Cardiac Muscle Cells: Structure and Function

Cardiac Muscle Cells

• Cardiac muscle cells are short, fat, branched, and interconnected
• Cardiac muscle is striated, like skeletal muscle, but less so
• Cardiac muscle contracts by the sliding filament theory
• Each cardiac cell has 1-2 nuclei
• 25-35% of a cardiac cell's volume is composed of large mitochondria
• Large amounts of mitochondria make the cell less prone to fatigue

Intercalated Discs

• Cardiac muscle fibers connect to the next via intercalated discs
• Each disc contains desmosomes for cell adhesion
• Each disc also contains gap junctions, which are plasma membrane channels for communication between cells

Desmosomes and Gap Junctions

• Desmosomes prevent adjacent cells from separating during contraction
• Gap junctions allow ions to pass between cells, transmitting current across the entire heart
• This creates a "functional syncytium"

Events of Contraction

• Modes of stimulation include automaticity, also called auto-rhythmicity
• 1% of cardiac muscle cells are self-excitable or autorhythmic, causing the heart to contract as a single unit
• The heart will beat even if nerve connections are severed
• Gap junctions and an "in house conduction system," known as pacemaker cells/autorhythmic cells, drive contraction
• Action potential spreads throughout the myocardium via gap junctions

Intrinsic Conduction System

• There are 5 areas where pacemaker cells are found:
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Atrioventricular bundle (Bundle of His)
• Bundle branches
• Subendocardial network/Purkinje fibers

Heart Physiology: Sequence of Excitation

• Impulses are generated about 75 times per minute (sinus rhythm) in the sinoatrial (SA) node (pacemaker)
• The SA node depolarizes faster than any other part of the myocardium
• Atrioventricular (AV) node has smaller diameter fibers with fewer gap junctions
• Impulses are delayed by ~0.1 second in the AV node
• The AV node depolarizes ~50 times per minute in absence of SA node input
• Only electrical connection between the atria and ventricles is the atrioventricular (AV) bundle (bundle of His)
• Right and left bundle branches are separated by two pathways in the interventricular septum
• These pathways carry impulses toward the apex of the heart
• Purkinje fibers complete the pathway into the apex and ventricular walls
• AV bundle and Purkinje fibers depolarize ~30 times per minute in absence of AV node input
• The Bundle is elaborate on the left side only due to the thickness of the ventricle

Electrical Events

• Impulses pass through the sinoatrial node through atrial myocytes to the atrioventricular node
• Impulses then go down the atrioventricular bundle (bundle of His) to the right and left bundle branches and on to the Purkinje fibers
• Ventricular contraction follows immediately ventricular depolarization wave
• About 70 mL of blood is displaced into large arteries during Ventricular contraction

Electrocardiogram Tracings

• Electrical signals of the heart are monitored and amplified during spread through the body by electrocardiographs
• Electrocardiographs give graphic records of heart activity
• The action potential of the heart is generated by all nodes and contractile cells
• An ECG has 3 waves of deflections: P, QRS, and T waves

ECG Tracing Waves

• The P wave results from the depolarization of the SA node through the atria
• After ~100 ms, the P wave begins, resulting in atrial contraction
• The QRS wave results from ventricular depolarization and proceeds with ventricular contraction
• The QRS wave lasts about ~80ms
• The T wave represents ventricular repolarization and lasts about ~160ms
• Repolarization is slower than depolarization, thus the T wave is more spread out with a lower amplitude than the QRS complex

Chemicals that Increase Heart Rate

• Chemicals that increase heart rate (tachycardia) include:
• Nicotine
• Adrenaline
• Thyroxin
• Caffeine
• Regular heart beat occurs at ~72 beats per minute (bpm)
• The heart can beat more than ~72 beats per minute during exercise or from the consumption of caffeine or nicotine
• A heartbeat over 100 bpm is called tachycardia
• Prolonged tachycardia may cause fibrillation, which is a rapid, irregular contraction of heart muscle due to a lack of synchronization

Chemicals that Decrease Heart Rate

• Chemicals that decrease heart rate (bradycardia) include:
• Carbon Monoxide
• Alcohol
• The heart can beat very slowly, less than 72 bpm, for athletes who have an elevated cardiac output even during rest
• A heartbeat below 60 bpm is called bradycardia

Cardiac Cycle

• The heart has four chambers: two atria and two ventricles
• The two atria contract at the same time, and then start to relax
• The two ventricles then contract
• Systole refers to ventricular contraction
• Diastole refers to ventricular relaxation
• The cardiac cycle is one complete heartbeat, which includes both atria and ventricles contracting and relaxing

Timing of Cardiac Cycle

• The heart beats ~75 times per minute
• One beat (cardiac cycle) is 0.8 seconds
• Atrial contraction takes 0.1 seconds
• Atrial relaxation follows
• Ventricular contraction takes 0.3 seconds
• Ventricular relaxation takes 0.4 seconds, creating a resting period
• Two phenomena occur during one cycle: contraction and relaxation of the myocardium, and the opening and closing of the valves

Important Features of the Cardiac Cycle

• P wave: depolarization of SA node and atria
• QRS complex: ventricular depolarization and atrial repolarization
• T wave: ventricular repolarization
• P-R interval: the beginning of atrial excitation to the beginning of ventricular excitation
• S-T segment: entire ventricular myocardium depolarized
• Q-T interval: beginning of ventricular depolarization through ventricular repolarization

Heart Sounds

• Heart sounds ("lub-dup, pause, lub-dup, pause") are associated with the closing of heart valves
• The first sounds (S1) occurs as the AV valves close and signifies the beginning of systole
• Ventricular pressure rises above the atrial pressure, which makes AV close
• The second sound (S2) occurs when the SL valves close at the beginning of ventricular diastole/relaxation
• The lub sound is louder, longer, and more resonate than the dup sound
• A pause occurs between heart beats when the heart is relaxing
• Murmurs occur if the valves are not tight enough, causing back flow of blood which makes a swishing sound

"Lub" Sound

• The "lub" sound indicates Closure of the AV valves (tricuspid and bicuspid)
• Increases in Pressure occur in the ventricles at this time
• Ventricles contract to Systole
• The QRS complex appears
• The "Lub" sound is Louder and longer than the "Dup" sound

"Dup" Sound

• The "Dup" sound indicates Closure of the semilunar valves (pulmonary and aortic)
• Pressure starts to drops
• Ventricles start to relax indicating Diastole
• This occurs in the Middle of the T wave
• The "Dup” sound is Softer and shorter than the "lub" sound

The Pulse

• The Pulse is the alternating pressure wave of contraction and relaxation on an artery
• This wave occurs in the left ventricle
• Pulse pressure is the difference between systolic and diastolic pressure
• At rest, pulse rate is equal to the heart rate, 70-76 bpm (pressure surge/min= bpm)

Blood Pressure

• BP- Pressure is the force the blood exerts against any unit area on the blood vessel walls
• Normal BP is approximately 120/80 mmHg
• Systolic pressure occurs during ventricular contraction
• Diastolic pressure occurs during ventricular relaxation

Cardiac Output

• CO is the amount of blood pumped by each ventricle in one minute ( ~ 5.25 1)
• HR is the number of heart beats per minute (75 bpm)
• SV is the amount of blood pumped out by a ventricle with each beat (70 ml)
• CO is the product of heart rate (HR) and stroke volume (SV) (70 ml)
• CO (ml/min) = HR (75 beats/min) x SV (70 ml/beat)
• CO = 5250 ml/min (5.25 L/min)
• Cardiac reserve is the difference between resting and maximal CO