The function of the autonomic nervous system (ANS) is to maintain homeostasis. It controls several systems such as digestion, respiration, cardiac activity, and exocrine glands among others. The ANS controls are exerted on targeted tissue and cells. The targets of the ANS are: cardiac tissue, smooth muscle, and exocrine glands. This system is made up of visceral motor (general visceral efferent [GVE]) components that operate automatically.
The ANS is divided into three major divisions: the parasympathetic, sympathetic, and enteric nervous system. The enteric system provides the intrinsic innervation for the gastrointestinal tract and mediates reflex peristaltic activity. Due to the unique and specific function of the enteric system, which is independent of the hypothalamus, that particular division will not be included in further discussions in this chapter.
Although the divisions are often considered as antagonistic, there are some situations where they work in concert or independently. For example, arrector pili muscles, which are associated with hair follicles in mammals, are solely innervated by sympathetic fibers. Conversely, reproduction requires both sympathetic (ejaculation) and parasympathetic (erection) activity.
Fig. 18.1 Circuit diagram of the autonomic nervous system. The central first (presynaptic) neuron uses acetylcholine as a transmitter in both the sympathetic and parasympathetic nervous systems neuron. Acetylcholine is also used as a neurotransmitter by the second (postsynaptic) neuron in the parasympathetic nervous system. In the sympathetic nervous system, norepinephrine is used by the noradrenergic neuron. Note: The target cell membrane contains different types of receptors (= transmitter sensors) for acetylcholine and norepinephrine. Each transmitter can produce entirely different effects, depending on the type of receptor. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Fig. 18.2 (a) The first order cell bodies of the sympathetic system are located in the intermediolateral cell column (IML) of the gray matter. (b) Organization of the gray matter, left oblique anterosuperior view. The gray matter of the spinal cord is divided into three columns (horns): (A) Anterior column (horn): Contains motor neurons (B) Lateral column (horn): Contains sympathetic or parasympathetic (visceromotor) neurons in select regions (C) Posterior column (horn): Contains sensory neurons. Afferent and efferent neurons within these columns are clustered in nuclei according to function. (Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
The function of the sympathetic system is to prepare the body for increased activity. It is commonly referred to as the “fight or flight response.” Activation of this division will increase heart rate, decrease digestion, and dilate pupils ().
The preganglionic fibers enter the sympathetic chain forming white communicating rami. White rami are named based on the fact that they contain preganglionic myelinated fibers. They are only present from T1 to L2/3 (a, b).
Some sympathetic preganglionic fibers terminate in the sympathetic chain by synapsing on second order neurons. They can synapse at the vertebral level they entered or they can ascend or descend within the sympathetic chain and synapse at a different level ().
Postganglionic fibers (unmyelinated) rejoin spinal nerves forming gray communicating rami. After joining the spinal nerve, the postganglionic fibers will travel to their target structure (cardiac tissue, smooth muscle, or glands). Gray rami are present the entire length of the spinal cord.
There are some preganglionic fibers that enter the sympathetic chain via white rami and exit without synapsing such as splanchnic nerves. Splanchnic nerves will ultimately synapse on second order neurons that are located in prevertebral ganglia (). From there, they will innervate abdominal and pelvic viscera. It should be noted that the term “splanchnic” denotes that the nerve innervates viscera. It does not indicate if it is sympathetic or parasympathetic or if it is pre- or postganglionic.
The greater, lesser, and least splanchnic nerves synapse on prevertebral ganglia in the abdomen. There is significant variability in terms of the specific target ganglia; however, the following relationships are fairly consistent:
Fig. 18.3 The sympathetic arm of the ANS is also called the thoracolumbar division due to the fact that the first order cell bodies are located in the spinal cord at the thoracic and lumbar levels. ANS, autonomic nervous system. (a: Reproduced with permission from Mattle H, Mumenthaler M, Taub E. Fundamentals of Neurology: An Illustrated Guide. 2nd Edition. © Thieme 2016.
Fig. 18.6 (a) Preganglionic fibers from first order sympathetic cell bodies enter into the sympathetic chain forming white communicating rami. Postganglionic fibers rejoin spinal nerves forming gray communicating rami. (b) White rami are named based on the fact that these axons are preganglionic and therefore myelinated. (a: Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
The function of the parasympathetic system is to conserve and restore energy. It is often called the “rest and digest response.” Activation of this division will slow heart rate, constrict pupils, increase peristalsis, and increase glandular activity ().
The parasympathetic division originates in the brainstem for most of the body and in the sacral area (IML S2–S4) for the hindgut and pelvic structures. The parasympathetics for the hindgut and pelvic structures are called the pelvic splanchnics ().
The efferent parasympathetic fibers travel in certain cranial nerves (CNs III, VII, IX, and X) and sacral spinal nerves (S2–S4). Due to its origin, it is sometimes called the craniosacral division ( ).
First order neurons (preganglionic) of the parasympathetic division are located in brainstem nuclei. The parasympathetic nuclei lie in close proximity to the nuclei of the cranial nerve they travel with. It should be noted that CNs III, VII, IX, and X all have a nucleus of their own motor. The preganglionic fibers from the parasympathetic nuclei travel with cranial nerves to reach their respective ganglia () (). The parasympathetic nuclei:cranial nerves associations are: