Pharmacology of the Autonomic Nervous System

Physiology

Organization the central portions of the ANS are at the hypothalamus, brain stem & spinal cord. It is an efferent part of the CNS comprised of the Sym & Psy systems, responsible for the involuntary actions while innervating smooth mm, heart & glands.

The preggl. Neurons are located either in the brain stem or spinal cord. Axons leave the cord as myelinated preggl. fibers to an autonomic ggl. where they synapse with a postggl. neuron. The latter passes as an unmyelinated axon to a visceral cell.

Sympathetic nervous system (Sym) the preggl. neurons come from the thoracolumbar segments (T₁-L₂) of the spinal cord. Axons leave the cord to enter the Sym chain, where they can synapse, or ascend/descend in the chain before synapting in another segment. They can also leave the chain without synapting and enter a splanchnic nerve, which synapses at a prevertebral ggl.
From either option will come a postggl. neuron, which ends at the target organ.
Preggl. Sym fibers also reach the adrenal medulla, stimulating E & NE secretion into the blood. Sym alarm/stress response occurs in response to rage, fear or pain. It is characterized by ↑BP, muscle blood flow, metabolic rate, blood glucose level and glycogenolysis, as well as a decrease in blood flow to the kidneys & GI and coagulation time.
These effects allow us to perform much more actively than normal.

Parasympathetic nervous system (Psy) the preggl. neurons are located at the craniosacral segments (brainstem & S_2-4). The axons leave through nerves III, VII, IX, X & the pelvic splanchnic nerves. The Psy ggl. are found closer or in the target organ. Nerve III reaches the pupil and ciliary muscle of the eye. Nerve VII reaches the lacrimal, nasal & submandibular glands. Nerve IX reaches the parotid gland and nerve X supplies the visceral organs of the thorax & abdomen. The sacral part reaches the descending colon, the lower portion of the uterus and ext. genitalia.
Characteristics of the ANS

The major neurotransmitters are ACh & NE. All preggl. fibers of both divisions are cholinergic. Almost all postggl. Psy. fibers are cholinergic and most postggl. Sym. fibers are adrenergic, except for few, which are cholinergic through M-receptors (innervating sweat glands, piloerectors & some vessels in skeletal mm).

Both systems either excite or inhibit different structures and they might antagonize each other. Most organs though are dominantly controlled by one division. Always one of them will be active, even at resting state (autonomic tone).