Chapter 12: Skeletal muscle
➢ Describe the different levels of muscle structure and the actions of skeletal muscles
What is the structure of skeletal muscles?
- Skeletal muscles are surrounded by a fibrous epimysium
- Connective tissue called perimysium subdivides the muscle into fascicles
- Each fascicle is subdivided into muscle fibers (myofibers) surrounded by endomysium
What is the muscle fiber structure?
- Have many of the organelles found in other cells
- Have plasma membranes called sarcolemma
- Are multinucleated; form a syncytium
- Are striated
- I bands: light bands
-A bands: dark bands
-Z lines (discs): dark lines in the middle of the I bands
What is the action of skeletal muscles?
➢ Describe motor units and explain significance of recruitment of motor units
What is a motor unit?
- Consists of one motor neuron and all the muscle fibers it innervates
- A muscle may have many motor units of different types
- All the muscle fibers in a motor unit contract at once
1. Graded/varied strength due to different numbers of motor units being stimulated
2. Neuromuscular junction: site where a motor neuron stimulates a muscle fiber
3. Motor end plate: area of the muscle fiber sarcolemma where a motor neuron stimulates it using
the neurotransmitter, acetylcholine
What is the control of motor units?
- Contraction strength comes from motor unit recruitment
- Finer muscle control requires smaller motor units (fewer muscle fibers)
1. Eye muscles may have ~23 muscle fibers/motor units
2. Larger, stronger muscles may have motor units with 1000s of muscle fibers
3. Control and strength are trade-offs
Motor unit summary:
, - Motor nerves extend from the spinal cord to the muscle fibers
- Each fiber is activated through impulses delivered via motor end plate
- Motor unit: a group of fibers activated via the same neuron
- All muscle fibers of one particular motor unit are always of the same fiber type
- Muscles needed to perform precise (fine) movements generally consist of a large number of
motor units and few muscle fibers
- Less precise movements (gross) are carried out by muscles composed of fewer motor units with
many fibers per unit
What is a nerve?
- An enclosed, cable-like bundle of nerve fibers or axons. It provides a common pathway for the
electrochemical nerve impulses transmitted along each of the axons.
What is the significance of recruitment of motor units?
- Process by which different motor units are activated to produce a given level and type of muscle
contraction
- The higher the recruitment, the stronger the muscle contraction will be
- Finer muscle control requires smaller motor units (few muscle fibers)
What is the neuromuscular junction summary?
- Axon terminal of motor neuron forms neuromuscular junction with a single muscle fiber
- Signals are passed between nerve terminal and muscle fiber by any means of neurotransmitter
ACH
- Released ACH binds to receptor sites on motor end plate of muscle cell membrane
- Binding triggers opening of specific channels in motor end plate
- Ion movements depolarize motor end plate, producing end-plate potential
- Local current flow between depolarized end plate and adjacent muscle cell membrane brings
adjacent areas to threshold
- Action potential is initiated and propagated throughout the muscle the fiber
➢ Describe the banding pattern of a myofibril and how these bands change length during
muscle contraction
What are myofibrils?
- Densely packed subunits that run the length of the muscle fiber
- Stacked in register so that the dark and light bands align
- Composed of thick and thin myofilaments
What are striations?
- Highly organized tissues produced by thick and thin filaments
- I bands: contain only thin filaments, primarily of the protein, actin
- A bands: contain all the thick filament with some thin filament overlap; the thick filament is the
protein, myosin
- H bands: center of the A band with no thin filament overlap
- Z discs (lines) are found in the center of each I band
What are myofilaments (proteins) composed of?
➢ Thick: composed of protein myosin
- Each protein has 2 globular heads with actin binding sites and ATP binding sites
➢ Thin: composed of protein actin
- Have proteins called tropomyosin and troponin that prevent myosin binding at rest
➢ Describe the different levels of muscle structure and the actions of skeletal muscles
What is the structure of skeletal muscles?
- Skeletal muscles are surrounded by a fibrous epimysium
- Connective tissue called perimysium subdivides the muscle into fascicles
- Each fascicle is subdivided into muscle fibers (myofibers) surrounded by endomysium
What is the muscle fiber structure?
- Have many of the organelles found in other cells
- Have plasma membranes called sarcolemma
- Are multinucleated; form a syncytium
- Are striated
- I bands: light bands
-A bands: dark bands
-Z lines (discs): dark lines in the middle of the I bands
What is the action of skeletal muscles?
➢ Describe motor units and explain significance of recruitment of motor units
What is a motor unit?
- Consists of one motor neuron and all the muscle fibers it innervates
- A muscle may have many motor units of different types
- All the muscle fibers in a motor unit contract at once
1. Graded/varied strength due to different numbers of motor units being stimulated
2. Neuromuscular junction: site where a motor neuron stimulates a muscle fiber
3. Motor end plate: area of the muscle fiber sarcolemma where a motor neuron stimulates it using
the neurotransmitter, acetylcholine
What is the control of motor units?
- Contraction strength comes from motor unit recruitment
- Finer muscle control requires smaller motor units (fewer muscle fibers)
1. Eye muscles may have ~23 muscle fibers/motor units
2. Larger, stronger muscles may have motor units with 1000s of muscle fibers
3. Control and strength are trade-offs
Motor unit summary:
, - Motor nerves extend from the spinal cord to the muscle fibers
- Each fiber is activated through impulses delivered via motor end plate
- Motor unit: a group of fibers activated via the same neuron
- All muscle fibers of one particular motor unit are always of the same fiber type
- Muscles needed to perform precise (fine) movements generally consist of a large number of
motor units and few muscle fibers
- Less precise movements (gross) are carried out by muscles composed of fewer motor units with
many fibers per unit
What is a nerve?
- An enclosed, cable-like bundle of nerve fibers or axons. It provides a common pathway for the
electrochemical nerve impulses transmitted along each of the axons.
What is the significance of recruitment of motor units?
- Process by which different motor units are activated to produce a given level and type of muscle
contraction
- The higher the recruitment, the stronger the muscle contraction will be
- Finer muscle control requires smaller motor units (few muscle fibers)
What is the neuromuscular junction summary?
- Axon terminal of motor neuron forms neuromuscular junction with a single muscle fiber
- Signals are passed between nerve terminal and muscle fiber by any means of neurotransmitter
ACH
- Released ACH binds to receptor sites on motor end plate of muscle cell membrane
- Binding triggers opening of specific channels in motor end plate
- Ion movements depolarize motor end plate, producing end-plate potential
- Local current flow between depolarized end plate and adjacent muscle cell membrane brings
adjacent areas to threshold
- Action potential is initiated and propagated throughout the muscle the fiber
➢ Describe the banding pattern of a myofibril and how these bands change length during
muscle contraction
What are myofibrils?
- Densely packed subunits that run the length of the muscle fiber
- Stacked in register so that the dark and light bands align
- Composed of thick and thin myofilaments
What are striations?
- Highly organized tissues produced by thick and thin filaments
- I bands: contain only thin filaments, primarily of the protein, actin
- A bands: contain all the thick filament with some thin filament overlap; the thick filament is the
protein, myosin
- H bands: center of the A band with no thin filament overlap
- Z discs (lines) are found in the center of each I band
What are myofilaments (proteins) composed of?
➢ Thick: composed of protein myosin
- Each protein has 2 globular heads with actin binding sites and ATP binding sites
➢ Thin: composed of protein actin
- Have proteins called tropomyosin and troponin that prevent myosin binding at rest
