Sympathetic nervous system

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:

• Structural Organization

  • Central nervous system (CNS)

    • Brain

    • Spinal cord

  • Peripheral nervous system (PNS)

    • Parts of nervous system found outside the CNS (Spinal nerves, cranial nerves and ganglia)

• Functional Organization

  • 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

      • Parasympathetic 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:

• preganglionic
• ganglionic
• postganglionic

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.

Preganglionic components

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).

Ganglionic components

Postganglionic 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.

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.

Function

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.