Questions and Answers
What is a motor unit?
A motor unit consists of one motor neuron and all the muscle fibers it innervates or supplies.
In response to stimulation by a motor neuron, do all of the muscle cells in one motor unit contract?
True
The number of muscle cells per motor unit may vary from what to what?
They vary from as high as several hundred or as few as four.
Give examples of skeletal muscles that exert fine control with small motor units.
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Provide an example of large, weight-bearing skeletal muscles that have large motor units.
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Are the muscle cells in a single motor unit spread throughout the muscle?
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What does stimulation of a single motor unit cause?
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Not all motor units of one entire skeletal muscle contract at once.
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What do motor units allow in terms of force of contraction?
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Why do motor units help prevent muscle fatigue?
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What is muscle tension?
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What does the term 'load' refer to in muscle physiology?
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What happens to muscle length, the load, and tension in isotonic contractions?
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What occurs in isometric contractions?
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What is muscle tone?
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What is muscle tone due to?
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How does muscle tone benefit muscles?
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How long can ATP provide energy for muscle contraction?
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What must ATP be regenerated from?
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What can be used to produce ATP by aerobic and anaerobic cellular respiration?
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The force of muscle contraction depends on what?
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What factors affect the number of myosin crossbridges?
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What is the relationship between the number of motor units recruited and muscle force?
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What is the relationship between muscle cell size and muscle force?
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How does regular exercise affect muscle force?
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The more a muscle is stimulated, the greater the force it exerts.
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How are skeletal muscles maintained near their optimal length to generate maximum force?
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What do joints normally prevent?
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What is muscle hypertrophy?
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What is muscle hyperplasia?
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What happens to skeletal and cardiac muscle tissue's ability to undergo cell division after birth?
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Which muscle type retains the ability to undergo cell division and growth?
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What happens to the amount of connective tissue and the number of muscle fibers as we age?
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Study Notes
Motor Units
- A motor unit is comprised of one motor neuron and all the muscle fibers it innervates.
- When stimulated, all muscle fibers within a motor unit contract simultaneously.
- The number of muscle fibers per motor unit can range from four to several hundred.
- Fine control muscles, like those in fingers and eyes, have smaller motor units.
- Larger muscles, such as those in the hips, have larger motor units for powerful but less precise movements.
- Muscle cells in a motor unit are distributed throughout the muscle, leading to weak overall contractions when a single unit is stimulated.
- Not all motor units in a muscle contract at the same time, allowing for finer control.
- Motor units enable graded contractions; increased recruitment of motor units results in stronger muscle contractions.
- Alternating contractions and relaxations of motor units prevent fatigue and allow sustained muscle contractions.
Muscle Contraction Types
- Muscle tension is the force exerted by a contracting muscle.
- Load refers to the opposing force on the muscle from the object being moved.
- Isotonic contractions involve changes in muscle length while moving a load; tension remains constant after sufficient force is reached.
- Isometric contractions involve increased tension without a change in muscle length.
- Muscle tone is the slight contraction present in relaxed muscles, maintaining firmness, readiness, and supporting posture.
Energy Metabolism in Muscles
- ATP in muscle cells sustains contraction for only 4-6 seconds.
- ATP must be regenerated from the breakdown of creatine phosphate, using the enzyme creatine kinase.
- Glucose from blood and glycogen can be converted to ATP through aerobic and anaerobic respiration.
- The number of myosin crossbridges attached impacts muscle contraction force, influenced by fiber stimulation, fiber size, stimulation frequency, and muscle stretch.
Exercise Effects
- Increased motor unit recruitment leads to greater muscle force.
- Larger muscle fibers correlate with greater strength, thanks to an increased cross-sectional area.
- Regular exercise promotes muscle hypertrophy, enhancing force production.
- Optimal muscle length is maintained through skeletal attachment to bones.
- Joint structures prevent overstretching of muscles beyond their optimal length.
Smooth Muscle Characteristics
- Smooth muscle cells typically measure 5-10 µm in diameter and 30-200 µm in length, contrasting with the larger skeletal muscle fibers.
- Smooth muscle is organized into sheets that form the walls of blood vessels and hollow organs (respiratory, digestive, urinary, reproductive tracts).
- Smooth muscle exhibits two layers: a longitudinal layer (contraction shortens and dilates organ) and a circular layer (contraction constricts organ lumen).
- Peristalsis refers to wave-like contractions that move content through hollow organs.
- Smooth muscle cells lack highly structured neuromuscular junctions and instead have autonomic nerve endings with varicosities that release neurotransmitters.
Muscle Cell Structures
- Smooth muscle lacks striations due to the different arrangement of myofilaments compared to skeletal muscle.
- Dense bodies and intermediate filaments replace the sarcomere organization in smooth muscle, leading to contraction that shortens and twists the cell.
- Synchronized contractions of neighboring smooth muscle cells result from electrical coupling via gap junctions.
- Contraction triggers in smooth muscle include rises in calcium ion levels, with ATP driving the sliding filament mechanism.
- Regulation of smooth muscle contraction is influenced by nerves, hormones, or local chemical changes.
Function and Adaptation
- Unitary smooth muscle, also called visceral muscle, is predominant, found in hollow organs and possesses electrically coupled fibers that contract as a unit.
- Multi-unit smooth muscle allows for more precise control found in large airways, blood vessels, and eye muscles.
- Muscle hypertrophy indicates an increase in muscle cell size, while hyperplasia refers to an increase in muscle cell number.
- After birth, skeletal and cardiac muscle primarily grow through hypertrophy, whereas smooth muscle can proliferate through hyperplasia in specific regions like the uterus.
- Aging typically leads to increased connective tissue and decreased muscle fiber count in skeletal muscles.
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Test your knowledge of Chapter 9 in Anatomy and Physiology I with these flashcards. This section focuses on motor units and their functions, including the contraction of muscle fibers. Ideal for students looking to reinforce their understanding of muscular physiology.
