Lecture 9: Smooth Muscle Flashcards
Characteristics of smooth muscle vs. other types
Smooth muscle:
-Has no striations
-Is innervated by the ANS (involuntary)
-Also uses sliding filaments, Ca++, and cross bridges
-Filaments are organized differently
-Excitation-contraction coupling is different
Characteristics of a smooth muscle cell
-Spindle shaped (much smaller vs sk. muscle fiber)
-Interconnected in sheet-like layers
-Mononucleated (maintain mitotic ability)
-No troponin (unknown tropomyosin function)
-Caldesmon on thin-f’s to regulate contraction(?)
-Myosin thick-f and actin thin-f
Where do smooth muscle thin-f’s anchor?
Either to the plasma membrane or to cytoplasmic dense bodies (Z-line analog)
How are smooth muscle filaments arranged? How does this shape contraction?
Thin-f’s are organized diagonal to the long axis of the cell; during contraction, plasma membrane between anchor points balloons out
Smooth muscle isometric tension vs length
Smooth muscle has an optimal length but can generate significant force at a broader range of lengths, retaining tension while vessels/bodies change volume
How is smooth muscle contraction activated/regulated?
Myosin light chain kinase vs myosin light chain phosphatase activity.
Ca++ binds calmodulin to activate MLCK; myosin-Pi moves away from thick-f and can bind to actin
How does smooth muscle myosin compare to skeletal muscle?
Smooth muscle has much less myosin/actin and much slower myosin ATPase
Smooth muscle force generation vs skeletal muscle
Smooth muscle has similar max tension per cross-sectional area but shortens much slower (more economical, less ATP per time) and does not fatigue
Latch state
Smooth muscle can maintain tension without much cross-bridge activity for long periods of time (v. low ATP use); X-bridge actin release just occurs very slowly. Happens w/ persistent stimulation/elevated Ca++
Sources of smooth muscle Ca++
- Sarcoplasmic reticulum
- Extracellular Ca++ via membrane channels
Smooth muscle sarcoplasmic reticulum
Less present, no t-tubules. Responds to APs at associations near plasma membrane or to secondary messengers from EC chemical messenger binding
How are smooth muscle APs mediated?
Voltage-gated Ca++ channels, not Na+
Characteristics of smooth muscle cytosolic Ca++ removal
Pumped into SR or out of cell; occurs much slower so that 1 twitch may last several seconds
Smooth muscle tension grading
Tension in smooth muscle can be graded by varying [Ca++], whereas in sk. muscle 1 AP = full troponin saturation
Smooth muscle tone
When Ca++ is sufficient to maintain low-level but super-basal X-bridge/MLCK activity
Pacemaker potentials
Some smooth muscle cells spontaneously generate APs without input, producing a rhythmic state of contractile activity
How are pacemaker potentials generated?
- Gradual depolarization to threshold, and again after repolarization
- Slow waves, where regular variation in ion flux makes membrane potential drift up and down. A superimposed ion flux then reaches threshold.
Varicosities
Swellings on axon branches of post-ganglionic autonomic nerves. 1 axon will have multiple varicosities for multiple cells
Dual innervation of smooth muscle cells
1 smooth muscle cell might be near varicosities from multiple neurons, sympathetic and parasympathetic
Local factors
Typically lead to smooth muscle relaxation. Allows local responses without long-distance signals. Includes:
-Paracrine signals
-pH
-pO2/pCO2
-Osmolarity
-Ionic comp. of ECF
e.g. nitric oxide
Stretch responses
Stretch-gated ion channels open and are amplified by nearby voltage-gated channels, producing an opposing contraction. Stretch response is intrinsic and common to almost all muscle.
Single-unit smooth muscle
Whole muscle tissue responds as a single unit; cells undergo synchronous electro-mechanical activity
Features of single-unit smooth muscle
-Cells linked by gap junctions
-Often pacemaker cells connected to non-pacemakers
-Entire muscle can be controlled by innervating/regulating pacemaker AP frequency; fewer autonomic axons
-Influenced by nerves, hormones, local factors
E.g. GI tract, uterus
Multi-unit smooth muscle
Muscle tissue acts as multiple units; each cell responds independently
Features of multi-unit smooth muscle
-No gap junctions
-Richly innervated by autonomic branches
-Tissue contraction response depends on frequency of stimulation and number of activated cells
-Often no APs, no stretch response
-Influenced by circulating hormones
E.g. large airways/arteries, piloerectors
Smooth muscle filaments
Thin-f’s are very long for unrestricted thick-f sliding and large % length changes
Thick filaments are sidepolar (sarcomeres have bipolar); myosin head orientation is always correct
