Smooth Muscle Anatomy and Physiology

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Questions and Answers

What role does myosin light-chain kinase play in smooth muscle contraction?

It adds a phosphate to myosin, allowing cross-bridge formation. (correct)

It removes phosphate from myosin.

It initiates the release of calcium from the sarcoplasmic reticulum.

It prevents cross-bridge cycling.

Which process is involved in the relaxation of smooth muscle?

Inhibition of ATPase activity.

Removal of calcium from the cytoplasm. (correct)

Activation of calcium channels.

Phosphorylation of actin filaments.

How does the regulation of myosin activity differ between phosphorylated and unphosphorylated forms?

Unphosphorylated myosin forms more cross-bridges.

Unphosphorylated myosin enhances calcium binding.

Phosphorylated myosin has no ATPase activity.

Phosphorylated myosin allows for contraction. (correct)

What governs the neural regulation of smooth muscle contraction?

It may be excitatory or inhibitory based on receptor type. Signup and view all the answers

Which factor does NOT influence intracellular calcium levels in smooth muscle?

Light exposure. Signup and view all the answers

What is a distinguishing feature of smooth muscle regarding its cellular arrangement?

It is arranged in sheets, often in multiple layers. Signup and view all the answers

What is unique about the myofilaments in smooth muscle compared to skeletal muscle?

They are arranged in a diagonal pattern. Signup and view all the answers

Which component is primarily responsible for triggering contractions in smooth muscle?

Calcium ions binding to calmodulin. Signup and view all the answers

How does the contraction mechanism in smooth muscle differ from that in skeletal muscle?

Smooth muscle has longer actin and myosin filaments. Signup and view all the answers

What structural characteristic of smooth muscle cells contributes to their strength during contractions?

Dense bodies acting as attachment points. Signup and view all the answers

What role do caveolae play in smooth muscle function?

Storing small amounts of calcium for contraction. Signup and view all the answers

Which of the following is true regarding the contraction speed of smooth muscle?

It is significantly slower than skeletal muscle contraction. Signup and view all the answers

What type of calcium channels are involved in the excitation-contraction coupling of smooth muscle?

Mechanically-gated, voltage-gated, and ligand-gated channels. Signup and view all the answers

What is a characteristic feature of single-unit smooth muscle?

Cells are connected by gap junctions and contract as a unit. Signup and view all the answers

Which statement correctly describes multi-unit smooth muscle?

Each muscle cell acts individually and receives its own innervation. Signup and view all the answers

What initiates the contraction process in cardiac muscle cells?

Action potentials cause voltage gated calcium channels to open. Signup and view all the answers

Which property distinguishes auto-rhythmic cells in cardiac muscle from contractile cells?

They are responsible for initiating action potentials. Signup and view all the answers

What happens to calcium during the relaxation phase of cardiac muscle contraction?

Calcium ATPases pump it back into the SR and extracellular fluid. Signup and view all the answers

What role does digitalis play in cardiac muscle function?

It increases intracellular calcium, enhancing contraction strength. Signup and view all the answers

What is the function of pacemaker potentials in single-unit smooth muscle?

They allow for spontaneous depolarization that initiates muscle contraction. Signup and view all the answers

Which of the following structures is involved in the excitation-contraction coupling in cardiac muscle?

T tubules Signup and view all the answers

Which characteristic is NOT associated with cardiac muscle?

Muscle fibers are larger than those found in skeletal muscle. Signup and view all the answers

Study Notes

Smooth Muscle

Found in internal organs, blood vessels, the iris of the eye, and other locations
Under involuntary control by the autonomic nervous system
Usually arranged in sheets, often in multiple layers
Spindle-shaped
One nucleus
Small, approximately 1/10 the size of skeletal muscle
Have thick and thin myofilaments
- No sarcomeres; myofilaments are not arranged into sarcomeres
- Appears "smooth" under a microscope
- Thick and thin myofilaments are arranged diagonally
Thin myofilaments contain tropomyosin, but no troponin
Dense bodies are analogous to Z disks (thin myofilaments attach to dense bodies)
Slow myosin ATPase
Myosin ATPase is 10-100 times slower in smooth muscle compared to skeletal muscle; slow contraction time
Little sarcoplasmic reticulum (SR)
Diagonal organization of actin/myosin (gets wider and shorter)
Actin and myosin are longer in smooth than skeletal muscle (slide over a greater area)
Myosin heads are along the whole length of thick filaments
Longer range of contraction
Calcium is the trigger for contraction
- No troponin
- Uses a different regulatory protein than skeletal muscle
- Calcium first binds to a cytoplasmic protein called calmodulin

Source Of Calcium

Most calcium required for contraction comes from the extracellular fluid (ECF)
Large concentration gradient for calcium (high ECF; low ICF)
Smooth muscle cells do not have an extensive SR (different than skeletal muscle)
- Some (little) calcium stored in caveolae (small, membrane-bound vesicles; usually found beneath the sarcolemma)

Steps of Excitation-Contraction Coupling in Smooth Muscle

Opening of calcium channels in the plasma membrane
- Voltage-gated
- Ligand (chemical)-gated
- Mechanically-gated
Calcium influx into the cell (a little calcium is released from caveolae and SR)
Calcium binds to calmodulin
Ca2+ -calmodulin complex activates an enzyme called myosin light-chain kinase
Myosin light-chain kinase phosphorylates myosin (adds a phosphate); myosin heads can now bind to actin on thin filaments
Cross-bridge cycling

Relaxation Of Smooth Muscle

Phosphatase removes phosphate from myosin
- No longer forms crossbridges with actin
Calcium is removed from the cytoplasm
- Ca2+ -ATPase is pumped out of the cell
- Ca2+ -Na+ counter transport; Ca2+ transported out of the muscle cell; Na+ into the cell

Neural Regulation Of Smooth Muscle Contraction

Innervated by the autonomic nervous system
- Sympathetic and/or parasympathetic
May be excitatory or inhibitory, depending on receptor type
Target cell response depends on receptor type
Neurotransmitter released from varicosities
Diffuse binding of neurotransmitter to receptors

Non-Neural Regulation of Contraction

Intracellular [Ca2+] determines tension
Intracellular [Ca2+] is influenced by:
- Hormonal control - many different hormones
  - Many open ligand-gated calcium channels (stimulate contraction); others close calcium channels (inhibit contraction)
  - Example: Certain hormones produced in the gastrointestinal tract influence contraction of intestinal smooth muscle cells
- Paracrines (chemicals produced by other cell types near smooth muscle cells) - open calcium channels
- Physical stretch - some smooth muscle respond to being stretched by contracting (opens Ca2+ channels)

Classification of Smooth Muscle

Single-unit smooth muscle
Multi-unit smooth muscle

Single-Unit Smooth Muscle

Most common type
Location
- Intestinal tract
- Urinary bladder
- Uterus
Muscle cells activated synchronously
- Cells connected by gap junctions
- Contract together as a single unit
  - Stimulate one cell to contract, all cells in the unit will contract

Properties of Single-Unit Smooth Muscle

Gap junctions
Pacemaker cells with spontaneous depolarizations
Innervation to few cells; others stimulated via gap junctions
Tone = level of contraction without stimulation
Contract for long periods

Pacemaker Activity

Single-unit smooth muscle
Pacemaker potentials
- Spontaneous depolarizations to threshold
- Lead to the opening of voltage-gated calcium channels
- Depolarizations spread to neighboring smooth muscle cells via gap junctions, leading to the contraction of these cells as well

Properties of Multi-Unit Smooth Muscle

Located in large airways, the eye (ciliary muscle and iris), blood vessels
Few if any gap junctions
Each muscle cell acts individually
- Receives its own innervation
- No tone

Cardiac Muscle

Cardiac muscle cells are only found in the heart wall
Two specialized types of cardiac muscle cells
- Contractile cells (99%)
- Auto-rhythmic cells (1%)
  - Both types connected by gap junctions

Properties of Contractile Cells

Smaller than skeletal muscle; cells often branch
Contain thick and thin myofilaments arranged into sarcomeres (similar to skeletal muscle)
- Cardiac muscle is also striated muscle
Contractile mechanism is similar to skeletal muscle cells with one exception:
- Action potentials travelling across the surface membrane and down T tubules cause voltage-gated calcium channels in these membranes to open
- Calcium enters the cells from the extracellular fluid
The calcium from the ECF, however, is not enough to bring about complete contraction of the cell
Calcium entering from ECF causes calcium channels on the SR to open - known as Ca2+-induced, calcium release from SR
Calcium from the SR plus calcium from the ECF binds to troponin; contraction just like in skeletal muscle
Also, Ca2+ released from the SR by action potentials in T tubules (just like in skeletal muscle)

Excitation-Contraction Coupling in Cardiac Muscle

Voltage-gated Ca2+ channel

Relaxation of Cardiac Muscle

Remove calcium from cytosol
- Ca2+ ATPase in sarcoplasmic reticulum: pumps Ca2+ back into the SR
- Ca2+ ATPase in the plasma membrane: pumps back into the extracellular fluid
- Na+ - Ca2+ exchanger in plasma membrane: pumps back into the ECF
Calcium leaves troponin
- Tropomyosin returns to its position covering myosin binding sites on actin

Digitalis

Drug that increases heart contraction strength (increases intracellular calcium)
Decreases Na+/K+ pump activity; decreases Na+ concentration gradient
Reduces Na+ - Ca2+ exchange
More calcium remains in the cells and increases the strength of heart contractions

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Description

This quiz focuses on the characteristics and functions of smooth muscle, including its structure, control mechanisms, and contraction properties. Learn about the key differences between smooth and skeletal muscle, as well as the roles of myofilaments and the autonomic nervous system in muscle function.