Twenty Flashcards
Which is the most important of the cranial nerves for general sensation of the head? Which areas of the head does it receive impulses from? Describe the nuclear column related to somatosensory impulses. Describe the parts of the trigeminal nuclei that relate to somatosensory impulses.
General sensations from the face, orbit, oral and nasal cavities, sinuses, teeth, supratentorial dura, etc. are conducted centrally in several cranial nerves (V, VII, IX, X),
the most important of which is the trigeminal.
A virtually continuous nuclear column related to somatosensory impulses extends from the level of the superior colliculus caudad through the brainstem and the entire spinal cord. At spinal levels this nuclear column is represented by the dorsal horn nuclei subserving pain and temperature sensations. In the brainstem this nuclear column is represented by the trigeminal nuclei.
At mid-pontine levels, where the trigeminal nerve enters, is the principal trigeminal nucleus. Extending caudally from this nucleus is the spinal trigeminal nucleus which becomes continuous with the dorsal horn of the spinal cord. The spinal trigeminal nucleus consists of three parts: oral, interpolar, and caudal. The oral and interpolar parts are chiefly associated with trigeminal reflexes related to blinking, lacrimation, salivation, etc. The caudal part is associated primarily with pain and temperature impulses destined for perception in higher centers.
Describe the synapses and tracts for the cranial temp./pain pathway.
Although some pain and temperature impulses travel in the facial, glossopharyngeal, and vagus nerves, the most important cranial nerve for these modalities is the trigeminal. Unipolar neurons in the trigeminal ganglion have axons whose central branches travel through the sensory root to enter the pons. As these approach the principal sensory nucleus they bifurcate into short ascending branches (for reflex purposes) and long
descending branches. The latter form a prominent tract, the spinal trigeminal tract, which descends through the pons and medulla and into the spinal cord where it intermingles with the Tract of Lissauer. Pain and temperature fibers from the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve descend to the most caudal parts of this tract and synapse on secondary neurons in the spinal trigeminal nucleus, pars caudalis, and the reticular formation just medial to this nucleus.
Axons from the secondary neurons in and just medial to the caudal part of the spinal trigeminal nucleus and in the upper cervical segments of the cord cross and ascend contralaterally in the (ventral) trigeminothalamic tract, located near the medial lemniscus. Upon reaching the thalamus this tract terminates in the ventral posteromedial nucleus.
Thalamocortical projections from tertiary neurons in the ventral posteromedial nucleus pass through the posterior limb of the internal capsule and terminate in the ventral half of the postcentral gyrus, the SI region for the head.
Describe the synapses and tracts for the cranial touch pathway.
The primary neurons for touch sensations from the head are unipolar cells in the trigeminal ganglion. The impulses pass centrally in the sensory root and enter the brainstem at mid-pontine levels. At the level of their entrance is the principal trigeminal nucleus, which contains the secondary neurons in this pathway. Most axons from this nucleus cross and ascend in the (ventral) trigeminothalamic tract. These trigeminothalamic fibers terminate in the ventral posteromedial nucleus. Thalamocortical projections from these tertiary neurons traverse the posterior limb of the internal capsule to reach the face region of the postcentral gyrus.
