Autonomic nervous system

The ANS operates without our conscious awareness. The two divisions of the ANS work in opposition.

Sympathetic division: stimulatory
The sympathetic division arouses the body, stimulating the heart to beat faster and inhibiting the digestion. It's not controlled directly by the spinal cord, but through autonomic control centers, gathered in the sympathetic chain ganglion. The chain is close to the spinal cord.

Parasympathetic division: Inhibitory
The parasympathetic division calms the body, slowing the heartbeat down and stimulating digestion. It's connected to the cranial and spinal nerves, but the ganglia between those nerves and the target organ are close to the organ, in contrast to the sympathetic division.

Neurotransmission
ACh from the CNS stimulates the parasympathetic division, and stimulates neurons from the sympathetic division containing norepinephrine, which stimulates SD. The effects on organs can be excitatory or inhibitory, depending on its receptors.