Smooth Muscle Physiology and Contraction

Smooth muscle is a type of muscle tissue found in the walls of internal organs. It contracts involuntarily, lacks striations, and is controlled by the autonomic nervous system and hormones.

Smooth muscle cells are spindle-shaped, meaning they are wider in the middle and tapered at the ends. They also have a single nucleus located in the center.

Smooth muscles lack the organized sarcomere arrangement found in striated muscle. This is why they appear smooth under a microscope.

Dense bodies are structures in smooth muscle cells where actin filaments attach. They act like Z-discs in striated muscle.

Calcium ions play a crucial role in smooth muscle contraction. When calcium enters the cell, it activates an enzyme called myosin light chain kinase (MLCK).

Smooth muscle contractions are typically slow but can be sustained for long periods. This makes them efficient for tasks like regulating blood vessel pressure or moving food through the digestive system.

Smooth muscle can be classified into two types: single-unit (visceral) and multi-unit. Single-unit smooth muscle has gap junctions allowing synchronized contractions, seen in organs like the intestines and uterus.

The autonomic nervous system, specifically the sympathetic and parasympathetic branches, controls smooth muscle contraction.

Calcium ions (Ca²⁺) are crucial for smooth muscle contraction. They change the membrane potential and activate enzymes like MLCK, leading to actin-myosin interaction.

Smooth muscle action potentials are slower to rise and last longer than those in skeletal muscle or neurons. This contributes to the energy-efficient and prolonged contractions typical of smooth muscle.

Some smooth muscles can generate action potentials without external stimulation. This is often seen in pacemaker cells, which initiate rhythmic contractions in smooth muscle organs like the intestines.

The autonomic nervous system, through its sympathetic and parasympathetic branches, can stimulate smooth muscle contraction.

Hormonal signals like oxytocin (uterine contractions) or adrenaline (contraction/relaxation) can influence smooth muscle function.

Physical stretching of smooth muscle cells opens calcium channels, triggering contraction.

Local factors such as low oxygen levels, pH changes, or increased carbon dioxide can influence smooth muscle activity.

The calcium-calmodulin-MLCK pathway regulates smooth muscle contraction using calcium and a protein called calmodulin to activate the enzyme MLCK, leading to myosin phosphorylation and cross-bridge formation.

Smooth muscle fibers contract independently, allowing fine control. This is because they lack gap junctions responsible for coordinated contractions. Examples: Iris muscles of the eye, arrector pili muscles.

A specialized type of smooth muscle cell found in the digestive system, responsible for initiating slow waves. These cells act as pacemakers, generating rhythmic fluctuations in membrane potential.

Slow waves are spontaneous, rhythmic fluctuations in smooth muscle membrane potential. They occur in most areas of the digestive system except the esophagus and proximal stomach. They are NOT action potentials, but rather fluctuations in resting membrane potential.

Spike potentials are true action potentials that occur on top of slow waves. They are triggered by neurotransmitters or hormones and lead to increased smooth muscle tension or contraction.

The resting membrane potential of smooth muscle cells is between -50mV and -60mV. This is more positive compared to skeletal muscle and nerve cells. At rest, the membrane is partially permeable to potassium ions, and calcium channels remain closed.

The depolarization phase of the smooth muscle action potential is characterized by the influx of sodium and calcium ions. This influx makes the inside of the cell more positive, causing the membrane potential to rise towards zero.

The plateau phase of the smooth muscle action potential is a period of prolonged depolarization. It is maintained by the influx of calcium ions and is often involved in long-duration contractions. This phase is optional and not always present in all smooth muscle cells.

The repolarization phase of the smooth muscle action potential is characterized by the efflux of potassium ions and the closure of calcium channels. This outward movement of positive ions makes the inside of the cell more negative, restoring the membrane potential to its resting state.