BIOENGENHARIA MÉDICA MÚSCULO CARDÍACO Abril 31, 2008 Eduardo Infante de Oliveira Instituto de Fisiologia, FML

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Orientation of cardiac muscle fibres: Unlike skeletal muscles, cardiac muscles have to contract in more than one direction. Cardiac muscle cells are striated, meaning they will only contract along their long axis. In order to get contraction in two axis, the fibres wrap around.

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TYPES OF MUSCLE LOCATIONMICROSCOPIC APPEARANCE RELATIONSHIP WITH THE NERVOUS SYSTEM SPEED OF CONTRATION SKELETALHEAVYILY STRIATED VOLUNTARYSLOW TO FAST CONTRACTIONS VISCERALNONSTRIATED (SMOOTH) INVOLUNTARYVERY SLOW CONTRACTIONS CARDIAC LIGHTLY STRIATED AUTORHYTHMIC SLOW CONTRACTIONS

Characteristics of Skeletal, Cardiac, and Smooth Muscle Table 10–4

Cardiac Tissue Cardiac muscle is striated, found only in the heart Figure 10–22 7 Characteristics of Cardiocytes Unlike skeletal muscle, cardiac muscle cells (cardiocytes): –are small –have a single nucleus –have short, wide T tubules –have no triads –have SR with no terminal cisternae –are aerobic (high in myoglobin, mitochondria) –have intercalated discs

Intercalated Discs Are specialized contact points between cardiocytes Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes) Functions –Maintain structure –Enhance molecular and electrical connections –Conduct action potentials Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

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15 Structure of Cardiac Muscle Cell

One distinguishes cardiac muscle from skeletal muscle by the branching fibers, presence of intercalated discs, and centrally-placed single nuclei/cell. Cardiac muscle intercalated disc Cardiac muscle – section – H&E – 40x objective centrally-located nucleus striations

Cardiac muscle –section – silver – 20x objective This stain clearly shows the single central nucleus, branching fibers, intercalated discs, and striations. Cardiac muscle branching nucleus intercalated disc

contractile unit: sarcomere Z line: the actin filaments are attached I: band of actin filaments, titin and Z line A: band of actin-myosin overlap H: clear central zone containing only myosin T: T tubules mit: mitochondria g: glycogen

AP-contraction relationship: AP in skeletal muscle is very short-lived AP is basically over before an increase in muscle tension can be measured. AP in cardiac muscle is very long-lived AP has an extra component, which extends the duration. The contraction is almost over before the action potential has finished.

Cardiac myocyte action potential:

Refractory Period Absolute: Cardiac muscle cell completely insensitive to further stimulation Absolute: Cardiac muscle cell completely insensitive to further stimulation Relative: Cell exhibits reduced sensitivity to additional stimulation Relative: Cell exhibits reduced sensitivity to additional stimulation Long refractory period prevents tetanic contractions Long refractory period prevents tetanic contractions

Cardiac vs. Skeletal Muscle Contraction Cardiac Muscle Skeletal Muscle Action Potential Duration: msec Duration: 10 msec Extracellular Calcium Ions Delay repolarization & initiate contraction Initiate contraction Type of contraction Long refractory period which continues while relaxing = no tetanus! Tetanus occurs because short refractory period

Cardiac conducting system:

Pacemaker potential:

Pacemaker regulation: Once the pacemaker cells reach threshold, the magnitude and duration of the AP is always the same. In order to change the frequency, the time between APs must vary. The interval can only be changed in two ways. The rate of depolarization can be changed The amount of depolarization required to reach threshold can be changed.

EE-515 Bioelectricity & Biomagnetism 2002 Fall - Murat Eyüboğlu The conduction system of the heart.

EE-515 Bioelectricity & Biomagnetism 2002 Fall - Murat Eyüboğlu Electrophysiology of the heart The different waveforms for each of the specialized cells

Principle of Continuity: Conservation of mass in a closed hydraulic system Blood is an incompressible fluid Vascular system is a closed hydraulic loop Vol ejected from left heart = vol received in R heart

Pressure relationships:

Curva Pressão-Volume Ventricular

Cardiac Output and EDV

Regulation of the Heart Intrinsic regulation: Results from normal functional characteristics, not on neural or hormonal regulation Intrinsic regulation: Results from normal functional characteristics, not on neural or hormonal regulation Starling’s law of the heart Starling’s law of the heart Extrinsic regulation: Involves neural and hormonal control Extrinsic regulation: Involves neural and hormonal control Parasympathetic stimulation Parasympathetic stimulation Supplied by vagus nerve, decreases heart rate, acetylcholine secreted Supplied by vagus nerve, decreases heart rate, acetylcholine secreted Sympathetic stimulation Sympathetic stimulation Supplied by cardiac nerves, increases heart rate and force of contraction, epinephrine and norepinephrine released Supplied by cardiac nerves, increases heart rate and force of contraction, epinephrine and norepinephrine released

Heart Homeostasis Effect of blood pressure Effect of blood pressure Baroreceptors monitor blood pressure Baroreceptors monitor blood pressure Effect of pH, carbon dioxide, oxygen Effect of pH, carbon dioxide, oxygen Chemoreceptors monitor Chemoreceptors monitor Effect of extracellular ion concentration Effect of extracellular ion concentration Increase or decrease in extracellular K + decreases heart rate Increase or decrease in extracellular K + decreases heart rate Effect of body temperature Effect of body temperature Heart rate increases when body temperature increases, heart rate decreases when body temperature decreases Heart rate increases when body temperature increases, heart rate decreases when body temperature decreases