Intraocular muscles & nerves (anatomy, pupils and lens)
Summary
TLDRThis educational script delves into the intricate workings of the eye's internal structures, focusing on the autonomic nervous system's role in controlling pupil size and lens shape. It explains how sympathetic and parasympathetic nerves innervate the iris and ciliary body, respectively, to regulate focus and light response. The script also touches on the trigeminal nerve's role in general sensation, particularly the blink reflex, and briefly mentions the optic nerve's function in transmitting visual information.
Takeaways
- 👁 The script discusses the anatomy of the eye, focusing on the nerves and muscles within the eyeball.
- 🌟 The iris contains two muscles: the sphincter pupillae and the dilator pupillae, which control pupil size in response to light.
- 🔍 The sympathetic nervous system is responsible for pupil dilation, while the parasympathetic system causes pupil constriction.
- 🔄 The lens changes shape to focus light on the retina, with its thickness varying between looking at distant and near objects.
- 💪 The ciliary body contains smooth muscles that adjust the shape of the lens, controlled by autonomic nerves.
- 🧠 The oculomotor nerve (cranial nerve III) provides parasympathetic innervation to the ciliary body and the sphincter pupillae.
- 🌐 The ciliary ganglion is where the parasympathetic preganglionic neurons from the oculomotor nerve synapse with postganglionic neurons.
- 🚀 Sympathetic nerves originate from the thoracic levels of the spinal cord and follow arteries to reach the eye, affecting the iris and other structures.
- 🕊️ The trigeminal nerve (cranial nerve V) sends a branch, the nasociliary nerve, into the eye to provide general sensation to the cornea.
- 👀 The optic nerve (cranial nerve II) carries visual information from the retina to the brain and is considered an extension of the central nervous system.
- 🔬 The script suggests that understanding the paths that nerves take within the eye can make complex diagrams less confusing.
Q & A
What are the two types of nerves discussed in the script that get into the eyeball?
-The script discusses sympathetic and parasympathetic nerves and how they get into the eyeball.
What is the role of the pupil in the eye?
-The pupil changes size in response to the amount of light available, getting smaller in bright light and dilating in dim light.
Which muscle in the iris is responsible for pupil dilation and is innervated by sympathetic nerves?
-The dilator pupillae muscle, with radial fibers, is responsible for pupil dilation and is innervated by sympathetic nerve fibers.
How can the mnemonic 'Robbie and headlights' help remember the function of the sympathetic nervous system in pupil response?
-The mnemonic 'Robbie and headlights' helps remember that the sympathetic nervous system, associated with the fight-or-flight response, dilates the pupils, as if Robbie is caught in the headlights with wide, dilated eyes.
What is the role of the parasympathetic nerves in the eye?
-The parasympathetic nerves innervate the sphincter pupillae muscle, causing the pupil to constrict in response to bright light.
What is the function of the lens in the eye?
-The lens changes shape to focus light on the retina, allowing us to see sharp images of both distant and near objects.
How does the ciliary body relate to the lens and its function?
-The ciliary body contains smooth muscle that, when it contracts or relaxes, affects the shape of the lens, enabling focus on near or distant objects, respectively.
What are the Zonular fibers, and how do they connect the lens to the ciliary body?
-Zonular fibers, also known as suspensory ligaments, are collagen fibers that connect the lens to the ciliary body, allowing the lens to change shape.
Which cranial nerve is responsible for innervating the intraocular muscles and where does it originate from?
-The oculomotor nerve (cranial nerve III) is responsible for innervating the intraocular muscles and originates from the midbrain.
How do sympathetic nerves enter the eye and what is their role?
-Sympathetic nerves enter the eye by following the internal carotid artery and then running through the superior orbital fissure. They are involved in various functions, including dilation of the pupil and regulation of blood flow within the eye.
What is the role of the trigeminal nerve in the eye and how does it relate to the cornea?
-The trigeminal nerve sends a branch called the naso ciliary nerve into the eye, which carries general sensory fibers to the cornea, allowing us to sense touch and triggering the blink reflex.
What is the function of the optic nerve and how is it different from other cranial nerves?
-The optic nerve (cranial nerve II) carries visual information from the retina to the brain. It is considered a direct extension of the central nervous system and is covered by the same connective tissue layers as the brain.
Outlines
👁️ Anatomy of the Eyeball and Nerve Innervation
This paragraph delves into the anatomy of the eyeball, focusing on the inner structures and the role of nerves. It begins with the iris and pupil, explaining the autonomic control of pupil dilation and constriction by the sympathetic and parasympathetic nervous systems. The sympathetic nerves cause pupil dilation, likened to a rabbit in the headlights, while the parasympathetic nerves induce constriction. The lens and its function in focusing light on the retina are discussed, with the ciliary body and its muscles, controlled by the autonomic nervous system, playing a key role in adjusting the lens shape for near and far vision.
🔍 The Mechanics of Lens Focus and Nerve Pathways
The second paragraph explains how mammals, unlike birds of prey, adjust the lens shape for vision. It details the role of the ciliary body, a muscular structure surrounding the lens, and the zonular fibers or suspensory ligaments that connect the lens to the ciliary body. The autonomic control of these structures allows the lens to thin for distant vision and thicken for near vision. The parasympathetic nerves from the oculomotor nerve (cranial nerve III) are highlighted as the primary controllers of this process, with the ciliary ganglion serving as a key synapse point for these nerves.
🚀 Sympathetic Nerve Pathways and Their Role in the Eye
This paragraph explores the sympathetic nerve pathways that enter the eye, originating from the thoracic levels of the spinal cord and traveling along the arteries to reach the orbit. The sympathetic nerves are described as having various roles within the eye, including affecting the ciliary body and lens, and regulating blood flow in the retina. The superior orbital fissure is identified as a critical pathway for these nerves to enter the eye, with some passing through the ciliary ganglion and others following the long ciliary nerves.
🌐 The Complex Network of Nerves in the Eyeball
The fourth paragraph discusses the intricate network of nerves within the eyeball, including the sympathetic and parasympathetic nerves, and their respective roles. It also introduces the trigeminal nerve's role in conveying general sensory information from the cornea to the brain, which is crucial for the blink reflex. The long ciliary nerves are revealed as the primary pathway for these sensory fibers, with the sympathetic nerves also utilizing this route. The optic nerve's function in transmitting visual information is briefly mentioned, setting the stage for a deeper discussion in a future session.
🛣️ Nerve Pathways and the Optic Nerve's Special Status
The final paragraph wraps up the discussion by summarizing the various nerve pathways into the eyeball, emphasizing the optic nerve's unique status as a direct extension of the central nervous system. It mentions the central retinal artery and vein within the optic nerve, which supply blood to the retina. The optic nerve's journey from the retina through the optic canal and its protective coverings are also highlighted. The paragraph concludes by suggesting a future discussion on the optic nerve's full pathway and the implications of its damage.
Mindmap
Keywords
💡Eyeball
💡Pupil
💡Iris
💡Lens
💡Ciliary Body
💡Zonular Fibers
💡Parasympathetic Nerves
💡Sympathetic Nerves
💡Oculomotor Nerve
💡Trigeminal Nerve
💡Optic Nerve
Highlights
Introduction to the anatomy of the eye, focusing on the nerves and muscles within the eyeball.
Explanation of how sympathetic and parasympathetic nerves enter the eyeball and their roles in pupil dilation and constriction.
Description of the iris muscles, sphincter pupillae, and dilator pupillae, and their autonomic nerve innervation.
Mnemonic 'Robbie and headlights' to remember the sympathetic and parasympathetic roles in pupil response.
Discussion on the lens's function in focusing light on the retina and the role of the ciliary body.
Mechanism of how the ciliary body's contraction and relaxation affect the lens shape for near and far vision.
Clarification on the autonomic control of lens shape change, distinguishing it from conscious focus control.
Identification of the oculomotor nerve as a source of parasympathetic innervation for intraocular muscles.
Details on the Edinger-Westphal nucleus's role in parasympathetic control of the eye.
Explanation of the ciliary ganglion's function as a parasympathetic ganglion for the eye.
Description of the short and long ciliary nerves and their roles in transmitting parasympathetic signals.
Pathway of sympathetic nerve fibers from the spinal cord to the eye and their function.
Role of sympathetic nerves in blood flow regulation within the eye's blood vessels and retina.
Discussion on the trigeminal nerve's branch, the naso ciliary nerve, and its role in general sensation of the cornea.
Importance of the blink reflex triggered by general sensory neurons in the cornea.
Overview of the optic nerve's function as a direct extension of the central nervous system, carrying visual information.
Mention of the central retinal artery and vein within the optic nerve supplying the retina.
Conclusion summarizing the complex nerve pathways and their functions within the eye.
Transcripts
[Music]
okay so in previous weeks we've been
doing various things with the eye
we stayed outside the eyeball this week
we're going to get inside the eyeball
and we're sticking with nerves this time
we've got a workout how sympathetic
nerves get into the eyeball how
parasympathetic nerves get into the
eyeball and how the intra ocular muscles
are innervated so there's still quite a
lot to do I think what we'll do is we'll
start anteriorly so we'll start with the
bit we can see we'll start with the
pupil and the iris because you know the
pupil gets larger and smaller and then
we'll go a step back and look at the
lens and the muscles that affect the
lens and then we'll do the last bit of
the retina which I'm not going to do in
any detail at all I'm just gonna say
retina optic nerve that's about it and
then we should have there's also a
general sensation as well remember the
trigeminal nerve that sends a branch
inside the eyeball to get to the cornea
if we layer it up like that the
complexity that you see on some diagrams
should seem less confusing all right
good luck okay so I've got yeah I got a
lot more thus today let's have a look
inside this eye beware this one this is
just the eye balled without the muscles
and stuff you can see you get an idea at
the ballet look all the little nerves
that are going in there if I take this
apart so the see-through bit is the
cornea and then of course we've got the
iris here and the pupil in the middle
and you know the pupil changes size in
response to the amount of light
available so if it's very bright
the pupil gets smaller and if it's not
very bright the people slowly dilate
right if you flash a light if you shine
light into somebody's eyes you should
see that in fact if you shine light into
one eye you should see the same reaction
in both pupils so what we must have is
in the iris then
there are two muscles we have one
circular muscle which is sphincter
poopley
and then we have a muscle like a radial
muscle with the fibers running in this
direction around the outside kind of in
the in the direction of lines that you
see the iris in and that's dilator pupil
e1 is innovated by sympathetic nerve
fibers and one is innovated by
parasympathetic nerve fibers so both of
those are motor autonomic nerves so
obviously you don't have control over
pupilary dilation and constriction it's
an autonomic response now which one does
which the way to remember is Robbie and
headlights right so you know the
sympathetic nervous system and
adrenaline are responsible for the
fight-or-flight response part of our
fight-or-flight response if you think
about the rabbit in the headlights who
stopped still wide eyes dilated pupils
as well maybe so dilated pupil II the
muscle with the radial fibers like that
that's innovated by the sympathetic
nerves and that causes the pupil to to
widen also remember we talked about the
the muscle up here the the the superior
tarsal muscle which also has sympathetic
fibers which is causes the lifting of
the oil it's and but the but see so that
means that the opposite then the
parasympathetic nerves innervate sinter
poopley
which then running radially around here
cause the sphincter to close in the
pupils to shrink in response to bright
daylight those are all smooth muscles
within the iris on the other side of the
colored part which kind of aren't really
shown on here the next bit thank you see
there's the lens so the pupil is a hole
and the lens is on the other side this
is the anterior chamber under here so
what about the lens then you're probably
aware that the lens changes shape the
job of the lens is to focus light on the
retina at the back of the eye right and
we're not going to go into the physics
of all of that but there is a lot
physics ER
when we look at something far away when
we focus on a distant object the lens is
quite thin and it focuses that distant
light onto the retina so that we see a
sharp image whereas when we're reading
when we're looking at something close up
when we're focusing on a close object
the lens fattens and by fattening it
again focuses the light from that near
object on the retina to give us a sharp
image now that's what asked mammals do
is we should change the shape of the
lens I think birds of prey have got I
think they can even change the shape of
their cornea and they might even have
two fovea on the on the retina they
which accounts for their amazing
eyesight but us mammals we do that by
changing the shape of the lens if you
change the shape of the lens how are we
going to do that it's going to be
muscular and again it's autonomic
because we're not I mean you can kind of
you you can kind of control the focus a
bit can you but essentially it's under
autonomic control that the way in which
this works is a little bit backwards
inside the eye there are three layers of
tissues one of those layers is called
the vascular layer and the vascular
layer where it exists around the lens
because obviously it's going to leave a
hole for the lens there is smooth muscle
arranged within that vascular layer the
muscle in this vascular layer is called
the ciliary body so the ciliary body
when you see in an illustration of
course you only ever see in 2d like
cross-sectional things so it's like it
looks like a flat thing but we've
imagined the ciliary body as a sphincter
as a circular sphincter like other
sphincters in the body and it runs
around the lens now the lens is
suspended from the ciliary body by
fibers by like ligamentous fibers by
collagen fibers these fibers get called
zoella fibers or zhonya nerve fibers
this area gets
called the zone new lives in which is a
great name so these fibers of Zin if you
want to the contemporary term would be
to call this as suspensory ligament
they're rather suspensory ligaments
around the body there's another one
outside the eye you've come across lots
of suspensory ligaments at all it's a
good general term but um
the lens is suspended from the ciliary
body by fibers running around in between
them and that would be the suspensory
ligament so again that's a circular
suspensory ligament so now what happens
right when the ciliary body contracts
it's going to shrink cuz it's a
sphincter right and when the ciliary
body relaxes its gonna open up because
it's a sphincter and that's what
sphincters do what effect is that going
to have on the lens but if the lens is
in the middle and it's it's suspended
from the ciliary body by those collagen
fibers by the suspensory ligament when
the ciliary body relaxes and gets bigger
the lens is going to get stretched and
flattened so when the ciliary body is
relaxed you're going to be able to focus
on distant objects when the ciliary body
contracts then those that suspensory
ligament is also going to relax and the
lens is going to shrink and it's going
to fatten which means that when the
ciliary body contracts you're able to
focus on near objects this is
essentially controlled by
parasympathetic innervation so these
thus the muscles in the ciliary body are
controlled by parasympathetic nerves so
when there is parasympathetic
innervation it contracts and you focus
on near objects and when that
parasympathetic innervation is removed
the ciliary body relaxes the the the
lens gets stretched and you focus on
distant objects hopefully the way I've
explained it seems really
straightforward because the
time you came to Italy it often seems a
bit backwards if I describe to you why
it's confusing you're gonna get confused
if you've not be confused by it then
you're not confuse nice that's good
right
next question where do those
parasympathetic nerve fibers come form
well we looked at the extra ocular
muscles and we saw that most of the
extraocular muscles moving the orbit
were innovated by the oculomotor nerve
very well named coriander 3 and also
know the trochlear in abducens nerve
well it turns out that the intra ocular
muscles are also innovated by the
oculomotor nerve good huh so the
oculomotor nerve produces mostly somatic
motor neurons for those extra ocular
muscles but also parasympathetic neurons
it produces a bunch of preganglionic
parasympathetic neurons which go out and
make their destination are the Paris of
a smooth muscle fiber smooth muscle
fibers of the of the ciliary body and
the sphincter poopley
muscles right do you remember where the
oculomotor nerve comes from it's a bit
bigger on this model than it was on the
other model but the oculomotor nerve
comes out of the midbrain here and some
reason the parasympathetic nucleus
involved in this is is quite famous and
people seem to remember its name the
heading of Westphal nucleus and do you
remember last time we were talking about
the cerebral aqueduct here and the
midbrain here
well the eddying Avesta foul nucleus is
very close to the other nuclei of the
oculomotor nerve it's in the midbrain
and a pop site as we saw there dives
through the superior orbital fissure at
the back of the orbit because that's how
pretty much everything gets from the
cranial cavity into the orbit unless
you're the optic nerve or similarly
special
and then those preganglionic
parasympathetic nerves get to the
ciliary ganglion and the ciliary
ganglion is one of the four
parasympathetic ganglia of the head say
four there's four on each side so
parasympathetic and sympathetic nerves
we have pre ganglionic neurons coming
out of the central nervous system they
find their way to a ganglion a
collection of cell bodies they synapse
with another neuron and that
postganglionic neuron goes off to his
target organ now in the case of the the
the parasympathetic nerves going to the
the eye going inside the eye that off
that's the ciliary ganglion and in the
ciliary ganglion and that's a
parasympathetic ganglion those
preganglionic parasympathetic neurons
parts of the oculomotor nerve cranial
nerve three they've run in there they
meet our postganglionic parasympathetic
neurons synapse with it and that neuron
shoots off and through these nerves here
it's getting to the orbit these nerves
get called the short ciliary nerves
because stuff randy gets called ciliary
but if they're a short ciliary nerves
then there must be longer ciliary nerves
these other nerves going up here these
longer ones those are the long sinewy
nerves
so those postganglionic parasympathetic
neurons run with the short ciliary
nerves could get into the eyeball and
then they whip off to those Targhee
muscles that we were talking about
earlier cool huh
next question if that's how the
parasympathetic neurons gain their how
do these sympathetic neurons get into
the eye well all sympathetic neurons
come out of the spinal cord in the
thoracic levels
one right so that preganglionic
sympathetic neuron comes out of the
spinal cord and you know about the
sympathetic trunk right the trunk of
sympathetic ganglia running down the the
posterior thoracic cage the posterior
thoracic wall posterior pelvic posture
you neck and what have you right well
the main way the sympathetic neurons
travel around the body or one of the
main ways is by following arteries so of
course all the orbit all right
and then here's a huge red blood vessel
here and you can see that it's covered
in little white lines what's the blood
vessel that's the internal carotid
artery because that's gone inside the
cranial cavity and makes this room
s-shape and at the end of our earth
shape it one's posterior to the superior
orbital fissure so postganglionic
sympathetic neurons work their way up
the sympathetic trunk and then they
follow the common carotid artery an
internal carotid artery up into the
cranial cavity and they stay with it
until it runs posterior to the orbit and
then they jump off those sympathetic
nerves jump off run through the superior
orbital fissure because everything goes
through that right and then they run
auntie merely through here and they'll
take whatever route they can get to get
into the eye which means that they can
on what some of them will run through
the ciliary ganglion they won't sign up
they're just running through it's just
cabling it's just a convenient route for
them to follow and they they're within
also then the short ciliary nerves and
they run into the eyeball and they do
they're sympathetic jobs within the on
label which is the motor bit we just we
just talked about there is some
discussion about sympathetic nerves also
affecting the the ciliary body and the
the lens you know opposing the
parasympathetic innervation so that when
you're startled you also focus on
distant objects nine times
how true that is but there you go so
they'll do those sympathetic motor jobs
we've been talking about but also of
course sympathetic nerves one of the
reasons they're following the arteries
is because they're going to affect the
smooth muscle in the artery walls and
and be involved in regulation of where
blood goes in blood flow so they're also
gonna go into the retina and into the
blood vessels within the retina and
control blood flow through there some of
them are going to run through the long
ciliary nerves because why not you just
trying to find any way you can get from
here to the eyeballs you go through the
long ciliary nerves and that's how the
sympathetic nerves and the
parasympathetic nerves get into the
eyeball but there is a nother nerve
going into the eyeball and it's kind of
the real reason the long ciliary nerves
exist but I've got to pick up this big
model again I could do with the big do
it like a tall thing he couldn't write
the short ciliary nerves run between the
ciliary ganglion and the eyeball those
are the short ciliary nerves now really
the long ciliary nerves are running
across here and this nerve here these
are branches of the trigeminal nerve
that we talked about some weeks ago we
talked about the trigeminal nerve and
how it innovates you know carriage
general sensory innervation from the
skin of the face and the eyelid in the
conjunctive or an ass or thing right now
the mezzo ciliary nerve is a branch
that's going to send some general
sensory fibers through these long
ciliary nerves to get into the eyeball
and the ultimate destination of those
neurons is is the cornea right so the
the cornea has general sensation and you
know if you try and touch her you really
can you really don't want to and that's
the the blink reflex so the I mean you
know even if you were to touch the
cornea likely with the bud of Cornwall
you should trigger the blink blink
reflex so general sensation is going to
travel through those general sensory
neurons through the long ciliary nerves
through the
nay's of ciliary nerves and then back
through the ophthalmic branch of the
trigeminal nerve and back to the brain
and then trigger the facial nerve to
close the oil herbs that that's the
blink reflex they should happen but both
eyes should happen on both sides so that
was the real reason that the long
ciliary nerves are there they're
actually a branch of the nazo ciliary
nerves carrying general sensation from
the cornea the sympathetic nerves just
run through there just for fun I think
as most of the stuff that goes through
the eyeball there's obviously one big
one that we haven't talked about and
that's the optic nerve by the way those
that the general sensory nerves of the
nazo ciliary know if they'll actually
run through some of those will run
through the ciliary ganglion as well
once I napped with it they'll just run
through and through the short silly wee
nerves getting the eyeball these nerves
they just seem to follow whatever path
they can it's probably because in the
embryo they're following similar
chemotactic
cues anyway the the optic nerve cranial
nerve to the optic nerve is as I'm sure
you're aware carrying special sensory
information back from the retina
carrying vision and I'm not going to
talk about it too much because that's
another very minute talk all in its own
suffice to say that cranial nerve two is
considered a direct extension of the
central nervous system so it's not like
a peripheral nerve in normal sense the
retina and the optic nerve are are an
outgrowth and extension of the forebrain
so they're quite special and because of
that if you follow the optic nerve
through the optic canal and into the
orbit it's covered by the three layers
of connective tissue of PMA - arachnoid
mater and dura mater with a bit of CSF
and what have you and it yeah that's all
I'm going to say is it runs to the eye
carries vision one thing is worth
mentioning is that within the nerve
there is a little artery and a little
vein and those are the central retinal
artery and central retinal vein carrying
blood vessels to the retina
but that's it those are the nerves that
enter the eye so now we've talked about
the intra ocular muscles the sympathetic
and parasympathetic nerves that
innervate those muscles and how they get
there we've added on a bit of general
sensation and we've briefly mentioned
the optic nerve if ever we talk about
the optic nerve what we really should do
is talk about the retina the optic nerve
the optic tract tract the lateral
geniculate nucleus and the thalamus and
the yeah we should follow the whole
tractor back and how all crosses over
and what happens when you damage
different bits because that's a whole
topic in its own right but we'll save
that for another day right
so if you see complicated diagrams of
nerves going to and from the Ani ball
and passing through the ciliary ganglion
don't get to befuddle just think about
the nerves are going from one place and
trying to get to another place and
they're just taking whatever route they
can and the only parasympathetic neurons
synapse in the ciliary ganglion other
nerves do pass through the single
ciliary ganglion but they're just
passing through
it's just cabling at that point all
right I've made a right mess now like
you guys next week
[Music]
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