physiological aspects of muscle contraction is discussed and sliding mechanism theory

1. PHYSIOLOGY OF MUSCLE
CONTRACTION
Dr. Priyanka Srivastava

2. Muscle tissue
• Skeletal Muscle
• Cardiac Muscle
• Smooth Muscle

3. Smooth muscle tissue
 Non striated
 Involuntary
 Fusiform cells
 One nucleus per cell
 Walls of hollow organs i.e. stomach
 Walls of tubes and passage ways i.e.
bronchioles, blood vessels, oesophagus
 Slow, wave-like contractions

5. Cardiac muscle tissue
 Striated
 Involuntary
 Fit together with tight junctions that promote
fast conduction of electrical signals.
 Heart Branching cells
 One or two nuclei per cell
 Medium speed contractions

7. Skeletal muscle tissue
 Striated
 Voluntary
 Attached to bone via tendons, fascia or skin.
 Long cylindrical cells
 Many nuclei per cell
 Rapid contractions

9. Skeletal muscle
function Produce heat
 Produce movement
 Maintain body posture
 Assists in joint stability

10. Contraction of Skeletal Ms.
1. The skeletal muscle is contractile in nature i.e it
contracts when excited.
2. Excitation(depolarization) of ms. fiber causes
development of action potential which through
sequence of events produce ms. Contraction.
3. These two phenomena are coupled together by Ca
ions.

11. Contraction of Skeletal Ms.
4. Thus, the process by which depolarization of the muscle
fiber initiates contraction is called Excitation contraction
coupling.

12. Actin and Myosin
 Actin – thin and light
 Myosin – thick and dark
 Arranged in bands that slide past each other.
 Calcium and Adenosine Triphosphate are
required.

21. Electrical Phenomena and Ionic fluxes
• The electrical events in the skeletal ms. and the ionic
fluxes is quite similar to that in nerve with quantitative
difference in timings and magnitude.
• The RMP of skeletal ms. Is about -90mV.
• The AP lasts 2-4ms and is conducted along the muscle
fiber at about 5m/s.
• The absolute refractory period is 1-3ms long and after-
polarizations are relatively prolonged.

22. Ionic distribution and Fluxes
• The distribution of ions across the muscle fiber membrane
is similar to that across the nerve cell membrane.
• Approximate values for the various ions and their
equilibrium potentials are shown in Table .
• As in nerves, depolarization is largely a manifestation of
Na+ influx, and repolarization is largely a manifestation of
K+ efflux.
•

24. Contractile Responses
• Electrical events
• Mechanical events
• Although one response does not normally occur without
each other but their physiologic bases and characteristics
are different.
• The ms. Fiber membrane depolarization starts at the
motor nerve plate.
• The AP is transmitted along the ms. Fiber and initiates a
contractile response.

26. Molecular Basis of contraction
• Sliding filament theory-
• The process by which ms. contraction occurs is brought
about by sliding of the thin filaments over the thick
filaments.
• This shortening is not due to actual length but it is brought
about by increased overlap of the thin(actin) and thick
filaments(myosin) within the ms cell.

27. Molecular Basis of Ms. Contraction
• The sliding occurs when the myosin heads bind firmly to
actin, bends at the junction of the head with the neck and
detach.
• This power stroke depends on the simultaneous
hydrolysis of ATP.
• Myosin II molecule are dimmers have 2 heads but only 1
attaches to the actin at any given time.

28. Sequence of Events in Cross bridge
formation
• In resting ms troponin I bound to actin and tropomyosin
and covers the sites where myosin head binds with actin.
• At rest the myosin head contains tightly bound ADP.
• An AP generation increases cytosolic Ca++ and free
Ca++ binds to troponin C.
• This binding causes weakening of troponin I interaction
with actin and exposes the actin binding sites which
allows the formation of myosin actin cross bridges.

29. Sequence of Events in Cross bridge
formation
• Upon the formation of cross-bridge, ADP released
causing a conformational change in the myosin head.
• This moves the thin filament relatively to the thick filament
comprising the cross-bridge power stroke.
• ATP binds to the free site on myosin which causes
detachment of the myosin head from the thin filament.

30. • ATP is hydrolyzed and inorganic phosphate(P) is released
causing a re-cocking of the myosin head completing the
cycle.
• As long as the Ca++ remains elevated and sufficient ATP
is available the cycle repeats.
• Many myosin heads cycle at or near the same time and
they cycle repeatedly producing gross ms contraction.
• Each power stroke shortens the sarcomere about 10nm
and each thick filament has about 500 myo sin heads and
each head cycles about 5 times per second during a rapid
contraction.

31. • This process by which depolarization of ms fiber initiates
contraction is called Excitation contraction coupling.
• The AP is transmitted to all the fibrils in the fiber via the T
tubules.
• Ca++ pumped back into SR.
• Release of Ca++ from troponin
• Cessation of interaction between actin and myosin.

32. Types of Contraction
• Isometric contraction
• Isotonic contraction
• Summation of contraction- tetanic contraction.
• The contractile mechanism does not have refractory
period so repeated stimulus before relaxation can occur
and this phenomena is called summation of contractions.

33. Tetanus
• With rapidly repeated stimulation, activation of contractile
mechanism occurs repeatedly before any relaxation and
individual responses fuse into one continuous contraction
i.e tetanus.
• Complete tetanus
• Incomplete tetanus.

35. Motor Unit
• Each single neuron and the muscle fibers it innervates
constitute a motor unit.
• Properties-
• 1.The no. of ms. Fibers in a motor unit varies.
• The ms concerned with the fine, graded and precise
movements motor unit innervates few ms. fibers.
• 2.All the efferent fibers passing to skeletal ms. are
excitatory.

36. Motor Unit
3.Each spinal motor neuron innervates only 1 kind of ms.
fiber, all the ms fiber in a motor unit are if same type.
4.Motor units are of 2 types slow and fast depending on
the type of ms fibers they innervate.
5.Recruitment of motor units- with minimal voluntary
activity few motor units discharge and with increasing
voluntary effort more units are brought into play.

37. Applied
• When motor nerve to muscle is cut.
• Disuse atrophy,
• Fibrous ms,
• Flaccid paralysis
• Fibrillation- fine irregular contraction of individual fibers
• Denervation hypersensitivity.
• Lower motor neuron lesion.
• Muscular dystrophy,
• Myotonia
• Metabolic myopathies.

38. Summary
• Muscles are of 3 types.
• Some are under voluntary control others are involuntary.
• Mechanism of muscle contraction depends upon the
availability of Ca ions presence.
• Actin and Myosin are muscle proteins.
• Muscle contraction depends on Sliding mechanism
Theory.
• It is an energy utilizing phenomenon requires ATP.
• Muscle contraction can be of 2 types Isometric & Isotonic
types.