autonomic divisions

Meaning: The autonomic nervous system is the motor output to involuntary effectors: smooth muscle, cardiac muscle and glands. The sympathetic division prepares the body for fight or flight: it raises heart rate, dilates bronchioles and pupils, diverts blood to skeletal muscle and slows digestion. The parasympathetic division favors rest and digest: it lowers heart rate, stimulates digestion and constricts pupils. Most organs receive both inputs, which usually oppose each other. Some texts add the enteric nervous system of the gut as a third division.

On the MCAT: Each pathway uses two neurons. Sympathetic: thoracolumbar origin, short preganglionic and long postganglionic fibers, ganglia near the spinal cord. Parasympathetic: craniosacral origin (including the vagus nerve), long preganglionic and short postganglionic fibers, ganglia near or in the target. Passages often describe a lesion or drug and ask for the effect on heart rate, pupil size or digestion.

Common confusion: Preganglionic neurons in both divisions release acetylcholine. Postganglionic parasympathetic neurons release acetylcholine, while postganglionic sympathetic neurons release norepinephrine at most targets; sweat glands are an exception. The adrenal medulla is innervated by preganglionic sympathetic fibers and releases epinephrine into the blood.