Table of Contents
Vision is such a blessing that we cannot fully praise it. Can you imagine a world full of audio but no visual element? That’s the misery of the people who are blind. One of the main causes of blindness is an injury to the optic nerve. In the following text, we shall discuss the origin, course, classification, and functional component(s) of the optic nerve. We shall also see what are the visual fields, visual pathways, and visual reflexes. Lastly, we shall discuss some diseases related to the optic nerve and optic pathway.
Cranial nerves that supply sensory and motor innervation to the area of the head and neck are 24 in number, i.e., twelve pairs. The optic nerve is the second cranial nerve (CN II). It carries a special somatic sensation, i.e., the sense of vision from the retina of the eye to the visual cortex of the occipital lobe of the forebrain. When the light coming from an object strikes the retina, the photoreceptive cells, the rods, and cons, transduce this light signal in the form of an electric signal which is then carried to the central nervous system via the optic nerve. These signals are then received, integrated, and analyzed in the visual cortex. The image formed in the visual cortex is the actual image that we see. This is the reason why a hallucinated person sees those things in the environment which do not exist in the real world.
Origin and Course of the Optic Nerve
The retina of the eye is composed of ten cellular layers. The outermost layer is formed by the ganglion cells. Axons of these ganglion cells converge at a point known as the optic disc (a physiological blind spot of the retina because it lacks rods and cons). From the optic disc, these axons form a bundle known as the optic nerve. Axons forming the optic nerve are myelinated, but this myelination is not done by the Schwann cells (myelinating cells found in the peripheral nervous system) but rather by the oligodendrocytes (myelinating cells found in the central nervous system). This clues that the optic nerve is not a true nerve; rather, it is a direct continuation of the brain.
The optic nerve leaves the orbital cavity through the optic canal and enters into the anterior cranial fossa of the skull. The two nerves from opposite sides meet at the center to form optic chiasm. In the optic chiasm fibers from the nasal halves of visual fields (discussed later) of both the eyes cross to the opposite side. From the optic chiasm, the optic tract is formed, which ascends to the lateral geniculate body. The lateral geniculate is a small projecting part of the thalamus where visual sensory information is relayed. From the lateral geniculate body, optic radiation emerges and reaches through the internal capsule to the visual cortex located in the occipital lobe of the forebrain. The visual cortex is divided into primary and secondary visual cortices, each having different functions. Some fibers from the visual cortex go to the frontal eye field area located in the frontal lobe of the forebrain. The frontal eye field area is involved in some reflexes (discussed later).
Some of the fibers from the optic tract leave the tract and go to the midbrain. In the midbrain, they synapse either in the pretectal nucleus or the superior colliculus. These neurons are important because they play an important role in different visual reflexes such as the light reflex, accommodation reflex, etc.
Classification and Functional Component
The optic nerve is exclusively a sensory nerve having no motor supply. It consists of special somatic afferents that carry one of the special somatic sensations, i.e., vision, to the central nervous system.
All of the visual information is relayed in the thalamus, the main sensory collection center of the brain. This is in contrast to the olfactory information that does not relay in the thalamus and goes directly to the olfactory cortex.
Vision and Visual Fields
Field of vision means the total area in which you can see in peripheral vision when your eye is focused on some central object. The sharpness of vision decreases from the center to the periphery of the visual field. The visual field of one eye is divided into two halves; the nasal half and the temporal half. The retina of the eye is also divided into two halves; nasal and temporal. The Nasal is towards the nose, and the temporal is towards the ear. We know that there is a convex lens present in our eyes. From the knowledge of basic physics, we also know that the image formed by the convex lens is inverted both horizontally and vertically. The result is that the nasal half of the visual field is projected on the temporal half of the retina, and the temporal half of the visual field is projected on the nasal half of the retina. These are also upside-down projections.
This is important in binocular vision (the ability to see an object with both eyes, creating a single image). If we consider the visual fields of both the eyes as right and left visual fields. The image of an object in the right visual field is projected on the nasal half of the right retina as well as the temporal half of the left retina. In the optic chiasm, fibers from the nasal half of the right retina cross over and move toward the left side. Then along with fibers from the temporal half of the left retina, they project the whole right visual field to the right lateral geniculate body. From LGB, the whole right visual field is projected to the visual cortex of the left half of the cerebral hemisphere. In the same manner, the left visual field is projected to the right half of the cerebral hemisphere. The whole information is then analyzed and integrated to create visual imagery.
Visual reflexes mean the reflexes in response to the stimulus of light. When light strikes the retina, its intensity, point of origin, and other parameters can cause changes in the pupillary diameter, lens convexity as well as body posture. These reflexes are mediated by the superior colliculi and pretectal nuclei of the midbrain.
To change the focus of the eye from a distant object to a near object, eyes are converged medially, pupils become constricted, and the convexity of the lens increases, i.e., the lens becomes more globular. All of this is done through accommodation reflex. Light enters the eye, and afferent information travels through the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiation, and ultimately to the visual cortex. From the visual cortex, some fibers reach the frontal eye field area. From the frontal eye field, cortical fibers descend down to the motor nuclei of the oculomotor nerve. Motor nuclei of both oculomotor nerves send impulses to the medial recti muscles of both eyeballs. Contraction of medial recti brings about the convergence of eyeballs. Some other cortical fibers synapse with the parasympathetic nucleus of the oculomotor nerve (Edinger Westphal Nucleus). This sends impulses to the constrictor pupillae and ciliary muscles resulting in constriction of the pupil and an increase in the convexity of the lens, respectively.
When light is thrown into the eye, such as using a torch, the pupil of that eye constricts; this is known as the direct light reflex. The pupil of the other eye also constricts, even if no light is thrown into that eye; this is known as the indirect or consensual light reflex. The afferent pathway for light reflex is the optic nerve, optic chiasm, optic tract, and pretectal nucleus lying in the midbrain. From the pretectal nucleus, efferent fibers pass onto the Edinger Westphal nucleus of the oculomotor nerve. Edinger Westphal nucleus provides parasympathetic supply to the constrictor papillae muscle, which causes constriction of the pupil. Consensual light reflex takes place because the pretectal nucleus sends impulses to both the Edinger Westphal nuclei.
Visual Body Reflexes
Turning your body towards or away from a visual stimulus, movement of the eyeball as well as head movement while reading a book, etc., is done by the visual body reflexes. The afferent pathway is the same as the light reflex, i.e., optic nerve, optic chiasm, and optic tract. Unlike the light reflex, for visual body reflexes, the fibers from the optic tract synapse with the neurons in the superior colliculi of the midbrain. These colliculi send impulses to the anterior motor neurons of the spinal cord via the tectospinal and retrobulbar tracts of the spinal cord. These motor neurons innervate the skeletal muscles, which bring about postural changes.
Diseases of the Optic Nerve and Visual Pathway
There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the optic nerve and optic pathway.
Optic neuropathy is a generalized term that denotes damage to the optic nerve by any cause. This damage can cause impaired visual functioning of the eye as well as partial blindness to full blindness.
The causes of optic neuropathy are diverse. Optic neuritis, or inflammation of the optic nerve, is one of the main causes of optic neuropathy. Inflammation may follow some infection (i.e., cat-scratch fever or syphilis) or other inflammatory triggers (i.e., autoimmune diseases, toxins, and drugs like ethambutol). Ischemia of the optic nerve is another cause. Ischemia can be caused by occlusion of the internal cerebral artery or carotid artery. The internal cerebral artery supplies not only the cerebral hemispheres but also supplies the optic nerve. This again justifies the statement which says that the optic nerve is the direct continuation of the brain. Ischemia causes necrosis of neurons leading to the loss of function of the optic nerve resulting in blindness. The extent of blindness always depends upon the extent of damage to the optic nerve.
Other causes of optic neuropathy include; pressure due to tumors (especially tumors of the pituitary gland), demyelinating diseases of the nervous system, non-arteritic ischemic optic neuropathy (NAION), trauma to the skull, etc.
Treatment of optic neuropathy lies in treating the underlying cause. Optic neuropathy caused by direct trauma resulting in crush injury or resection of the optic nerve is almost invariably irreversible. However, infectious or ischemic neuropathy can be reversed, before some serious damage to the neurons, by treating the underlying cause. In case of infections, antibiotics antiviral can be used. In the case of a tumor suppressing the optic nerve, surgery is usually recommended to remove the tumor.
If there is total resection of the optic nerve of one side occurs along its course before optic chiasm formation, complete blindness of that eye results. This can be caused by trauma to the skull (i.e., road traffic accidents), ischemic injury, or any mishap during some surgical procedure.
Bitemporal hemianopia is a condition in which there is blindness of the temporal halves of both eyes. This condition occurs due to the sagittal sectioning of the optic chiasm. The most common etiology is the pressure exerted by tumors of the pituitary gland.
Treatment involves the removal of the tumor. If damage to the optic nerve is subtle, good recovery is possible.
Nasal hemianopia occurs due to a partial lesion of the optic chiasm located at its lateral side. This results in blindness of the nasal half of the eye of the side involved.
Contralateral Homonymous Hemianopia
Contralateral homonymous hemianopia is caused by any of the lesions of the optic tract, optic radiation, or destruction of the visual cortex. This results in the same hemianopia for both eyes. If there is a lesion of the optic tract of the right side, it results in left temporal hemianopia and right nasal hemianopia. The same occurs in the case of lesions of optic radiation or the visual cortex.
The conditions mentioned above are usually caused by ischemic injury and are mostly irreversible, resulting in permanent blindness.
The optic nerve is the second cranial nerve (CN II). It is exclusively a sensory nerve that contains special somatic afferents as its functional component. Axons of these ganglion cells of the retina converge at a point called the optic disc.
From the optic disc, these axons form a bundle called the optic nerve. It enters the anterior cranial fossa through the optic canal. Both nerves meet at a point called an optic chiasm, where fibers from the nasal halves of both eyes cross to the opposite side. From the optic chiasm, the optic tract arises, which synapses in the lateral geniculate body.
From the lateral geniculate body, optic radiation arises, which carries visual information to the visual cortex located in the occipital lobe of the forebrain. Some fibers from the optic tract leave the tract and reach the midbrain, where the synapse is in the pretectal nuclei and superior colliculi.
The visual field is divided into two halves; the nasal half and the temporal half. Fibers from the nasal half of the right eye and the temporal half of the left eye contain information about the whole right visual field, which is projected to the visual cortex of the left hemisphere and vice versa for the left visual field. This is important in binocular vision.
The optic nerve is also involved in different reflexes like light reflex (constriction of the pupil in response to light), accommodation reflex (medial convergence of eyeballs, constriction of the pupil, and more globular shape of the lens, when the visual focus is changed from a distant object to a near one.
Diseases of the optic nerve include optic neuropathy, optic neuritis, complete blindness, bitemporal hemianopia, nasal hemianopia, and contralateral homonymous hemianopia. Causes of optic neuropathy include inflammation, pressure due to tumors, ischemic injury, traumatic injury, exposure to toxins or drugs and demyelinating diseases, etc.
Treatment involves the solution of the underlying cause by using either medications or surgical interventions. The extent of blindness depends on the extent of damage to the nerve. Complete resection of nerve or crush injury usually results in irreversible damage leading to permanent blindness.
Smith, Austen M.; Czyz, Craig N. (2021). “Neuroanatomy, Cranial Nerve 2 (Optic)”. StatPearls. Treasure Island (FL): StatPearls Publishing. Retrieved 14 June 2021.
Image source: Optic Nerve