Sympathetic Vs. Parasympathetic Nervous System

Parasympathetic nerve fibers arise from the central nervous system with the S2, S3, and S4 spinal nerves and from the third, seventh, ninth, and tenth cranial nerves. Because of its location, the parasympathetic system is commonly referred to as having “craniosacral outflow”, which stands in contrast to the sympathetic nervous system, which is said to have “thoracolumbar outflow”.

The parasympathetic nerves that arise from the S2, S3, and S4 spinal nerves are commonly referred to as the ‘pelvic splanchnic nerves’ or the “nervi erigentes”.

The vast majority of sweat glands in the body are innervated by sympathetic cholinergic neurons. Sympathetic cholinergic neurons are sympathetic postganglionic neurons that happen to release acetylcholine instead of norepinephrine.

A. i.e. Ventral rami of Sz, S₃ and S₄

C i.e. Increased refractory period

D i.e. Pupillary constriction

Ans. C i.e. Acetylcholine

In the autonomic nervous system, acetylcholine is released in the following sites:

• All pre- and post-ganglionic parasympathetic neurons
• All preganglionic sympathetic neurons

– Preganglionic sympathetic fibers to suprarenal medulla, the modified sympathetic ganglion; on stimulation by acetylcholine, the suprarenal medulla releases epinephrine and nor epinephrine

• Some postganglionic sympathetic fibers

– Sudomotor neurons to sweat glands.

Ans. a. Skin

Sympathetic stimulation decreases blood flow in skin.

Sympathetic Stimulation

• Stimulation of sympathetic fibers causes vasoconstriction but it is not important, as the vasocontricor system is not well developed in cerebral vascular bed.
• Sympathetic stimulation increases myocardial blood flow through an increased metabolic demand and a predominance of beta-receptor activation.

Ans. is ‘c’ i.e., Neural crest

Ans. is ‘c’ i.e., Decreased GFR with afferent arteriolar constriction

Sympathetic stimulation causes afferent arteriolar constriction with decrease in RBF and GFR.

Effect of sympathetic discharge on the renal regulation of body fluid and electrolytes

Sympathetic discharge on the kidney results in :-

• Decreased GRF.

1. Increased reabsorption of Na⁺ and water from the PCT.
2. Increased reabsorption of Na⁺ and water from the DCT.
3. Thirst (due to angiotensin II production).
4. The overall effect is fluid and electrolyte retention due to :-

• Decreased urinary output.

1. Decreased urinary Na’ excretion.
2. Increased water intake.

Ans. is ‘c’ i.e., Dorsolumbar

Ans. is ‘d’ i.e., Acetylcholine

• Neurotransmitter in all preganglionic autonomic nerves (both sympathetic and parasympathetic) is acetylcholine (ACH)
• Neurotransmitter in all ganglia (both sympathetic and parasympathetic) is acetylcholine.
• Neurotransmitter in postganglionic parasympathetic fibres is acetylcholine.
• In posganglionic sympathetic fibres, the major neurotransmitter is noradrenaline (NA) except in renal and mesentric vasculature where it is dopamine, in sweat glands, some blood vessels where it is acetylcholine and in adrenal medulla where it is adrenaline.
• Most of the visceral organs are supplied by both sympathetic and parasympathetic system except;
• Blood vessels, spleen, sweat glands and hair follicles receive only sympathetic innervation.
• Ciliary muscle gastric and pancreatic glands receive only parasympathetic innervation.
• In general sympathetic and parasympathetic systems are antagonistic except;
• Refractory period of atrial fibres is decreased by both.
• At almost all organs except heart, cholinergic system has excitatory activity and adrenergic system has relaxing properties Sympathetic system stimulates (Tachycardia, Positive inotropic) and parasympathetic system depresses (Bradycardia, Negative inotropic) the heart.