Properties of skeletal muscle

Properties of skeletal muscle - to teach medical students

1. Properties of skeletal muscle Dr Anu Priya J

2. ELECTRICAL & MECHANICAL PROPERTIES Excitability Conductivity Electrical properties Contractility Extensibility Elasticity Mechanical properties

3. Excitability Ability to receive and respond to a stimulus Stimulus – Electrical, Chemical Respond by producing electrical signals such as action potentials Muscle cells can be excited by a stimulus. RMP -90 mV Local potential – end plate potential Application of a threshold stimulus produces an action potential . Action potential of skeletal muscle has all the characteristics of action potential of a nerve.

4. Excitability Skeletal muscle action potential RMP -90 mV Firing level -50 mV Amplitude 120 mV Duration of Action Potential 2-4 ms Duration of Absolute Refractory Period 1-3 ms Velocity of conduction 3-5 m/s Chronaxie 0.25 – 1 ms

5. +30 -50

6. Excitability Ionic basis – same as that for nerve Action potential is a spike type of action potential Depolarisation is by Na+ influx Repolarisation is by K+ efflux

7. Excitability Action potential can be elicited/ produced only by a threshold stimulus Action potential is a self propagating process A subthreshold stimulus produces a local response called endplate potential/EPP EPP does not propagate

8. Excitability Action potential of skeletal muscle has all the characteristics of action potential of a nerve. Obeys all or none law – single muscle fiber In cardiac muscle – the whole muscle obeys all or none law as it functions as a syncytium Has a refractory period Absolute refractory period 1-3 ms Latent period

9. Excitability Muscle can be stimulated directly by keeping the electrode in the muscle & indirectly by stimulating the nerve supply

10. The mechanical activity begins only when the electrical activity is about to end.

11. Excitability Action potential is very brief The absolute refractory period ends before the mechanical response begins. So, it is possible to tetanize the skeletal muscle if a number of stimuli are applied one after the other in the contraction period of previous contractions. Here , fusion of contractions(muscle twitches) occur resulting in tetanus

12. Conductivity Propagation of action potential, the same way as in an unmyelinated nerve fiber Propagates along the sarcolemma Action potential spreads in both the directions from the point of stimulation Velocity of conduction is 3-5 m/s

13. Contractility Ability to shorten when stimulated i.e., ability to contract when stimulated by an action potential When a threshold stimulus is applied, the muscle responds to the stimulus by a contraction A single action potential causes a brief period of contraction followed by relaxation. This response is called as a muscle twitch/ simple muscle twitch .

14. In a muscle fiber , the depolarisation starts at the motor end plate & action potential is conducted along the muscle & this initiates contraction. Mechanical response occurs slightly after the electrical response. The twitch starts 2 ms after the start of depolarisation, before repolarisation is completed. Duration of twitch varies with the type of muscle stimulated Contractility

15. Fast muscles have short twitch duration – 7.5 ms Slow muscles have long twitch duration – 100 ms Contractility

16. Contractility – Types of contraction Types Isometric contraction Isotonic contraction Length of the muscle No change Shortening Tone of the muscle & tension developed Increased No change

17. Contractility – Types of contraction Types Isometric contraction Isotonic contraction External w ork done No. Since there is no movement of the object. Work done = force x distance Energy is converted to heat Yes. As the muscle length is shortened A part of the energy is converted to useful work & the remaining is converted to heat Examples Arm muscles when trying to push a wall Muscles which help in maintaining posture against gravity Contraction of leg muscles during walking, running Contraction of muscles while lifting a weight Contraction of muscles during flexion of arm

18. Contractility – Types of contraction Types Isometric contraction Isotonic contraction Changes in the 3 component model Length remains the same as the shortening produced by the contractile component of the muscle is compensated by the stretching of the series elastic component Length decreases as the contractile component & parallel elastic component shortens but the series elastic component does not stretch

19. FACTORS AFFECTING FORCE OF CONTRACTION Strength of stimulus – A weak stimulus can excite only the most excitable (low threshold) fiber . With increasing intensity of stimulus, more number of motor units come into play, hence the force of contraction increases. Frequency of stimulus – directly proportional Initial length of muscle fiber – force of contraction is directly proportional to the initial length of the muscle fiber within physiological limits. This is Starling’s law Contractility

20. Contractility Load and force of contraction - Load applied before contraction(preload) – force of contraction increases. Load applied to a contracting muscle ( afterload ) - force of contraction decreases Temperature – directly proportional - by accelerating the metabolism. Above 45 degree C , muscle proteins coagulate Type of muscle – increased in fast muscle

21. Give reason Increase in initial length increases the force of contraction Starling’s law Increased exposure of active binding sites i.e., myosin binding sites on actin molecule Optimum overlapping between the actin & myosin filaments So, maximum cross bridges are formed between them

22. Can a skeletal muscle be tetanised? If so, how? Tetanus – a state of sustained contraction The mechanical response begins only after the end of the absolute refractory period of the action potential . So, if repeated stimuli are applied one after the other in the contraction period of the previous contractions – fusion of contractions - tetanus

23. Skeletal muscle tone Even during the resting state of the muscle, there is a slow & steady discharge of impulses from the alpha motor neuron . Because of the discharge, the muscle is in a state of partial sustained contraction even at rest . This is the resting tone of the muscle & is called as muscle tone Decrease – hypotonia – alpha motor neuron lesion, injury to dorsal nerve root Increased – hypertonia – UMN lesion, extrapyramidal lesion.

24. Fatiguability Temporary inability of the muscle to respond satisfactorily after being repeatedly stimulated Temporary state of inexcitability of the muscle Can be reversed by rest, oxygen...

25. Elasticity Ability to recoil and resume resting length Ability to return to normal length after stretch Explained by the presence of the parallel elastic component Passive tension by elastic component Total tension by contractile & elastic component Active tension = total tension – passive tension

26. Elasticity

27. Extensibility Ability to stretch or to increase in length Series elastic component

28. EMG/ Electromyogram Recording of electrical activities of muscle Surface electrode placed over the skin Hypodermic needle electrodes

29. Properties of skeletal muscle

30. Atrophy Hypertrophy Hyperplasia Wasting Paralysis/ Plegia Dicoumarol

31. Thank you

32. Length tension relationship

33. Thermal changes during muscle contraction Muscle contractions are associated with heat changes Thermodynamically, energy supply to the muscle must be equal to the energy output which appear as Mechanical work done during isotonic contraction Resynthesis of ATP used Heat released – a wasteful form of energy

34. Work is done during isotonic contraction Work done = force x distance Mechanical efficiency of skeletal muscle =work done ----------------- x 100 Total energy expenditure Isotonic contraction 50% Isometric contraction – 0

35. Heat produced during muscle contraction can be measured using suitable thermocouples & the changes seen are known as thermal changes Types of heat produced Resting heat – heat evolved during rest – due to metabolism Initial heat – heat evolved in excess of resting heat, during contraction – 2 types – activation heat & shortening heat

36. Excitability Muscle can be stimulated directly by keeping the electrode in the muscle & indirectly by stimulating the nerve supply Chronaxie is of longer duration 0.5-1 ms Action potential of skeletal muscle has a long afterdepolarisation & during that time, the muscle fiber is highly excitable ( hyperexcitable ). This is called supernormal period.