Autonomic Nervous System

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What is the Autonomic Nervous System?

The autonomic nervous system (ANS), also known as vegetative nervous system previously, is a division of the peripheral nervous system (PNS) that regulates and controls internal organs without any awareness by the organism. It is a control system that unconsciously regulates bodily functions such as heart rate, digestion, urination, sexual arousal and fight-or-flight response.

The autonomic nervous system has three distinct divisions: sympathetic, parasympathetic and enteric. The system works through reflexes from the brainstem to the spinal cord and organs. It regulates functions such as cardiac regulation, vasomotor activity, respiration, reflex activities such as sneezing, swallowing, coughing and vomiting. It works in conjunction with the peripheral nervous system.

The sympathetic nervous system is associated with fight-or-flight response. On the other hand, the parasympathetic nervous system is associated with rest and digest. Both the divisions are opposite to each other as one initiates physiological response and the other inhibits it. However, this statement is often not considered because of many exceptions.

Structure

The sympathetic nervous system arises from the lumbar and thoracic regions of the spinal cord and terminates in the L2 or L3 areas. The parasympathetic system arises from the cranial nerves and ends at the sacral region of the spinal cord.

The sympathetic and parasympathetic nervous systems consist of afferent and efferent fibres that provide motor and sensory output to the central nervous system. Both the divisions have two types of neurons: a preganglionic neuron with its cell body in the CNS and a postganglionic neuron with its cell body innervating the target tissues.

The enteric system is independent from the other two systems. It has an extensive web-like structure that aids in the digestion process.

The motor neurons of the ANS are located in the autonomic ganglia. The parasympathetic motor neurons are located close to the target organs whereas the sympathetic motor neurons are located near the spinal cord.

Function

The sympathetic and parasympathetic divisions work complementary to each other, while one initiates quick fight-or-flight response, the other functions with actions that do not require quick or immediate response. If the sympathetic division is the accelerator, the parasympathetic division is the brake.

However, in some cases, the responses by the sympathetic and parasympathetic divisions are not entirely ‘quick response’ and ‘rest’. For example, the constant modulation of heart rate is done in conjunction by the sympathetic and parasympathetic divisions. These two systems can be better understood as working in antagonistic fashion to modulate bodily functions and maintain homeostasis.

The functions of the sympathetic division include:

• It diverts the blood flow from skin and gastrointestinal tract by vasoconstriction.
• It enhances blood flow to lungs and skeletal muscles.
• It allows greater oxygen exchange in the alveoli by dilating the bronchioles with the help of epinephrine.
• It enhances blood flow in the skeletal muscles by contracting cardiac cells and increasing heart rate.
• It dilates the pupils and relaxes the ciliary muscles, thus enhancing the vision in the eye.
• It vasodilates the coronary vessels of the heart.
• It constricts the urinary and intestinal sphincters.
• It helps in stimulating orgasm.
• It inhibits peristalsis.

The functions of the parasympathetic division include:

• It increases the blood flow towards the gastrointestinal tract by dilating the blood vessels.
• It constricts the bronchioles when there is no need of oxygen.
• The vagus nerves in the cardiac branches control the heart parasympathetically.
• It contracts the ciliary muscles and constricts the pupil allowing closer vision.
• It accelerates peristalsis by stimulating the salivary glands and thus enhances absorption of nutrients.
• They stimulate sexual arousal and also take part in the erection of genital tissues.

The enteric nervous system performs the following functions:

• Senses mechanical and chemical changes in the gut.
• It regulates secretions in the gut.
• Modulates the peristaltic movements.

The sympathetic neurons release neurotransmitters such as norepinephrine to the effector organs to act on the adrenergic receptors. Acetylcholine, a preganglionic neurotransmitter for both the divisions and is also postganglionic for parasympathetic division. It is cholinergic in nature. In the adrenal medulla, neurotransmitters are released to stimulate the nicotinic receptors.

Disorders of the ANS

1. Dysautonomia: Also known as autonomic dysfunction, in this condition the autonomic nervous system stops working properly. It directly affects the functioning of the heart, pupils, intestines, bladder, blood vessels and sweat glands. The primary symptoms of this condition include low blood pressure, anxiety, foggy brain, insomnia, dizziness, vertigo and constipation. It is caused either due to genetic reasons or injury to the autonomic nervous system.

Familial dysautonomia is a rare condition that is a mutation of the IKBKAP gene at the 9th chromosome. It is also known as Riley-Day Syndrome. It is a rare, progressive genetic disorder that affects the working of sympathetic and parasympathetic neurons.

2. Autonomic Neuropathy: It is a form of polyneuropathy that affects the autonomic nervous system, thus affecting the functioning of the heart, genital organs, bladder muscles and the digestive tract. The symptoms include urinary incontinence, vomiting, nausea, disturbed heart rate, dizziness, impotence and hypoglycemia.
3. Pure Autonomic Failure: It is a form of dysautonomia that affects men more than women in later stages of life. It is a degenerative disease that causes dizziness, neck pain, sexual dysfunction and fatigue.

Effect of Caffeine

Caffeine is a biochemical active compound found in beverages such as tea, coffee and sodas. Consumption of caffeine is known to cause increased sympathetic nerve outflow and heart beat. Caffeinated drinks increase parasympathetic activity. However, decaffeinated drinks inhibit parasympathetic activity.

It is believed that caffeine increases a person’s capacity to work while performing intensive tasks. The increase in body activity due to consumption of caffeine evokes other physiological functions to maintain homeostasis.

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Also Read:

• Peripheral Nervous System – Definition, Parts and Functions
• Nervous System Diseases – An Overview and Types
• What Is a Nerve? – Structure, Function and Types
• Important Solved NEET MCQs on Nervous System
• MCQs on Central Nervous System for NEET