14-15. Motor Units & Recruitment Flashcards
Define the motor unit. What does it consist of?
- Functional unit of the neuromuscular system
- Alpha motor neuron + muscle fibers innervated
Where are motoneurons located?
Ventral horn of spinal gray matter
From where does the alpha motor neuron cell body receive info? What kind of info does it receive?
- Receive input from neurons in the motor cortex and brainstem
- Receive additional excitatory and inhibitory input from spinal cord interneurons and muscle afferents
When is an action potential produced in the cell body?
When the membrane potential exceeds threshold potential
What is a motor pool? How many motor pools does a muscle have?
- Motoneurons innervating a specific muscle
- Every muscle has one motor pool
What is the innervation ratio?
Number of muscle fibers innervated by a single motor neuron
What is the relationship between innervation ratio and motor unit type?
Higher innervation ratio = larger alpha, beta, gamma
What are the 2 properties used to classify motor units? Based on these properties, what are the classifications of motor units?
- Fiber size and conduction velocity
- Alpha = largest, fastest
- Beta = next largest, fastest
- Gamma = smallest, slowest
What is the basic function of sensory receptors?
Provide info to the system on its own state and that of its surrounding
What is feedback?
The flow of info from sensory receptors to the CNS
Afferent vs. Efferent neurons
- Afferent provide feedback info to CNS
- Efferent involved in activating muscle
What are muscle spindles? Location?
- Intrafusal fiber = collection of miniature muscle fibers enclosed in a connective tissue capsule
- Located in parallel w/ extrafusal fibers of skeletal muscle
What do muscle spindles primarily sense?
Muscle length and rate of change of muscle length
What is the Ia afferent fiber? Function? Which sensory fiber is Ia afferent?
- Monosynaptic connection to alpha-motoneurons
- Increases excitability of alpha motoneurons w/ muscle from which it originates
- Muscle spindle
What is reciprocal-inhibition?
When a motoneuron excites muscle from which it came & inhibits antagonistic muscle
What are the parts of the reflex arc during the stretch reflex.
- Receptor = muscle spindle
- Afferent pathway = Ia sensory axons
- Integrating center = spinal cord (interneurons, motoneurons)
- Efferent pathway = alpha motor axons
- Effector organ = muscle
Which sensory receptor is Ib afferent?
Golgi tendon organ
What does the golgi tendon organ primarily sense?
Muscle tension
Where are golgi tendon organs located?
Primarily at musculotendinous junctions in series w/ muscle fibers
What is the function of golgi tendon organs?
Reduce the excitability of alpha and gamma motoneurons
What is the function of golgi tendon organs?
Reduce the excitability of alpha and gamma motoneurons
What reflex do muscle spindles respond to?
Stretch reflex
What reflex do golgi tendon organs respond to?
Disynaptic reflex
What are joint receptors? Function?
- Mechanoreceptors
- Provide info on position, displacement, velocity, acceleration of movement, noxious stimuli
What do cutaneous receptors sense?
Pressure and stretch of the skin
What do free nerve endings innervate? Function?
- Innervate almost all structures w/in the muscle belly
- Detect noxious stimuli (pressure, temp, chemicals)
What do free nerve endings innervate? Function?
- Innervate almost all structures w/in the muscle belly
- Detect noxious stimuli (pressure, temp, chemicals)
What are ERGO receptors? Location?
- Mechanoreceptors and metaboreceptors that provide rapid input (feed forward) from muscles to CNS
- In skeletal muscle
What do ERGO receptors sense?
- Tension
- Rate of tension development
- Metabolic rate
What variables do ERGO receptors influence?
HR, BP, rate of breathing (Ve)
How fast do mechanoreceptors respond? What do they respond to?
W/in about 200 msec from onset of contraction
How fast do metaboreceptors respond? What do they respond to?
- In about 4-5 secs
- Highly sensitive to metabolic rate
What are ERGO receptors? Location?
- Mechanoreceptors and metaboreceptors that provide rapid input (feed forward) from muscles to CNS (cardiovascular & respiratory centers)
- In skeletal muscle
How fast do metaboreceptors respond? What do they respond to?
- In about 4-5 secs
- Highly sensitive to metabolic rate
What is the equation for Fatigue Index
FI = force after 2 mins / initial force
What are the 3 classifications of motor units?
- S = slow
- FR = fast fatigue resistant
- FF = fast fatiguable
What is the fatigue index of slow motor units?
1.0
What is the fatigue index of FR motor units?
> 0.75
What are the 2 properties used to classify motor units? Based on these properties, what are the classifications of motor units?
- Fiber size and conduction velocity
- Alpha = largest, fastest
- Beta = next largest, fastest
- Gamma = smallest, slowest
What is the fatigue index of FF motor units?
What 3 properties control the maximum tension that can be produced by a motor unit?
- Innervation ratio
- Fiber CSA
- Specific tension
What property is the best predictor of the max tension of a motor unit?
Innervation ratio
What is the size principle?
Force development w/ increased motor unit recruitment is nonlinear
According to the size principle, in what order are motor units recruited?
- Smaller motor units that generate low force are recruited first
- Larger motor units that generate greater force are recruited later
What are the factors affecting total muscle tension development?
- Number of motor units recruited (as shown by total CSA of muscle that is active)
- Frequency of motor neuron firing
- Optimization of muscle fiber length
Why do FF units produce greater max tension than S units?
- Larger
- Innervate more muscle fibers
What is the relationship between motor unit types and muscle fiber types?
- Alpha = FF = FG = type IIx
- Beta = FR = FOG = type IIa
- Gamma = S = S = type I
