Thursday, April 14, 2011

Neuroanatomy- Post #7- Cranial Nerves

     Well group, this is the next to last blog I have to do! This is actually going to be the last one on neuroanatomy. I promise. This blog is going to strictly be on the cranial nerves. This is probably one of the most interesting topics in neuroanatomy. Or at least I think that it is. They should really let me teach a class on all this neuroanatomy stuff. Ha. I guess the best place to start would to be at the first.

CN I: Olfactory Nerve
      The olfactory nerves are short connections that project from the olfactory mucosa within the nose and the olfactory bulb within the cranial cavity. There are 9 to 15 of these nerves on each side of the brain. The olfactory bulb lies just superior to the cribriform plate and below the frontal lobe. Axons from the olfactory bulb run within the olfactory stalk, synapse in the anterior olfactory nucleus, and terminate in the primary olfactory cortex (pyriform cortex) as well as the entorhinal cortex and amygdala.

CN II: Optic Nerve
       The optic nerve contains myelinated axons that come from the ganglion cells in the retina. It passes through the optic papilla to the orbit, where it is contained within the meningeal sheaths. The nerve changes its name to optic tract when the fibers have passed through the optic chiasm. Optic tract axons project to the superior colliculus and to the lateral geniculate nucleus within the thalamus, which relays visual information to the cortex.

CN III: Oculomotor Nerve

       The oculomotor nerve contains axons that arise in the oculomotor nucleus (which innervates all of the oculomotor muscles except the superior oblique and lateral rectus) . The oculomotor nerve leaves the brain on the medial side of the cerebral peduncle, behind the posterior cerebral artery and in front of the superior cerebellar artery. It then passes anteriorly, parallel to the internal carotid artery in the lateral wall of the cavernous sinus, leaving the cranial cavity by way of the superior orbital fissure.
The somatic efferent portion of the nerve innervates the levator palpebrae superioris muscle; the superior, medial, and inferior rectus muscles; and the inferior oblique muscle. The visceral efferent portion innervates two smooth intraocular muscles: the ciliary and the constrictor pupillae.

CN IV: Trochlear Nerve
         The trochlear nerve is the only crossed cranial nerve. It originates from the trochlear nucleus, which is a group of specialized motor neurons located just caudal to the oculomotor nucleus within the lower midbrain. Trochlear nerve axons arise from these neurons, cross within the midbrain, and then emerge contralaterally on the dorsal surface of the brain stem. The trochlear nerve then curves ventrally between the posterior cerebral and superior cerebellar arteries. It continues anteriorly in the lateral wall of the cavernous sinus and enters the orbit via the superior orbital fissure. It innervates the superior oblique muscle.

CN V: Trigeminal Nerve

        The trigeminal nerve contains a large sensory root, which carries sensation from the skin and mucosa of most of the head and face, and a smaller motor root, which innervates most of the chewing muscles (masseter, temporalis, pterygoids, mylohyoid), and the tensor tympani muscle of the middle ear.

CN VI: Abducens Nerve

           The abducens nerve arises from neurons of the abducens nucleus located within the dorsomedial tegmentum within the caudal pons. These axons project through the body of the pons and leave it as the abducens nerve. This nerve emerges from the pontomedullary fissure, passes through the cavernous sinus close to the internal carotid, and exits from the cranial cavity via the superior orbital fissure. Its long intracranial course makes it vulnerable to pathologic processes in the posterior and middle cranial fossae. The nerve innervates the lateral rectus muscle.

CN VII: Facial Nerve

        Both parts of the facial nerve pass through the internal auditory meatus, where the geniculate ganglion for the taste component lies. The facial nerve contains axons that arise in the facial nucleus. The nerve exits through the stylomastoid foramen; it innervates the muscles of facial expression, the platysma muscle, and the stapedius muscle in the inner ear.

CN VIII: Vestibulocochlear Nerve
       Cranial nerve VIII  passes into the cranial cavity via the internal acoustic meatus and enters the brain stem behind the posterior edge of the middle cerebellar peduncle in the pontocerebellar angle. The cochlear nerve is concerned with hearing; the vestibular nerve is part of the system of equilibrium.
CN IX: Glossopharyngeal Nerve

Cranial nerve IX contains several types of fibers. Branchial efferent fibers from the ambiguus nucleus pass to the stylopharyngeal muscle.

CN X: Vagus Nerve

       Efferent fibers from the ambiguus nucleus contribute rootlets to the vagus nerve and the cranial component of the accessory nerve (XI). Those of the vagus nerve pass to the muscles of the soft palate and pharynx. Those of the accessory nerve join the vagus outside the skull and pass, via the recurrent laryngeal nerve, to the intrinsic muscles of the larynx.

CN XI: Accessory Nerve

        The accessory nerve consists of two separate components: the cranial component and the spinal component. In the cranial component, branchial efferent fibers (from the ambiguus nucleus to the intrinsic muscles of the larynx) join the accessory nerve inside the skull but are part of the vagus outside the skull. In the spinal component, the branchial efferent fibers from the lateral part of the anterior horns of the first five or six cervical cord segments ascend as the spinal root of the accessory nerve through the foramen magnum and leave the cranial cavity through the jugular foramen. These fibers supply the sternocleidomastoid muscle and partly supply the trapezius muscle. 



CN XII: Hypoglossal Nerve

            Somatic efferent fibers from the hypoglossal nucleus in the ventromedian portion of the gray matter of the medulla emerge between the pyramid and the olive to form the hypoglossal nerve . The nerve leaves the skull through the hypoglossal canal and passes to the muscles of the tongue. A few proprioceptive fibers from the tongue course in the hypoglossal nerve and end in the trigeminal nuclei of the brain stem. The hypoglossal nerve distributes motor branches to the geniohyoid and infrahyoid muscles with fibers derived from communicating branches of the first cervical nerve. A sensory recurrent meningeal branch of nerve XII innervates the dura of the posterior fossa of the skull.



The table below gives a great summary of the CN's. 

Table 8–1 Overview of Cranial Nerves.
  
Functions
Location of Cell bodies
  
Functional Type*
Motor Innervation
Sensory Function
Parasympathetic Function
Within Sensory Organ or Ganglia
Within Brain Stem
Major Connections
Special Sensory:
I Olfactory
SS
 
Sense of smell
 
Olfactory mucosa
 
Mucosa projects to olfactory bulb
 
II Optic
SS
 
Visual input from eye
 
Ganglion cells in retina
 
Projects to lateral geniculate; superior colliculus
 
VIII Vestibulocochlear
SS
 
Auditory and vestibular input from inner ear
 
Cochlear ganglion
 
Projects to cochlear nuclei, then inferior colliculi, medial geniculate
      
Vestibular ganglion
 
Projects to vestibular nuclei
Motor for Ocular System:
III Oculomotor
SE
Medial rectus, superior rectus, inferior rectus, inferior oblique
   
Oculomotor nucleus
Receives input from lateral gaze center (paramedial pontine recticular formation; PPRF) via median longitudinal fasciculus
  
VE
  
Constriction of pupil
 
Edinger–Westphal nucleus
Projects to ciliary ganglia, then to pupil
 
IV Trochlear
SE
Superior oblique
   
Trochlear nucleus
 
 
VI Abducens
SE
Lateral rectus
   
Abducens nucleus
Receives input from PPRF
Other Pure Motor:
XI Accessory
BE
Sternocleido-mastoid, trapezius
   
Ventral horns at C2–5
 
 
XII Hypoglossal
SE
Muscles of tongue, hyoid bone
   
Hypoglossal nucleus
 
Mixed:
V Trigeminal
SA
 
Sensation from face, cornea, teeth, gum, palate. General sensation from anterior 2/3 of tongue
 
Semilunar (= gasserian or trigeminal) ganglia
 
Projects to sensory nuclei and spinal tract of V, then to thalamus (VPM)
  
BE
Chewing muscles
   
Motor nucleus of V
 
 
VII Facial
BE
Muscles of facial expression, platysma, stapedius
   
Facial nucleus
 
  
VA
 
Taste, anterior 2/3 of tongue (via chorda tympani)
 
Geniculate ganglion
 
Projects to solitary tract and nucleus, then to thalamus (VPM)
  
VE
  
Submandibular, sublingual, lacrimal glands (via nervus intermedius)
 
Superior salivatory nucleus
 
 
IX Glossopharyngeal
VE
  
Parotid gland
 
Inferior salivatory nucleus
 
  
VA
 
General sensation from posterior 1/3 of tongue, soft palate, auditory tube. Sensory input from carotid bodies and sinus. Taste from posterior 1/3 of tongue
 
Inferior (petrosal) and superior glossopharyngeal ganglia
 
Projects to solitary tract and nucleus
  
BE
Stylopharyngeus muscle
   
Ambiguus nucleus
 
 
X Vagus
BE
Soft palate and pharynx
   
Ambiguus nucleus
 
  
VE
  
Autonomic control of thoracic and abdominal viscera
 
Dorsal motor nucleus
 
  
SA
 
External auditory meatus
 
Superior (jugular) ganglion
 
Projects to thalamus (VPM)
  
VA
 
Sensation from abdominal and thoracic viscera
 
Inferior vagal (nodose) and superior ganglia
 
Projects to solitary tract and nucleus

*Efferent (motor): SE—somatic; general SE; BE—branchial; special VE; VE—visceral; general VE. Afferent (sensory): VA—visceral; general VA, special VA; SA—somatic; general SA; SS—sensory.
*Most nerves with SE components have a few SA fibers for proprioception



In this blog I'm not going to list any pathologies because essentially everyone did the extra credit assigment that told you what to look for and how damage presents itself. 

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