Understanding the sympathetic and parasympathetic nervous systems
Knowledge of complex neural systems can help paramedics link medications to therapeutic goals
The human nervous system is complex, and it doesn’t take a brain surgeon to recognize that it controls just about every function within our bodies. Although there are numerous functions associated with the nervous system, this article focuses on the subdivisions of the sympathetic and parasympathetic nervous systems.
Understanding the sympathetic and parasympathetic nervous systems is especially relevant for paramedics because ALS personnel have access to medications that directly influence these two systems.
Overview of the nervous system
Before delving into the sympathetic and parasympathetic nervous systems, let’s look at the nervous system as a whole. The nervous system starts with the central nervous system, which includes the brain and spinal column. Our brain is the foundation of conscious and unconscious thought, as well as how our body interacts with the external environment.
The central nervous system branches into the peripheral nervous system. The peripheral nervous system entails the sensory division that brings information to the brain, and the motor division that sends information from the brain to the body. The sensory division can be referred to as the afferent division, and the motor division can be referred to as the efferent division.
The motor division is divided into the somatic system and autonomic system. The somatic nervous system controls body functions that are voluntary. Voluntary means we have conscious control over functions such as skeletal muscles, sight and hearing. The autonomic nervous system controls involuntary body responses like the heart, lungs and digestion.
The autonomic nervous system is then divided into the sympathetic and parasympathetic nervous systems. These divisions of the nervous system can be a little confusing; refer to the diagram that illustrates these divisions within the nervous system.
Sympathetic nervous system
Substances that activate the sympathetic nervous system are frequently referred to as catecholamines. The primary catecholamine responsible for activating the sympathetic nervous system is a neurotransmitter called norepinephrine.
The sympathetic nervous system rapidly directs the body’s involuntary response to a perceived and/or actual dangerous situation. It is also activated during increased physical activity. Many people refer to the sympathetic nervous system as the “fight or flight” system. This is because it helps our body physically “fight” or “flee” from a perceived and/or actual threat.
The physical reactions associated with the sympathetic nervous system are facilitated by the activation of cellular receptors. These receptors are frequently referred to as adrenergic receptors. The sympathetic/adrenergic receptors that will be addressed in this article are Alpha1, Alpha2, Beta1 and Beta2 receptors.
Alpha1 receptors are primarily located within the:
- Smooth muscle of the blood vessels in the brain, skin, gastrointestinal (GI) tract and kidneys
- Sweat glands
- Minimally scattered throughout skeletal muscle tissue
When activated, Alpha1 receptors cause the following physical responses:
- Pupil dilation
- Blood vessel constriction in the brain, skin, GI tract and kidneys
- Sweating (diaphoresis)
- Minimal effects on circulation to skeletal muscle tissue
Alpha1 receptors are primarily located within the:
- Stomach and GI tract
When activated, Alpha2 receptors cause the following physical responses:
- Decreased motility of the GI tract
- Decreased insulin secretion
- Blocked release of fat stores during increased catecholamine levels
Beta1 receptors are located within the heart and are responsible for causing the following responses:
- Increased chronotropy (heart rate)
- Increased electrical activity, referred to as dromotropy
- Increased muscle contraction, which is referred to as inotropy
Beta2 receptors are located within the:
- Smooth muscle of blood vessels in the skeletal muscles
- Smooth muscle of bronchioles of the lungs
When activated, Beta2 receptors cause the following physical responses:
- Blood vessel dilation in the smooth muscles
- Bronchiole dilation in the lungs
Cumulatively, all these physical responses increase the body’s ability to respond to actual and/or perceived stressful situations.
Parasympathetic nervous system
Substances that activate the parasympathetic nervous system are referred to as cholinergic agents. The primary cholinergic agent responsible for activating the parasympathetic nervous system is a neurotransmitter called acetylcholine, though there are other cholinergic agents that activate the parasympathetic nervous system.
The parasympathetic nervous system is responsible for controlling functions when the body is at rest. This is especially the case after eating and includes functions such as salivation, urination, defecation and sexual arousal. Because of these associated functions, many people refer to the parasympathetic nervous system as the “rest and digest” system. The parasympathetic nervous system is also responsible for but not limited to:
- Learning and memory
- Control of posture and body temperature regulation
- Endocrine and exocrine secretion
- Heart and blood vessels
Like the sympathetic nervous system, physical reactions are facilitated by the activation of cellular receptors. These receptors are frequently referred to as cholinergic receptors. The parasympathetic/cholinergic receptors that addressed in this article are muscarinic and nicotinic receptors.
There are five main types of muscarinic receptors. However, for the sake of illustration, there is little benefit in separately identifying the functions of each individual muscarinic receptor. It is more important to understand that these five muscarinic receptor groups are responsible for facilitating the physical responses associated with the parasympathetic nervous system. These muscarinic receptor functions include:
- Inhibiting central nervous system and nerve impulses
- Increasing GI function
- Decreasing cardiac heart rate (chronotropy), electrical impulse speed (dromotropy), and contraction strength (inotropy)
- Increasing exocrine gland secretion
At the neuromuscular junction, nicotinic receptors are the primary receptors found in skeletal muscles. Within this area of the body, they are responsible for transmitting an electrical impulse to the skeletal muscles. These nicotinic receptors play an important role in ensuring skeletal muscles contract properly. Nicotinic receptors get their designation because they respond to acetylcholine as well as the drug nicotine.
Human physiology and function
The sympathetic and parasympathetic nervous systems play multiple roles within human physiology and function. By using drugs that are typically available within a normal EMS medication formulary, EMS providers can directly, as well as indirectly, influence the sympathetic and parasympathetic nervous systems.
For this reason, it is important for advanced-level EMS providers to understand the general, along with specific, functions associated with each of these systems. This way EMS personnel will understand how these medications cause the therapeutic goals they are attempting to achieve within their patients.
- McCorry, LK (Aug 15, 2007). "Physiology of the autonomic nervous system." American Journal of Pharmaceutical Education. 71 (4): 78.
- Pocock, Gillian (2006). Human Physiology (3rd ed.). Oxford University Press. pp. 63–64. ISBN 978-0-19-856878-0.
- Schmidt, A; Thews, G (1989). "Autonomic Nervous System." Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.