Smooth Muscle Physiology and Contraction

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Questions and Answers

What effect does stretching cardiac cells have on the orientation of actin and myosin?

It has no significant effect on the proteins.

It leads to a random alignment of proteins.

It produces a more optimal orientation of actin and myosin. (correct)

It decreases contractile force generation.

According to Starling's Law of the Heart, what happens to the contractility of heart cells when more blood is pushed into the heart?

Contractility remains unchanged regardless of blood volume.

Contractility decreases with increased blood volume.

Contractility only depends on heart rate, not blood volume.

Contractility increases up to a certain point. (correct)

What is the relationship between ventricular filling with blood and cardiac muscle stretch?

Increased filling leads to decreased stretch.

There is no correlation between filling and muscle stretch.

Increased filling results in optimal stretch for contractility. (correct)

Decreased filling enhances muscle stretch and contractility.

What is the primary benefit of optimal actin and myosin orientation in cardiac cells?

It allows a greater contractile force to be generated. Signup and view all the answers

What happens when blood volume exceeds the normal limits in relation to cardiac performance?

The heart's contractility begins to decline. Signup and view all the answers

What initiates depolarization in cardiac muscle cells?

Influx of calcium ions Signup and view all the answers

What occurs after depolarization spreads through the atria?

Atrial contraction Signup and view all the answers

In which part of the heart is the action potential transmitted down the conducting system?

From the bottom to the apex of the ventricle Signup and view all the answers

Which aspect differentiates cardiac muscle contraction from skeletal muscle contraction?

The process of excitation-contraction coupling Signup and view all the answers

What role does the DHP receptor play in skeletal muscle cells?

Stimulating calcium release from the SR Signup and view all the answers

What event directly follows the action potential in cardiac muscle cells?

Opening of voltage-gated calcium channels Signup and view all the answers

What causes the atrial muscle cells to contract?

Atrial depolarization Signup and view all the answers

What is primarily responsible for the spread of action potentials in the heart?

Conducting system Signup and view all the answers

What initiates the contraction of smooth muscle after an elevation in intracellular Ca2+?

Calmodulin forming a complex with calcium Signup and view all the answers

Which of the following statements is true regarding single-unit smooth muscle?

Cells are connected by gap junctions for electrical linkage. Signup and view all the answers

In smooth muscle, what does the term 'functional syncytium' refer to?

Cells contracting as a coordinated unit Signup and view all the answers

What is the role of MLCK in smooth muscle contraction?

It phosphorylates myosin for activation. Signup and view all the answers

What characterizes the pacemaker potential in smooth muscle?

It involves gradual depolarization leading to action potentials. Signup and view all the answers

How do slow-wave potentials occur in smooth muscle?

By alternating ionic fluxes altering membrane potential. Signup and view all the answers

Which structure is formed by the autonomic nerve axons surrounding smooth muscle?

Plexus of varicosities Signup and view all the answers

What initiates contraction in the cardiac muscle cells of the atria and ventricles?

Spontaneous action potentials from the SA node Signup and view all the answers

What distinguishes smooth muscle's response to Ca2+ compared to skeletal muscle?

Smooth muscle has no troponin. Signup and view all the answers

What is the resting membrane potential of SA node cells?

-60 mV Signup and view all the answers

Which ions are primarily responsible for the spontaneous depolarization in SA node cells?

Na+ and Ca2+ Signup and view all the answers

What occurs immediately after the depolarization of SA node cells reaches 0 mV?

Rapid efflux of K+ occurs Signup and view all the answers

What happens during the cardiac cycle when the atria contract?

Blood is forced into the ventricles Signup and view all the answers

How does the SA node maintain heart rhythm?

By producing spontaneous, repetitive action potentials Signup and view all the answers

What triggers the production of an action potential in SA node cells?

A threshold of -40 mV Signup and view all the answers

What is the primary purpose of the sequential contraction and relaxation of the heart muscle?

To allow for filling and pumping of blood Signup and view all the answers

What effect does noradrenaline and adrenaline have on the SA node cells?

They depolarize the SA node cells. Signup and view all the answers

Which receptor does acetylcholine bind to in the parasympathetic nervous system?

M2 receptor Signup and view all the answers

How does the sympathetic nervous system affect ventricular contraction strength?

By enhancing Ca2+ release. Signup and view all the answers

What is the primary effect of parasympathetic stimulation on the heart rate?

Inhibition of the heart rate by reducing SA node firing. Signup and view all the answers

What is the result of stretching cardiac cells in the ventricles according to Starlings Law?

Enhanced force of contraction. Signup and view all the answers

Which neurotransmitter is released by the sympathetic nervous system?

Noradrenaline Signup and view all the answers

What is the effect of M2 receptor stimulation on the SA node?

Decreases heart rate. Signup and view all the answers

What role does Ca2+ play in cardiac muscle contraction?

It enhances force of contraction. Signup and view all the answers

Study Notes

Smooth Muscle Contraction

Smooth muscle (SM) contraction relies on the interaction of actin and myosin, similar to skeletal muscle.
However, smooth muscle does not contain troponin, the protein responsible for regulating actin-myosin binding in skeletal muscle.
Instead, calcium ions (Ca2+) bind to calmodulin, forming a complex that activates myosin light chain kinase (MLCK).
MLCK phosphorylates myosin light chains, activating myosin ATPase activity.
The activated myosin ATPase enables myosin cross-bridging with actin, leading to muscle contraction.

Single-Unit Smooth Muscle

Single-unit smooth muscle is also known as visceral smooth muscle.
It exhibits myogenic properties, meaning it can contract without nervous stimulation.
Cells are electrically linked by gap junctions, allowing coordinated contraction of a single unit.
This allows for efficient and slow contractions, well-suited for organs like the bladder and intestines.

Pacemaker Potential

The membrane potential of single-unit smooth muscle depolarizes on its own due to shifts in ionic fluxes.
When the membrane depolarizes to threshold, an action potential is generated.
This inherent rhythmicity allows for sustained smooth muscle contractions in the absence of nervous input.

Slow-Wave Potentials

Slow-wave potentials are characterized by periodic fluctuations in membrane potential, driven by changes in sodium ion transport across the membrane.

Smooth Muscle Innervation

Smooth muscle is innervated by the autonomic nervous system, with both sympathetic and parasympathetic branches.
Autonomic nerve axons form a network of varicosities surrounding the smooth muscle cells.
Varicosities release neurotransmitters that can modulate smooth muscle contraction and relaxation.

Cardiac Cycle

The cardiac cycle consists of sequential contraction and relaxation of the atria and ventricles, enabling blood circulation.
The atria fill with blood during relaxation, then contract to push blood into the ventricles.
After a brief delay, the ventricles contract simultaneously, propelling blood to the lungs and body.

Cardiac Conduction System

Specialized cardiac cells form a conduction system that initiates and spreads action potentials (APs) throughout the heart.
The sinoatrial (SA) node, considered the pacemaker, generates spontaneous, repetitive APs.
APs propagate through the atria, triggering their contraction, and then to the ventricles, leading to their contraction.

Excitation-Contraction Coupling in Cardiac vs Skeletal Muscle

In skeletal muscle, action potentials travel through T-tubules, triggering the release of calcium from the sarcoplasmic reticulum (SR) via the dihydropyridine receptor (DHP).
In cardiac muscle, action potentials depolarize the plasma membrane and T-tubules, activating voltage-gated calcium channels.
The influx of calcium into cardiac muscle cells further triggers the release of calcium from the SR, leading to muscle contraction.

Autonomic Nervous System Control of Heart Rate

The sympathetic nervous system releases noradrenaline and adrenaline, activating β1 receptors, which increase heart rate and contractility.
The parasympathetic nervous system releases acetylcholine, activating muscarinic (M2) receptors, which decrease heart rate.

Intrinsic Control of Ventricular Contraction - Starling's Law

Starling's Law of the Heart states that the force of ventricular contraction increases with the degree of ventricular filling.
Increased ventricular filling stretches the cardiac cells, optimizing the arrangement of actin and myosin, leading to stronger contractions.
This inherent mechanism allows the heart to adjust the force of contraction to maintain appropriate blood flow.

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Description

Explore the mechanisms of smooth muscle contraction including the role of calcium and calmodulin. Learn about single-unit smooth muscle and its unique properties, along with the concept of pacemaker potential. This quiz delves into the intricate functions that allow smooth muscle to operate effectively in the body.