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

2. 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

3. 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

4. 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 Multi-unit (left) and
Unitary (right) smooth muscle

5. 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%)

6. Fig. 8.2 Physical structure of smooth muscle

7. 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

8. 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

9. Fig. 8.3

10. 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

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

12. 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

13. 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

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

15. 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

16. 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)