The autonomic nervous system (ANS) mediates actions that occur without voluntary control such as heart rate or blood pressure. It consists of both the sympathetic nervous system (SNS) and parasympathetic nervous systems (PNS), and they often act in a complementary manner.
The neural system of all animals is composed of highly specialised cells called neurons which can detect, receive and transmit different kinds of stimuli. The neural organisation is very simple in lower invertebrates. For example, in Hydra it is composed of a network of neurons. The neural system is better organised in insects, where a brain is present along with a number of ganglia and neural tissues. The vertebrates have a more developed neural system.
The central nervous system includes the brain and the spinal cord and is the site of information processing and control. The peripheral nervous system comprises of all the nerves of the body associated with the central nervous system (brain and spinal cord).
The afferent nerve fibres transmit impulses from tissues/organs to the central nervous system and the efferent fibres transmit regulatory impulses from the central nervous system to the concerned peripheral tissues/organs.
The peripheral nervous system is divided into two divisions called somatic neural system and autonomic neural system. The somatic neural system relays impulses from the central nervous system to skeletal muscles while the autonomic neural system transmits impulses from the central nervous system to the involuntary organs and smooth muscles of the body. The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system.
Visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system.
The autonomic nervous system (ANS) mediates actions that occur without voluntary control such as heart rate or blood pressure. It consists of both the sympathetic nervous system (SNS) and parasympathetic nervous systems (PNS), and they often act in a complementary manner.
The sympathetic nervous system is part of the autonomic nervous system, an extensive network of neurons that regulate the body’s involuntary processes. Specifically, the sympathetic nervous system controls aspects of the body related to the flight-or-fight response, such as mobilizing fat reserves, increasing the heart rate, and releasing adrenaline.
The origin of the sympathetic nervous system is found within the thoracic and lumbar segments of the spinal cord also known as the thoracolumbar division (T1 to L2,3). Sympathetic neurons have cell bodies located in the intermediolateral columns, or lateral horns, of the spinal cord. The presynaptic fibers exit the spinal cord through anterior roots and enter the anterior rami of T1-L2 spinal nerves and onto the sympathetic trunks via white rami communicantes. From here, the fibers may ascend or descend the sympathetic trunk to a superior or inferior paravertebral ganglion, respectively, pass to adjacent anterior spinal nerve rami via gray rami communicantes, or cross through the trunk without synapsing and continue through an abdominopelvic splanchnic nerve to reach prevertebral ganglia. Because of the central location of the sympathetic ganglia, presynaptic fibers tend to be shorter than their postsynaptic counterparts.
Sympathetic activation causes the radial muscle of the iris to contract, which leads to mydriasis, allowing more light to enter. The ciliary muscle relaxes, allowing for far vision to improve. Sympathetic activation causes an increased heart rate, the force of contraction, and rate of conduction, allowing for increased cardiac output to supply the body with oxygenated blood. Bronchodilation and decreased pulmonary secretions occur to allow more airflow through the lungs.
Stomach and intestines, decreased motility and sphincter contraction, as well as contraction of the gallbladder, occur to slow down digestion to divert energy to other parts of the body. Exocrine and endocrine pancreas decreases both enzyme and insulin secretion. There is relaxation of the detrusor muscle and contraction of the urethral sphincter to help stop urine output during sympathetic activation. Kidney increases renin secretion to increase intravascular volume. Salivary glands work through small volume potassium and water secretion.
The actions of the sympathetic nervous system occur in concert with other neural or hormonal responses to stress, including increases in corticotropin and cortisol secretion. In humans, chronic stress results in long-term stimulation of the fight-or-flight response, which leads to constant production and secretion of catecholamines (e.g., epinephrine) and hormones such as cortisol. Long-term stress-induced secretion of these substances is associated with a variety of physiological consequences, including hyperglycemia (high blood glucose levels), which can lead to type 2 diabetes mellitus, and hypertension (high blood pressure), which can lead to cardiovascular disease.
