Muscle Control and Contraction Types

Control of Muscle Tension

• Muscle tension depends on the number of muscle fibers contracting and the tension developed by each contracting fiber.
• Factors influencing tension development:
  • Frequency of stimulation
  • Length of the fiber at the onset of contraction
  • Extent of fatigue
  • Thickness of the fiber

Twitch Summation and Tetanus

• A single action potential produces only one muscle twitch.
• Repeated fiber stimulation produces longer and greater tension.
• Stimulation of the muscle fiber by a second stimuli before it has completely relaxed causes a second contractile response that adds on top of the first twitch, known as twitch summation.
• Tetanus is a continuous contraction without relaxation due to rapid stimulation, which is 3-4 times stronger than a single twitch.
• Twitch summation is possible because the action potential duration is much shorter than the twitch duration.

Types of Contraction

• Isotonic Contraction: The load remains constant as the muscle changes length. Used for body movements and moving objects.
• Isometric Contraction: The muscle is prevented from shortening so tension develops at constant muscle length.

Concentric and Eccentric Contraction

• Concentric Contraction: Muscle shortening while contracting.
• Eccentric Contraction: Muscle fiber lengthening while still actively contracted in opposition to being passively stretched by the load.

Muscle Tone

• Ongoing, involuntary, low-level state of tension in a muscle even at rest.
• Important in maintaining postural stability.
• Results from elastic properties of the muscle and continuous minimal stimulation by motor neurons.
• Regulated by postural reflexes and output from the multineuronal motor system.

Muscle Sensory Receptors

• Proprioceptors are found in joints and muscles providing information about body part location and movement.
• Two types:
  • Golgi Tendon Organ: Detects changes in muscle tension.
  • Muscle Spindle: Monitors muscle length.

Muscle Spindles

• External muscle (skeletal muscle) is innervated by alpha motor neurons.
• Internal muscle (skeletal muscle with muscle spindles) is innervated by sensory neurons and other motor neurons.

Production of ATP in Muscle Fibers

• Three pathways supply additional ATP during muscle contraction:
  • Creatine Phosphate: Provides ATP rapidly, especially at the start of exercise.
  • Oxidative Phosphorylation: Takes place in the mitochondria with the presence of oxygen. Provides a rich yield of ATP but is relatively slow.
  • Glycolysis: Used when oxygen delivery or oxidative phosphorylation cannot keep up with ATP demand. Can occur anaerobically, but has a lower ATP yield and leads to lactate production.

Fatigue

• Muscle fatigue occurs when the muscle can no longer respond to stimulation with the same degree of contractile activity.
• Central fatigue occurs when the central nervous system no longer adequately activates motor neurons.

Types of Muscle Fibers

• Slow-Oxidative (Type I) Fibers: Slow contraction speed, primarily use oxidative phosphorylation for ATP.
• Fast-Oxidative (Type IIa) Fibers: Fast contraction speed, use a mix of oxidative phosphorylation and glycolysis for ATP.
• Fast-Glycolytic (Type IIx) Fibers: Fastest contraction speed, primarily use glycolysis for ATP.

Effects of Exercise on the Muscle

• Aerobic Exercise: Promotes metabolic changes within oxidative muscle fibers by increasing mitochondria and capillaries.
• Resistance Exercise: Promotes hypertrophy of muscle fibers, increasing the number of myofibrils and sarcomeres.

This quiz explores the control of muscle tension, focusing on factors that influence tension development. It covers concepts such as twitch summation and tetanus, along with the different types of muscle contractions. Test your understanding of how muscles respond to stimulation and the mechanics behind muscle contractions.