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The autonomic nervous system (ANS) is an essential part of the peripheral nervous system (PNS) concerned with the body’s involuntary processes, such as blood pressure, heart rate, breathing rate, digestion, sexual arousal, etc. It supplies primarily to smooth muscles, cardiac muscles, and glands and regulates the vascular tone, heart contractility, and glands secretions. It makes the subconscious regulation of the vital processes possible, without which the normal physiological balance of the body will be disturbed.
It has different divisions to stimulate or inhibit different processes and has a network of fibres that relay information from peripheries to the different nuclei in the central nervous system and convey back the information; all this happens at a subconscious level without any voluntary controls.
In this article, we will try to learn about the structure and functions of the autonomic nervous system. We will understand the functioning of the sympathetic and parasympathetic nervous systems. We will also review the enteric nervous system and cover common diseases involving the autonomic nervous system.
Autonomic Nervous System
All the functions we perform fall under two categories; voluntary, which are under our conscious control, like walking, running, writing, etc., and involuntary, which aren’t under our conscious control, such as breathing, sweating, regulation of heart rate, blood pressure, etc. All these functions must be regulated so the body’s normal physiological processes go on without disturbance. The nervous and endocrine systems play a crucial role in this regulation.
The nervous system includes the central nervous system and peripheral nervous system. The peripheral nervous system is divided into:
- Somatic nervous system
- Autonomic nervous system
The somatic nervous system regulates the voluntary processes of the body, while the autonomic nervous system regulates the involuntary processes of the body. The autonomic nervous system (ANS) maintains the smooth functioning of the body processes and also prepares the body to deal with emergencies. Different reflex actions, such as coughing, sneezing, vomiting, etc., are all included in the duties of ANS. ANS regulates the autonomic process of the body through integrated reflexes involving the brainstem, spinal cord, and organs. Hypothalamus, receiving input information from the limbic system, acts as the integrator of information for autonomic functions.
The system generally comprises two series of neurons; preganglionic neurons, which have cell bodies residing in the CNS, and postganglionic fibres, which have their cell bodies located in peripheral tissues. The principal neurotransmitters involved in autonomic conduction include acetylcholine, norepinephrine, nitrous oxide, and serotonin.
ANS has afferent and efferent fibres, which transmit sensory information from muscles and glands to CNS and motor information from CNS to the effectors (muscles and glands). Two divisions of ANS; the sympathetic nervous system and parasympathetic nervous system, have preganglionic and postganglionic neuronal series, while the third division; the enteric nervous system, has a complicated neuronal network like a spiderweb capable of working independently of the other parts of the nervous system. Neurons link each other by synapses which may excite or inhibit the next neurons in the circuit depending upon the neurotransmitter released at the synapse.
Divisions of the Autonomic Nervous System
The autonomic nervous system (ANS) has three divisions that differ in structure and function. These are:
- Sympathetic nervous system
- Parasympathetic nervous system
- Enteric nervous system
The sympathetic nervous system is involved in the “fight or flight” response, while the parasympathetic system is active during the relaxed state. These two systems work against each other while the enteric nervous system regulates gastrointestinal motility.
Sympathetic nervous system:
The sympathetic nervous system involves preganglionic fibres, sympathetic ganglia, and postganglionic fibres. It’s involved in the “fight or flight” response. Sympathetic neurons are located in the intermediolateral columns or the lateral horns of the spinal cord. They leave the spinal cord through the anterior roots and join the anterior rami of T1-L2 spinal nerves. Sympathetic ganglia include paravertebral ganglia (sympathetic trunk) and prevertebral ganglia (splanchnic ganglia).
Due to the central location of the sympathetic ganglia, presynaptic fibres of the sympathetic nervous system are smaller than the postganglionic fibres. Paravertebral ganglia are located in the sympathetic trunk adjacent to the spinal cord and receive preganglionic fibres from CNS and send postganglionic fibres to the effectors. There are usually three cervicals, twelve thoracics, four lumber, and five sacral ganglia in the thoracic sympathetic chain.
Prevertebral ganglia are the part of neuronal complexes located around the abdominal aorta and include celiac, aortic renal, superior and inferior mesenteric ganglia. Postganglionic fibres of the sympathetic nervous system give innervation to the head and neck by C2-C8 spinal nerves and the abdominopelvic viscera by the T1-L2 spinal nerves.
The sympathetic system prepares the body to handle any emergency. During an emergency, it increases adrenaline secretion, heart rate, blood pressure and breathing rate and dilates pupils. It constricts blood vessels of skin and GIT, diverting the blood flow towards muscles to increase muscular energy and slow down other processes like digestion. So, it coordinates the different processes for surviving the danger. It is termed the “fight or flight” response.
The SNS is also active constantly at rest. It innervates the blood vessels and maintains their tone. If sympathetic supply to a blood vessel increases, vasoconstriction occurs and vice versa. However, increased sympathetic supply in coronary arteries, skeletal muscles, and external genitalia decreases the vascular tone. SNS also regulates the breathing cycle. It regulates immunity by innervating the immune organs such as the spleen, lymph nodes, and thymus.
Parasympathetic nervous system
The parasympathetic nervous system (PSNS) is concerned with the “rest and digest” response of the body. It works opposite to the sympathetic system and makes the body’s conditions normal when sympathetic activity isn’t needed. The parasympathetic system involves preganglionic and postganglionic fibres. The presynaptic neurons of PSNS are located in the medulla and the sacral segments of the spinal cord. They give long presynaptic fibres that leave the CNS and synapse with postganglionic neurons in the parasympathetic ganglia of the head or inside or adjacent to the target organs.
There are four parasympathetic ganglia in the head portion: ciliary ganglia, pterygopalatine ganglia, otic ganglia, and submandibular ganglia. They regulate the autonomic supply and functioning of salivary glands, lacrimal glands, and vasomotor tone of blood vessels in the head.
PSNS fibres leave the CNS via cranial nerves III, VII, IX, X and S2-S4 spinal nerves. The vagus nerve (cranial nerve X) forms the biggest portion of the parasympathetic nervous system. It gives parasympathetic supply to the major organs in thoracic and abdominal cavities. The sacral nerves give parasympathetic supply to the descending colon, sigmoid colon, and rectum.
The most common neurotransmitter involved in PSNS is acetylcholine; its pathways are called cholinergic pathways. The postsynaptic neurons of the parasympathetic nervous system are located in the parasympathetic ganglia located within or near the target organs. That’s why PSNS has longer presynaptic fibres and shorter postsynaptic fibres. The PSNS slows the heart rate, decreases blood pressure, and constricts the pupil. It promotes digestion by increasing peristalsis and GIT secretions.
Enteric nervous system
The enteric nervous system is a type of autonomic nervous system concerned with GIT. It controls the peristaltic movements of GIT and the movements of water and electrolytes across the intestinal surface. It is a complex network of neurons divided into two parts:
- Meissner’s plexus
- Myenteric plexus
Meissner’s plexus is present in the submucosa of GIT and regulates the movement of water and different electrolytes across the intestinal wall.
The myenteric plexus is located between the longitudinal and circular layers of GIT smooth muscles. It produces the peristaltic movements of GIT by local reflexes involving excitatory and inhibitory circuits. When the bolus enters the tract, it stretches the circular muscles in the propulsive segment, which contract while the longitudinal muscles of the segment relax. At the same time, the longitudinal muscles of receiving segment contract while circular muscles relax and the bolus is pushed forward. This coordinated movement of GIT by local myenteric reflexes pushes the bolus forward.
The enteric nervous system makes many connections with the sympathetic and parasympathetic nervous systems, which can affect the excitability of GIT.
Functions of the Autonomic Nervous System
Until now, we studied the structure and divisions of the autonomic nervous system. Now we will discuss the important physiological functions of ANS in detail.
The autonomic nervous system is crucial for the process of digestion. The food we eat is broken down into smaller pieces and then into building blocks while moving through the GIT. The smaller particles are absorbed while the undigested material is defecated out. All this movement of food from the mouth to the anus is under the control of ANS. The autonomic nervous system can slow down or speed up digestion according to the body’s needs. If the body is resting, the parasympathetic system dominates, and the digestion of food is fast. However, any disturbing stimuli or emergency activating the sympathetic system slows the digestive process. So the digestion process is greatly influenced by the sympathetic and parasympathetic nervous systems.
The autonomic nervous system greatly influences heart rate. Sympathetic and parasympathetic fibres supply the heart musculature, the sinoatrial node, and the AV node. The sinoatrial node is the natural cardiac pacemaker that doesn’t need any innervation to fire. However, the sympathetic and parasympathetic systems highly affect their depolarization rate.
Sympathetic fibres come from the superior, middle, and inferior cervical ganglia, while parasympathetic fibres come from the vagus nerve. When sympathetic activity increases during emergency conditions, sinoatrial firing increases, and the heart rate speeds up.
However, increasing parasympathetic activity decreases the SA activity and depresses the heart rate; it is the basis of carotid massage. It helps to decrease the heart rate in supraventricular tachycardia. In supraventricular tachycardia, the atrial contraction rate is greater than normal. Massage is performed at the carotid sinus to decrease the heart rate. It activates the parasympathetic system and decreases AV nodal conduction. However, in the case of an overactive carotid sinus, it can block heart contractility.
The autonomic nervous system handles the amount of blood flow in different body parts. If it’s necessary to decrease the blood from a body part, the sympathetic system constricts its blood vessels to decrease the blood flow or vice versa. For example, increasing the muscles’ nutrients and oxygen supply is necessary for emergencies. Hence, the autonomic nervous system increases the blood flow to muscles and decreases blood flow in GIT and skin. In this way, it can alter the blood flow in different body parts.
The autonomic nervous system is also involved in the regulation of blood pressure. Baroreceptors in carotid arteries are a good example to understand the influence of ANS on blood pressure. When blood pressure decreases, baroreceptors send a signal to CNS. The sympathetic activity is increased, so heart rate and peripheral vascular tone increase to maintain the blood pressure. Similarly, when blood pressure increases, baroreceptors again send signals to decrease sympathetic activity and maintain blood pressure. So ANS helps the body maintain blood pressure by altering the heart rate and peripheral vascular tone.
ANS regulates the amount of light entering the eyes by regulating the pupillary diameter. Increasing the diameter increases the light entering the eyes and vice versa. When we enter a dark room, ANS dilates the pupils to allow maximum light to enter the eyes so that we can see clearly. In contrast, when we enter in bright daylight, ANS constricts the pupils to limit the amount of light entering the eyes. It also regulates the pupillary diameter in emergencies.
When there is a “fight or flight” response, the sympathetic system dilates the pupils so that the person can easily observe the things to take a quick decision. During the resting stage, the parasympathetic system constricts the pupils to decrease light entering the eyes so that the person can rest.
The breathing rate is under the control of ANS. ANS regulates the blood supply, mucus amount and air passageways’ diameter. The ANS increases the respiratory rate during emergencies and decreases during rest. Involuntary reflexes such as coughing and sneezing are also under the control of ANS.
ANS also influences normal sexual responses of the human body. ANS controls the functions such as sexual arousal, ejaculation and vaginal secretions.
The autonomic nervous system (ANS) works to keep the body temperature at an optimum level. When it is hot outside, it starts the sweating process to reduce the body temperature and also increases the blood flow to the skin to increase heat loss. When external temperature decreases, ANS reduces blood flow to the skin to minimize heat loss.
Secretion of body fluids
The ANS controls normal body secretions. Different glands, including sweat, respiratory, and salivary glands, are under ANS control. Sympathetic and parasympathetic nervous systems work in a coordinated way to regulate their secretions.
Disorders of ANS
The ANS is crucial to maintain the normal physiological functioning of the body. Any abnormality in its functioning alters the normal body processes. Following are a few abnormalities of ANS:
Autonomic paralysis may arise due to spinal cord damage that disrupts the normal distribution of ANS fibres. It can cause serious consequences such as breathing failure, irregular heart rate and strokes if not treated timely.
Multiple system atrophy
Multiple system atrophy is a degenerative condition affecting the autonomic nervous system. It causes motor dysfunction, respiratory failure, bladder abnormalities and poor body coordination.
Orthostatic hypotension is when blood pressure drops when a person stands up from a sitting position. It causes dizziness or light-headedness. However, in severe cases, it can cause unconsciousness due to decreased cerebral blood supply.
The autonomic nervous system is the part of the nervous system controlling involuntary processes of the body, such as breathing, heart rate, blood pressure, sexual arousal and temperature regulation. It is divided into the sympathetic, parasympathetic and enteric nervous systems. Sympathetic and parasympathetic systems comprise preganglionic fibres, ganglia and postganglionic fibres and innervate smooth muscles, cardiac muscles and glands. The sympathetic nervous system prepares the body to cope with emergencies by increasing heart rate, blood pressure, breathing rate and dilating pupils. It increases muscle blood flow and inhibits digestion. PSNS works opposite to SNS, decreasing heart rate, blood pressure, and breathing rate and constricting pupils.
The enteric system comprises Meissner’s plexus and myenteric plexus. Miessner’s plexus controls the movements of substances across the intestinal wall, while the myenteric plexus controls the peristaltic movements of GIT. The sympathetic and parasympathetic nervous systems also influence the enteric nervous system.
The autonomic nervous system also regulates body temperature, body fluids secretions, coughing, sneezing and vomiting reflexes. Damage to the spinal cord can cause autonomic paralysis, disrupted cardiac rhythm and blood circulation and lead to respiratory failure. Neurodegenerative conditions such as multiple system atrophy damage ANS and cause respiratory failure. Orthostatic hypotension may arise due to disturbance in body fluids which affects the ANS response.
Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janis, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370
Image source: The Autonomic Nervous System