Motor Innervations

Motor pathway: other cortical motor areas ↔ primary motor cortex → (upper motor neurons) → brainstem → cranial nerves → lower motor neurons → muscles. This lecture starts at the cranial nerves.

Nerves covered: CN V, VII, IX, X, XI, XII, and the cervical plexus (C1, ansa cervicalis).

CN V: Trigeminal (mandibular branch)

• muscles of mastication

Clinical tip: Suprahyoids also help open the jaw, but the lateral pterygoid is the primary contributor.

CN VII: Facial

Functions: facial expression and taste.

Branches (mnemonic: "Ten Zombies Bothered My Colleague"):

• Temporal: eyebrow and forehead

• Zygomatic: eyelid

• Buccal: cheek and lip (important for swallowing)

• Mandibular: chin (important for swallowing)

• Cervical: platysma

Most important muscles for swallowing:

• Buccinator: provides tone, flattens the cheek, and holds food in contact with the cheek.

• Orbicularis oris: closes, opens, protrudes, inverts, and twists the lips (lip seal).

• Stylohyoid (suprahyoid): elevates the hyoid and base of tongue (BOT) and helps retract the tongue.

• Other CN VII muscles include the posterior belly of the digastric and the stapedius.

UMN (upper motor neuron) innervation of the face:

• Upper face: bilateral UMN innervation

• Lower face: contralateral UMN innervation

Parasympathetic: submandibular gland (Wharton's duct) and sublingual gland (ducts of Rivinus).

CN IX: Glossopharyngeal

• Stylopharyngeus is the only motor portion of CN IX.

  • It is one of the "long" (longitudinal) pharyngeal muscles and elevates and dilates the pharynx.

• Combines with CN X to form the pharyngeal plexus.

• Parasympathetic: parotid gland (Stensen's duct).

CN X: Vagus (only abductor!!)

Clinical tip: important for talking and the pharyngeal swallow events.

1. Pharyngeal branch

Forms the pharyngeal plexus with IX, which innervates the soft palate and pharyngeal musculature.

Exceptions to remember: stylopharyngeus is CN IX, and tensor veli palatini is CN V.

Pharyngeal muscle groups:

• Three semicircular constrictors produce constriction.

• Two or three paired longitudinal muscles produce shortening and elevation (salpingopharyngeus, stylopharyngeus, and palatopharyngeus).

2. Superior laryngeal nerve (SLN)

• External branch (SLNe): cricothyroid.

  • It has no direct role in swallowing, but it is testable, so it can be used to infer the likely integrity of the inferior pharyngeal constrictor.

• Internal branch (SLNi): sensation to the hypopharynx and larynx (above the vocal folds).

3. Recurrent laryngeal nerve (RLN)

• Sensation to the subglottic region (below the vocal folds).

• Intrinsic laryngeal muscles (except CT):

  • Adductors: thyroarytenoid (shortens the folds; with arytenoid adduction it produces glottal closure), oblique arytenoid, transverse arytenoid (interarytenoids), and lateral cricoarytenoid.

  • Abductor: posterior cricoarytenoid is the only muscle that abducts the vocal folds and opens the glottis.

Clinical tip: The vocal folds are the dividing line for sensory innervation between the SLNi (above) and the RLN (below).

CN XI: Accessory (Spinal)- only motor

• Sternocleidomastoid (SCM): head movement.

• Trapezius: shoulder shrug.

• Role in swallowing: it has no direct role, but damage can change postural strategies in dysphagia management (e.g., head turn or tilt may not be feasible).

• Jugular foramen: CN IX, X, and XI all exit here, so a lesion there can damage IX and X too, which does affect swallowing.

CN XII: Hypoglossal

Clinical tip: intrinsics change tongue size and shape, while extrinsics move the tongue's location.

Intrinsic muscles (form the body of the tongue, with no external origin):

Extrinsic muscles (originate outside the tongue and produce protrusion, retraction, lateral motion, elevation, and depression):

Exception: palatoglossus is an extrinsic muscle but is innervated by CN X, not XII.

Cervical Plexus

C1: Geniohyoid

• Suprahyoid muscle; elevates the hyoid and moves it anteriorly.

• It is the primary contributor to anterior hyoid movement.

• The other suprahyoids (mylohyoid, digastric, stylohyoid) are not innervated by C1.

Ansa cervicalis (C1–C3/C4):

A nerve loop that innervates 3 of the 4 infrahyoid muscles (depressors).

Clinical tips:

1. Supra- and infrahyoids act as "secondary" muscles of mastication. The infrahyoids stabilize the hyoid so the suprahyoids can help depress the mandible.

2. The ansa cervicalis is a candidate for laryngeal reinnervation in RLN paralysis. It can also be damaged during neck surgery.

Parasympathetic Control: Salivary Glands

Quick Reference Table (the lecture's "make a table" exercise)

Two things worth double-checking in the slides:

• The pharyngeal constrictors slide says "superior, medial, and inferior." The standard term is middle.

• Standard anatomy texts usually describe thyrohyoid as innervated by C1 fibers traveling with CN XII rather than by the ansa proper. The lecture groups it with the ansa, so follow the lecture for exams, but be aware of the difference.