The entire model illustrates several organs in the head: the vessels of the oral and nasal mucosae, the iris of the eye, and salivary glands and vessels. The heart and respiratory system are illustrated in the thoracic region.
The parasympathetic division (blue neurons) reaches all these organs -- except blood vessels -- as outflow from the cranium. The sympathetic division (orange neurons) reach these organs -- except the respiratory organs -- after synapsing in one of the three cervical ganglia.
The sympathetic ganglionic chains (small gold circles beside spinal cord) lie on each side of the vertebral column extending from the neck to the coccyx. However, preganglionic axons (solid orange lines) only reach the chain from the thoracolumbar region. To synapse in the upper and lower regions of the chains, these axons must first pass through thoracolumbar ganglia of the chain.
The three paired neck ganglia are named, respectively, the upper, middle and lower cervical ganglia. The postganglionic axons (dashed orange lines) that innervate targets in the head and thorax originate in these ganglia. Even though the model implies that the internal organs are only innervated from one of the two chains, anatomically the postganglionic axons from each side usually unite to form plexi (not shown in the model) around blood vessels until reaching their targets.
The inset to the right shows the preganglionic cell body (orange circle) in the spinal cord. Its axon (solid orange line) exits the cord and enters a thoracolumbar chain ganglion (lowest gold circle) before turning and passing through the lower and middle cervical ganglia. It synapses within the upper cervical ganglion and the postganglionic axon (dashed orange line) exits the ganglion to synapse with vascular smooth muscle (donut) encircling blood vessels.
The predominant receptor type on vascular muscles in these locations is the stimulatory alpha 1. This type receptor is more sensitive to norepinephrine (NE) that is released from sympathetic neuron terminals (orange bar) or delivered by the blood. The hormone epinephrine (E) also activates these receptors but to a lesser extent. The vessels respond by vasoconstricting and decreasing blood flow to the region. The predominance of this receptor type explains why sympathetic stimulation results in a "cotton mouth" sensation.
The pupil constricts when circularly arranged smooth muscles of the iris contract; the pupil dilates when radially arranged smooth muscles of the iris contract. The constrictor muscle is innervated by the parasympathetic division (blue) and the dilator muscle is innervated by the sympathetic division (orange).
The parasympathetic preganglionic neurons exit the cranium within the occulomotor nerve (III) to synapse within the ciliary ganglion (light blue circle); the neurotransmitter at this junction is acetylcholine -- not shown. The postganglionic neurons innervate the constrictor muscle and release acetylcholine (ACh) onto the stimulatory (color-code green) muscarinic 3 (M3) receptors. The response of the circularly arranged muscle fibers is to contract thus reducing the diameter of the pupil.
The sympathetic preganglionic neurons exit the thoracic region of the spinal cord, enter and pass through thoracic chain ganglia without synapsing, and travel to the superior cervical ganglia. There they synapse -- acetylcholine is the transmitter, not shown -- with postganglionic neurons which travel to the dilator muscle of the iris where they release norepinephrine (NE) onto stimulatory (color-code green) alpha 1 receptors. These radially arranged fibers contract thus increasing the diameter of the pupil. This explains why sympathetic stimulation causes the "pupils to dilate".
There are three sets of salivary glands: parotid, submandibular & sublingual. Only a 'generic' gland is illustrated in the model. These glands have different proportions of mucous and/or serous acini.
Acetylcholine (ACh) released from postganglionic parasympathetic terminals (blue bar) combines with stimulatory (color-code green) muscarinic 3 (M3) receptors on the glandular cells causing the release of mucin and/or amylase, depending on the gland, along with a fluid similar to interstitial fluid. As this mixture flows through the ducts its ionic composition is modified by active and passive transport processes.
The effect of sympathetic stimulation is indirect, affecting the stimulatory (color-code green) alpha 1 receptors on the vascular smooth muscle of blood vessels and not the actual glands. This causes vasoconstriction and the reduced blood flow provides less interstitial fluid to the glands. When the volume of saliva is low the suspended mucin causes it to be thick and viscous as opposed to high volume saliva which is thin and watery. This explains why "parasympathetic saliva" is abundant and watery while "sympathetic saliva" is scant and thick.
Myocardial cells (cardiac muscle) make up the bulk of the heart; the pacemaker (circle inside heart) consists of autorhythmic cells. The coronary arteries are shown inside the heart illustration.
The pacemaker, consisting of autorhythmic cells (see Cardiac Autorhythmic Cells in "Index" above), sets the heart rate. However, this intrinsic rate can be decreased when acetylcholine (not shown) is released from parasympathetic terminals (blue bar) and binds with inhibitory (color-code red) muscarinic 2 (M2) receptors. These nerves are the cardiac plexus derived from the vagus (X) nerve.
The heart's atria (upper chambers), consisting of myocardial cells (see Cardiac Myocardial Cells in "Index" above), also have muscarinic 2 (M2) receptors. When acetylcholine binds to these cells their force of contraction is decreased. There is no parasympathetic innervation of the heart's ventricles (lower chambers).
The heart rate can be increased when either norepinephrine (NE) or epinephrine (E) binds with stimulatory (color-code green) beta 1 (B1) receptors on the pacemaker's cells. Norepinephrine (NE) can be secreted directly on these cells by the sympathetic division (orange neuron); it can also diffuse (arrow) to these receptor sites from the blood supply (vessel illustrated on heart). The hormone epinephrine (E), also diffusing from the blood supply (arrow), is only slightly more potent in its effect on these receptors.
All myocardial cells also have beta 1 (B1) receptors and respond to both epinephrine (E) and norepinephrine (NE) by increasing the force of their contractions. Because the ventricles (lower chambers) are thicker-walled than the atria the overall force is greatest in this region of the heart. Sympathetic stimulation increases both the heart rate and force of contraction.
The parasympathetic division (blue neurons) is the dominant neural control mechanism of this system -- no sympathetic neural involvement is illustrated in the model.
Airway diameter, and therefore air flow, is regulated by circular smooth muscles (donuts); a normal tonic constriction is due to parasympathetic (blue neurons) stimulation. Acetylcholine (not shown) is secreted onto stimulatory (color-code green) muscarinic 3 (M3) receptors located on these smooth muscles. Excess bronchoconstriction is prevented due to inhibitory (color-code red) muscarinic 2 (M2) receptors located on the terminal (blue bar) -- the site of acetylcholine release. Acetylcholine molecules bind with the M2 receptors and cause a reduction in the subsequent amount of acetylcholine secreted.
Beta 2 (B2) receptors are also located on the smooth muscle cells (donuts) of the airways. These inhibitory (color-code red) receptors relax when the hormone epinephrine (E) binds to them; the result is bronchodilation and increased air flow.
Notice the lack of receptors on the vascular smooth muscle of the pulmonary blood supply. Blood flow to the lungs us governed by local control, i.e., respiratory gases. See "Respiratory System" in the INDEX above.
Submucosal bronchial glands secrete mucous and defensive chemicals onto the inner lining of the airways. The mucous and serous cells that secrete these components have stimulatory (color-code green) muscarinic 3 (M3) receptors; acetylcholine binding is responsible for the secretion of these compounds.