Autonomics of the head and neck

Overview of the autonomic nervous system

General information

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    The autonomic nervous system (ANS) has control over the functions of many organ systems and tissues

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    Provides innervation to:

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      Cardiac muscle

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      Smooth muscle

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      Glands

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    Also provides innervation to the organs of the immune system and metabolic organs (mainly through the sympathetics)

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    The hypothalamus exerts control over the ANS and helps the body maintain homeostasis

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    The ANS uses a 2-neuron chain system:

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      Preganglionic neurons—the cell bodies are located in the central nervous system (CNS) (i.e., the brain and spinal cord), and their myelinated axons pass out to the autonomic ganglia

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      Postganglionic neurons—the cell bodies are located in the autonomic ganglia, which are outside of the CNS, and their unmyelinated axons travel to the effector organ

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    The ANS is divided into 2 parts:

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      Parasympathetic—the portion responsible for preserving and restoring energy

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      Sympathetic—the portion responsible for preparing the body for emergency situations

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    Organs typically receive dual innervation, which has an antagonistic action, although there are some notable exceptions, such as the arrector pili muscles (which are sympathetic only) and the male sexual response (erection is parasympathetic, ejaculation is sympathetic)

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    Acetylcholine and norepinephrine are the 2 major neurotransmitters used in synapses of the ANS

Figure 20-1

Figure 20-2

Divisions and functions

DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM
Parasympathetic Sympathetic
Referred to as craniosacral fibers Referred to as thoracolumbar fibers
Arise from:

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    Cranial nerves III, VII, IX, and X

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    Sacral fibers 2–4

Arise from:

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    Thoracic fibers 1–12

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    Lumbar fibers 1–2/3

Preganglionic fibers are myelinated and travel from the CNS to their autonomic ganglia (located near their respective effector organ in the head and neck), utilizing acetylcholine as the neurotransmitter at the synapse with the nicotinic receptor Preganglionic fibers are myelinated and travel from the CNS to their autonomic ganglia (located in the sympathetic chain for the head and neck), utilizing acetylcholine as the neurotransmitter at the synapse with the nicotinic receptor
Postganglionic fibers are unmyelinated and travel from the autonomic ganglia to the effector organ, utilizing acetylcholine as the neurotransmitter at the synapse with the muscarinic receptor Postganglionic fibers are unmyelinated and travel from the autonomic ganglia to the effector organ, typically utilizing norepinephrine * as the neurotransmitter at the synapse with the α or β receptor

* Main exception to this is in the adrenal gland, where chromaffin cells secrete epinephrine and norepinephrine into the blood.

FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM
Parasympathetic Sympathetic
Responsible for preserving and restoring energy Responsible for preparing the body for emergency situations
Discharges focally, not as a complete system Discharges as a complete system
Activated in response to the specific body function that needs to be adjusted (peristalsis, pupillary accommodation) Activated in response to stressful situations (helps to increase cardiac output, get blood to muscles, and decrease blood flow to the skin and viscera)

Figure 20-3

Figure 20-4

Figure 20-5

Sympathetics of the head and neck

General anatomic pathway

Type of Neuron Name of Cell Body Characteristics of Cell Body Course of Neuron
Preganglionic fibers Intermediolateral horn nucleus Collection of nerve cell bodies located in the lateral horn nucleus of the spinal cord between spinal segments T1 and T3 (and possibly T4)

  • Fibers arise from the intermediolateral horn nuclei from T1 to T3 (or T4)

  • Travel through the ventral root of the spinal cord to the spinal n.

  • Enter the sympathetic chain via a white ramus communicans

  • Once in the sympathetic chain, the preganglionic fibers will ascend and synapse with postganglionic fibers in the various sympathetic chain ganglia

  • A majority of the preganglionic fibers will synapse with postganglionic fibers in the superior cervical ganglion, located at the base of the skull

Postganglionic fibers Superior cervical ganglion * (this is where a majority of postganglionic sympathetic fibers to the head and neck begin)

  • Collection of nerve cell bodies located in the sympathetic chain

  • The location of the nerve cell body for a majority of the postganglionic neurons to the head and neck is the superior cervical ganglion

  • The locations of the cell bodies for other postganglionic sympathetics include the middle and inferior cervical ganglia

  • Postganglionic fibers arise in their respective sympathetic chain ganglia (e.g., superior cervical, middle cervical, inferior cervical ganglia)

  • Some of the postganglionic fibers that travel to the periphery (e.g., skin of the neck, blood vessels) will rejoin the spinal nerves in the cervical region via a gray ramus communicans, to be distributed along the path of the peripheral nerves following the path with blood vessels

  • A majority of the postganglionic fibers join the major blood vessels of the head (namely, the internal carotid a. and the external carotid a. and its branches) to follow the vessel until reaching their final effector organ (e.g., dilator pupillae m. of the eye)

* Location of the cell body for the postganglionic is variable and depends on the course of this neuron.

Figure 20-6

Autonomic pathways

Parasympathetics of cranial nerve III with corresponding sympathetics

ANATOMIC PATHWAY FOR PARASYMPATHETICS OF THE EYE
Type of Neuron Name of Cell Body Characteristics of Cell Body Course of Neuron
Preganglionic neuron Edinger-Westphal nucleus

  • A collection of nerve cell bodies located in the midbrain

  • The Edinger-Westphal nucleus is found medial to the oculomotor nucleus and lateral to the cerebral aqueduct

  • Fibers arise from the Edinger-Westphal nucleus in the midbrain from the oculomotor n.

  • Oculomotor n. passes anteriorly on the lateral wall of the cavernous sinus immediately superior to the trochlear n.

  • Immediately before entering the orbit, the nerve divides into superior and inferior divisions of the oculomotor

  • Both the superior and the inferior divisions of the oculomotor enter the orbit through the superior orbital fissure

  • Preganglionic parasympathetic fibers travel in the inferior division

  • A small parasympathetic root passes from the inferior division of the oculomotor to the ciliary ganglion, carrying the preganglionic parasympathetic fibers

Postganglionic neuron Ciliary ganglion

  • Located anterior to the optic foramen between the optic n. and the lateral rectus m.

  • 3 roots connect to the ciliary ganglion:

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    Sensory root from the ophthalmic division of the trigeminal n., which carries general sensation fibers to the eye via the short ciliary n.

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    Parasympathetic root from the inferior division of the oculomotor n., carrying preganglionic parasympathetic fibers to the ganglion

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    Sympathetic root, which arises from the postganglionic sympathetic fibers that were carried by the internal carotid a.

  • The short ciliary nn., usually numbering about 8 total, arise from the ciliary ganglion to enter the posterior portion of the eye

  • Fibers from all 3 roots pass through the ciliary ganglion and short ciliary nn. to enter the eye

  • Only the parasympathetic fibers synapse in the ciliary ganglion

  • Fibers arise in the ciliary ganglion after a synapse with the preganglionic parasympathetic fibers

  • Travel through the short ciliary nn. to enter the eye’s posterior portion

  • Innervate the:

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    Sphincter pupillae m.—constricts the pupil

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    Ciliary m.—changes the shape of the lens during accommodation

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Feb 15, 2025 | Posted by in General Dentistry | Comments Off on Autonomics of the head and neck

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