Books/Autonomic nervous system/Physiology of autonomic nervous system

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Cardiovascular system

Both sympathetic and parasympathetic nerves innervate heart. The activation of the sympathetic outflow to the heart results in increased heart rate (tachycardia), force of contraction (positive ionotropy) and conductivity in the atrioventricular region (A-V region). On the other hand, activation of parasympathetic out flow of the heart results in decrease a in heart rate (bradycardia) and prolongation in the AV conduction time.

Role of Reflex action in regulation of heart activity

Reflexes regularize the heart activity. When there is sudden change in mean arterial blood pressure because of increased sympathetic activity leads to the activation of baroreceptor reflex and increased out flow of parasympathetic system. This reflex action slows the heart. The opposite mechanism works in case of hypotension in which parasympathetic system has increased activity.

Reproductive system

In male sympathetic stimulation causes vasoconstriction and contraction of the smooth muscles of the prostrate, seminal vesicles, prostatic urethra and vas deferens. The parasympathetic system promotes vasodilatation of blood vessels of the cavernous tissue of the penis and thus promotes penile errection therefore sympathetic system is important for ejaculation and the parasympathetic is for the penile errection.

Gastro intestinal tract

Both sympathetic and parasympathetic nerves sympathetic stimulation to the GIT inhibits peristaltic movements and increases the tone of the sphincter richly innervate the GIT. It does not inhibit gastric secretion. Parasympathetic stimulation increases peristaltic activity and the secretion of gastric and other digestive juices, but decreases the tone of sphincter.

Urinary bladder

The stimulation of sympathetic fibers innervating the vasculature of the bladder causes the urethral orifice to close. Parasymsympathetic stimulation causes contraction of the detrusor muscle and relaxation of the sphincter leading to emptying of the bladder. The micturation is a complex mechanism involving autonomic nervous system and partly voluntary control.

Eye

The smooth muscles that control the size of the pupil and the degree of the visual accommodation are supplied by autonomic nervous system. The radial muscle of the iris (dilator papillae) is innervated by sympathetic fibers. These fibers arise from cells in the super cervical ganglion, and their stimulation causes the contraction of the radial muscle fibers leading to dilation of the pupil (mydriasis). The dilatation of pupil so produced is known as active mydriasis. The circular muscle of the iris (constrictor pupillae) is innervated by Para sympathetic nerves arising from cervical ganglia. Stimulation of the cholinergic fibers causes contraction of the circular smooth muscle of the iris and this produces he contraction of the pupil (miosis)

The lens, which aids in visual accommodation, is attached to the cillary body by suspensory ligaments. When the smooth muscles of the cillary body are relaxed, the cillary body exerts tension on the lens causing it to flatten. Now this accommodated for far vision. Stimulation of the parasympathetic nerves causing contraction of the smooth muscles of the cilary body; this decreases the lateral tension on the lens. The lens thickens and the eye accommodates for near vision (cycloplegia). In this condition the pupil is widely dilated and the power for accommodation is lost simultaneously. Cholinergic and adrenergic receptors agonist and antagonist are very good target for treatment of various dysfunction as agonist or antagonist like heart disorder, glaucoma, urinary dysfunction, peptic ulcer, mydriatic and cyclopegic, preanesthetic medication Parkinson’s diseases, spasm, motion sickness etc.

Table 1. Organ and the receptors simulation

Fig.13 Mechanism of direct acting adrenergic drugs

Fig.14(a) Mechanism of indirect acting adrenergic drugs

Fig 14(b) Mechanism of indirect acting adrenergic drugs adrenergic

This is a free Pharmpedia textbook Author Prof.A.N.Nagappa