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Muscle Physiology
文章出處
  Feng Chou Tsai
日期&閱覽
 張貼日期:2004/10/21  閱覽次數:7149


Myofilament Structure
Myofilament is the term for the chains of (primarily) actin and myosin that pack a muscle fiber. These are the force generating structures.

Component Molecules

Although there are still gaps in what we know of the structure and functional significance of the myofilament lattice, some of the key proteins include:

Myosin
This peptide is responsible for force generation. It is composed of a globular head with both ATP and actin binding sites, and a long tail involved in its polymerization into myosin filaments. 
 

Figure 1: Three-dimensional crystal structure of the S-1 portion of the myosin heavy chain molecule. (A) High resolution ribbon diagram of chicken S-1 myosin determined using X-ray crystallography. Labeled are the key features of the molecule, the ATP and actin binding sites, the cleft between them, and the a-helical region to which the myosin light chains binds. (Ribbon diagram kindly provided by Dr. Ivan Rayment, University of Wisconsin). (B) Schematic drawing of the myosin S-1 portion based on the ribbon diagram presented in (A) and used to illustrate the cross-bridge cycle.

Actin
The other major component in force production. Actin, when polymerized into filaments, forms the "ladder" along which the myosin filaments "climb" to generate motion.
Troponin
Apparently the major regulator of force production. Its three subunits lie in the groove of each actin filament blocking the myosin binding site, in the absence of ionic calcium.
Titin
An enormous (2500 kD) peptide that appears to be involve in maintaining the neatly ordered striation pattern. Closely associated with the myosin molecule, it appears to anchor the myosin network to the actin network.
Nebulin
Another actin associated molcule, nebulin appears to act as a molecular ruler regulating the length of actin filaments.

Interlocking Mesh Structure

Seen in longitudinal section (with an electron microscope), a myofilament shows several distict bands, each of which has been given a special letter. The lightest (least electron dense) band is known as the I band and consists mostly of actin. The wide, dark band, known as the A band, is composed primarily of myosin. In the center of the I band is an electron dense line, known as the Z-line. In the middle of the A band is another dense line known as the M line.

In cross section, under very high magnification, both A and I bands can be seen to be hexagonal networks. These networks are apparently ordered and fixed at the M- and Z-lines. In the region where the A and I bands overlap (sometimes known as the H band) the two hexagonal networks intermesh so that each myosin filament is surrounded by six actin filaments.

These networks appear to be anchored to (and through) the cell membrane in two ways. At the ends of fibrils, special structures anchor the terminal actin filaments to the membrane. There also appear to be connections between the Z and M lines and the cell membrane.

Muscle Contraction



Energy Supply



@ Sarcopenia:
(1) definition= involuntary loss of skeletal muscle mass
    ---- using DEXA (dual-energy x-ray absorptiometry)

    ---- SMM(skeletal muscle mass) index(kg/m2)
         <2 SD below mean
         or <5.45 for women / <7.26 for men
(2) prevalence=9.5%

@ Soleus muscle-- type I M.

Origin: Posterior aspect of fibular head, upper 1/4 - 1/3 of posterior surface of fibula, middle 1/3 of medial border of tibial shaft, and from posterior surface of a tendinous arch spanning the two sites of bone origin

Insertion
: Eventually unites with the gastrocnemius aponeurosis to form the Achilles tendon, inserting on the middle 1/3 of the posterior calcaneal surface

Action
: Powerful plantar flexor of ankle

Innervation
: Tibial nerve (S1, S2)

Arterial Supply
: Posterior tibial, peroneal, and sural arteries

Accessory soleus muscle. A clinical and radiographic presentation of eleven cases. Romanus-B; Lindahl-S; Stener-B J-Bone-Joint-Surg-Am. 1986 Jun; 68(5): 731-4 When an accessory soleus muscle is present, it consists of a soft-tissue mass bulging medially between the distal part of the tibia and the Achilles tendon. It usually inserts with a separate tendon on the calcaneus anteromedial to the Achilles insertion, and may be a cause of pain on exercise. One may suspect a soft-tissue tumor, such as lipoma, hemangioma, and even sarcoma, but the anomalous muscle has a typical appearance on plain radiographs, and the appearance on computed tomography is diagnostic. If the patient is asymptomatic, no therapy is required, but if pain or other discomfort is provoked by exercise, exploration with fasciotomy or excision of the accessory muscle is recommended, as was done in six of our eleven patients who were seen between 1968 and 1985.

@ Gastrocnemius muscle-- type II M.

Origin
:
Medial head from posterior nonarticular surface of medial femoral condyle;
Lateral head from lateral surface of femoral lateral condyle

Insertion: The two heads unite into a broad aponeurosis which eventually unites with the deep tendon of the soleus to form the Achilles tendon, inserting on the middle 1/3 of the posterior calcaneal surface

Action: Powerful plantar flexor of ankle

Innervation: Tibial nerve (S1, S2)

Arterial Supply: Each head supplied by a sural branch of the popliteal artery

@ Two Major Fiber Types: type I and type II fibers;
    - in lower mammals and other animals, muscles are generally composed entirely type I or type II fibers;
    - human muscle is made up of a mixture of type I and type II fibers;
    - muscle fiber type depends not on any intrinsic feature of the fiber itself but on the motor neuron supplying that particular fiber;
    - all muscle fibers supplied by one particular axon will be of either type I or type II;
- Type I:
    - there are significant differences in the fiber types;
    - type I muscle fibers are rich in the enzymes necessary for oxidative metabolism and are darker in appearance;
    - when stimulated, they have a slow contraction or "twitch" time;
    - these fibers have increased resistance to fatigue;
    - type I fibers are well suited for activities related to physical effort requiring strength and endurance that depend for energy metabolism on oxidative processes;
- Type II Fibers:
    - type II fibers obtain their energy through much faster glycolytic process;
    - as glycogen stores are more rapidly depleted than oxygen supplies, type II fibers are less suited to continuous types of activity and are more suited to rapid alternating effort;
          - those muscles most accustomed to slow, continuous work have a lower percentage of type II muscle fibers;
    - type II fibers may be more prone to anatomic changes following altered energy demands than are type I;
    - type II fibers tend to be smaller than type I in children and in adults who do not carry out strenuous physical exercise, although they increase in size with repeated physical demands on the muscle;
    - type II fibers are subdivided into type IIa & type IIb;
    - type IIa fibers have an admixture of glycolytic and oxidative enzymes and show an intermediate twitch time;
    - type IIb fibers (fast glycolytic fibers) have the largest motor unit size, have the fastest rate of contraction, and are most susecptible to fatigue;
    - muscle injury:
          - type II muscle fibers are more suseptible to injury than type I fibers.
          - other risk factors for injury include muscle that cross two joints and eccentric loading;

@ superficial posterior compartment contians gastrocnemius, soleus, popliteus, & plantaris muscles.
    - sural nerve, short & long saphenous veins are also w/in this compartment, but there are no arterial structures of significance;
    - popliteus functions as a leg flexor, internal rotator of tibia, & initiator of knee flexion;
    - gastrocnemius, crossing both knee and ankle, is a pimary flexor of knee and tibiotalar joints;
    - soleus tendon joins gastrocnemius tendon in distal third of leg to form triceps surae or Achilles tendon;
    - plantaris has no f(x) significance, but may serve as a source for tendon graft donation;

@ Achilles Tendon Rupture
ruptures typically occur after age 30, more often in people beyond middle age;
    - rupture at musculous tendinous junction occurs most often in young people, but ruptures near the calcaneus also occur;
          - inciting event may be related to atrophy of the soleus muscle, and commonly occur in weekend athletes;
          - in 5-33% of patients there will be a prodromal incident of pain several days beforhand;
          - mechanism usually involves eccentric loading on a dorsiflexed ankle with the knee extended (soleus and gastroc on maximal stretch);
    - unlike flexor tendons in the hand, the Achilles tendon has no true synovial sheath, but rather is covered only by a paratenon;
          - hence, exogenous healing (from synovial fluid) would not be expected to occur;
    - inciting causes:
          - consider systemic conditions such as gout or hyperparathyroidism (esp with pure avulsion injury);
          - inquire about previous steroid injections;
                - injections of steroids into or around the Achilles tendon may provoke rupture and should generally be avoided;
          - flouroquinolones may also contribute to tendon rupture;

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