1.13 - Excitation - Contraction Coupling In Muscle

Question 1

What type of motor neurons are motor fibres supplied by?

Answer 1

Alpha

Question 2

What is the innervation ratio?

Answer 2

The number of muscle fibres a single motor

neuron innervates. Smaller for fine movement (ocular muscleS), larger for strong movement (e.g. leg muscles)

Question 3

What is in a muscle triad?

Answer 3

The two opposing sarcoplasmic reticulums and the T tubule.

Question 4

Describe Excitation-Contraction Coupling

Answer 4

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

Question 5

what causes the muscle to relax after contraction?

Answer 5

Muscle relaxation occurs as intracellular Ca2+ is removed from the cell.

Question 6

How does the cell lower the intracellular Ca2+ concentration?

Answer 6

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

Question 7

What is a muscle fibre?

Answer 7

A single multi-nucleated muscle cell

Question 8

What is the myofibril divided longitudinally into?

Answer 8

Sarcomeres

Question 9

What is contained within the myofibrils?

Answer 9

Actin & myosin

Question 10

What does the Z line demarcate?

Answer 10

The Sarcomere

Question 11

What does the I band contain?

Answer 11

Thin actin filaments

Question 12

What does the A band contain?

Answer 12

Thick myosin filaments

Question 13

What is actin anchored to?

Answer 13

The Z line

Question 14

What happens to actin and myosin during contraction? And what happens to the A, I and H band?

Answer 14

They slide over each other causing the sarcomere to shorten. The A band remains the same length, while the I and H bands shorten.

Question 15

Describe Actin

Answer 15

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

Question 16

Describe Myosin

Answer 16

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

Question 17

Describe the effect of Ca2+ on tropomyosin

Answer 17

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

Question 18

What are the stages in the Cross-Bridge Cycle

Answer 18

An ATP-dependent conformational
change results in the movement of the actin filament closer to the centre of the sarcomereà contraction of
muscle fibre

1. Resting state: In the resting state, myosin has partially hydrolysed ATP
2. Cross bridge state: When Ca2+ binds to TnC, myosin can bind with actin forming a cross-bridge
3. 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
4. Attached state: Myosin then releases the ADP, but remains attached to actin
5. Released State: ATP then binds to the
   myosin head, which reduces myosin’s affinity for its binding site on actin –> myosin releases from actin
6. Resting state: ATP is partially
   hydrolysed and returns to its initial resting state

Question 19

What are the different skeletal muscle types and how do they differ?

Answer 19

Fast Twitch (Type II):

• SERCA 1
• Faster cross bridge cycling

Slow Twitch (Type 2):

• SERCA 2
• Slower cross bridge cycling

Question 20

Describe the coordination of cardiac muscle contraction

Answer 20

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.

Question 21

Describe how Ca2+ initiates contraction in cardiac muscle and how it compares to skeletal

Answer 21

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

Question 22

How is Ca2+ removed in cardiac muscle?

Answer 22

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)

Question 23

What are the two patterns of activity of smooth muscle?

Answer 23

Phasic smooth muscle contracts rhythmically or intermittently. E.g. gastrointestinal tract o Tonic smooth muscle contracts continuously. E.g. Vessels and some sphincters

Question 24

Smooth muscle contraction is dependent on [Ca2+]i. How doe the smooth muscle cell control this?

Answer 24

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)

Question 25

Smooth muscle cells are connected by a variety of structures (Gap junctions, Dense bodies, Adherens junctions). Compare multiunit and single-unit smooth muscle.

Answer 25

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