Contraction and Excitation of Smooth Muscle

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CONTRACTION OF SMOOTH MUSCLE
The same principles as with Skeletal muscle, actin and myosin. The internal physical structure is different.

TYPES OF SMOOTH MUSCLE: 
MULTI-UNIT SMOOTH MUSCLE
Each fiber can contract independently of the others and that the control is exerted mainly by nerve signals. Smooth muscle fibers of the ciliary muscle of the eye, the iris of the eye.
  
SINGLE UNIT SMOOTH MUSCLE 
Unitary smooth muscle, syncytial smooth muscle and viseral smooth muscle. Millions of fibers contract together as a single unit. The cell membranes are joined together by gap junctions so that action potentials can travel from fiber can to the next and cause muscle fibers to contract together. Walls of the gut, bile ducts, ureters, uterus, and blood vessels.


Physical Basis for Smooth Muscle Contraction
Smooth muscle does not have the same stritated arrangement of actin and myosin filaments as that found in skeletal muscle.
Actin filaments attach to dense bodies.  Some of the dense bodies are dispensed inside the cell and held in place by a scaffold of structural protein lining one dense body to another. Others attach to the cell membrane, and form bonds with dense bodies of adjacent cells, 
Myosin Filaments are interspersed amongst the actin filaments. The myosin filaments have a diameter twice that of actin filaments
Contractile units The individual contractile units consist of actin filaments radiating from two dense bodies; these filaments overlap a single myosin filament that is located midway between the dense bodies.


Comparison of Smooth Muscle contraction with Skeletal Muscle contraction: 
Unlike skeletal muscle contractions, most smooth muscle contractions are prolonged tonic ones that sometimes last for hours or even days. 
Slow cycling of the cross-bridges. The rate of cross-bridge cycling is much more slower in smooth muscle.
Low Energy requirement. Only 1/10 to 1/300 of the energy required.
Slow Onset of contraction and relaxation. Contracts later but lasts longer (1 to 3 seconds)
Increase maximum force of contraction. 


Smooth muscle can shorten a far greater percentage of its length than can skeletal muscle. The smooth muscle in the Gut, bladder, and blood vessels can change their lumen diameters from very large to almost zero.

The "latch mechanism" facilitates prolonged holding contractions. Once the smooth muscle has developed full contraction, the degree of activation of the muscle can usually be reduced to far less than the initial level, yet the muscle will maintain its full force of contraction. This is called the "latch mechanism." The importance of the latch mechanism is that it can maintain prolonged tonic contractions in smooth muscle for hours with little energy use.

Actin and Myosin are arranged in a Mashwork

Smooth muscle is of two types: 
          a. Single-unit (myogenic) 
          b. Multi-unit (neurogenic)





Single-unit smooth muscle has rhythmic pacemaker cells






Very slow and sustained contraction




Regulation of Contraction by calcium ions. 


Calcium ions combine with calmodulin to cause activation of myosin kinase and phosphophorylation of the myosin head.
Smooth muscle does not contain troponin but instead contains calmodulin, another regulatory protein. Although this protein reacts with calcium ions, it is different from troponin in the manner it initiates the contraction. calmodulin does this by activating the myosin cross bridges.


  1. The calcium ions bind with calmodulin; the calmodulin-calcium combination then joins with and activates myosin kinase, a phosphoralating enzyme.
    2. 

  2. Once the light chains of each light head, called the regulatory chain, becomes phosphoralated in response to the myosin kinase.
    3. 

  3. When the regulatory chain is phosphoralated, teh head has the capability of binding to the actin filament, thus causing muscle contraction. When the chain is not phosphoralated, the attachment-detachment cycling of the head with the actin filament does not occur.
    4. 

Myosin phosphatase is important for cessation of contraction. When the calcium ion concentration falls below a critical level, the contraction stops except for the phosphoralation of the myosin head - which requires myosin phophatase to split the phosphate from the regulatory head.
  1. Role of Ca++ in smooth muscle contraction
    2. 



Neural and hormonal control of smooth muscle contraction


Neuromuscular junctions of smooth muscle. These are not highly structured like in skeletal muscle.
Diffuse junctions: The autonomic nerve fibers secrete their transmitter substance in the matrix coating of the smooth muscle, the transmitter substance then diffuses to the cell. 
Varicosities on the axons:  The fine terminal axons have varicosities filled with neurotransmitter substance.
Contact junctions: In the multi-unit type smooth muscle vericosities lie directly on the muscle fiber membrane. 


Acetylcholine and norepinephrine can have excitatory or inhibitory effects at smooth muscle neuromuscular junction. These transmitter substances are secreted by the autonomic nerves innervating smooth muscle, but they are never secreted by the same nerve fibers. When Acetylcholine excites smooth muscle, norepinepharine ordinarily inhibits it, and vice versa.
  1. 

    2. 

Membrane potentials and action potentials in smooth muscle.
The resting membrane potential is -50 to -60 mV (-90 mV Skeletal Muscle)


Action potentials occur in single unit smooth muscle, such as viseral muscle, in the same way they occur in skeletal muscle. Spike potentials: Occur in most single unit smooth muscle. Pacemaker activity.
Action potentials with plateus. This accounts for the prolonged period of contraction that occur in the ureter, the uterus and some vascular smooth muscle.


Calcium ions are important for the generating of smooth muscle action potential. 


Slow wave potentials in single-unit smooth muscle cause the spontaneous generation of action potentials. 
Causes of slow waves: 
  1. oscillations of the  sodium pump activity, which cause the membrane potential to become more negative when sodium is pumped rapidly and less negative when sodium is pumped slowly, and 
    2. 

  2. the conductances of ion channels, which may increase and decrease rhythmically.
    3. 

Importance of slow waves. Action potentials can be initiated when potential of slow wave rises above threshold (about- 35 mV). The action potential spreads over the muscle mass, and contraction occurs.


Spontaneous action potentials are often generated when viceral (single-unit) smooth muscle is stretched. Spontaneous action potentials result from a combination of the normal slow wave potentials and a decrease in the negativity of the membrane potential caused by the stretch itself. This response to stretch  allows the gut wall when excessively stretched, to contract automatically.


SMOOTH MUSCLE CONTRACTION WITHOUT ACTION POTENTIALS - EFFECT OF LOCAL TISSUE FACTORS AND HORMONES


Smooth muscle relaxation in blood vessels occurs in response to local tissue factors.  The vasodilatory response in important for loval blood flow.


Most circulating hormones effect smooth muscle contraction to some degree. 


Source of Calcium Ions that cause contraction (1) Through the Cell Membrane and (2) from the sarcoplasmic Retineculum.
Most of the calcum ions that cause smooth muscl contraction enter the muscle from extracellular fluid. The sarcoplasmic reticulum is rudimentary in most smooth muscle. The contraction s highly dependent on the extracellular fluid calcium.


Calcium pumps remove calcium ions from the intracellular fluid and thereby terminate concentration.  Calcium is removed by calcium pumps that pump the calcium out of the smooth muscle or into the sarcoplasmic reticulum. 
  


B. Cardiac muscle 







     1. Myogenic tissue with pacemaker cells 
          a. Refractory period (250 msec) 
          b. No tetany
     2. Entire muscle organ obeys the "all-or-none" law
     3. Intercalated disks 
     4. Molecular mechanism of contraction is similar to skeletal muscle

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