Neuromuscular II

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What initiates the action potential in muscle fibers?

Release of calcium ions from the sarcoplasmic reticulum

Entry of sodium ions through voltage-gated channels

Binding of acetylcholine to ACh receptors (correct)

Increase of local potential by ~50 mV

What effect does a strong end plate potential have on muscle fibers?

It directly stimulates muscle contraction

It triggers an action potential through a positive feedback loop (correct)

It decreases the threshold for calcium ion release

It inhibits sodium ion entry into the muscle fiber

Which step occurs directly after the action potential travels down the T-tubules?

Voltage-gated Na+ channels open in the plasmalemma

Calcium ions are released from the sarcoplasmic reticulum (correct)

Acetylcholine is released from the axon terminal

Sodium ions pour into the muscle fiber

What is the role of acetylcholine in muscle fiber excitation?

It facilitates the flow of sodium ions into the muscle fiber Signup and view all the answers

How does the muscle fiber membrane potential change during the excitation process?

It increases as sodium ions enter the muscle fiber Signup and view all the answers

What is the primary role of Ca2+ in the process of muscle contraction?

It binds to troponin, leading to the exposure of actin's active sites. Signup and view all the answers

What is the primary function of acetylcholinesterase in the neuromuscular junction?

To destroy acetylcholine after its release Signup and view all the answers

What initiates the mass release of calcium from the sarcoplasmic reticulum?

The arrival of an action potential from the T tubule. Signup and view all the answers

Where is the neuromuscular junction located on a muscle fiber?

At the motor end plate Signup and view all the answers

Which of the following best describes the synaptic cleft?

A narrow gap filled with acetylcholinesterase Signup and view all the answers

During excitation-contraction coupling, which structure is responsible for transmitting action potentials deep into the muscle fiber?

T tubules. Signup and view all the answers

What occurs when ATP attaches to myosin during the contraction cycle?

It breaks the cross-bridge between actin and myosin. Signup and view all the answers

What key role do mitochondria play at the neuromuscular junction?

They provide energy for neurotransmitter synthesis. Signup and view all the answers

Which component of the muscle fiber is primarily involved in the storage of calcium ions?

Sarcoplasmic reticulum. Signup and view all the answers

What structure at the neuromuscular junction increases the surface area of the postsynaptic membrane?

Subneural clefts Signup and view all the answers

How does excitation-contraction coupling begin in skeletal muscle?

Propagation of nerve impulses to the terminal Signup and view all the answers

What structural change occurs in tropomyosin when calcium binds to troponin?

It undergoes a conformational change that reveals the active sites on actin. Signup and view all the answers

What must be present for the muscle contraction cycle to continue?

ATP and calcium ions. Signup and view all the answers

What is the maximum number of neuromuscular junctions that can occur on a single muscle fiber?

One Signup and view all the answers

Which ion channel is primarily located in the bottom half of the motor end plate's structure?

Voltage-gated Na+ channels Signup and view all the answers

What is the initial state of myosin cross-bridges before muscle contraction occurs?

Weak binding state. Signup and view all the answers

What role does the SR calcium ATPase pump play in muscle relaxation?

It transports Ca2+ back into the sarcoplasmic reticulum. Signup and view all the answers

How does the ability to release and re-sequester Ca2+ affect muscular fatigue with exercise?

It may contribute to the onset of fatigue during extended exercise. Signup and view all the answers

Which of the following muscle fiber types contains SERCA2a isoform?

Cardiac muscle fibers Signup and view all the answers

What is the primary source of ATP during the first few seconds of intense muscle contraction?

Phosphocreatine Signup and view all the answers

Rate coding in muscle units refers to what?

The frequency of action potentials discharged by motor units. Signup and view all the answers

Which factor does NOT belong to neural factors influencing force generation?

Skeletal muscle architecture Signup and view all the answers

What happens to the myosin-binding site on actin when Ca2+ is not present?

It is covered by troponin and tropomyosin. Signup and view all the answers

What does an increase in the amount of SERCA isoforms indicate in athletes following a training program?

Improved recovery ability and repeated intense activity capability. Signup and view all the answers

What is the main advantage of having small motor units?

They allow for more precise control. Signup and view all the answers

What does the size principle refer to in motor unit recruitment?

Motoneurons with progressively larger axons are recruited as force increases. Signup and view all the answers

Which of the following muscle fiber types is characterized by a high myoglobin content?

Type I fibers Signup and view all the answers

What key difference exists between fast and slow muscle fibers regarding their mitochondrial content?

Slow fibers have more mitochondria than fast fibers. Signup and view all the answers

Which property of muscle fibers is primarily associated with their speed of contraction?

Sarcoplasmic reticulum function Signup and view all the answers

Why is there a lack of strong correlation between fiber type and motor unit size?

Different muscle functions require varied fiber types within units. Signup and view all the answers

What mechanism serves to achieve coordinated muscle responses?

Selective recruitment and firing patterns of motor units. Signup and view all the answers

Which factor does NOT influence the peak power output of muscle fibers?

Number of capillaries present Signup and view all the answers

What role do calcium ions (Ca2+) play in the release of acetylcholine (ACh) at the neuromuscular junction?

They activate synapsin proteins that free ACh vesicles. Signup and view all the answers

What effect does the binding of acetylcholine (ACh) to its receptors have on the muscle membrane?

It opens channels allowing cations such as Na+ to flow inward, depolarizing the membrane. Signup and view all the answers

Which of the following correctly describes the process of synaptic transmission at the neuromuscular junction?

ACh binds to receptor sites, causing vesicles to undergo exocytosis into the muscle fiber. Signup and view all the answers

What is the primary role of acetylcholinesterase at the neuromuscular junction?

To degrade acetycholine into acetate and choline. Signup and view all the answers

What initiates the action potential (AP) along the muscle membrane after ACh binds to its receptors?

Local depolarization triggering adjacent voltage-gated sodium channels. Signup and view all the answers

Which of the following statements is NOT true regarding the structure of acetylcholine receptors?

They selectively permit only sodium ions to cross. Signup and view all the answers

What happens to the excess choline and acetate after the degradation of acetylcholine in the synaptic cleft?

They are actively reabsorbed into the nerve terminal. Signup and view all the answers

What is the consequence of a failure in the synapsin proteins during neuromuscular transmission?

Reduced release of ACh from presynaptic neurons. Signup and view all the answers

Study Notes

Neuromuscular System II

The neuromuscular junction is a specialized synapse between a motor neuron and a muscle fiber.
It occurs at the motor end plate, typically one per fiber.
The synaptic cleft is 20-30nm wide and contains acetylcholinesterase (AChE).
Subneural clefts increase the surface area of the postsynaptic membrane.
Acetylcholine (ACh) is stored in small synaptic vesicles.
ACh excites the muscle fiber membrane.
The synaptic space contains large amounts of acetylcholinesterase which destroys ACh milliseconds after release.

Excitation of Skeletal Muscle

Nerve impulses reach the terminal causing the release of acetylcholine into the synaptic cleft/space.
Voltage-gated calcium channels allow calcium from the synaptic space to the inside of the nerve.
Ca2+ activates Ca2+-Calmodulin dependent protein kinase which phosphorylates synapsin proteins.
This frees ACh vesicles anchored to the cytoskeleton.
ACh vesicles move, dock at release sites, fuse, and release ACh into the synaptic space (exocytosis).
ACh attaches to the alpha subunits of acetylcholine receptors.
Acetylcholinesterase degrades ACh into acetate and choline which is reabsorbed into the terminal for re-use.
Acetylcholinesterase mostly lines the connective tissue in the synaptic space, though a small amount might be free in the space.
These acetylcholine-gated ion channels are near the openings of the subneural clefts, comprised of 5 transmembrane protein subunits.
Two ACh molecules attach to the alpha subunits to open the channel allowing Na+, K+, and Ca2+ ions (cations) to flow inward (especially Na+) while repelling anions.
The resulting positive potential change (end plate potential) causes adjacent voltage-gated sodium channels to open, triggering greater Na+ influx. This propagates an action potential along the muscle membrane leading to contraction.

End Plate Potential to Muscle Fiber Excitation

Some end-plate potentials are strong enough to cause sodium channels to open to create a positive feedback loop, opening more sodium channels, causing an action potential.

Muscle Fiber Contraction: Excitation-Contraction Coupling

Action potentials (APs) start in the brain.
APs arrive at axon terminals, releasing acetylcholine (ACh).
ACh crosses the synapse and binds to ACh receptors on the plasmalemma.
Na+ pours into the muscle fiber.
The local area potential in the muscle increases.
Voltage-gated Na+ channels open.
APs travel down the plasmalemma and T-tubules.
Ca2+ is released from the sarcoplasmic reticulum (SR).

Transverse Tubules and Excitation-Contraction Coupling

Transverse Tubules (T tubules) transmit APs from the surface of muscle fibers deep to the level of the myofibrils.
They run transverse to the myofibrils, starting at the cell membrane and penetrating all the way from one side of the muscle fiber to the other.
AP travels along T tubules and near the sarcoplasmic reticulum (SR) .
AP allows Ca2+ release from the SR.
The influx of Ca2+ allows muscle contraction.

Role of Ca2+ in Muscle Contraction

AP arrives at the SR from the T-tubule.
SR is sensitive to electrical charge.
Causes mass release of Ca2+ into the sarcoplasm.
Ca2+ binds to troponin on the thin filament.
Tropomyosin covers the myosin-binding site, blocking actin-myosin attraction.
Troponin-Ca2+ complex moves, exposing actin-binding sites on actin.
Myosin binds to actin and contraction occurs.

Excitation-Contraction Coupling

Nerve impulse travels down T tubules and causes release of Ca2+ from the sarcoplasmic reticulum (SR).
Ca2+ binds to troponin and causes a positional change in tropomyosin, exposing active sites on actin.
This allows strong binding between actin and myosin, causing contraction to occur.

Contraction

At rest, myosin cross-bridges occur in a weak binding state.
Ca2+ binds to troponin, causing a shift in tropomyosin to uncover active sites, forming a strong binding state.
P1 is released from myosin, resulting in cross-bridge movement.
ADP is released from myosin.
ATP attaches to myosin, breaking the cross-bridge and forming a weak binding state. ATP is broken down to ADP & P1, energizing the myosin.
This cycle continues as long as Ca2+ and ATP are available

Muscle Relaxation

AP ends, halting electrical stimulation of the SR.
Ca2+ is pumped back into the SR, stored until the next AP arrives.
Requires ATP.
Called the SR calcium ATPase pump.
Without Ca2+, troponin and tropomyosin return to their resting conformation.
Myosin-binding sites are covered, preventing actin-myosin cross-bridging.

SR Calcium ATPase Pump

Two isoforms found in adults: SERCA1 and SERCA2a.
SERCA2a is found in Type I, cardiac, and smooth muscle.
The ability to release and re-sequester Ca2+ plays a role in exercise-induced fatigue. Studies show that training programs with all-out sprints affect the amount of SERCA isoforms and can improve recovery and ability to repeat the activity.

Factors Influencing Force Generation

Neural factors: orderly recruitment, rate coding, and synchronization.
Contractile factors: skeletal muscle size, architecture, length-tension relationship, contractile speed, and history.

Motor Units

α-motor neurons innervate muscle fibers.
A motor unit is a single α-motor neuron and all the fibers it innervates.
More operating motor units = more contractile force.
The finer the control required, the fewer the number of fibers innervated by a motor unit.
Motor units overlap for better coordination.
Not a strong relationship between fiber type and motor unit size.
Small units (e.g., larynx, extraocular) have 10 fibers/motor unit, and have rapid control, while large units (e.g., quadriceps) have 1000 fibers/motor unit and have slower, coarse control.

Motor Unit Properties

Fast fatigable (FF: type 2x): fast twitch, high fatigue rate.
Fast fatigue-resistant (FR: type lla): fast twitch, moderate fatigue rate.
Slow (S: type 1): slow twitch, low fatigue rate

Motor Unit Recruitment

The process of adding motor units to increase force.
Size principle: motor neurons with progressively larger axons are recruited as muscle force increases.
Selective recruitment and firing pattern of fast-twitch and slow-twitch motor units controls movement by producing a coordinated response.

Muscle Fiber Types

Most muscles contain both fast and slow fibers in different proportions depending on function.
Fiber types in a motor unit are all the same type (fast or slow).
Slow fibers (Type I): oxidative, small diameter, high myoglobin content, high capillary density, many mitochondria, and low glycolytic enzyme content.
Fast fibers (Type II): glycolytic, large diameter, low myoglobin content, low capillary density, few mitochondria, and high glycolytic enzyme content.

Fiber Type Determinants

Histochemistry can differentiate fast and slow muscle fibers.
Motor units containing slow fibers are recruited first for normal contractions, while fast fibers are used for forceful contractions.
Genetic and training factors influence fiber type proportions.
Endurance training may lead to some (10%) changes in fiber type proportions but not complete changes.
Aging can cause loss of type II motor units.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers key concepts regarding action potentials in muscle fibers, including the initiation processes, the effects of end plate potentials, and the role of acetylcholine in excitation. Test your understanding of how muscle fibers respond to electrical signals and the subsequent changes in membrane potential.