Smooth Muscle Types, Contraction Mechanisms, and Regulation

Distinguish between multiunit and unitary smooth muscle

Multiunit smooth muscle functions independently (e.g., iris, piloerector muscles); unitary smooth muscle contracts as a single unit (e.g., GI tract, uterus)

What is multi-unit smooth muscle?

Discrete smooth muscle fibers that contract independently; controlled mainly by nerve signals

What is unitary smooth muscle?

Many fibers contract together as a single unit; connected via gap junctions

Describe how smooth muscle contracts

Ca2+ binds to calmodulin → activates myosin kinase → phosphorylates myosin → cross-bridge forms with actin

What protein does Ca2+ bind to in smooth muscle?

Calmodulin

What enzyme is activated by calmodulin?

Myosin light chain kinase (MLCK)

What happens after myosin is phosphorylated?

Myosin binds actin and initiates cross-bridge cycling

Compare energy efficiency of smooth vs skeletal muscle

Smooth muscle contracts slower but uses <1% of the energy; can sustain contraction for long periods

Why does smooth muscle contraction take longer?

Slower calcium reuptake into SR and more complex activation steps

Define "smooth muscle tone"

Constant, low-level tension maintained by smooth muscle over time

What happens when intracellular calcium declines in smooth muscle?

Muscle relaxes

What signals can trigger smooth muscle contraction?

Nervous signals, hormones, and local factors

How does neural regulation affect smooth muscle?

Autonomic nerves release neurotransmitters that can excite (e.g., acetylcholine) or inhibit (e.g., norepinephrine)

What is an example of opposing neurotransmitter effects in smooth muscle?

Bronchioles constrict with acetylcholine; dilate with norepinephrine

How do hormones affect smooth muscle?

Some smooth muscle contracts in response to hormones without APs (e.g., oxytocin in uterus, histamine in airways)

What are local factors that affect smooth muscle?

Low O2, high CO2, low pH, temperature changes, stretch

Compare and contrast skeletal and smooth muscle contraction

Skeletal: fast, striated, voluntary, only neural input; Smooth: slow, non-striated, involuntary, multiple input types

What are caveolae in smooth muscle?

Pouch-like infoldings in sarcolemma that store calcium near the membrane

How are smooth muscle filaments arranged?

Not in sarcomeres; arranged diagonally with dense bodies anchoring actin

Does smooth muscle have troponin?

No; uses calmodulin instead

What anchors actin in smooth muscle?

Dense bodies

Does smooth muscle have T-tubules?

No

How does smooth muscle compare to skeletal in terms of fatigue?

Smooth muscle resists fatigue and maintains contraction longer

Multi-unit smooth muscle

Smooth muscle fibers that contract independently; not electrically coupled

Unitary smooth muscle

Smooth muscle fibers that contract as a group; electrically coupled via gap junctions

Dense bodies

Anchor points for actin in smooth muscle (functionally like Z-discs)

Calmodulin

Calcium-binding protein that activates myosin kinase in smooth muscle