Clase 9 Fisiología - Contracción y excitación del músculo liso (IG:@doctor.paiva)

New Section

In this section, the instructor introduces the topic of smooth muscle contraction and excitation.

General Characteristics of Smooth Muscle

• Smooth muscle is involuntary and controlled by the autonomic nervous system.

• It is found in reproductive and urinary systems, blood vessels, and internal organs.

• Contrasts between smooth and skeletal muscles include diameter, length, and control mechanisms.

Types of Smooth Muscle

• Two types: multiunitary and unitary smooth muscle.

• Multiunitary muscle fibers act independently.

• Unitary muscle fibers contract as a single unit due to gap junctions.

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• Insightful point or key concept discussed in this part of the video.

Muscle Contraction in Smooth Muscle Cells

The discussion delves into the mechanisms of muscle contraction in smooth muscle cells, highlighting the role of calcium and the differences between smooth and skeletal muscle contractions.

Calcium Dependence and Contractions

• Smooth muscle relies on calcium for contraction but lacks troponin found in skeletal muscle. Instead, it utilizes calmodulin for regulation.

Calmodulin Function and Contraction Process

• Calmodulin, akin to troponin, activates myosin cross-bridges by binding with calcium. This interaction triggers myosin light chain phosphorylation.

Structural Differences and Contraction Characteristics

• Smooth muscles contain actin and myosin filaments similar to skeletal muscles but lack the typical troponin complex.

• Contrary to skeletal muscles, smooth muscles contract more forcefully (up to 80% of their length) but with less strength compared to skeletal muscles.

Differences Between Skeletal and Smooth Muscle Contractions

This segment contrasts the mechanisms of contraction and relaxation between skeletal and smooth muscles, emphasizing their dependency on different regulatory proteins.

Calcium Dependency in Contractions

• Skeletal muscle relies on troponin for contraction while smooth muscle depends on calmodulin. Troponin mediates relaxation in skeletal muscle, whereas phosphatases facilitate relaxation in smooth muscle.

Components of Contraction and Relaxation

• Key components involved in smooth muscle contraction include calcium, actin, myosin, calmodulin (replacing troponin), myosin light chain kinase (for contraction), and myosin light chain phosphatase (for relaxation).

Contraction Mechanisms in Smooth Muscle Cells

Exploring the detailed processes underlying contraction and relaxation within smooth muscle cells through a step-by-step breakdown.

Calcium Regulation Pathway

• Intracellular calcium levels are crucial for initiating contractions. The interaction between calcium-calmodulin activates myosin light chain kinase leading to phosphorylation of myosin light chains.

Relaxation Process

• Following contraction, calcium is removed via SERCA pumps from the sarcoplasmic reticulum. Myosin light chain phosphatase then dephosphorylates myosins, inducing relaxation.

Summary Diagram Explanation

• A comprehensive diagram illustrates the sequential events during both contraction and relaxation phases in smooth muscles.

New Section

In this section, the speaker discusses the typical action potential of muscle cells, focusing on spike potentials and slow wave potentials in the intestinal wall.

Action Potentials in Muscle Cells

• Spike potentials are seen in the image as a typical action potential of muscle cells triggered by electrical stimulation.

• The image also shows repetitive spike potentials occurring in a sequence due to slow waves.

• Slow wave potentials are observed in the intestinal wall during peristalsis, with relaxation lasting for several seconds.

New Section

This part delves into mesenteric potentials, highlighting their similarities and differences with spike potentials.

Mesenteric Potentials

• Mesenteric potentials at their onset resemble spike potentials with sodium influx during depolarization.

• However, repolarization is significantly delayed, leading to a plateau phase known as a "meseta."

• The meseta is influenced by two factors: slow calcium-sodium channels and delayed potassium efflux.

• This prolonged depolarization contributes to sustained contractions in various smooth muscles like those found in the uterus.

New Section

Here, the speaker explains how meseta contributes to prolonged contractions in different types of smooth muscles.

Role of Meseta in Muscle Contraction

• Meseta results from slow-opening potassium channels during repolarization after sodium influx.

• These delayed potassium channels aid in maintaining the plateau phase of the action potential.

• The sustained depolarization caused by meseta leads to extended contractions in muscles like uterine and cardiac cells.

New Section

This segment focuses on observing action potentials similar to those discussed earlier but now present in cardiac cells.

Action Potentials in Cardiac Cells

• Similar action potentials are noted in cardiac cells compared to other muscle types like smooth muscles.