Unit Two: Membrane Physiology, Nerve, and Muscle Chapter 8: Excitation and Contraction of Smooth Muscle Guyton and Hall, Textbook of Medical Physiology, 12th edition

Types of Smooth Muscle Distinctive Structure-smooth muscle of each organ is distinctive from that of other organs Physical dimensions Organization into bundles or sheets Response to different types of stimuli Characteristics of innervation Function

Types (cont.) Multi-unit smooth muscle Composed of discrete separate fibers Each fiber operates independently Often innervated by a single nerve Contracts independently i.e. ciliary muscle of the eye, iris muscle of the eye, piloerector muscles Unitary smooth muscle (synctial or visceral) Mass of fibers that contract together as a unit

b. Usually arranged as sheets or bundles Types (cont.) b. Usually arranged as sheets or bundles Synctial interconnections and lots of gap junctions d. Found in the walls of most of the viscera Fig. 8.1 Multi-unit (left) and Unitary (right) smooth muscle

Contractile Mechanism Chemical Basis-contains actin and myosin but no troponin complex Physical Basis Dense bodies Side bridges-hinge in opposite directions Can contract up to 80% of their length as compared to skeletal muscle (30%)

Fig. 8.2 Physical structure of smooth muscle

Comparison of Smooth and Skeletal Muscle Contraction Slow cycling of the myosin cross-bridges in sm Low energy requirement to sustain the contraction in sm Greater maximum force of contraction in sm Prolonged “holding” of contraction in sm Can return to original force of contraction after being stretched for a long time

Regulation of Contraction Calcium ions and calmodulin Ca++ bind to calmodulin Ca-calmodulin complex activates myosin light chain kinase The myosin head becomes phosphorylated Binding to actin occurs Myosin phosphatase splits the phosphate from the myosin, cycling stops, and contraction ceases

Fig. 8.3

Nervous and Hormonal Control Neuromuscular Junctions: Physiologic Anatomy Autonomic fibers branch extensively on top of the muscle fibers Form diffuse junctions where neurotransmitter diffuses through extracellular matrix Axons terminate in varicosities Neurotransmitter can be AcH or norepinephrine

Control (cont.) Fig. 8.4 Innervation of Smooth Muscle

Control (cont.) Transmitters Secreted AcH-may be either excitatory or inhibitory Norepinephrine-may be excitatory or inhibitory Response depends on the nature of the receptor Membrane and APs Membrane-depends on the momentary condition of the muscle; intracellular potential is usually -50 to -60 mV

Control (cont.) b. APs occur in unitary smooth muscle but not usually in multi-unit Occur in two forms (1) spike potentials or (2) APs with plateaus

Fig. 8.5 A: Typical smooth muscle AP spike potential); B: Repetitive spike potentials; C: Plateau AP

Control (cont.) Calcium channels are important in generating the SM Action Potential Slow wave potentials in unitary SM lead to spontaneous generation of action potentials Excitation of visceral SM by muscle stretch

Effects of Local Tissue Factors SM contraction responds to local tissue factors Circulating hormones may affect contraction Source of calcium ions (sarcoplasmic reticulum and extracellular fluids)