Optic disc

Key takeaways

Optic disc belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Optic disc to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Optic disc from memory before moving on to harder problems.

Reference excerpt

The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye. The ganglion cell axons form the optic nerve after they leave the eye. The optic disc represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together. The optic disc in a normal human eye carries 1–1.2 million afferent nerve fibers from the eye toward the brain. The optic disc is also the entry point for the major arteries that supply the retina with blood, and the exit point for the veins from the retina.

Structure The optic disc is located 3 to 4 mm to the nasal side of the fovea. It is a vertical oval, with average dimensions of 1.76mm horizontally by 1.92mm vertically. There is a central depression, of variable size, called the optic cup. This depression can be a variety of shapes from a shallow indentation to a bean pot—this shape can be significant for diagnosis of some retinal diseases.

Function The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye.

Clinical significance Almost all eye structures can be examined with appropriate optical equipment and lenses. Using a modern direct ophthalmoscope gives a view of the optic disc using the principle of reversibility of light. A slit lamp biomicroscopic examination along with an appropriate aspheric focusing lens (+66D, +78D or +90D) is required for a detailed stereoscopic view of the optic disc and structures inside the eye. A biomicroscopic exam can indicate the health of the optic nerve. In particular, the eye care physician notes the colour, cupping size (as a cup-to-disc ratio), sharpness of edge, swelling, hemorrhages, notching in the optic disc and any other unusual anomalies. It is useful for finding evidence corroborating the diagnosis of glaucoma and other optic neuropathies, optic neuritis, anterior ischemic optic neuropathy or papilledema (i.e. optic disc swelling produced by raised intracranial pressure), and optic disc drusen. Women in an advanced stage of pregnancy with pre-eclampsia should be screened by an ophthalmoscopic examination of the optic disc for early evidence of a rise in intracranial pressure.

Pale disc

A normal optic disc is orange to pink in colour . A pale disc is an optic disc which varies in colour from a pale pink or orange colour to white. A pale disc is an indication of a disease condition.

Imaging

Traditional colour-film camera images are the reference standard in imaging, requiring an expert ophthalmic photographer, ophthalmic technician, optometrist or ophthalmologist for taking standardised pictures of the optic disc. Stereoscopic images offer an excellent investigative tool for serial follow-up of suspected changes in the hands of an expert optometrist or ophthalmologist. Automated techniques have also been developed to allow for more efficient and less expensive imaging. Heidelberg retinal tomography (HRT), scanning laser polarimetry and optical coherence tomography are computerised techniques for imaging various structures of the eyes, including the optic disc. They quantify the nerve fiber layer of the disc and surrounding retina and statistically correlate the findings with a database of previously screened population of normals. They are useful for baseline and serial follow-up to monitor minute changes in optic disc morphology. Imaging will not provide conclusive evidence for clinical diagnosis however, and the evidence needs to be supplanted by serial physiological testing for functional changes. Such tests may include visual field charting and final clinical interpretation of the complete eye examination by an eye care physician. Ophthalmologists and optometrists are able to provide this service. Blood flow in the retina and choroid in the optic disc region can be revealed non invasively by near-infrared laser Doppler imaging. Laser Doppler imaging can enable mapping of the local arterial resistivity index, and the possibility to perform unambiguous identification of retinal arteries and veins on the basis of their systole-diastole variations, and reveal ocular hemodynamics in human eyes. Furthermore, the Doppler spectrum asymmetry reveals the local direction of blood flow with respect to the optical axis. This directional information is overlaid on standard grayscale blood flow images to depict flow in the central artery and vein. A systematic review of 106 studies and 16,260 eyes compared the performance of the imaging techniques, and found that all three imaging tests performed very similarly when detecting for glaucoma. The review found that in 1,000 patients subjected to imaging tests, with 200 having manifest glaucoma, the best imaging tests would miss 60 cases out of the 200 patients with glaucoma, and incorrectly refer 50 out of 800 patients without glaucoma.

Abnormalities Megalopapilla: a non-progressive condition in which the optic disc is enlarged (diameter exceeding 2.1 mm) with no other morphological abnormalities. Morning glory disc anomaly: a unilateral congenital deformity resulting from failure of the optic nerve to completely form in utero. The term was coined in 1970 by Kindler, noting a resemblance of the malformed optic nerve to the morning glory flower. Optic disc coloboma: a congenital deformity resulting from improper closure of the choroid fissure, usually bilateral. Optic pit: congenital excavation of the optic disc resulting from a malformation during development of the eye.

Gallery

Comparative anatomy The optic disc has different shapes among vertebrates. It may be circular, oval, reniform, triangular, or linear. The linear form is the case for the squirrels, most birds, and the predaceous pikes, salmonoids, and percoids among the teleost fishes. Most squirrels have a very long and thin linear optic disc, placed horizontally and dorsally in the retina. This allows the squirrel to see the sky without blind spots. Generally, the brighter the environment that the squirrel is active in, the longer the optic disc. The flying squirrel Glaucomys volans is nocturnal, and has a circular optic disc at the center of the fundus.

Illustrations

Worked examples

Example 1 — a first encounter with Optic disc

Start with the simplest possible case. Write down what Optic disc claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Optic disc before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Optic disc ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Optic disc

In research: Optic disc appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Optic disc in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Optic disc is common in secondary-school and first-year university syllabi. It links to neighbouring topics Human eye anatomy, so understanding it makes those chapters shorter.
In everyday life: Look for Optic disc outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Optic disc in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Optic disc means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Optic disc out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Optic disc in simple terms?

The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye.

Why does Optic disc matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Optic disc?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Optic disc.