Skeletal muscle phsiology 10/16

Question 1

Muscle Twitch

Answer 1

• 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)

Question 2

Total Force produced by muscle

Answer 2

• directly related by home many cycling cross-bridges are formed: sum of forces
• length-tension relationship
• frequency/twitch summation
• multiple motor unit summation

Question 3

Isometric Contractions

Answer 3

• Muscle length is constant (“same length”)
• Stimulation causes increase in tension, but no shortening
• Force production is equal to resistance (no movement)

Question 4

Isotonic Contractions

Answer 4

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

Question 5

Length-tension Relationship

Answer 5

• 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

Question 6

increased/decreased fiber length

Answer 6

• 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

Question 7

Total Tension

Answer 7

Passive + Active tension

Question 8

what is force-velocity relationship with isotonic contractions?

Answer 8

• 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.

Question 9

What is work vs. power?

Answer 9

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

Question 10

What is frequency summation (twitch summation)?

Answer 10

• 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

Question 11

Motor Unit vs. Motor pool

Answer 11

• 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

Question 12

what does whole muscle tension depend on?

Answer 12

• 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

Question 13

Multiple-fiber/Multiple Motor Unit Summation (MMUS)

Answer 13

• 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

Question 14

Asychronous Recruitment

Answer 14

• 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

Question 15

Electromyography

Answer 15

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

Question 16

What is Henneman’s Size Principle?

Answer 16

• 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

Question 17

Slow-twitch vs fast-twitch fatigue/fatigue resistant

Answer 17

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**

Question 18

Muscle fatigue: central vs. peripheral fatigue

Answer 18

• 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

Question 19

Anaerobic Sources of Energy

Answer 19

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

Question 20

Aerobic source of energy

Answer 20

Oxidative Phosphorylation

• ATP produced in mitochondria under aerobic conditions
• Supports aerobic exercise
• when exercising time to depletion: creatine phosphate < glycolysis < oxidative phos

Question 21

Difference between fast and slow twitch muscles…

Answer 21

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:

1. Slow oxidative (Type I) fibers
2. Fast-oxidative (Type IIA) fibers ~ fast fatigue resistant
3. 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)

Question 22

Slow- Twitch Fibers

Answer 22

• 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

Question 23

Fast-Twitch Fibers

Answer 23

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

Question 24

See fiber type chart

Answer 24

slide 44

Question 25

Slow twitch vs. fast twitch

Answer 25

• 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

Question 26

summary of factors influencing tension

Answer 26

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)

Question 27

Proprioceptors: 2 types

Answer 27

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

Question 28

Muscle Spindle

Answer 28

• 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

Question 29

DRAW OUT THE MUSCLE SPINDLE: SLIDE 58

Question 30

Why does the muscle spindle have motor innervation?

Answer 30

• 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

Question 31

Monosynpatic reflex

Answer 31

• 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.

Question 32

How does monosynaptic reflex occur?

Answer 32

1. recptor = muscle spindle: recognizes initial stretch
2. Affarent = AP to dorsal horn of spinal cord
3. integration center = spinal cord (interneurons) [and inhibitory interneurons]
4. Efferent = AP of alpha-motor neuron from ventral horn
5. 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

Question 33

Golgi Tendon Organs

Answer 33

• 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