Human Anatomy and Physiology - Muscles and Muscle Tissue

Whole Muscle Contraction

• Contraction principles are similar for single fibers and whole muscles.
• Muscle Tension: The force exerted on a load/object during contraction.
• Contraction may lead to muscle shortening or not:
  • Isometric Contraction: No shortening; muscle tension increases but does not exceed the load.
  • Isotonic Contraction: Muscle shortens as muscle tension exceeds the load.

Motor Unit

• A motor unit consists of a motor neuron and all the muscle fibers it innervates (4 to several hundred fibers).
• Fine Control: Smaller fiber number allows greater fine control.
• Motor fibers from a motor unit are spread throughout the muscle, leading to only a weak contraction when a single motor unit is stimulated.

Muscle Twitch

• A muscle twitch is the simplest contraction resulting from a muscle fiber’s response to a single action potential from a motor neuron.
• Myogram: A tracing that records contraction activity.
• Three Phases of Muscle Twitch:
  1. Latent Period: Events of excitation-contraction coupling occur, but no muscle tension is observed.
  2. Period of Contraction: Cross-bridge formation occurs, and tension increases.
  3. Period of Relaxation: Calcium reenters the sarcoplasmic reticulum (SR), and tension declines to zero.
• Muscle contracts faster than it relaxes.

Graded Muscle Responses

• Graded muscle responses provide smooth contractions and vary in strength according to needs.
• Responses are graded through:
  • Changing frequency of stimulation
  • Changing strength of stimulation

Temporal Summation

• Occurs when two stimuli are received by a muscle in rapid succession without complete relaxation. This results in:
  • Increased twitch force due to additional calcium release from the second stimulus.

Tetanus

• Unfused Tetanus: Higher stimulation frequency leads to continuous contractions that are not smoothly fused.
• Fused Tetanus: Even higher frequency results in a smooth, sustained contraction with no relaxation between stimuli.

Recruitment of Motor Units

• Recruitment involves sending stimuli to more muscle fibers for precise control. Types of stimuli:
  • Subthreshold: Not strong enough for contraction.
  • Threshold: Causes the first observable contraction.
  • Maximal: Generates the maximum contractile force.
• Size Principle: Smaller motor units are recruited first, followed by larger ones for stronger contractions.

Muscle Tone

• The constant, slightly contracted state of all muscles, due to spinal reflexes that keep muscles firm and ready to respond.

Isotonic vs. Isometric Contractions

• Isotonic Contraction: Muscle changes length while moving a load.
  • Concentric: Muscle shortens (e.g., lifting a weight).
  • Eccentric: Muscle lengthens while generating force (e.g., lowering a weight).
• Isometric Contraction: Muscle tension increases but the muscle does not shorten or lengthen due to the load being greater than the maximum tension.

Energy for Contraction

• ATP: Essential for muscle functions; it allows for cross-bridge detachment, calcium pumping back into SR, and ionic balance restoration. ATP depletes in 4-6 seconds.
• ATP is regenerated through three mechanisms:
  1. Creatine Phosphate (CP): Directly phosphorylates ADP to form ATP using creatine kinase.
  2. Anaerobic Pathway: Glycolysis converts glucose into ATP when oxygen is not available, producing lactic acid.
  3. Aerobic Pathway: Requires oxygen, yielding more ATP (32 molecules per glucose).

Muscle Fatigue

• Defined as the physiological inability to contract despite continued stimulation.
• Can be caused by ionic imbalances, increased inorganic phosphage, decreased ATP, or decreased glycogen.

Excess Postexercise Oxygen Consumption (EPOC)

• Refers to oxygen required to restore muscle to its pre-exercise state, including replenishing oxygen reserves, converting lactic acid back to pyruvic acid, and resynthesizing ATP and creatine phosphate.