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Home , Cardiac muscle, Pacemaker potential

CARDIOVASCULAR SYSTEM: : STRUCTURE AND PROPERTIES

For: Semester II CC2TH/ GEN 2TH

Prepared and Compiled By: OLIVIA CHOWDHURY DEPARTMENT OF PHYSIOLOGY SURENDRANATH COLLEGE April 29, 2020 OLIVIA CHOWDHURY • of The

April 29, 2020 OLIVIA CHOWDHURY •The Layers Of The Heart

Three layers:

• Epicardium . – a double serous membrane . Visceral pericardium (Next to heart) . Parietal pericardium (Outside layer) . Serous fluid fills the space between the layers of pericardium . Connective layer • Myocardium . Middle layer . Mostly cardiac muscle • . Inner layer . April 29, 2020 OLIVIA CHOWDHURY • The Heart Valves

 Allows to flow in only one direction  Four valves:

 Atrioventricular valves– between atria and ventricles

Bicuspid/ between LA and LV

between RA and RV

 Semilunar valves between ventricles and arteries

Pulmonary semilunar valve

Aortic semilunar valve April 29, 2020 OLIVIA CHOWDHURY •Direction Of Blood Flow In The Heart

April 29, 2020 OLIVIA CHOWDHURY Right side of the heart: • receives venous blood from systemic circulation via superior and inferior vena cava into right

• pumps blood to from right Left side of the Heart: • receives oxygenated blood from pulmonary • pumps blood into systemic circulation

April 29, 2020 OLIVIA CHOWDHURY •The Cardiac Muscle

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

April 29, 2020 OLIVIA CHOWDHURY •The Cardiac Muscle

 Striated and resemble the fibre

is the functional unit

 Sarcomere of the cardiac muscle has all the contractile proteins, namely , , .

 Cardiac muscle fibre is bound by . It has a centrally placed nucleus. are embedded in the .

is less abundant than in skeletal muscle.  Sarcolemma of cardiac muscle has specialized channels that skeletal muscle does not have: voltage-gated Ca2+ channels.

April 29, 2020 OLIVIA CHOWDHURY • 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 (Atrial syncytium and ventricular syncytium)

The discs contain several gap junctions providing cytoplasmic continuity.

Rapid transmission of impulse.

when one cardiac cell undergoes an , the electrical impulse spreads to all other cells that are joined by gap junctions April 29, 2020 OLIVIA CHOWDHURY •Muscle Fibres which Form the Pacemaker

 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.

April 29, 2020 OLIVIA CHOWDHURY •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

April 29, 2020 OLIVIA CHOWDHURY •Conducting System Of The Heart

April 29, 2020 OLIVIA CHOWDHURY • Properties of Cardiac Muscle

Autorhythmicity

Excitability (Bathmotropic action)

Conductivity ( action)

Contractility (Inotropic action)

April 29, 2020 OLIVIA CHOWDHURY • Autorhythmicity

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

April 29, 2020 OLIVIA CHOWDHURY •Locations Of Autorhythmic Cells

(SA node) Specialized region in right atrial wall near opening of .

(AV node) Small bundle of specialized cardiac cells located at base of right atrium near septum

(atrioventricular bundle) Cells originate at AV node and enters . Divides to form right and left which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls

Small, terminal fibers that extend from bundle of His and April 29, 2020 OLIVIA CHOWDHURY spread throughout ventricular myocardium Autorhythmic cells do not have stable resting (RMP)

Natural leakiness to Na & Ca spontaneous and gradual depolarization

Unstable resting membrane potential = / prepotential)

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

April 29, 2020 OLIVIA CHOWDHURY •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

April 29, 2020 OLIVIA CHOWDHURY April 29, 2020 OLIVIA CHOWDHURY •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.

April 29, 2020 OLIVIA CHOWDHURY • Excitability

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

along conductive pathway

excites atrial & ventricular muscle fibres

April 29, 2020 OLIVIA CHOWDHURY •Action Potential In Contractile Fibers

April 29, 2020 OLIVIA CHOWDHURY •Refractory Period

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

 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.

April 29, 2020 OLIVIA CHOWDHURY • 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 , prevents tetanus

• Gives time to heart to relax after each contraction, prevents fatigue

• It allows time for the heart chambers to fill during before next contraction

April 29, 2020 OLIVIA CHOWDHURY •Contractility

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.

April 29, 2020 OLIVIA CHOWDHURY Treppe or Staircase Phenomenon

• 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 .

April 29, 2020 OLIVIA CHOWDHURY Summation of Subliminal Stimuli

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.

April 29, 2020 OLIVIA CHOWDHURY •Conductivity

Definition: property by which excitation is conducted through the cardiac tissue

April 29, 2020 OLIVIA CHOWDHURY 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

April 29, 2020 OLIVIA CHOWDHURY •Effects of Autonomic Nerve Activity on the Heart

April 29, 2020 OLIVIA CHOWDHURY THANK YOU!

April 29, 2020 OLIVIA CHOWDHURY