Question

In sleep, K-complex occurs spontaneously or in response to:

• A. Very slowly changing stimulus
• B. Very high stimulus
• C. Very low stimulus
• D. Sudden stimulus

Hint:

K-complexes are the hallmark of Stage 2 NREM sleep. They can happen spontaneously or be evoked immediately by an external, sudden stimulus (like a sudden sound).

Answer 1

The Correct Option is D

Concept: In polysomnography (the clinical study of sleep physiology), sleep architecture is classified into distinct stages by continuously analyzing electroencephalographic (EEG) activity. During Stage 2 Non-Rapid Eye Movement (NREM) sleep, the EEG trace displays two distinct neurophysiological signatures: sleep spindles and K-complexes. A K-complex is a large, distinct waveform composed of a sharp, high-voltage, brief negative deflection (upward wave on traditional EEG setups) followed immediately by a slower, broader positive deflection (downward wave). The entire complex must have a total duration of at least 0.5 seconds and stands out clearly from the background EEG activity.

Step 1: Mechanisms of K-Complex activation.
K-complexes are generated within cortical networks and serve a dual physiological purpose: they help protect sleep continuity (sleep gating) and contribute to memory consolidation. They can be triggered via two distinct pathways:
• Spontaneous K-Complexes: These are generated automatically by internal thalamocortical networks without any external environmental trigger, occurring periodically every few minutes.
• Evoked K-Complexes: These are triggered immediately in response to an abrupt sensory event in the sleeper's environment.

Step 2: Analyzing the role of external stimuli.
When an abrupt sensory disturbance occurs during sleep—such as a sudden auditory click, a sharp knock, a bump against the bed frame, or a quick tactile touch—the peripheral nervous system transmits this sensory signal to the brain. Instead of causing an immediate awakening (cortical arousal), the cerebral cortex generates an evoked K-complex. This waveform suppresses further cortical processing of the sound, acting as a filter that helps the brain ignore the sudden disturbance and maintain deep sleep. Because this mechanism is specifically triggered by abrupt environmental changes, the K-complex occurs in response to a sudden stimulus, validating Option (D).