Cardiac Muscle Cells + Autorhythmicity

Autorhythmicity

Cardiac muscle sets is own rhythm without a need for input from the nervous system

done with pacemaker cells

Pacemaker Cells

Electrical activity is coordinated by a small, unique population of modified cardiac muscle cells called Pacemaker Cells that rhythmically and spontaneously generate Action Potentials

Action potentials trigger the other type of cardiac muscle cell, known as Contractile Cells, to also have action potentials

Cardiac Muscle Cells

Have Striations, or light and dark bands that show the arrangement of the contractile proteins, transverse tubules and a sarcoplasmic reticulum, which share the same functions as skeletal muscle and generate tension through the sliding filament mechanism

have individual short, branched cells with a single nucleus, abundant Myoglobin, which carries oxygen, and multiple mitochondria

intercalated discs

Intercalated Discs

Structures with desmosomes and gap junctions that link adjacent cells and allow ions to pass from one cell to the next

Functional Syncytium

Gap junctions allow the electrical activity generated by the pacemaker cells to spread rapidly to all cardiac muscle cells which permits the heart to contract as a unit and produce a Heart Beat

cells of the heart are referred to as a Functional Syncytium

Voltage-Gated Sodium Ion Channels

sarcolemma contains sodium ion channels (voltage, calcium, potassium, nonselective)

Open in response to voltage changes across the membrane

found in all cardiac muscle cells except certain pacemaker cells

Calcium Ion Channels

Exhibit voltage-gated opening but time-gated closing, so will close after a certain period regardless of the voltage

present in all cells

Potassium Ion Channels

One or more types per cell

some are ligand-gated while others are voltage-gated

Nonselective Cation Channels

HCN Channel found on some pacemaker cells that is activated by hyperpolarization

allows sodium ions to enter and potassium ions to exit simultaneously

Voltage

Difference in electrical potential between two points

Membrane Potential

Voltage or charge difference that exists across the membranes of all cells, including excitable cells

Resting Membrane Potential

Membrane potential of an excitable cell at rest—averages between −60 and −90 mV

Ion Gradient

Difference in the concentration of ions across a plasma membrane

Current

Flow of ions or electrons with a chemical or electrical gradient

Depolarization

Change in resting membrane potential to a less negative value

occurs when positive charges (usually sodium or calcium ions) rush into the cell

Repolarization

Return of a cell to its negative resting membrane potential

occurs when positive charges (potassium ions) leave the cell

Hyperpolarization

Change in resting membrane potential to a more negative value

occurs when positive charges (potassium ions) exit the cell or when negative charges enter the cell

cause its potential to become more negative than the resting membrane potential

Remember that the concentration of sodium and calcium ions in the ECF is higher than in the cytosol, and the concentration of potassium ions in the ECF is lower than in the cytosol

gradients are maintained by the Na+/K+ pump and a active transport pump for calcium