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 isdistinctive 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

a. Composed of discrete separate fibersb. Each fiber operates independentlyc. Often innervated by a single nerved. Contracts independentlye. i.e. ciliary muscle of the eye, iris muscle

of the eye, piloerector muscles

• Unitary smooth muscle (synctial or visceral)

a. Mass of fibers that contract together as a unit

Types (cont.)

b. Usually arranged as sheets or bundlesc. Synctial interconnections and lots of gap

junctionsd. 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 notroponin complex

• Physical Basis

a. Dense bodiesb. Side bridges-hinge in opposite directionsc. 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

a. Slow cycling of the myosin cross-bridges in smb. Low energy requirement to sustain the

contraction in smc. Greater maximum force of contraction in smd. Prolonged “holding” of contraction in sme. Can return to original force of contraction after

being stretched for a long time

Regulation of Contraction

• Calcium ions and calmodulin

1. Ca++ bind to calmodulin2. Ca-calmodulin complex activates myosin

light chain kinase3. The myosin head becomes phosphorylated4. Binding to actin occurs

• Myosin phosphatase splits the phosphate fromthe myosin, cycling stops, and contraction ceases

Fig. 8.3

Nervous and Hormonal Control

• Neuromuscular Junctions: Physiologic Anatomy

a. Autonomic fibers branch extensively on top ofthe muscle fibers

b. Form diffuse junctions where neurotransmitterdiffuses through extracellular matrix

c. Axons terminate in varicositiesd. Neurotransmitter can be AcH or norepinephrine

Control (cont.)

Fig. 8.4 Innervation of Smooth Muscle

Control (cont.)

• Transmitters Secreted

a. AcH-may be either excitatory or inhibitoryb. Norepinephrine-may be excitatory or inhibitoryc. Response depends on the nature of the receptor

• Membrane and APs

a. Membrane-depends on the momentary conditionof 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

c. 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 reticulumand extracellular fluids)