hello anatomy colleagues this is dr alsup and in this video we are talking the
very basics of every single cranial nerve so 12 pairs of them in all you will get more details
as pertinent throughout the curriculum and some we've already discussed in detail vegas anyone
but this learning objective is meant to be a quick overview of basic function so the
basics will include the name as well as the roman numeral associated with them basic characteristics
of each so understanding what fibers make up the nerve is it sensory fibers motor fibers or both
and then of course we need to get a basic understanding of what these nerves will innervate
so this may seem like a lot but i hope this feels more like a nice leisurely stroll through the
cranial nerves instead so let's get started recall that the cranial nerves are a component of the
peripheral nervous system as these are nerves thus nervous tissue outside the central nervous system
there are 12 pairs in all so quite a few less than what we have with spinal nerves
recall there are 31 pairs of spinal nerves and they're also different from spinal nerves which
are composed of both sensory and motor fibers some of the cranial nerves are composed of
only sensory fibers or only motor fibers while some will have both sensory and motor the cranial
nerves are closely associated with what are called cranial nerve nuclei which are groups of
neurons in the brain where the efferent fibers originate and afferent fibers will terminate
so as you might have guessed we're going to start with cranial nerve one and make our way
to cranial nerve 12. so what is cranial nerve one why it's the olfactory nerve of course this
nerve is entirely sensory in fact it is what we call special sensory so what does
that even mean what is why is it so special all this means is that this nerve or any
special sensory nerve is composed of axons or fibers transmitting unique sensations
think of the five senses you likely grew up learning plus some bonuses and it's a
little bit more complex than that but i think for our purposes that's where we're gonna go
the olfactory nerve is associated with the sense of smell so sensation of odors
that results from the detection of odorous substances aerialized from the environment
the origin and journey of the olfactory nerves and associated structures such as the olfactory bulbs
and the olfactory tracts which you can see here it's typically these olfactory tracks that are
most grossly visible component of the olfactory nerves so it's pretty complex
but for our purposes we want you to understand that the olfactory nerve is cranial nerve one
and is composed of special sensory fibers associated with the sense of smell
also entirely sensory and special sensory to boot is the optic nerve or cranial
nerve two which functions in vision as you would probably guess from the name
the optic nerve begins where the axons of the retinal ganglion pierce the sclera and so the
sclera is the whites of the eyes and we'll talk about this in great detail in the next session
from here the optic nerve will exit the optic canal to enter the middle cranial fossa where
there will be this formation of the optic chiasm which you can see a bit of it here it's been cut
and fibers from the medial half of each retina will deco site or cross in the chiasm and join
uncross fibers from the other side so this partial crossing of certain optic nerve
fibers is essential in binocular vision so a bit more to come on this in the next session
moving to cranial nerve 3 or the oculomotor nerve which is unlike the other two entirely motor or
efferent the oculomotor nerve sends somatic motor fibers to all extra ocular eye muscles except for
two the superior oblique and the lateral rectus which are going to be innervated by different
cranial nerves so up until now the only cranial nerve that we knew had parasympathetic fibers was
the vagus well that is about to change because the oculomotor nerve also has preganglionic
parasympathetic fibers associated with it and it will synapse in the ciliary ganglion
which is within the orbital region and it is those postganglionic fibers are destined for
the ciliary muscles and um the sphincter papillae muscle so the ciliary muscle plays a role in terms
of accommodation so keeping things in focus and the sphincter papillae muscle does exactly what
it sounds like it will do it will constrict the pupil it will eventually split into superior
and inferior divisions the oculomotor nerve which you can see the inferior division right here
and so more to come on the oculomotor as well in the next session of the orbiting eye
next we have the trochlear nerve or cranial nerve four which is also is
going to be entirely motor or efferent this is the smallest cranial nerve and it will innervate
the superior oblique muscle which is one of the extra eye muscles
excuse me the trochlear nerve is named for the trochlea which is associated with the superior
oblique muscle so it kind of helps change the direction of the tendon for the muscle this nerve
is unique in the fact that it emerges from the posterior aspect of the brain stem so it's the
only one that looks like it's kind of poking out from that posterior region and wrapping around
all right our first nerve with both afferent and efferent fibers and this also happens to
be the largest cranial nerve and this is the trigeminal nerve or cranial nerve five
it's divided into three divisions which are quite large as well
two of the three divisions are entirely sensory these are v1 or the ophthalmic nerve
you can see here v2 also known as the maxillary nerve these are entirely sensory whereas v3
is going or the mandibular nerve has both afferent and efferent fibers and those efferent fibers
are going to be associated with the muscles of mastication or chewing muscles in addition to a
few neighboring muscles the trigeminal nerve will provide affair and innervation to a relatively
huge expansive area so including but not limited to the skin of the face teeth gingiva paranasal
sinuses and portions of the mouth so a very large very important nerve associated with
the head the abducens nerve or cranial nerve 6 is relatively simple compared to the trigeminal nerve
this nerve is entirely efferent and will innervate the lateral rectus muscle so here's your abducens
here's your lateral rectus which is one of the extra ocular eye muscles the lateral rectus muscle
will abduct the eye thus the name abducens recall that the abducens nerve is the cranial nerve
that travels directly through the cavernous sinus near the internal carotid artery before
exiting the superior orbital fissure to enter the orbital region the facial nerve or cranial nerve
7 is another nerve that has both sensory and motor fibers the sensory fibers include special sensory
fibers for tastes to the anterior two-thirds of the tongue so anterior portion of the tongue
the somatic motor fibers innervate muscles of facial expression so how we change the
expressions in our face it's because these muscles are contracting as well as a few
neighboring muscles and you will see all these small branches kind of throughout in this image
here these are all branches of the facial nerve heading to those muscles of facial expression
and that's really where it gets its name the visceral motor fibers include pre-ganglionic
parasympathetic fibers which will synapse and peripheral parasympathetic ganglia
and these five or these are going to send fibers to initiate secretion
from the lacrimal gland so thinking tears as well as the submandibular and sublingual glands so
thinking um salivary glands or secreting saliva so a very very complex nerve here in the facial nerve
one of my favorite cranial nerves is cranial nerve eight or vestibulocochlear nerve which is entirely
sensory specifically special sensory involved in hearing and equilibrium the vestibulocochlear
nerve will enter into the internal acoustic meatus so heading into the petrous part of the
temporal bone and it will divide into cochlear and vestibular nerves so you can see the cochlea here
the glossopharyngeal nerve is cranial nerve nine and has both afferent and efferent fibers and
there is a lot going on with the glossopharyngeal nerve there are going to be special sensory fibers
for taste for the posterior one third of the tongue so whereas the facial nerve is
going to have special sensory fibers for the anterior two-thirds it's the glossopharyngeal
that has the posterior one-third it will also provide general sensory fibers for that area
we already know that the glossopharyngeal provides sensory innervation to the pharynx
we talked about that in great detail but it's been a while since we did so it's time for a
reminder additionally there will be preganglionic parasympathetic fibers that originate in the
glossopharyngeal so we've had that in oculomotor facial glossopharyngeal and then of course
vagus those are the four nerves excuse me that have parasympathetics associated with them
and so these preganglionic parasympathetic fibers are going to synapse in the otic ganglion
and it will be heading towards the largest salivary gland the parotid gland
so we'll initiate secretion of saliva and lastly it provides somatic motor innervation for the
stylopharyngeus which we can see right here and hopefully we recall that from the pharynx session
at this point i hope you are very familiar with the vagus nerve or cranial nerve tin
in fact we have already described the major roles of the vagus nerve throughout the year
but if you need a review of this very complex nerve you have this short summary here
the accessory nerve or cranial nerve 11 we have also discussed as being
entirely efferent or motor and it will innervate the muscles sternocleidomastoid and the trapezius
muscles this nerve was traditionally called the spinal accessory nerve so you may hear it referred
to in that way and last but certainly not least we have the hypoglossal nerve or cranial nerve
12. and when you think of hypoglossal nerve i want you to think efferent and i want you
to think tongue so it is going to innervate the muscles of the tongue both extrinsic and
intrinsic muscles except for one but for the most part just generalized think tongue muscles
all right excellent we have made it to the summary slide with everyone's favorite a nice
beautiful table that succinctly summarizes the information we discussed in this video
please do take time to review on your own and always feel free to
reach out with any questions have a great rest of your day