Topic 7: Physiology of the Muscular System / บทที่ 7: สรีรวิทยาระบบกล้ามเนื้อ และรีเฟล็กซ์

A muscle twitch is a single brief contraction and relaxation cycle elicited by a single electrical or neural threshold stimulus.

Three Phases of Twitch Myogram:

1. Latent Period (~2 milliseconds): Delay between stimulus application and onset of measurable force. Events: AP travels along sarcolemma and down T-tubules; DHPR activates RyR1; $Ca^{2+}$ is released from SR into sarcoplasm; $Ca^{2+}$ binds Troponin C; tropomyosin moves; cross-bridge attachment begins. No visible shortening.
2. Contraction Phase (10–100 msec): Active cross-bridge power strokes generate tension; thin filaments slide past thick filaments; peak tension is reached.
3. Relaxation Phase (10–100 msec): Tension falls back to baseline; SERCA pumps actively resequester $Ca^{2+}$ back into SR terminal cisternae; cytosolic $[Ca^{2+}]$ drops; tropomyosin re-blocks actin active sites; elastic elements return fiber to resting length.

Effects of Stimulation Frequency on Tension

• Wave (Temporal) Summation: If a second stimulus is applied before the relaxation phase of the first twitch finishes, the second twitch adds directly to the first, producing higher peak tension. Cause: residual $Ca^{2+}$ remaining in cytosol from first twitch combines with newly released $Ca^{2+}$.
• Treppe ("Staircase Effect"): When identical stimuli are delivered at moderate rates to a rested muscle, each successive twitch generates slightly higher tension until a plateau is reached ("warming up" effect; caused by increased SR $Ca^{2+}$ availability and warming of myosin ATPase enzymes).
• Incomplete (Unfused) Tetanus: Produced by frequencies (~20–30 Hz) allowing only brief, partial relaxation between twitches. Graph shows an oscillating, wavy plateau of high tension.
• Complete (Fused) Tetanus: Produced at high stimulation frequencies (≥50 Hz); individual twitches fuse into a single, smooth, continuous maximal sustained contraction with NO relaxation phase between stimuli.

Henneman's Size Principle of Motor Unit Recruitment

Small motor units (small α-motor neurons innervating few Type I slow fibers) have the lowest threshold and are recruited first for light, precise control. As stimulus intensity increases, progressively larger motor units (larger fast Type II fibers) are recruited to generate greater force.

Skeletal muscles contain a mosaic mixture of three fiber types tailored to different functional demands:

The 5 Essential Elements of a Reflex Arc

1. Sensory Receptor: Responds to stimulus by producing a generator potential.
2. Sensory (Afferent) Neuron: Conducts nerve impulse through dorsal root into the spinal cord/brainstem.
3. Integration Center: Synapse within CNS gray matter (Monosynaptic = direct sensory-to-motor synapse; Polysynaptic = involving interneurons).
4. Motor (Efferent) Neuron: Transmits action potential from ventral horn out to effector organ.
5. Effector: Muscle or gland that carries out the reflex motor response.

Muscle Spindle vs. Golgi Tendon Organ (GTO)

• Muscle Spindle (in parallel): Monitors muscle stretch (length and velocity of change). Tapping patellar tendon stretches quadriceps ➔ activates Ia sensory afferents ➔ excites alpha motor neurons ➔ Monosynaptic Stretch Reflex (Knee Jerk; L2–L4).
• Golgi Tendon Organ (in series at tendon): Monitors muscle tension / contraction force. Excessive tension activates Ib afferents ➔ synapses on inhibitory interneurons ➔ causes autogenic relaxation of contracting muscle (Inverse Stretch Reflex; protects against tendon avulsion).

Clinical Reflex Grading & Jendrassik's Maneuver

Clinical deep tendon reflex testing with percussion hammer and Jendrassik maneuver reinforcement