Physiology of the skeletal muscle Objectives Mechanical properties of the skeletal muscle Physiological properties of the skeletal muscle Organization of the skeletal muscle Mechanism of muscle contraction and relaxation Tetanus The all or nothing law in skeletal muscle Types of muscle fibres Mechanisms of skeletal muscle strength Practical tasks Determination of work and fatigue in human Determination of skeletal muscle strength in a human Katarína Babinská, MD, PhD. 2016
Physiological properties of skeletal muscle tissue 1. Excitability (irritability) - refers to the ability of a muscle to respond to stimulation - in the human body the muscle activity is regulated by the nervous system and in some muscle types by the endocrine system - (in experiment the electric current is a suitable stimulation) 2. Contractility - refers to the capacity of muscle to contract (shorten) - contraction = response of a muscle to stimulation Skeletal muscle mechanical characteristics 1. Strength (firmness) can be expressed as a maximum weight that can be kept by contracted muscle (or group of muscles) in balance against gravity
2. Elasticity (Extensibility) - ability to return to the resting length after contraction - if load is put on the muscle gradually, it becomes progressively longer - the increase of length per each kilo added gradually decreases, (the muscle resists the stretch at first slowly, then more rapidly) - when taking the weight off the muscle becomes shorter, however it reaches its resting length just after some time 0 kg kg + kg + kg + kg + kg removing the load
Organization of the skeletal muscle - muscle - fascicles - fibres (cells) - myofibrils - myofilaments (actin, myosin) myofibrils a sequence of sarcomeres a sarcomere a basic longitudinal contractile unit of the striated muscle H demarcated by two successive Z lines I A I main components (myofilaments) thin filaments (actin) thick filaments (myosin) (fixed by titin to the Z lines) cross striae formed by: I band - actin A band actin + myosin overlapped H band myosin only http://www.sport-fitness-advisor.com/images/actin_myosin.jpg
Myofilaments Thick filament Myosin molecules in shape of golf clubs long tails bundled together, heads sticking out Thin filament actin globules arranged into fibres - helix of 2 filaments tropomyosin spreads along actin and covers the binding sites for myosin troponin (C, I, T) complex is present on each tropomyosin dimer
Neuro-muscular junction The mechanism of AP transmission from a nerve to a muscle - motor end-plate - the point of junction of a motor nerve fibre and a muscle fibre - modified area of the muscle fibre membrane at which a synapse occurs 1. nerve impulse reaches the end of a motor neuron 2. Ca 2+ volt. gated canals in the axon terminal open, Ca 2+ moves inside the terminal 3. this triggers release of acetylcholine into the synaptic cleft 4. acetylcholine binds to receptors on the motor endplate 5. this causes opening of Na + channels in motor endplate 6. motor endplate potential is generated 7. action potential is generated that travels along the sarcolemma
Excitation contraction coupling the AP travels along the membrane of the muscle cell (sarcolemma) through T tubules (invaginations of the sarcolemma) pass to sacs of sarcoplasmic (endoplasmic) reticulum = store of Ca 2+ Ca ++ is released from sarcoplasmic reticulum into the sarcoplasm Ca ++ binds to troponin molecules tropomyosin fibres shift and expose the actin s active sites http://t1.gstatic.com/images?q=tbn:and9gcs2b26vzq_y1gpw03zdkl-1lrwnkf9qkss9mme3j0rjulq8xeog http://www.blobs.org/science/cells/sr.gif
https://classconnection.s3.amazonaws.com/216/flashcards/1 042216/jpg/power_stroke1327356421108.jpg Actin myosin interaction
The mechanism of muscle relaxation after the impulse is over, the sarcoplasmic reticulum begins actively pumping Ca ++ into sacs as Ca ++ is released from troponin, tropomyosin returns to its resting position blocking actin s active sites myosin cross bridges are prevented the contraction can no longer sustain the muscle returns to its resting length http://www.blobs.org/science/cells/sr.gif
Isotonic contractions - generate force by changing the length of the muscle Types of muscle contractions a concentric contraction causes muscles to shorten, thereby generating force eccentric contractions cause muscles to elongate in response to a greater opposing force Isometric contractions generate force without changing the length of the muscle Auxotonic contraction combination of isotonic and isometric contraction this type occurs mostly in real life Muscle tone a continual partial contraction of the muscle involuntary activation of a small number of motor units causes small contractions that give firmness to the muscle important for maintaining posture higher when awake https://figures.boundless-cdn.com/32705/large/uvqvhggorgilmckvon5b.jpe
Muscle contraction strength mechanisms The force of contraction depends on: 1. Motor Unit Recruitment 2. Increase in firing frequency 3. Muscle length _ tension relationship 4. The graded strength principle 5. Type of muscle fibres
1. Motor unit recruitment in the human body skeletal muscles are stimulated by signals (action potentials) transmitted via motor neurons to muscles axon of a single motor neuron branches and is attached to more muscle fibres motor unit = one motor neuron + muscle fibres to which it is attached when a motor unit is activated, all of its fibres contract small motor units motor neuron is attached to fewer muscle fibres (eye, face, fingers) allow for fine and precise movements they produce little force large motor units involve several hundreds of muscles e.g. in postural muscles produce large force allow for less precise movements http://www.muaythaischolar.com/motor-unit/
If more motor units are recruited to contract - muscle strength increases
2. Increase in firing frequency The twitch contraction is a quick jerk (contraction) of the muscle fibre that occurs after stimulation (e.g. electric stimulation) can be recorded by myograph (see picture) the curve of a twitch contraction includes 3 phases: latent period time between stimulation and beginning of contraction contraction phase relaxation phase http://t2.gstatic.com/images?q=tbn:and9gcq0qya15dfqeietqelpvygrunlzt_ixrytcfxuxrci_41b3ruva
if a series of stimuli come in longer intervals, the muscle has enough time to relax completely before next contraction a series of individual twitch contractions can be observed if stimulation is fast, and the next stimulus arrives before the relaxation phase has ended summation of twitches occurs muscle is in tetanus
Tetanus - sustained contraction of a skeletal muscle, result of stimulation with high frequency of stimuli Incomplete tetanus the next stimulus arrives before the relaxation phase has ended muscle gets only partially relaxed summation of twitches occurs and the force of contraction increases Complete tetanus the next stimulus comes at the peak of the previous contraction the muscle is instantly contracted strength of contraction increases even more Incomplete tetanus Complete tetanus
Increase in firing frequency increases the strength of contraction single stimulus release of Ca ++ from sarcoplasmic reticulum twitch twitch is terminated by reuptake of Ca ++ into sarcoplasmic reticulum repeated stimulation in high frequency insufficient time to reaccumulate Ca ++ into sarcoplasmic reticulum remains in sarcoplasm sustained contraction = tetanus incomplete next stimulus occurs in relaxation period of a twitch complete next stimulus comes on the top of the twitch Incomplete tetanus Complete tetanus
3. Muscle Length Length tension relationship the strength or maximum force produced by a muscle depends on the number of cross bridges per unit area to increase the maximum force, increase the number of cross bridges the number of cross bridges depends on the starting position of actin and myosin http://ffden- 2.phys.uaf.edu/211_fall2004.web.dir/Katherine_ Van_Duine/actin%20and%20myosin.jpg
Optimum muscle lenght greatest force - when the muscle is at an optimal length = indicated by the greatest possible overlap of thick and thin filaments, maximal strength is produced. -CNS maintains optimum length producing adequate muscle tone Overly contracted - thick filaments too close to Z discs cannot slide more Too stretched -little overlap of thin and thick filaments does not allow for very many crossbridges to form
4. The graded strength principle The all or nothing law in skeletal muscle individual muscle fibres operate according the all or nothing law: insufficient stimulation (subthreshold stimulus) causes no contraction (no response) sufficient stimulation (threshold or suprathreshold stimulus) causes maximum contraction subtheshold threshold suprathreshold stimulus stimulus stimulus no response maximum contraction maximum contraction
The graded strength principle in a muscle skeletal muscle = a bundle of muscle fibres muscle as a bundle of fibres operates according to graded strength principle (fibres respond to the stimulus gradually depending on their sensitivity) subthreshold stimulus no response threshold stimulus first response then the stronger the stimulus, the stronger response (still more muscle fibres are recruited and respond) maximal stimulus all the muscle fibres respond further increase of intensity of the stimulus (supramaximal stimulus) does not increase the response
5. Muscle fibre type Fast twith fibres (type II, white) Contraction velocity High Low Capillarization Low high Myoglobin content Low High Mitochondrial content Low High Aerobic energy production Low High Anaerobic energy production High Low Glycogen stores High Low Fatigue Fast Slow Generation of speed and power High Low Slow twitch fibres (type I) Suited for Explosive sports Endurance sports - Muscle fibre type is determined genetically + by training
1. type I muscle fibers (slow-twitch fibers, red) typically smaller motor units 2. type II fibers (fast-twitch fibers, white) - typically larger than motor units containing type I fibers - i.e. when a single type II motor unit is stimulated, more muscle fibers contract - since more fibers are stimulated to contract in type II motor units, more force is produced by type II fibers.
Determination of skeletal muscle strength in a human Muscle strength can be expressed as a maximum weight that can be kept by contracted muscle (or group of muscles) in balance against gravity can be measured by hand dynamometer average (normal) value of the dominant hand 50 55 kg in an adult male 31 36 kg in an adult female value of the non-dominant hand approx. 10 % less http://www.getprice.com.au/images/uploadimg/2434/ Hydraulic-Hand-Dynamometer-Left-Side- View_545_320x320.jpg
Procedure 1. Rotate the peak-hold needle counter to 0 2. Let the right upper extremity with dynamometer hang freely along the body (in the standing position). 3. Compress the dynamometer by the right hand maximally 4. Record the value in kg (the peak-hold needle records the max force) 5. Repeat the measurement 3 times and calculate average value. 6. Repeat for the left hand. Report A. Which of your hands is dominant? B. Write down the measured and average values: Right hand Left hand measurement-1: measurement-1: measurement-2: measurement-2: measurement-3: measurement-3: average value: average value: C. Compare your average value for dominant hand with normal values
Muscle fatigue - the transient/reversible decrease in performance capacity of muscles induced by eercise - evidenced by a failure to maintain or develop a certain expected force or power, or to sustain the task - depends on the intensity/duration of the performance aerobic/anaerobic metabolism types of muscle fibres personal fitness - experienced mainly in sustained and/or close to maximum activities Sites, causes and mechanisms of fatigue: Neuromuscular depression (fatigue of synapses) - synapse most prone to fatigue - every successive stimulation of a motor nerve causes weaker response in the postsynaptic muscle fibre - acetylcholine synthesis slower than required by fast firing rate
Cellular fatigue - accumulation of extracellular K + - due to repeated action potentials and the Na + - K + pump can not rapidly transport K + back to the muscle - failure to reestablish the resting membrane potential on a synapse - rise in lactic acid concentration = lowering of ph inhibits the cross-bridge formation - depletion of glycogen/glucose - decrease in availability of Ca 2+ ions - results in decreased Ca 2+ release from sarcoplasmic reticulum - excessive accumulation of inorganic phosphate (ATP breakdown in cross-bridge formation) in cytoplasm - glycolytic fibres more prone to fatigue (slower Ca uptake) Central fatigue - subjective feeling of tiredness and a desire to stop the activity - lack of motivation due to failure of cerebral cortex to send excitatory signals to the motor neurons - low ph seems to play role (in close to maximum physical activities)
Task: Determination of work and fatigue in a human Principle the volunteer lifts 2 kg load with m. flexor digitorum superficialis signs of fatigue are observed Moss ergograph serves for fixing the forearm and hand fixing the cable with load recording the contractions Procedure the forearm of examinee is fixed in the ergograph holder the examinee holds the handle with his hand a leather ring is put on the second finger the examinee lifts a 2 kg load in pace given by a metronome the series of contractions are registered and evaluated
1. ask the subject for any feelings of fatigue (pain, weakness of the finger ) 2. observe signs of fatigue on the record the curve is flattened, the contractions are irregular (the examinee continues to lift the load for another 30-60 s) 3. start to encourage the volunteer observe the effect of motivation on the performance 4. calculate the work done per unit of time - at the beginning of performance - the period when sifns of of fatigue are visible - in the period when the subject is encouraged - unit of time = a segment, e.g. 15 cm - select 3 segments from the beginning of the performance and the end - when signs of fatigue are seen)
size of contractions count of contractions frequency size trajectory gravitational acceleration load work done beginning fatigue encouraging Work done (J) = load (kg). gravitational acceleration (m.s -2 ). trajectory (m) trajectory = count of contractions. size of 1 contraction Positive dynamic work - work done during muscle contraction - e.g. load ligting Negative dynamic work -prohibits falling down (e.g. load releasing, going downstairs) this is not taken into account in this task
The smooth muscle http://faculty.ccri.edu/kamontgomery/muscle%20tissues.jpg
Types of muscles Skeletal muscle Smooth muscle Cardiac muscle Skeletal muscle and smooth muscle basic comparison Skeletal muscle Bigger cells, long and thin Syncytium (multinuclear cells) Smooth muscle Smaller cells, spindle shaped Single nucleus Stiated muscle, sarcomere basic unit No striations, no sarcomeres Voluntary control Involuntary control http://faculty.ccri.edu/kamontgomery/muscle%20tissues.jpg
Types of the smooth muscles Multi unit smooth muscle composed of separate smooth muscle fibers (separated by a glycoprotein/collagen layer) only a few fibres innervated by a single nerve ending small units of fibers can contract independently of the others their control is exerted mainly by nerve signals. e.g. iris, ciliary muscle, erectores pili Single unit (unitary) smooth muscle mass of hundreds/ thousands of smooth muscle cells that contract together as a single unit. syncytium - cell membranes joined by gap junctions - ions can flow freely from one cell to another and cause depolarisation /contraction Often controled by non-nervous stimuli (e.g.hormones) e.g. in the viscera, vessels
Smooth muscle contraction Stimuli for a smooth muscle cells: Nervous Humoral norepinephrine, epinephrine, acetylcholine, oxytocin, etc. Passive stretch Local tissue factors excess of H +, CO 2, lactate, deficit of O 2 Innervation of the smooth muscle Skeletal Motor neurons Synapse - motor end plate 1muscle cell 1 synapse Smooth Autonomic nerves Synapse varicosities 1 muscle cell may have several synapses
Types of potentials in the smooth muscle Spike potentials Slow wave potentials - GIT Potentials with plateau ureters, uterus, some vessels - Calcium channels play the role (instead of Na) slow channels therefore slow AP - Ca ions only from ECT, not form sarcoplasmic reticulum
Contractile mechanism in the smooth muscle contraction interaction of actin and myosin filaments (different arrangement of the filaments than in the skeletal muscle) actin filaments attached to the dense bodies (in the cell membrane, inside the cell) the smooth muscle does not contain troponin calmodulin is the regulatory protein - initiates contraction in a different manner This activation and contraction occur in the following sequence: 1. The calcium ions bind with calmodulin 2. The calmodulin-calcium combination joins with and activates myosin kinase, a phosphorylating enzyme 3. Myosin heads become phosphorylated and are capable of interaction with actin
Other differences in the smooth muscle contraction Skeletal muscle contraction by 30% of their lenght Fast (10 300x) More energy required Lower force of contraction Smooth muscle contraction by 80% of their lenght Slow (cycling of the cross bridges, long lasting attachment of A-M, less ATP-ase activity) Less energy required to sustain the contraction Greater force of contraction http://www.interactive-biology.com/wp-content/uploads/2012/04/muscle-cells- 1024x1024.jpg