Download Unit One: Introduction to Physiology: The Cell and General Physiology

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
a.
b.
c.
d.
e.
•
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)
a. Mass of fibers that contract together as a
unit
Types (cont.)
b. Usually arranged as sheets or bundles
c. 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
a. Dense bodies
b. Side bridges-hinge in opposite directions
c. 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.
b.
c.
d.
e.
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
1. Ca++ bind to calmodulin
2. Ca-calmodulin complex activates myosin
light chain kinase
3. The myosin head becomes phosphorylated
4. 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
a. Autonomic fibers branch extensively on top of
the muscle fibers
b. Form diffuse junctions where neurotransmitter
diffuses through extracellular matrix
c. Axons terminate in varicosities
d. 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 inhibitory
b. Norepinephrine-may be excitatory or inhibitory
c. Response depends on the nature of the receptor
•
Membrane and APs
a. 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
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 reticulum
and extracellular fluids)