Sympathetic nervous system
The autonomic system is made up of two divisions, the sympathetic and parasympathetic systems. They usually work antagonistically in the organs, but in a well integrated manner. It is the balance of the actions of both divisions that maintains a stable internal environment in the body. While the sympathetic system is also important at rest, it is essential for preparing us for emergencies, in other words, for “fight-or-flight” responses. If you have ever been scared or anxious, attacked or threatened, you have basically experienced activation of your sympathetic system. To prepare yourself for an emergency in a “fight-or-flight” response, the sympathetic system activates numerous complex pathways and components to achieve faster breathing, increased heart rate and blood pressure, dilation of pupils, changes in blood flow, so blood leaves the skin, stomach and intestines and goes to the brain, heart and muscles where it’s needed, increased sweating and “goose-bumps” as the hair on your skin stands on end; all those things you feel during a fight-or-flight response. This article will discuss the components of the sympathetic nervous system, how the fibers reach their targets and the resulting effects.
Visceral nervous system
The human nervous system is the most complex product of evolution. It enables the body to react to continuous internal and external or environmental changes. It also controls and integrates various internal activities of the body. The nervous system can be divided structurally or functionally, as follows:
Sensory nervous system
Somatic sensory system
Visceral sensory system
Motor nervous system
Somatic motor system
Visceral motor system (Autonomic nervous system or ANS)
Sympathetic nervous system
The visceral nervous system is commonly known as the autonomic nervous system (ANS). It is involved in regulating the involuntary functions of organs and other visceral components, by mediating the activity of smooth muscle fibers, cardiac muscle fibers and glands. In order for the ANS to effectively regulate heart rate, blood pressure, digestion, etc., it needs to first detect changes within these visceral components. The sensing part is performed by the visceral sensory component, which consists of visceral sensory neurons and general visceral afferent fibres. Information from the continuous monitoring activities of the visceral sensory component are sent to the ANS, so that the visceral motor component can make the needed adjustments for the correct functioning of organs. The visceral motor component contains visceral motor neurons and general visceral efferent fibres. The sensory (afferent) and motor (efferent) fibers of the visceral system accompany each other in their trajectories.
The ANS consists of strictly visceral motor neurons and is divided into two divisions called sympathetic (SNS) and parasympathetic. The anatomical distinction between the divisions is given by the location of the presynaptic cell bodies and the types of nerves conducting presynaptic nerve fibers.
General sympathetic pathway
The general sympathetic pathway can be simplified into the following components:
The preganglionic components consist of preganglionic neurons located inside the spinal cord and their fibers (axons), which are called preganglionic fibers. The axons synapse with the postganglionic neuron inside sympathetic ganglia. These ganglia are actually a collection of cell bodies of postganglionic neurons, usually situated outside the CNS. Postganglionic components consist of postganglionic neurons and their fibers. The axons leave the ganglia and project onto visceral effectors, where they release the neurotransmitter norepinephrine. Both preganglionic and postganglionic neurons are multipolar.
The cell bodies of the preganglionic neurons of the SNS are found only in the intermediolateral cell columns (ICLs) of the spinal cord, one on the left side and on the right. ICLs are part of the lateral horns of the gray matter of the thoracic (T1-12) and upper lumbar (L1-2 or 3) spinal cord segments, hence the alternative name “thoracolumbar” for the sympathetic division. This region consists of the visceral motor region of the spinal gray matter. You can think of the ICLs as longitudinal tubes passing through the respective lateral horns of the spinal cord. The preganglionic SNS cell bodies are organised somatotopically, meaning the arrangement of the cell bodies is a close representation to that of the body. Basically, the T1-6 cell bodies that are located superiorly innervate the head, upper limb and thoracic viscera. T7-11 located in the middle innervate the body wall and abdominal viscera, while T11-L2(3) located inferiorly innervate the lower limb and pelvic viscera.
The preganglionic fibers leave the ICLs and thus, the spinal cord through the anterior roots. They travel very briefly through the anterior rami of spinal nerves T1-L2(3), before leaving them and passing to the sympathetic trunks (more details later) through the white rami communicantes (white because nerve fibers are covered with white myelin).
Types of ganglia
The ganglionic compartment is actually composed of the cell bodies of the postganglionic neurons. It consists of two types, paravertebral and prevertebral ganglia.
Paravertebral ganglia (“para” = alongside, beside) occur on either side of the vertebral column and are independently linked on either side, forming two sympathetic trunks (chains). The paravertebral ganglia are the site where preganglionic fibers synapse with postganglionic neurons. The trunks extend the entire length of the column, from the base of the cranium to the coccyx. They converge anteriorly to the coccyx, forming the ganglion impar (ganglion of Walther). Each trunk is attached to the anterior rami of the T1-L2(3) spinal nerves.
Prevertebral ganglia (splanchnic ganglia) are located in the abdominal cavity around the origin of the major branches of the abdominal aorta. The prevertebral ganglia form aggregations around the abdominal prevertebral plexus and are referred to as the celiac, aorticorenal and superior and inferior mesenteric ganglia. Various nerve plexuses branch from these ganglia.
Course of fibers
In general, after passing briefly through the anterior rami, preganglionic fibers enter the sympathetic trunk via white rami communicantes. Inside the trunk, preganglionic fibers can follow one of four courses:
Ascend and synapse in a higher paravertebral ganglion
Within the sympathetic trunk, preganglionic fibers usually from T1-5 spinal cord levels can ascend to other vertebral levels and synapse inside ganglia located at a more superior level. The ganglia might not necessarily be associated with inputs directly from the spinal cord (other nerves than T1-L2/3 can participate in the synapse).
Descend and synapse in a lower paravertebral ganglion
These are similar to the ascending preganglionic, but in contrast, they descend to ganglia located at a more inferior level. This pathway usually involves fibers from T5-L2(3). The ascending and descending preganglionic fibers gives the sympathetic trunk the appearance of a chain with connections between the ganglia.
Synapse directly in a paravertebral ganglion at the same level
After synapsing inside the ganglion, postganglionic fibers leave through a gray ramus communicans (grey due to absence of myelin) and re-enters the same anterior ramus, which it initially travelled through. The fibers are subsequently distributed to effector structures with peripheral branches of the anterior and posterior rami of the same spinal nerve. The fibers can also combine with fibers from other levels to form splanchnic nerves, which then pass onto the thoracic viscera (more details later).
Travel without synapsing all the way to the prevertebral ganglia
Preganglionic fibers can also pass through the sympathetic trunk without synapsing. These fibers are usually derived from the spinal cord levels T5 to L2(3). Once they pass through the sympathetic trunk, they combine with fibers from other levels to form and exit the trunk as a splanchnic nerve. Splanchnic nerves synapse on a prevertebral ganglia, and the postsynaptic fibers then pass onto the abdomen and pelvic viscera via a visceral motor nerve plexus.
The postganglionic compartment consists of postganglionic fibres travelling to effectors. The number of postganglionic fibers are greater than preganglionic ones. Approximately one preganglionic fiber synapses with at least thirty postganglionic fibers. After synapsing, postganglionic fibers leave the ganglia through gray rami communicantes and travel through the anterior and posterior rami of the spinal nerves. These rami carry the fibers all the way to the periphery and visceral components.
Ascending sympathetic fibers through the sympathetic trunk join peripheral nerves from C2-8 spinal nerves. These project onto effectors in the head, neck, upper limbs and thoracic cavity. For example, a cephalic arterial nervous branch leaves the superior cervical ganglion and projects onto the peri-arterial plexus on the carotid arteries. From here they project onto the dilator muscle of iris.
Descending sympathetic fibers through the sympathetic trunk join peripheral nerves from L3 to coccyx spinal nerves. These project onto the skin in the lower limbs, where they stimulate vasomotion, sudomotion and pilomotion.
Sympathetic fibers that enter and leave the trunk at the same level join peripheral nerves from T1-L2(3) spinal nerves. These project onto the body wall via cutaneous branches, but also via visceral motor nerves to sweat glands, smooth muscle and arrector pili muscles. Postganglionic fibers can also combine to form splanchnic nerves. These nerve types convey visceral efferent and afferent fibers to and from the viscera. Postganglionic fibers projecting onto thoracic viscera (e.g., heart, lungs, esophagus) pass through cardiopulmonary splanchnic nerves.
Sympathetic fibers which pass through the trunk without synapsing also combine with other fibers to form splanchnic nerves, of which there are five: greater, lesser, least, lumbar and sacral splanchnic nerves. Collectively these are called abdominopelvic splanchnic nerves. In this case, the synapsing happens in prevertebral ganglia rather than paravertebral ganglia. Postganglionic fibers from these prevertebral ganglia follow the main branches of the aorta and subsequently project onto all the organs (except adrenal glands) in the abdominal and pelvic cavities.
The adrenal glands are an exception. For every single human body organ, the postganglionic fibers synapse and release norepinephrine for regulation. However, for these glands, the nerves project directly onto the medullary cells without synapsing. The cells themselves play the role of the postganglionic neurons by releasing neurotransmitters, such as epinephrine (adrenaline), directly into the bloodstream. This results in a widespread sympathetic response.
The reach of the sympathetic system is extremely broad within the human body. It is a component of virtually all spinal nerves and peri-arterial plexuses, and sympathetic fibers innervate all the blood vessels, sweat glands, arrector pili and viscera. The only structures the sympathetic system does not reach are avascular structures, like nails and cartilage.
The sympathetic and parasympathetic divisions of the nervous system work in very close association, with contrasting, yet tightly coordinated effects. The sympathetic system is involved in energy-expending (catabolism), enabling the body to use energy appropriately to respond to stressful situations and emergencies, as in the “fight or flight” response. Activation of the sympathetic system results in pupil dilation, piloerection, vasoconstriction of cutaneous blood vessels, sweating, release of adrenaline, bronchodilation, increased cardiac contraction and reduced digestion.
During normal conditions, blood vessels are tonically maintained in a resting state of moderate vasoconstriction. If sympathetic signals are increased, vasoconstriction increases and vice-versa. However, in coronary vessels, skeletal muscles and vessels of the external genitalia, sympathetic stimulation results in vasodilation.