12.1
Regulatory & Contractile Proteins
Myosin & actin are contractile proteins
they do work of shortening the muscle
Tropomyosin & troponin are regulatory proteins
act like a switch that starts & stops shortening
@Jastifer2010SoftTissue
Overlap of Thick & Thin Filaments
@Jastifer2010SoftTissue
Beer is always the answer Length-Tension Curve
@Jastifer2010SoftTissue
Types of contraction
@Jastifer2010SoftTissue
Muscle Twitch
Threshold is minimum voltage necessary to produce contraction
a single brief stimulus at that voltage produces a quick cycle of contraction & relaxation called a twitch
Phases of a twitch contraction
latent period (2 ms) is delay between stimulus & onset of twitch contraction phase is period during which tension develops and shortens relaxation phase shows a loss of tension & return to resting length refractory period is period when muscle will not respond to new stimulus
@Jastifer2010SoftTissue
Production of Variable Contraction Strengths
Stimulating the nerve with higher voltage get stronger contractions because recruit more motor units Stimulate the muscle at higher frequencies (stimuli/sec) up to 10Hz, produces twitch contractions with full recovery between twitches 10 - 20, each twitch develops more tension than the one before due to failure to remove all Ca+2 20 - 40, each stimulus arrives before the previous twitch is over temporal or wave summation produces incomplete tetanus 40 - 50, no time to relax between stimuli so twitches fuse into smooth prolonged contraction called complete tetanus (normal smooth movements)
@Jastifer2010SoftTissue
Recruitment & Stimulus Intensity
Maximal recruitment
@Jastifer2010SoftTissue
Histology of muscle
Type IIA Type IIB Type I
Eye muscle (Rectus lateralis); Myofibrillar ATPase stain, PH 4.3
@Jastifer2010SoftTissue
Muscle Differentiation (types of fibers) I (slow-twitch oxidative)
IIA (fast-twitch oxidative glycolytic)
IIB fast-twitch glycolytic
Contraction speed
Slow
fast
fast
Myosin-ATPase activity
Low
High
High
Primary source of ATP Oxidative production phosphorylation
Oxidative phosphorylation
Anaerobic glycolysis
Glycolytic enzyme activity
Low
Intermediate
High
No. of mitochondria
Many
Many
Few
Capillaries
Many
Many
Few
Myoglobin contents
High
High
Low
Muscle Color
Red
Red
White
Glycogen content
Low
Intermediate
High
Fiber diameter
small
Intermediate
Large
Rate of fatigue
slow
Intermediate
Fast
@Jastifer2010SoftTissue
Functional arrangement of muscle
pinnated angle of muscle
@Jastifer2010SoftTissue
The Musculotendinous Unit
PEC: parallel elastic component CC: contractile component SEC: series elastic component
Tendon- spring-like elastic component in series with contractile component (proteins)
Parallel elastic component (epimysium, perimysium, endomysium, sarcolemma)
@Jastifer2010SoftTissue
Muscle mechanics Static contractions Twitch, summation, tetanus: 60hz
Brooks et al.
@Jastifer2010SoftTissue
Motor unit recruitment All-or-nothing event 2 ways to increase tension: - Stimulation rate - Recruitment of more motor unit
Size principle Smallest m.u. recruited first Largest m.u. last
@Jastifer2010SoftTissue
Influence of parallel elastic component
Fc
Ft
Ft
Fp
Fp l0
Fc
l0
@Jastifer2010SoftTissue
Force and Velocity •Force diminishes with increase in velocity •Maximum force can actually be generated with eccentric contractions (1.8 times isometric max) •Power= Force * Velocity
@Jastifer2010SoftTissue
Force-Velocity Characteristics
Concentric contraction
Muscle contracts and shortening, positive work was done on external load by muscle Tension in a muscle decreases as it shortens
Eccentric contraction
Muscle contracts and lengthening, external load does work on muscle or negative work done by muscle. Tension in a muscle increases as it lengthens by external load
@Jastifer2010SoftTissue
Length and velocity versus Force
Note: maximum contraction condition; normal contractions are fraction of the maximum force
@Jastifer2010SoftTissue
Muscle fatigue
Drop in tension followed prolonged stimulation. Fatigue occurs when the stimulation frequency outstrips rate of replacement of ATP, the twitch force decreases with time
@Jastifer2010SoftTissue
Muscle mechanics To review, determinants of force/power production by a muscle 1. # of motor units recruited (i.e., the cross-sectional area of the active muscle) 2. frequency of stimulation 3. length of the fibers relative to Lo 4. velocity (shortening and lengthening) a. myosin ATPase activity b. SR concentration 5. muscle architecture a. orientation of fibers to the long axis b. the # of sarcomeres in series
@Jastifer2010SoftTissue
Obesity
There is a dose-response relationship between obesity and Osteoarthritis.
Increase in body weight proportionatly increases forces across weight bearing joints.
@Jastifer2010SoftTissue
Changes in obesity
@Jastifer2010SoftTissue