Special senses

Vision

The eye is made of three layers - the outer, thick and fibrous white sclera, conjunctiva and clear cornea; the middle pigmented and vascular uvea comprised of iris, ciliary body and choroid, and the inner retina. The cornea is anterior to the iris, ciliary body and lens. The eye has three chambers - anterior, posterior and vitreous. The anterior chamber lies between the cornea and iris. The posterior chamber lies between the iris and lens. The vitreous chamber is between the lens and the retina. The anterior and posterior chambers are filled with clear aqueous humor while the vitreous chamber is filled with gelatinous vitreous humor. Aqueous humor is continually being produced by the ciliary body. It then flows through the anterior and posterior chambers to provide nourishment to the cornea, lens etc. It is drained through the trabecular meshwork into the canal of Schlemm and into the systemic circulation.

Light enters the eye through the transparent cornea and lens. It gets refracted and falls on the retina where it stimulates photoreceptors in the rods and cones ultimately giving rise to action potentials in the optic nerve. The optic nerve and visual pathway carry this information to the cerebral cortex resulting in the ability to see, appreciate color and movement.

The iris has two important muscles called sphincter pupillae and dilator pupillae. These two muscles along with their autonomic innervation control the amount of light entering the eye by changing the diameter of the pupillary aperture. The dilator pupillae is under sympathetic control, innervated by T1 spinal segment through the superior cervical ganglion. The sphincter pupillae is under parasympathetic control from the Edinger Westphal nucleus of the oculomotor nerve relayed through the ciliary ganglion.

The lens is suspended by the suspensory ligaments attached to the ciliary body which contains the ciliary muscle. When the ciliary muscle contracts the suspensory ligaments are lax allowing the lens to increase its curvature as seen in accommodation. It is under parasympathetic control and innervated by the oculomotor nerve.

The rods and cones are the photoreceptors of the eye. They have an outer segment for signal transduction and inner segment containing cell organelles. Rods outnumber the cones. Rods have high light sensitivity but low spatial resolution. Their role is scotopic or night vision. Cones are less sensitive to light and their role is daytime or photopic vision, color vision and visual acuity. L cones respond to red wavelengths, M cones respond to green and S cones to blue wavelengths. In moonlight or starlight both rods and cones are stimulated, called mesopic vision.

On fundoscopy, in the center of the retina, the optic nerve can be seen as a circular to oval white area. This area is called the optic disc or optic nerve head. Blood vessels can be seen radiating out of the optic nerve. Rods and cones are absent on the optic disc, hence it is the blind spot of the eye. An oval, blood vessel free reddish spot can be seen a little to the left of the optic nerve, called the fovea. A depression in the center of the fovea is called the foveal pit and it has only cones in a densely packed pattern hence it is the area of highest visual acuity. The yellow tinged ,circular macula lutea surrounds the fovea. Carotenoid pigments like lutein give the macula its yellow color. It protects the delicate fovea from harmful ultraviolet radiation. The part of the retina around the fovea is called the central retina while the peripheral retina is away from it. Central retina has a higher number of cones while the peripheral retina has more rods. In retinitis pigmentosa, the rods degenerate first causing tunnel vision. In macular degeneration, the retinal pigment epithelium degenerates, forming drusen and affecting the macula and fovea leading to death of cones with a detrimental impact on visual acuity.

Fundus photograph showing the blood vessels in a normal human retina. Veins are darker and slightly wider than corresponding arteries. The optic disc is at right, and the macula lutea is near the centre.

The retina is a multilayered structure made of pigment cell, photoreceptor, outer nuclear, outer plexiform, inner nuclear, inner plexiform, ganglion cell and optic nerve layers. Axons of the retinal ganglion cells form the optic nerve.

The outer segments of the photoreceptors contain photopigments. These pigments are made of 11 cis-retinal derived from vitamin A which is bound to an opsin protein which is rhodopsin in rods and iodopsin in cones. When photons of light strike the outer membrane it causes the photoisomerisation of 11-cis retinal to all-trans retinal. This activates a G protein called transducin which activates a phosphodiesterase converting cGMP to 5’ GMP. Reduced levels of cGMP close Na channels so that inward flux of sodium is blocked, leading to hyperpolarization of the cell membrane. The opposite is seen in dark conditions i.e. increased cGMP, opening of Na channels and depolarization. These changes influence the secretion of excitatory and inhibitory neurotransmitters. Rods and cones synapse with bipolar cells which constitute the first order neurons in the visual pathway. There are ON and OFF bipolar cells which are depolarized and hyperpolarized in response to light respectively. Bipolar cells connect to ganglion cells which form the second order neurons in the visual pathway. P type ganglion cells or Midget cells are specialized for color vision and spatial resolution. M type ganglion cells or Parasol cells specialize in movement detection. The axons of ganglion cells converge at the optic disc to form the optic nerve. The optic nerve is developmentally and functionally a part of the CNS. Its sheath is laid down by oligodendrocytes. A special type of retinal ganglion cell expressing melanopsin are photosensitive cells that do not contribute to image-processing or vision but are involved in circadian rhythms.

The retina is tentatively divided into a medial or nasal half and a lateral or temporal half. The nasal half of the retina receives visual input from the lateral or temporal visual field while the temporal retina receives input from the nasal visual fields. Each optic nerve carries inputs from the respective nasal and temporal visual fields. The optic nerves meet above the pituitary gland to form the optic chiasma. Crossing over of a few nerve fibres happens at the optic chiasma. The fibres arising from each nasal retina, which is the same as fibres carrying inputs from the temporal visual fields, will cross over to the opposite side in the optic chiasma. This gives rise to the optic tract.

The optic tract on each side will terminate in the corresponding lateral geniculate nucleus (LGN) of the thalamus, which is the third order neuron of the visual pathway. The visual radiation arises from the LGN and projects to the primary visual cortex, Brodmann area 17, in the occipital lobe. The cuneus gyrus is located superior to , and the lingual gyrus is located inferior to the calcarine sulcus. The cuneus gyrus receives lower quadrants of the visual hemifields. The lingual gyrus receives superior quadrants of the visual hemifields. The inferior part of the visual radiation forms the Meyer’s loop which travels through the temporal lobe before terminating in the lingual gyrus.

In the primary visual cortex, the peripheral retina is represented inwards and the central retina including the fovea and macula are represented outwards.

*Heteronymous means both right and left. Homonymous means only right or only left.

** Initially these lesions may manifest as quadrantanopia i.e. only a quadrant of the visual field will be involved before involving the hemifield.

*** The Macular cortex has dual blood supply from PCA and MCA.

Pupillary light reflex: When light is directed at one eye, the pupil of that eye constricts due to a direct light reflex. At the same time, the pupil of the opposite eye constricts due to consensual or indirect light reflex. The sensory limb of the light reflex is the optic nerve. Nerve fibres related to the light reflex travel all the way to the pretectal nuclei which project bilaterally to both Edinger Westphal nuclei in the midbrain. It is the parasympathetic nucleus of the oculomotor or III cranial nerve. The Edinger Westphal nuclei innervate the parasympathetic ciliary ganglion through the oculomotor nerve. Postganglionic parasympathetic fibres from the ciliary ganglion travel in the short ciliary nerves to innervate the constrictor pupillae muscle leading to constriction of the pupils.

Accommodation reflex: When we focus on a near object, three things happen simultaneously - both eyes converge, the lens becomes more convex and the pupil constricts. The afferent limb consists of the optic nerve. The motor nucleus of the oculomotor nerve and Edinger Westphal nucleus then innervate the medial rectus muscle and sphincter pupillae plus ciliary muscle respectively, causing them to contract. Efferents are carried in the oculomotor nerve.