Neurotransmitters are chemical messengers that transmit signals between two neurons or from neurons to effector organs. Neurons in the Autonomic Nervous System or ANS are cholinergic or adrenergic, depending on the released neurotransmitter acetylcholine or noradrenaline. Acetylcholine facilitates signal transmission between preganglionic neurons and the autonomic ganglia of both the sympathetic and parasympathetic nervous systems. Acetylcholine also mediates neuromuscular transmission inducing voluntary muscle contraction in the SNS. Further, the postganglionic neurons of parasympathetic division are cholinergic in nature. In the sympathetic division, the postganglionic neurons are adrenergic and release noradrenaline. Also, the adrenal medulla releases a mixture of adrenaline and noradrenaline. A few NANC (Non-adrenergic non-cholinergic) transmitters, such as vasoactive intestinal peptide or VIP, also mediate autonomic transmission. Most neurons release more than one neurotransmitter simultaneously. For example, in cholinergic neurons, VIP is co-transmitted with acetylcholine. Each neurotransmitter stimulates specific receptors and modulates the pre- or postsynaptic effects.