Fiber at Rest
- Available calcium ions are stored in the SR
- ATP molecules are bound to myosin (thick filaments)
- Thin filaments are intact
- Sarcolemma is polarized (ions are leaking across while the Na/K pump is pushing sodium out and potassium in to maintain a resting potential)
Role of the Stimulus
- Release of ACh into the synaptic cleft triggers the process.
- ACh contacts the motor end plate of the muscle cell, and it binds to receptors. The chemical bond triggers the action potential.
- Action potential travels along the sarcolemma, down T tubules, and to the SR.
- The SR membrane increases its permeability to calcium
- ions, allowing calcium ions to flow out and into the sarcoplasm.
Arrival of calcium ions at myofibrils leads to binding to troponin (in thin filaments).
Binding of calcium to troponin causes actin and troponin to change shape, exposing binding sites on the thin filaments.
Cross bridges from thick filaments bind to the exposed sites on the thin filaments.
Calcium ions activate the breakdown of ATP that is bound to myosin
Myosin acts as an enzyme to catalyze the reaction ATPà ADP + PO42- + energy
Part of the energy released is used to move cross bridges and the rest is released as heat
Cross bridges shift, moving the attached thin filaments toward the sarcomere center=power stoke
Another ATP molecule binds to myosin, providing the energy needed to cause the cross bridge to break its connection to the thin filament.
The cross bridge returns to its upright position.
If calcium is still available, the cross bridge will attach to another thin filament binding site, and the cycle begins again.
The cycle of cross bridge attachment, shift, release, and reattachment to a new site repeats as long as ATP and calcium ions are available, shifting the thin filaments toward the sarcomere center.
As a result, the Z lines of the sarcomere approach each other and the sarcomere shortens in length.
The simultaneous shortening of all sarcomeres forming a myofibril cause the myofibril to shorten in length.
Return to Rest
The stream of action potentials from the motor neuron stops, causing ACh to be enzymatically removed from the motor end plate by AChE.
Calcium ions are pumped back into the SR by enzymatic action (requires ATP).
Unavailability of calcium ions restores thin filaments, covering the binding sites.
Remaining cross bridge connections are broken, resulting in thin filaments returning to resting position.
Energy for Contraction
- Energy (ATP) is required to:
- -Shift the cross bridges
- -Break the cross bridge attachments
- -Return calcium ions to the SR
Four phases of ATP utilization
Conversion of available ATP in muscle fibers
Uses a muscle protein called myoglobin to convert ATP for immediate use (6-10 seconds of exercise).
Creatine phosphate system also converts ATP
Conversion of available glucose -> ATP
Conversion of stored glycogen -> glucose-> ATP
Conversion of stored lipids -> glucose -> ATP
with oxygen, converts glucose to 36 ATPs
without oxygen, converts glucose to 3 ATPs + lactic acid
how does oxygen debt get repaid
- 1. deep breathing -- to bring more oxygen into cells
- 2. rest
- 3. restore glucose and glycogen -- like eating an apple
When a muscle fiber receives a stimulus, it will either contract completely or not at all.
Threshold stimulus = weakest stimulus that can initiate a muscle contraction.
Subthreshold stimulus = a weak stimulus unable to initiate a muscle contraction.
Enables a whole muscle organ to modify its contraction strength; to maximize strength, many motor units are recruited to achieve simultaneous contraction, and to minimize strength, only one motor unit may be activated.
Types of Contractions
Rapid response of a muscle to a single stimulus.
- -Latent period = time required for calcium ion release from SR, myosin activation, and cross bridge attachment.
- -Period of contraction = upward tracing that indicates shortening of muscle.
- -Period of relaxation = downward tracing.
Stimulation of a muscle to produce single twitches results in a gradual increase in force of contraction.
If a muscle receives additional stimuli before the preceding contraction cycle is complete, successive contractions are slightly stronger than the first.
If successive stimuli are received before each contraction cycle is completed, the contraction will eventually sustain at its maximum value.
produces movement as the muscle pulls on a bone by applying tension to the tendon.
produces no movement, but only tension.