1.13 - Excitation - Contraction Coupling In Muscle Flashcards
What type of motor neurons are motor fibres supplied by?
Alpha
What is the innervation ratio?
The number of muscle fibres a single motor
neuron innervates. Smaller for fine movement (ocular muscleS), larger for strong movement (e.g. leg muscles)
What is in a muscle triad?
The two opposing sarcoplasmic reticulums and the T tubule.
Describe Excitation-Contraction Coupling
Depolarisation of the triad activates L-type V-G Ca2+ channels (DHP) in the T-Tubules.
The DHP channel is mechanically coupled to the Ryanodine receptor (RYR; in the SR) which opens and releases Ca2+ when the DHP channel is opened.
Ca2+ is then released from the Sarcoplasmic reticulum –> promotes interaction of actin and myosin –> muscle contraction
what causes the muscle to relax after contraction?
Muscle relaxation occurs as intracellular Ca2+ is removed from the cell.
How does the cell lower the intracellular Ca2+ concentration?
Ca2+ is taken up in to the SR via a Ca2+ ATPase pump called SERCA.
Calsequestirn (in the SR) can store large pools of Ca2+ near the release site and maintains free Ca2+ at low levels.
Sarcalumenin is present in longitudinal tubules and as a low affinity Ca2+ binding protein. May facilitate SERCE, or play a role in transfer from sites of uptake to sites of release
What is a muscle fibre?
A single multi-nucleated muscle cell
What is the myofibril divided longitudinally into?
Sarcomeres
What is contained within the myofibrils?
Actin & myosin
What does the Z line demarcate?
The Sarcomere
What does the I band contain?
Thin actin filaments
What does the A band contain?
Thick myosin filaments
What is actin anchored to?
The Z line
What happens to actin and myosin during contraction? And what happens to the A, I and H band?
They slide over each other causing the sarcomere to shorten. The A band remains the same length, while the I and H bands shorten.
Describe Actin
Actin filaments make the backbone of the thin filament.
Tropomyosin extends over the actin filaments covering its myosin binding sites
The troponin complex (troponin T, troponin C & troponin I) control the position of tropomyosin:
TnT bind to tropomyosin
TcC binds Ca2+ and promotes the exposure of the myosin binding site
TnI facilitates the inhibition of the myosin binding site
Describe Myosin
Myosin makes up the thick filaments, arranged in a tail like configuration
Myosin has a tail region and a cross-bridge region
The cross-bridge region comprised a neck and globular heads
The head region is what binds to actin and has light chains:
- The essential light chain has an ATPase
- The regulatory light chain is thought to influence the kinetics of myosin with actin
The neck region is where the mechanical change occur than enable contraction
Describe the effect of Ca2+ on tropomyosin
When Ca2+ is present it binds to troponin C (TnC)
This causes the tropomyosin to move, exposing the myosin binding site on the actin filament
What are the stages in the Cross-Bridge Cycle
An ATP-dependent conformational
change results in the movement of the actin filament closer to the centre of the sarcomereà contraction of
muscle fibre
- Resting state: In the resting state, myosin has partially hydrolysed ATP
- Cross bridge state: When Ca2+ binds to TnC, myosin can bind with actin forming a cross-bridge
- Power-stroke state: Once bound, the myosin head will release Pi, and undergoes a conformational change. The angle of the myosin head pulls the actin filament toward the centre of the sarcomere
- Attached state: Myosin then releases the ADP, but remains attached to actin
- Released State: ATP then binds to the
myosin head, which reduces myosin’s affinity for its binding site on actin –> myosin releases from actin - Resting state: ATP is partially
hydrolysed and returns to its initial resting state
What are the different skeletal muscle types and how do they differ?
Fast Twitch (Type II):
- SERCA 1
- Faster cross bridge cycling
Slow Twitch (Type 2):
- SERCA 2
- Slower cross bridge cycling
Describe the coordination of cardiac muscle contraction
The sinoatrial node sets the pace, or frequency, of the beats. It sets off a depolarisation wave to the atria.
The depolarisation reaches the atrioventricular (AV) node (specialised conducting fibres) that slows down conduction to allow the atria to finish contracting and the ventricle to fill
The depolarizing wave passes through to the purkinje fibres that coordinate the contraction of the ventricles
Being electrically coupled (intercalated discs), adjacent Cardiomyocytes spread their APs to adjacent cells.
Describe how Ca2+ initiates contraction in cardiac muscle and how it compares to skeletal
In cardiac muscle, the L-type Ca2+ channel allows the inflow of Ca2+ which initiates the release of Ca2+ from the SR –> electrochemical coupling
In skeletal muscle, the voltage gated L-type Ca2+ channels (DHPR) cause a mechanical interaction with the Ca2+ release channel (RYR) in the SR –> electromechanical coupling
How is Ca2+ removed in cardiac muscle?
SERCA pump operates to pump Ca2+ back in to the SR using ATP (as skeletal muscle)
3Na+-1 Ca2+ exchanger (uses Na+ gradient)
Sarcolemmal Ca2+ pump (ATP dependent)
What are the two patterns of activity of smooth muscle?
Phasic smooth muscle contracts rhythmically or intermittently. E.g. gastrointestinal tract o Tonic smooth muscle contracts continuously. E.g. Vessels and some sphincters
Smooth muscle contraction is dependent on [Ca2+]i. How doe the smooth muscle cell control this?
Smooth muscle tone can be directly related to Vm, e.g. vascular smooth muscle Calcium influx from L-type Ca2+ channels (as per skeletal muscle but with caveolae rather than at the triad) Calcium induced calcium release (sometimes Ca2+ can relax muscle by opening Ca2+-dependent K+ channels) Inositol triphosophate (IP3)-gated Ca2+ (G-protein controlled)
Smooth muscle cells are connected by a variety of structures (Gap junctions, Dense bodies, Adherens junctions). Compare multiunit and single-unit smooth muscle.
Multiunit smooth muscle: no electrical coupling between cells (similar to skeletal)
Single-unit smooth muscles: has electrical gap junctions that propagates electrical activity to adjacent cells (similar to cardiac)
Most smooth muscle cells probably use a combination of both
