cardiovascular system: cardiac muscle

CARDIOVASCULAR SYSTEM:

CARDIAC MUSCLE: STRUCTURE

AND PROPERTIES

For: Semester II

CC2TH/ GEN 2TH

Prepared and Compiled By:

OLIVIA CHOWDHURY

DEPARTMENT OF PHYSIOLOGY

SURENDRANATH COLLEGEApril 29, 2020 OLIVIA CHOWDHURY

•Anatomy of The Heart

April 29, 2020 OLIVIA CHOWDHURY

Three layers:

• Epicardium

Pericardium – a double serous

membrane

Visceral pericardium (Next to heart)

Parietal pericardium (Outside layer)

Serous fluid fills the space between

the layers of pericardium

Connective tissue layer

• Myocardium

Middle layer

Mostly cardiac muscle

• Endocardium

Inner layer

Endothelium

•The Layers Of The Heart


Allows blood to flow in only one direction

Four valves:

Atrioventricular valves– between atria and ventricles

Bicuspid/ Mitral valve between LA and LV

Tricuspid valve between RA and RV

Semilunar valves between ventricles and arteries

Pulmonary semilunar valve

Aortic semilunar valve

• The Heart Valves


•Direction Of Blood Flow In The Heart


Right side of the heart:

• receives venous blood from systemic circulation via

superior and inferior vena cava into right atrium

• pumps blood to pulmonary circulation from right

ventricle

Left side of the Heart:

• receives oxygenated blood from pulmonary veins

• pumps blood into systemic circulation


Myocardium has three types of muscle fibers:

Muscle fibers which form contractile unit of heart

Muscle fibers which form the pacemaker

Muscle fibers which form conductive system

•The Cardiac Muscle


Striated and resemble the skeletal muscle fibre

Sarcomere is the functional unit

Sarcomere of the cardiac muscle has all the contractile proteins,

namely actin, myosin, troponin tropomyosin.

Cardiac muscle fibre is bound by sarcolemma. It has a centrally

placed nucleus. Myofibrils are embedded in the sarcoplasm.

Sarcoplasmic reticulum is less abundant than in skeletal muscle.

Sarcolemma of cardiac muscle has specialized ion channels that skeletal muscle does not have: voltage-gated Ca2+ channels.

•The Cardiac Muscle


• The Cardiac Muscle:

Presence Of Intercalated Discs

Fibers are branched

Adjacent cardiac cells are joined end to end by

specialized structures known as intercalated

discs

Heart functions as a syncytium (Atrial

syncytium and ventricular syncytium)

The discs contain several gap junctions

providing cytoplasmic continuity.

Rapid transmission of impulse.

when one cardiac cell undergoes an action

potential, the electrical impulse spreads to all

other cells that are joined by gap junctionsApril 29, 2020 OLIVIA CHOWDHURY

Some of the muscle fibres of heart are modified into a specialized

structure known as pacemaker.

These muscle fibres forming the pacemaker have less striation.

They are named pacemaker cells or P cells.

Sino-atrial (SA) node forms the pacemaker in human heart.

•Muscle Fibres which Form the Pacemaker


•Muscle Fibres Which Form Conductive System

Conductive system of the heart is formed by modified cardiac

muscle fibres

Impulses from SA node are transmitted to the atria directly. However,

the impulses are transmitted to ventricles through various components

of conducting system


•Conducting System Of The Heart


• Properties of Cardiac Muscle

Autorhythmicity

Excitability (Bathmotropic action)

Conductivity (Dromotropic action)

Contractility (Inotropic action)


Definition: the ability of the heart to initiate its beat continuously and

regularly without external stimulation

myogenic (independent of nerve supply)

due to the specialized excitatory & conductive system of the

heart

intrinsic ability of self-excitation

(waves of depolarization)

cardiac impulses

•Autorhythmicity


•Locations Of Autorhythmic Cells Sinoatrial node (SA node) Specialized region in right

atrial wall near opening of superior

vena cava.

Atrioventricular node (AV node) Small bundle of

specialized cardiac cells located at base of right

atrium near septum

Bundle of His (atrioventricular bundle)

Cells originate at AV node and enters

interventricular septum. Divides to form right and

left bundle branches which travel down septum,

curve around tip of ventricular chambers, travel

back toward atria along outer walls

Purkinje fibers

Small, terminal fibers that extend from bundle of His and

spread throughout ventricular myocardiumApril 29, 2020 OLIVIA CHOWDHURY

Autorhythmic cells do not have stable resting membrane potential (RMP)

Natural leakiness to Na & Ca spontaneous and gradual depolarization

Unstable resting membrane potential = pacemaker potential/ prepotential)

Gradual depolarization reaches threshold (-40 mv) spontaneous AP generation


•Rate Of Generation Of AP At Different Sites Of TheHeart

SA node acts as the pacemaker because it has the fastest rate ofgenerating action

potential


•Autorhymicity-Effect Of Stannius Ligature In

Frog’s Heart

• The properties of cardiac muscle are demonstrated using a quiescent heart.

• A quiescent heart is a heart which has stopped beating but is still alive.

• Such a preparation can be obtained by tying a Stannius Ligature in the frog’s

heart.


Definition: The ability of cardiac muscle to respond to a stimulus of

adequate strength & duration by generating an AP

AP initiated by SA node travels

excites atrial & ventricular muscle fibres

along conductive pathway

• Excitability


•Action Potential In Contractile Fibers


•Refractory Period

It is that period during which a second stimulus fails to evoke aresponse.

Absolute Refractory Period : It is that period during which a

second stimulus however strong , fails to evoke a response.

Relative Refractory Period : It is that period during which a

second stimulus evokes a response if it is sufficiently high.


• Long refractory period

(250msec) compared to

skeletal muscle (3msec)

• During this period membrane is

refractory to further stimulation until

contraction is over.

• It lasts longer than muscle

contraction, prevents tetanus

• Gives time to heart to relax after each

contraction, prevents fatigue

• It allows time for the heart chambers

to fill during diastole before next

contraction


Definition: Ability of cardiac muscle to contract in response to

stimulation.

All Or None Law

• The response to a threshold stimulus is maximal. If the stimulus is

below threshold there is no response

• The cardiac muscle follows the all or none law as a whole.

• In the case of skeletal muscle, all-or-none law is applicable

only to a single muscle fiber.

•Contractility


• When stimuli of same strength are applied at short intervals, a gradual

increase in the height of contraction is observed for the first few

contractions, after which there is no further increase. This gradual increase in

the force of contraction is called staircase phenomenon.

• This is due to the BENEFICIAL EFFECT- decrease in viscosity, mild increase

in temperature and increase in the level of calcium ions.

Treppe or Staircase Phenomenon


When a stimulus of subliminal strength is applied the cardiac muscle

does not show any response. When a series of subliminal stimuli are

applied in succession, the muscle responds with a contraction. It is

due to the summation of all the subliminal stimuli that produce a

threshold stimulus.

Summation of Subliminal Stimuli


Definition: property by which excitation is conducted through

the cardiac tissue

•Conductivity


Tissue Conduction rate (m/s)

Atrial muscle 0.3

Atrial pathways 1

AV node 0.05

Bundle of His 1

Purkinje system 4

Ventricular muscle 0.3-0.5

Thus, the velocity of impulses is maximum in Purkinje fibers and minimum in the AV

node


•Effects of Autonomic Nerve Activity on the Heart


THANK YOU!


cardiovascular system: cardiac muscle

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