Inspection of the entire model shows these nerves, consisting of two-neuron , color-coded orange & gold. The cell bodies of the first or preganglionic neurons (orange circles) are located within the thoracolumbar region of the spinal cord. Their axons (solid orange lines) leave the cord to synapse with the cell bodies of the second or postganglionic neurons (gold circles); axons of these neurons are shown as dashed orange lines.
Two chains of ganglia lie along each side, i.e. paravertebral, of the spinal cord --only one is shown in the model. Each ganglion is actually a cluster of postganglionic cell bodies but only one cell body ( gold circle) represents each ganglion in the model. The nerve that links the ganglia into a chain consists of hundreds of preganglionic axons (orange lines) running up and down between the ganglia.
The illustration at the right shows how the preganglionic axon (top solid orange line) exits the spinal cord but does not synapse in the nearest ganglion. Instead, it bends and passes through other ganglia before synapsing. The postganglionic axon (dotted orange line) exits the ganglion to travel to the target organ. The lower preganglionic neuron shows an axon that does synapse with the nearest ganglion.
The illustration at the left shows the course of a postganglionic axon (dashed orange line) when its target is in the somatic region of the body. Inspection of the lower left side of the entire model shows these targets to be vascular smooth muscle within skeletal muscles and the dermis of the skin, the arrector pili muscle of hair follicles, and eccrine sweat glands.
The preganglionic neuron always originates (orange circle) in the thoracolumbar region of the spinal cord. Its axon (solid orange line) leaves the cord as the white ramus to synapse within the nearest sympathetic ganglion (gold circle). The postganglionic axon (dashed orange line) exits the ganglion as the grey ramus and enters the associated spinal nerve (not shown) to travel within it to the target organ.
The exception to the 'two neuron rule' is the innervation of the adrenal medulla; there is no postganglionic neuron involved...there is only a preganglionic neuron (orange). Its axon (solid orange line) exits the cord, passes through the nearest sympathetic ganglion (gold circle), penetrates the cortex (i.e., outer most part) of the gland to terminate within the medulla (i.e., inner most part) of the gland .
Inspection of the central area of the entire model shows three large, gold circles; these represent prevertebral/collateral ganglia which are at the midline of the body atop the aorta. They are called "prevertebral" because they lie "in front of the vertebrae." "Collateral" means "side by side" which probably refers to the celiac ganglia which are usually paired.
The three preganglionic axons (solid orange lines) entering them from the left represent the greater, lesser and least splanchnic nerves. The top most ganglion is called the celiac, the middle is called the superior mesenteric, and the lower most is called the inferior mesenteric. Their names are derived from the arteries upon which they sit.
The image to the left depicts the basic pattern that the two-neuron pathway uses to reach targets in the abdominal and pelvic regions. The preganglionic axon (solid orange line) passes through the nearby paravertebral/chain ganglion (gold circle) after exiting the spinal cord. This axon is long and continues toward the midline of the body where collateral ganglia (gold circle) are located. Numerous preganglionic axons are bundled together forming a splanchnic nerve.
The postganglionic axons (dashed orange lines) exit the ganglia and proceed to their target organs. These are the organs of the gastrointestinal and urogenital tracts.
Last update: 9/21/2005