Questions: Smooth Muscle Structure and Distribution

Question 1

A patient is treated with a drug that blocks myosin light chain kinase (MLCK). Which physiological effect is most likely?

• A. Skeletal muscle paralysis, because MLCK is the primary regulator of all muscle contraction
• B. Relaxation of smooth muscle in blood vessel walls and GI tract, with little effect on skeletal muscle
• C. Enhanced smooth muscle contraction, because MLCK normally inhibits myosin
• D. Decreased heart rate, because MLCK controls cardiac pacemaker activity

Answer: (B)

MLCK is the key regulatory kinase in smooth muscle: it phosphorylates myosin light chains to enable myosin-actin interaction and force generation. Blocking MLCK prevents myosin phosphorylation, inhibiting smooth muscle contraction. The primary locations of smooth muscle — blood vessel walls (maintaining vascular tone) and the GI tract — would relax. Skeletal muscle uses a completely different regulatory mechanism: troponin-based regulation where calcium directly unmasks actin binding sites, not MLCK. So MLCK inhibition selectively targets smooth muscle. Cardiac muscle is also striated and uses troponin-based regulation, so option D is wrong.

Question 2

Why can a smooth muscle cell in the bladder wall shorten to a much greater fraction of its resting length than a skeletal muscle fiber can?

• A. Smooth muscle cells have more mitochondria, providing more ATP for sustained contraction
• B. Smooth muscle uses actin and myosin arranged obliquely with no fixed sarcomere register, allowing greater total shortening range
• C. Bladder smooth muscle is innervated by more motor neurons, enabling stronger tetanic contraction
• D. Smooth muscle expresses a special isoform of actin that is more elastic than skeletal actin

Answer: (B)

The sarcomere architecture of skeletal muscle is highly ordered and limits the range over which thick and thin filaments can overlap productively — skeletal muscle can only shorten to about 60–70% of resting length before the filaments interfere. Smooth muscle abandons the sarcomere: actin and myosin filaments are anchored to dense bodies and dense plaques, arranged obliquely, and the whole cell shortens in a corkscrew pattern. Without the geometric constraints of sarcomere periodicity, smooth muscle can shorten to as little as 10–20% of resting length — essential for a bladder that expands from nearly empty to full. This structural flexibility is a direct consequence of lacking striations.

Question 3

Smooth muscle contracts more slowly than skeletal muscle because it lacks the regulatory protein troponin.

• T. True
• F. False

Answer: False

Smooth muscle contracts slowly primarily because its regulatory mechanism is slower, not merely because it lacks troponin. In smooth muscle, calcium binds calmodulin, which must then activate MLCK, which must phosphorylate myosin light chains before cross-bridge cycling can begin — each enzymatic step adds latency. The calmodulin-MLCK pathway is inherently slower than the troponin-mediated mechanism in skeletal muscle where calcium binding to troponin C immediately and directly unmasks actin binding sites. Lacking troponin is a description of the difference, not the explanation for why it's slower.

Question 4

Smooth muscle in the GI tract can generate coordinated peristaltic contractions even after all extrinsic autonomic nerve connections are severed.

• T. True
• F. False

Answer: True

The GI tract contains the enteric nervous system (ENS) — often called the 'second brain' — which is an intrinsic neural network embedded in the gut wall capable of coordinating peristalsis independently of input from the brain or spinal cord. Even after all extrinsic connections are cut, the ENS integrates local signals (stretch, chemistry of luminal contents) and drives coordinated smooth muscle contractions. This intrinsic autonomy is unique to the GI tract among smooth muscle-containing organs; blood vessel smooth muscle and uterine smooth muscle do not have comparable intrinsic nervous systems.

Question 5

Why is the calmodulin-MLCK regulatory mechanism better suited to smooth muscle's physiological role than the troponin mechanism used in skeletal muscle?

Model answer: Smooth muscle needs to sustain contraction for long periods with minimal ATP expenditure — blood vessels must maintain vascular tone continuously, and the bladder must hold volume. The MLCK pathway achieves this through the 'latch state': once myosin is phosphorylated and cross-bridges form, the myosin can remain attached and generate force even as MLCK activity decreases and myosin becomes dephosphorylated. This attached, slowly cycling state maintains force cheaply. The troponin mechanism, by contrast, requires continued calcium elevation and rapid cross-bridge cycling to sustain force — efficient for fast, brief skeletal muscle contractions but energetically wasteful for sustained tonic contractions.

The latch state is a key concept: phosphorylated myosin cross-bridges can be 'locked' in force-generating positions by the action of a myosin light chain phosphatase that dephosphorylates them while they remain attached. These 'latch bridges' generate force slowly and with very low ATP consumption. This makes smooth muscle extraordinarily economical for sustained contractions — a smooth muscle can maintain a given force for 300× less ATP than a skeletal muscle generating the same force, making it ideal for organs that must maintain continuous tone.