Download Smooth Muscle Contraction

Physiology 121, Lecture 8
Smooth Muscle Contraction (Dr. Donald B. Thomason)
Objectives:
The student should be able to:
1. Match the phases of shortening (isometric and isotonic) with the tension developed during these
phases.
2. State the difference between syncytial and multi-unit smooth muscle.
3. State the electrical conditions that modulate smooth muscle contraction.
4. Name the type of innervation characteristic of smooth muscle.
5. List the key proteins that regulate smooth muscle contraction.
6. List the two ways to produce more force by smooth muscle.
Outline:
I. The Diversity of Smooth Muscle Function
Diversity of Smooth Muscle Function
Examples:
regulation of vascular resistance and blood flow
propulsion and compartmentalization of the
digestive system
pilomotor function
optics of vision
II. Organization of Smooth Muscle
Smooth Muscle Cell Types
A. Smooth muscle cells are primarily mononuclear
Syncytial or Unitary
Autonomic Nerve
B. Smooth muscles are mechanically and electrically diverse
Autonomic Nerve
Multi-unit
1. Syncytial or “single-unit”
smooth muscle
2. Multi-unit smooth muscle
C Smooth muscle cells do not
normally exhibit well-structured
sarcomeres
Muscle Fiber Organization
Striated
Muscle
Smooth
Muscle
III. Contractile Activity and its Regulation
A. The action potential and ion fluxes in smooth muscle
1. Membrane conductances are different from skeletal muscle
2. Calcium currents can play an important role
IK
membrane current
membrane potential (mV)
Smooth Muscle Action Potential
0
-50
0
100
200
I Na
100
200
time (msec)
IK
membrane current
membrane potential (mV)
Smooth Muscle Action Potential
0
-50
0
I Na
100
200
time (msec)
I Ca
3. An action potential need not occur for contraction
tension
potential (mV)
potential (mV)
tension
Smooth Muscle Contraction
0
-50
0
1
2
time (sec)
0
-50
0
1
2
time (sec)
4. Pacemaker activity causes spontaneous contraction
potential (mV)
tension
Smooth Muscle Pacemaker Activity
0
-50
0
1
2
time (sec)
3
B. Nervous control of smooth muscle contraction
1. Innervation is exclusively autonomic
2. Modulation of contractile activation by nerves
a. excitatory effects
b. inhibitory effects
Neurotransmitter Effects
Substance
ACh
Effect
Excitatory
Mechanism
depolarization
g
Inhibitory
Example
Na
g Ca
hyperpolarization
gK
NE
Excitatory
depolarization
g
Inhibitory
Ca
g Na
gK
increased
peristalsis
coronary
vasodilation
vasoconstriction
g Cl
hyperpolarization
g
pacemaker
K
E-C coupling
decreased
peristalsis
(human)
C. Non-neural control of smooth muscle contraction
1. Local and distant activation
2. Receptor-mediated and non-receptor mediated mechanisms
Local and Distant Activation
Hormones
e.g., cholesystokinin, estrogen,
PDGF, oxytocin
Metabolites
e.g., H+, CO2 ,
adenosine
IV. Mechanism of Contraction of Smooth Muscle
A. Calcium and the permissive effect of myosin light chain-phosphorylation
1. Calcium-calmodulin interaction
2. Myosin light-chain kinase
3. Cross-bridge cycling
Smooth Muscle Contractile Mechanism
Ca
intracellular
stores
Ca
2+
2+
receptor-activated
potential sensitive
+ CM
CM-Ca
2+
+ MLCK
actin + myosin -P
contraction
myosin + ATP
phosphatase
Pi
B. The central role of calcium and its various sources
α-Adrenoreceptor-mediated
Vasoconstriction
Ca
2+
Ca
α2
2+
α1
G
G
Ca
2+
G
Ca
inhibitors:
nifedipine
pertussis toxin
PLC
2+
IP3
C. Phosphatase activity and the cessation of cross-bridge cycling
Factors Which Regulate Smooth
Muscle Contraction
Velocity : Activation of MLCK/phosphatase
Magnitude :
1. frequency of stimulation
2. number of cells stimulated
PIP2
D. The latch mechanism
E. The stress-relaxation mechanism
Stretch-Relaxation Phenomenon
Length
Force
time
Length
Force