Why can some ions move across membranes but others cannot

because the membranes are selectively permeable.

Explanation:

Meaning it only allows certain ions to move across.

well, you see. the answer to your question is quite simple. the thing that is big, powerful, and over 9,000 is your mom.

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what do the different brain waves do?

explanation:

brainwaves are produced by synchronised electrical pulses from masses of neurons communicating with each other. brainwaves are detected using sensors placed on the scalp. brainwave speed is measured in hertz (cycles per second) and they are divided into bands delineating slow, moderate, and fast waves.