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Skeletal muscle PhysiologyBy
Dr. Mudassar Ali Roomi (MBBS, M. Phil)
TETANIZATION: Definition: Summation of
contraction/twitches Sustained contraction without relaxation.
Complete tetanus is produced when a muscle is stimulated at a very rapid rate. Example: 60-70 stimuli/sec.
Frequency of stimulation at which complete tetanus is produced is called tetanizing frequency.
Muscle tension produced in complete tetanization is greater than that in single muscle twitch.
Clostridium Tetani alpha motor neuron repeated discharge and tetanus.
INCOMPLETE & COMPLTE TETANUS
INCOMPLETE
Repeated stimuli at a fast rate relaxation of each twitch remains incomplete incomplete tetanus.
COMPLETE
Repeated stimuli at still higher rate relaxation phase disappears altogether sustained contraction phase is obtained complete tetanus.
TETANY (increased excitability of motor nerves)
CAUSES: Parathyroidectomy (during thyroid surgery) lack
of PTH plasma Ca+ level falls signs of neuromuscular hyperexcitability appear.
Alkalosis plasma proteins behave as anions bind cations including Ca++ decreased ionized calcium hypocalcemia less calcium available in ECF for membrane stabilization no blocking of sodium channels by calcium cations (negatively charged on inside) increased excitability of motor nerves tetany.
Hyperventilation hypocapnia (decreased CO2 concentration) respiratory alkalosis plasma ionized calcium falls carpopedal spasm, a positive Chvostek sign & other signs of tetany).
SIGNS OF TETANY:
CHVOSTEK’S SIGN: A quick contraction of ipsilateral facial muscles elicited by tapping over the facial nerve at the angle of the jaw.
SIGNS OF TETANY:
TROUSSEAU’S SIGN: A spasm of muscles of the upper extremity that cause flexion of the wrist & thumb with extension of fingers.
In individuals with mild tetany with no obvious spasm, trousseau’s sign may be produced by occluding circulation for few minutes with a B.P cuff.
TREPPE / STAIRCASE PHENOMENON Definition: When a muscle
is stimulated by maximum stimuli at a frequency less than tetanizing frequency progressive increase in muscle tension with repeated stimuli, till it becomes constant.
If threshold stimuli are applied so that each stimulus reaches the muscle when the muscle twitch due to previous stimulus has completed each successive twitch shows an increased amplitude till a maximum height is reached this is called as staircase phenomenon.
Ca Gluconate IV Slowly over 20 minutes
Vitamin D
Mechanism of Treppe
Greater availability of calcium in sarcoplasm.
Accumulation of metabolites.
Rise in local temperature.
All these exert beneficial effect on contraction.
Muscle Atrophy
Definition of atrophy: decrease in the size of a tissue due to decrease in size of its cells.
There is weakening and shrinking of a muscle
Mechanism: decrease in the actin, myosin filaments and the diameter of myofiber.
May be caused by:
Immobilization of muscles e.g. in cases of bed ridden patient
Loss of neural stimulation e.g. in cases of nerve injury
Muscle Hypertrophy
Definition of hypertrophy: increase in the size of a tissue due to increase in size of its cells.
Enlargement of a muscle
More capillaries
More mitochondria
Caused by: Strenuous exercise
Anabolic Steroid hormones
RIGOR MORTIS:
after death muscles of dead body become rigid rigor mortis.
Its onset depends on:
Temperature: increased temperature rapid onset.
Activity: Vigorous activity of muscle before death rapid onset.
Mechanism of Rigor Mortis
• After death ATP is not available no detachment of cross-bridges of myosin from active site of actin filaments rigor mortis.
• After 16-24 hours rigor mortis disappears due to autolysis of muscle proteins (resulting from hydrolytic enzymes released from lysosomes).
SIGNIFICANCE of Rigor Mortis
Forensic significance: can give us a clue about cause of death. In cases of suicide, the gun or dagger is locked in the hand.
Time since death: it gives us some idea about time since death. rigor mortis remains for about 16 hours after death. It disappears after 16-24 hrs of death.
SLOW AND FAST MUSCLE FIBERS
SOURCES OF ENERGY DURING MUSCLE CONTRACTION
