Skeletal muscle phsiology 10/16 Flashcards
1
Q
Muscle Twitch
A
- mechanical response to 1 AP (100 msec)
- Latent Period: time from AP initiation to cross-bridge formation (1-3 msec)
- Contraction time: Beginning of contraction to beginning of rlaxtion (peak tension) (50 msec)
- when active sites are exposed until they begin to be covered up, due to high [Ca2+]
- Relaxation time: peak tension to complete relaxation- [Ca2+] is sequestered into the SR (50 msec)
2
Q
Total Force produced by muscle
A
- directly related by home many cycling cross-bridges are formed: sum of forces
- length-tension relationship
- frequency/twitch summation
- multiple motor unit summation
3
Q
Isometric Contractions
A
- Muscle length is constant (“same length”)
- Stimulation causes increase in tension, but no shortening
- Force production is equal to resistance (no movement)
4
Q
Isotonic Contractions
A
Tension exceeds the load
- 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 amount of force production also changes through the ROM
2 phases:
- *concentric** - muscle shortens as tension is produced
- *eccentric** - muscle lengthens as tension is produced
5
Q
Length-tension Relationship
A
- Isometric contractions: Force production depends on initial fiber length
- Muscle length influences tension development by determining region of overlap between actin & myosin
- Passive tension: tension measured prior to muscle contraction (the longer you stretch it the more stiff it becomes indep. of actin contribution to tension)
- Active tension: total tension - passive tension (when muscle is stimulated to contract at any fixed length - isometric - cross bridge cycling produces active tension in addition to passive tension)
- active tension is max near 100% normal muscle length
6
Q
increased/decreased fiber length
A
- increased: no interaction/overlap occurs between actin and myosin and no development of active tension
- decreased: actin and myosin increase overlap, resulting in a distorted relationship between actin and myosin, decreasing tension
- you want somewhere in between for optimal amount of force
7
Q
Total Tension
A
Passive + Active tension
8
Q
what is force-velocity relationship with isotonic contractions?
A
- as load increases the velocity decreases–> lighter loads can be lifted faster
- Maximum Velocity: primarily determined by myosin ATPase enzyme (this varies with fiber type)
- the maximal velocity is dependent of fiber length (solely dep. on ATPase) - while the maximal load carried is dependent upon fiber length.
9
Q
What is work vs. power?
A
work = load x displacement (only when muscle displaces a load)
power = work/time (rate work is performed)
- maximal at intermediate loads
- zero at maximum load
- zero at zero load
10
Q
What is frequency summation (twitch summation)?
A
- tension developed by a single fiber, which depends on stimulation frequency. repetitive stimulation results in increasing tummation
- contractile responses (twitches) can be summed if APs fire rapidly, resulting in no fiber relaxation between stimuli due to sustained elevation of Ca2+
–> tetanus = smooth sustained contraction
- duration of each twitch is long vs. duration of AP: as frequency of stimulation increases, no Ca2+ is sequestered and tension is maintained
11
Q
Motor Unit vs. Motor pool
A
- motor unit = a single motor neuron and the muscle cells it innervates
- motor pool = many motor neurons which innervate a motor unit with the muscle
12
Q
what does whole muscle tension depend on?
A
- size of muscle
- number of motor units recruited
- size of each motor unit being recruited
- Muscles performing refined, delicate movements have few muscle fibers per motor unit
- Muscles performing stronger, coarser movements have a large number of fibers per motor unit
13
Q
Multiple-fiber/Multiple Motor Unit Summation (MMUS)
A
- Excitation of additional motor neuron cell bodies recruits the fibers of the motor unit, adding them to the contractile pool
- Whole Skeletal Muscle:
Increased force production via summation of multiple fibers - this is how CNS can control muscle force by the number of individual fibers that it stimulates
14
Q
Asychronous Recruitment
A
- cycle through which motor units are being recruited, cycel through to delay fatigue during submaximal contractions
- Multiple-fiber summation: Mechanism allowing whole-muscle force development to be relatively constant
- Asynchronous activation of individual motor units by the CNS so that some units develop tension while others relax
15
Q
Electromyography
A
Surface EMG:
- Gross measure of electrical activity, projected to skin surface
- Records sum of all electrical activity (APs of all activated fibers in all activated motor units)
central fatigue = electical activity would drop off
peripheral fatigue = electrical activity would remain constant - electrical activity is still there
16
Q
What is Henneman’s Size Principle?
A
- the order by which motor units are recruited has to do with size of motor neurons
- Size of cell body dictates excitability (smaller 1st)
- Given same input: smaller neurons threshold reached sooner
- Motor unit excitability determines degree of use
- Degree of use influences fiber type
- size principle: small motor units are recuirted with minimal neuronal stimulation because a given excitatory stimulus will generate a larger EPSP in motor neurons with small cell bodies
- as neuronal stimulation intensifies, larger motor neurons innervating larger motor units are also recruited
17
Q
Slow-twitch vs fast-twitch fatigue/fatigue resistant
A
Slow-Twitch Motor Units (I):
Small amount of force, prolonged period of time
- Type I
- small cell diameter - why recruited first
- fast conduction velocity
- high excitability
- oxidative
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
- Type II
- large diameter - why recruited later
- very fast conduction velocity
- low excitability
- glycolytic
Order of Recruitment:
** I –> FR –>FF**
18
Q
Muscle fatigue: central vs. peripheral fatigue
A
- inability to maintain desired power output, decline in force production and shortening velocity
- decline in maximal force production, resulting from reduction in number of cross bridges
- lower rates of force production and relaxation: due to impaired release and reuptake of Ca2+ from the SR
- Central Fatigue: reflects changes in the CNS (brain–>motor cell bodies)
- altered input from muscle sensory fibers, reduced excitatory input to motor control centers of the brain and SC, altered excitability of alpha and gamma motor neurons
- ex. mental conditioning to stress and discomfort
- Peripheral Fatigue: motor neuron axon –> NMJ –> fiber
- possible involvement of impaired initiation of APs, impaired release/handling of Ca2+, depletion of energy metabolism, accumulation of metabolic byproducts
19
Q
Anaerobic Sources of Energy
A
1st: Creatine phosphate
* Creatine phosphate + ADP → Creatine + ATP (<< 1 minute of exercise)
2nd : Glycolysis
- 2 ATP/glucose molecule
- 1 glucose molecule → 2 molecules of pyruvic acid
- In the absence of O2: pyruvic acid → Lactic Acid
- Supports anaerobic exercise
- Accumulation of LA likely contributes to muscle soreness
20
Q
Aerobic source of energy
A
Oxidative Phosphorylation
- ATP produced in mitochondria under aerobic conditions
- Supports aerobic exercise
- when exercising time to depletion: creatine phosphate < glycolysis < oxidative phos
21
Q
Difference between fast and slow twitch muscles…
A
Different fiber types expresses different myosin heavy chain (MHC) isoforms
- Difference in mATPase activity corresponds to rate of contraction
- Many fibers express combination of different isoforms; hybrid fibers have contractile rates intermediate between pure fiber types (intermediate between Type I and Type II)
Three major types:
- Slow oxidative (Type I) fibers
- Fast-oxidative (Type IIA) fibers ~ fast fatigue resistant
- Fast-glycolytic (Type IIX) fibers
Classification by:
- Pathway for ATP synthesis (oxidative vs. glycolytic)
- Rate of ATP hydrolysis (mATPase isoform)
- Contractile velocity (fast vs. slow)
22
Q
Slow- Twitch Fibers
A
- 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
- tetanize at lower stimulation frequencies, partly due to longer duration of contraction
23
Q
Fast-Twitch Fibers
A
Type IIA:
- 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
Type IIX (IIB)
- Fatigable
- Rely on glycolysis
- Few mitochondria; low concentrations of myoglobin and oxidative enzymes (white)
- High glycolytic enzyme content
- “couch potato” fibers
- can’t switch between fast twitch and slow switch - but can switch between IIA and IIX
- develop larger maximal force due to greater twitch tension and larger motor units
24
Q
See fiber type chart
A
slide 44
25
Q
Slow twitch vs. fast twitch
A
- properties of fibers as well as firing frequency pattern of motor neuron
Type I:
- Slow twitch, resistant to fatigue, red color, metabolism is oxidative, mitochondria is high, glycogen is low
Type IIa:
- Fast twitch, resistant to fatigue, red myoglobin color, metabolism is oxidative, highish mitochondria, abundant glycogen
Type IIb:
- Fast twitch, fatigable, white (low myoglobin), glycolytic, fewer mitochondria, high glycogen
26
Q
summary of factors influencing tension
A
Number of fibers contracting:
- # motor units recruited
- # muscle fibers per motor unit
- size of muscle (# of muscle fibers available to contract)
tension developed by each contracting fiber:
- freq. of stimulation (twitch summation and tetanus)
- fiber length at onset of contraction (length-tension relationship)
- extent of fatigue (duration of activity, amount of asynchronous recruitment of motor units, type of fiber)
- thickness of fiber (pattern of neural activity, hypertrophy, atrophy, amount of testosterone)
27
Q
Proprioceptors: 2 types
A
Procprioception: detailed info. sensed about location in space; dxn and speed of mvmt.
- Muscle Spindles: detect changes in length and rate of stretch
- “intrafusal muscle fibers” aligned in parallel with force-generating extrafusal fibers
- Golgi tendon organs: detect muscle tension in muscle tendon (near MTJ)
- aligned in series with extrafusal fibers - located at myotendinous junction
28
Q
Muscle Spindle
A
- send proprioceptive info about muscle to CNS
- respond to muscle stretching
- modified skeletal muscle fibers (intrafusal fibers)
- afferent and efferent innervation
- sensory in function - no sig. force contribution
- 2 kinds of sensory endings: primary and secondary endings.
- primary sensory endings: sensitive to changes in length (innervate bag fibers)
- secondary sensory endings: transduce static length: slowly adapting receptors (mostly innervate chain fibers)
- firing rate of sensory fibers increases when the muscle is stretched
29
Q
DRAW OUT THE MUSCLE SPINDLE: SLIDE 58
A
30
Q
Why does the muscle spindle have motor innervation?
A
- When α-motor neurons stimulate force-generating extrafusal fibers to contract, the spindle becomes slackened
- A slackened spindle is insensitive to any further changes in length
- For continued maintenance of spindle sensitivity, γ-motor neurons cause intrafusal fibers to contract in parallel with extrafusal fibers
- The spindle fiber’s ability to change length greatly increases the range of lengths over which the spindle can function
- Sensory response of the spindle depends on BOTH the length of the whole muscle AND on the contractile state of the intrafusal fiber itself
–> need alpha gamma coactivation
31
Q
Monosynpatic reflex
A
- stretch reflex (myotatic reflex
- 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 and opposes the initial stretch
- Stretch reflex is especially strong in extensor muscles
ex. : Patellar tendon reflex (a stretch reflex). Tapping the patellar tendon with a rubber mallet stretches the muscle spindles in the quadriceps femoris muscle. The resulting monosynaptic stretch reflex results in contraction of this extensor muscle, causing the characteristic knee-jerk response.
32
Q
How does monosynaptic reflex occur?
A
- recptor = muscle spindle: recognizes initial stretch
- Affarent = AP to dorsal horn of spinal cord
- integration center = spinal cord (interneurons) [and inhibitory interneurons]
- Efferent = AP of alpha-motor neuron from ventral horn
- Effector: stretched muscle contracts
- Increased muscle length stimulates spindle afferents (particularly group Ia axons from the 1° sensory endings)
- Group Ia sensory axons terminate monosynaptically in the spinal cord on α-motor neurons innervating the same m.
- Stretching a muscle causes rapid feedback & excitation of the same muscle through 1 sensory neuron, 1 synapse, and 1 motor neuron
Reciprocal innervation:
- Simultaneously: As the stretched muscle is being stimulated to contract, parallel circuits inhibit the α- motor neurons of the antagonist
- Inhibition: Branches of group Ia sensory axons excite inhibitory interneurons that synapse with α-motor neurons of the antagonist
- Reciprocal innervation increases the effectiveness of the stretch reflex by minimizing the antagonistic forces against the stretched muscle
33
Q
Golgi Tendon Organs
A
- autogenic inhibition; protective reflex
- GTO group Ib sensory axons excite both excitatory and inhibitory interneurons
- Autogenic Inhibition:
- In most cases, the GTO circuit inhibits the muscle in which tension increased & excites the antagonist
- GTO response is generally opposite the stretch reflex
- In general, GTO-mediated reflexes act to control muscle force & joint stability
