Muscle Physiology

·Muscle form 40 to 50% of body mass.
·About 600 muscles are identified.
Classification of Muscles:
·Depending on striations: Striated(Skeletal & Cardiac) & Nonstriated(smooth)
·Depending on Control: voluntary(Skeletal) & Involuntary(Cardiac & smooth)
·Depending on situation: Skeletal, Cardiac & Smooth.
·Muscle fiber or cell has a length b/w 1 – 4cm (Average 3cm) & diameter from 10 – 100microns.
·1 myofibril contains 1500 myosin filaments & 3000 actin filaments
·Sarcomere is the structural & functional unit of skeletal muscle.
  Length – 2.5 to 3.2 mm. 
  Sarcomere lies b/w two ‘Z’ lines
·Each myofibril consists of alternate light (I or J band) & dark band (A or Q band).
·Each sarcomere consist of thin (Actin) & thick (myosin) filaments.
Myosin filament is present throughout the ‘A’ band.
No movement of myosin during muscular contraction.
·Actin filaments are formed by 3 types of proteins called actin, tropomyosin & troponin.
·The skeletal muscle is formed by 75% of water, 20% of proteins & 5%of organic substances.
·Troponin: is formed by 3 subunits; Troponin I – attached to F actin; Troponin T – attached to tropomyosin; Troponin C – attached to calcium ions.
·Following are the muscle proteins; Myosin; Actin; Tropomyosin; Troponin; Actinin; Titin; Desmin; Myogen & Myoglobulin.
·There is no movement in the myosin filament during muscle contraction.
·Actin filaments slide over the myosin filament during muscle contraction.
·Tropomyosin covers the active sites of actin.
·Rheobase: this is the least possible, i.e. (minimum) strength (Voltage) of stimulus which can excite the tissue.
·Chronaxie: it is the minimum time, at which a stimulus with double the rheobasic strength (voltage) can excite the tissue.
Chronaxie of skeletal muscle is shorter than that of cardiac and smooth muscles.
Cold lengthens chronaxie. whereas vagal stimulation shortens chronaxie.
·Refractory period is the period at which the muscle does not show any response to a stimulus.
·Skeletal muscles are purely aerobic & don’t have any fuel reserve.
·Dark, light bands & troponin are absent in smooth muscle.
·The study of electrical activity of the muscle is done by electromyography.
·The muscle ruptures when it is stretched to about 3 times its equilibrium length.
Cardiovascular system

·The syncytium of called physiological syncytium because there is no anatomical continuity of the fibers.

·SA node the pace maker is a small strip of modified cardiac muscle is situated in the superior part of lateral wall of right

  atrium, just below the opening of superior vena cava.

· AV node is situated in the right posterior portion of interatrial septum.

· Bundle of his run on either side of the interventricular septum.

· Rhythmicity of different parts of heart:

o SA node                        : 70 to 80 / min

o AV node                        : 40 to 60 / min

o Atrial muscle                  : 40 to 60 / min

o Ventricular muscle          : 20 to 40 / min

· Velocity of impulse at different parts of the conductive system.

o Atrial muscle fibres         : 0.3 meter / second

o Internodal fibers            : 1.0 meter / second

o AV node                        : 0.05 meter / second

o Bundle of his                  : 0.12 meter / second

o Purkinje fibers               : 4 meter / second

o Ventricular muscle fibers : 0.5 meter / second

· Cardiac cycle includes systole & diastole which practically includes the events of ventricles.

· When heart beats at the normal rate of 72/min, the duration of cardiac cycle is 0.8 sec. 

· The duration of systole is 0.27 sec & that of diastole is 0.53 sec.

· The subdivision with duration are

o Systole

§   Isometric contraction      = 0.05 sec

§   Ejection period   = 0.22 sec

o Diastole

§   Protodiastole                  = 0.04 sec

§   Isometric relaxation       = 0.08 sec

§   Rapid filling                    = 0.11 sec

§   Slow filling                     = 0.19 sec

§   Atrial systole                  = 0.11 sec

§   Total duration of cardiac cycle = 0.27 + 0.53 = 0.8 sec.

· Atrial systole: atrial contract & a small amount of blood enter the ventricles.

· Isometric contraction: all the valves are closed, ventricles undergo isometric contraction & pressure in the ventricles

 is increased.

· Ejection period: semilunar valves opened, ventricles contract & blood is ejected out.

· Protodiastole: this is the first diastole. The semilunar valves are closed at the end of this period.

· Isometric relaxation: all the valves are closed, ventricles undergo isometric relaxation & pressure in the ventricles is reduced.

· Rapid & slow filling: Atrioventricular valves are opened, ventricles relax & filling occurs.

·  Pressure difference

· End systolic volume is 70 – 90 ml

· End diastolic volume is 130 – 150 ml.

· Cause for cardiac murmurs 

· Electrocardiography:

Duration

o    Interval B/W 2 thick lines: (5mm) = 0.2 second

o    Interval B/W 2 thin lines (1mm) = 0.04 second

Amplitude

o    Interval B/W 2 thick lines: (5mm) = 0.5 mV

o    Interval B/W 2 thin lines (1mm) = 0.1 mV

Speed of the paper

o    25 mm  or 50 mm / second

§   Normal heart rate is 72 / min

§   Tachycardia: increase heart rate above 100 / min.

Waves of normal ECG

· In hyperkalemia, ECG shows a tall T wave.

· In hypokalemia, ECG shows depressed S-T segment.

· Heart sounds.

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