Can someone PLEASE help me in how neurones help a pupil dilate ? - a level biology

I'm assuming this question is asking about the pupil response in the dark. When light hits the photoreceptor (i.e. rod/cone), it actually causes the photoreceptor cell to become hyperpolarised. Therefore in the dark, the photoreceptor is depolarised. The photoreceptor cell synapses with a bipolar cell, and the bipolar cell then synapses with the ganglion cell (this is the sensory neurone in this case). Therefore, in the dark the ganglion cell is not depolarised and does not fire an action potential. As macpatgh-Sheldon has already said, the ganglion cells make up the optic nerve. When light is shone in the eye, the impulse then travels along the optic nerve to the midbrain (part of the brainstem) and synapses with motor neurones which innervate the radial pupillary muscles (via the oculomotor nerve). This then causes the pupillary sphincter muscle to contract, causing the pupil to constrict. In the dark however, I don't believe the dilation of the pupil is due to an increase in sympathetic activity. Yes, sympathetic fibres innervate the radial muscles which, when contracted, dilate the pupil. However, in the absence of light, there will be less parasympathetic activity, and the size of the pupil is determined by the balance between sympathetic and parasympathetic activity. Therefore, a decrease in parasympathetic activity will cause the pupil to dilate, even though the sympathetic activity remains unchanged.