Chapter 38 Main Points

🔑 Key Definitions

          Biological Rhythms: Regular cyclical changes in biological processes (circadian, ultradian, infradian)

          Circadian Rhythm: Approximately 24-hour cycle in biological processes; from Latin "circa" (about) + "diem" (day)

          Zeitgeber: Environmental time cue that synchronizes biological clock (German: "time giver"); light is primary zeitgeber

          Entrainment: Synchronization of biological rhythm to environmental cycle

🧠 Rhythm Characteristics

Period: Duration of one complete cycle (e.g., 24 hours for circadian)
Frequency: Number of cycles per unit time (inverse of period)
Amplitude: Magnitude of variation from baseline
Phase: Timing of peak or trough within cycle

📊 Circadian Control

SCN (Suprachiasmatic Nucleus): Master circadian pacemaker in hypothalamus
Retinal Input: Special photoreceptors send light information directly to SCN
Free-Running Period: Natural cycle without zeitgebers; usually slightly longer than 24 hours (~25 hours in humans)
Entrainment: Light-dark cycle synchronizes SCN to exactly 24 hours

📊 Phase Shifts

Phase Advance: Rhythm shifts earlier (e.g., travel east, earlier light exposure)
Phase Delay: Rhythm shifts later (e.g., travel west, later light exposure)
Jet Lag: Mismatch between internal clock and external time after rapid time zone change
Shift Work: Disrupted circadian rhythms from irregular schedules

💡 Exam Tips

SCN = master clock; lesion SCN = lose circadian rhythms
Zeitgeber = time cue; LIGHT is the most powerful zeitgeber
Free-running period in humans is ~25 hours (not exactly 24)
Remember rhythm parameters: Period, Frequency, Amplitude, Phase
Phase advance (earlier) harder to adapt to than phase delay (later)
Entrainment = synchronization to external cycle