Smooth Muscle Physiology Quiz

Smooth Muscle Physiology

• Smooth muscle lacks striations, unlike skeletal muscle
• Smooth muscle is found in various locations, including blood vessel walls, digestive tract, bladder, and urinary tracts, airways, and reproductive organs (uterus)
• Regulation of smooth muscle contraction/relaxation is primarily demonstrated by contrasting its behavior with skeletal muscle
• Smooth muscle cells are spindle-shaped, with a single nucleus centrally located
• Smooth muscle cells are often grouped in sheets or clusters
• Smooth muscles are innervated by the autonomic nervous system, not the somatic nervous system.
• Smooth muscle is under involuntary control
• Smooth muscle can be activated by stretch
• Smooth muscle can generate tension over a wider range of muscle lengths compared to skeletal muscle.
• Smooth muscle can maintain tension for a longer period.

Overview of Muscle Types

• Skeletal muscles are connected to bones, enabling movement
• Cardiac muscle is located in the heart, responsible for pumping blood
• Smooth muscle is primarily found in the walls of internal organs and blood vessels

Smooth Muscle: General Characteristics

• Smooth muscle cells are not arranged in sarcomeres, and are therefore not striated

• Smooth muscle contains actin and myosin proteins

• Smooth muscle cells are spindle-shaped and possess one central nucleus

• Smooth muscles are typically found clustered together

• Smooth muscle is controlled by the autonomic nervous system.

• Smooth muscle contraction is involuntary

• Smooth muscle can be excited or inhibited

Differences between Skeletal and Smooth Muscle

• Smooth muscles contract and relax more slowly than skeletal muscles
• Contractile activity in smooth muscles can change without a change in membrane potential
• Smooth muscles can maintain tension for longer periods
• Smooth muscles can be activated by stretching
• Smooth muscle can develop active tension through a much wider range of lengths than skeletal muscle

Manipulating smooth muscle contraction and relaxation

• Smooth muscle contraction and relaxation are crucial in managing numerous pathologies.
• Controlling smooth muscle is significant in hypertension management.
• Vasoconstriction and vasodilation, processes involving smooth muscle, are essential for blood vessel function
• Different forms of smooth muscle exist, including multi-unit and single-unit, each with specific characteristics relevant to their function.

Location of Major Types of Smooth Muscle

• Smooth muscle is found in blood vessels, digestive tract, urinary (bladder and ureters), respiratory (airways), reproductive (uterus in females), and eye (ciliary and iris muscle).

Two Types of Smooth Muscle

• Multi-unit smooth muscle cells are electrically isolated, permitting finer motor control.
• Single-unit (visceral) smooth muscles are interconnected by gap junctions, coordinating contraction. Multi-unit smooth muscle is an example of fine motor control.

Differences between Skeletal and Smooth Muscle

• Skeletal muscle's sarcoplasmic reticulum (SR) is well developed, whereas the smooth muscle SR is less developed
• A primary difference in smooth muscle action-potential initiation is that the majority of Calcium (Ca++) required for smooth muscle contraction originates from the extracellular fluid (ECF) rather than the SR, which is more common in skeletal muscle cells.
• Smooth muscle cells contain a specialized protein called Ca2+ rather than troponin C in skeletal muscles.

Differences between Skeletal and Smooth Muscle- Myosin ATPase activity

• Smooth muscle myosin ATPase activity is lower, leading to a slower cross-bridging rate and a longer contraction duration.
• Myosin in smooth muscle exists as a continuous chain of heads, which allows for greater stretching and lengthening than in skeletal muscle.
• This can be seen in organs like the bladder, where size changes significantly.

Differences between Skeletal and Smooth Muscle – Nervous Signals

• Smooth muscle contraction can be triggered by autonomic nervous signals, hormonal stimulation, and changes in local chemical conditions, while skeletal muscle contraction only depends on somatic nervous input.

Differences between Skeletal and Smooth Muscle - Example: Gastric smooth muscle

• Factors influencing gastric smooth muscle activity include the smell of food, gastrin hormone levels, and stomach fullness.

Differences between Skeletal and Smooth Muscle - No NMJ in smooth muscle

• Smooth muscle lacks neuromuscular junctions (NMJs), unlike skeletal muscle.
• In smooth muscle, depolarization is primarily caused by increased intracellular Ca++, not by changes in Na+, as seen in skeletal muscles.

How does smooth muscle contract/relax?

• Smooth muscle contracts through an increase in cytosolic calcium (Ca++) levels, either from the ECF and SR.
• Ca++ binds to calmodulin, forming a complex that activates the enzyme myosin light chain kinase (MLCK).
• MLCK phosphorylates myosin light chains, which enhance myosin ATPase activity to create cross-bridges
• Smooth muscle relaxation occurs when intracellular Ca++ levels decrease. This deactivates the calmodulin-MLCK complex, and hence myosin gets dephosphorylated to release from actin

How does smooth muscle contract? - Additional details

• The increase in cytosolic Ca++ levels, which initiates smooth muscle contraction, can occur through three distinct pathways.
• IP3 receptors allow intracellular calcium release from the sarcoplasmic reticulum.
• Ligand-gated Ca++ channels allow extracellular Ca++ influx.
• Voltage-gated Ca++ channels allow extracellular Ca++ to pass into the cell.

Differences between Skeletal and Smooth Muscle

• Smooth muscles don't only respond to nervous signals; circulating hormones like epinephrine, acetylcholine, angiotensin, endothelin, vasopressin, oxytocin, serotonin, and histamine also affect their activity.
• Contraction occurs when excitatory receptors are bound by these substances
• Relaxation occurs when inhibitory receptors are bound.

Differences between Skeletal and Smooth Muscle - Calcium

• Smooth muscle contraction involves intracellular calcium (Ca++) binding to calmodulin, while skeletal muscle contraction involves calcium binding to troponin C.

What about pathology of Smooth muscle?

• Smooth muscle in blood vessels, the digestive tract, bladder/ureters, airways, uterus (females), and eye is important
• Pathologies of smooth muscle can affect these systems.

How we regulate contraction force in smooth muscle

• Contraction force regulation in smooth muscle is unique, relying on the balance of myosin light chain phosphorylation and dephosphorylation.

How we regulate contraction force in smooth Muscle - Details

• Smooth muscle contraction force regulation is often controlled at a second level by factors that influence Ca++ sensitivity of regulatory proteins. This can make contraction either easier or harder, depending on conditions and needs.
• Using PKA to phosphorylate MLCK serves as an example of this second-tier control. This phosphorylation typically reduces MLCK activity.

Test your knowledge on smooth muscle physiology with this quiz. Explore topics including contraction forces, latch bridges, and distinguishing features of smooth muscle dysfunction syndrome. Perfect for biology students looking to solidify their understanding of smooth muscle mechanics.