Innervation of masticatory muscles and the trigeminal nerve
The trigeminal nerve is responsible for the sensation of the face, mucous membranes, and teeth, as well as the motor function of the masticatory muscles. Its motor nucleus is located in the pontine tegmentum and receives impulses from the precentral gyrus via corticobulbar tracts. Motor fibers exit as part of the mandibular nerve.
Damage to the motor component presents with:
- Atrophy and decreased tone of the masseter and temporalis muscles.
- Deviation of the mandible towards the affected side upon opening the mouth due to the action of the healthy pterygoid muscle.
- Absence of the mandibular reflex when tapping the chin with a reflex hammer.
Oculomotor functions and pupillary reactions
Eyeball movements and pupillary reactions are mediated by complex reflex arcs involving cranial nerves. The oculomotor nerve is responsible for the function of the sphincter pupillae, ciliary muscle, and medial rectus muscles.
Key manifestations and tests:
- Convergence: simultaneous inward movement of the visual axes with simultaneous pupillary constriction.
- Diplopia (double vision): occurs when looking toward the paralyzed muscle and disappears when one eye is covered (which distinguishes neurological pathology from ophthalmological conditions).
- Pupillary reflexes: direct and consensual reactions to light, as well as the convergence reaction, normally cause pupillary constriction.
Radiculopathies and radicular syndromes
Lumbosacral root lesions are accompanied by motor, reflex, and sensory disorders in the limbs:
- L5 radiculopathy: characterized by pain radiating to the big toe, hypoesthesia on the outer surface of the lower leg, preserved reflexes, and weakness of the extensor hallucis longus.
- S1 radiculopathy: distinguished by pain extending to the lateral toes and the outer edge of the foot, loss of the Achilles reflex, hypoesthesia of the outer surface of the lower leg, and weakness of the gastrocnemius muscle.
Pain syndromes and neurovascular conflicts
Neurological disorders are frequently associated with nerve trunk compression:
- Trigeminal neuralgia: manifests as paroxysms of piercing pain in the innervation zones of the V nerve branches. The main cause is a neurovascular conflict, most commonly compression of the root by the superior cerebellar artery at its exit from the pons. The presence of trigger points on the face is characteristic.
- Myofascial syndromes and lumboischialgia: are accompanied by local tenderness of the gluteal, piriformis, and lumbar muscles without involvement of the foot and toes, which differentiates them from discogenic radiculopathy.
Zelder zones on the face are examined "from the ear to the nose", like a wave motion during washing.
Class and exam questions
Where is the motor nucleus of the trigeminal nerve located?
The motor nucleus of the trigeminal nerve (nucleus motorius) is located in the tegmentum of the pons.
Which reflex is lost in an S1 root lesion?
Lesion of the S1 root results in the loss of the Achilles reflex and weakness of the gastrocnemius muscle.
Why does the jaw deviate to one side in masticatory muscle paresis?
Deviation of the jaw toward the affected side occurs under the dominant influence of the healthy contralateral pterygoid muscle.
How to distinguish neurological double vision from ophthalmological diplopia?
In neurological pathology, double vision completely disappears when closing one eye, whereas visual blurring in ophthalmological issues persists.
What else to study on this topic
- Anatomical and physiological features of the trigeminal nerve branches (ophthalmic and maxillary nerves)
- Pathophysiology of neurovascular conflict and microvascular decompression
- Method for testing the corneal reflex and the role of the facial nerve in its efferent limb
- Differential diagnosis of lumbalgia, lumboischialgia, and discogenic radiculopathy
- Physiology of accommodation and pupillary reactions to light and convergence
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