Show The optic nerve (CN II) is the second cranial nerve, responsible for transmitting the special sensory information for vision. It is developed from the optic vesicle, an outpocketing of the forebrain. The optic nerve can therefore be considered part of the central nervous system, and examination of the nerve enables an assessment of intracranial health. Due to its unique anatomical relation to the brain, the optic nerve is surrounded by the cranial meninges (not by epi-, peri- and endoneurium like most other nerves). In this article, we shall look at the anatomy of the optic nerve – its course, sensory functions and clinical relevance. Fig 1 – Overview of the anatomical course of the optic nerve. Anatomical CourseThe anatomical course of the optic nerve describes the transmission of special sensory information from the retina of the eye to the primary visual cortex of the brain. It can be divided into extracranial (outside the cranial cavity) and intracranial components. The optic nerve is formed by the convergence of axons from the retinal ganglion cells. These cells in turn receive impulses from the photoreceptors of the eye (the rods and cones). After its formation, the nerve leaves the bony orbit via the optic canal, a passageway through the sphenoid bone. It enters the cranial cavity, running along the surface of the middle cranial fossa (in close proximity to the pituitary gland). Intracranial (The Visual Pathway)Within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm. At the chiasm, fibres from the nasal (medial) half of each retina cross over to the contralateral optic tract, while fibres from the temporal (lateral) halves remain ipsilateral:
Fig 2 – The nasal retinal fibres crossing over at the optic chiasm. Each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus (LGN), a relay system located in the thalamus; the fibres synapse here. Axons from the LGN then carry visual information via a pathway known as the optic radiation. The pathway itself can be divided into:
Once at the visual cortex, the brain processes the sensory data and responds appropriately. By Ratznium [CC-BY-SA-3.0], from Wikimedia Commons Fig 3 – The optic pathway. Clinical Relevance: Pituitary AdenomaA pituitary adenoma is a tumour of the pituitary gland. Within the middle cranial fossa, the pituitary gland lies in close proximity to the optic chiasm. Enlargement of the pituitary gland can therefore affect the functioning of the optic nerve. Compression to the optic chiasm particularly affects the fibres that are crossing over from the nasal half of each retina. This produces visual defect affecting the peripheral vision in both eyes, known as a bitemporal hemianopia. Surgical intervention is commonly required. To access the gland, the surgeon uses a transsphenoidal approach, accessing the gland via the sphenoid sinus. Fig 4 – Bitemporal hemianopia, affecting the lateral visual fields in both eyes. How is the visual pathway from the eye different from the ear or hand?How is the visual pathway from the eye different from that of the ear or hand? Each eye is not primarily connected to one hemisphere only.
What is the visual pathway from the eye to the brain?Optic nerve: This cranial nerve sends visual information from your retina to your brain. It consists of more than 1 million nerve fibers.
How does the visual pathway work?The visual pathway starts with light as its stimulus. As light enters the eye, photoreceptors in the retina convert the stimulus into an electrical potential. There, the potential is transmitted from retinal photoreceptors to bipolar neurons, which form the large optic nerve (cranial nerve II).
What are the steps of the visual pathway?Introduction. When light passes through the lens it reaches the. the retina, where the formed image is. ... . Summary of conscious vision perception. retina → optic nerve → optic chiasm → optic tract → lateral geniculate body → optic radiation to primary visual cortex.. Additional pathways.. |