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E NEUROSURGERY
ESSENTIAL INFORMATION FOR THE BUDDING NEUROSURGEON
TRIGEMINAL NERVE

Trigeminal Reflexes:
·
Corneal/tearing: afferent is V1, efferent is VII
·
Jaw jerk: both afferent and efferent V3
·
Oculocardiac: afferent is V1,  efferent is X
·

·
Gardenigo’s syndrome : pain in the distribution of the ophthalmic nerve with palsies of cranial nerves V-VII, caused by infectio of the petrous apex.

TRIGEMINAL NERVE NUCLEI

 

The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.

 

The nucleus is divided into three parts, from rostral to caudal (top to bottom in humans):

 

   * The mesencephalic nucleus

   * The chief sensory nucleus (or "pontine nucleus" or "main sensory nucleus" or "primary nucleus")

   * The spinal trigeminal nucleus

 

The spinal trigeminal nucleus is further subdivided into three parts, from rostral to caudal:

 

   * Pars Oralis

   * Pars Interpolaris

   * Pars Caudalis

 

There is also a distinct trigeminal motor nucleus that is medial to the chief sensory nucleus.

 

The mesencephalic nucleus is involved with proprioception  of the face, that is, the feeling of position of the muscles. Unlike many nuclei within the CNS, the mesencephalic nucleus contains no chemical synapses but are electrically coupled.Instead, neurons of this nucleus are pseudounipolar cells receiving proprioceptive information from the jaw, and sending projections to the motor trigeminal nucleus to mediate monosynaptic jaw jerk reflexes. It is also the only structure in the CNS to contain the cell bodies of a primary afferent, which are usually contained within ganglia (like the trigeminal ganglion).

 

The principal sensory nucleus (or chief sensory nucleus of V) is a group of second order neurons which have cell bodies in the dorsal Pons.

 

It receives information about discriminative sensation and light touch of the face as well as conscious proprioception of the jaw via first order neurons of CN V.

 

   * Most of the sensory information crosses the midline and travels to the contralateral ventral posteriomedial (VPM) of the thalamus via the Ventral trigeminothalamic tract.

   * However, information of the oral cavity travels to the ipsilateral Ventral Posteriomedial (VPM) of the thalamus via the Dorsal trigeminothalamic tract.

 

The spinal trigeminal nucleus is a nucleus in the medulla that receives information about deep/crude touch, pain, and temperature from the ipsilateral face. The facial, glossopharyngeal, and vagus nerves also convey pain information from their areas to the spinal trigeminal nucleus.

 

Motor branches of the trigeminal nerve

 

Motor branches of the trigeminal nerve are distributed in the mandibular nerve. These fibers originate in the motor nucleus of the fifth nerve, which is located near the main trigeminal nucleus in the pons. Motor nerves are functionally quite different from sensory nerves, and their association in the peripheral branches of the mandibular nerve is more a matter of convenience than of necessity.

 

In classical anatomy, the trigeminal nerve is said to have general somatic afferent (sensory) components, as well as special visceral efferent (motor) components. The motor branches of the trigeminal nerve control the movement of eight muscles, including the four muscles of mastication.

 

   * Muscles of mastication

 

           * masseter

           * temporalis

           * medial pterygoid

           * lateral pterygoid

 

   * Other

 

           * tensor veli palatini

           * mylohyoid

           * anterior belly of digastric

           * tensor tympani

 

With the exception of tensor tympani, all of these muscles are involved in biting, chewing and swallowing. All have bilateral cortical representation. A central lesion (e.g., a stroke), no matter how large, is unlikely to produce any observable deficit. However, injury to the peripheral nerve can cause paralysis of muscles on one side of the jaw. The jaw deviates to the paralyzed side when it opens.the reason is lower motor neuron paralysis of nerve causes loss of activity, where as upper motor neuron as it has bilateral even though it has damage in one side of cortex the other side helps in function as lower motor neuron is intact.

 

Somatotopic representation

Onion Skin Distribution of the Trigeminal Nerve

 

Exactly how pain/temperature fibers from the face are distributed to the spinal trigeminal nucleus has been a subject of considerable controversy. The present understanding is that all pain/temperature information from all areas of the human body is represented (in the spinal cord and brainstem) in an ascending, caudal-to-rostral fashion. Information from the lower extremities is represented in the lumbar cord. Information from the upper extremities is represented in the thoracic cord. Information from the neck and the back of the head is represented in the cervical cord. Information from the face and mouth is represented in the spinal trigeminal nucleus. [

Within the spinal trigeminal nucleus, information is represented in an onion skin fashion. The lowest levels of the nucleus (in the upper cervical cord and lower medulla) represent peripheral areas of the face (the scalp, ears and chin). Higher levels (in the upper medulla) represent more central areas (nose, cheeks, lips). The highest levels (in the pons) represent the mouth, teeth, and pharyngeal cavity.

 

The onion skin distribution is entirely different from the dermatome distribution of the peripheral branches of the fifth nerve. Lesions that destroy lower areas of the spinal trigeminal nucleus (but which spare higher areas) preserve pain/temperature sensation in the nose (V1), upper lip (V2) and mouth (V3) while removing pain/temperature sensation from the forehead (V1), cheeks (V2) and chin (V3). Analgesia in this distribution is “nonphysiologic” in the traditional sense, because it crosses over several dermatomes. Nevertheless, analgesia in exactly this distribution is found in humans after surgical sectioning of the spinal tract of the trigeminal nucleus.

 

The spinal trigeminal nucleus sends pain/temperature information to the thalamus. It also sends information to the mesencephalon and the reticular formation of the brainstem. The latter pathways are analogous to the spinomesencephalic and spinoreticular tracts of spinal cord, which send pain/temperature information from the rest of the body to the same areas. The mesencephalon modulates painful input before it reaches the level of consciousness. The reticular formation is responsible for the automatic (unconscious) orientation of the body to painful stimuli.

 

Wallenberg syndrome

 

Wallenberg syndrome (also called the lateral medullary syndrome) is a classic clinical demonstration of the anatomy of the fifth nerve. It provides a useful summary of essential points about the processing of sensory information by the trigeminal nerve.

 

A stroke usually affects only one side of the body. If a stroke causes loss of sensation, the deficit will be lateralized to the right side or the left side of the body. The only exceptions to this rule are certain spinal cord lesions and the medullary syndromes, of which Wallenberg syndrome is the most famous example. In Wallenberg syndrome, a stroke causes loss of pain/temperature sensation from one side of the face and the other side of the body.

 

The explanation involves the anatomy of the brainstem. In the medulla, the ascending spinothalamic tract (which carries pain/temperature information from the opposite side of the body) is adjacent to the descending spinal tract of the fifth nerve (which carries pain/temperature information from the same side of the face). A stroke that cuts off the blood supply to this area (e.g., a clot in the posterior inferior cerebellar artery) destroys both tracts simultaneously. The result is loss of pain/temperature sensation (but not touch/position sensation) in a unique “checkerboard” pattern (ipsilateral face, contralateral body) that is entirely diagnostic.