Smooth Muscle Structure and Function
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Questions and Answers

Which type of muscle has a poorly developed sarcoplasmic reticulum?

  • All types of muscle
  • Smooth muscle (correct)
  • Cardiac muscle
  • Skeletal muscle

Which muscle type can contract without nerve stimulation?

  • Skeletal muscle
  • Smooth muscle
  • Both B and C (correct)
  • Cardiac muscle

What is the primary function of calmodulin in smooth muscle?

  • Bind to Ca2+ to activate myosin light-chain kinase (correct)
  • Inhibit muscle contraction
  • Facilitate nerve stimulation
  • Store calcium ions

What type of muscle contains intercalated discs?

<p>Cardiac muscle (A)</p> Signup and view all the answers

Which muscle type is characterized by striations and the presence of troponin?

<p>Both skeletal and cardiac muscle (D)</p> Signup and view all the answers

What happens to skeletal muscle if denervation occurs?

<p>It results in muscle atrophy (A)</p> Signup and view all the answers

In smooth muscle, where can calcium ions enter the cytoplasm from?

<p>From extracellular fluid, sarcoplasmic reticulum, and mitochondria (A)</p> Signup and view all the answers

Which statement about muscle fibers in skeletal muscle is true?

<p>They are stimulated independently. (C)</p> Signup and view all the answers

What shape are smooth muscle cells?

<p>Spindle (C)</p> Signup and view all the answers

Which of the following is true regarding the arrangement of myofilaments in smooth muscle?

<p>They are not arranged into sarcomeres. (C)</p> Signup and view all the answers

What is the primary factor that allows smooth muscle to contract when stretched?

<p>Absence of sarcomeres (D)</p> Signup and view all the answers

How does smooth muscle contraction differ from skeletal muscle contraction?

<p>Smooth muscle contraction is involuntary. (C)</p> Signup and view all the answers

Which structural component is absent in smooth muscle but present in skeletal muscle?

<p>Troponin (C)</p> Signup and view all the answers

What initiates or modulates smooth muscle contraction?

<p>Membrane potential changes (A)</p> Signup and view all the answers

What type of ion channels are essential for smooth muscle contraction?

<p>L-type Ca2+ channels (B)</p> Signup and view all the answers

What is the role of calmodulin in smooth muscle cells?

<p>It replaces troponin to regulate contraction. (D)</p> Signup and view all the answers

What is the main function of gap junctions in smooth muscle?

<p>They create low-resistance channels for nerve impulse transmission. (A)</p> Signup and view all the answers

Which type of smooth muscle exhibits pacemaker activity?

<p>Single-unit smooth muscle (D)</p> Signup and view all the answers

What initiates the contraction of smooth muscle cells?

<p>Increase in cytoplasmic calcium levels. (C)</p> Signup and view all the answers

How does the regulation of multiunit smooth muscle differ from single-unit smooth muscle?

<p>Multiunit smooth muscle lacks gap junctions. (B)</p> Signup and view all the answers

Which protein complex is formed when calcium combines with calmodulin in smooth muscle?

<p>Calcium-calmodulin (Ca2+-CaM) complex (B)</p> Signup and view all the answers

What is primarily responsible for cross-bridging during smooth muscle contraction?

<p>Phosphorylation of myosin by myosin light-chain kinase. (C)</p> Signup and view all the answers

What happens during the relaxation phase of smooth muscle contraction?

<p>Calcium is pumped back into the sarcoplasmic reticulum. (B)</p> Signup and view all the answers

Which of the following statements about the autonomic nervous system's role in smooth muscle is true?

<p>Multiunit smooth muscle contraction is dependent on the autonomic nervous system. (B)</p> Signup and view all the answers

What is the approximate ratio of thin to thick filaments in smooth muscle cells?

<p>16 to 1 (C)</p> Signup and view all the answers

How do smooth muscle cells differ in filament arrangement from striated muscle cells?

<p>Smooth muscle myosin proteins are stacked vertically. (D)</p> Signup and view all the answers

Which structure in smooth muscle is analogous to the Z discs found in striated muscle?

<p>Dense bodies (A)</p> Signup and view all the answers

What is a primary functional difference between smooth muscle and striated muscle during stretching?

<p>Smooth muscles maintain the ability to contract even when stretched. (B)</p> Signup and view all the answers

In which of the following locations is single-unit smooth muscle predominantly found?

<p>Gastrointestinal tract (D)</p> Signup and view all the answers

What allows smooth muscle cells to function effectively when stretched?

<p>Lack of organized sarcomeres (A)</p> Signup and view all the answers

What biochemical role does Ca2+ play in smooth muscle contraction?

<p>It initiates a biochemical cascade. (A)</p> Signup and view all the answers

What percentage of the volume of a smooth muscle cell do myofilaments and dense bodies occupy?

<p>90% (B)</p> Signup and view all the answers

What is the primary role of sympathetic nerves in the fight-or-flight response?

<p>Pupillary dilation and bronchial dilation (D)</p> Signup and view all the answers

How does the innervation of smooth muscles differ from that of skeletal muscles?

<p>Smooth muscles contain neurotransmitter receptors on their entire surface. (D)</p> Signup and view all the answers

What are varicosities in the context of autonomic nerve fibers?

<p>Regions of autonomic fibers that release neurotransmitters (C)</p> Signup and view all the answers

In terms of neurotransmission, what does 'synapses en passant' refer to?

<p>Synapses formed with smooth muscle cells along stretches of autonomic fibers (A)</p> Signup and view all the answers

Which of the following statements accurately describes the roles of the sympathetic and parasympathetic nerves?

<p>Sympathetic nerves generally stimulate while parasympathetic nerves typically inhibit. (C)</p> Signup and view all the answers

Where are neurotransmitter receptor proteins primarily located in skeletal muscle fibers?

<p>Only at the neuromuscular junction (D)</p> Signup and view all the answers

What physiological effect does sympathetic stimulation have on intestinal motility?

<p>It decreases motility to conserve energy. (D)</p> Signup and view all the answers

What mechanism do smooth muscle cells use to receive signals from autonomic nerves?

<p>Release of neurotransmitters along stretches of nerve fibers (A)</p> Signup and view all the answers

Flashcards

Skeletal Muscle

Striated muscle tissue with sarcomeres; requires nerve stimulation to contract; denervation leads to atrophy; stimulated independently.

Cardiac Muscle

Striated muscle tissue with sarcomeres; can contract without nerve stimulation; action potentials originate in pacemaker cells.

Smooth Muscle

Non-striated muscle; relies less on nerve stimulation; maintains tone in the absence of stimulation; uses calmodulin, not troponin.

Sarcomeres

Basic units of striated muscle; organized arrangement of actin and myosin filaments.

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Troponin

Protein in the thin filaments of striated muscle, involved in regulating muscle contraction.

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Calmodulin

Protein involved in smooth muscle contraction, activated by Ca2+.

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Sarcoplasmic Reticulum

Specialized network of membranes in muscle cells that stores and releases calcium ions.

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Gap Junctions

Connections between muscle cells that allow for the rapid transmission of signals.

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Smooth Muscle Contraction Mechanism

Smooth muscle contraction differs from striated muscle, utilizing a biochemical cascade instead of a steric mechanism, and involving calmodulin to regulate the process.

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Smooth Muscle Filament Ratio

Smooth muscle cells have a significantly different ratio of thin to thick filaments (approximately 16:1) compared to striated muscles (2:1).

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Smooth Muscle Thin Filaments

Unlike striated muscles, smooth muscle has longer thin filaments that attach to dense bodies, analogous to striated muscles' Z-discs.

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Smooth Muscle Myosin Arrangement

Smooth muscle myosin is arranged vertically, allowing cross-bridges along the entire thick filament length. This differs from the horizontal arrangement in striated muscles.

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Smooth Muscle Organization

Smooth muscle lacks sarcomeres, organized for greater extensibility and the ability to contract forcefully even with significant stretch.

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Smooth Muscle Extensibility

Smooth muscles can contract effectively, even when significantly stretched, like those in the bladder and uterus.

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Single-Unit Smooth Muscle

The predominant type of smooth muscle in the body; organized as a syncytium, found in digestive and excretory systems, enabling coordinated contractions.

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Striated Muscle's Contraction Stretch Limit

Striated muscle loses its ability to contract when stretched to a point that actin and myosin filaments no longer overlap.

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Smooth Muscle Structure

Smooth muscle cells are spindle-shaped, arranged in layers around hollow organs and blood vessels. They lack sarcomeres and are therefore non-striated.

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Smooth Muscle Contraction

Smooth muscle contraction is slow and sustained, due to slower ATPase activity compared to skeletal muscle, and can contract even when stretched significantly.

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Smooth Muscle vs Skeletal Muscle

Smooth muscle contraction is involuntary and doesn't require nerve stimulation (like skeletal muscle does) to initiate or modulate. It can maintain tone in absence of stimulation.

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Smooth Muscle Action Potentials

Smooth muscle action potentials are varied. They can be triggered by different factors, including local humoral factors, circulating hormones, and mechanical stimulation (like stretching).

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Smooth Muscle Calcium Mechanism

Smooth muscle contraction involves calcium influx, activating calmodulin, which regulates the interaction between actin and myosin filaments. The process does not use troponin.

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Smooth Muscle Layers

Smooth muscle in the digestive tract, ureters, and other tubes are often arranged circularly and longitudinally. Contraction of these layers produces peristaltic waves.

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Smooth Muscle Cell Density Bodies

Smooth muscle cells do not have sarcomeres, but they do have dense bodies which are analogous to Z-disks in skeletal muscles and serve in the regulation of the myofilament organization.

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Smooth Muscle's Role in Hollow Organs

Smooth muscle regulates movement within hollow organs by adjusting the size and pressure of these organs, enabling functions like digestion, urination, and childbirth.

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Smooth Muscle Types

Smooth muscle is categorized into low-resistance (single-unit) and multiunit types. Single-unit muscle contracts in a coordinated manner, while multiunit muscle contracts independently.

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Single-unit Smooth Muscle

Smooth muscle type characterized by gap junctions that facilitate synchronized contraction.

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Multiunit Smooth Muscle

Smooth muscle type where each fiber is innervated and contracts independently; no gap junctions.

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Smooth Muscle Contraction Mechanism

Smooth muscle contraction is triggered by calcium influx, activating calmodulin and myosin light-chain kinase, leading to cross-bridge formation and muscle shortening.

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Slow Waves in Smooth Muscle

Rhythmic fluctuations in membrane potential that act as a pacemaker, leading to spike potentials in single-unit smooth muscle, initiating contraction.

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Smooth Muscle Regulation

While some smooth muscle has intrinsic pacemaker activity, autonomic nervous system input significantly influences its contraction and relaxation.

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Ca2+-CaM complex

Calcium combines with calmodulin to activate other proteins initiating contraction.

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Myosin Light Chain Kinase (MLCK)

An enzyme activated by the Ca2+-CaM complex that phosphorylates myosin, causing contraction.

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Autonomic Nerves and Smooth Muscle

Nerves that control smooth muscle activity in multiple organs simultaneously; sympathetic and parasympathetic nerves affect many organs in the body together.

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Fight-or-Flight Response

A response triggered by sympathetic nerves, causing various bodily changes like dilated pupils and rapid heartbeat to prepare for action.

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Synapses en passant

Connections forming between autonomic nerve fibers and smooth muscle cells along the nerve fiber, not just at a specific junction as with skeletal muscle.

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Varicosities

Bulges along autonomic nerve fibers where neurotransmitters are released to stimulate smooth muscle cells.

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Skeletal vs. Smooth Muscle Control

Skeletal muscles have a direct, single connection to motor neurons, while smooth muscles have multiple points of contact along the nerve fiber, which releases neurotransmitters.

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Neurotransmitter Receptors (Smooth Muscle)

Neurotransmitter receptors found on the entire surface of smooth muscle cells

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Neuromuscular Junction

Specific region of contact between a motor neuron and skeletal muscle; not involved in autonomic stimulation of smooth muscles

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Parasympathetic Stimulation

Nerve stimulation that generally opposes the effects of sympathetic stimulation

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Study Notes

Smooth Muscle Structure

  • Smooth muscle, also known as visceral muscle, is arranged in layers around hollow organs and blood vessels.
  • Its contraction reduces the size of these structures.
  • Smooth muscle cells are spindle-shaped.
  • Actin-myosin myofilaments are not arranged into sarcomeres resulting in a non-striated appearance.
  • Smooth muscle can contract even when significantly stretched, unlike skeletal muscle where contraction is limited by length.
  • The sarcoplasmic reticulum is loosely arranged within the cells and lacks T tubules.
  • Dense bodies, analogous to Z discs in skeletal muscle, are present.

Smooth Muscle Function

  • Smooth muscle is involuntary.
  • Contraction is slow and sustained due to slow ATPase activity.
  • Circular and longitudinal layers produce peristaltic waves in the digestive tract, ureters, etc, to move contents.
  • Smooth muscle responds to various stimuli such as hormones, local factors, and mechanical stretch.
  • Its contraction is triggered by membrane depolarization that opens Ca2+ channels allowing influx of calcium (Ca2+).
  • Ionotropic receptors or membrane oscillators can induce depolarization.

Types of Smooth Muscle

  • Single-unit (visceral) smooth muscle: This is the predominant type found in the GI tract, bladder, uterus, and ureters. Its low-resistance channels between cells form a functional syncytium enabling coordinated contractions. Slow wave potentials generate spike potentials and trigger contraction. The activity is adjusted by the autonomic nervous system, and rhythmic fluctuations of membrane potential occur, serving as a pacemaker function.
  • Multiunit smooth muscle: This type is located in the iris, ciliary muscle, arrector pili, and vas deferens. Each muscle fiber is innervated and functions independently. Gap junctions are absent, and activity is regulated by the autonomic nervous system.

Mechanism of Smooth Muscle Contraction

  • Slow waves can stimulate spike potentials, triggering contraction in single-unit smooth muscle cells.
  • Initial phase of contraction from intracellular Ca2+ released from the sarcoplasmic reticulum ( similar to skeletal ).
  • Sustained contraction by Ca2+ influx via voltage-gated channels from interstitial fluid.
  • Calcium combines with calmodulin to form a complex, activating myosin light-chain kinase (MLCK).
  • MLCK phosphorylates myosin cross-bridges, exposing actin binding sites, allowing cross bridge formation and subsequent contraction.
  • Relaxation occurs when calcium is pumped back to its storage sites, the complex dissolves, and the muscle relaxes.

Regulation of Contraction

  • Smooth muscle has inherent pacemaker activity
  • Sympathetic and parasympathetic autonomic nerves regulate smooth muscle activity in various organs simultaneously. Examples of actions include pupil dilation, coronary artery dilation, and reduced intestinal motility resulting from sympathetic stimulation.
  • Parasympathetic effects are often the opposite of sympathetic for example increased intestinal motility.

Autonomic Innervation

  • Smooth muscle cells have neurotransmitter receptors across their entire surface.
  • Neurotransmitters are released along the length of autonomic nerve fibers.
  • These fibers branch into varicosities (bulges) releasing neurotransmitters, each stimulating multiple smooth muscle cells in a localized way.

Smooth Muscle vs. Skeletal/Cardiac Muscle (Comparison)

  • Key differences in contraction mechanisms, innervation, structure, and responsiveness to stimuli are noted in a comparison table (note: a table isn't requested but you should be able to piece one together).

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Description

This quiz explores the structure and function of smooth muscle, also known as visceral muscle. It discusses the unique characteristics of smooth muscle cells, their contraction mechanisms, and their role in various bodily functions. Test your knowledge on how smooth muscle differs from other muscle types and its importance in organ function.

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