The Autonomic Nervous System is comprised of two distinct divisions: the sympathetic and parasympathetic divisions. Each division is responsible for different functions, typically acting as an accelerator and brake for actions that require a quick reaction. As such, the sympathetic division is often referred to as the “fight or flight” system while the parasympathetic division is responsible for actions that are more relaxed and require more time to act.
Sympathetic nervous system
The sympathetic nervous system is a key part of the autonomic nervous system, which controls many of the body’s automatic functions. It helps regulate your heart rate and blood pressure. It also controls digestion and sweating. However, the sympathetic nervous system is perhaps best known for its role in response to danger. When you are in danger, the sympathetic nervous system will activate to speed up the heart rate and deliver more blood to the muscles. In times of rest, the parasympathetic nervous system kicks in to maintain normal body functions.
The sympathetic nervous system is very important for regulating the body’s homeostatic processes. It is comprised of fibers that innervate almost every system of the body, including the heart and blood vessels. It is also responsible for regulating the pupil diameter, gut motility, and urinary output and function. In addition, sympathetic nerves mediate the neuronal stress response by secreting substances such as acetylcholine. This chemical then triggers the release of other neurotransmitters, such as adrenaline and noradrenaline.
The sympathetic nervous system originates in the spinal cord. It is composed of cells called preganglionic neurons, which extend in two columns near the spinal cord. These neurons send signals to other cells, including the postsynaptic cells. The message is then carried to its destination, the muscle or gland.
Visceral afferents
Visceral afferents project information from the visceral organs to the CNS via spinal afferent pathways. They are associated with reflex and affective functions. Their inputs influence integrator adjustments and responses at several levels. They may function as input arms of polysynaptic central relays and as sources of axon responses.
In addition to delivering information to the central nervous system, visceral afferents send information to effector organs. The reflex arc of these fibers involves sensory limbs, central neural processing, and efferent motor systems. Among other functions, visceral afferents transmit information to the PSNS and SNS, and are involved in maintaining homeostasis and normal digestive and bowel function.
Visceral afferents project to organs located in the abdominal, thorax, and pelvis. They are sensitive to temperature, chemical and mechanical signals. These impulses then lead to reflex control of organs, such as digestion and excretion.
Visceral afferents are distinct from cutaneous afferents. They have a higher number of terminal swellings, suggesting that they receive more information from the viscera.
Hypothalamus
The hypothalamus is a region of the brain that controls the autonomic nervous system. It integrates efferent and afferent fibers. These nerve fibers originate in the gray matter of the spinal column. They are divided into two subtypes, sympathetic and parasympathetic, based on the basic differences between them. The sympathetic division of the central nervous system derives from the first thoracic (T1) and second or third lumbar (L3) sections of the spinal cord, while the parasympathetic division originates from the medulla oblongata and sacral portion of the spinal cord.
The hypothalamus controls a variety of bodily processes, including temperature regulation. It initiates appropriate autonomic responses to maintain body temperature, including vasoconstriction, which redistributes blood to vital organs, and shivering, which produces heat.
The hypothalamus controls many aspects of behavior, including hunger, thirst, and pain. It also controls the release of hormones from the pituitary gland. The pituitary gland is located just below the hypothalamus. It has two lobes, the anterior pituitary and posterior pituitary, which are linked to the hypothalamus and secrete hormones.
Vagal nerve
The vagal nerve controls involuntary functions such as digestion. When it becomes damaged, digestion can become a problem. In such cases, healthcare providers may use electrical stimulation to treat the condition. This method helps the brain relax and calms irregular electrical activity in the vagal nerve.
The vagal nerve passes through the neck and connects the lower part of the brain with the chest and abdomen. This nerve also controls motor functions in the voice box and diaphragm, as well as sensory functions in the larynx and pharynx.
In some patients, the vagal nerve is underrepresented in the autonomic nervous system, and this can lead to severe disorders. In these cases, the remaining organs must compensate for the impaired organ’s decreased function. This is known as intrinsic autonomic dysfunction, which occurs due to primary damage to autonomic nerve fibers.
Another important function of the vagal nerve is its anti-inflammatory properties. It stimulates the splenic sympathetic nerve, which in turn releases noradrenaline at the distal end. This noradrenaline inhibits the production of TNF-a by spleen macrophages. This pathway is also important for the regulation of the immune response, particularly in diseases such as colitis.
Glossopharyngeal afferents
The glossopharyngeal nerve is a bundle of threadlike fibers that travels from the brainstem to the mouth and throat. Its function is to provide sensation to the organs and muscles of the mouth and throat. It originates in the brainstem’s medulla, the lower half of which contains the heart, respiratory, and vomiting centers. Glossopharyngeal ganglia are also a source of sensation.
In addition to providing sensory input to the tongue and throat, the glossopharyngeal nerve supplies organs and muscles in the mouth and throat. It also helps us taste food and detect pain. However, the nerve can be damaged by tumors, cancer, or medical procedures. However, a healthy lifestyle and surgical care by a qualified surgeon can reduce the risk of nerve damage.
The glossopharyngeal nerve originates in the brainstem and descends through the pharynx, alongside the jugular vein. Its branches are located in the lingual and hypoglossal regions of the tongue. They are responsible for controlling taste and saliva production. They receive sensory fibers from the tonsils and the posterior third of the tongue. They also contribute to the pharyngeal plexus.
Preganglionic autonomic neurons
Preganglionic autonomic neurons have long axons and a cell body in the central nervous system (CNS). They synapse with postganglionic neurons that are located in the autonomic ganglia. The axons of these neurons innervate the organs that they control.
Parasympathetic neurons originate from the central nervous system. They act on muscarinic and nicotinic receptors to regulate heart rate, digestive function, and salivation. Some release nitric oxide, while others act on muscarinic receptors.
The preganglionic autonomic fibers of the autonomic nervous system are located in the midbrain and hindbrain. They connect to the spinal cord and the cells that control the effector organs. They are either inhibitory or stimulatory depending on the receptors they send signals to.
Parasympathetic motor fibres
The sympathetic motor fibres are located in the spinal cord and are responsible for initiating the “fight or flight” response, which is a reflex response to a threat. These fibres release the neurotransmitter noradrenaline, which triggers the body to increase blood flow to the muscles and dilate blood vessels. These fibres also cause the heart to beat faster and increase the body’s temperature. They also control sweating and ejaculation. These fibres are part of the sympathetic and parasympathetic nervous systems, and in certain situations, they work together to control the various physiological events that prepare the body for self-defence.
The parasympathetic nervous system uses acetylcholine as its neurotransmitter. This neurotransmitter acts on both muscarinic and nicotinic receptors. Most transmissions are two-stage processes: first, the preganglionic neuron releases ACh to stimulate the postganglionic neuron, and then, the postganglionic neuron releases ACh to the muscarinic receptor. These receptors are located on the lateral sides of the spinal cord and brain stem.
Parasympathetic nerves also modulate smooth muscle tone, mucus secretion in the airways, and post-ganglionic efferent fibres. While the sympathetic and parasympathetic nervous systems are opposite in their functions, their dynamic interaction is crucial to the body’s survival. Inhibition of parasympathetic nerve traffic inhibits the activity of the sympathetic nerve, and vice versa.
