Neuromuscular Junction and Motor Unit Recruitment

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Questions and Answers

What is the term for the ability to generate spontaneous potentials in cardiac muscle?

Rate of Recruitment

Neuromuscular junction

Autorhythmia (correct)

Desmosome connection

Which type of muscle fiber activation spreads through gap junctions for a quick response?

Fast glycolytic fibers

Visceral smooth muscle (correct)

Endomysium present fibers

Multi-unit smooth muscle

In the sliding filament model, which type of muscle fiber activation is dependent on the recruitment of motor units?

Cardiac muscle fibers

Slow oxidative fibers

Fast glycolytic fibers (correct)

Multi-unit smooth muscle

What leads to hypertrophy in muscles as a result of weight training?

Increase in myofilaments (C) Signup and view all the answers

What is the main cause of fatigue according to the text?

Decrease in neurotransmitters (Ach) (D) Signup and view all the answers

Which type of muscle fiber has few gap junctions, causing only a specific fiber to fire upon stimulation?

Multi-unit smooth muscle (C) Signup and view all the answers

What is the first step in the mechanism of skeletal muscle contraction?

Motor neuron releases neurotransmitter acetyl choline (ACh) at neuromuscular junction (C) Signup and view all the answers

When Ca2+ attaches to troponin in the sliding filament model, what does troponin do?

Pulls tropomyosin rope off actin (D) Signup and view all the answers

What happens during the attachment phase in the sliding filament model?

Myosin binds to the exposed binding site on actin (D) Signup and view all the answers

What is the role of acetylcholinesterase (AChE) in muscle contraction?

Stimulates the muscle (A) Signup and view all the answers

Which component of the neuromuscular junction contains neurotransmitter?

Synaptic vesicles (C) Signup and view all the answers

What happens after myosin uses ADP as energy in the power stroke during muscle contraction?

Myosin binds to ATP (A) Signup and view all the answers

What is the function of the sarcoplasmic reticulum in a muscle fiber?

Storage site of calcium ions for muscle contraction (A) Signup and view all the answers

Which connective tissue layer surrounds each individual muscle and separates them from each other?

Deep fascia (A) Signup and view all the answers

What is the function of T-tubules in muscle fibers?

Extend the sarcolemma deep into the muscle fiber (A) Signup and view all the answers

Which organelle in a muscle fiber is responsible for providing ATP for muscle contraction?

Mitochondria (C) Signup and view all the answers

What is the function of myoglobin in muscle fibers?

Enhance oxygen storage and delivery (B) Signup and view all the answers

What part of the muscle fiber serves as the contractile unit?

Sarcomere (D) Signup and view all the answers

What is the main function of the slow-twitch (Type I) muscle fibers?

They are used for endurance activities that rely on aerobic energy pathways. (A) Signup and view all the answers

What is the primary difference between fast-twitch (Type IIb) and fast-oxidative (Type IIa) muscle fibers?

Type IIb fibers rely more on anaerobic energy pathways like glycolysis. (A) Signup and view all the answers

According to the sliding filament model, what happens during the contraction of a muscle fiber?

The myosin filaments slide past the actin filaments, shortening the sarcomere. (C) Signup and view all the answers

What is the main role of creatine phosphate (stored in muscle) in the energy systems described in the text?

It is converted to creatine and ATP, providing a fast source of energy for short, submaximal contractions. (A) Signup and view all the answers

What is the relationship between the size of muscle fibers and their contractile properties?

Larger muscle fibers (Type IIb) have faster contraction speeds but lower fatigue resistance. (C) Signup and view all the answers

What is the main function of the sarcoplasmic reticulum in muscle contraction?

It stores and releases calcium ions, which bind to the myosin heads to initiate contraction. (B) Signup and view all the answers

Which of the following statements about myosin is correct?

Its tail coils around another myosin tail. (A) Signup and view all the answers

What is the function of tropomyosin in the thin filaments?

It covers the myosin binding sites on actin. (B) Signup and view all the answers

Which of the following statements about the organization of thick and thin filaments is correct?

The H zone is the area where thin filaments overlap with thick filaments. (B) Signup and view all the answers

What is the function of titin in the sarcomere?

It binds the myosin to the Z-disc. (B) Signup and view all the answers

What is the significance of the 'all or none' response in motor units?

All muscle fibers in a motor unit contract at the same time. (C) Signup and view all the answers

How is the graded response of muscle contraction achieved?

By varying the number of motor units recruited. (A) Signup and view all the answers

Study Notes

Muscle Physiology

We are 1.3-1.5 times stronger eccentrically than concentrically
Eccentric contractions rip myosin from actin, leading to muscle soreness and repair, which increases strength
The force of contraction is less than the force of the load during eccentric contractions

Muscle Tension

The amount of muscle tension is dependent on two factors:
- Recruitment: the number of motor units (MUs) activated
- Rate of recruitment (coding frequency): how rapidly the nervous system stimulates the muscle

Muscle Fiber Types

Slow-twitch (Type I, SO): aerobic pathways, dark in color, small motor units, slower, high endurance, high myoglobin, and dense capillary beds
Fast-oxidative (Type IIa, FOG): intermediate between Type I and Type IIb, similar to fast-twitch but with more mitochondria
Fast-twitch (Type IIb/x, FG): anaerobic pathways, light in color, large motor units, fast, fewer mitochondria, and little/no myoglobin

Muscle Fiber Distribution

Skeletal muscles typically have a mix of all three fiber types, but the proportion varies from muscle to muscle
Single motor units have single types of fibers
Postural muscles tend to have slow-twitch fibers (e.g., soleus)
Eye and hand muscles tend to have fast-twitch fibers only

Muscle Contractions

Isotonic contractions:
- Concentric: muscle contracts as it shortens (force of contraction > force of load)
- Eccentric: muscle contracts as it lengthens (force of load > force of contraction)

Muscle Function and Aging

Between ages 30-50, there is a 10% loss of muscle mass
By age 80, there is a 40% loss of muscle mass

Muscle Structure

Thick filaments:
- Made up of myosin, with a binding site for actin and ATP
- Elongated tail with a golf club head
Thin filaments:
- Made up of actin, tropomyosin, and troponin
- Double-stranded helix with binding sites for myosin
Sarcomere structure:
- A-bands: isotropic, area of thick filaments
- I-bands: area of no overlap between thin filaments
- H-zone: area of non-overlap between thick filaments
- M-line: proteins in the middle of the sarcomere, anchoring myosin tails
- Z-disc: proteins separating sarcomeres, anchoring thin filaments

Motor Units and Neuromuscular Junction

Motor unit: motor neuron and all the muscle fibers it innervates
Motor neuron can stimulate a few to 2,000 muscle fibers
All or none response: all muscle fibers of a motor unit contract at the same time
Recruitment: smallest motor units are recruited first, and as demand increases, larger motor units are recruited
Fatigue sets in when motor units can no longer be recruited