Muscle Physiology

Motor Units and Neuromuscular Junctions

The control of skeletal muscle contraction begins with the interaction between motor neurons and muscle fibers at specialized synapses called neuromuscular junctions (NMJs). Understanding these structures is essential for grasping how voluntary movement is initiated and regulated.

• Motor Unit: A motor unit consists of a single motor neuron and all the muscle fibers it innervates. All fibers in a motor unit contract simultaneously when the neuron fires.

• Neuromuscular Junction (NMJ): The synapse between a motor neuron and a skeletal muscle fiber. It includes the axon terminal, synaptic cleft, and the motor end plate.

• Motor End Plate: A specialized region of the sarcolemma (muscle cell membrane) containing nicotinic acetylcholine (ACh) receptors.

• Role of Acetylcholine (ACh): ACh is released from the motor neuron, diffuses across the synaptic cleft, and binds to nicotinic receptors on the motor end plate. This opens ligand-gated channels, allowing Na+ influx, generating an end plate potential that triggers a muscle action potential.

Example: Fine motor control (e.g., eye movements) involves small motor units, while powerful movements (e.g., jumping) involve large motor units.

Toxins and Drugs Affecting Skeletal Muscle Contraction

Certain toxins and drugs can disrupt normal muscle contraction by interfering with synaptic transmission or muscle excitability. The physiological effect depends on the site of action.

• Botulinum toxin: Inhibits ACh release, causing paralysis.

• Curare: Blocks ACh receptors, preventing muscle contraction.

• α-Bungarotoxin: Binds ACh receptors, blocking ACh binding.

• Saxitoxin and Tetrodotoxin: Block voltage-gated Na+ channels, inhibiting action potentials.

• Nerve gas and Neostigmine: Inhibit acetylcholinesterase, prolonging ACh action.

• Strychnine: Prevents inhibitory postsynaptic potentials (IPSPs) in the spinal cord, leading to excessive contraction.

Sliding Filament Model of Skeletal Muscle Contraction

Skeletal muscle contraction is explained by the sliding filament model, where thin (actin) filaments slide past thick (myosin) filaments, shortening the sarcomere without changing filament length. Both calcium ions (Ca2+) and ATP are required for this process.

• Sarcomere: The contractile unit, extending from one Z disc to the next.

• Key Bands: I band (thin filaments only), A band (thick filaments, with overlap), H band (thick filaments only, no overlap).

• Titin: Protein anchoring myosin to Z discs.

Phases of Contraction:

1. Initiation: ACh at NMJ triggers an action potential in the muscle fiber.

2. Excitation-Contraction Coupling: Action potential travels down T-tubules, causing Ca2+ release from the sarcoplasmic reticulum (SR). Ca2+ binds troponin, moving tropomyosin and exposing myosin-binding sites on actin.

3. Cross-Bridge Cycle:

   • ATP binds myosin, causing detachment from actin.

   • ATP hydrolysis repositions myosin head.

   • Myosin binds actin; phosphate release triggers the power stroke, pulling actin inward.

   • ADP release leaves myosin attached until another ATP binds.

4. Relaxation: Action potentials stop, Ca2+ is pumped back into the SR by SERCA pumps, and tropomyosin covers binding sites, leading to muscle relaxation.

Equation (ATP hydrolysis):

Muscle Contraction: Twitch, Summation, Treppe, and Tetanus

The force generated by muscle depends on the timing and frequency of stimulation.

• Twitch: A single contraction-relaxation cycle.

• Latent Period: Delay between stimulus and contraction.

• Summation: Increased force when a second stimulus arrives before full relaxation.

• Treppe: Gradual increase in contraction strength with repeated stimulation ("staircase effect").

• Incomplete Tetanus: High-frequency stimulation with partial relaxation.

• Complete Tetanus: Sustained contraction with no relaxation between stimuli.

Recruitment and Force Generation

Muscle force increases as more motor units are activated, a process called recruitment, which follows the size principle.

• Recruitment: Activation of additional motor units (multiple motor unit summation).

• Small motor units are recruited first; larger ones are added as needed for greater force.

• Asynchronous recruitment helps delay fatigue during sustained contractions.

Muscle Fiber Types and Fatigue

Skeletal muscle contains different fiber types, each with unique metabolic and functional properties. Muscle fatigue is a reversible decline in the ability to generate force.

• Slow-Twitch Fibers (Type I): Use oxidative phosphorylation, are fatigue resistant, have many mitochondria and blood vessels, and are important for endurance.

• Fast-Twitch Fibers (Type II): Rely more on glycolysis, develop tension quickly, fatigue easily, and have fewer mitochondria and blood vessels.

• Causes of Fatigue:

  • Central fatigue (nervous system origin)

  • Decreased ACh or receptor activation at NMJ

  • Extracellular K+ accumulation

  • Reduced SR Ca2+ release

  • Increased phosphate, lack of ATP, decreased glycogen, ADP buildup

  • Lactic acid accumulation is likely not the primary cause

Metabolic Requirements and Oxygen Deficit During Exercise

The source of energy for muscle contraction changes with exercise intensity, and an oxygen deficit occurs during intense activity.

• Low-Intensity Exercise (<70% max): Muscles primarily metabolize fats, sparing glycogen.

• High-Intensity Exercise (>70% max): Carbohydrate metabolism predominates; stored glycogen is used first, then plasma glucose.

• GLUT4: Glucose transporter inserted into the sarcolemma to increase glucose uptake during exercise.

• Oxygen Deficit: The difference between oxygen required and oxygen immediately available at the onset of exercise.

• Excess Post-Exercise Oxygen Consumption (EPOC): Elevated oxygen consumption after exercise, used to replenish ATP, phosphocreatine, and oxygenated myoglobin.

Reflex Arcs and Their Physiological Importance

Reflexes are rapid, involuntary responses that protect the body and help maintain posture. They follow a specific neural pathway called a reflex arc.

• Components of a Reflex Arc: Receptor, sensory neuron, CNS integration center, motor neuron, effector (muscle tissue).

• Reflexes are automatic and consistent, minimizing damage and contributing to posture and fall prevention.

• Knee Jerk Reflex Example: Stretch receptor activates sensory neuron, which bifurcates in the spinal cord; one branch activates quadriceps, the other inhibits hamstrings via interneuron.

Excitation-Contraction Coupling in Skeletal, Cardiac, and Smooth Muscle

While all muscle types use actin, myosin, calcium, and cross-bridge cycling, the mechanisms of excitation-contraction coupling differ.

• Skeletal Muscle: AP triggers Ca2+ release from SR; Ca2+ binds troponin; sliding filament mechanism.

• Cardiac Muscle: AP opens voltage-gated Ca2+ channels; Ca2+ influx triggers further Ca2+ release from SR; slower coupling allows heart filling.

• Smooth Muscle: Ca2+ binds calmodulin, activating MLCK, which phosphorylates myosin; relaxation via MLCP.

Autonomic Regulation of Smooth Muscle in Organ Systems

Smooth muscle is regulated by the autonomic nervous system, with sympathetic and parasympathetic divisions often producing opposite effects depending on the organ system.

Key Terms and Definitions

• Motor End Plate: Specialized region of sarcolemma with nicotinic ACh receptors.

• Sarcomere: Contractile unit from one Z disc to the next.

• MLCK (Myosin Light-Chain Kinase): Enzyme activated by Ca2+-calmodulin in smooth muscle.

• EPOC (Excess Post-Exercise Oxygen Consumption): Continued heavy breathing after exercise.

Practice Questions (Selected)

• Multiple Choice Example: During skeletal muscle contraction, calcium binds to troponin.

• Matching Example: SERCA pump – Active transport pump that returns Ca2+ to the sarcoplasmic reticulum.

• Fill-in-the-Blank Example: The contractile unit extending from one Z disc to the next is the sarcomere.

Summary Table: Muscle Fiber Types

Additional info: For a comprehensive understanding, students should review diagrams of the sarcomere, NMJ, and excitation-contraction coupling, as well as practice applying these concepts to clinical scenarios and laboratory data.