Question

can someone explain the 2 parts of the autonomic nervous system.

Answer 1

The classical division of the autonomic nervous system (ANS or involuntary NS) is into the sympathetic nervous system (SNS) and the parasympathetic (PSNS). The action of the sympathetic is largely to prepare for "flight or fight" responses such as increased heart rate, decreased blood flow to the gut, opening of bronchioles. This occurs by release of noradrenaline (norepinephrine if you're in the USA) and it's action on adrenergic receptors on target cells.
On the other hand, these responses need to be damped down once the need is passed so the parasympathetic nervous sytstem is more active. This has been termed "rest and digest" because blood flow is increased to the gut and away from the muscles.

Anatomically, the neurons responsible for both arms of the ANS start at different levels of the spinal column - the parasympathetic come out higher up than the sympathetic ones. Note that outflow (efferent pathway) from the spinal cord occurs through two neurones - a preganglionic and a postganglionic neuron. In both the SNS and the PSNS the preganglionic neuron releases the neurotransmitter acetyl choline whereas only the PSNS has acetyl choline released from the postganglionic.

I hope this clears things up a bit - I haven't touched on nitric oxide but there is some more to the system than I have described. Best Wishes