The Autonomic Nervous System and Its Two Outflows

A drug prediction needs a location as well as a receptor name. Activating an alpha-1 receptor in radial iris muscle widens the pupil. Blocking that receptor reduces this dilating response. Phenylephrine is an alpha-1 agonist used to produce pupil dilation during an eye examination.

The heart provides a different example. Sympathetic beta-1 activation increases heart rate. If sympathetic input is already active, a selective beta-1 antagonist tends to reduce that acceleration. This does not mean the drug directly blocks alpha-1 receptors in blood vessels. A prediction about the whole person's blood pressure also needs information about other tissues and reflex responses.

Muscarinic receptors carry parasympathetic signals at many organ targets. In the heart, their activation slows pacing. In circular iris muscle, it produces contraction and pupil constriction. Blocking iris muscarinic receptors therefore reduces constriction. Pupil dilation can result from activating one pathway or blocking an opposing pathway.

The divisions share some chemistry. Preganglionic neurons in both release acetylcholine onto nicotinic receptors in ganglia. Most sympathetic postganglionic neurons release norepinephrine onto adrenergic receptors, while parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors. Sympathetic nerves to most sweat glands also use acetylcholine and muscarinic receptors.

These examples explain mechanisms. They are not instructions for choosing, taking or changing a medicine.

To trace an autonomic reflex, identify what detects the change, where the sensory information is integrated, and which tissue responds. These are the sensor, integrator and effector. A nerve carrying a command is part of the output pathway. It is not the responding muscle or gland.

Consider a rise in arterial pressure. Stretch increases in the aortic arch and carotid sinuses, increasing baroreceptor firing. Aortic signals travel through the vagus nerve. Carotid sinus signals travel through the glossopharyngeal nerve. Both reach brainstem circuits that coordinate the baroreflex.

The response includes less sympathetic output and more parasympathetic output to the heart. The heart slows. Reduced sympathetic vasoconstrictor input also allows many blood vessels to relax. These changes tend to lower arterial pressure. Vessel relaxation here does not require direct parasympathetic stimulation of most systemic vessels.

Count the motor branch separately from the whole reflex. Its usual route contains a preganglionic neuron from the central nervous system to a ganglion, followed by a postganglionic neuron to the target. Sensory and integrating neurons are additional parts of the reflex, not extra links in this two-neuron motor count.

A digestive reflex can have a different route. Enteric circuits in the gut wall can process local sensory information and control muscles or glands without requiring a signal to enter the brain or spinal cord. Such circuits can contain sensory neurons, interneurons and motor neurons. The usual central autonomic motor chain does not describe every local enteric circuit.

An outflow's origin tells you where its preganglionic neuron cell bodies lie. It does not name every nerve or ganglion through which the signal later travels. Peripheral nerves can carry sensory information toward the central nervous system and motor information toward tissues.

Sympathetic preganglionic neurons begin in the thoracic and upper lumbar spinal cord. Their fibers can synapse in chain ganglia beside the vertebral column. Other fibers pass to prevertebral ganglia in front of it. Postganglionic neurons then supply targets. Some of their fibers join spinal nerves to reach structures such as sweat glands in the body wall and limbs.

Parasympathetic origins are in the brainstem and sacral spinal cord. Brainstem outflow travels in selected cranial nerves. Sacral outflow travels in pelvic splanchnic nerves. Parasympathetic ganglia lie near or within the organs they serve.

The distinction from somatic motor output concerns the target and pathway. Somatic motor neurons supply skeletal muscle, including during involuntary reflex movements. Autonomic motor pathways regulate cardiac muscle, smooth muscle and glands. Calling somatic movement exclusively voluntary would leave out those reflexes.

The usual autonomic motor route has two neurons, but there are exceptions. Sympathetic preganglionic fibers reaching the adrenal medulla activate secretory cells. Those cells release epinephrine and norepinephrine into the blood rather than sending a postganglionic axon to another organ.

Enteric networks are located in the gut wall. They receive sympathetic and parasympathetic input and can also coordinate local digestive responses without a command from the central nervous system.