Muscle Excitation and Contraction Flashcards
Describe the ionic distribution across cell membranes
More K+ ions in cell than out (150mM vs 5 mM)
Less Na+ in cells than out (12mM vs 140mM)
Less Cl- in cell than out (10mM vs 105mM)
More Organic anions (-ve ions) inside cell than out
The body is electrically neutral, cells have excess negative inos on isede but matching positive ones on the outside
Electrical gradient exists across cell membrane but remains in osmotic equilibrium
Resting membrane potential: the electrical gradient across the cell memrbane
How is the ionic distribution across cell membranes maintained using metabolic energy?
ATP is used to actively transport positive ions out of the cell to create an electrical chemical gradient.
Na+ is pumped out and K+ ions pumped in by Na+/K+ ATPase. Pumps 3 Na+ out and 2 K+ in
What is the consequence of the ionic distribution in resting cells?
Resting membrane potential is between -40 and -90 in nerve and muscle cells. (usually -70)
Able to be depolarised
What is the electrical consequences in electrically active nerve or muscle cells?
Depolarises - forming positive potential difference
How is a signal transmitted from nerve to skeletal muscle?
- Nerve impulse reaches neuromuscular junction
- Acetylcholine(Ach) released from motor neurone
- Ach binds with receptors in muscle membrane to allow sodium to enter
- Sodium influx generates action potential in sarcolemma
- Action potential travels down T tubules
- Sarcoplasmic reticulum releases calcium
- Calcium binds with troponin moving the troponin/tropomyosin complex
- Binding sites in actin filament are exposed
How is a signal propogated along the length of an individual cell?
Changing resting potential in cell causes adjacent voltage gated sodium ion channels to open
What is the microscopic structure of skeletal muscle fibre?
Each muscle cell is called a muscle fibre. Each muscle fibre contains:
- Many myofibrils in a single cell (actin and myosin)
-Membrane called the sarcolemma surrounds bundles of myofibrils
- Nucleus (can be multinucleate)
- Mitochondria
- Sarcoplasmic reticulum
- T tubules: indentations in the sarcoplasmic reticulum
How are the contractile proteins in a muscle cell organged?
Myofibrils are made of smaller myofilaments. Made of bundles of actin (thin filament), myosin (thick filament), titin (elastic filament).
Sacromeres are the functional unit of a muscle cell
I band: area between myosin filaments, only actin
A band: length of mysin filament (appears dark), consists of both myosin and overlapping actin
H band: area between 2 actin filaments, only myosin
Z line: mark the boundary of each sacromere, made of tinin which holds sacromeres together
M-line: centre of myosin, no globular heads
How are contractile proteins involved in muscle length change?
I & H bands get shorter as sacromere contracts
Actin filaments are pulled over myosin filaments and sacromere length shorted
In full contraction I and H bands almost non-existant
How do contractile proteins make muscle contract?
- Myosin head attaches to actin myofilament to create crossbridge
- Working stroke: Myosin heads bend to move actin filament towards the M line, ADP and Pi released
- New ATP attaches to myosin, breaking crossbridge
- As ATP is split into ADP and Pi (not released yet) head cocks and ready to reattach
How does electrical excitation lead to contraction?
- Action potential passes down T tubule
- Sarcoplasmic reticulum releases calcium
- Calcium binds with troponin to move tropomyosin
- Binding sites in actin filament are exposed
- Myosin head attaches to binding sites and creates power stroke
- ATP detaches myosin heads and energizes them for another contraction
- When action potentials cease muscle stops contracting
What is the resting membrane potential difference?
Resting - not changing, steady state
Potentail - active transport of ions to cause electrochemical gradient is source of potential/ stored energy. When oppositely charged ions come together again they release energy
Difference - difference in electrical charge inside and out of cell
Measured using volt cell
Structure of a myosin filament:
Myosin molecule:
- Elongated with enlarged head at end
- Two myosin molecules twisted together (2 heads = smooth activation)
- Myosin head is ATP enzyme (hydrolyses ATP causing conformational change in head allowing shortening)
Thick myosin filament:
- Many myosin molecules bundled together form thick myosin filament
- Many heads projecting away from main molecule
Structure of thin actin filament:
3 parts: globular actin, tropomyosin, troponin
Globular actin:
- low calcium levels allow globular actin to form filament
- 2 filaments form helical structure with groove in midle (provides binidng site)
Tropomyosin:
- long chain
- inhibitory molecule
- sits in groove and actin and myosin can’t connect
Troponin complex:
- made of 3 molecules
- troponin C binds to calcium
- troponin I holds tropomyosin in position
- troponin T lifts tropomyosin away from binding site when calcium is released
Sources of ATP for muscle contraction
ATP:
- Chemical bond between last 2 phosphates has enough energy to unhood myosin heads and energize for next contraciton
- Energy from food to regenerate ATP
Creatine:
- Molecule capable of storing ATP
- Creatine + ATP –> Creatine phosphate + ATP
- Creatine phosphate + ADP –> Creatine + ATP
- The second enables production of ATP which is quicker than syntheising it from food
