5.7: Sliding Filament Theory of Contraction
- Page ID
- 99997
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere.
- Identify the main parts of a sarcomere: Z-line, A-band, I-band, H-band, and M-line, and describe what each represents.
- Explain the sliding filament model of muscle contraction, including how the various bands change or stay the same.
Sarcomere Structure Review
To understand the sliding filament model requires an understanding of sarcomere structure. Remember from page 5.4 of the study guide that a sarcomere is the basic contractile unit of striated muscle, meaning it's the smallest segment of muscle fiber capable of contracting and generating force. Sarcomeres are arranged end-to-end along myofibrils, which are long thread-like organelles found inside muscle fibers. Each sarcomere sits between two Z-discs (or Z-lines), which define its boundaries.
Within each sarcomere, there are two main kinds of protein filaments:
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Thin filaments made mostly of actin.
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Thick filaments made mostly of myosin.
These filaments are organized in a precise, overlapping pattern, giving skeletal and cardiac muscles their striped (striated) appearance under the microscope. When a muscle contracts, the filaments slide past one another, pulling the Z-discs closer together and shortening the sarcomere, which shortens the entire muscle.
The sarcomere also contains:
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I bands (light bands) — with only thin filaments,
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A bands (dark bands) — where thick and thin filaments overlap,
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H band (or zone) — a lighter area in the center of the A band with only thick filaments,
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M line (the central point of the sarcomere).
This careful arrangement allows muscle contraction to be powerful, coordinated, and efficient.
Figure \(\PageIndex{1}\): Sarcomere Structure. Diagram of skeletal muscle showing myofiber (muscle cell) —> myofibril (organelle) —> sarcomere with overlapping actin (microfilament) and myosin (motor protein).
Sliding Filament Model and Band Changes
The sliding filament model (aka theory) of contraction describes how — when signaled by a motor neuron — thin filaments (actin) slide past thick filaments (myosin) to shorten the sarcomere. You can see the structural changes under a light microscope. Specifically during contraction:
- The Z lines move closer together.
- The I-Band shortens.
- The H-Zone (or H-band) shortens/disappears.
- The A-Band stays the same width.
- At full contraction, the thin and thick filaments overlap completely.
Figure \(\PageIndex{2}\): Band Shortening During Muscle Contraction. As the sarcomere shortens, the Z lines move closer together and the I Band (actin only) gets smaller or disappears. The A band (containing actin and myosin filaments) stays the same.