CoursesPhysiologyMuscular systemSmooth and cardiac muscleSmooth and cardiac muscle

Smooth and cardiac muscle

Learning objectives

After completing this study unit, you will be able to:

Compare and contrast the structure of smooth and cardiac muscle.
Explain excitation-contraction coupling in smooth muscle.
Discuss calcium-induced calcium release in cardiac muscle.

Introduction

Smooth and cardiac muscle contract involuntarily to regulate internal environments. Their functions include peristalsis, continence and blood circulation.

Smooth muscle has spindle-like cells with a single nucleus. Since myofilaments are not organized into sarcomeres, smooth muscle lacks striations. There are two primary types of smooth muscle cells:

Single-unit smooth muscle typically forms the walls of hollow organs and most blood vessels. These cells are electrically and mechanically connected, so that action potentials and mechanical forces spread rapidly to make the tissue contract as a single unit. Single-unit muscle can respond to electrical, chemical or mechanical stimuli.
Multi-unit smooth muscle is found in the eye and in the skin. Cells are electrically and mechanically independent, and are directly activated by the autonomic nervous system.

Smooth muscle lacks troponin, so its excitation-contraction coupling relies on a different pathway. As cytosolic calcium levels rise, Ca²⁺ binds calmodulin, which activates MLCK, ultimately turning on myosin ATPase and triggering crossbridge cycling.

Cardiac muscle cells are branched, striated, and typically contain a single nucleus. Intercalated discs connect them electrically and mechanically to ensure that the heart contracts as a single unit.

Cardiac muscle excitation-contraction coupling is similar to that of skeletal muscle, but with a different triggering mechanism. Action potentials open voltage-gated channels that let Ca²⁺ into the cell. This Ca²⁺ binds to ligand-gated channels on the sarcoplasmic reticulum, which release Ca²⁺ into the cytosol, leading to crossbridge cycling. This mechanism is called calcium-induced calcium release.

Explore concepts

Characteristics of smooth and cardiac muscle

Structural differences between cardiac muscle cells, single-unit and multi-unit smooth muscle cells support their function.

Smooth muscle cell contraction

Like other muscle tissue types, smooth muscle regulates crossbridge cycling via calcium ions. However, it relies on an enzymatic pathway instead of troponin.

Cardiac muscle cell contraction

The excitation-contraction coupling of cardiac muscle cells is characterized by calcium-induced calcium release.

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Test your knowledge about smooth and cardiac muscle with this quiz!

Summary

Key points about smooth and cardiac muscle
Stimulus	Single-unit smooth muscle: electrical, chemical, mechanical Multi-unit smooth muscle: primarily chemical Cardiac muscle: electrical (autorhythmic)
Electrical coupling	Single-unit smooth muscle: yes (gap junctions) Multi-unit smooth muscle: no Cardiac muscle: yes (gap junctions in intercalated discs)
Mechanical coupling	Single-unit smooth muscle: yes (linked sarcolemmal dense bodies) Multi-unit smooth muscle: no Cardiac muscle: yes (desmosomes in intercalated discs)
Smooth muscle contraction	1- Ca²⁺ inflow from extracellular fluid or sarcoplasmic reticulum 2- Ca²⁺ binds to calmodulin 3- Calmodulin-Ca²⁺ complex binds to myosin light chain kinase 4- Myosin light chain kinase phosphorylates myosin and activates ATPase 5- Myosin hydrolyzes ATP to power crossbridge cycling
Smooth muscle relaxation	Ca²⁺ levels fall -> Ca²⁺ dissociates from calmodulin -> myosin light chain kinase is deactivated -> myosin light chain phosphatase dephosphorylates myosin
Cardiac muscle contraction	1- Action potential in T-tubule activates voltage-gated Ca²⁺ channel 2- Ca²⁺ opens ligand-gated channels on sarcoplasmic reticulum 3- Ca²⁺ binds to troponin; tropomyosin reveals myosin-binding site 4- Myosin hydrolyzes ATP to power crossbridge cycling
Cardiac muscle relaxation	Ca²⁺ levels fall (SERCA returns Ca²⁺ to sarcoplasmic reticulum; NCX exchanger to extracellular fluid) -> Ca²⁺ dissociates from troponin -> tropomyosin covers myosin-binding site