Smooth Muscle Overview and Function

Smooth Muscle Overview

• Smooth muscle is found in hollow internal organs and tubes (vascular, gastrointestinal, urinary, respiratory, and reproductive tracts)

• It also found in the pupil of eye (ciliary muscles), attached to hair follicles (pilo motor muscles), and controls material movement throughout the body

• Smooth muscle is involuntary, controlled by the autonomic nervous system and can be modulated by the endocrine system.

Smooth Muscle Contraction

• Unlike skeletal muscle, smooth muscle lacks sarcomeres
• Actin filaments are attached to dense bodies (analogous to Z-discs)
• Some myosin filaments are present
• The amount of sarcoplasmic reticulum is variable and less organized than in skeletal muscle.
• No transverse tubules are present Arrangement allows contraction even when greatly stretched

Smooth Muscle Organization

• Intermediate filaments and protein dense bodies form a cytoskeleton

• Actin is attached to these dense bodies

• Each myosin molecule is surrounded by actin filaments

• Myosin filaments in smooth muscle are longer than those in skeletal muscle

• They are completely covered with myosin heads (unlike skeletal muscle)

• Unique arrangement develops even when stretched (e.g., bladder filling)

Types of Smooth Muscle

• Single-unit smooth muscle : multiple gap junctions cause neighbouring cells to behave as a unit. Most smooth muscle is single-unit which displays pacemaker activity, moderated by stretch or autonomic innervation. Only a few cells receive acetylcholine stimulation.
• Multi-unit smooth muscle : requires individual nerve innervation (no pacemaker activity). Fewer or no gap junctions. Arrector pili muscles and the ciliary muscle in the eye are examples

Smooth Muscle Innervation

• Neurotransmitters are released along the length of an autonomic neuron from varicosities.
• A number of smooth muscle cells are stimulated at once via synapses en passant (along the way)
• Sympathetic and parasympathetic innervation may be present in different tissues and may allow for up or down regulation of autorhythmic activity. The effect is dependent on tissue and levels of alpha or beta adrenergic receptors

Smooth Muscle Contraction and Relaxation

• Increased cytosolic calcium is the signal for contraction
• Ca2+ binds to calmodulin
• The Ca2+ signal initiates cascade that ends with phosphorylation of myosin light chain and activation of myosin ATPase
• Relaxation occurs when Ca2+ is removed from the cytosol causing the myosin light chain kinase (MLCK) to be inactive , myosin phosphatase to activate, and a decrease in muscle tension

Smooth Muscle vs. Skeletal Muscle Comparison

• Smooth muscles operate across a wider range of lengths
• Smooth muscle layers run in multiple directions within an organ
• Smooth muscle contracts and relaxes much slower than skeletal muscle
• Smooth muscle uses less energy to maintain a given amount of force
• Smooth muscle can sustain contractions for extended periods without fatigue
• Smooth muscle cells are small, spindle-shaped with a single nucleus
• Smooth muscle contractions can be initiated by electrical or chemical signals or both
• Smooth muscle lacks specialized receptor regions, and the Ca2+ for contraction comes from outside or from the sarcoplasmic reticulum, unlike skeletal muscle.
• The structure of smooth muscle is not arranged in sarcomeres as in skeletal muscles.

Smooth Muscle Contractions

• Phasic smooth muscles: periodic contraction and relaxation (e.g., esophagus, intestinal wall)
• Tonic smooth muscles: continuously contracted, maintaining some tone (e.g., esophageal sphincter, urinary bladder sphincter)

Membrane Potentials in Smooth Muscle

• Some smooth muscles have unstable membrane potentials (slow-wave potentials, pacemaker potentials)
• Slow-wave potentials are graded oscillations reaching threshold, followed by action potentials
• Pacemaker potentials continually depolarize to threshold, firing action potentials.
• Pharmacomechanical coupling: signals change muscle tension (no or little change in membrane potential)

Clinical Implications

• Ca2+ channel blockers, such as nifedipine, cause smooth muscle relaxation (dilation of blood vessels) used to treat hypertension and angina.
• "Latch state": dephosphorylation of myosin may result in sustained contractions without fatigue; in this state, myosin may remain attached to actin for a period of time

Explore the essential features of smooth muscle, including its unique characteristics, organization, and role in involuntary movements. This quiz covers the contraction mechanics and various functions of smooth muscle in the human body. Test your understanding of this vital muscle type and its contribution to physiological processes.