Sympathetic and Parasympathetic Nervous Systems Essay

Sympathetic and Parasympathetic Nervous Systems - Essay Example The nerve fibers of the  parasympathetic nervous system  are the  cranial nerves, primarily the  vagus nerve, and the lumbar spinal nerves. When stimulated, these nerves increase digestive secretions and reduce the heartbeat. The sympathetic nervous system activates what is often termed the fight or flight response. Like other parts of the nervous system, the sympathetic nervous system operates through a series of interconnected neurons. Sympathetic neurons are frequently considered part of the peripheral nervous system (PNS), although there are many that lie within the central nervous system (CNS). Sympathetic neurons of the spinal cord (which is part of the CNS) communicate with peripheral sympathetic neurons via a series of sympathetic ganglia. Within the ganglia, spinal cord sympathetic neurons join peripheral sympathetic neurons through chemical synapses. Spinal cord sympathetic neurons are therefore called presynaptic (or preganglionic) neurons, while peripheral sympathetic neurons are called postsynaptic (or postganglionic) neurons. At synapses within the sympathetic ganglia, preganglionic sympathetic neurons release acetylcholine, a chemical messenger that binds and activates nicotinic acetylchol ine receptors on postganglionic neurons. In response to this stimulus, postganglionic neurons principally release noradrenalin (norepinephrine). Prolonged activation can elicit the release of adrenaline from the adrenal medulla. Once released, noradrenalin and adrenaline bind adrenergic receptors on peripheral tissues. Binding to adrenergic receptors causes the effects seen during the fight-or-flight response. These include pupil dilation, increased sweating, increased heart rate, and increased blood pressure. Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the