Have you ever wondered what part of our nervous system acts when we are in a situation of imminent risk?
Our nervous system is so extensive and deep that it has multiple divisions. Among them, the autonomic nervous system, responsible for involuntary structures such as the heart, smooth muscle and glands.
For this reason it is also known as the involuntary nervous system, and can be divided into: the sympathetic system and the parasympathetic system.
Next, we explain in detail the sympathetic system, its components and functions.
What is the sympathetic nervous system?
The sympathetic nervous system is one of the divisions of the autonomic nervous system. It is responsible for preparing and mobilizing the body in situations of emergency, exercise, fear or anger.
Both have different afferent and efferent nerve fibers. Afferent nerve fibers transmit nerve stimuli from a part of the body to the central nervous system. The efferent from the nervous system to the effector organ.
The sympathetic system and the parasympathetic system are physiological antagonists. That is, they cause opposite reactions in most organs.
However, both systems work synergistically, helping to keep the internal environment of our body in balance.
Structure of the sympathetic nervous system
The effector pathways of the sympathetic system and parasympathetic system are generally constituted by a chain of two neurons, they are the preganglionic and postganglionic ones.
Neurons communicate with other neurons of the autonomic nervous system through synapses, which are located in peripheral structures called autonomic ganglia.
The ganglion within contains the terminal axons of preganglionic neurons and cell bodies, and the dendrites of postganglionic neurons, as explained below.
Preganglionic neurons
Preganglionic neurons originate in the thoracic and lumbar spinal cord, in a region of gray matter called the lateral horn.
Preganglionic neurons pass from the spinal cord to the autonomic ganglia and are myelinated. Myelin is a protein sheath that covers nerve endings and makes nerve impulses faster.
Postganglionic neurons
Postganglionic neurons are those that go from the autonomic ganglia to the effector organs, and present unmyelinated postganglionic fibers, that is, they do not have myelin, conducting the impulse in a different way.
The sympathetic system presents long postganglionic fibers, this is because the ganglia of the sympathetic system are located in the vicinity of the spinal cord. For this reason, the journey to the target organ is longer.
Pathway of the sympathetic nervous system
In the autonomic nervous system division, the larger of the two parts is the sympathetic system, distributed widely throughout the body.
The sympathetic nervous system is made up of an efferent pathway that arises from the spinal cord, two ganglionic sympathetic trunks, nerve plexuses, and ganglia.
Next, we will detail its distribution throughout our body.
departure area
The sympathetic neuron cell bodies are found in the lateral horns of the spinal cord, running from the first thoracic segment to the lumbar segment of the spinal cord of the human body.
That is to say, that the body of the neuron is located within the central nervous system.
The myelinated axons of these neurons are those that leave the spinal cord through the anterior nerve roots, in the direction of the ganglia belonging to the sympathetic ganglion chain.
The preganglionic fiber reaches the ganglion in the sympathetic trunk, distributing in a cephalocaudal direction, that is, from top to bottom in the body.
Cervical Sympathetic Zone
Sympathetic postganglionic nerve fibers pass through the gray communicating rami to join the 8 cervical spinal nerves.
The cervical sympathetic trunk ganglia chain is located on each side of the cervical spine, made up of 3 ganglia: the superior ganglion, the middle ganglion (inconstant), and the inferior ganglion or stellate ganglion.
In terms of their function, the sympathetic nerves are directed to target organs within the head. For example, the pineal gland, the choroid plexus, the eyes, the lacrimal glands, the salivary glands, and the thyroid gland.
Upper Heart Zone
From the thoracic ganglion chain, a series of nerves called splanchnic cardiopulmonary are detached.
The nerves that go to the heart are intermingled with the vagus nerve fibers. Together, they form two nerve plexuses: the superficial (in front) and deep (behind) cardiac plexuses.
Similarly, the sympathetic and vagus nerves unite, forming an anterior and posterior pulmonary plexus. They are also in close relationship with the cardiac plexus and parasympathetic fibers.
Therefore, any stimulation of respiratory function also increases cardiac function.
Thoracic sympathetic zone
The sympathetic chain runs along the ribcage. There are a total of 10 to 12 ganglia that start from the entrance or beginning of the thorax to the pillars of the diaphragm.
They are two in number (left and right) and are located on each side of the spine.
The first thoracic nerve ganglion often fuses with the last cervical ganglion to form the cervicothoracic ganglion (stellate ganglion).
They supply nerves to the dorsum of the thorax, controlling superficial blood vessels, piloerection muscles, and sweat glands.
Regarding the functions of this system, it is in charge of the upper extremities, viscera of the thorax and the fibers that are part of the celiac plexus, whose nerves are distributed in the abdominal cavity.
Lumbar area
The sympathetic system acts on the viscera of the abdominal region through the abdominopelvic splanchnic nerves and the prevertebral and paravertebral ganglion chain.
In the abdominal region, these are the greater, lesser, and inferior splanchnic nerves. Together with the vagus nerve (the parasympathetic system), they form the celiac or solar plexus.
From the latter, derive the aortic plexus, the superior and inferior mesenteric plexus, the hepatic plexus, the adrenal plexus, and others.
Thus, this sympathetic system acts on the liver, spleen, lower esophagus, stomach, pancreas, kidney, adrenal glands, small intestine, and part of the large intestine.
Pelvic Area
The pelvic viscera are innervated by nerves from the sympathetic system, the parasympathetic system, and somatic nerves responsible for transmitting stimuli and sending responses.
Together, they form the inferior hypogastric plexus. It is a network of nerves, located on both sides of the body. In the man, they are located on the sides of the rectum. In women, on the sides of the rectum and vagina.
Nerve fibers from this system control the function of the descending colon, sigmoid colon, rectum, sweat glands in the pelvic region, bladder, gonads, and external genitalia.
Any injury to this point can cause urinary dysfunction (such as incontinence).
Functions of the sympathetic nervous system
The integration of the components of the autonomic nervous system, that is, the sympathetic nervous system, the parasympathetic system and the enteric system, control almost all the organs of our body.
The functions of the autonomic nervous system cannot be controlled. We cannot control the dilation of our pupil, nor the heartbeat when we experience fear.
The functions fulfilled by the fibers of the sympathetic system are multiple. However, we present its best-known functions.
increases heart rate
The sympathetic fibers form a network of nerves known as the cardiac plexus.
In moments of stress, fear, anguish or during physical activity, the fibers of the sympathetic system send stimuli to the cardiac fibers. This signal is transformed, in turn, into an electrical stimulus that increases the activity of the heart.
The heart fibers contract with greater force, pumping blood into the circulation with greater force and speed.
Promotes the release of adrenaline and noradrenaline
Epinephrine and noradrenaline are catecholamines. They are substances produced in the adrenal glands (mainly), pituitary glands, and other body tissues due to stressful factors.
Both are the main neurotransmitters released by postganglionic sympathetic fibers. When they are secreted, they are recognized by the membranes of different tissues.
In other words, the effects of the sympathetic system in our body are mediated by these two substances.
Adrenaline stimulates cardiac function, circulation, oxygen and glucose transport to tissues, especially muscle, heart and brain.
It also modifies sodium absorption in the kidney, decreasing the excretion of this metabolite.
When the kidney reabsorbs the sodium it filters, water is also reabsorbed. This keeps blood volume and, by extension, blood pressure elevated.
dilates the pupils
In the eyeball there are a series of muscles that regulate the diameter of your pupil, they are called ciliary muscles.
These are controlled by nerve fibers from the sympathetic. Intense light stimuli contract our pupil and, when we find ourselves in the dark, they dilate.
Also when we look at something we like (food, a person or an object), our pupils dilate. This is because oxytocin induces the release of adrenaline.
Also excessive consumption of alcohol or narcotics dilates the pupil.
increase breathing
The frequency and depth of respirations increase under stress. This is because the sympathetic nerves stimulate the phrenic nerve that goes to the diaphragm.
The diaphragm is a muscle that separates the abdominal and thoracic viscera. In addition, it is the main muscle of respiration. When taking in air, the diaphragm descends. When we expel the air, the muscle rises.
When we exercise or find ourselves in a stressful situation, the sympathetic nerve fibers stimulate the diaphragm to contract and relax faster and more forcefully, increasing the respiratory rate.
They also cause bronchodilation, allowing more air to pass into the lungs.
Limit unnecessary features
When our body prepares to face the stressful stimulus, certain functions are inhibited, favoring others.
How does it work? As we have already explained, the sympathetic system strictly regulates blood circulation and, therefore, the transport of oxygen and metabolites.
During exercise or when confronted with danger, the arteries that supply blood to the kidney, liver, and gastrointestinal tract constrict, reducing blood flow.
These tissues then receive only the minimum amount necessary for them to work and not enter a state of hypoxia.
Gastric emptying and gastrointestinal motility are decreased. Also the amount of blood filtered by the kidneys decreases and, therefore, the desire to urinate.
On the other hand, the metabolism of the muscle, brain and heart rise exponentially. This set of actions occurs to carry out the effective use of our body’s caloric reserves.
Increases glucose release
For evolutionary reasons, the body generally seeks to store as much energy as possible. Glucose is metabolized to glycogen, which is stored in muscle.
Under stress, the sympathetic inhibits glycogenogenesis, the formation of glycogen, and stimulates glycogenolysis, which is the release of glucose from glycogen.
It also stimulates, in the liver, glycogenesis, which is the production of more glucose from other metabolites.
Glucose is the body’s main fuel. It can be created and used quickly, making it an effective source of energy for the muscles, brain, and heart.
contract the sphincters
A sphincter is a group of muscle fibers that are arranged in a ring. They act as gates that regulate the passage of liquid or solid substances within our body.
An example is the sphincter that regulates the passage of the food bolus from the stomach to the intestine. In the bladder there are sphincters that allow or prevent the passage of urine.
The stimulation of the fibers of the sympathetic system acts on said sphincters and contracts them.
In this way, the passage of food to the intestine decreases, as well as that of urine from the bladder to the urethra.
Engages in sexual intercourse
Physiological responses to sexual stimuli (whether audiovisual or physical) are mediated by the autonomic (involuntary) nervous system and somatic (voluntary) innervation.
Preganglionic sympathetic nerve fibers emerge from the lower thoracic and lumbar segments of the spinal cord.
In a state of rest (that is, without sexual stimulation), the sympathetic system keeps the blood vessels of the genitals partially contracted. This reduces blood flow to them.
When stimulation occurs, the sympathetic system is inhibited and the parasympathetic system acts, dilating the blood vessels, increasing circulation to the genitals, and erection (either penile or clitoral) occurs.
In this way, the sympathetic is not an effector of sexual functions, but an inhibitor.
University Professional in the area of Human Resources, Postgraduate in Occupational Health and Hygiene of the Work Environment, 14 years of experience in the area of health. Interested in topics of Psychology, Occupational Health, and General Medicine.