RK Goit, LecturerDepartment of Physiology

Structure of Cardiac MuscleExcitation-Contraction Coupling

Properties of Cardiac Muscle

• heart is composed of 3 major types of cardiac muscle: atrial muscle, ventricular muscle, & specialized excitatory & conductive muscle fibers

• atrial & ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer

• excitatory & conductive fibers contract only feebly

• cardiac muscle fibers are made up of many individual cells connected in series & in parallel with one another

• cardiac muscle fibers arranged in a latticework, with the fibers dividing, recombining, & then spreading again

• cardiac muscle is striated in same manner as in skeletal muscle

• cardiac muscle has myofibrils that contain actin & myosin filaments almost identical to those found in skeletal muscle– these filaments lie side by side & slide along one another during

contraction in the same manner as occurs in skeletal muscle

Cardiac Muscle as a Syncytium • cardiac cells are so interconnected that when one of

these cells becomes excited, the action potential spreads to all of them, from cell to cell throughout the latticework interconnections

• heart actually is composed of two syncytiums: the atrial syncytium, which constitutes the walls of the two atria, & the ventricular syncytium, which constitutes the walls of the two ventricles

• division of the muscle of the heart into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction

properties of heart can be divided into 2 groups:Beating heart Quiescent heart

• Automaticity• Rhythmicity• Contractibility• Excitability• Conductivity• Distensibility• Functional syncitium• Long refractory period• Extrasystole & compensatory pause

• All or none law• The staircase phenomenon• Length-tension relationship• Summation of subminimal stimuli

Properties of cardiac muscle• Automaticity– capability of contract even in the absence of neural control

• Rhythmicity– heart beats are extremely regular

• Contractibility– cardiac muscle contracts in response to a stimulus

• Excitability – ability of the cardiac muscle to respond to different stimuli

• Conductivity– impulses produced in the SA node is conducted by the

specialized conducting pathway

• Distensibility– occurs due to compliance of the cardiac muscle

• Functional syncytium– due to the presence of numerous gap junctions

• Long refractory period

• Extrasystole & compensatory pause– when the ventricle is stimulated in the relaxation period

(relative refractory period), the cardiac muscle may contract

– It may occurs because a papillary muscle may fire an impulse before normal impulse reaches the ventricles

• All or none law– if the heart is stimulated with subthreshold stimuli no response

is seen

– a threshold stimulus is the weakest stimulus that evokes a response

– amplitude of contraction in response to the suprathreshold stimuli remains the same as that with the threshold stimuli

• Staircase phenomenon– if a quiescent ventricle is stimulated repeatedly such that the

interval between consecutive stimuli is less than 10 s, the first 3-4 contractions are progressively more forceful

• Length-tension relationship (Frank-Starling law) – the force of contraction of cardiac muscle is directly proportional

to the initial length of the muscle fibers

• Summation of subminimal stimuli– when subminimal stimuli are applied repeatedly, the stimuli

summate & produce a response

• Frequency force relationship– changes in cardiac rate & rhythm also affect myocardial

contractility

• Load velocity relationship

References

• Ganong Review of Medical Physiology, 23/E• Textbook of Medical Physiology, 12/E Guyton & Hall • Understanding Medical Physiology, 4/E Bijlani &

Manjunatha

Thank You