30 Questions
What is the term for the ability to generate spontaneous potentials in cardiac muscle?
Autorhythmia
Which type of muscle fiber activation spreads through gap junctions for a quick response?
Visceral smooth muscle
In the sliding filament model, which type of muscle fiber activation is dependent on the recruitment of motor units?
Fast glycolytic fibers
What leads to hypertrophy in muscles as a result of weight training?
Increase in myofilaments
What is the main cause of fatigue according to the text?
Decrease in neurotransmitters (Ach)
Which type of muscle fiber has few gap junctions, causing only a specific fiber to fire upon stimulation?
Multi-unit smooth muscle
What is the first step in the mechanism of skeletal muscle contraction?
Motor neuron releases neurotransmitter acetyl choline (ACh) at neuromuscular junction
When Ca2+ attaches to troponin in the sliding filament model, what does troponin do?
Pulls tropomyosin rope off actin
What happens during the attachment phase in the sliding filament model?
Myosin binds to the exposed binding site on actin
What is the role of acetylcholinesterase (AChE) in muscle contraction?
Stimulates the muscle
Which component of the neuromuscular junction contains neurotransmitter?
Synaptic vesicles
What happens after myosin uses ADP as energy in the power stroke during muscle contraction?
Myosin binds to ATP
What is the function of the sarcoplasmic reticulum in a muscle fiber?
Storage site of calcium ions for muscle contraction
Which connective tissue layer surrounds each individual muscle and separates them from each other?
Deep fascia
What is the function of T-tubules in muscle fibers?
Extend the sarcolemma deep into the muscle fiber
Which organelle in a muscle fiber is responsible for providing ATP for muscle contraction?
Mitochondria
What is the function of myoglobin in muscle fibers?
Enhance oxygen storage and delivery
What part of the muscle fiber serves as the contractile unit?
Sarcomere
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.
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.
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.
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.
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.
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.
Which of the following statements about myosin is correct?
Its tail coils around another myosin tail.
What is the function of tropomyosin in the thin filaments?
It covers the myosin binding sites on actin.
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.
What is the function of titin in the sarcomere?
It binds the myosin to the Z-disc.
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.
How is the graded response of muscle contraction achieved?
By varying the number of motor units recruited.
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
Neuromuscular Junction
- Contact site of motor neuron on muscle fiber
- Components:
- Synaptic knob: end of axon, secretes neurotransmitter
- Synaptic vesicles: contain neurotransmitter
- Synaptic cleft: space between synaptic knob and post-synaptic membrane
- Motor end plate: post-synaptic membrane on muscle fiber
- Acetylcholine receptors: on motor end plate, receives neurotransmitter
- Acetylcholinesterase: breaks down neurotransmitter
Explore key concepts related to neuromuscular junctions and motor unit recruitment in the nervous system. Learn about the structure of the neuromuscular junction, the process of recruitment of motor units, and the connection between muscle fibers and the nervous system.
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