This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Physiology
Sleep spindles are seen in which stage 
Select and explain. 
A. Stage 1, B. Stage 2, C. Stage 3, D. Stage 4
B: Stage 2
Answer: B: Stage 2 
Explanation: Sleep spindles are bursts of neural oscillatory activity that are generated by interplay of the thalamic reticular nucleus (TRN) and other thalamic nuclei during stage 2 NREM sleep in a frequency range of ~11 to 16 Hz (usually 12-14 Hz ) with a duration of 0.5 seconds or greater (usually 0.5-1.5 seconds) After generation in the TRN, spindles are sustained and relayed to the coex by a thalamo-thalamic and thalamo-coical feedback loops regulated by both GABAergic and NMDA-receptor mediated glutamatergic neurotransmission. Sleep spindles have been found in all tested mammalian species and in vitro cells. Ref: guyton and hall textbook of medical physiology 12 edition page number: 822,823,824