Excitation and contraction of smooth muscle

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Two major smooth muscle types

1) Multi-unit smooth muscle - composed of discrete, separate smooth muscle fibers. Each fiber operates independently of the others and often is innervated by a single nerve ending, as occurs for skeletal muscle fibers
-like skeletal muscle they are covered on their outside by a thin layer of basement membrane-like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another
*most importantly = each FIBER can contract independently of the others, and their control is exerted mainly by nerve signals

2) unitary smooth muscle
-a major share of control is exerted by non-nervous stimuli
-examples (ciliary muscle of eye, the iris, pilo-erector muscles
-also called SYNCYTIAL SMOOTH MUSCLE or VISCERAL SMOOTH MUSCLE
-it is a mass of hundreds to thousands of SMFs that contract together as a SINGLE UNIT
-fibers usually arranged in sheets or bundles
-and their cell membranes are adherent to one another at multiple points (more force transmission
-many GAP JUNCTIONS (for action potentials) to work in "sync"
-found in viscera including GI tract, bile ducts, ureters, uterus, and many BLOOD VESSELS)

Physical basis of SMC

-actin filaments attach to DENSE BODIES (same role as Z disk) (some are attached to cell membranes, others dispersed in the cell, other dispersed inside the cell)
*some of the membrane dense bodies are bonded together by INTERCELLULAR PROTEIN BRIDGES - its is mainly through these bonds that the force of contraction is transmitted from one cell to the next
-Myosin is interspersed (usually 5-10 X more actin)
-myosin have "sidepolar" cross-bridges - for multiple pulling actions simultaneously
-allows SM to contract as much as 80% of their length vs. 30% in skeletal muscle

Differences of SM to skeletal muscle

1) slow cycling of the myosin cross-bridges -for slow tonic contractions

2) low energy requirement to sustain smooth muscle contraction - from slow attachment/detachment cycling

3) slowness of onset of contraction and relaxation of the total smooth muscle tissue - also caused by slow attachment and detachment of myosin/actin

4) Maximum force of contraction is often greater in smooth muscle than in skeletal muscle - results from prolonged period of attachment (tonicity)

5) "Latch" mechanism facilitates prolonged holding of contractions of smooth muscle - once latched; muscle does not need continued excitation or energy to hold on (myosin to actin) *can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy and little excitatory signals from nerve fibers and hormones. Possibly due to deactivation of the enzymes needed to cycle the contractions

6) Stress-Relaxation of smooth muscle - allows for pressures inside organs to regulate itself and remain constant (takes 15 secs to a minute)

Regulation of contraction by calcium ions

-as is true for skeletal muscle, the initiating stimulus for most smooth muscle contraction is an increase in intracellular calcium ions
*this increase can be caused in different types of smooth muscle by nerve stimulation of the smooth muscle fiber, hormonal stimulation, stretch of the fiber, or even change in the chemical environment of the fiber
***Yet smooth muscle does not contain troponin (which is the regulatory protein that is activated by calcium ions) INSTEAD - SMC is activated by:

In place of troponin, smooth muscle cells contain a large amount of another regulatory protein called ____

-calmodulin
*initiates contraction by activating the myosin cross-bridges

1) Calcium binds with calmodulin

2) The calmodulin-calcium complex then joins with and activates MYOSIN LIGHT CHAIN KINASE, a phosphorylating enzyme

3) One of the light chains of each myosin head, called the REGULATORY CHAIN, becomes phosphorylated in response to this myosin kinase. when this chain is not phosphorylated, the attachment-detachment cycling of the mysoin head with the actin filament does not occur. But when the regulatory chain is PHOSPHORYLATED, the head has the capability of binding repetitively with the actin filament and proceeding through the entire cycling process of intermittent "pulls," the same as occurs for skeletal muscle, thus causing muscle contraction

Myosin Phosphatase

-when the calcium ion concentration falls below a critical value, the aforementioned processes automatically reverse except for phosphorylation of the myosin head
**This requires MYOSIN PHOSPHATASE (located in the cytosol of the smooth muscle cell) which splits the phosphate from the regulatory light chain
-the time required for relaxation is ~ the amount of MYOSIN PHOSPHATASE in the cell

Neuromuscular junctions of smooth muscle

-the autonomic nerve fibers branch diffusely on top of a sheet of muscle fibers
-most of the time do not make direct contact with the smooth muscle fiber cell membranes but instead form so called DIFFUSE JUNCTIONS that secrete their transmitter into the matrix coating of the smooth muscle often a few nanometers to a few micrometers away from the muscle cells -->diffuses into the cell
-where there are many layers of muscle cells, the nerve fibers often innervate only the outer layer. muscle excitation travels from this outer layer to the inner layers by action potential conduction in the muscle mass or by additional diffusion of the NT substance

Varicosities - interruptions in the Schwann cells of the axons so that transmitter substances can be secreted through the walls of the varicosities. Varicosities have vesicles that may contain ACETYLCHOLINE or *NOREPINEPHRINE or occasionally some other substances
*acetylcholine and norepinephrine are never secreted by the same nerve fiber

Acetylcholine

-an excitatory NT for smooth muscle fibers in some organs; but an inhibitory NT in other organs
*when acetylcholine excites a muscle fiber, norepinephrine ordinarily inhibits it. opposite is true

***there are inhibitory and excitatory receptors on the surface of muscle cell membranes. Thus the type of receptor determines whether smooth muscle is inhibited or excited and also determines which of the two NTs, is effective in causing either of these two actions

Membrane potential in smooth muscle

resting intracellular = -50 to -60 mV which is about 30 mV less negative than in skeletal muscle

Action potentials with plateaus vs. Spike Potentials

Spike: 10-50 milliseconds

Plateaus: onset is similar to that of typical spike potential BUT repolarization is delayed more
-accounts for prolonged contraction that occurs in some types of smooth muscle, such as URETER, the UTERUS and certain types of vascular smooth muscle

$ Action Potentials occur in unitary smooth muscle (such as visceral muscle) also in cardiac muscle fibers

Smooth muscle Calcium channels

-far more Calcium channels than skeletal muscle but FEW voltage gates sodium channels
-therefore sodium participates little in the generation of the action potential of most smooth muscle
**Calcium inflow = the action potential
-acts DIRECTLY on the smooth muscle contractile mechanism
-Calcium channels open more slowly and remain open longer --> plateau

Slow wave rhythm

-local property of some smooth muscle fibers
-may be due to waxing and waning of the pumping of positive ions
-when they are strong enough they can initiate an action potential
-also called "pacemaker waves"

Visceral smooth muscle stretch

--> spontaneous action potentials
1) the normal slow wave potentials

2) decrease in overall negativity of the membrane potential cause by the stretch itself

Multi-Unit smooth muscle

-Action Potentials usually do not develop
-the fibers are too small to generate an action potential
-the junctional potential caused by the NT itself spreads "electronically" over the entire fiber and is all that is necessary to cause muscle contraction

Local tissue chemical factors

1) lack of oxygen

2) excess CO2

3) increased hydrogen concentration

Adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temp. all can cause vasodilation

Hormones and SMC

-norepniephrine, epnipehrine, acetylcholine, angiotensin, endothelin, vasopressin, oxytocin, serotonin, histamine

-A hormone causes contraction of smooth muscle when the muscle cell membrane contains hormone-gated excitatory receptors

-causes inhibition if the membrane contains inhibitory receptors for the hormone (closing of the sodium and calcium channels; also the normally close potassium channels are opened --> hyperpolarization)
-Inhibition w/out affecting membrane potential: hormones bind to receptors that are couple to adenylate cyclase and guanylate cyclase -->cAMP and cGMP which change the degree of phosphorylation of several enzymes that indirectly inhibit contraction (pump that moves calcium for sarcoplasm into the sarcoplasmic reticulum, etc)

Calcium source for SMC

-the sarcoplasmic reticulum is much less developed in smooth muscle compared to skeletal muscle
-most of the calcium comes from the EXTRACELLULAR FLUID (where Ca [] is high) --> rapid diffusion when channels open at time of contraction
*Time required for calcium diffusion = LATENT PERIOD (50X as great for smooth muscle)

$$ Smooth muscle contraction is dependent on EXTRACELLULAR CALCIUM ION CONCENTRATION (effects force of contraction on smooth muscle drastically/ not for skeletal muscle)

Caveolae

-small invaginations of the cell membrane that abut the surfaces the Sarcoplasmic reticulum tubules

Calcium Pump

**Required to cause relaxation of smooth muscle
-calcium ions must be removed from the intracellular fluids
-pumps them into the ECF or Sarcoplasmic reticulum (if present)
-pump is slow acting compared with the fast acting SARCOPLASMIC RETICULUM PUMP of SKELETAL MUSCLE
-therefore a single smooth muscle contraction usually lasts SECONDS compared with hundredths to tenths of a second

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