Muscle Physiology: Contraction, Energy, and Control

Muscle Contraction and Relaxation

Nervous Stimulation and Muscle Relaxation

Muscle contraction is initiated by nervous stimulation and ceases when the stimulus is removed. Several biochemical processes ensure that muscles return to their resting state after contraction.

• Nervous stimulation ceases: The signal from the motor neuron stops, ending muscle activation.

• Acetylcholinesterase (enzyme) breaks down acetylcholine (Ach): This prevents further stimulation of the muscle membrane.

• Calcium is pumped back into the sarcoplasmic reticulum (SR): Removal of calcium ions leads to the breaking of cross-bridge linkages between actin and myosin.

• Tropomyosin recovers the active sites: Tropomyosin covers actin binding sites, preventing further myosin attachment and contraction.

Motor Units and Muscle Control

Motor Unit Structure and Function

A motor unit consists of all muscle fibers controlled by a single motor neuron. The size and number of motor units recruited determine the precision and strength of muscle movements.

• Size of unit: Related to the degree of control; smaller units allow finer control.

• Recruitment: Movements begin with small units and add more as needed for increased force.

• Muscle tone: Some units are always activated, maintaining a baseline level of contraction even at rest.

Muscle Energy Metabolism

ATP Production and Usage in Muscle Cells

Muscle cells require ATP for contraction. The source and regeneration of ATP depend on the intensity and duration of activity.

• Resting cell:

  • Low ATP demand; surplus ATP is produced.

  • Reserves of creatine phosphate (CP) and glycogen are built up.

• Initial contraction:

  • Powered by ATP reserves, but these do not last long.

  • CP is used to regenerate ATP from ADP:

  • CP cannot be directly used by cells for energy, but donates its phosphate to ADP to form ATP.

  • This process supports maximum contraction for about 10 seconds, after which cellular respiration is required for new ATP.

Cellular Respiration Pathways

ATP is regenerated through two main pathways: anaerobic glycolysis and aerobic respiration.

• Glycolysis (anaerobic):

  • First step in cellular respiration; does not require oxygen.

  • Produces only 2 ATP per glucose molecule.

• Citric acid cycle and electron transport chain (ETC) (aerobic):

  • Requires oxygen; breaks down the rest of the glucose molecule.

  • Most ATP is produced in these steps.

  • Oxygen is supplied by hemoglobin (blood) and myoglobin (muscle).

Energy Supply During Different Activity Levels

The method of ATP production varies with the intensity of muscle activity.

• Rest and moderate activity: ATP demand is met by mitochondria using oxygen (aerobic metabolism).

• Strenuous activity:

  • Oxygen supply is outpaced by demand; glycogen, lipids, and amino acid reserves are exhausted.

  • Only about 1/3 of needed ATP can be provided aerobically.

  • Glycolysis is relied upon for ATP production (anaerobic).

  • Lactic acid threshold: When glycolysis produces pyruvate faster than mitochondria can use, pyruvate is converted to lactic acid.

  • Lactic acid lowers pH, causing muscle fatigue and inability to contract (e.g., sprinters' fatigue).

• Glycolysis under low oxygen:

  • Inefficient; only 4-6% of glucose energy is used.

  • Creates lactic acid and increases body temperature.

• Recovery:

  • Oxygen is used to convert lactate back to pyruvate.

  • About 30% of lactic acid is broken down to ATP; the rest is converted back to glucose.

  • Oxygen debt: The amount of oxygen needed to convert lactic acid and reset ATP and CP concentrations in muscle.

Muscle Adaptation and Training

Metabolic Capacity and Training Effects

Muscle metabolic capacity can be improved through training, affecting enzyme production and vascularization.

• High intensity training: Increases production of glycolytic enzymes.

• Aerobic training: Increases vascularization and stimulates production of more mitochondria.

Muscle Contraction Types and Force Generation

Muscle Twitch and Summation

A single impulse to a muscle fiber results in a muscle twitch, which is a brief contractile response. Multiple impulses can lead to sustained contractions.

• Muscle twitch: Can be recorded via a myogram; includes a latent period (delay between impulse and contraction).

• Summation: High-frequency impulses prevent full relaxation, causing contractions to combine and become sustained.

• Tetanus: At very high frequencies, relaxation is minimal, resulting in a continuous contraction.

Force Generation in Skeletal Muscle

Skeletal muscles are capable of generating significant force, depending on their size and activation.

• Maximum force: Up to 50 lbs per square inch of cross section.

• Large muscles: Can pull with several hundred pounds of force; excessive contraction can pull tendons away from bone.

Muscle Tone

Muscle tone refers to the continuous and passive partial contraction of muscles, even at rest.

• Function: Maintains posture and readiness for action.

• If muscle tone is lost, the body collapses due to lack of support.

Key Terms and Definitions

• Acetylcholinesterase: Enzyme that breaks down acetylcholine in the synaptic cleft.

• Creatine phosphate (CP): High-energy compound used to regenerate ATP.

• Glycolysis: Anaerobic breakdown of glucose to produce ATP.

• Myogram: Recording of muscle contraction.

• Summation: Increased force of contraction due to multiple stimuli.

• Tetanus: Sustained muscle contraction due to high-frequency stimulation.

• Oxygen debt: Extra oxygen required after exercise to restore muscle to resting state.

Summary Table: Muscle Energy Pathways

Pathway	Oxygen Required?	ATP Yield	Byproducts	Duration Supported
Creatine Phosphate	No	1 ATP per CP	Creatine, ADP	~10 seconds
Glycolysis	No	2 ATP per glucose	Lactic acid	Short-term, up to 1-2 minutes
Aerobic Respiration (Citric Acid Cycle & ETC)	Yes	~36 ATP per glucose	CO2, H2O	Long-term, hours

Key Equations

• ATP regeneration from creatine phosphate: $\text{CP} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• Glycolysis (anaerobic): $\text{Glucose} \rightarrow 2\ \text{ATP} + \text{Lactic Acid}$

• Aerobic respiration: $\text{Glucose} + 6\ \text{O}_2 \rightarrow 36\ \text{ATP} + 6\ \text{CO}_2 + 6\ \text{H}_2\text{O}$