PDF Download

1. Describe the basic divisions of the autonomic nervous system (ANS).

The autonomic nervous system (ANS) is an anatomically continuous component of the nervous system and is composed of two divisions: sympathetic and parasympathetic.

Both sympathetic and parasympathetic divisions consist of visceral motor fibers. These fibers differ from somatic motor fibers in the structures that they will stimulate: smooth and cardiac muscle, and glandular tissue. Somatic motor fibers innervate skeletal muscle.

Another significant difference from somatic fibers is that the ANS is composed of two neuron pathways, connected by ganglia:

• Preganglionic neurons, which originate in the CNS and conduct signals to ganglia, &

• Postganglionic neurons, which originate in ganglia and conduct signals to target organs.

Visceral sensory fibers are not technically autonomic structures, but they do conduct information from target organs back to the CNS.

Most organs of the body are antagonistically innervated by both sympathetic (“fight or flight”) and parasympathetic (“rest & digest”) divisions of the ANS.

Sympathetic division basics

• ‘Fight or flight’ stimulation:

  • Often referred to as an adrenergic response

  • Short preganglionic, long postganglionic fibers

  • Contracts smooth muscle of arteries (vasoconstriction)

  • Increases Heart & Respiratory rates

  • Relaxes smooth muscle of the respiratory tract (bronchodilation)

  • Increases concentration of blood glucose

  • Dilates pupils

  • Relaxes smooth muscle of the wall of the gut

  • Stimulates sudoriferous (sweat) glands

• Preganglionic fibers originate in intermediolateral nucleus (IML) columns (aka lateral horns) of spinal levels T1-L2

  • Thoracolumbar outflow (T1-L2)

  • Described as short

    • Connect CNS to paravertebral ganglia (sympathetic trunks) via white rami communicantes

  • Secrete acetylcholine as a neurotransmitter

• Postganglionic fibers originate in the ganglia of the sympathetic trunks at all spinal levels

  • Trunks run the length of the vertebral column

    • Skull base to coccyx

  • Described as long

    • Connect sympathetic trunks to target organs via grey rami communicantes that conduct fibers to spinal nn.

  • Secrete norepinephrine

Parasympathetic division basics

• ‘Rest & digest’ stimulation:

  • Often referred to as a cholinergic response

  • Long preganglionic, short postganglionic fibers

  • Decreases Heart & Respiratory rates

  • Constricts smooth muscle of the respiratory tract (bronchoconstriction)

  • Decreases concentration of blood glucose

  • Constricts pupils

  • Relaxes smooth muscle of arteries (vasodilation)

  • Constricts smooth muscle of the wall of the gut

• Preganglionic fibers originate in the brainstem or sacral spinal cord (S2-4)

  • Craniosacral outflow

  • Described as long: connect either brainstem or sacral spinal cord to ganglia within walls of target organs. Parasympathetic fibers are found as components in the following nerves:

    • Cranial nn.: III, VII, IX, & X

    • Spinal nn.: S2-4

  • Secrete Acetylcholine as a neurotransmitter

• Postganglionic fibers originate in ganglia within the walls of target organs

  • Described as short

  • Secrete Acetylcholine as a neurotransmitter

2. How is the sympathetic division of the ANS structured? 

The sympathetic division of the autonomic nervous system consists of pre- and postganglionic fibers which typically (but not always) synapse in ganglia of the sympathetic trunk. 

All preganglionic sympathetic fibers originate in the intermediolateral (IML) nucleus (lateral horn) of the spinal cord at levels T1-L2.

These fibers travel via the ventral roots of spinal nerves to the ventral primary rami (VPR) of spinal nerves, and then leave the VPR via white rami communicantes (at levels T1-L2) to the sympathetic trunk. All preganglionic sympathetic fibers go to the sympathetic trunk (white rami communicantes at levels T1-L2) but do not necessarily synapse there.

Once in the trunk, preganglionic fibers may:

• synapse at that level of the sympathetic trunk and return to the VPR via a gray ramus communicans,

• ascend the sympathetic trunk (and then either synapse and return to the VPR via a gray ramus communicans), or

• descend the sympathetic trunk (and then either synapse and return to the VPR via a gray ramus communicans).

From the synapse, postganglionic fibers carry signals back to the VPR of the spinal nerve via gray rami communicantes. These fibers can then be distributed by the VPR.

 While white rami communicantes are restricted to the T1-L2 levels, the sympathetic trunks extend the entire length of the spinal cord, and gray rami communicantes conduct postganglionic fibers back to VPRs at every level of the cord.

 In the neck, there are typically 2-3 sympathetic ganglia, whereas in the thorax there are typically sympathetic ganglia at every spinal level (T1-T12).

Splanchnic nerves

In certain areas, there are deviations from the typical autonomic neuron organizations as described above. There are instances of preganglionic fibers traveling through the sympathetic trunks without synapsing, and continuing on to synapse in prevertebral ganglia associated with the major arteries of the abdomen. 

4. What is Horner’s syndrome, and how do Horner’s syndrome symptoms help illustrate the targets and functions of the sympathetic nervous system in the head?

Horner’s syndrome is an interruption of the sympathetic pathways to the head, especially the face. Recall that preganglionic fibers serving the sympathetic trunk exit VPR of spinal nerves T1-L2, thus all fibers within the cervical sympathetic trunk have ascended from the thoracic trunks. Postganglionic fibers serving the head are distributed via perivascular plexuses originating with the superior cervical ganglion. Any disruption of the pathways of sympathetic fibers to the head may lead to Horner’s syndrome. Horner’s syndrome may be classified by the portion of the pathway affected:

• First order (central fibers) - fibers descending from the hypothalamus to the thoracic spine

• Second order (preganglionic fibers) - preganglionic fibers exiting the spinal cord (white rami communicantes), ascending the sympathetic trunk to the superior ganglion

• Third order (postganglionic fibers) - postganglionic fibers traveling from the superior ganglion to their target organs

Potential symptoms of Horner’s syndrome include unilateral (only on one side):

• Miosis (constricted pupil): sympathetic fibers innervate the dilator pupillae mm. of the eye. With denervation of the muscles, the sphincter pupillae mm. (innervated by parasympathetic fibers) are left unbalanced, so an affected pupil will be constricted.

• Ptosis ('droopy' eyelid): sympathetic fibers innervate a small slip of smooth muscle which attaches the levator palpebrae superioris m. to the upper eyelid. When denervated, the lack of muscle tone fails to support the superior eyelid, which gives a ‘droopy’ appearance.

• Pseudo-enophthalmos (appearance of sunken globe of eye): concomitant with ptosis, the eye appears to be sunken.

• Anhidrosis (lack of sweating): sudoriferous (sweat) glands are exclusively innervated by (sudomotor) sympathetic fibers. When denervated, sweat glands will not secrete.

Hyperemia (flushed skin): the smooth muscle of the walls of blood vessels is under autonomic control. Vasomotor (sympathetic) fibers will typically cause the smooth muscle to contract, thus limiting the flow of blood. When denervated, the walls of blood vessels will relax, and give the skin of the face a ‘flushed’ appearance.

5. Describe the boundaries of the naso-, oro-, and laryngopharynx, and identify the major contents of these spaces. 

The pharynx is a common space for the conductive pathways of the respiratory (gasses) and digestive (food and drink) systems. As such, the pharynx shares borders with the nasal cavity , oral cavity , larynx , and esophagus.

The pharynx is divided into three regions, which reflect the above borders. They are the:

• Nasopharynx (choanae; soft palate),

  • Contents:

    • Pharyngeal tonsil (adenoid)

    • Opening to auditory (Eustachian) tube

• Oropharynx (soft palate; palatoglossal arch; epiglottis & pharyngo-epiglottic folds),

  • Contents:

    • Uvula

    • Fauces: region bounded anteriorly by the palatoglossal arch (folds) and posteriorly by the palatopharyngeal arch (folds). These folds are muscles of the soft palate covered by mucosa. 

      • Tonsillar fossa w/palatine tonsils

      • Root (pharyngeal part) of tongue and lingual tonsil

      • Epiglottic valleculae - shallow depressions between the root of the tongue and epiglottis

        • Collect saliva to prevent deglutition reflex

• Laryngopharynx (epiglottis & pharyngo-epiglottic folds; entrance to esophagus; laryngeal inlet).

6. Explain the relationship of Waldeyer’s (Tonsillar) Ring to the pharynx.

Tonsils are collections of mucosa associated lymphoid tissue (MALT) in the pharynx. Covered by pharyngeal mucosa (stratified non-keratinized squamous epithelium or ciliated pseudostratified epithelium), the tonsils are loosely situated in a ring-shape, surrounding the naso- and oropharyngeal region. Such an arrangement brings inspired or consumed pathogens into contact with lymphoid tissue, which provides an opportunity to raise an immune response.

The tonsils involved in Waldeyer’s (Tonsillar) Ring include:

• Pharyngeal tonsil (adenoid) -  located in the roof of the nasopharynx, inferior to the sphenoid bone. Involutes during late adolescence. May be involved with obstructive sleep apnea.

• Tubal tonsils - associated posteriorly with the openings of the auditory tubes.

• Palatine tonsils “the tonsils”- located in the tonsillar fossa of the walls of the fauces. May project into the oropharynx. Recurrent acute tonsillitis and/or airway obstruction may indicate need for tonsillectomy. Atrophies during early puberty. Lymphatic drainage passes through the jugulodigastric node of the deep cervical lymph nodes. The glossopharyngeal n. (CN IX) is found just deep to the tonsillar fossa.

• Lingual tonsil - associated with the dorsal surface of the root (pharyngeal part) of the tongue. Lymphatic drainage passes through the jugulodigastric node of the deep cervical lymph nodes.

7. Describe the muscles of the wall of the pharynx. 

The muscular wall of the pharynx consists of two layers: an outer layer of predominantly circularly-oriented constrictors, and an inner layer of longitudinally-oriented elevators that together shorten and widen the pharynx, and elevate the larynx.

The three pharyngeal constrictor muscles in the external layer of the pharynx surround the pharynx, and meet along a posterior midline pharyngeal raphe. A raphe is a seam where two elements fuse together. The pharyngeal raphe is a posterior midline seam of investing muscular fascia of the pharyngeal constrictor muscles. 

Each muscle has a unique attachment to an osteological structure, which helps to reciprocally reinforce spatial relationships, including the:

• Superior pharyngeal constrictor m. attaches to the skull,

• Middle pharyngeal constrictor m. attaches to the hyoid bone, &

• Inferior pharyngeal constrictor m. attaches to the larynx.

When activated, the pharyngeal constrictor mm. serially constrict the lumen of the pharynx.

The three longitudinal muscles of the inner layer have a less coordinated anatomical arrangement than the constrictor muscles. These muscles elevate the larynx and by doing so shorten the pharynx. The most obvious among these muscles is the stylopharyngeus, which has classic anatomical relationships of:

• Traveling between superior & middle pharyngeal constrictor mm., &

• Traveling closely with the glossopharyngeal n. (CN IX). 

8. Diagram the contributions and neural modalities (somatic sensory, somatic motor, autonomics) of the pharyngeal plexus.

With the exception of the stylopharyngeus mm., the muscles of the pharynx are innervated by the pharyngeal (neural) plexus, which includes contributions of:

• somatic motor (efferent) fibers from the vagus nn. (CN X)

• somatic sensory (afferent) fibers from the glossopharyngeal nn. (CN IX).

• autonomic fibers

  • vagus nn. (parasympathetic) - increase mucosa secretions & blood flow,

  • superior cervical ganglia of the cervical sympathetic trunks - decrease mucosal secretions & blood flow. 

The stylopharyngeus mm. are innervated exclusively (motor and sensory) by the glossopharyngeal nn. (CN IX).

The pharyngeal mucosa is afferently innervated by the pharyngeal plexus as well, with the exception of the mucosa of the nasopharynx, which is afferently innervated by a branch of V2, the maxillary division of the trigeminal n. (CN V).

Thyroid cartilage:

• Most prominent cartilage

• Formed from two laminae, united anteriorly along the midline (posteriorly, the cartilage is incomplete)

• Suspended from the hyoid bone by the thyrohyoid membrane

• Inferior horns form the cricothyroid joint with the cricoid cartilage

  • Joints with cricoid allow for thyroid cartilage to ‘tip’ forwards

Cricoid cartilage:

• Forms a complete ring around the airway

• Most robust posteriorly

Epiglottic cartilage:

• Leaf-shaped cartilage

• Sits posterior to thyroid cartilage

• Forms the epiglottis

  • Forms a lid to cover the laryngeal inlet during deglutition (swallowing)

Also important to the function of the larynx in phonation and the regulation of airflow are the:

Arytenoid cartilages:

• Pyramid-shaped

• Sit atop the posterior cricoid 

• Vocal process (attaches to vocal ligament)

• Muscular process (allows for intrinsic laryngeal muscle attachment & control)

• Capable of either rotation or gliding

  • For either abduction (wide open airway), or adduction (narrower airway) of vocal cords

Vocal ligaments connect arytenoid cartilages to the thyroid cartilage and are covered by mucosa to form the (true) vocal folds (=vocal cords). The vocal folds vibrate with exhaled air (in a slightly adducted position) for phonation. The vocal folds may be abducted to allow for a more patent (open) pathway for air to move. 

The arytenoid cartilages are acted upon by most (but not all) of the intrinsic laryngeal mm. 

Posterior crico-artytenoid mm.

• Abduction of vocal folds (only muscles that allow this)

  • Opens the airway

Cricothyroid mm.

• No connection to arytenoid cartilages

• Tip thyroid cartilage anteriorly, which leads to increased tension on vocal ligaments

  • Increases pitch

Emergency Airway Access

The (median) cricothyroid ligament spans the distance between the anterior cricoid and thyroid cartilages., and is pierced during a cricothyrotomy. A cricothyrotomy is a procedure used to create an airway during an emergency involving an upper airway obstruction. The skin, subcutaneous layer, and (median) cricothyroid membrane are incised and the opening is cannulated to maintain patency. There are good landmarks for this procedure (the laryngeal prominence of the thyroid cartilage superiorly & the cricoid cartilage inferiorly), and there are no typical problematic vascular elements to endanger.

10. Diagram the neurovasculature serving the larynx.

The larynx is exclusively innervated by the vagus n., specifically by means of the:

• Superior laryngeal n.:

  • External br.

    • Efferent to cricothyroid m. (intrinsic laryngeal muscle)

  • Internal br.

    • Afferent from (and secretomotor to) laryngeal mucosa proximal to vocal folds

• Recurrent laryngeal n.:

  • Efferent to all intrinsic laryngeal mm. (except cricothyroid m.)

  • Afferent from (and secretomotor to) laryngeal mucosa distal to vocal folds

Blood supply to the larynx comes from two major sources:

• Superior laryngeal a. (External carotid a. → Superior thyroid a. → Superior laryngeal a.)

  • Accompanied by the internal br. of the superior laryngeal n. through the thyrohyoid membrane.

• Inferior laryngeal a. (1st part of Subclavian a. → Thyrocervical trunk → Inferior thyroid a. → Inferior laryngeal a.)