cardiac muscle cells and contraction

What type of muscle is found in the myocardium?

Cardiac muscle.

Cardiac muscle characteristics

Striated, branched, usually one nucleus, involuntary, connected by intercalated discs.

What are the two major types of cardiac myocytes?

Contractile cells and autorhythmic cells.

What do contractile cardiac cells do?

Contract to pump blood.

What do autorhythmic cardiac cells do?

Generate spontaneous electrical impulses.

What is autorhythmicity?

The ability of some cardiac cells to depolarize on their own without nervous stimulation.

Why does the heart still beat without direct nerve stimulation?

Autorhythmic cells can generate their own action potentials.

What are intercalated discs?

Specialized junctions between cardiac muscle cells.

What two important structures are in intercalated discs?

Desmosomes and gap junctions.

Function of desmosomes in cardiac muscle

Hold cells together during strong contractions.

Function of gap junctions in cardiac muscle

Allow ions to pass cell-to-cell so electrical impulses spread quickly.

Why does cardiac muscle act like a functional syncytium?

Gap junctions allow many cells to behave like one coordinated unit.

How does cardiac muscle contraction differ from skeletal muscle contraction?

Cardiac muscle is involuntary, uses gap junctions, has long refractory periods, requires extracellular calcium, and some cells are autorhythmic.

Does cardiac muscle require calcium from outside the cell?

Yes. Extracellular calcium entry helps trigger more calcium release from the sarcoplasmic reticulum.

What is calcium-induced calcium release?

Calcium entering the cell triggers release of more calcium from the sarcoplasmic reticulum.

What structure stores calcium inside muscle cells?

Sarcoplasmic reticulum.

What does calcium bind to during contraction?

Troponin.

What happens after calcium binds troponin?

Tropomyosin moves, exposing actin binding sites so myosin can attach.

What forms cross-bridges during contraction?

Myosin heads bind to actin.

What powers cross-bridge cycling?

ATP.

What causes cardiac muscle relaxation?

Calcium is removed from the cytosol, causing calcium to detach from troponin.

Where does calcium go during relaxation?

Back into the sarcoplasmic reticulum and out of the cell.

What is the role of L-type calcium channels?

They allow calcium to enter during the plateau phase of the cardiac action potential.

Why is the cardiac refractory period long?

The plateau phase keeps the cell depolarized longer.

Why is a long refractory period important in cardiac muscle?

It prevents tetanus and allows the heart to relax and fill between beats.

Can cardiac muscle normally undergo tetanus?

No.

Why would tetanus be dangerous in the heart?

The heart would stay contracted and could not fill with blood.

What happens first: electrical action potential or mechanical contraction?

Electrical action potential happens first.

Basic sequence of cardiac contraction

Action potential → calcium enters → calcium release from SR → calcium binds troponin → cross-bridge cycling → contraction.

Basic sequence of cardiac relaxation

Calcium removed from cytosol → calcium leaves troponin → tropomyosin blocks actin → cross-bridges stop → relaxation.