Muscle Physiology
Lecture 4: Smooth and cardiac muscle
Allied
Allied Health
Health Science
Science Physiology
Physiology
Dr.
Dr. Daniel
Daniel Ulrich
Ulrich
Trinity
Trinity College
College Dublin
Dublin
Lecture Outline:
I.
II.
Smooth muscle
–
Smooth muscle anatomy
–
Classification
–
Excitation-contraction coupling
–
Slow-wave and pacemaker potentials
–
Neural and non-neural control
Cardiac muscle
–
Similarities with smooth and skeletal muscle
2
Properties of Smooth Muscle
•
•
•
•
•
Found in internal organs, blood
vessels
Under involuntary control by
autonomic nervous system
Spindle-shaped
Small, approximately 1/10 skeletal
Contains actin and myosin
– No sarcomeres (no striations)
– Higher actin:myosin ratio
– Actin and myosin much longer
– Myosin heads over entire length
– Arranged diagonally
(contraction along different
axes)
3
Smooth Muscle Cell
-One nucleus
-Tropomyosin
-No troponin
-Dense bodies
(analogous to Z
line)
-Slow myosin
ATPase
-Myosin has light
chains
-Little sarcoplasmic
reticulum
Figure 12.33
4
Classification of Smooth Muscle:
a) Single-Unit Muscle
Figure 12.35b
•Most common type
•Location
–Intestinal tract
–Uterus
•Muscle fibers activated synchronously
–Fibers connected by gap junctions
–Contract together as a single unit
5
Properties of Single-Unit Smooth Muscle
• Gap junctions
• Pacemaker cells
with spontaneous
depolarizations
• Innervation to few
cells (few neurones)
• Tone = level of
contraction without
stimulation
• Graded Contractions
– No recruitment
– Vary by intracellular
calcium
• Stretch Reflex
– Relaxation in
response to sudden
or prolonged stretch
• Increases/decreases
in tension
6
b) Multi-Unit Muscle
•
•
•
Located in large airways and arteries, eye (ciliary muscle and
iris)
Few if any gap junctions
Each fiber acts individually
– Receives own innervation
– No tone
– Recruitment
Figure 12.35a
7
Spontaneous Depolarizations
•
Single-unit smooth muscle
•
Pacemaker potentials
•
–
Spontaneous
depolarizations to
threshold
–
Due to permeability
changes in Ca++ and Na+
–
Influenced by neural
activity but can occur in
absence of neural
activation
Slow-wave potentials
–
Cycles in resting potential
(depol/hyperpol)
Figure 12.36
8
Excitation-Contraction Coupling
Ca2+
Endoplasmic
reticulum
Ca2+
Ca2+
Calmodulin
Ca-calmodulin
MLCK
Unphosphorylated
myosin light
chain
Phosphorylated
myosin light
chain
No myosin
ATPase activity
Myosin ATPase
active
No crossbridge
activity
Crossbridge
cycling
Smooth muscle cell
Contraction
Figure 12.34
9
Steps of Excitation-Contraction Coupling
1.
2.
3.
Opening of calcium channels in plasma membrane
– Voltage
– Receptor
– Mechanically-gated
Calcium triggers release of calcium from sarcoplasmic reticulum
Calcium binds to calmodulin
4.
Ca++-Calmodulin activates MLCK enzyme (Myosin Light Chain
Kinase)
5.
MLCK phosphorylates myosin
6.
Crossbridge cycling
- In skeletal m: Ca++ targets actin (troponin/tropomyosin system)
- In smooth m: Ca++ targets myosin
10
Relaxation of Smooth Muscle
•
Phosphatase enzyme removes phosphate from myosin
•
Calcium removed from cytoplasm
– Ca++ -ATPase
– Ca++ - Na+ exchanger (antiport)
Phosphatase continuously active and compete with MLKC.
Thus, high Ca++ concentration needed to activate MLKC
11
Regulation of Myosin Light Chain
Unphosphorylated
myosin light chain
MLCK
Phosphorylated
myosin light chain
phosphatase
ATPase activity
No ATPase activity
No Contraction
Myosin head, the light chains
are in yellow and orange
Contraction
Myosin ATPase 10–100 times slower in smooth
muscle compared to skeletal
12
Neural Regulation of Smooth Muscle Contraction
•
Innervated by autonomic nervous system
– Sympathetic and/or parasympathetic
•
May be excitatory or inhibitory
•
Target cell response depends on receptor type
•
Neurotransmitter released from varicosities
•
Gap junctions - allow transmission of electrical
signal from one cell to neighboring cells
13
Non-Neural Regulation of Contraction
•
Some smooth muscle able to actively exert
tension in absence of external stimulus:
Intracellular Ca++ levels high enough to maintain
constant level of crossbridge activity: TONE
•
Sometimes smooth muscle able to contract by
hormonal / other chemical stimulation
•
Sometimes smooth muscle able to contract by
chemical stress
14
Cardiac Muscle
• Similarities with smooth muscle
• Similarities with skeletal muscle
• Cardiac action potentials
15
Cardiac Muscle: Similarities with Skeletal Muscle
•
Striated
– Sarcomeres
•
Troponin AND tropomyosin regulation
•
T tubules
•
Sarcoplasmic reticulum, but not as well developed
•
Similar to slow oxidative fibers
– Myoglobin
– Mitochondria
– Slow to fatigue
16
Cardiac Muscle: Similarities with Smooth Muscle
• Gap junctions (within intercalated disks)
• Pacemaker cells
• Innervated by autonomic nervous systems
• Influenced by hormones, paracrines
• Calcium comes from extracellular fluid and
sarcoplasmic reticulum
17
No summation in Cardiac Muscle
Figure 12.37
•
•
•
Action potential lasts almost as long as tension
No summation due to long refractory period
Positive: summation would not allow the heart to relax after each
beat to fill up with blood
18
Muscle Comparisons
Table 12.2
19
Muscle Tissue Types
Figure 12.32
20