Describe and explain the differences in the function of the sympathetic and parasympathetic nervous systems.

The sympathetic and parasympathetic nervous systems tend to have opposite effects on the organs which are innervated by their nerves. The Sympathetic NS innervates organs via the sympathetic chain and the parasympathetic NS innervates only the viscera via the Vagus nerve.
The Sympathetic NS increases the heart rate, the force of contraction and thus the cardiac output. It dilates the pupils by causing contraction of the radial muscles of the iris. This allows more light to enter the eye. It relaxes the ciliary muscles whilst contracting the suspensory ligaments. This makes the lens long and thin for distant vision. Furthermore, the bronchioles dilate to allow a larger volume of air to be inhaled. The Parasympathetic NS decreases the heart's activity and excitability (Heart rate), constricts the pupils, by contracting the circular muscles of the pupil, and makes the lens thicker and shorter for near vision by contracting the ciliary muscles and relaxing the suspensory ligaments.
The Sympathetic NS does not affect the digestive system, but contracts the sphincter on the bladder and relaxes the detrusor muscle, delaying urination. It also increases glycogen mobilisation directly and via adrenaline production from the adrenal medulla. Smooth muscle contraction in arterioles leads to vasoconstriction and a rise in peripheral blood pressure. Ejaculation is controlled by the sympathetic nervous system.The parasympathetic NS increases the contraction of smooth muscle in the walls of the gut e.g. the small intestines, stimulates secretions of the digestive system e.g. bile. It contracts the detrusor muscle of the bladder and relaxes the sphincter of the bladder. It is also responsible for sexual arousal.
The differences in the actions of the sympathetic and parasympathetic NSs is explained by the sympathetic system preparing the body for physical activity and the parasympathetic system for resting and mobilisation of the gut and digestion.