6: Examination of the Motor Cranial Nerves V, VII, IX, X, XI, and XII

INTRODUCTION

I. V CRANIAL NERVE MOTOR FUNCTION: CHEWING

A. Functional anatomy of chewing

1. Motor-wise, cranial nerve (CrN) V only chews. Its motor axons innervate all and, for clinical purposes, only the chewing muscles: masseter, temporal, and lateral and medial pterygoids. CrN V conveys no efferents to glands or smooth muscle and no special sensory afferents (Table 2-7).

2. Jaw closure

   1. Place your fingertips about 2 cm above and in front of the angle of your mandible. Bite hard and relax several times. The muscle felt, the masseter, is the easiest of the chewing muscle to palpate.

   2. The other jaw-closing muscles are the temporalis muscles that originate in the temporal fossa and insert on the mandible and the medial pterygoid muscle.

3. Lateral jaw movement: Move your jaw from side to side. Chewing requires not only jaw closure but also a lateral, grinding action and jaw opening, caused by the lateral pterygoid muscles.

   1. In Fig. 6-1 notice that the lateral pterygoid muscle originates from the skull base and inserts near the mandibular condyle.

   2. Because the skull base is fixed, only the mandible moves when the pterygoids contract. Equal contractions of right and left pterygoids pull the mandible straightforward (Fig. 6-1).

   3. If only the right lateral pterygoid muscle contracts, the mandibular tip moves to the □ right/□ left.

      ☑ left

   4. If the patient (Pt) can move the jaw to the right but not to the left, the lateral pterygoid muscle on the □ right/□ left is paralyzed.

      ☑ right

   5. As a second action, the lateral pterygoid muscles act to open the jaw because they insert on the neck of the mandible (Fig. 6-2).

   6. If the Pt's mandible moves forward and down in the midline on jaw opening, both _______________________________ muscles have contracted equally.

      lateral pterygoid

   7. If the left lateral pterygoid muscle contracts, the tip of the mandible moves not only to the □ right/□ left but also □ up/□ down.

      ☑ right; ☑down

4. Name the two major actions of the lateral pterygoid muscles.

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   Lateral movement and opening of the jaw

5. The remaining mandibular muscles innervated by CrN V all close the jaw. Name these muscles.

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   Masseter, temporal, and medial pterygoid