Page 70 - 84 human physiolofy part-1

Chapter 4

as breathing, digestion (peristalsis), and regulation of the heartbeat. The Autonomic system consists of
the sympathetic and the parasympathetic divisions. Both divisions work without conscious effort,
and they have similar nerve pathways, but the sympathetic and parasympathetic systems generally have
opposite effects on target tissues (they are antagonistic). By controlling the relative input from each
division, the autonomic system regulates many aspects of homeostasis. One of the main nerves for the
parasympathetic autonomic system is Cranial Nerve X, the Vegas nerve.

The Sympathetic System

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.

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.

70 | Human Physiology