for a brief revision of basic anatomy of Autonomic Nervous System.

1. Introduction to Autonomic Nervous System: Anatomy & physiology
Submitted by: Mehrab Tahir
S19-0324
Our Autonomic Nervous System is a part of the Motor Division of our Peripheral Nervous System, bifurcating into two sub-branches;
the Sympathetic Nervous System or the Fight/Flight/Fright responses and the Parasympathetic Nervous System or the
Rest/Digest/Repair/Defecate/Reproduce responses, producing its effects by means of neurotransmitters and hormones.
Sympathetic preganglionic fibers originate from the Thoraco-Lumbar (T1-L3/L4) region of our spinal cord and mostly synapse close to
the spinal cord in pairs of paravertebral ganglia present around the vertebral column. Some sympathetic preganglionic fibers may
bypass synapse in paravertebral ganglia and face either of the two fates; they may synapse at prevertebral ganglia located anterior
to the vertebral column or may directly synapse with the effector organs i.e. at intramural ganglia (e.g. w/ chromaffin cells of adrenal
medulla to release epinephrine (hormone) in the blood). It is to be noted that all sympathetic preganglionic fibers release acetyl
choline (Ach) at the ganglia while sympathetic postganglionic fibers release norepinephrine (NE) at effector cell’s adrenergic
receptors. The type of receptor mainly determines the type of response the effector will be showing. Alpha receptors mainly cause
constriction of smooth muscles upon activation with NE (e.g. constriction of blood vessels of GIT) while beta receptors mainly cause
smooth muscle relaxation (relaxation of blood vessels of skeletal muscles).
On the other hand, Parasympathetic preganglionic fibers originate from the cranio-sacral region (CN III, CN VII, CN IX, CN X and S2-
S4) of CNS. These synapse pretty far from their origin, either near their effector organs through terminal ganglia or directly with the
effector organs at intramural ganglia. It is to be noted here that all preganglionic and postganglionic parasympathetic fibers release
Ach as their neurotransmitter only. The parasympathetic/cholinergic receptors also are of different types (Nicotinic and Muscarinic)
where each type is specified for a certain effector cell/organ and particular effector response. It is noteworthy to know the
significance of the Vagus nerve (CN X) among all the other 12 cranial nerves as it is a mixed (motor + sensory) and most extensively
distributed (throughout the Thoraco-abdominal region) nerve of the parasympathetic nerve circuitry.
To maintain homeostasis, there ought to be a balance between the sympathetic and the parasympathetic tone (extent of
activation or deactivation) of the ANS normally regarded as the Autonomic Tone of a person. Physiologically,
parasympathetic tone is dominant over sympathetic tone to ensure smooth running of body processes.