Skeletal Muscle Physiology


     Skeletal muscle is an example of muscle tissue, one of the four types of basic tissue. The essential characteristic of muscle tissue is that it shortens or contracts. There are three kinds of muscle tissue, skeletal, cardiac and smooth. In the next two lectures we will focus only on skeletal muscle physiology and anatomy.
Characteristics 
     Skeletal muscle is striated and voluntary. The word striated means “striped” and the significance of this term will become apparent when we consider the histology below. Skeletal muscle is the only type of muscle that we can consciously control through our nervous system. This is the reason it is also voluntary. 
     Skeletal muscle cells are also long and cylindrical. For this reason a skeletal muscle cell can also be referred to as a skeletal muscle fiber. A skeletal muscle fiber can be up to a foot long! These long, highly specialized cells result from the fusion of many cells and after the cells fuse their individual nuclei are retained. As a result, skeletal muscle fibers are multinucleate.
Connective Tissue Components
     Skeletal muscles as organs consist of muscle fibers bound by connective tissue. The connective tissue also attaches skeletal muscle to the skeleton and other tissues and transmits the force of a contraction to the moving part.
     Connective tissue binds skeletal muscle fibers in a hierarchical pattern. Individual muscle cell fibers are surrounded by delicate connective tissue called endomysium. Skeletal muscle fibers are aligned in bundles called fascicles and these fascicles are in turn surrounded by a stronger sheath of connective tissue called the perimysium. The fascicles are finally packaged in yet a stronger connective tissue encasement called the epimysium.
     At the attachment points of the muscle all the connective tissue elements combine to form the connective tissue attachment of the muscle to bone or other tissue. If this attachment is round and cord-like it is called a tendon. If the attachment is broad and sheet-like it is called an aponeurosis. 
Microscopic Anatomy 
     The plasma membrane of the skeletal muscle fiber is called a sarcolemma. The muscle fiber contains long cylindrical structures, the myofibrils. 
The myofibrils almost entirely fill the cell and push the nuclei to the outer edges of the cell under the sarcolemma. The many myofibrils each have light and dark bands and are aligned with one another so that the light and dark bands are next to one another. This gives the cell its striated appearance.
     The light bands are called I bands and the dark bands are called A bands. In the middle of the I bands there is a line called the Z line (or disc). In the middle of the A bands (or dark bands) there is a light zone called the H zone. In the middle of the H zone there is another line, the M line. The precise arrangement of these features is due to a chain of functional units in the myofibrils, sarcomeres.
     The sarcomere consists of a number of individual protein elements. Some of these proteins are thread-like proteins called myofilaments. There are two major types of myofilaments:
1. Thick (myosin) myofilaments
  Thick myofilaments are made up of proteins molecules called myosin. The myosin molecules are shaped like golf clubs with long shafts. Myosin forms the thick myofilaments by forming bundles in which the heads of the “golf clubs” stick out at either end of the filament and the shafts form a “bare” zone in the middle of the filaments.
  The heads of the thick myofilaments form attachments with the other type of myofilaments, the thin actin myofilaments. These attachments are called cross bridges. The heads are also the places on the thick myofilaments that use the energy in the ATP molecule to power the muscle contraction.
2. Thin (actin) myofilaments
  The thin myofilaments are composed of the protein actin. The thin myofilaments have the binding sites to which the heads of the thick myofilaments attach.
actin polymerization
Banding Pattern and Sarcomere
     Now we can relate the banding pattern and sarcomere to the myofilaments:
SarcomereAn individual sarcomere extends from one Z line to the next.
I band – The I band corresponds to a region that overlaps two adjacent sarcomeres where there are only thin myofilaments.
Z line or disc – The Z line in the center of the I band is where proteins hold the thin myofilaments in position.  
A band – The A band is where the thick myofilaments are positioned.
H zone – The H zone is the region in the middle of the sarcomere where the thin myofilaments fail to overlap the thick myofilaments.
M line – The M line in the center of the sarcomere and A band is where proteins hold the thick myofilaments in position.
Sliding Filament Theory Tutorial
     The skeletal muscle fibers contract when the sarcomere in the myofibrils contract. The contraction of the sarcomeres is explained by the sliding filament theory. (The word “theory” is used here in its scientific sense as meaning generally accepted laws and principles, as in the theory of evolution.)
     According to the sliding filament theory, the myosin heads become energized by using the energy contained in ATP. The energized myosin head then attaches to a binding site on the actin myofilaments to form a cross bridge. The energy contained in the myosin head is then released as the head swivels toward the middle of the sarcomere pulling the attached actin myofilaments with it. The cross bridge detaches only when another molecule of ATP attaches to the myosin head. The energy in the ATP is then used again to energize the myosin head.
     This cycle by which the myosin heads become energized, form an attachment, swivel and then detach is repeated many times in all the sarcomeres of all the myofibrils within the cell. The net effect of all this molecular movement is muscle contraction!
Role of Calcium
     The calcium ion (Ca++) plays a key role in determining when contraction occurs
     Ca++ is concentrated in smooth endoplasmic reticulum called sarcoplasmic reticulum which surrounds the myofibrils like the sleeve of a very loose knit sweater might surround your arm. When a nerve impulse arrives at the muscle cell, the impulse to contract spreads throughout the skeletal muscle cell and causes channels in the membrane of the sarcoplasmic reticulum to open. This causes the Ca++ to rush out of the sarcoplasmic reticulum down its concentration gradient.
     Ca++ attaches to a protein called tropomyosin that covers the attachment site on the actin myofilaments. This causes the tropomyosin to uncover the attachment site which permits the myosin head to bind to the attachment site and begin the cycle described above. As long as the Ca++ concentration remains high cycling, or contraction, continues.
Skeletal Muscle Contraction
     Contraction of a skeletal muscle as a whole depends upon the contraction of individual skeletal muscle cells. An individual skeletal muscle cell will either contract or not contract if it is stimulated. This is referred to as the “all or none” response. However, because each muscle consists of a number of individual muscle cells, the contraction of whole muscles can vary.  
     The different degree of contraction that can occur in a whole muscle results in graded responses to different degrees of stimuli. Graded responses are achieved in two ways:
1) Changing the frequency of stimulation;
2) Changing the number of muscle cells stimulated to contract.
Tetanus
     Contractions of skeletal muscles result from the impulses delivered to them by nerves. The impulses normally are normally delivered at a high frequency and results in the phenomenon called tetanus.
     If only a single impulse or stimulus is delivered to a muscle a contraction occurs and is quickly followed by relaxation of the muscle. This is called a muscle twitch. If many impulses or stimuli are delivered to the muscle the muscle contracts but does not have time to relax before it contracts again. This is called tetanus. If the frequency of stimulation permits the muscle to relax to an even slight degree between contractions, the tetanus is unfused or incomplete. If the frequency is so high that relaxation does not occur during contraction, the tetanus is fused or complete (see Fig. 6.9).
     Our ability to produce smooth and sustained movements when we use our muscles in the result of tetanus.