Skeletal Muscle Structure And Funciton 2

Question 1

What is a neuromuscular junction

Answer 1

The myelinated axon of a motor neuron that terminates at a single point on the sarcolemma (cell membrane surrounding skeletal muscle fibre)

Question 2

How many motor neuron fibres does a muscle fibre make contact with

Answer 2

One

Question 3

How many muscle fibres does a motor neuron axon make contact with

Answer 3

Each motor neuron axon branches in the muscle to make contact with many muscle fibres - dozens to hundreds.

Question 4

Where are motor neuron cell bodies located in the spinal cord

Answer 4

The ventral part of the spinal cord

Question 5

Where do axons project together, and then branch out

Answer 5

Axons project together to target the muscle, and in the muscle the branch out so that each axon innervates many fibres.

Question 6

What are the characteristics of very large motor units (hundreds of fibres)

Answer 6

They develop a lot of force, but do not provide much control over small increments in force

Question 7

What are the characteristics of small motor units (20 or less fibres)

Answer 7

Do not develop much force, but provide fine control over force because activation of each unit only yields a small increase to the total muscle force

Question 8

What is recruitment

Answer 8

The activation of additional muscle fibres to accomplish an increase in contractile strength

Question 9

Where is the neuromuscular junction located, and why

Answer 9

The NMJ is normally located in the middle third of the fibres length, so that the wave of depolarisation (action potential) spreads over the sarcolemma away from the NMJ in both directions

Question 10

What is actin

Answer 10

A globular protein (G actin). Globules assemble to form filamentous protein strand (F actin)

Question 11

What is a thin filament

Answer 11

2 filamentous protein strands of actin wound like a rope

Question 12

What are tropomyosin

Answer 12

A protein that forms a complex with troponin regulating the interaction of actin and myosin in muscular contraction

Question 13

What is the roll of Troponin

Answer 13

Troponin binds tropomyosin and actin, and has an additional important role in regulating the interaction between actin and myosin during the cross bridge cycle

Question 14

What is excitation contraction coupling

Answer 14

A term used to describe the steps from plasma membrane excitation to calcium release to muscle contraction

Question 15

What are the 7 steps of the excitation contraction coupling

Answer 15

-The action potential from nerve causes synaptic transmission at the NMJ to trigger AP in the muscle fibre
-Muscle AP spreads over the surface of the sarcolemma and invades the transverse tubular system
-Depolarisation writhing transverse tubular system triggers release of Ca2+ from nearby terminals of the Sarcoplasmic reticulum
-Ca2+ release into cytoplasm promotes binding of Ca2+ to subunit of troponin. Causes change in shape in troponin. Tropomyosin moves and exposes binding site.
-Myosin then binds to actin and the filaments slide. Force/movement is generated
-Calcium is pumped from the intracellular space back into sarcoplasmic reticulum via SERCA (calcium unbinds from contractile apparatus)
-Muscle is relaxed

Question 16

Outline the difference between the excitation-contraction coupling and the cross bridge cycle

Answer 16

The Excitation-contraction coupling is the term used to describe the steps from plasma membrane excitation, to calcium release, to muscle contraction

The cross bridge cycle explains the mechanism of muscle contraction based on proteins that slide past each other to generate movement

Question 17

What is the first step in the cross bridge cycle

Answer 17

ADP bound myosin head is the “cocked” position add ready to bind to actin
At recast Ca2+ levels in muscle fibre cytoplasm are low.

Question 18

What is the second step in the cross bridge cycle

Answer 18

Action potential (received down T tubules) triggers release of Ca2+ from sarcoplasmic reticulum
Ca2+ concentration in cytoplasm rapidly increases
Ca2+ binds to troponin
Troponin changes shape, causing tropomyosin to “roll”, exposing myosin binding sites
Molecular interaction between actin and myosin now possible

Question 19

What is the third stage of the cross bridge cycle

Answer 19

Myosin binds to the actin site, forming Cross Bridge
After cross bridge forms, the bound ADP and Pi are released from the myosin head
Results in a change in the shape of the myosin head - it flexes
Flexing of myosin is called power stroke
Power stroke results in a tiny shift in the position of the thick and thin filaments. The ends of the sarcomere are pulled towards each other

Question 20

What is the fourth stage of the bridge cycle

Answer 20

After ADP and Pi are released from the myosin head, the myosin ATP binding site is exposed
If ATP is available, ATP binds to the myosin head
This lowers the affinity of myosin for actin, myosin detaches, the cross bridge is broken

Question 21

What is the fifth stage of the cross bridge cycle

Answer 21

Myosin ATPase hydrolyses the ATP to ADP + Pi, the myosin head is recycled in readiness for another cycle

Question 22

What two things does muscle tension (force) depend on

Answer 22

The rate at which the muscle is stimulated
The number of muscle fibres recruited

Question 23

What does a single action potential result in

Answer 23

A single action potential will trigger a pulse of Ca2+ release from SR into cytoplasm, triggering a short period of tension development called a twitch

Question 24

What do many action potentials fired in rapid sequence trigger

Answer 24

Many action potentials in rapid sequence trigger sustained release of Ca2+ from the SR into the cytoplasm, resulting in a sustained period of contraction. Sustained contraction is called Tetanus

Question 25

Outline the relationship between muscle tension and muscle length

Answer 25

When length is optimal - maximum force is created
When muscle cell is stretched - minimal overlap of actin/myosin thus fewer myosin heads can bind and flex, thus creating less tension
When muscle cell is slack - no more room left in sarcomere for muscle to contract thus less tension can be produced

Question 26

What regulates the number of muscle fibres activated

Answer 26

How many neurons are active