Pons and Cranial Nerves V, VII, and VIII Flashcards
Describe the two divisions of the Pons:
1) Tegmentum: Portion of grey matter containg cranial nerve nuclei and ascending and descending tracts. This region lies between the fourth ventricle and the more ventrally located basis points (or ventral pons).
2) Basis Pontis: ventral portions containing pontine nuclei and crossing pontine fibers running transversely that become the middle cerebellar peduncle. This ventral portion is marked by large fascicles of fibers containing corticospinal, corticobulbar and corticopontine axons. These axons are running in the caudal direction.
Describe the cerebellar peduncles:
all three (inferior, middle, and superior) cerebellar peduncles can be seen at the pontine level. As a general principle, these allow for cerebellar afferents to enter the cerebellum (inferior and middle cerebellar peduncles) and cerebellar efferents to exit the cerebellum (superior cerebellar peduncles)
What cranial nerves are associated with the pons?
- CN VIII Vestibulocochlear (cochlear pathways)
- CN VII Facial
- CN VI Abducens (discussed with the oculomotor system)
- CN V Trigeminal (sensory with somatosensation)
Describe the Cochlear Division of the Vestibulocochlear Nerve (CN VIII):
- Cochlear Division is a relay of auditory information from specialized receptor, the cochlea.
- Intro: auditory information is distributed bilaterally through the brain stem and cortical auditory areas. Therefore, unilateral lesions of CNS auditory structures do not result in unilateral deafness. Damage to the CN VIII or both cochlear nuclei will result in ipsilateral deafness.
- -The entire system is tonotopically organized.
- Auditory Pathways: the ultimate goal of cerebral cortex is for perception of the spoken word and sound. Primary auditory cortex is located on the superior surface of the superior temporal gyrus (within the lateral sulcus). Brodmann’s area 41 and 42 “TRANSVERSE TEMPORAL GYRI OF HESCHL”. Much Cross-communication takes place between sides before arriving at the cortex. The bilateral representation is complex and the actual relay steps are unclear. It does however support the ability to localize sound to one side
What is meant by the Vestibulocochlear Nerve system being entirely tonotopically organized?
means that cells responsive to different frequencies are found in different places at each level of the central auditory system, and that there is a standard (logarithmic) relationship between this position and frequency.
Describe the Cochlear Nerve; what it consists of? Where it enters the brainstem? and where the nuclei lie?
- consists of axons or central processes of neurons bringing afferent information form the inner ear.
- enters brainstem at cerebellopontine angle, then bifurcates to synapse in both dorsal and ventral cochlear nuclei
- nuclei are superficial and located in rostral (open) medulla, adjacent to the base of the inferior cerebellar peduncle and alongside the lateral aperture of the 4th ventricle
Describe the 7 landmarks of ascending system of the Cochlear Nerve:
1) Dorsal and ventral cochlear nuclei (a number of synapses occur after the cochlear nuclei)
2) Superior Olivary Nucleus
3) Lateral Lemniscus (bilateral pathway for sound localization)
4) Nucleus of inferior colliculus
5) Brachium of inferior colliculus
6) Medial geniculate nucleus
7) primary auditory cortex
Describe the Protective Auditory Reflex of the cochlear nerve:
- Inferior colliculus -> Superior solliculus -> tectospinal tract = reflex turning of head to sound
- IMPORTANT FOR protection against loud noise. Superior olivary nucleus projects to CN VII - stapedius muscle - dampening of noise at the stapes; and also to CN V - tensor tympani muscle - dampening of noise at the malleus
Describe Acoustic Neuroma:
- Tumor growing in the cerebellopontine angle, usually benign and beginning on the VIIIth nerve.
1) Vestibular component - irritative lesion; changes in perception of head position - spinning sensation, vertigo.
2) Tinnitus (ringing of the ears) - irritative lesion
Describe the Trigeminal nerve (CN V)
A mixed nerve for general sensation of the face and head area, as well as motor output to the muscles of mastication. The sensory parts of this nerve carry the same modalities as those carried in both the dorsal column system and the spinothalamic tract.
Describe the functional components of the Trigeminal Nerve:
- General Somatosensation - anterior 2/3 of the (distributed in three parts): [each division carries fibers of discriminative touch, proprioception, pain, and temperature]; V1 - ophthalmic division = above the eye, V2 - maxillary division = below the eye above the mouth, V3 - mandibular division = jaw
- Lower Motor Neurons innervating the muscles of mastication are restricted to the mandibular division of the CN V lies in Motor nucleus V.. This also innervates a couple of other muscles, nerve to the mylohyoid and tensor tympani
Describe the Lower Motor Neurons of the Facial Nerve (VII):
- LMNs in the motor nucleus of VII innervate IPSILATERAL muscles of facial expression, platysma and stapedius. Axons travel AROUND the Abducens nucleus to form the Facial Colliculus (bump on the floor of the 4th ventricle)
- location: exits at anterolateral part of tegmentum of lower pons. Somatotopic organization of motor neurons in motor nucleus for muscles on ipsilateral half of face. The lower motor neurons in the facial motor nucleus have a topographical organization, to the upper part of the face and to the lower part of the face. The lower motor neuron input from BOTH hemispheres (bilateral upper motor neuron control). The lower motor neurons that innervate the lower part of the face receive upper motor neuron input from ONLY the contralateral hemisphere.
- FINAL COMMON PATHWAY: lower motor neurons are the final common pathway, and therefore a lesion of the nucleus or the fibers of the facial nerve result in lower motor neuron signs in the innervated muscles.
What are the signs of Lower Motor Neuron Lesion in the Facial Nerve?
- Flaccid paralysis (atonia or hypotonia)
- Areflexia or hyporeflexia
- Atrophy of muscle (muscle wasting)
- Fasciculations and fibrillations
ipsilateral signs of lower motor neuron lesion would show in all muscles of the face.
Describe the Preganglionic Parasympathetic Neurons of the Facial Nerve:
- located in the reticular formation in the caudal pons
- tear production; postganglionic cell bodies in pterygopalatine ganglion (lacrimal gland)
- salivation; postganglionic cell bodies in submandibular ganglia (submandibular and sublingual glands)
The facial nerve transmits taste sensations from:
the anterior 2/3 of the tongue (chorda tympani)
