity
- Innervate intrafusal muscle fibers
17
Q
Intrafusal muscle fibers
A
- Smaller diameter
- Effectively no force generation
- Affect stiffness of sensory region
- Nuclear bag and nuclear chain types
- Central (bag) region: Ia receptors (fastest)
- Distal regions: type II receptors
18
Q
Type Ia axons
A
- Dynamic, intense firing with a stretch onset and then slows down
- Nonlinear
19
Q
Type II axons
A
- Steady firing, increases with increased stretch
- Linear
20
Q
Stretch reflex
A
- Muscles typically work as agonist/antagonist pairs
- Increased load causes stretch of intrafusal fibers (spindle)
- Afferent signals sent to spinal cord, activating alpha motoneurons
- Original (homonymous) muscle excited
- Synergist (heteronymous) muscles also excited
- Antagonist muscle inhibited by Ia inhibitory interneuron
- Stretch corrected
21
Q
Gamma motoneurons and the stretch reflex
A
- When extrafusal fibers contract they fire to shorten intra-fusal fibers
- Afferent endings remain sensitive to further changes in muscle length
22
Q
Golgi tendon organ
A
- Type Ib receptor in series with muscle extrafusal fibers (ie connects muscle to tendon)
- Active muscle contraction causes strong GTO firing–can sense force generation
- Ib afferent fires Ib inhibitory interneuron which inhibits the homonymous muscle
- Ib afferent also fires disynaptic pathway that excites the antagonist muscle
23
Q
Flexion crossed-extension reflex
A
- Protective–faster limb withdrawal than voluntary reaction time
- Opposite activity across the midline
- ie stimulated leg flexes to withdraw and opposite leg extends to support
24
Q
Central pattern generator
A
- Distributed network of neurons in spinal cord that can produce coordinated movements in the absence of higher inputs
- Cat example: flexors activated in swing phase, extensors in stance phase
25
Paraparesis/plegia
-Injury caudal to T1
26
-Quadriparesis/plegi
-Neck injury
27
Immediate acute consequences of SCI
- Weakness
- Sensory loss
- Sensory abnormalities--parasthesia, neuropathic pain
- Hypotension (low BP)
- Spinal cord ischemia
- Urinary retention
- Orthopedic pain
28
Long-term chronic consequences of SCI
- Involuntary movements (spasticity, myoclonus)
- Bladder problems--number 1 complaint 5 yrs post SCI
- Spastic or flaccid bladder
- Sexual dysfunction
- Blood clots
- Pressure ulcers
- Cutaneous ischemia--repositioning is vital
- Autonomic dysfunction
- Thermoregulation largely absent
- Autonomic dysreflexia
- Metabolic disorders
- Chronic hypertension
- Musculoskeletal breakdown
- Fractures
29
Spastic (neurogenic bladder)
- Injury at or above T10
- Voluntary relaxation of external sphincter is lost
- Detrusor contracts with continued filling
- Urine flows in dribbles only when detrusor emptying pressure>sphincter closure pressure
- Incomplete bladder emptying--chronic infections
30
Male sexual dysfunction after SCI
- Erection unlikely, ejaculation rare
- Sperm viability impaired
- Fertility reduced
31
Female sexual dysfunction after SCI
- Fertility largely unaffected
| - C-section strongly recommended due to autonomic dysreflexia
32
Autonomic dysreflexia
- Crazy high increased in BP (like, 240/160) for some sensory inputs along with bradycardia (40bpm), for no apparent reason
- Paradoxical hypertension
- Noxious inputs commonly cause it
- Typically seen after cervical or high thoracic SCI and in complete SCI
33
Possible causes of autonomic dysreflexia
- Loss of inhibition from brain/brainstem regions
- Excessive sensory response to stimuli
- Excessive sympathetic response to normal afferent activity*****Sensory afferents making novel synaptic contacts onto preganglionic sympathetic neurons in high thoracic spinal cord
- Excessive vascular response to normal sympathetic activity
34
ASIA A Complete
-No motor or sensory function below injury
35
ASIA B Incomplete
-Sensory only below the injury, including S4-S5 segment
36
ASIA C Incomplete
-Sensation and limited motor function below the injury level
37
ASIA D Incomplete
-Sensation and significant motor function below the injury level
38
ASIA E Normal
-Sensory and motor function normal
39
Factors that may damage nerves within region of injury
- Interrupted blood flow--ischemia
- Local toxins
- Glutamate release (excitotoxicity)
- O2 radicals (membrane breakdown)
40
3 approaches to SCI treatment
- Protect surviving cells and axons--neuroprotection
- Replace cells, establish a growth-permissive environment--neurorestoration
- Strengthen existing systems--neurorehabilitation
41
CPG for stepping in humans
- Low gain (high threshold)
- Spontaneous expression always associated with pathology
- Noxious input to cord
- Can't train to improve voluntary walking in persons with incomplete SCI--not due to neuroplasticity at least
42
Interlimb reflex characteristics
- In ALL personals with chronic cervical SCI
- Common in distal limb muscles
- Excitatory response
- Appear months to years later
43
Plasticity and SCI--Interlimb reflexes mechanism
-LMN lose inneveration from UMN
-Sensory fibers from more caudal regions denied targets in higher level, prompts new growth caudal to level or injury
-New contacts made to LMN--regenerative sprouting
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