Skeletal Muscle Structure And Funciton 2 Flashcards
What is a neuromuscular junction
The myelinated axon of a motor neuron that terminates at a single point on the sarcolemma (cell membrane surrounding skeletal muscle fibre)
How many motor neuron fibres does a muscle fibre make contact with
One
How many muscle fibres does a motor neuron axon make contact with
Each motor neuron axon branches in the muscle to make contact with many muscle fibres - dozens to hundreds.
Where are motor neuron cell bodies located in the spinal cord
The ventral part of the spinal cord
Where do axons project together, and then branch out
Axons project together to target the muscle, and in the muscle the branch out so that each axon innervates many fibres.
What are the characteristics of very large motor units (hundreds of fibres)
They develop a lot of force, but do not provide much control over small increments in force
What are the characteristics of small motor units (20 or less fibres)
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
What is recruitment
The activation of additional muscle fibres to accomplish an increase in contractile strength
Where is the neuromuscular junction located, and why
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
What is actin
A globular protein (G actin). Globules assemble to form filamentous protein strand (F actin)
What is a thin filament
2 filamentous protein strands of actin wound like a rope
What are tropomyosin
A protein that forms a complex with troponin regulating the interaction of actin and myosin in muscular contraction
What is the roll of Troponin
Troponin binds tropomyosin and actin, and has an additional important role in regulating the interaction between actin and myosin during the cross bridge cycle
What is excitation contraction coupling
A term used to describe the steps from plasma membrane excitation to calcium release to muscle contraction
What are the 7 steps of the excitation contraction coupling
-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
Outline the difference between the excitation-contraction coupling and the cross bridge cycle
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
What is the first step in the cross bridge cycle
ADP bound myosin head is the “cocked” position add ready to bind to actin
At recast Ca2+ levels in muscle fibre cytoplasm are low.
What is the second step in the cross bridge cycle
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
What is the third stage of the cross bridge cycle
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
What is the fourth stage of the bridge cycle
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
What is the fifth stage of the cross bridge cycle
Myosin ATPase hydrolyses the ATP to ADP + Pi, the myosin head is recycled in readiness for another cycle
What two things does muscle tension (force) depend on
The rate at which the muscle is stimulated
The number of muscle fibres recruited
What does a single action potential result in
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
What do many action potentials fired in rapid sequence trigger
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
Outline the relationship between muscle tension and muscle length
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
What regulates the number of muscle fibres activated
How many neurons are active
