Biology · Nervous Systems, Senses & Movement
Muscle contraction, excitation and energy use
A muscle can exert force while holding the same overall length. When a sarcomere does shorten, its filaments overlap more; the filaments do not need to become shorter. Keep the electrical trigger, molecular cycle and mechanical outcome distinct.
Cells, Membranes & Organisation supplies boundaries, ion gradients and transport. Gene Regulation & Cell Signalling introduces receptors and regulatory relationships; Homeostasis, Nutrition & Biological Organisation supplies coordinated physiology and energy accounts. This chapter reintroduces voltage and neural vocabulary before using them. Physics’s Electricity and Magnetism and Optics and Photonics are optional return paths for electrical and lens relations; Chemistry’s Solutions, Concentration & Solubility supports the distinction between ion amount and distribution.
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Trace selected skeletal-muscle excitation, filament sliding and ATP-dependent cycling without confusing contraction with automatic shortening.
An electrical event changes access to a molecular interaction
In the selected vertebrate skeletal-muscle route, a motor terminal releases acetylcholine at a neuromuscular junction. Receptor activity produces an end-plate response that can initiate a muscle action potential. The signal spreads along the muscle-cell membrane and its T-tubule extensions.
Excitation–contraction coupling links that membrane event to calcium release from the sarcoplasmic reticulum. Cytosolic Ca2+ binds troponin, contributing to movement of tropomyosin so that myosin can interact with actin. The transmitter is not the chemical fuel for pulling, and calcium does not itself substitute for ATP.
Calcium removal back into the storage compartment supports relaxation and uses ATP-dependent pumping. Thus stopping activity is not necessarily free of energy cost. Cardiac and smooth muscle have different coupling and regulation details; the drawing is specifically a simplified skeletal-muscle route.
Excitation, calcium access and relaxation
Read the diagram in words
Motor-terminal signalling leads to muscle excitation and a T-tubule voltage route. Coupling permits calcium release from the sarcoplasmic reticulum; calcium binding to troponin changes access for actin–myosin interaction. A separate ATP-dependent route returns calcium to the store.
| Compartment | Assigned x coordinate | Assigned y coordinate | Assigned width | Assigned height |
|---|---|---|---|---|
| Selected muscle-cell interior | 0 | 2 | 12 | 5 |
| Sarcoplasmic-reticulum calcium store | 4 | 2.5 | 3 | 1.5 |
- Muscle-cell surface membrane and selected T-tubule route
- 2
| Displayed structure | Assigned x coordinate | Assigned y coordinate |
|---|---|---|
| Motor-terminal transmitter release | 0 | 0 |
| End-plate response and muscle excitation | 2 | 2 |
| T-tubule voltage route | 4 | 2 |
| Stored Ca2+ | 5.5 | 3.2 |
| Cytosolic Ca2+ | 8 | 5 |
| Ca2+ binding to troponin | 5 | 5.5 |
| Changed actin access | 2 | 5.5 |
| Selected actin–myosin interaction | 0.5 | 6 |
| ATP-dependent calcium return | 10 | 5 |
| Selected ATP-dependent calcium pump | 7 | 3.2 |
| From | To | Selected relationship | Direction | Passage through | Selected condition | Incoming symbol | Released symbol |
|---|---|---|---|---|---|---|---|
| Motor-terminal transmitter release | End-plate response and muscle excitation | Transmitter influences the muscle membrane | Directed route | — | — | — | — |
| End-plate response and muscle excitation | T-tubule voltage route | Muscle excitation spreads along the selected membrane route | Directed route | — | — | — | — |
| T-tubule voltage route | Stored Ca2+ | Coupling permits calcium release | Directed route | — | — | — | — |
| Stored Ca2+ | Cytosolic Ca2+ | Ca2+ enters the cytosolic compartment | Directed route | — | — | — | — |
| Cytosolic Ca2+ | Ca2+ binding to troponin | Ca2+ binds the regulatory protein | Directed route | — | — | — | — |
| Ca2+ binding to troponin | Changed actin access | Tropomyosin position changes | Directed route | — | — | — | — |
| Changed actin access | Selected actin–myosin interaction | Actin–myosin interaction is permitted | Directed route | — | — | — | — |
| Cytosolic Ca2+ | Stored Ca2+ | Calcium returns to the store | Directed route | (7, 3.2) | — | — | — |
| ATP-dependent calcium return | Selected ATP-dependent calcium pump | ATP-dependent pumping supports calcium removal | Directed route | — | — | — | — |
- Model conditions
- Simplified vertebrate skeletal-muscle route. Cardiac/smooth mechanisms, quantitative currents, calcium inventory, force and ATP totals are unassigned.
Contraction changes overlap, not filament length
A sarcomere is a repeating contractile unit of striated muscle between Z boundaries. Thin actin-containing filaments extend from those boundaries, and thick myosin-containing filaments occupy a central region. Myosin–actin interactions can produce sliding and force when the regulatory conditions permit.
In the original geometric comparison, Z boundaries move from 0/2.4 µm to 0.3/2.1 µm around the same centre. Sarcomere length falls from 2.4 to 1.8 µm. Thick-filament length remains 1.2 µm and each selected thin filament remains 0.9 µm. Their overlap increases while the central thin-filament-free region decreases.
The A band corresponds to the thick-filament region and keeps its assigned length. The combined thin-only regions fall from 1.2 to 0.6 µm, and the selected central H zone falls from 0.6 to zero. This simplified one-dimensional geometry omits the filament lattice, elastic structures and load dependence; zero here is a constructed endpoint, not every contracting muscle.
Boundaries move while filament lengths stay fixed
Read the diagram in words
Z boundaries change from 0/2.4 to 0.3/2.1 µm. Sarcomere length changes 2.4 to 1.8; each thin filament stays 0.9 and the thick region stays 1.2. A band remains 1.2, combined thin-only lengths fall 1.2 to 0.6 and H zone 0.6 to zero.
| Minimum | Maximum | Axis ticks | Unit |
|---|---|---|---|
| 0 | 2.4 | 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 | µm |
| Assigned state | Left Z boundary | Right Z boundary | Z-to-Z length | A-band length | Combined thin-only regions | Central H-zone length | Left selected overlap | Right selected overlap | Unit |
|---|---|---|---|---|---|---|---|---|---|
| Initial assigned sarcomere | 0 | 2.4 | 2.4 | 1.2 | 1.2 | 0.6 | 0.3 | 0.3 | µm |
| Shorter assigned sarcomere | 0.3 | 2.1 | 1.8 | 1.2 | 0.6 | 0 | 0.6 | 0.6 | µm |
| Displayed identity | Displayed structure | Start | End | Assigned length | Unit |
|---|---|---|---|---|---|
| TL | Left selected thin filament | 0 | 0.9 | 0.9 | µm |
| TR | Right selected thin filament | 1.5 | 2.4 | 0.9 | µm |
| M | Selected thick-filament region | 0.6 | 1.8 | 1.2 | µm |
| Displayed identity | Displayed structure | Start | End | Assigned length | Unit |
|---|---|---|---|---|---|
| TL | Left selected thin filament | 0.3 | 1.2 | 0.9 | µm |
| TR | Right selected thin filament | 1.2 | 2.1 | 0.9 | µm |
| M | Selected thick-filament region | 0.6 | 1.8 | 1.2 | µm |
- Fixed centre
- 1.2 µm
- Each selected thin-filament length
- 0.9
- Thick-filament length
- 1.2
- Model conditions
- Original one-dimensional geometry around a fixed centre. One selected thin filament from each side and one thick-filament region represent the arrangement, not the full molecular lattice or a measured contraction.
A cycle has several chemically different events
The selected cycle begins with detached ATP-bound myosin. ATP hydrolysis produces an ADP-and-phosphate-bound primed state. When actin access is permitted, binding is followed by a force-producing transition associated with phosphate release; ADP release leaves a strongly bound nucleotide-free state.
Binding a new ATP permits detachment and another cycle. The diagram separates hydrolysis, actin binding, phosphate-associated mechanical transition, ADP release and ATP-dependent detachment. It does not claim that ATP binding alone supplies the whole power stroke or that calcium binds the actin active site.
At the muscle level, contraction means active force production and can accompany shortening, constant length or lengthening under an opposing load. Force, shortening speed and external work are different quantities. ATP is also used in ion handling and other processes, so counting one ATP per ideal cross-bridge cycle cannot yield total whole-muscle energy use.
Five distinct events in one selected cycle
Read the diagram in words
Detached ATP-bound myosin becomes primed with ADP and Pi, binds accessible actin, undergoes a phosphate-associated force-producing transition, releases ADP and binds new ATP to detach. The ideal completed cycle consumes one ATP.
| Displayed structure | Assigned x coordinate | Assigned y coordinate |
|---|---|---|
| Detached, ATP bound | 0 | 0 |
| Primed, ADP and Pi bound | 4 | 0 |
| Actin-bound, ADP and Pi present | 6 | 3 |
| Post-stroke, ADP present | 3 | 5 |
| Strongly bound, nucleotide-free | 0 | 3 |
| From | To | Selected relationship | Direction | Passage through | Selected condition | Incoming symbol | Released symbol |
|---|---|---|---|---|---|---|---|
| Detached, ATP bound | Primed, ADP and Pi bound | ATP hydrolysis primes the selected state | Directed route | — | — | — | — |
| Primed, ADP and Pi bound | Actin-bound, ADP and Pi present | Binding when actin access is permitted | Directed route | — | — | — | — |
| Actin-bound, ADP and Pi present | Post-stroke, ADP present | Phosphate-associated force-producing transition | Directed route | — | — | — | Pi |
| Post-stroke, ADP present | Strongly bound, nucleotide-free | ADP release | Directed route | — | — | — | ADP |
| Strongly bound, nucleotide-free | Detached, ATP bound | New ATP binding permits detachment | Directed route | — | — | ATP | — |
| Selected molecular state | Bound nucleotide state | Actin binding |
|---|---|---|
| Detached, ATP bound | ATP | Detached |
| Primed, ADP and Pi bound | ADP+Pi | Detached |
| Actin-bound, ADP and Pi present | ADP+Pi | Bound |
| Post-stroke, ADP present | ADP | Bound |
| Strongly bound, nucleotide-free | Strongly bound, nucleotide-free | Bound |
- One selected myosin head followed through the states
- myosin-1
- One ATP consumed per completed ideal cycle
- 1
- Model conditions
- Idealised selected actin–myosin cycle with access permitted. Not complete kinetics, a measured stroke distance or whole-muscle ATP accounting.
Worked example
Check boundaries and lengths before interpreting the outcome
The Z-to-Z distance changes from 2.4−0=2.4 µm to 2.1−0.3=1.8 µm, a decrease of 0.6 µm or 25%. The central thick filament stays from 0.6 to 1.8 µm. Both selected thin filaments retain 0.9 µm length as their anchored positions shift.
Greater overlap is therefore compatible with unchanged filament lengths. The separate chemical cycle consumes one ATP per completed ideal cycle, but no number of heads, cycles, time interval or calcium-pumping cost is assigned. Neither that geometry nor one molecular cycle supplies an ATP total or an external-work value.
Check your understanding
Do the selected filaments shorten when the sarcomere shortens?
Show reasoning
No. Thick length stays 1.2 µm and each thin length 0.9 µm. The boundaries move inward and overlap changes, reducing the sarcomere from 2.4 to 1.8 µm.
Why does relaxation still depend on metabolism?
Show reasoning
ATP-dependent calcium handling helps restore low cytosolic calcium, alongside other cellular processes. Removing the contraction trigger is not necessarily an energy-free event.
Does contraction guarantee shortening or external work?
Show reasoning
No. Active muscle can produce force at constant length or be lengthened by a load. Molecular ATP cycling, force and external displacement are distinct observations.
What to carry forward
Trace excitation, calcium regulation and ATP cycling separately, then use the actual load and geometry to interpret force or movement.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2eMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook · 2018Biology 2e35.1 Neurons and Glial Cells35.2 How Neurons Communicate36.1 Sensory Processes36.3 Taste and Smell36.4 Hearing and Vestibular Sensation36.5 Vision38.1 Types of Skeletal Systems38.4 Muscle Contraction and Locomotion
- Structural mechanism of muscle contractionM. A. Geeves and K. C. HolmesStructural biochemistry review · 1999