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

9. 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

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

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

12. 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

13. 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

14. 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.

15. • 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

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

17. 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

18. Contractility – Types of contraction
Types Isometric contraction Isotonic contraction
External work 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

19. 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

20. FACTORS AFFECTING FORCE OF CONTRACTION
1. 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.
2. Frequency of stimulus – directly proportional
3. 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

21. Contractility
4. 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
5. Temperature – directly proportional - by
accelerating the metabolism. Above 45
degree C , muscle proteins coagulate
6. Type of muscle – increased in fast muscle

22. 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

23. 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

24. 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.

25. 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...

26. 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

27. Elasticity

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

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

31. Properties of skeletal muscle

32. • Atrophy
• Hypertrophy
• Hyperplasia
• Wasting
• Paralysis/ Plegia
• Dicoumarol

33. Thank you

34. Length tension relationship

35. 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
1. Mechanical work done during isotonic
contraction
2. Resynthesis of ATP used
3. Heat released – a wasteful form of energy

36. • 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

38. • 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

39. 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.