Neuromuscular Junction and Muscle Fibers

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Questions and Answers

What is the definition of a synapse?

A synapse is a junction between two neurons or between a neuron and a muscle where signals are transmitted.

What is an alpha motor neuron?

An alpha motor neuron is a type of lower motor neuron that directly innervates skeletal muscle fibers to facilitate movement.

Identify the main components of the neuromuscular junction (NMJ).

The main components of the NMJ include the motor nerve terminal, synaptic cleft, and postsynaptic muscle membrane.

Describe the process of neurotransmitter release at the NMJ.

Neurotransmitter release occurs when an action potential arrives at the terminal, causing calcium influx and the release of acetylcholine (ACh) into the synaptic cleft. Signup and view all the answers

What happens after the release of acetylcholine into the synaptic cleft?

After ACh release, it binds to receptors on the muscle fiber, generating an action potential that leads to muscle contraction. Signup and view all the answers

List the different components of a muscle fiber.

Components of a muscle fiber include myofibrils, sarcoplasmic reticulum, and mitochondria. Signup and view all the answers

Outline the sequence of events during skeletal muscle contraction.

The sequence begins with ACh release, leads to muscle depolarization, calcium release from the sarcoplasmic reticulum, and cross-bridge formation. Signup and view all the answers

What role does calcium play in muscle contraction?

Calcium ions bind to troponin, causing a conformational change that allows cross-bridge cycling between actin and myosin. Signup and view all the answers

How is the signal terminated at the NMJ?

The signal is terminated by the breakdown of ACh by the enzyme acetylcholinesterase, stopping the action potential. Signup and view all the answers

What is the significance of the synaptic cleft width in the NMJ?

The synaptic cleft width of about 20 µm ensures no direct contact, allowing for the diffusion of neurotransmitters. Signup and view all the answers

What neurotransmitter is released into the synaptic cleft to initiate muscle contraction?

Acetylcholine (ACh) Signup and view all the answers

What is the contractile unit of skeletal muscle called?

Sarcomere Signup and view all the answers

Which type of muscle fibers are characterized by high myoglobin content and resistance to fatigue?

Type I (Slow twitch) fibers Signup and view all the answers

What is the process that breaks down acetylcholine after it facilitates muscle contraction?

Hydrolysis by acetylcholinesterase (AChE) Signup and view all the answers

What types of muscle fibers would be found in postural muscles?

Type I (Slow twitch) fibers Signup and view all the answers

What role do T-tubules play in muscle contraction?

They spread the muscle action potential throughout the muscle fiber. Signup and view all the answers

What is the primary function of mitochondria in skeletal muscle fibers?

They produce ATP. Signup and view all the answers

Describe the composition of a thin filament in skeletal muscle.

It is mainly composed of actin, along with troponin and tropomyosin. Signup and view all the answers

Which type of muscle fibers would be more prone to fatigue?

Type II (Fast twitch) fibers Signup and view all the answers

How does the myosin ATPase isoform differ between Type I and Type II muscle fibers?

Type I hydrolyzes ATP slowly, while Type II hydrolyzes ATP quickly. Signup and view all the answers

Study Notes

Neuromuscular Junction (NMJ)

NMJ is an example of a synapse, where a motor nerve meets a muscle.
The motor nerve is also known as a lower motor neurone or an alpha (α) motor neurone.
The NMJ is a chemical synapse, meaning there is no physical contact between the nerve and the muscle.
There is a synaptic cleft (a gap of about 20um) between the nerve and the muscle.
The message is transferred across the synaptic cleft via the release of neurotransmitter acetylcholine (ACh).
ACh is produced and stored in synaptic vesicles in the nerve terminal.
ACh is released into the synaptic cleft and binds to receptors on the muscle.
This binding triggers an end-plate potential (EPP) in the muscle cell.
The EPP sets up an action potential in the skeletal muscle, causing it to contract.
ACh is then broken down by acetylcholinesterase (AChE), and the components are recycled.

Muscle Fiber Types

There are two main types of skeletal muscle fibers:
- Type I Slow twitch (Red) muscle fibers
- Type II Fast twitch (White) muscle fibers
Type I fibers are found in postural muscles, while Type II fibers are found in extraocular muscles.
Type I fibers are rich in myoglobin, mitochondria, and perform oxidative metabolism.
Type I fibers are fatigue resistant.
Type II fibers have less myoglobin and mitochondria, and perform more glycolytic metabolism
Type II fibers are more fatigue prone.

Muscle Structure

Muscle fibers are made up of myofibrils, which are composed of myofilaments of Actin and Myosin.
The proteins Actin and Myosin give muscles their striated appearance.
Myofibrils are surrounded by T-tubules, which help to spread the muscle action potential.
Muscle fibers also contain mitochondria, which are responsible for ATP production.
Muscle fibers also have a sarcoplasmic reticulum, which stores calcium.

Sarcomere

The sarcomere is the contractile unit of skeletal muscle.
It extends from one Z disc to the next and gives muscle its striated appearance.
The sarcomere is composed of:
- Actin (thin filament)
- Myosin (thick filament)
- Titin
- Troponin
- Tropomyosin

Cross-bridge Formation

The cross-bridge cycle describes the sequence of events within a sarcomere when a muscle contracts and relaxes.
The cycle involves:
- Attachment: Myosin binds to actin, forming a cross-bridge
- Power Stroke: The myosin head pivots, pulling the actin filament toward the center of the sarcomere.
- Detachment: The myosin detaches from actin.
- Re-cocking: Myosin is reset to its original position and energy is restored.
This cycle continues as long as calcium is present, causing the muscle to contract.
Relaxation occurs when calcium is removed from the sarcomere, allowing troponin and tropomyosin to block the active sites on actin.
This prevents further cross-bridge formation and the muscle returns to its resting length.

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Description

Explore the fascinating details of the neuromuscular junction (NMJ) and the different types of muscle fibers. This quiz covers the chemical synapse between motor nerves and muscles, focusing on neurotransmitter functions and muscle contraction. Test your knowledge of NMJ physiology and muscle fiber classifications!