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• Human Nervous System comprises of Central Nervous System (CNS) & Peripheral Nervous System (PNS).
  • Central Nervous System includes Brain & Spinal Cord.
  • Peripheral Nervous System includes Somatic Nervous System & Autonomic Nervous System
    • Autonomic Nervous System is again divided into Sympathetic Nervous System & Parasympathetic Nervous System

Hierarchy of PNS

• Autonomic nervous system consists of output pathways of CNS which control involuntary functions like heart rate, blood flow, sweating, and digestive activities.

• Sympathetic and Parasympathetic divisions of the autonomic nervous system have opposite functions in regulating organ. One results in an increase and the other in a decrease in activity.

• Sympathetic division corresponds to arousal and energy generation (“fight-or-flight” response). E.g. faster heart beat, increase in blood pressure, increase in cardiac output, digestion is inhibited, liver converts glycogen to glucose, secretion of epinephrine (adrenaline) increases from adrenal medulla, prepares the body for emergencies.
• Parasympathetic division generally causes opposite responses that promote calming and return to self-maintenance functions (“rest and digest”). E. g. heart rate decreases, lowers blood pressure, reduces cardiac output, digestion is enhanced, and glycogen production increases.

• Both the sympathetic and parasympathetic divisions, involves a Pre-ganglionic and a Post-ganglionic neuron.
• Sympathetic pre-ganglionic neurons exit the CNS midway in thoracic and lumber regions of the spinal cord so called as Thoracolumbar system, and sends its axon to sympathetic chain of ganglia located just outside of the spinal cord.
• Parasympathetic pre-ganglionic neurons exit the CNS at the base of the brain and sacral region of spinal cord so also called as Craniosacral. These neurons travel in the vagus (tenth cranial) nerve and sends its axon to parasympathetic ganglion located near or within an internal organ.

• Pre-ganglionic neuron of both release acetylcholine as its neurotransmitter at their synapse.

• Post-ganglionic neuron of both has its cell body in the ganglion and sends its axon to synapse with smooth muscle in the target organs. Post-ganglion of sympathetic division release Norepinephrine while those of parasympathetic division release Acetylcholine as its neurotransmitter at their synapse.
• In Sympathetic nervous system, pre-ganglionic fibers are short and myelinated, post-ganglionic fibers are long and unmyelinated.
• In Parasympathetic nervous system, pre-ganglionic fibers are long and myelinated, post-ganglionic fibers are short and unmyelinated.

Exception:  Axons of some pre-ganglionic sympathetic neurons pass through the sympathetic chain of ganglia without synapsing and terminate within the medulla of the adrenal gland.

• In response to action potentials, adrenal medulla cells secrete the hormone epinephrine (adrenaline) and post-ganglionic neurons release norepinephrine.

Parasympathetic and sympathetic divisions of the Autonomic Nervous System