Graph Identification

Q1. Identify the graph. Why is the second curve taller?

• P₂ (the second stimulus) is delivered during the relaxation phase of the first contraction — this is called wave summation in skeletal muscle.
• More importantly, the staircase/treppe effect occurs because each successive stimulus causes more Ca²⁺ to accumulate in the cytosol. With each action potential, Ca²⁺ is released from the sarcoplasmic reticulum, but the SR cannot fully recapture all of it before the next stimulus arrives. This progressively elevated intracellular Ca²⁺ results in stronger cross-bridge cycling and therefore greater force of contraction.

"When a muscle begins to contract after a long period of rest, its initial strength of contraction may be as little as one-half its strength 10 to 50 muscle twitches later... the strength of contraction increases to a plateau, a phenomenon called the staircase effect, or treppe." — Guyton & Hall Textbook of Medical Physiology

Q2. Is this effect possible in cardiac muscle? Justify.

Why tetanus/wave summation is NOT possible in cardiac muscle:

"Cardiac muscle cannot be excited again; that is, it is in its absolute refractory period... Therefore, tetanus of the type seen in skeletal muscle cannot occur. Of course, tetanization of cardiac muscle for any length of time would have lethal consequences." — Ganong's Review of Medical Physiology

The Bowditch Staircase (cardiac equivalent):

Summary Table

GRAPH 1 — Treppe (Staircase Effect) in Frog Muscle

Identification

• P₁ = first stimulus → produces a simple twitch
• P₂ = second stimulus applied after a short interval → produces a taller (stronger) contraction
• The wavy line at the bottom = stimulus marker trace on the kymograph drum

Q1. Why is the second curve taller?

1. The first few twitches are weaker, and each successive twitch is slightly stronger, forming a staircase pattern (hence "Treppe" = German for "staircase").
2. The second contraction is taller because:
   • The second stimulus arrives during the relaxation phase of the first twitch — this is temporal summation.
   • More importantly, with each stimulus, Ca²⁺ accumulates progressively in the sarcoplasm because the sarcoplasmic reticulum cannot fully resequester it before the next impulse. More Ca²⁺ → more actin-myosin cross-bridge formation → stronger contraction.
   • There is also a "warming up" effect — increased metabolic activity and optimal enzyme function contribute to the increasing force.

This is also called the Bowditch effect or beneficial effect of successive stimuli.

Q2. Is this effect possible in cardiac muscle?

• In cardiac muscle, the absolute refractory period is so long that it lasts almost as long as the mechanical contraction. So a second stimulus cannot excite the muscle while it is still contracting — wave summation and tetanus are impossible.
• A modified staircase (Bowditch staircase) can occur in cardiac muscle when heart rate increases — each beat becomes progressively stronger due to Ca²⁺ accumulation — but this happens between beats, not by summation within a single contraction.
• Tetanization of cardiac muscle would be fatal (the heart would stop pumping blood).

GRAPH 2 — Genesis of Tetanus (Frog Gastrocnemius Muscle Experiment)

Graph Identification

1. Simple twitches — individual contractions at low frequency (muscle relaxes fully between each)
2. Clonus (Incomplete Tetanus) — frequency increases; the muscle partially relaxes between contractions, producing a saw-tooth/wavy pattern
3. Complete Tetanus — frequency high enough that contractions fuse completely; the tracing becomes a smooth, sustained, elevated plateau
4. Fatigue (far right, if shown) — force begins to decline with prolonged stimulation

Q1. Identify the graph

Q2. Identify Graph B

• Clonus is the stage where stimuli arrive before complete relaxation but after partial relaxation of each twitch.
• The muscle cannot relax fully between stimuli, producing a characteristic oscillating/saw-tooth wave pattern.
• Each contraction summates partially on the previous one → overall tension is higher than a simple twitch.

Q3. Reason for the effect (Genesis of Tetanus)

For Clonus (Incomplete Tetanus):

• Stimulus frequency is greater than the frequency needed for simple twitches, but less than the frequency needed for complete tetanus.
• Each new stimulus arrives before the muscle has fully relaxed (i.e., during the relaxation phase of the previous twitch).
• The contractions partially summate — partial overlap of Ca²⁺ availability and cross-bridge cycling.
• The result: an oscillating, partially fused contraction (saw-tooth pattern).

For Complete Tetanus:

• Stimulus frequency is very high (in frog muscle: ~20–30/sec; in mammalian muscle: ~40–50/sec).
• Each new stimulus arrives before any relaxation occurs — contractions completely fuse.
• Sufficient Ca²⁺ is maintained in the sarcoplasm continuously between action potentials, keeping troponin-C saturated and cross-bridges perpetually cycling.
• The result: a smooth, sustained, maximum force contraction with no visible oscillations.

"When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together, and the whole muscle contraction appears to be completely smooth and continuous... Tetany occurs because enough Ca²⁺ ions are maintained in the muscle sarcoplasm, even between action potentials, so that a full contractile state is sustained without allowing any relaxation between the action potentials." — Guyton & Hall Textbook of Medical Physiology

Summary Table

Key Point for Exam

• Clonus frequency < Complete tetanus frequency
• In frog muscle, clonus occurs at ~10–15 stimuli/sec; complete tetanus at ~20–30 stimuli/sec
• The student's answer is correct: Clonus = frequency less than complete tetanus; Tetanus = frequency more than or equal to the critical fusion frequency