10.8 Smooth Muscle

Smooth Muscle Structure and Function

Smooth muscle lines the walls of hollow organs and passageways, controlling the movement of substances through your body. Understanding how it differs from skeletal and cardiac muscle is a major focus of this unit, since its contraction mechanism, organization, and regulation are all distinct.

Dense Bodies and Force Transmission

Smooth muscle cells don't have sarcomeres, so they don't have Z-discs. Instead, they use dense bodies, which are protein structures made of α-actinin (among other proteins) that anchor actin filaments inside the cell. Some dense bodies are scattered throughout the cytoplasm, while others are attached to the cell membrane.

Here's how force gets transmitted during contraction:

1. Myosin heads pull on actin filaments (the sliding filament mechanism still applies here).
2. Actin filaments transmit that pulling force to the dense bodies they're anchored to.
3. Dense bodies connected to the cell membrane transfer force to the membrane itself and to the extracellular matrix outside the cell.
4. The entire cell shortens, pulling inward from multiple directions rather than along a single axis like skeletal muscle.

Because actin and myosin aren't arranged in neat parallel rows (no sarcomeres), smooth muscle cells contract in a twisting, wringing pattern rather than shortening strictly end-to-end. This is why smooth muscle lacks the striped appearance of skeletal and cardiac muscle.

Functions of Smooth Muscle

Smooth muscle is found wherever your body needs to move substances through tubes or regulate the diameter of a passageway:

• Digestive tract: contracts to mix food and push it along (peristalsis)
• Respiratory tract: adjusts airway diameter to control airflow
• Urinary tract: propels urine from kidneys to bladder and out of the body
• Blood vessels: regulates blood pressure and distributes blood flow to different regions
• Skin vasculature: controls blood flow to the skin surface, contributing to thermoregulation (more flow = more heat loss; less flow = heat conservation)

The common theme is that smooth muscle contraction is slow, sustained, and involuntary. You don't consciously decide to constrict a blood vessel or push food through your intestines.

Comparison of Muscle Types

Smooth vs. Skeletal and Cardiac Muscle

All three muscle types share some fundamentals: they contain actin and myosin, they contract via the sliding filament mechanism, and they require calcium to initiate contraction. The differences matter more for exams, though.

Single-Unit vs. Multi-Unit Smooth Muscle

These two types differ in how their cells communicate and where they're found.

Single-unit smooth muscle has cells connected by gap junctions, so when one cell depolarizes, the electrical signal spreads to neighboring cells. The whole sheet of muscle contracts together as a coordinated unit.

• Found in the walls of hollow organs: digestive tract, uterus, ureters, small blood vessels
• Responsible for peristalsis, labor contractions, and similar wave-like movements
• Can exhibit spontaneous contractions without nervous system input (some cells act as pacemakers)

Multi-unit smooth muscle has cells that are not electrically coupled. Each cell contracts independently, only when it receives its own signal from the autonomic nervous system.

• Found in larger blood vessels, airways, the iris of the eye, and arrector pili muscles
• Allows for fine, graded control (think of your pupil adjusting precisely to light levels)
• Requires direct innervation for contraction, unlike single-unit muscle

A quick way to remember: single-unit acts like a team (all cells contract together), while multi-unit acts like individuals (each cell responds on its own).

Smooth Muscle Contraction Mechanism

The contraction pathway in smooth muscle differs from skeletal muscle at several key steps. Here's the sequence:

1. A stimulus arrives (autonomic nerve signal, hormone, or local chemical signal such as stretch).
2. Calcium ions ($Ca^{2+}$) enter the cytoplasm. In smooth muscle, calcium comes from both the sarcoplasmic reticulum and from extracellular fluid through membrane calcium channels. This is different from skeletal muscle, which relies almost entirely on SR calcium release.
3. $Ca^{2+}$ binds to calmodulin (not troponin, which is the skeletal/cardiac pathway).
4. The calcium-calmodulin complex activates an enzyme called myosin light chain kinase (MLCK).
5. MLCK phosphorylates the myosin light chain on the myosin head, which allows myosin to bind actin and begin cross-bridge cycling.
6. Contraction continues as long as the myosin light chain stays phosphorylated.
7. Relaxation occurs when myosin light chain phosphatase removes the phosphate group, preventing further cross-bridge cycling.

The autonomic nervous system controls smooth muscle by releasing neurotransmitters that either increase or decrease intracellular calcium levels. Unlike the neuromuscular junction in skeletal muscle, smooth muscle doesn't have motor end plates. Instead, autonomic nerve fibers release neurotransmitters from swellings called varicosities that diffuse across a wider area to reach the muscle cells.