Lecture 13, 09 Oct 2003 Chapter 10 Muscles. Vertebrate Physiology ECOL 437 University of Arizona Fall instr: Kevin Bonine t.a.

Transcription

1 Lecture 13, 09 Oct 2003 Chapter 10 Muscles Vertebrate Physiology ECOL 437 University of Arizona Fall 2003 instr: Kevin Bonine t.a.: Bret Pasch

2 Vertebrate Physiology Muscles (Ch10) 2. Announcements

3 Review of EC Coupling and Muscle Contraction Sherwood, 1997 (Also a nice summary on p. 387 of your text)

4 Cross Bridges and Force Production ATP required for the (3) dissociation of actin and myosin (else rigor mortis) Myosin acts as an ATPase, hydrolyzing ATP to ADP + P i (4) (Energy of ATP hydrolysis cocks the myosin head) Actomyosin complex forms (= crossbridge) (1) ATP hydrolysis Cross bridge stronger when Pi released, then myosin head rotates Myosin releases ADP and P i (very slowly (2) unless bound to actin) Vander et al., 2001 ATP binds to myosin Cycle repeats until Ca++ resequestered or run out of energy Movement about 1000 nm

5 Control of Muscle Force Two primary factors can be adjusted to increase whole-muscle force: the force developed by each contracting fiber (summation) the number of muscle fibers contracting within a muscle (recruitment)

6 See text Fig Summation It s all about CALCIUM Increase force by decreasing time between individual action potentials (increase rate of stimulation)

7 Control of Muscle Force Two primary factors can be adjusted to increase whole-muscle force: the force developed by each contracting fiber (summation) the number of muscle fibers contracting within a muscle (recruitment)

8 Motor Unit Motor unit = motor neuron and all of the muscle fibers it innervates AP in motor neuron causes all innervated fibers to contract simultaneously

9 Each muscle consists of many intermingled motor units Recruitment Muscle fibers Motor Neurons Increase force by adding more motor units

10 See text Fig a Muscle & Tendon Bone Parallel Elastic Component (sarcolemma, connective tissue within muscle) Bone Muscle Model Randall et al., 2002 Contractile Unit (sarcomeres) Series Elastic Component (tendon, connective tissue linking muscle fibers to tendon, titin, Z-line material, crossbridge links)

11 See text Fig and Fig Isometric Contraction iso = same metric = length Randall et al., 2002

12 See text Fig and Fig b Isotonic Contraction iso = same tonic = tension Purely isotonic contraction Randall et al., 2002

13 See text Fig Force-Velocity Curve Greatest force during isometric contraction Randall et al., 2002 Greatest velocity when muscle is unloaded

14 See text Fig Muscles can produce power Muscle fiber types vary in their mechanical properties (see text Table 10-1) Power = force * velocity Maximum power output is found at intermediate force and velocity (~40%) Randall et al., 2002

15 Different Muscle Fiber-Types Randall et al., 2002

16 Cellular Energetics See text Fig Myosin ATPase Actin + Myosin crossbridge movement 75% ATP ADP + P Immediate energy source i PCr + ADP Cr + ATP Non-oxidative energy source Glucose 2 Lactate + 2 ATP Oxidative energy source Glucose + O 2 CO 2 + H 2 O + 36 ATP Ca 2+ ATPase Ca 2+ pumped back into SR 25% Energy systems differ in their rate of and capacity for producing ATP

17 Fatigue Fatigue can result from many factors including; -decreased motivation -failure of neuromuscular transmission -accumulation of metabolic end-products -dehydration Cause of fatigue depends on intensity & duration of exercise

18 Fatigue Continuous exercise at moderate speeds results in net accumulation of P i PCr + ADP + H + Cr + ATP ATP + H 2 O ADP + P i + H + + energy Exercise also produces net accumulation of lactic acid Correlation vs. Causation

19 P i accumulation is correlated with development of fatigue, as is lactic acid accumulation (drop in ph) Wilson et al., 1988

20 Muscle Biopsy prepare homogenate & perform enzymatic analysis of homogenate (e.g., creatine phosphate, ATP, P i, lactate, glucose, glycogen) Pros: low cost per assay Cons: many samples required for time course

21 31 P-Magnetic Resonance Spectroscopy Intact muscle (e.g., creatine phosphate, ATP, P i, ph) Pros: multiple time points for each preparation Cons: high cost per preparation ph can be determined from position of P i peak

22 31 P-Magnetic Resonance Spectroscopy Rat muscle P i PCr ATP rest recovery Time Kushmerick & Meyer, 1985

23 Postulated Mechanisms of P i Effect on Force Reduced cross-bridge force development Reduced Ca 2+ release from sarcoplasmic reticulum Reduced Ca 2+ sensitivity of myofilaments Decreased ph (e.g., lactic acid) does not seem to have much effect on contractility - but may cause pain! Cooke & Pate, 1985; Allen & Westerblad, 2001; Westerblad et al. 2002

24 Is steady-state locomotion the only strategy? Ray Harryhausen

25 Intermittent Exercise When a heavy (i.e. supramaximal) workload is divided into short exercise and pause periods, the heavy workload is transformed to a submaximal load on circulation and respiration and (can) be well tolerated. Astrand et al., 1960

26 Foraging animals Cruise desertusa.com Distance Saltatory Justin W. Moore Ambush J.L. Gingerich Time O Brien et al., 1990

27 Benefits of moving intermittently Reduced attack rate by predators predators more likely to attack moving prey Increased detection of prey and/or predators Remove potential conflict between running & breathing Increased endurance (range) Lima & Dill, 1990 O Brien et al., 1990; McAdam & Kramer, 1998 Carrier, 1987 pauses permit net clearance of metabolic end-products Weinstein & Full, 1992

28 Integration of locomotor behavior and muscle stimulation Continuous locomotion one stride Time Intermittent locomotion Exercise Pause Exercise Pause Time

29 Metabolite Changes Correlate with Performance Lactate (µmol g leg -1 ) sec 120 sec * * Arginine Phosphate (µmol g leg -1 ) Rest E P Rest E P * Rest E P Rest E P Weinstein & Full, 2000

30 Cyclic Contractions In cyclic motions muscle contractions are not purely isometric or isotonic. Instead, muscles shorten and lengthen during each cycle. How much work does a muscle do during one cycle?

31 End