Action Potential- Muscle Contraction Flashcards
Steps (Diagramed explained
Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh on the sarcolemma
- Net enter of Na+ initiates an action potential which is propagated along the sarcolemma and down the T tubules
- An action potential in T tubule activates voltage sensitive receptors, which trigger Ca2+ release from terminal cisternae of SR (sarcoplasmic reticulum) cytosol
- Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed
- Contraction; myosin heads alternatively attach to actin and detach, pulling the actin filaments toward the centre of the sarcomere; release of energy by ATP hydrolysis powers the cycling process
- Removal of Ca2+ by active transport into the SR after the action potential ends (Uses ATP)
- Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fibre relaxes
OR
- ACh release
- Na+ in / K+ out
- AP travels along T- Tubules
- Ca++ is released from the SR
- Troponin changes shape, exposing A-M binding sites
Anatomy of the sarcomere
Sarcomeres are essentially short pieces of myofibril attached end to end separated by regions called Z-lines
-One sarcomere extends from one Z-line to the next Z-line
With each sarcomere, there are bands of light and dark regions:
- Dark regions- called A-band
- Light regions- I-band
H zone: only myosin
A-zone: region from one myosin to other
I-band: only actin
Actin
Globular protein (G-actin) that binds together in a chain to form microfilaments (F-actin)
Tropomyosin
Long, double-stranded protein that weaves around the F-actin molecules in a double helical arrangement
Troponin
A globular protein that is made up of 3 subunits
Myosin
Two globular head regions and rod-like tail regions and is the major protein in thick filaments
Type of contractions
Static and Dynamic
Static (isometric)
contraction where force is generated, but there is no movement or shortening of the muscle
Dynamic
contractions that result in movements:
- Concentric (shortening)
ex. lifting cup to mouth - Eccentric (lengthening)
ex. slowly lowering weight to ground
Muscle Fibre Contraction
- Motor neuron connects and innervates muscle fibres
- Contraction is triggered by an ACTION POTENTIAL
- The AP travels down the axon terminal in the myofibrils
- AP must be generated before the muscle can contract
- The message of the brain is sent down the spinal cord to the nerves and motor neurons
- The AP travels down the motor neuron and causes the release of acetylcholine into the neuromuscular junction
- The AP then travels inside the muscle via the T tubules causing the voltage calcium channels on the SR to open and release calcium ions
- The calcium binds to troponin C, causing tropin T to change shape allowing tropomyosin to move an unblock the binding sites on actin and the actin-myosin binding and Powerstroke to occur
Ca2+ and the ‘thin’ filament
- Ca2+ binds to troponin, change its shape which pulls tropomyosin away from the cross-bridge binding site
- When Ca2+ is removed, tropomyosin moves back to block cross-bridge binding sites
- Shape of actin determines your maximal strength
Sliding Filament Theory
- Attachment of the cross bridge to a thin filament
- Movement of the cross bridge, producing tension in the thin filament, also known as the ‘power stroke’
- Detachment of the cross-bridge so it can attach to a thin filament and repeat the cycle
Requirements for cross-bridge cycling
- Energy: ATP
2. Calcium (Ca2+)
Muscle contraction: main players
- Actin filament: pulled by myosin, causing contraction
- Ca2+: exposes myosin binding site on actin
- Myosin filaments: pulls actin filaments by means of cross bridges
- ATP: supplies energy for muscle contraction
