Biology a level - nervous system

how does a stimulus provide energy for depolarisation?

Thanks

how does a stimulus provide energy for depolarisation?
Thanks

It depends on the receptor - so I will give an example, using pressure receptor.
When you apply pressure onto a pressure receptor, it causes the layers of tissue, with fluid between, within the pacinian corpuscle to push inwards - changing the conformation of stretch mediated Na+ channels in the membrane. Apply pressure, more Na+ enters. At -40mV (threshold potential), voltage gated Na+ channels open… start of depolarisation.