Chapter 9 B -Muscle Contraction, Energy Systems, and Muscle Fiber Types: Study Notes for Anatomy & Physiology

Muscle Contraction: Principles and Types

General Principles of Muscle Contraction

Muscle contraction involves the generation of tension within muscle fibers, enabling movement and force production. The same principles apply to both single fibers and whole muscles.

• Muscle tension: The force exerted by a muscle on an object.

• Contraction may or may not shorten the muscle.

• Isometric contraction: Muscle tension increases but does not exceed load; muscle does not shorten.

• Isotonic contraction: Muscle shortens because muscle tension exceeds load.

Force and Duration of Contraction

• Force and duration vary in response to different frequencies and intensities of stimulation.

• Each muscle is served by at least one motor nerve.

• Motor nerve: Contains axons of up to hundreds of motor neurons.

• Axons branch into terminals, each of which forms a neuromuscular junction (NMJ) with a single muscle fiber.

• Motor unit: A neuron and all the muscle fibers it innervates; functional unit of muscle contraction.

The Motor Unit

Structure and Function

A motor unit consists of one motor neuron and all the muscle fibers it supplies. The number of fibers per motor unit varies:

• Smaller motor units allow for finer control.

• Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of the entire muscle.

The Muscle Twitch

Definition and Phases

A muscle twitch is the simplest contraction resulting from a muscle fiber's response to a single action potential from its motor neuron.

• Muscle fiber contracts quickly, then relaxes.

• Twitches can be observed and recorded as a myogram.

• Three phases of a muscle twitch:

  • Latent period: Events of excitation-contraction coupling; no tension increase.

  • Period of contraction: Cross bridge formation; tension increases.

  • Period of relaxation: Ca2+ reentry into SR; tension declines to zero.

• Muscle contracts faster than it relaxes.

• Differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles.

Graded Muscle Responses

Types and Mechanisms

Graded muscle responses vary strength of contraction for different demands, essential for proper control of skeletal movement.

• Responses are graded by:

  • Changing strength of stimulation

  • Changing stimulus frequency

• Muscle response to changes in stimulus frequency:

  • Single stimulus results in single contractile response (twitch).

  • Wave (temporal) summation: If two stimuli are received by a muscle in rapid succession, the second twitch will be stronger than the first.

  • If stimuli frequency increases, muscle tension reaches near maximum (incomplete tetanus).

  • If stimuli frequency further increases, muscle tension reaches maximum (fused/complete tetanus).

• Muscle response to changes in stimulus strength:

  • Recruitment: Multiple motor units are stimulated; more units lead to more precise control.

  • Motor units with smallest muscle fibers are recruited first.

Types of Muscle Contractions

Isotonic and Isometric Contractions

• Isotonic contractions: Muscle changes length and moves load.

  • Concentric contractions: Muscle shortens and does work (e.g., biceps contract to pick up a book).

  • Eccentric contractions: Muscle lengthens and generates force (e.g., lowering a book causes biceps to lengthen while generating force).

• Isometric contractions: Load is greater than the maximum tension muscle can generate; muscle neither shortens nor lengthens.

Energy for Contraction and ATP

ATP Supply and Regeneration

ATP supplies the energy needed for the muscle fiber to:

• Move and detach cross bridges

• Pump calcium back into SR

• Return Na+ and K+ after excitation-contraction coupling

• Available stores of ATP are depleted in 4-6 seconds

Mechanisms of ATP Regeneration

• Direct phosphorylation of ADP by creatine phosphate (CP):

  • Creatine kinase transfers phosphate from CP to ADP.

  • Muscle fibers have enough ATP and CP reserves to power cell for about 15 seconds.

• Anaerobic pathway: Glycolysis and lactic acid formation

  • ATP is generated for each glucose broken down.

  • Low oxygen levels prevent pyruvic acid from entering aerobic respiration; converted to lactic acid.

  • Lactic acid diffuses into bloodstream, used as fuel by liver, kidneys, and heart.

  • Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2.5 times faster.

• Aerobic pathway: Aerobic respiration

  • Produces 95% of ATP during rest and light-to-moderate exercise.

  • Slower than anaerobic pathway.

  • Consists of series of chemical reactions that occur in mitochondria and require oxygen.

  • Breaks glucose into CO2, H2O, and large amounts of ATP (32 can be produced).

  • Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids after 30 minutes of exercise.

Muscle Fatigue

Causes and Effects

• Fatigue is the physiological inability to contract despite continued stimulation.

• Possible causes include:

  • Ionic imbalances (levels of K+, Na+, and Ca2+ can change, disrupting membrane potential).

  • Prolonged exercise may damage SR and interfere with Ca2+ regulation.

  • Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles.

Excess Postexercise Oxygen Consumption (EPOC)

Definition and Importance

• For a muscle to return to its pre-exercise state:

  • Oxygen reserves are replenished.

  • Lactic acid is reconverted to pyruvic acid.

  • Glycogen stores are replenished.

  • ATP and creatine phosphate reserves are resynthesized.

• All replenishing steps require extra oxygen; this is referred to as excess postexercise oxygen consumption (EPOC), formerly known as "oxygen debt."

Factors of Muscle Contraction

Force of Muscle Contractions

• Force depends on number of cross bridges attached, affected by:

  • Number of muscle fibers stimulated (recruitment): More motor units recruited, greater force.

  • Relative size of fibers: Bulkier muscle, more tension.

  • Frequency of stimulation: Higher frequency, greater force.

  • Degree of muscle stretch: Sarcomeres at 80-120% of normal resting length generate more force.

Velocity and Duration of Contraction

• Influenced by:

  • Load: Greater load, slower contraction, shorter duration.

  • Recruitment: More motor units contracting, faster and more prolonged contraction.

  • Muscle fiber type: Classified according to two characteristics: speed of contraction and metabolic pathways for ATP synthesis.

Muscle Fiber Types

Classification and Properties

• Three types: slow oxidative fibers, fast oxidative fibers, fast glycolytic fibers.

• Most muscles contain mixture of fiber types, resulting in a range of contractile speed and fatigue resistance.

• Different muscle types are better suited for different jobs:

  • Slow oxidative fibers: endurance activities (e.g., maintaining posture).

  • Fast oxidative fibers: medium-intensity activities (e.g., sprinting, walking).

  • Fast glycolytic fibers: short-term intense or powerful movements (e.g., hitting a baseball).

• Load and recruitment:

  • Greater load, shorter duration of contraction.

  • Greater load, slower contraction.

  • More motor units contracting, faster and more prolonged contraction.

Adaptation to Exercise

Aerobic (Endurance) Exercise

• Leads to increased muscle capillaries, number of mitochondria, and myoglobin synthesis.

• Results in greater endurance, strength, and resistance to fatigue.

• May convert fast glycolytic fibers into fast oxidative fibers.

Resistance Exercise

• Typically anaerobic (e.g., weight lifting, isometric exercises).

• Leads to muscle hypertrophy (increase in fiber size), increased mitochondria, myofilaments, glycogen stores, and connective tissue.

• Increases muscle strength and size.

Clinical – Homeostatic Imbalance

Muscle Health and Disease

• Muscles must be active to remain healthy.

• Disease (atrophy/degeneration and loss of mass) can result from immobilization or loss of neural stimulation.

• Muscle strength can decline 5% per day.

• Paralyzed muscles may atrophy to one-fourth initial size.

• Fibrous connective tissue replaces lost muscle tissue; rehabilitation is impossible at this point.

Smooth Muscle

Location and Structure

• Found in walls of most hollow organs (except heart).

• Most organs contain two layers of sheets with fibers oriented at right angles to each other:

  • Longitudinal layer: Fibers parallel to long axis of organ.

  • Circular layer: Fibers run around circumference of organ.

• Alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs.

Differences Between Smooth and Skeletal Muscle Fibers

• Smooth muscle fibers are spindle-shaped, have one nucleus, and lack striations.

• Contain varicosities (bulbous swellings) of nerve fibers instead of neuromuscular junctions.

• Innervated by the autonomic nervous system.

Microscopic Structure of Smooth Muscle

• Thick filaments are fewer and have myosin heads along entire length.

• No troponin complex; protein calmodulin binds Ca2+.

• Thick and thin filaments arranged diagonally, causing smooth muscle to contract in a corkscrew manner.

• Intermediate filament-dense body network contains lattice-like arrangement of noncontractile intermediate filaments that resist tension.

Contraction of Smooth Muscle

• Mechanism of contraction:

  • Slow, synchronized contractions.

  • Cells electrically coupled by gap junctions.

  • Action potentials transmitted from fiber to fiber.

  • Some cells are self-excitatory (depolarize without external stimuli).

  • Rate and intensity of contraction may be modified by neural and chemical stimuli.

• Contraction in smooth muscle is similar to skeletal muscle contraction in the following ways:

  • Actin and myosin interact by sliding filament mechanism.

  • Final trigger is increased intracellular Ca2+ level.

  • ATP energizes sliding process.

  • Contraction stops when Ca2+ is no longer available.

Role of Calcium Ions in Smooth Muscle

• Ca2+ binds to and activates calmodulin.