NMJ/Excitation-Contraction Coupling

somatic motor neuron

Each muscle fiber receives only one single axon terminal from one _____.

branches

Each axon of one somatic motor neuron can produce many collateral ______ to innervate individual muscle fibers.

motor unit

Each motor neuron, together with all of the muscle fibers that it innervates, is known as a what?

contract

When one motor neuron is activated, all of the muscle fibers that it innervates will be stimulated to do what?

Neuromuscular Junctions (NMJs)

What are the synapses between motor neurons and skeletal muscle cells?

Motor end plate

What is the postsynaptic membrane of NMJ, a specialized sarcolemma of a skeletal muscle fiber with saucer-like appearance that interacts with the motor neuron axon terminals?

Presynaptic terminal

Where are the many terminals of the motor neuron axon branch that innervates a given skeletal muscle fiber, having many synaptic vesicle containing neurotransmitter ACh (acetylcholine)?

Postsynaptic membrane

What is another term for the motor end plates, a specialized area of the membrane (sarcolemma) of a skeletal muscle fiber (cell), having many ACh receptors (AChR, nicotine type) and membrane-bound ACh esterase (AChE), an enzyme degrading excessive ACh?

Synaptic cleft

What is the space between the pre and postsynaptic parts of the neuromuscular junction, filled with extracellular matrix known as a basal lamina (BL), contains matrix-bound AChE?

Action potentials in a motor neuron cause the release of neurotransmitter acetylcholine (ACh) at the neuromuscular junction (one NMJ per muscle fiber).

What is the first step in the excitation-contraction coupling pathway?

The released ACh binds to its receptor on the postsynaptic membrane of NMJ. Upon bond with ACh, the receptor becomes open to allow Na+ and Ca2+ to flow in, K+ to flow out, thus depolarizes the post-synaptic muscle membrane which causes the voltage-gated Na+ channels open and action potential is generated in the muscle membrane.

What is the second step in the excitation-contraction coupling pathway?

Action potential spreads deep down to T tubules and stimulates SR to release Ca2+.

What is the third step in the excitation-contraction coupling pathway?

Ca2+ binds with troponin, causing a change in its structure.

What is the fourth step in the excitation-contraction coupling pathway?

The shape change in tropnin causes its attached tropmyosin to shift its position in the actin filament, thus exposing binding sites for the myosin cross bridges.

What is the fifth step in the excitation-contraction coupling pathway?

Myosin cross bridges, previously activated by the hydrolysis of ATP, attach to actin filament, generating a power stroke and pull the thin filaments over the thick filaments.

What is the sixth step in the excitation coupling pathway?

Attachment of a fresh ATP allows the cross bridge to detach from actin filaments and repeat the contraction cycle as long as Ca2+ remains bonding with troponin.

What is the seventh step in the excitation coupling pathway?

After contraction, Ca2+ is actively pumped back to SR and tropomyosin returns to it inhibitory position, muscle gets relaxed.

What is the eighth step in the excitation coupling pathway?

ACh is cleared up by its degrading enzyme- ACh esterase present in both synaptic cleft (as the matrix-bound form) and postsynaptic muscle membrane (as the membrane-bound form).

What is the ninth step in the excitation coupling pathway?

Waiting for the new nerve impulses (stimuli) to start the new contraction cycle.

What is the tenth step in the excitation coupling pathway?

Type I

What type of muscle fibers are slow-twitch or red fibers?

Type II

What type of muscle fibers are fast-twitch or white muscle fibers?

slow-twitch

Red muscle (such as postural muscle, soleus) has more ___________________ fibers, where fibers are relatively small and controlled by smaller motor unit.

fast-twitch

White muscle (such as gluteal muscles) has more _____________ fibers, and they are large fibers for greater strength of contraction and controlled by larger motor units

- Extensive blood vessels for oxygen supply

- High concentration of iron containing myoglobins which combine with O2

- Numerous mitochondria

- High concentration of mitochondrial enzymes

What features of slow-twitch fibers endow them with high oxidative capacity for aerobic energy generation?

- Low myosin ATPase activity

- Slow speed contraction

What features of red fibers make them suited for prolonged aerobic (endurance) excercises?

Distance running (marathon)

What would red fibers help more with: sprinting or distance running?

- Extensive SR for rapid Ca2+ release

- Fewer mitochondria

- Well developed glycolytic system(more glycogens)

- Less extensive blood supply (less oxidative metabolism)

- Low concentration of iron containing myoglobins

What features of white fibers endows them with better generation of anaerobic energy?

High myosin ATPase activity = generates energy rapidly, forceful contraction

What feature of white fibers makes them suited for explosive sport activity such as basketball, weight lifting, field hockey, etc.?