Skeletal Muscle Physiology Flashcards
Describe Type IIX fibers
- Fatigable
- Rely on glycolysis
- Few mitochondria; low concentrations of myoglobin and oxidative enzymes (white)
- High glycolytic enzyme content
At what fruequencys do slow twitch fibers tetanize?
Why?
Lower stimulation frequencies, partly due to longer duration of contraction.
Why do fast twitch fibers develop larger maximal force?
Greater twitch tension and larger motor units
Identify fiber type of each graph
Which fiber types are represented on the fatigability graphs shown?
(Left to right) - Type I, Type IIa, Type IIX(b)
Speed of shortening is correlated with…
myosin ATPase activity and firing rate
What changes can be seen as a result of adaptation to training?
•Skeletal muscle plasticity
•Response to changes in functional demands
–Fiber-type composition, MHC isoform content
–Fiber size
–Satellite cells and myonuclei
–Myonuclear domain
–Mitochondrial content
–Capillary density
–Oxidative enzymes
The two main purposes of proprioceptors are?
- Identification of external objects
- Accurately guide movement
What are the two muscle proprioceptors, what do they do?
- Muscle spindles:Detect changes in muscle length and rate of stretch
- Golgi tendon organs:Detect muscle tension/strain in m. tendon (near MTJ)
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Describe the fiber orientation of muscle spindles
Intrafusal muscle fibers aligned in parallel with force-generating extrafusal fibers
How are golgi tendon organs aligned?
aligned in series with extrafusal fibers
Describe the sensory components of muscle spindle fibers
•Afferent and efferent innervation
(primarily sensory in function)
Describe the shown phases of the pathways involved in the monosynaptic stretch reflex and coactivation of alpha and gamma motor neurons
- Afferent input from sensory endings of muscle spindle fiber
- Alpha motor neuron output to regular skeletal-muscle fiber
- Stretch reflex pathway
- Gamma motor-neuron output to contractile end region of spindle fiber
- Descending pathways coactivating alpha and gamma motor neurons
What is alpha-gamma coactivation important for?
Important for maintenance of Proprioceptor sensitivity
What are the 3 types of basic spinal reflexes?
- monosyaptic reflex/stretch reflex
- reciprocal inhibition
- autogenic inhibition
Define motor reflex
Rapid, stereotyped motor response to a particular sensory stimulus
Motor neurons receive many synaptic inputs within the brain and spinal cord
What are the 5 Components of a Reflex Arc?
- Receptor
- Afferent Pathway
- Integration Center (CNS)
- Efferent Pathway
- Effector
Describe the stretch reflex
What is being activated here?
- Most simple reflex, involves 1 synapse
- When skeletal m. is abruptly stretched,a rapid, reflexive contraction of the same muscle can occur
- Contraction increases muscle tension & opposes the initial stretch
- Stretch reflex is especially strong in extensor muscles
Activation of muscle spindle fibers
What are the 5 components of this reflex arc?
- Receptor - muscle spindle
- Afferent - AP to spinal cord
- Integration center - Spinal cord, (interneurons if not monosynaptic, which this is)
- Efferent - AP of alpha motor neuron
- Effector - Biceps Brachii (Plus inhibition of triceps brachii)
What are the two types of inhibition we covered?
- Reciprocal inhibition
- Autogenic Inhibition
Describe reciprocal inhibition
As a stretched muscle is being stimulated to contract, parallel circuits inhibit the α- motor neurons of the antagonist
Group Ia afferent axons excite inhibitory interneurons which synapse with α-motor neurons of the antagonist muscle.
Describe Autogenic inhibition
The GTO circuit inhibits the muscle with increased tension & excites the antagonist (multiple synapses)
GTO group Ib afferent axons excite both excitatory AND inhibitory interneurons
Describe the golgi tendon organ reflexes
GTO response is generally opposite the stretch reflex
GTO-mediated reflexes may act to control muscle force & joint stability
Describe the nerve endings and location of the golgi tendon organ
- Bare nerve endings of group Ib axons
- Endings invested in encapsulated collagen matrix
- Located at musculotendinous junction
mechanosensitive nerve endings of the GTO → action potential
Define Twitch…
Mechanical response to 1 AP (~100msec)
Describe the latent phase of muscle twitch
Time from AP initiation to cross-bridge formation, start of contraction (~1-3 msec
Describe the contraction time of muscle twitch
Beginning of contraction to beginning of relaxation (peak tension) (~50msec)
- when active sites are exposed until they begin to be covered up
- when [Ca2+] is high enough to keepactive sites exposed
Describe the relaxation time of muscle twitch
Peak tension to complete relaxation (~50msec)
- ↓ tension as active sites are covered
- time for Ca2+ sequestering into the SR
What is the total force developed by a muscle?
•Total force (tension) generated = Sum of forces independently produced by many cycling cross-bridges
The Number of simultaneously cycling cross-bridges can vary with:
– Initial length of muscle fiber: (Length-Tension Relationship)
–Pattern or frequency of muscle fiber stimulation
•Frequency or Twitch Summation (frequency)
•Multiple-Fiber or Multiple Motor Unit Summation
Describe the factors influencing tension development in skeletal muscle with regard to the number of fibers contracting.
Describe the factors influencing tension development in skeletal muscle with regard to the tension developed by each fiber.
Describe isometric contractions
- Muscle length is constant (“same length”)
- Stimulation causes increase in tension, but no shortening
- Force production is equal to resistance (no movement)
Describe isotonic contractions
- Muscle contraction occurs at “constant” load (“same tone”)
- Not really a constant force (dynamic movement)
–Length changes during production of tension: muscle length, joint angle & leverage changes with ROM à amount of force production also changes through the ROM
What are the 2 phases of isotonic contractions?
Concentric - muscle shortens as tension is produced
Eccentric - muscle lengthens as tension is produced
How does muscle length influence tension development? What is force production dependent on in isometric contractions?
By determining region of overlap between actin and myosin.
Force production in isometric contraction depends on initial fiber length.
What is total tension?
Passive + active tension
Describe passive tension
- Tension measured prior to muscle contraction
- Passive tension increases as a fiber is progressively lengthened (stretched). Muscle becomes stiffer as it is distended.
Describe active tension
•When muscle is stimulated to contract at any fixed length (isometric), cross-bridge cycling produces active tension in addition to passive tension
- Small when fiber length is < ~70-85 % of resting length
- Small when fiber length is > ~ 130-150 % of resting length
When is active tension maximal?
Near 100% of normal muscle length, where there is maximal overlap of actin and myosin filaments
What is maximum velocity (Vmax) determined by?
Determined primarily by the maximum velocity of the myosin ATPase enzyme
- Vmax varies with fiber type
Define Work
work = load x displacement
define power
power = work/time
Where is power maximal? Where is it zero?
At intermediate loads, where both forve and velocity are moderate.
Zero at maximum load, where V = 0, and zero load where F = 0
Describe the frequency (twitch) summation known as tetanus (not to be confused with the pathological conditions due to toxins or hypocalcemia)
twitches merge into a smooth sustained maximal contraction
Describe low stimulation frequencies
Tension developed falls to resting level between individual twitches
- Duration of each twitch is long vs. duration of AP
- Possible to initiate a second action potential before the tension of the first contraction has fully decreased
- The next AP stimulates a twitch which is added to the residual tension of the first twitch = greater total tension
Describe high stimulation frequencies
Individual twitches fuse resulting in a steady plateau of muscle tension
• Time between successive APs is too short to return enough Ca2+ to the SR to lower [Ca2+]i below relaxation level
- ↑[Ca2+]i is sustained until tetanic stimulus ceases
- Tension increases very little at stimulation frequencies greater than the fusion frequency that causes tetanus
What three things does whole muscle tension depend on?
- Size of the muscle
- Number motor units recruited
- Size of each motor unit being recruited
What muscles have few muscle fibers per motor unit?
muscles performing refined, delicate movements
What muscles have larger numbers of fibers per motor unit?
muscles performing stronger, coarser movements
Define multiple-fiber/multiple motor unit summation
What is the motor neuron pool?
Excitation of additional motor neuron cell bodies recruits the fibers of the motor unit, adding them to the contractile pool
Group of all motor neurons innervating a single muscle
How can CNS control the degree of force?
through the number of individual fibers it stimulates
What purposes does asynchronous recruitment serve?
- Asynchronous activation of individual motor units by the CNS so that some units develop tension while others relax
- Delays or prevents muscle fatigue during submaximal contractions
What does Henneman’s size principle say?
Progressive recruitment small —> larger motor units
Characterize the stimulation required to recruit small motor units
• Small motor units are recruited with minimal neuronal stimulation:
A given excitatory stimulus will generate a larger EPSP in motor neurons with smaller cell bodies
What are the motor unit types? Describe them, and their order of recruitment.
•Slow-Twitch Motor Units (I):
–Small amount of force, prolonged period of time
•Fast-Twitch Fatigue-Resistant Motor Units (FR):
–Moderate amount of force, sustained for moderate amount of time
•Fast-Twitch Fatigable Motor Units (FF):
–Large amount of force, brief period of time•Order of Recruitment:
I ---\> FR ---\> FF
Describe the following properties of type I and II nerves
Cell Diameter
Conduction velocity
Excitability
Type I
Small cell diameter
Fast conduction velocity
High excitability
Type II
Large cell diameter
Very fast conduction velocity
low excitability
Define muscle fatigue
nability to maintain desired power output, decline in force production & shortening velocity
During fatigue, what causes the decline in maximal force production?
What causes the diminished rates of force production and relaxation?
- Decline in maximal force production: results from reduction in number of active cross-bridges
- Lower rates of force production and relaxation: due to impaired release and reuptake of Ca2+ from the sarcoplasmic reticulum (SR
What factors contribute to fatigue?
Motivation, physical fitness, nutritional status, and the type of motor units (i.e., fibers) recruited with respect to the intensity and duration of activity
Describe central fatigue
Reflects changes in the CNS (Brain à motor neuron cell bodies)
Possible involvement of:
–Altered input from muscle sensory nerve fibers
–Reduced excitatory input to motor control centers of the brain & SC
–Resulting in altered excitability of α - and γ - motor neurons
What can help individuals overcome central fatigue?
Training
Ex: Minor role in limiting performance of highly trained athletes who have learned to pace and are mentally conditioned to stress and discomfort
Will
Ex: External sensory input (cheering) can increase force production and performance, indicating that pathways superior to corticospinal outputs can oppose central fatigue
Describe peripheral fatigue
Motor neuron axon —> NMJ —> Fiber
Possible involvement of:
–Impaired initiation/propagation of muscle APs
–Impaired release/handling of [Ca2+]i
–Depletion of energy metabolism substrates
–Accumulation of metabolic byproducts
What factors lead to variability in peripheral fatigue and recovery time?
Recruitment pattern of motor units
Metabolic properties of muscle fiber type
What are the three sources of ATP in skeletal muscle.
Anaerobic - fast
- creatine phosphate
- Glycolysis
Aerobic
Oxidative phosphorylation
Describe creatine posphate’s role in ATP production
–Creatine phosphate + ADP → Creatine + ATP
(<< 1 minute of exercise)
What are the 3 major human skeletal muscle fiber types?
–Slow oxidative (Type I) fibers
–Fast-oxidative (Type IIA) fibers
–Fast-glycolytic (Type IIX) fibers
Differences in what correspond to the rate of contraction a muscle fiber is capable of?
mATPase
Describe slow-twitch fibers
•Generally smaller cross-sectional area (CSA)
•Greater oxygen transport capability as oxidative metabolism is 1° source of ATP
–Greater capillary density
–Appear red because due to high concentration of myoglobin ( oxygen-binding protein) in the sarcoplasm
–Low glycogen content and glycolytic enzyme activity
–High mitochondrial and oxidative enzyme content
–Slow but efficient contraction
–Resistant to fatigue
Describe type IIA fast twitch fibers
•Fatigue resistant
•Oxidative metabolism
–similar to Type I in myoglobin content (red) and metabolic machinery
–similar number of mitochondria vs. slow-twitch fibers
•Abundant glycogen
•Ensures adequate ATP generation to compensate for the increased rate of ATP hydrolysis for rapid contraction
What are the two types of fast-twitch fibers?
Type IIA and Type IIX (Previously IIB)
