Motor Functions of the Cranial Nerves I

(Somatic Motor Functions)

Just as the trigeminal system parallels spinal sensory (ascending) pathways, [cranial nerves nuclei are analogous to ventral horn nuclei].

E.g., especially evident in the lowest n. XI – spinal accessory.

Review 3 Functional Categories (we will go over 2 today).

-located medially relative to sensory nuclei, in columns.

-review the cranial nerves associated with these [Fig. 6-1, 3].

We will also review the cerebral control of the functions of these motor nuclei:

1. The long-associated corticocerebellar system

2. The other cerebral areas responsible for controlling eye muscles.

Fig. 6-1

Columnar Organization of Cranial Nerve Nuclei:

Fig. 6-3A

Columnar organization (cont’d)[Fig. 6-3]

Fig. 11-1

I. The 3 classes of cranial nerve motor nuclei and their columnar organization.

II. Cortical control of cranial motor functions.

A. Corticobulbar tracts (head, neck muscles).

B. Cortical areas involved in eye movement control.

III. General somatic motor column (eye, tongue muscles) of somatic origin.

IV. Special visceral motor column (face, throat, neck muscles) derived from branchial arches.

V. Regional anatomy of the brainstem nuclei reviewed today.

3 Classes of Cranial Nerve Motor Nuclei and their Columnar Organization

• Review Figs 6-1, 3, 11-1.

A. General somatic motor (eye, tongue): III, IV, VI, XII – developmentally derived from occipital somites.

B. Special visceral motor (face, throat, neck): V, VII, IX, X, XI – also striated muscle under voluntary control, but developmentally derived from branchial arches.

C. General visceral motor: autonomous control of cranial glands and body organs: III (pupil), VII, IX, X – under hypothalamic control – will cover in next lecture, which will be a good seguay into the hypothalamus and the ANS.

II. Cortical Control of Cranial Motor Function

A. Corticobulbar tract

Review Note: Regarding unilateral and bilateral projections from cortex to sc (or bs nuc sc) that we viewed when examining the descending motor systems of the limbs and axes.

Note that the trend for lateral descending tracts serving the limbs to have unilateral (crossed) projections, and medial tracts to serving the axis to have bilateral.

Functional or clinical significance? Muscles working in concert – unilateral lesion no unilateral deficit.

This is frequently the scenario with descending tracts for the head as well: bilateral projections

Fig. 11-2.Corticobulbar tract(XII)

Fig. 11-4.Corticobulbar tract(V, VII, IX, X, XI)

B. Cortical areas involved in eye movement control.

Rapid and slow eye movements:

Rapid: saccades: quick movements of eyes in tandem to bring the fovea to an image.

Slow: smooth pursuit: eyes in tandem to track a moving object

Slow: convergence: disconjugate eye movement for viewing an object at a close distance.

These movements are not controlled by 1° motor cortex (Note no eyes in homonculus, Fig. 10-7), but rather, by a number of important areas throughout the cortex:

Fig. 10-7

Look!! No eyes!

Look! No eyes!

Cortical areas for Rapid Eye Movement Control

Fig. 12-5.For REMs (saccades): frontal eye field, post parietal cortex(area 7).

Cortical Areas for Slow Eye Movement Control

Fig. 12-6.For slow movements: middle temporal and mid-superior temporal(involvement of cortical visual axons andcerebellum).

III. General Somatic Motor Column Nuclei[eyes, tongue (from somatic)]

III. Oculomotor n. 4/6 extraocular muscles and one elevator of eyelid (Fig. 12-3):

IV. Trochlear n. superior oblique muscle (Fig. 12-3).

In Fig. 12-4 (not shown here), note that in the dorsal brainstem, all axons decussate.

VI. Abducens n. lateral rectus muscle.

XII. Hypoglossal n. all intrinsic tongue muscles and travel in corticobulbar tract (Fig. 11-2).

Note the bilateral projections

For tongue disruption on exam, lesion would have to be relatively peripheral.

IV. Special Visceral Motor Column Nuclei – Branchial Arch-derived.

V. Trigeminal motor muscles of mastication bilateral projection (chewing is bilaterally coordinated).

VII. Facial motor muscles of facial expression laterally of projection – more complex.

Upper face: bilateral projections from both hemispheres.

Lower face: projection only from contralateral motor cortex (IX, X, XI).

Fig. 6-8

Nucleus ambiguous (IX, X, XI) striatal muscles of palate, pharynx, and larynx.

Most neurons project through X.

IX 1 pharyngeal muscle

XI cranial root comes from n. ambiguous and joins vagus n. (X) to innervate some muscles.

Nucleus ambiguous andspinal accessory nucleus.

Spinal accessory nucleus 2 upper shoulder muscles(sternocleidomastoid, trapesiuz) – part of transitionbetween cranial and spinalnuclei [really part of ventralhorn of upper cervical sc].

Note: unilateral projection (part of limbs) and separate sc root.

Note: branchial arch-derived.

V. Regional Anatomy of the Brainstem Nuclei

• Spinal accessory nucleus at pyramidal decussation (spinal-medullary junction).

Fig. 11-4

• Medial medullary section

Compare this figure to Fig. 11-1 columns (hypoglossal, motor vagus, n. ambiguous).

And sensory: vestibular, solitary n., trigeminal.

Fig. 11-12

Rostral medulla (Fig. 11-12): n. ambiguous, sensory nuclei, exit of IX (glossopharyngeal)

Pons: Abducens, facial n., trigeminal motor

Fig. 11-11

Pons: Abducens, facial n., trigeminal motor

Fig. 11-11

• Midbrain: (level of inferior colliculus): trochlear n. Fig. 11-9.

Midbrain (at level of superior colliculus): oculomotor n. Fig. 11-8.

Sensory: mesencephalic trigeminal n.: corticospinal, corticobulbar,corticopontine tracts.

Motor Functions of the Cranial Nerves II

Visceral Motor Functions

I. Functional Anatomy – the General Visceral Motor Cell Column:

This column contains neurons that regulate the function of smooth muscle and endocrine glands.

II. Regional Anatomy - Review

Fig. 6-4

I. Functional Anatomy

• Once again – look at columnar organization: general visceral motor = the most lateral of the 3 motor columns (Fig. 6-4):

X – dorsal motor nerve of vagus.

VII, IX superior and inferior salivatory in pons.

III Edinger-Westphal – midbrain (level of superior colliculus).

These neurons are part of the parasympathetic nervous system (ANS).

• Recall that skeletal muscle has 1 important innervation by a single motor neuron.

• Smooth muscle and glands: innervation by 2 separate neurons (ganglia involved = preganglionic and postganglionic).

• Preganglionic neurons: located in the brainstem or spinal cord nuclei.

• Postganglionic neurons: located in peripheral autonomic ganglion.

Extraocular Eye Muscles

A. Edinger-Westfall Nucleus: located in pretectal region and midbrain (near III cn ciliary ganglion) ciliary muscle, constrictor muscle of iris.

Functions of 2 important visual reflexes:

1. Pupillary light reflex pupil constriction.

2. Accommodation reflex curvature of lens in response to near vision (“plump up”).

Fig. 11-8Pupillary reflex:Visual input from retina pretectal nuclei preganglionicneurons of EWN via bilateralprojections ciliary ganglion constriction of muscle of iris.What is the importance or consequence of bilateral projection?

B. Preganglionic Neurons of Pons and Medulla.

General scheme:

Nuclei cn ganglia target

Terminal ganglion associated with vagus nerve: located in visceral of thoracic and abdominal cavities (proximal to splenic flexure).

Function: regulate heart rate, gastric motility, bronchial muscle control, bronchial secretions.

Distal to flexure: innervated by preganglionic n of sacral spinal cord.

II. Regional Anatomy

• General Visceral Motor column