Term
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Definition
-Composed of discrete smooth muscle fibers.
-Innervated by individual nerves that are insulated (like skeletal muscle).
-Higher degree of control, activated by nerve signals.
-Examples include: ciliary muscle of eye, iris of eye, piloerector. muscles
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Term
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Definition
-Large conglomerate of smooth muscle
fibers
-Aggregated into sheets or bundles.
-Fibers are connected at multiple points.
-Have cytoplasmic connections called gap junctions.
-Also known as syncytial smooth muscle.
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Term
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Definition
-Contains both actin and myosin.
-Ratio: ~ 15 actin: 1 myosin filament
-Contractile process initiated by calcium
-ATP is required.
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Term
Organization of Smooth Muscle |
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Definition
-Not as organized as skeletal muscle.
-Z disk replaced by dense bodies.
-Much reduced sarcoplasmic reticulum.
-Ca2+ comes predominantly from outside the cell.
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Term
Termination of Contraction |
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Definition
-Ca2+ channels close when stimulus ceases.
-Cytosolic Ca2+ levels fall below “critical level”.
-Myosin light chain is dephosphorylated by myosin phosphatase.
-Cross-bridge cycling stops. and contraction ceasesa
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Term
Characteristics of
Smooth Muscle Contraction |
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Definition
-Slow cycling of cross bridges.
-Uses much less energy to sustain contraction.
-Much slower onset of contraction and relaxation.
-Maximum force of contraction greater than skeletal muscle.
-Greater degree of shortening.
-Ability to “latch” at full contraction.
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Term
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Definition
-Once contracted, muscle can sustain contraction with much less energy.
-Uses 1/300 the energy of skeletal muscle to sustain a contraction.
-Allows for tonic contraction of such things as blood vessels without using large amounts of energy or requiring continuous stimulus input.
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Term
Control of Muscle Contraction |
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Definition
-Neural and hormonal mechanisms
-Membranes contain many different receptor types.
-Also have receptors which can inhibit contraction (not found in skeletal muscle).
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Term
Neural Control Mechanisms |
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Definition
-No discrete neuromuscular junctions.
-Nerves terminate over the muscle fibers.
-Form diffuse junctions ® secrete their transmitters into fluid surrounding fibers.
-Nerve fibers only usually innervate outer layers ® Gap junctions transmit the signal.
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Term
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Definition
-Autonomic nerves: Acetylcholine and Norepinephrine
-Bind to receptors on muscle cell membrane.
-Never released by the same nerve.
-Action varies with organ type
–Rule: If Ach excites, NE inhibits and vice versa for a given smooth muscle.
–Ultimate action of Ach and NE depends on type of receptor present.
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Term
Membrane Potentials
and Action Potentials |
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Definition
-Resting smooth muscle cell = -50 mV
-Action potentials occur in unitary smooth muscle just as they did in skeletal muscle.
Two types:
1. Spike potential
2. Action potentials with plateaus
* Most multiunit smooth muscles do not have action potentials.
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Term
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Definition
-Duration = 10 - 50 milliseconds
-Elicited by electrical stimulation, hormonal actions, neuro-transmitters, stretch, or spontaneous generation (ion leak).
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Term
Action Potentials with Plateaus |
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Definition
-Onset similar to normal action potential.
-Very slow repolarization of muscle cell membrane.
-Found in muscles requiring extended periods of contraction.
-Found in the smooth muscle of ureter, uterus, and vascular smooth muscle (also in cardiac muscle).
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Term
Calcium in the Action Potential |
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Definition
-Ca2+ channels far exceed Na+ channels in smooth muscle.
-Flow of Ca2+ responsible for AP generation.
-Difference: Ca2+ channels are much slower opening.
-Couples action potential generation directly with the contraction process.
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Term
Slow Waves in
Unitary Smooth Muscle |
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Definition
-Self excitatory
-No extrinsic stimulus needed.
-Source of slow waves unknown
-Possibilities:
1. Changes in pumping of Na+
2. Ion conductance changes
3. Leak of ions (either Na+ or Ca2+)
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Term
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Definition
-Caused by combination of normal slow waves and a progressive increase in membrane potential (less negative) caused by stretch.
-Found in stomach and intestine.
-Prevents too much stretch and often sets up peristalic waves that push contents.
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Term
Effects of Local Tissue Factors |
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Definition
-1/2 of smooth muscle contraction is initiated by hormones and local tissue factors
-Local tissue factors include:
1. Oxygen
2. Carbon dioxide
3. Acidity
4. Lactic acid
5. Prostanoids
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Term
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Definition
-Include norepinephrine, epinephrine, acetylcholine, angiotensin, etc.
-Bind to specific receptors, either excitatory or inhibitory.
-Operate usually by altering ion conductance.
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Term
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Definition
-Almost all Ca2+ used in contraction comes from outside (are exceptions).
-Some smooth muscle contains a useable sarcoplasmic reticulum.
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