5.2: Microscopic Anatomy of Skeletal Muscle Fibers
- Page ID
- 99990
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
- Describe the structural organization of skeletal muscle, from myofiber to myofibril to sarcomere to myofilaments.
- Define and briefly explain the function of sarcoplasm, sarcolemma, sarcoplasmic reticulum, and transverse tubules.
Myofiber Structure
Myocytes, or in the case of skeletal muscle often called muscle fibers or myofibers, are the cells that form the bulk of muscle tissue. These cells are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes in general contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation of terminology which differentiates them from generic animal cells. Specific terms are:
- Generic muscle cell = Myocyte
- Skeletal muscle cell = Myofiber
- Cytoplasm = Sarcoplasm
- Cell membrane = Sarcolemma
- Smooth endoplasmic reticulum = Sarcoplasmic reticulum
Myofibers can be very large, with diameters up to 100 micrometers and lengths exceeding 40 centimeters. Remember from the previous section, the longest muscle in the human body is the sartorius, a long, thin, strap-like muscle that runs diagonally across the front of the thigh.
The sarcoplasm of a myofiber is rich in glycogen (a storage form of glucose) and myoglobin (an oxygen-binding protein), which provide the energy resources needed for contraction. Most of the space inside the cell is filled with myofibrils, long organelles composed of myofilaments that are responsible for muscle contraction.
The sarcolemma of myocytes contains numerous invaginations (pits) called transverse tubules which are usually perpendicular to the length of the myofiber. Transverse tubules play an important role in supplying the myocyte with Ca+ ions, which are key for muscle contraction.
As you know, each myofiber contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to mature muscle cells.
Figure \(\PageIndex{1}\): Myofiber = Skeletal muscle cell. A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm called the sarcoplasm. A muscle fiber is composed of many myofibrils, packaged into orderly units.
Myofibril Structure
Each myofiber can contain many thousands of myofibrils. Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end. Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules. The sarcoplasmic reticulum acts as a sink of Ca+ ions, which are released upon signaling from the transverse tubules.
Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins. These proteins are organized into regions termed sarcomeres, the functional contractile unit of the myofiber. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction. The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance.
Sarcomeres
Figure \(\PageIndex{2}\): Sarcomere. The sarcomere is the functional unit of the muscle cell. It defines the region of interaction between a set of thick (myosin) and thin (active) filaments.
Myofilament (Thick and Thin Filament) Structure
Myofibrils are composed of smaller subunits called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments.
- Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments.
- Thin filaments are composed of actin protein subunits, as well as tropomyosin and troponin proteins.
The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.
Figure \(\PageIndex{3}\): The two myofilaments. Interaction of the thin filament (actin, tropomyosin, and troponin) with the thick filament (myosin) leads to muscle contraction. The myosin head attaches to actin to generate force. More details to come.
Key Point Summary
- Muscles are composed of long bundles of myofibers.
- Myofibers contain thousands of myofibrils.
- Each myofibril is composed of numerous sarcomeres, the functional contractile region of a striated muscle. Sarcomeres are composed of myofilaments of myosin and actin, which interact using the sliding filament theory (model) and cross-bridge cycle to contract.