Muscle Physiology Lecture 5: Smooth Muscle

Smooth Muscle

• Smooth muscle fibers are different from skeletal muscle fibers in terms of internal physical arrangement, physical dimensions, organization into bundles or sheets, response to different stimuli, characteristics of innervation, and function.
• There are two types of smooth muscle:
  • Multi-unit smooth muscle: discrete, separate smooth muscle fibers, each contracting independently, and innervated individually.
  • Single-unit (visceral) smooth muscle: fibers are arranged in sheets or bundles, cell membranes are adherent to one another, and contract together as a single unit.

Gap Junctions

• Allow the passage of small water-soluble molecules from cell to cell without having to pass through the plasma membrane.
• Ions can flow freely from one muscle cell to the next through gap junctions, which are narrow (2-4 nm) and very important in tissues containing electrically excitable cells.

Microscopic Structure of Smooth Muscle Fibers

• Small, spindle-shaped, non-striated cells with one central nucleus and lack of coarse connective tissue covering.
• Fibers are arranged into sheets, and each fiber has a large number of actin filaments attached to dense bodies (Z-disk similar).
• Myosin filaments are intercalated among the actin filaments, and myosin filaments have side-polar cross-bridges that allow smooth muscle to contract as much as 80% of their length.

Sarcoplasmic Reticulum

• Not the major source of Ca for smooth muscle contraction; extracellular fluid (ECF) is the primary source of Ca.
• Sarcoplasmic reticulum is slightly developed, lies near the cell membrane in some larger smooth muscle cells, and has small invaginations of the cell membrane called caveolae.
• The more extensive the SR in the smooth muscle fiber, the more rapidly it contracts.

Neural and Hormonal Control

• Smooth muscle can be stimulated to contract in different ways:
  • Nervous stimulation
  • Hormonal stimulation
  • Local tissue chemical factors
  • Self-excitation
• The autonomic nervous system (ANS) influences the function of internal organs, and has two branches:
  • Sympathetic nervous system (SNS) - "fight or flight" system
  • Parasympathetic nervous system (PNS) - "rest and digest" system

Nervous Stimulation

• Smooth muscle is innervated by autonomic nerve fibers, which form diffuse junctions that secrete neurotransmitters into the matrix coating of the smooth muscle.
• Neurotransmitters diffuse to the cells, and in multi-unit smooth muscle, neurotransmitter (ACh or NE) causes depolarization of the muscle membrane and contraction without generating an action potential.
• In single-unit (visceral) smooth muscle, action potentials occur similar to those presented in skeletal muscle.

Action Potential in Visceral Smooth Muscle

• Action potentials occur in one of two forms:
  1. Spike potentials: similar to those seen in skeletal muscle, can be elicited by electrical stimulation, hormones, stretch, or spontaneously.
  2. Action potentials with plateau: onset is similar to that of the typical spike potential, but instead of rapid repolarization, the muscle fiber undergoes a delayed repolarization phase.

Calcium Ions and Contraction

• Calcium ions perform two important tasks: generating the action potential and causing contraction.
• The contraction of smooth muscle occurs in 5 steps:
  1. Calcium concentration in the cytosol increases.
  2. Calcium ions bind reversibly with calmodulin.
  3. The Ca-calmodulin complex joins with and activates myosin light chain kinase (MLCK).
  4. MLCK phosphorylates one of the light chains of each myosin head (regulatory chain).
  5. Contraction occurs, depending on extracellular calcium concentration.