What is Dual Innervation?

Dual innervation is the instruction that an organ receives from both sympathetic and parasympathetic systems. Most of the organs in our body are innervated by both divisions of the autonomic nervous system (ANS). For example, the vagal parasympathetic innervation slows the heart rate, and sympathetic innervation increases the heart rate.

Do all the Organs have Dual Innervation?

Most of the body parts receive dual innervation, such as the effector organs adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system. Moreover, the control of blood pressure is considered to be a sympathetic activity, without the involvement of the parasympathetic system.

Organs with Dual Innervation

Most visceral organs have dual innervation from the autonomic nervous system:

• Antagonistic effects:The sympathetic and parasympathetic fibers innervate the same cells and the actions are opposite to each other. For example, the heart rate is increased by the sympathetic division and slowed down by the parasympathetic division. Likewise, digestion is inhibited by the sympathetic division and stimulated by the parasympathetic division. The dilation and constriction of the pupil are also regulated by the sympathetic and parasympathetic divisions respectively. Some organs, such as the heart are exerted through dual innervations of the same effector cells. In contrast, some organs are exerted because each division innervates different cells. For instance, the pupil dilator muscle dilates the pupil (sympathetic) and constrictor papillae constrict the pupil (parasympathetic).
• Cooperative effects:The sympathetic and parasympathetic stimulation produce different effects that work together to produce the desired effect. During salivation, the parasympathetic division stimulates serous cells of the salivary glands to secrete a watery, enzyme-rich secretion, while the sympathetic division stimulates mucous cells of the same glands to secrete mucus. The enzymes and mucus are the important components of saliva. Even when both divisions of the ANS innervate a single organ, they do not always innervate it equally or exert an equal effect. For example, an extensive plexus in the wall of the digestive tract is formed by the parasympathetic division that exerts much more influence over it than the sympathetic division does. In contrast to the digestive tract, there is more sympathetic innervation than the parasympathetic within the ventricles of the heart. Another example is ejaculation that takes place by the innervation of the sympathetic nerve. Both the sympathetic and parasympathetic fibers do function cooperatively in males for sexual function by stimulating the penis to become erect and ejaculating the sperms. In females. The parasympathetic nerves stimulate clitoral erection and vaginal secretions, and the sympathetic nerves cause an orgasm.

• Complementary effect: One of the examples of complementary effect is salivary gland secretion. The parasympathetic nerves are involved in stimulating the secretion of watery saliva, and also other components of digestive juices. But the sympathetic nervous system innervates the constriction of blood vessels of the digestive tract which results in decreased blood flow to the salivary glands and thus results in the production of viscous saliva.

Organs without Dual Innervation

Dual innervations are not always required for the autonomous nervous system to produce opposite effects on an organ.

Some effector cells such as the adrenal medulla, sweat glands, arrector pili muscles, and several blood vessels receive only sympathetic fibers. An example includes the regulation of blood pressure and routes of blood flow. The sympathetic fibers innervating a blood vessel have a baseline sympathetic tone that helps keep maintain the vasomotor tone(a state of partial constriction).

The fire rate is regulated by the following mechanisms:

• The blood vessels are maintained in a partially constricted condition.
• Vasoconstriction causes an increase in the firing frequency.
• Vasodilation causes a decrease in the firing frequency.

• The blood flow is shifted from one organ to another as per the need.

Sympathetic divisions that act alone exert possible effects on the target organ through the control of blood vessels. In case of stress, blood vessels to the heart and muscle dilate, and blood vessels to the skin constrict. The processes such as digestion, nutrient absorption, and urine formation can wait. Therefore, the sympathetic division constricts arteries to the gastrointestinal tract and kidneys. Blood vessels to the skin also constrict in case of any injury to minimize the bleeding. Moreover, due to an insufficient supply of blood to all the organ systems at once, it is required to temporarily divert blood away from some organs to supply a sufficient amount to the muscular system.

How Heart and Lungs are Dually Innervated?

Both the heart and lungs are dually innervated. A slight chance of the pace and the force of contraction of heart muscles are influenced by the effect of the vagus nerve on the pacemaker cells of the heart. Parasympathetic input to both right and left vagus (cranial nerve X) nerves reach the cardiac plexus supplying the heart. The preganglionic sympathetic fibers innervating the heart emerges from the T1-T4/T5 of the spinal cord. A normal human heart rhythm is 60-100 beats per min (bpm). Under resting conditions, the vagus nerve, through parasympathetic input, has the dominant influence that maintains the heart rate within 60-80 bpm. When there is stress or excitement or exertion, sympathetic stimulation can shift the heart rate to more than 100 bpm.  When the heart rate is slower which is less than 60 bpm, mediated by the vagus nerve and the parasympathetic nervous system, or an illness is known as bradycardia.  When the heart beats faster than the normal which is mediated by sympathetic nervous systems or can be due to illness, the condition is known as tachycardia.

In the case of lung innervations, the sympathetic and parasympathetic divisions of the autonomic nervous system nerves are arranged as the pulmonary plexus and innervate the smooth muscle and glands of the bronchi and pulmonary blood vessels.  The sympathetic preganglionic fibers supplying the lungs originate from T1-T4.  The sympathetic stimulation of the bronchial smooth muscles occurs through the release of epinephrine from adrenal glands. Thus, vasoconstriction and bronchodilation in the lungs are a result of sympathetic stimulation, whereas vasodilation and bronchoconstriction are caused by parasympathetic stimulation.     

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for      

• B.Sc. Biochemistry 
• M.Sc. Biological science
• M.Sc. in Biochemistry

Related Concepts 

Sympathetic, Parasympathetic