Human Physiology/The Nervous System

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Figure 1: 

The right sympathetic chain and its

connections with the thoracic, abdominal, and

pelvic plexuses. (After Schwalbe.)

The Sympathetic and Parasympathetic Systems

The sympathetic nervous system activates what is often termed the

fight or flight response, as it is most active under sudden stressful

circumstances (such as being attacked). This response is also known as

sympathetico-adrenal response of the body, as the pre-ganglionic

sympathetic fibers that end in the adrenal medulla (but also all other
sympathetic fibers) secrete acetylcholine, which activates the secretion

of adrenaline (epinephrine) and to a lesser extent noradrenaline

(norepinephrine) from it. Therefore, this response that acts primarily on

the cardiovascular system is mediated directly via impulses transmitted

through the sympathetic nervous system and indirectly via

catecholamines secreted from the adrenal medulla.

Western science typically looks at the SNS as an automatic regulation

system, that is, one that operates without the intervention of conscious

thought. Some evolutionary theorists suggest that the sympathetic

nervous system operated in early organisms to maintain survival

(Origins of Consciousness, Robert Ornstein; et al.), as the sympathetic

nervous system is responsible for priming the body for action. One

example of this priming is in the moments before waking, in which

sympathetic outflow spontaneously increases in preparation for action.

The parasympathetic nervous system is part of the autonomic nervous system. Sometimes called the rest and digest

system or feed and breed. The parasympathetic system conserves energy as it slows the heart rate, increases

intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.

After high stress situations (ie: fighting for your life) the parasympathetic nervous system has a backlash reaction

that balances out the reaction of the sympathetic nervous system. For example, the increase in heart rate that comes

along with a sympathetic reaction will result in an abnormally slow heart rate during a parasympathetic reaction.

Organization

Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the

intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord and

extending into the second or third lumbar segments. Because its cells begin in the thoracic and lumbar regions of the

spinal cord, the SNS is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord in the

ventral branches (rami) of the spinal nerves, and then separate out as 'white rami' (so called from the shiny white

sheaths of myelin around each axon) which connect to two chain ganglia extending alongside the vertebral column

on the left and right. These elongated ganglia are also known as paravertebral ganglia or sympathetic trunks. In these

hubs, connections (synapses) are made which then distribute the nerves to major organs, glands, and other parts of

the body. [1]

In order to reach the target organs and glands, the axons must travel long distances in the body, and, to accomplish

this, many axons link up with the axon of a second cell. The ends of the axons do not make direct contact, but rather

link across a space, the synapse.

In the SNS and other components of the peripheral nervous system, these synapses are made at sites called ganglia.

The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is called a

postganglionic cell. As mentioned previously, the preganglionic cells of the SNS are located between the first

thoracic segment and the second or third lumbar segments of the spinal cord. Postganglionic cells have their cell