Smooth Muscle Anatomy and Function

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Questions and Answers

What characteristic property allows smooth muscle fibers to gradually drift towards action potential?

Neuro-muscular junction activity

Pacemaker activity (correct)

Repolarization activity

Independent innervation

Which type of smooth muscle fibers are independent and not connected by gap junctions?

Single unit fibers

Multi-unit fibers (correct)

Skeletal fibers

Cardiac fibers

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

To regulate thin filaments

To activate the autonomic nervous system

To enable cross bridge formation with thick filament (correct)

To initiate action potentials

Where are single unit smooth muscle fibers primarily found?

GI tract Signup and view all the answers

What mechanism primarily drives smooth muscle contraction?

Nervous stimuli and hormones Signup and view all the answers

How do single unit smooth muscle fibers react to the depolarization of one fiber?

All fibers contract together Signup and view all the answers

What type of muscle is smooth muscle categorized as?

Involuntary and non-striated Signup and view all the answers

Which muscle type is primarily associated with the heart wall?

Cardiac muscle Signup and view all the answers

What distinguishes multi-unit smooth muscle fibers in terms of contraction from single unit fibers?

They are independently innervated Signup and view all the answers

Which location is NOT typically associated with multi-unit smooth muscle fibers?

Walls of small blood vessels Signup and view all the answers

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

It phosphorylates the light chain to initiate contraction. Signup and view all the answers

Which of the following mechanisms does NOT initiate smooth muscle contraction?

Direct exposure to light. Signup and view all the answers

What percentage of ATP does smooth muscle use compared to skeletal muscle for the same contractions?

10% Signup and view all the answers

What type of activation does NOT occur in smooth muscle for contraction?

Voluntary activation through motor neurons. Signup and view all the answers

Which statement correctly describes the characteristic structure of smooth muscle?

Smooth muscle contains actin and myosin but lacks T tubules. Signup and view all the answers

Which of the following channels allows calcium to enter smooth muscle cells primarily through mechanical stimulation?

Mechano-gated channels. Signup and view all the answers

Which term describes smooth muscle that contracts as a single unit?

Single unit smooth muscle. Signup and view all the answers

What is the primary role of calcium ions (Ca++) in smooth muscle contraction?

To form cross bridges between actin and myosin. Signup and view all the answers

What distinguishes spontaneous electrical activity in smooth muscle?

It can occur in the absence of nerve or hormonal stimuli. Signup and view all the answers

How does smooth muscle differ from skeletal muscle in terms of ATP utilization?

Smooth muscle is more efficient and uses less ATP. Signup and view all the answers

Study Notes

Smooth Muscle Overview

Smooth muscle is an involuntary, non-striated muscle type found in blood vessels and internal organs.
Cardiac muscle is involuntary, striated, and forms the heart wall.

Learning Objectives

Describe the structure and function of smooth muscle types.
Explain how smooth muscle myofilaments are regulated.
Name three ways in which smooth muscle contraction is initiated.
Explain spontaneous electrical activity (pacemaker) in smooth muscle.
Contrast single unit and multiunit smooth muscles.

Smooth Muscle Structure

Smooth muscle cells are spindle-shaped with a single nucleus.
They have thin filaments (actin) and thick filaments (myosin).
Intermediate filament bundles are attached to dense bodies.
Caveoli are present in the plasma membrane.

Smooth Muscle Myofilament Regulation

Myosin has two subunits: a heavy chain (containing ATPase activity) and a light chain (regulating ATPase).
Myosin light chain kinase (MLCK) is activated by Ca++.
Phosphorylation by MLCK allows cross-bridge formation between myosin and actin, initiating contraction.

Smooth Muscle Contraction Initiation

Three primary ways:
- Autonomic nervous system (parasympathetic, sympathetic, and enteric) via voltage-gated Ca++ channels.
- Hormones via ligand-gated Ca++ channels.
- Stretch via mechano-gated Ca++ channels.

Pacemaker Activity

Some smooth muscle exhibits spontaneous contractile activity without nerve or hormonal stimulation.
The resting membrane potential gradually drifts toward threshold, triggering action potentials.
Following repolarization, the membrane depolarizes again. This spontaneous activity is called pacemaker activity.
Common in the GI tract.

Single-unit vs. Multi-unit Smooth Muscle

Single-unit smooth muscle:
- Cells are connected by gap junctions, allowing synchronized depolarization and contraction.
- Often found in the walls of internal organs (e.g., intestines, blood vessels) and the uterus.
- Stretching one cell can initiate a coordinated contraction in the entire sheet.
Multi-unit smooth muscle:
- Fibers are innervated independently.
- Depolarization of one fiber triggers contraction only of that fiber.
- Located in areas requiring fine control, like large arteries, airways, and hair follicles.
- Autonomic nerves and hormones regulate contraction; not stretch.

Membrane Activation

Smooth muscle contraction depends on a rise in cytosolic calcium.
Calcium enters the cytoplasm through calcium channels (voltage-gated, ligand-gated, and mechano-gated).
Smooth muscle lacks t-tubules.

Characteristics of Smooth Muscle

Smooth muscle contractions are slow and sustained using less ATP than skeletal muscle.
Smooth muscle exhibits variability in tension.

Relaxation of Smooth Muscle

Relaxation is achieved by removing calcium from the cytoplasm.

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Description

This quiz covers the essential aspects of smooth muscle, including its structure, function, and contraction mechanisms. Learn about the differences between single unit and multiunit smooth muscles and the regulation of myofilaments. Test your understanding of spontaneous electrical activity in these unique muscular tissues.