Muscle Physiology Lecture 5: Smooth Muscle

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Questions and Answers

What is the main difference between the internal physical arrangement of smooth muscle fibers and skeletal muscle fibers?

The arrangement of fibers into bundles or sheets (correct)

The size of the cells

The presence of coarse connective tissue covering

The number of central nuclei

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

To maintain the resting membrane potential

To allow the passage of small water-soluble molecules from cell to cell (correct)

To regulate the calcium ion concentration in the cell

To facilitate the contraction of individual muscle fibers

Which type of smooth muscle has fibers that are arranged in sheets or bundles and contract together as a single unit?

Single Unit smooth muscle

Visceral smooth muscle

A and B (correct)

Multi-unit smooth muscle

What is the main characteristic of myosin filaments in smooth muscle cells?

They have side-polar cross-bridges Signup and view all the answers

What is the maximum amount of contraction that smooth muscle can achieve?

80% of their length Signup and view all the answers

What is the main source of calcium ions for smooth muscle contraction?

Extracellular fluid (ECF) Signup and view all the answers

What is the characteristic of the microscopic structure of smooth muscle fibers?

Spindle-shaped cells with a single central nucleus Signup and view all the answers

Where are gap junctions typically located in the body?

In connective tissue, epithelial tissue, and cardiac muscle Signup and view all the answers

What is the function of dense bodies in smooth muscle cells?

To anchor actin filaments to the cell membrane Signup and view all the answers

What are the small invaginations of the cell membrane that abut the surface of the sarcoplasmic reticulum (SR)?

Caveolae Signup and view all the answers

What is the effect of having more extensive SR in smooth muscle fibers?

More rapid contraction Signup and view all the answers

What is the function of the autonomic nervous system (ANS)?

Influence of internal organs Signup and view all the answers

What is the difference between the sympathetic and parasympathetic nervous systems?

SNS is for 'fight or flight' and PNS is for 'rest and digest' Signup and view all the answers

What is the function of varicosities in smooth muscle?

To store neurotransmitters Signup and view all the answers

What is the effect of neurotransmitters on multi-unit smooth muscle?

Causes depolarization of the muscle membrane and contraction without generating an action potential Signup and view all the answers

What is the difference between multi-unit smooth muscle and single-unit smooth muscle?

Single-unit smooth muscle has action potentials, while multi-unit smooth muscle does not Signup and view all the answers

What is the name of the potential generated by the neurotransmitter in multi-unit smooth muscle?

Junctional potential Signup and view all the answers

What is the characteristic of single-unit smooth muscle?

Action potentials occur similar to those in skeletal muscle Signup and view all the answers

What is similar to those seen in skeletal muscle in visceral smooth muscle?

Spike potentials Signup and view all the answers

What can elicit spike potentials in visceral smooth muscle?

Hormones, stretch, or electrical stimulation Signup and view all the answers

What is associated with a basic slow wave rhythm of the membrane potential?

Spike potentials Signup and view all the answers

What initiates an action potential whenever it gets strong enough?

Basic slow wave rhythm Signup and view all the answers

What is the result of the action potential in visceral smooth muscle?

Rhythmical contraction of the intestinal wall Signup and view all the answers

How many forms do action potentials of visceral smooth muscle occur in?

Two Signup and view all the answers

What is characteristic of action potentials with plateau?

Delayed repolarization phase Signup and view all the answers

What is the difference between action potentials with plateau and typical spike potentials?

The repolarization phase Signup and view all the answers

What is the primary source of calcium ions that increase the concentration in the cytosol?

Influx from ECF Signup and view all the answers

What is the significance of the plateau in smooth muscle contraction?

It enables prolonged contraction in certain types of smooth muscle Signup and view all the answers

What is the function of calmodulin in smooth muscle contraction?

To bind with calcium ions Signup and view all the answers

Which type of muscle fiber has a similar action potential to smooth muscle?

Cardiac muscle fibers Signup and view all the answers

What is the role of myosin light chain kinase (MLCK) in smooth muscle contraction?

To phosphorylate the myosin light chain Signup and view all the answers

What is the primary function of calcium channels in smooth muscle

To generate the action potential and cause contraction Signup and view all the answers

What is the result of phosphorylation of the regulatory light chain of myosin?

Activation of myosin ATPase Signup and view all the answers

What is responsible for the prolonged plateau in smooth muscle action potentials?

Slow opening and long-lasting opening of calcium channels Signup and view all the answers

What is the role of ATP in smooth muscle contraction?

To provide energy for contraction Signup and view all the answers

In small vessels, what is the primary mechanism of smooth muscle contraction?

Response to local chemical conditions and stretch Signup and view all the answers

What is the result of the power stroke in smooth muscle contraction?

Shortening of the muscle Signup and view all the answers

What is the function of the regulatory light chain of myosin?

To regulate the attachment-detachment cycling of myosin Signup and view all the answers

What is the effect of increased oxygen in local tissues on small vessels?

Relaxation of smooth muscle Signup and view all the answers

What is the state where myosin is bound to actin?

Rigor state Signup and view all the answers

What is the role of hydrogen ions in regulating smooth muscle contraction?

They increase smooth muscle contraction Signup and view all the answers

What is the requirement for smooth muscle contraction?

Extracellular calcium concentration Signup and view all the answers

What is the effect of increased body temperature on smooth muscle contraction?

It increases smooth muscle contraction Signup and view all the answers

What is the primary role of calcium ions in smooth muscle contraction?

They participate in the contraction of smooth muscle fibers Signup and view all the answers

Study Notes

Smooth Muscle

Smooth muscle fibers are different from skeletal muscle fibers in terms of internal physical arrangement, physical dimensions, organization into bundles or sheets, response to different stimuli, characteristics of innervation, and function.
There are two types of smooth muscle:
- Multi-unit smooth muscle: discrete, separate smooth muscle fibers, each contracting independently, and innervated individually.
- Single-unit (visceral) smooth muscle: fibers are arranged in sheets or bundles, cell membranes are adherent to one another, and contract together as a single unit.

Gap Junctions

Allow the passage of small water-soluble molecules from cell to cell without having to pass through the plasma membrane.
Ions can flow freely from one muscle cell to the next through gap junctions, which are narrow (2-4 nm) and very important in tissues containing electrically excitable cells.

Microscopic Structure of Smooth Muscle Fibers

Small, spindle-shaped, non-striated cells with one central nucleus and lack of coarse connective tissue covering.
Fibers are arranged into sheets, and each fiber has a large number of actin filaments attached to dense bodies (Z-disk similar).
Myosin filaments are intercalated among the actin filaments, and myosin filaments have side-polar cross-bridges that allow smooth muscle to contract as much as 80% of their length.

Sarcoplasmic Reticulum

Not the major source of Ca for smooth muscle contraction; extracellular fluid (ECF) is the primary source of Ca.
Sarcoplasmic reticulum is slightly developed, lies near the cell membrane in some larger smooth muscle cells, and has small invaginations of the cell membrane called caveolae.
The more extensive the SR in the smooth muscle fiber, the more rapidly it contracts.

Neural and Hormonal Control

Smooth muscle can be stimulated to contract in different ways:
- Nervous stimulation
- Hormonal stimulation
- Local tissue chemical factors
- Self-excitation
The autonomic nervous system (ANS) influences the function of internal organs, and has two branches:
- Sympathetic nervous system (SNS) - "fight or flight" system
- Parasympathetic nervous system (PNS) - "rest and digest" system

Nervous Stimulation

Smooth muscle is innervated by autonomic nerve fibers, which form diffuse junctions that secrete neurotransmitters into the matrix coating of the smooth muscle.
Neurotransmitters diffuse to the cells, and in multi-unit smooth muscle, neurotransmitter (ACh or NE) causes depolarization of the muscle membrane and contraction without generating an action potential.
In single-unit (visceral) smooth muscle, action potentials occur similar to those presented in skeletal muscle.

Action Potential in Visceral Smooth Muscle

Action potentials occur in one of two forms:
1. Spike potentials: similar to those seen in skeletal muscle, can be elicited by electrical stimulation, hormones, stretch, or spontaneously.
2. Action potentials with plateau: onset is similar to that of the typical spike potential, but instead of rapid repolarization, the muscle fiber undergoes a delayed repolarization phase.

Calcium Ions and Contraction

Calcium ions perform two important tasks: generating the action potential and causing contraction.
The contraction of smooth muscle occurs in 5 steps:
1. Calcium concentration in the cytosol increases.
2. Calcium ions bind reversibly with calmodulin.
3. The Ca-calmodulin complex joins with and activates myosin light chain kinase (MLCK).
4. MLCK phosphorylates one of the light chains of each myosin head (regulatory chain).
5. Contraction occurs, depending on extracellular calcium concentration.

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Description

Learn about the two types of smooth muscle, their particularities, and the factors that can initiate contraction in smooth muscle.