Smooth Muscle and Muscle Tissue Overview

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Questions and Answers

What initiates smooth muscle contraction?

Increased levels of intracellular sodium

Rise in intracellular calcium concentrations (correct)

Decrease in extracellular fluid

Release of troponin from muscle cells

Which channels are primarily responsible for calcium entry in smooth muscle?

Only voltage-gated calcium channels

Only ligand-gated calcium channels

Calcium release channels from the sarcoplasmic reticulum

Stretch-activated and voltage-gated calcium channels (correct)

Which statement is true regarding the relaxation of smooth muscle?

Calcium must be continuously present for relaxation

Calmodulin directly activates troponin to facilitate relaxation

Calcium is removed solely through the sarcoplasmic reticulum

Calcium can be removed back to the extracellular fluid (correct)

What is the term used for contraction caused by chemical signaling in smooth muscle?

Pharmacomechanical coupling Signup and view all the answers

What role does calmodulin play in smooth muscle function?

It is involved in the phosphorylation of myosin light chains Signup and view all the answers

Which muscle type requires ACh from a motor neuron to initiate contraction?

Skeletal muscle Signup and view all the answers

What characterizes the contraction speed of cardiac muscle compared to other muscle types?

It is slower than skeletal muscle but faster than smooth muscle Signup and view all the answers

Which of the following statements regarding the calcium source for muscle contraction is correct?

Cardiac muscle uses calcium from extracellular fluid (ECF) and sarcoplasmic reticulum (SR) Signup and view all the answers

Which muscle type is characterized by graded contraction force of single fiber twitches?

Both smooth and cardiac muscle Signup and view all the answers

How is the contraction of smooth muscle primarily controlled?

By autonomic neurons and chemical signals Signup and view all the answers

What shape are smooth muscle cells?

Spindle-shaped Signup and view all the answers

Which muscle type lacks sarcomeres?

Smooth muscle Signup and view all the answers

What initiates contraction in smooth muscle?

Both electrical and chemical signals Signup and view all the answers

Which system controls smooth muscle contraction?

Autonomic nervous system Signup and view all the answers

What is the origin of Ca2+ for contraction in smooth muscle?

From both the extracellular fluid and the sarcoplasmic reticulum Signup and view all the answers

What fiber arrangement is absent in smooth muscle?

Striated arrangement Signup and view all the answers

How many nuclei do smooth muscle fibers possess?

Uninucleate Signup and view all the answers

What ends the calcium signal in smooth muscle contraction?

Phosphorylation of myosin light chain Signup and view all the answers

Which of the following organs primarily utilize smooth muscle for propulsion?

Esophagus Signup and view all the answers

What is a characteristic of tonic smooth muscles?

Maintains continuous contraction Signup and view all the answers

Which of the following best describes the arrangement of myosin filaments in smooth muscle?

Entire length covered with myosin heads Signup and view all the answers

What function does smooth muscle serve in the bladder?

Allows filling and storage of urine Signup and view all the answers

What is the primary influence on smooth muscle contraction?

Autonomic nervous system Signup and view all the answers

How do phasic smooth muscles differ from tonic smooth muscles?

Phasic muscles exhibit periodic contraction and relaxation. Signup and view all the answers

Which of the following statements is true regarding the sarcoplasmic reticulum in smooth muscle?

It varies in amount and is less organized. Signup and view all the answers

What role do ciliary muscles, classified as smooth muscle, play in the body?

Regulate pupil size Signup and view all the answers

Which statement correctly describes the latch state in smooth muscle?

Allows smooth muscle to maintain tension with low ATP consumption. Signup and view all the answers

What is the primary action of Ca2+ channel blockers like nifedipine?

Cause smooth muscle relaxation and dilation of blood vessels. Signup and view all the answers

What type of smooth muscle predominantly behaves as a unit due to multiple gap junctions?

Single-unit smooth muscle Signup and view all the answers

Which mechanism plays a critical role in the phosphorylation of myosin light chains during smooth muscle contraction?

Calmodulin activating myosin light chain kinase Signup and view all the answers

Which of the following terms describes the unstable membrane potentials found in some smooth muscles?

Slow-wave potentials Signup and view all the answers

What directly affects the ratio of myosin light chain kinase to myosin light chain phosphatase activity?

Calcium levels in the cytosol Signup and view all the answers

What is the primary role of myosin light chain phosphatase in smooth muscle contraction?

To dephosphorylate myosin and trigger relaxation. Signup and view all the answers

What is a key characteristic of multi-unit smooth muscles?

They require individual nerve innervation. Signup and view all the answers

Which of the following statements about autorhythmic activity in smooth muscle is true?

Autorhythmic activity can be regulated by the autonomic nervous system. Signup and view all the answers

How do smooth muscles differ from skeletal muscles in terms of contraction speed?

Smooth muscles contract and relax much more slowly than skeletal muscles. Signup and view all the answers

What role do gap junctions play in single-unit smooth muscles?

They link smooth muscle cells, allowing coordinated contraction. Signup and view all the answers

Which type of receptor is primarily involved in regulating smooth muscle activity via the autonomic nervous system?

Muscarinic receptors Signup and view all the answers

What is an advantage of the electrical isolation of multi-unit smooth muscles?

It enables finer motor control over individual muscle cells. Signup and view all the answers

Which statement best describes the energy efficiency of smooth muscles?

Smooth muscles maintain a given amount of force using less energy. Signup and view all the answers

Which of the following best describes the length range operation of smooth muscles?

Smooth muscles operate over a wide range of lengths. Signup and view all the answers

Study Notes

Smooth Muscle

Smooth muscle is found in the walls of hollow internal organs and tubes (e.g., vascular, gastrointestinal, urinary, respiratory, and reproductive tracts)
It is also found in the pupils of the eyes (ciliary muscles), and attached to the base of hair follicles (pilo motor muscles)
Smooth muscle controls the movement of materials into, out of, and within the body; it contracts involuntarily, responding to autonomic nerves and modulated by the endocrine system
Smooth muscle exhibits spontaneous contraction to control peristaltic waves for propelling materials within organs like the intestines
The movement of materials within the body is controlled by smooth muscle contractions

Types of Muscle Tissue

Muscle tissue can be classified by control mode, anatomical location, and histological characteristics
Voluntary muscles (skeletal) are controlled by the somatic nervous system
Involuntary muscles (smooth and cardiac) are controlled by the autonomic nervous system

Smooth Muscle Structure

Smooth muscle fibers are elongated, spindle-shaped cells with a single nucleus; they lack striations.
Contraction is influenced by the presence of myosin filaments, attached to dense bodies (analogous to Z-discs)
Thin filaments with actin are attached to the dense bodies
Smooth muscle lacks sarcomeres and transverse tubules (T-tubules); the amount of sarcoplasmic reticulum (SR) also varies

Smooth Muscle Contraction Mechanisms

Contraction in smooth muscle is initiated by an increase in cytosolic calcium (Ca²⁺) concentrations, which results from extracellular fluid, as well as calcium released from sarcoplasmic reticulum
A cascade begins with Ca²⁺ binding to calmodulin, which activates myosin light-chain kinase (MLCK), phosphorylating myosin light chains
This enhances myosin ATPase activity, leading to cross-bridge cycling and contraction
Relaxation occurs when Ca²⁺ levels decrease, leading to dephosphorylation of the myosin light chain by myosin light chain phosphatase (MLCP).

Smooth Muscle Contraction Characteristics

Smooth muscle differs from skeletal muscle in several key characteristics:
Smooth muscle contractions are slower than in skeletal muscle.
Smooth muscle can maintain prolonged contractions with low energy consumption (latch state): dephosphorylated myosin may remain attached to actin for extended periods under certain conditions
It can operate over a range of lengths and adjust muscle tension accordingly.
Smooth muscle layers exhibit various orientations within an organ.
Different smooth muscle types can exhibit varying membrane potentials (slow-wave potentials, pacemaker potentials), influencing contraction

Smooth Muscle Classification

Phasic smooth muscles: These muscles exhibit periodic contraction and relaxation cycles, and are involved in activities like the wall of the intestine and the esophagus
Tonic smooth muscles: These muscles maintain a state of sustained contraction (always maintaining some tone) to carry out functions like the esophageal sphincter and the urinary bladder sphincter

Smooth Muscle Innervation

Smooth muscle is innervated by the autonomic nervous system.
Neurotransmitters are released along the length of autonomic neurons from structures called varicosities.
A number of smooth muscle cells are stimulated at once using this mechanism (synapses en passant).
Smooth muscle groups contract spontaneously in many cases, and these contractions can be adjusted by the autonomic nervous system (ANS); dual innervation occurs by sympathetic and parasympathetic nerves, leading to regulated up or down-regulation of autorhythmicity.

Smooth Muscle Control

Smooth muscle relaxation and contraction are affected by different transmitters based on target tissue, along with hormones such as epinephrine

Clinical Implications

Calcium channel blockers such as nifedipine are used for smooth muscle relaxation, and to treat hypertension and angina
Smooth muscle relaxation is caused by dilation of blood vessels, leading to a therapeutic benefit

Membrane Potentials

Some smooth muscle cells have unstable membrane potentials due to slow-wave potentials or pacemaker potentials.
These potentials vary based on individual cells, enabling a range of responses in the same tissue.

Types of Smooth Muscle

Single-unit smooth muscle: Multiple gap junctions connect neighboring cells, enabling coordinated contractions. Most smooth muscles function as single units.
Multi-unit smooth muscle: Individual cells receive independent innervation; found in certain places such as the skin and the eyes..

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Description

Explore the fascinating world of smooth muscle and its crucial role in the human body. This quiz covers the types of muscle tissue, their functions, and their control mechanisms. Test your understanding of how smooth muscle operates within various organ systems.