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[image]
This image shows the interior of the skull, the base of the skull, and reminds us about the position and organization of the nerves and reminds us that they leave the brain in one region, but they travel to their cranial exit between the various layers, the two layers of the dura. And many of them travel together, as we can see here, whereas others of them travel separately and then branch, and others travel together as they leave the brain. |
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Olfactory Nerve, Sensory
Olfactory receptor cells are located in the most posterior and superior portion of the nasal cavity. These bipolar neurons have cell bodies and dendrites within the nasal epithelium and axons that penetrate the base of the skull to enter the brain. The nerve is categorized as special somatic sensory. |
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[image]
And what we see here, we see this is an image of the lateral nasal cavity here and the medial nasal cavity here. And up here in the most superior part of the nasal cavity, it shows where the olfactory receptors are located.
And what we can see here is they penetrate what's called the cribriform plate of the ethmoid bone in a bunch of little fascicles, a dozen or so for each olfactory bulb. And we see here the brain, of course, is sitting up here in this region. And the reason this can be important for us to know is that a blow to the head here can cause the brain to shift backwards and shear these little receptor axons. So anosmia, the loss of olfactory sensation, is often associated with blows to the head.
And furthermore, if the cribriform plate is fractured, there's possibility cerebrospinal fluid can dribble into the nasal cavity. And a cerebrospinal fluid leak is something that needs to be treated very seriously. |
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Optic Nerve, Sensory
The optic nerve begins at the retina and carries visual information to the cortex. The designation “nerve” is technically incorrect since the retina is actually a part of the brain. By convention, this tract of the brain is called the optic nerve until the optic chiasm, where it is called the optic tract. The nerve is categorized as special somatic sensory. |
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[image]
And what this shows simply is some of the beautiful anatomy of the eye. This is a schematic cross-section. We see the retina here in yellow here. And we see the optic nerve gathering up to carry information back to the brain. And we note also that coming through the optic nerve here is the central artery of the retina, which is extremely important, because it's the sole blood supply to the retina. And an embolus in the central artery of the retina causes sudden blindness. So for that reason, this artery, the central artery of the retina, is sometimes called the artery of sudden blindness.
And I emphasize also that ophthalmoscopic exam allows us the rare ability to look directly at the brain. So an eye exam can turn out to be extremely important for reasons other than simply examining vision. |
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Auditory Nerve, Sensory
The auditory nerve has cochlear and vestibular parts. The cell bodies of the cochlear division are located in the spiral ganglion within the cochlea. The dendrites of these cells contact the receptor organ of the cochlea and the axons travel to the brainstem. The vestibular division arises from the vestibular ganglia adjacent to the semicircular canals. Its axons travel with the auditory nerve to reach the brainstem. The nerve is categorized as special somatic sensory. |
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[image]
So this shows an example now of the cochlea. We see our receptor axons here in this spiral ganglion. We see they have a process that goes into the receptors here of the basilar membrane, which you'll learn about subsequently. And the cochlear nerve, also called the auditory nerve, passes to the brain stem. |
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Oculomotor Nerve, Motor
The oculomotor nerve innervates all but two of the extraocular muscles. These muscles are voluntary muscles that control the orientation of the eyeball and raise the eyelid.
The oculomotor nerve also has a parasympathetic component. The preganglionic cell bodies are located in the brainstem. Their axons terminate on the ciliary ganglion. From the ciliary ganglion, the postganglionic axons innervate the ciliary body and the sphincter pupillae muscle. The ciliary body controls the tension on the lens to adjust for near vision. The sphincter pupillae constricts the pupil. |
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[image]
So this diagram here shows the oculomotor nerve. And as it enters the eyeball, it has branches which go to the voluntary muscle. But it also has branches which come here and terminate in the ciliary ganglion. And from the ciliary ganglion then, these postganglionic fibers travel around the eye to serve their function. We note also that there are sensory nerves, as well.
We're all aware how sensitive our eyes are, so we have the nasociliary nerve, which is a branch of the trigeminal nerve. And we also have sympathetic axons coming in. And these, we'll talk about their pathway later. But we see at the posterior of the eye, there is a lot going on.
We have a parasympathetic innervation of the ciliary body and the sphincter pupillae. We have sensory innervation of the cornea and surrounding areas. And we have a sympathetic innervation going to the dilator pupilae muscles. So there's a lot going on here. And as we'll see, there is a very important mutual relationship between the sympathetic and parasympathetic roots in this region. |
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Trochlear Nerve, Motor
The trochlear nerve supplies only one muscle, the superior oblique. |
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Abducens Nerve, Motor
The abducens nerve supplies only one muscle, the lateral rectus. |
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[image]
And this diagram of the orbit from above shows the trochlea here and the superior oblique. And here we have the lateral rectus, supplied by the abducens nerve, the superior oblique supplied by the trochlea nerve. It also happens to show the superior rectus, as well as the remaining recti, and inferior oblique, which are supplied by the ocular motor nerve. |
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[image]
This is a lateral view here showing more of the muscles. We see the inferior rectus, the superior rectus, medial rectus, levator palpebrae superioris, and inferior oblique. All of these supplied by the oculomotor nerve. |
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Accessory Nerve, Motor
This nerve originates in the upper cervical spinal cord, enters the skull, and then exits the skull to innervate the trapezius and the sternocleidomastoid muscles. |
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Hypoglossal Nerve, Motor
This nerve supplies all of the muscles of the tongue. |
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Trigeminal Nerve, Mixed
This nerve supplies sensation to the face and most of the head. The cell bodies of these somatic sensory axons are located in the trigeminal (semilunar) ganglion. Like other sensory ganglia mentioned above, the semilunar ganglia are functionally equivalent to dorsal root ganglia. The trigeminal nerve also supplies the muscles of mastication and a few other muscles. |
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[image]
We see here now that this shows an example of the important sensation here, sensation on the forehead, on the cheek, on the jaw, on the side of the head. These are all regions which you can test in your patients. Recall that we have the ophthalmic branch here, tap the patient on the forehead. We have a maxillary branch here, tap them on the cheek. And the mandibular branch here, tap them on the chin. So if you will, a quick and dirty neurologic exam. Forehead, cheek, and chin, testing the functions. And you can ask the person to-- you can try to have them move their jaw against force, for instance, to test the motor function. |
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Facial Nerve, Mixed
• Supplies the muscles of facial expression. The most clinically important muscles of facial expression are the orbicularis oculi, which close the eye, and the orbicularis oris, which close the mouth to prevent drooling.
• Supplies taste to the anterior two-thirds of the tongue. The cell bodies for these special visceral sensory axons are located in the geniculate ganglion.
• Has a parasympathetic component that supplies the lacrimal, nasal, and palatine glands, as well as the submandibular and sublingual salivary glands.
• Supplies sensation to the external acoustic meatus. |
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[image]
So we see here the branches of the facial nerve. We see it leaving the skull here, and we see it branching through the parotid gland, and one of the many ironies of anatomy is that although the facial nerve travels through the parotid gland, the innervation of the parotid gland is actually through the glossopharyngeal nerve. But what that means here with its relationship to the parotid gland means that surgery of the parotid gland often for benign tumors involves very careful identification of the facial nerve in order to keep from damaging the muscles of facial expression, which damage to them can be very devastating to your patients. |
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Glossopharyngeal Nerve, Mixed
• Supplies (general) visceral sensation to the posterior third of the tongue and upper pharynx. There are also physiologically important sensory branches to the carotid body and sinus. The cell bodies of these axons are located in the glossopharyngeal ganglion at the base of the skull.
• Supplies taste to the posterior third of the tongue. The cell bodies of these axons are located in the glossopharyngeal ganglion at the base of the skull.
• Carries preganglionic parasympathetic axons that innervate the parotid gland. These axons synapse in the otic ganglion, close to the external ear.
• Supplies one muscle, the stylopharyngeus. |
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[image]
So here we show the cartoon, here, talking about sensation. It shows that the nasal pharynx here behind the nasal cavity, the pharyngeal region behind the nasal cavity, supplied by the maxillary nerve. The oral pharynx is supplied by the glossopharyngeal nerve, and we talk about, now, the importance of the glossopharyngeal nerve being a sensory component of the gag reflex. And then the region below the oropharynx or the laryngopharynx and below is supplied by the vagus nerve. |
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Vagus Nerve, Mixed
• Supplies general sensation to the pharynx, larynx, thoracic viscera, and abdominal viscera as far as the end of the midgut. Their cell bodies are located in the vagal ganglia at the base of the skull.
• Innervates taste buds around the epiglottis. These special visceral sensory axons have cell bodies in vagal ganglia at the base of the skull.
• General somatic sensation to the region around the ear and dura of the posterior cranial fossa.
• Provides parasympathetic innervation of thoracic and abdominal viscera as far as the end of the midgut.
• Innervates striated muscle of palate, pharynx, larynx, and the proximal portion of the esophagus. |
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| Which is NOT a function of the trigeminal nerve? (vs what is) |
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| Sensation in the oropharynx. functions: Sensation in the nasal cavity, Sensation on the anterior two-thirds of the tongue, Sensation on the cornea |
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| Which is NOT a function of the facial nerve? (vs what is) |
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| Innervation of the levator palpebrae superioris. Functions: Salivation, Lacrimation, Innervation of the orbicularis oculi |
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| Damage to the oculomotor nerve will produce lateral deviation and depression of the eyeball, due to the unopposed actions of the lateral rectus and the superior oblique. The pupil will be dilated due to the unopposed actions of the dilator pupillae muscle innervated by the sympathetic nerves. There will also be loss of accommodation. |
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| Injury will decrease range of motion for depression of the eyeball when gaze is medial. |
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| Damage to the trigeminal nerve will result in loss of sensation over the affected area and paralysis of the muscles of mastication, if the motor root is involved. If the ophthalmic nerve is damaged, there will be loss of the corneal blink reflex. |
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| Damage to this nerve will produce a medially directed eyeball. The range of motion for lateral gaze will be decreased. |
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Damage to the facial nerve may produce a variety of symptoms depending on the location of the injury. These include paralysis of the muscles of facial expression, dry cornea (loss of lacrimation), loss of taste on the anterior two-thirds of the tongue, and increased sensitivity to noise.
From a clinical standpoint, it is the inability to shut the eye, due to paralysis of the orbicularis oculi and the dry cornea that produce the most significant problems.
The relationship of the motor branches of the facial nerve to the parotid gland is important, because of the incidence of tumors there. |
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| Injury may produce deafness or tinnitus (ringing in the ears) by damage to the auditory division and dizziness and nystagmus due to damage to the vestibular division. |
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| Glossopharyngeal Nerve Damage |
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| Damage to the nerve will produce loss of taste on the posterior third of the tongue, and loss of sensation on the pharynx and posterior soft palate. This sensory loss can be seen clinically as an absent (or asymmetric) gag reflex, since the glossopharyngeal provides the sensory limb of that reflex. |
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In the neck, damage to the vagus may produce several symptoms including hoarseness of the voice due to unilateral paralysis of the vocal muscles.
The gag reflex may be asymmetric. There may be difficulty in swallowing.
Because the heart and the GI tract have their own nervous systems, injury of the vagus would not be debilitating, but the CNS would lose some of its ability to modulate the coronary and enteric systems. |
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| Paralysis of the trapezius will produce a drooping shoulder and a rotated scapula. Paralysis of the sternocleidomastoid will produce a weakness in turning the head toward the intact side. |
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| Damage to the hypoglossal nerve will produce slurred speech. The tongue will deviate to the damaged side when protruded. This deviation occurs because the intact genioglossus (a protruder of the tongue) is unopposed. |
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| A patient presents with a dilated pupil on the left, and the eye is directed laterally and downward. What nerve do you suspect? |
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| A patient has slightly slurred speech and her tongue deviates from the midline when protruded. What nerve do you suspect? |
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| A patient has an asymmetric gag reflex, a hoarse voice, and a droopy shoulder. What nerves do you suspect (more than one answer)? |
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CN IX CN X CN XI
Yes, all of them may be compressed as they pass through the jugular canal. |
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