Nerve depolarization is due to -

Ans. is 'a' i.e., Opening of sodium channels Milk ejection reflexo Milk ejection is normally initiated by a neuroendocrine reflex.o The receptors involved are the touch receptors, which are plentiful in the breast-especially around the nipple,o Impulses generated in these receptors are relayed to the supraoptic and paraventricular nuclei,o Discharge of the neurons causes secretion of oxytocin from the posterior pituitary.o Oxytocin causes contraction of the myoepithelial cells, smooth muscle - like cells that line the ducts of the breast,o This squeezes the milk out of the alveoli of the lactating breast into the large ducts (sinuses) and thence out of the nipple ready to flow into the mouth of the waiting infant,o In lactating women, genital stimulation and emotional stimuli also produce oxytocin secretion, sometimes causing milk to spurt from the breasts.Phases of Action Potentialo The action potential, which is recorded using an intracellular electrode, has following phases : -Resting stage : - This is the resting membrane potential before the action potential. At rest, membrane is said to be "polarized" because - 70 mV negative membrane potential (RMP) that is present. Strictly speaking, it is not a part of the action potential.Depolarization phase : - The potential shoots up to + 35 mV in less than a millisecond. It is due to opening of "voltage gated" Na' channels which causes influx of sodiumQ, so inside negative charge becomes positive. From - 70 mV to -55 mV (firing Ivel or threshold point) the voltage gated sodium channles open partially. This portion of AP (- 70 mV to -55 mV) is called "prepotential" or "foot ofAP". Once the threshold level (- 55 mV) is reached voltage gated Na-i- channels open completely and the potential shoots up suddenly. It is called depolarization, since it represents loss of the original polarity of cell membrane.Repolarization phase : - The potential drops to near resting level, i.e., reversal to original polarity (repolarization). At the start of repolarization (end of depolarization), Voltage - gated Na+ channels close andVoltage - gated K+ channels open which cause efflux of K+ out of the cellQ. So, inside positive charge once again becomes negative due to efflux of positive charge (K+).After depolarization : - The repolarization is very fast till 70% of repolarization has been acheived. After that it is slows down. This slowrer phase of repolarization is called 'after depolarization \After-hyperpolarization : - The intracellular negativity overshoots the normal RMP of - 70 mV to reach a more negative value of about - 75 mV and then slowly retruns to the normal RMP of - 70 mV.