Muscle Physiology Lecture Notes PDF

MUSCLE PHYSIOLOGY 7. Muscle strength and length-tension relationship Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Des...

MUSCLE PHYSIOLOGY 7. Muscle strength and length-tension relationship Andre Azevedo, DVM, MSc Visiting Professor of Veterinary Physiology [email protected]  Learning objectives for this lecture Describe how muscle changes its strength of contraction Describe the length-tension relationship List the sources of energy for muscle contraction relationship engthtensile ffectofresting fiber on length contraction muscular of emÉ e 9nf this mail.it  Muscle contraction strength A muscle fiber contracts completely when exposed to a threshold stimulus or not at all - ALL-OR-NONE RESPONSE The muscle fiber always contracts maximally The strength of contraction is controlled in the body by changing the number of motor units recruited and or the frequency of stimulation of the muscle  Muscle contraction strength The strength of skeletal muscle contractions can be separated into: TWITCH with SUMMATION summation TETANUS tetanus The time between a stimulus to the motor neuron and the subsequent contraction of the innervated muscle is called the LATENT PERIOD  Muscle contraction strength A TWITCH is a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself If another action potential comes before the complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous one, thereby producing a SUMMATION  Muscle contraction strength SUMMATION occurs in 2 ways: 1. MULTIPLE FIBER SUMMATION Increase in the number of motor units recruited 2. FREQUENCY SUMMATION Increase in the frequency of contraction  Muscle contraction strength 1. MULTIPLE FIBER SUMMATION all activated same time Is the increase in the number of motor units contracting simultaneously As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger (THE SIZE PRINCIPLE)  Muscle contraction strength 1. MULTIPLE FIBER SUMMATION If the central nervous system sends a weak signal to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. THE SIZE OF MOTOR UNITS REQUIRED TO HOLD A PEN IS DIFFERENT THAN THE SIZE REQUIRED TO PICKING UP A HEAVY DUMBELL  Muscle contraction strength 2. FREQUENCY SUMMATION Is the increase in frequency of contraction The force exerted by the skeletal muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers Action potentials do not arrive at muscles synchronously At lower frequency stimulation, contractions occur one after another If a muscle fiber is re-stimulated after it has completely relaxed, the second twitch is the same magnitude as the first twitch  Muscle contraction strength 2. FREQUENCY SUMMATION As the frequency increases, there comes the point where each new contraction occurs before the preceding one is over re-stimulation before complete relaxation the second contraction is added partially to the first the total strength of contraction rises progressively with the increasing frequency When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together The fiber is stimulated so fast that it does not have a chance to relax at all The whole muscle contraction appears to be completely smooth and continuous This is called TETANIZATION – a contraction of maximal strength Muscle contraction strength  Length-tension relationship Skeletal muscles operate with the greatest active tension (force) when close to an ideal length (often their resting length). If the muscle is shortened or stretched the strength of contraction will be diminished. LENGTH-TENSION RELATIONSHIP is the relation between the length of the muscle before the onset of contraction and the tension that each contracting fiber can subsequently develop at that length. In a muscle fiber, the amount of tension generated during a contraction depends on the number of cross-bridge interactions that occur in all of the sarcomeres along all of the myofibrils. The number of cross-bridge interactions is determined by the degree of overlap between thick and thin filaments  Length-tension relationship For every muscle, there is an optimum length at which maximal force can be achieved Muscle fibers can contract forcefully when stimulated over a relatively narrow range of resting lengths When the sarcomere is in the optimal resting length (b), there is an optimal rate of cross-bridges formed, and contraction is optimal  maximal tension MUSCLE TOO SHORTENED MUSCLE TOO STRETCHED High degree of overlap between Little or no interaction between the thin and thick filaments the thin and thick filaments Muscle contraction cannot Muscle contraction is weak or progress does not happen The fiber cannot actively produce tension, and contraction cannot occur  Source of energy Muscle tissue needs energy for: Walk along mechanism Calcium pump in the SR Sodium-potassium pump in the sarcolemma The amount of ATP in muscle is limited, and rephosphorylation of ADP must occur so that contraction can continue to remove ca ca pump in SR energy req  rem phosphorylate Source of energy add phosphate There are 3 sources of energy for muscle contraction: 1. PHOSPHOCREATINE or CREATINE PHOSPHATE Creatine produced from AAs in the liver is phosphorylated in the muscle by the enzyme creatine phosphokinase to produce phosphocreatine Carries a high-energy phosphate bond similar to the bonds of ATP Used to reconstitute the ATP molecule The cleavage of phosphocreatine releases energy that is used to bond a new phosphate ion to ADP to reconstitute the ATP The amount of muscle phosphocreatine is small  Source of energy 2. GLYCOLYSIS Enzymatic breakdown of carbohydrates as glucose and glycogen, then with the release of energy and the production of pyruvic acid and lactic acid Liberated energy is used to reconstitute both ATP and phosphocreatine Process can be done in the absence of oxygen Muscle contraction can be sustained for more than a minute even without O2 delivery from the blood The rate of ATP formation by glycolysis is 2.5x as rapid as ATP formation by oxidative metabolism Many end products accumulate (ex.: Lactate) just anaerobic  part Source of energy 3. OXIDATIVE METABOLISM Responsible for 95% of the energy used for contraction Combining of O2 with the end products of glycolysis (ex: lactate) and with other sources of energy (proteins, lipids, and carbohydrates) to produce ATP For short periods – carbohydrates For long-term activity – fatty acids  Source of energy – cardiac muscle The main energy source for cardiac muscle is normally derived from the oxidative metabolism of fatty acids Mitochondria make up about 40% of the cytoplasm volume in cardiac fibers Only 2% of skeletal muscle fibers Numerous lipid droplets containing triglycerides Storage form of Fatty acids Only 10 – 30% of the energy comes from glucose and lactate Glycogen granules are found in cardiac muscle Cardiac cells stop contracting after 30 sec of O2 deprivation  cardiac muscle is derived from oxidative the main energysource for metabolism of fatty acids