Trigeminal Ganglion and its Connections with Human Brain and Neck!
The trigeminal ganglion (semilunar, Gasserian) is a sensory ganglion of trigeminal nerve, and corresponds with the dorsal root ganglion of a spinal nerve.
It contains the cell bodies of pseudo- unipolar first sensory neurons (Figs. 8.9, 8.10).
The ganglion is situated in an impression above the apex of the petrous part of temporal bone on the floor of the middle cranial fossa, just outside the posterior part of the lateral wall of cavernous sinus. It lies about 5 cm deep to the pre-auricular point.
It is semilunar with the convex margin directed forward and laterally.
The ganglion is contained in a dural pouch known as the cavum trigeminale (Meckel’s cave), which extends forward as a ballooning of the meningeal layer of Dura mater from the posterior cranial to the middle cranial fossae below the lateral attached margin of tentorium cerebelli.
The roof of the cavum is formed by two meningeal layers, and the floor by one meningeal and one endosteal layer. The mouth of the cave is directed behind and transmits sensory and motor roots of the trigeminal nerve; the mouth is bounded above by the superior petrosal sinus and below by the trigeminal notch at the upper margin of the petrous temporal just lateral to its apex.
The cavum contains trigeminal ganglion and its sensory and motor roots. The dura and arachnoid are adherent to the epineurium of the anterior part of trigeminal ganglion and its three divisions; but the posterior part of the ganglion and its sensory and motor roots are surrounded by the pia-arachnoid and are bathed in the cerebrospinal fluid.
Motor root of the 5th nerve, greater petrosal nerve, apex of petrous temporal, and foramen lacerum;
Free margin of tentorium cerebelli, superior petrosal sinus;
Posterior part of the lateral wall of cavernous sinus, internal carotid artery surrounded by a plexus of sympathetic nerves;
Uncus of the temporal lobe, middle meningeal vessels, and nervous spinosus.
The ganglion receives communications from the sympathetic plexus around internal carotid artery, and gives meningeal twigs to the tentorium cerebelli and floor of the middle cranial fossa.
1. Peripheral branches:
These are the peripheral processes of the unipolar cells of the ganglion and form the ophthalmic, maxillary and sensory parts of mandibular nerves. The three divisions pierce the cavum trigeminale and are attached to the convex margin of the ganglion.
2. Central branches:
These are the central processes of the cells of the ganglion and form a sensory root which is attached to the concave margin of the ganglion. The sensory root passes backward and medially and is attached to the anterior surface of the pons as its junction with the middle cerebellar peduncle.
Within the substance of the pons about 50% fibres of the sensory root divide into ascending and descending branches; the remaining fibres ascend or descend without division (Fig. 8.11).
These are less in number and consist of thick fibres which make synapses with the principal sensory nucleus of the 5th nerve. They convey discriminative touch sensation from the trigeminal field.
A few ascending fibres carrying proprioceptive impulses from the muscles of mastication and extraocular muscles pass through the ganglion without interruption and end as peripheral processes to the unipolar cell bodies of the mesencephalic nucleus of the 5th nerve.
The termination of central processes of these neurons is not yet established; the fibres may end in the motor nucleus of the 5th nerve as mono-synaptic reflex for muscle tone, in the principal sensory nucleus of the 5th nerve, and in the cerebellum.
Electro- physiological recording strongly suggests that the cell bodies of proprioceptive neurons from the extra-ocular muscles are located in the trigeminal ganglion; however, it needs further confirmation.
These are more numerous, consisting of thinly myelinated and unmyelinated fibres, and form a spinal tract which accompanies the lateral side of the spinal nucleus of the 5th nerve. The terminals and collaterals of the spinal tract terminate in the spinal nucleus.
They convey the fibres for pain, temperature and light touch from the trigeminal area. Touch from the mouth is received by the pars oralis, nociceptive fibres from the teeth by the pars interpolaris, and pain, temperature and touch from all trigeminal areas by the pars caudalis.
The fibres of the sensory root and the spinal tract are somatotopically arranged, corresponding to the three divisions of the trigeminal nerve. In the sensory root ophthalmic fibres are dorsal, mandibular fibres are ventral, and maxillary fibres are in between.
Rotation of fibres takes place as they enter the brain stem, so that in the spinal tract ophthalmic fibres are ventral, maxillary fibres are in the central part and mandibular fibres occupy dorsal position.
The ophthalmic fibres descend to the upper two or three cervical cord segments, the maxillary fibres extend upto the lower limit of the medulla, and the mandibular fibres extend upto the middle of the medulla.
The second order of sensory neurons reach the ventral posteromedial nucleus of the thalamus (VPM) by two pathways:
(a) From the principal sensory nucleus and nucleus of the spinal tract as crossed ventral trigemino-thalamic tract;
(b) A few fibres from the principal nucleus ascend as uncrossed dorsal trigemino-thalamic tract.
The third order of neurons from the thalamus terminate in the lower part of post-central gyrus to evoke consciousness.
A few collaterals from the trigemino-thalamic tract make synapses with the reticular nuclei of the brain stem, and thence to the entire grey matter of cerebral cortex through the thalamus for arousal response. This explains why some patients with fainting attacks regain consciousness after sprinkling cold water to the face and forehead.
The ganglion is supplied by the ganglionic braches of internal carotid, middle meningeal and accessory meningeal arteries.