Smooth Muscle and Muscle Tissue Overview

The process by which an increase in intracellular calcium levels triggers contraction in smooth muscle.

Smooth muscle relies on a mix of calcium sources: some from the sarcoplasmic reticulum (SR) and a larger amount from the extracellular fluid (ECF).

In smooth muscle, calcium binds to calmodulin, a protein that activates myosin light-chain kinase (MLCK), triggering muscle contraction.

Smooth muscle contraction initiated by electrical signaling.

Smooth muscle contraction initiated by chemical signaling.

Smooth muscle type where each cell requires individual nerve innervation, allowing for fine motor control. They are characterized by few or no gap junctions, resulting in a lack of coordinated contraction. Examples are arrector pili muscles and ciliary muscles.

Smooth muscle type where cells are connected by gap junctions, allowing electrical signals to pass between them and contract as a single unit. This leads to coordinated contraction. Found in tissues like the digestive tract.

A type of synapse in smooth muscle where neurotransmitters are released along the length of the autonomic neuron from swellings called varicosities. This allows for a diffuse stimulation of multiple smooth muscle cells.

The ability of some smooth muscle groups to contract without external nerve stimulation. This intrinsic activity can be modulated by the autonomic nervous system.

The control of smooth muscle activity by both the sympathetic and parasympathetic nervous systems. This allows for fine-tuning of muscle contraction.

Receptors on smooth muscle cells that bind to acetylcholine, a neurotransmitter released by the parasympathetic nervous system. They are involved in various physiological processes like slowing heart rate, constricting pupils, and stimulating digestion.

Smooth muscles are the slowest to contract and relax compared to skeletal and cardiac muscles. This allows for sustained contractions and a more gradual response.

Smooth muscle functions differently from skeletal muscle. It can operate over a range of lengths, has multi-directional layers, contracts slowly, uses less energy, sustains contractions longer, and does not fatigue easily.

The three types of muscle tissue are skeletal, smooth, and cardiac. Each type has unique characteristics related to its structure, function, and control.

Skeletal muscle contraction is initiated by a signal from the somatic motor neuron, releasing acetylcholine (ACh) which triggers calcium release from the sarcoplasmic reticulum (SR).

Smooth muscle contraction can be initiated by various stimuli, including stretch, chemical signals, or autorhythmicity. Calcium for contraction comes from both the extracellular fluid (ECF) and the SR.

Cardiac muscle contraction is regulated by a combination of autorhythmicity and autonomic nervous system input. Calcium comes from both the ECF and SR.

Skeletal muscle contracts the fastest, followed by cardiac muscle, and then smooth muscle, which contracts the slowest.

Occur in some smooth muscles, they are rhythmic fluctuations in membrane potential that do not reach threshold for action potential generation. They contribute to smooth muscle's ability to contract rhythmically.

Occur in some smooth muscles and spontaneously depolarize the membrane potential until it reaches threshold, triggering an action potential. This contributes to smooth muscle's ability to contract automatically.

Calcium plays a crucial role in triggering smooth muscle contraction. It enters the cytosol from both the sarcoplasmic reticulum (SR) and extracellular fluid (ECF). Inside the cytosol, Ca2+ binds to calmodulin, activating myosin light chain kinase (MLCK) and leading to the phosphorylation of myosin light chains (MLC). This phosphorylation enhances myosin ATPase activity, allowing for smooth muscle contraction.

MLCK activates myosin by phosphorylating it, leading to contraction. MLCP dephosphorylates myosin, promoting relaxation. The balance between MLCK and MLCP activities ultimately determines the smooth muscle's contraction level.

The dephosphorylation of myosin doesn't automatically lead to relaxation. In a latch state, dephosphorylated myosin remains attached to actin, allowing smooth muscle to maintain tension with minimal energy expenditure. This is crucial for sustained contractions without fatigue.

While both use calcium, smooth muscle contraction doesn't rely on the troponin-tropomyosin interaction. Instead, the balance between MLCK and MLCP activity regulates smooth muscle contraction. In addition, smooth muscle can maintain tension in the 'latch state' without actively consuming much energy.

Calcium channel blockers like nifedipine prevent calcium from entering smooth muscle cells, leading to relaxation. This is used to dilate blood vessels, lowering blood pressure and treating conditions like hypertension and angina.

Smooth muscle fibers lack the organized sarcomere structure found in skeletal and cardiac muscle, resulting in a smooth appearance under a microscope.

Smooth muscle contraction can be triggered by electrical signals (electromechanical coupling), chemical signals (pharmacomechanical coupling), or a combination of both.

Smooth muscle is primarily controlled by the autonomic nervous system, which regulates involuntary functions like digestion and blood pressure.

Smooth muscle cells lack specialized receptor regions like those found in skeletal muscle. This means that the entire cell surface can respond to stimuli.

Smooth muscle obtains calcium for contraction from both extracellular fluid and the sarcoplasmic reticulum.

Calcium binds to calmodulin, activating myosin light-chain kinase (MLCK). MLCK then phosphorylates myosin light chains, triggering myosin ATPase activation and muscle contraction.

Skeletal muscle is striated, has sarcomeres, and is attached to bones. Smooth muscle is non-striated, lacks sarcomeres, and forms the walls of hollow organs.

Cardiac muscle is striated, has sarcomeres, and is found in the heart. Smooth muscle is non-striated, lacks sarcomeres, and forms the walls of hollow organs.

There are three main types of muscles: skeletal, cardiac, and smooth.

Smooth muscle is found in the walls of hollow internal organs and tubes, such as the digestive tract, blood vessels, and bladder.

Smooth muscle contracts to control the movement of materials within and out of the body, like pushing food through the digestive system or narrowing blood vessels.

Smooth muscle cells lack sarcomeres, the organized structures found in skeletal muscle, but have dense bodies that act like Z-discs.

Smooth muscle contraction is triggered by calcium binding to calmodulin, which activates myosin light-chain kinase (MLCK), leading to myosin and actin interaction.

Phasic smooth muscles undergo periodic contraction and relaxation cycles, such as in the esophagus and intestinal wall.

Tonic smooth muscles maintain a continuous state of contraction, helping to regulate sphincters and other organs.