Special Sense: Vision

We have previously discussed about the special senses; Taste, Hearing and Olfaction. In this blog we shall discuss the special sense Vision.

Vision:

The special organs dedicated for perceiving the vision are the Eyes. Each individual has a pair of eye, which are located within the protective bony cavity of the skull called orbit. It is externally protected with eyelids, eyelash, lacrimal gland, etc.

• Internal structure of eye:

Internally eye is an organized structure made up of different parts, namely;

– Cornea:

The cornea is the outermost layer (fig 1) of the eye. It is the transparent and dome­shaped layer covering the eyes from the front. The cornea transmits and focuses light into the eye.

(Just for info: Read more about cornea)

– Iris:

This is the colored part (fig 1). It helps in regulation of the amount of light that enters the eye, like diaphragm of camera. The iris opens or closes to allow the entry appropriate amount of the light onto the retina.

(Just for info: Read about the artificial iris)

– Pupil:

Pupil is the hole in the centre of the iris (fig 1). It appears as a dark center in the middle of the iris. The pupil determines how much light is let into the eye by changing its size, and works similar to the aperture of the camera. In bright light, the iris closes, constricting the pupil, so that less light enters inside. Similarly in low light conditions the iris opens up, dilating the pupil and allowing more light to enter.

– Lens:

Lens in the eyes is a biconvex lens (fig 1). It is a transparent structure. The function of the lens is to focus light onto the retina. The suspensory ligament attaches the lens to the ciliary muscles (fig 1). This ligament help change the shape of the lens and hence focus the light on the retina at different distance. For close vision, the suspensory ligaments contracts making the lens rounder, while during far vision the suspensory ligaments relaxes and flattens the lens.

– Aqueous humor:
It is the fluid present in the area between the lens and the cornea. This provides nourishment to both lens and cornea and maintains the intraocular (within eye) pressure.

– Vitreous Humor:
This is a transparent semi-solid, jelly-like substance that fills the interior of the eyes (fig 1). It helps maintains the shape of the eye and also refracts light before it reaches the retina.

– Sclera:
This is dense, white visible portion of the eye (fig 1). This covers the entire eye structure except at the opening at posterior through which the optic nerve and retinal vessels enter into the eye. Anteriorly it is continuous with the cornea. It forms a fibrous protective outer covering of the eye.

– Uvea:
The uvea is the middle layer of the eye located beneath the sclera. It is collectively made up of the iris, ciliary body, and choroid (red middle layer in fig 1). This layer helps in various eye functions, including adaptation to different intensity of light and distance of the object.

– Retina:

This is the actual light sensitive layer of the eye (yellow layer in fig 1). The retina of all vertebrates can be broadly divided into three layers of cells: made up of photoreceptors, bipolar interneuron cells and ganglion cells (fig 2).

From the point of entry of light, the layer made up ganglion appears first, then the bipolar neurons and finally the photoreceptors. That is the light passes through the ganglion and neuron layer and then interacts with the photoreceptors.

1. Photoreceptors

Photoreceptors have a basic cellular structure which consists of a outer segment, narrow stalk, an inner segment, nucleus and finally the synaptic terminal.

~Outer segment:
This portion is packed with a number of flattened discs piled one over the other. The light capturing receptors i.e. the photoreceptor molecules are present on the membranes of these discs.

~Narrow stalk:

Narrow stalk connects the inner and outer segment.

~Inner segment:

This portion is rich in mitochondria and contain vesicles filled with neurotransmitter.

~Nucleus lies below the inner segment

~Axon is located at the end of the cell which terminates with the Synaptic region.

Now, the photoreceptors are of two types which vary slightly in their structural features; Rods and Cones.

Rods:

These are longer and more slender than the cones (fig 3) and enables visualisation of black, white and shades of grey. These cells help visualisation in the dim light. The photopigment in these cells are the rhodopsin, made up of opsin protein, scotopsin and a molecule of chromophore, cis-retinal. The absorption maximum of rhodopsin is at around 500 nm.

(Just for info: Know more about opsin)

Cones:

These are shorter and more tapered than the rods (fig 3). There are three different types of cones in humans which help perceive three different colours red, green and blue. Cones work in bright light. Photopigments present in the cones are known as photopsins which varies from rhodopsin by few amino acids, due to which the absorption maximum changes. Photopsin is made up of opsin moiety, iodopsin and cis-retinal.

In humans, there are three kinds of cones having the cone photopsins I, II and III
with different absorption maxima at 455 nm (blue-absorbing), 530 nm (green-absorbing)
and 625 nm (red-absorbing). These cones are known as blue, green, and red cone.

2. Bipolar interneuron cells:
These cells form the middle layer of the retina (fig 2). They receive signal from the photoreceptors and transmit it to the ganglion cells. This layer also have two other types of cells; Horizontal interneurons and
Amacrine cells.

Horizontal interneurons:

These cells are interneurons and are mainly inhibitory. They modulate signaling between photoreceptors and bipolar cells.

(Just for info: See how horizontal cells challenge many long-standing assumptions in neural development and cancer biology)

Amacrine cells:

These are also inhibitory interneurons. They modulate signalling between bipolar cells and the ganglions.

(Just for info: Read more about the intriguing amacrine cells)

3. Ganglion cells:

These neurons (see fig 2) receive information from photoreceptors (via bipolar cells and amacrine cells). Their long axons form optic nerve, optic chiasm and optic tract. They send information to the brain.

• Physiology of Vision:

In the dark, the ion channels (Sodium and Calcium) are open and the photoreceptor cells are depolarised. On depolarisation, the photoreceptor cells release neurotransmitter glutamate, which inhibits the bipolar neurons and no information is sent to the brain. In the presence of the light, they get hyperpolarized and the release of glutamate stops allowing the bipolar neurons to send the information to the rain. Rod photoreceptors are able to respond to a single photon, whereas cones require higher amount of light to lower the amount of glutamate released.

The protein moiety of rhodopsin and photopsin, i.e. the opsins are G-protein-coupled receptors (GPCRs). The chromophore, 11-cis retinal, absorbs light energy and is converted to all-trans isomer, all-trans retinal. This brings about
conformation changes in the opsin (protein) part, which in turn stimulates the G protein component, transducin (Gt). When stimulated, the α subunit of Gt dissociates (fig 4).

(Just for info: see how Transducin and the cGMP Phosphodiesterase amplify the input from rhodopsin molecule in more details)

The dissociated α subunit (GTP bound) activates the enzyme cGMP phosphodiesterase (PDE) which hydrolyzes cGMP to GMP.

As the concentration of cGMP decreases the cGMP-dependent ion channels, for cations sodium and calcium, close (fig 4).
As the extracellular cations (positive ions) stop entering the cell, the interior of the cell becomes more negative. Due to the negative charges within, the photoreceptor cell hyperpolarizes and discontinues the release of inhibitory neurotransmitters.
The (disinhibited) bipolar neurons then pass the information to the ganglion neurons and finally to the brain.

• Nerve pathway and the Visual fields:

The lens in the human eye is a convex lens.
Visual images are inverted as they pass through the lens (as in fig 5).

We can consider each retina screen divided into two halves, left and right. The side towards the nose is known as the nasal or medial side (shown in red in fig 6) and the one towards the temple is known as the temporal or lateral side (shown in blue in fig 6).

As the inverted image are formed, the left halves of the retina of both the eyes see overlapping images. Similarly both the right halves of retina of both the eyes also see overlapping images (see fig 7).

In other words, the temporal half (T in fig 7) of left eye see similar image as the nasal half (N in the fig 7) of the right eye and vice versa.

After the image formation, the nerves travel from the orbit area via the optic canal (fig 8).

The optic nerves from the nasal half of each retina cross over each other, at the optic chiasma (fig 7) and then travel in form the optic tracts as,
Left optic tract – contains fibres from the left temporal retina, and the right nasal retina.
Right optic tract – contains fibres from the right temporal retina, and the left nasal retina (fig 9).

Hence the overlapping information of images of each visual field is sent to the same side of the brain (information of blue object is all sent to the left of the brain in image 9, similarly information of green object is sent to right side of brain).

Each optic tract then reach Lateral Geniculate Nucleus (LGN) in the thalamus which act as a relay centre. Axons from the LGN neurons then carry visual information in form of the optic radiation to the visual cortex (fig 9).

Once the information reaches the visual cortex, data is perceived and the image is visualised.

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Image Source:

Fig1: PDQ Pediatric Treatment Editorial Board. https://www.ncbi.nlm.nih.gov/books/NBK65835.11/

Fig 2: Modified from MacGillivray et al (2014). Retinal Imaging as a Source of Biomarkers for Diagnosis, Characterisation and Prognosis of Chronic Illness or Long-Term Conditions. The British journal of radiology. 87.

Fig 3: Bibliowicz et al (2011) Chapter 7 – Toward a Better Understanding of Human Eye Disease: Insights From the Zebrafish, Danio rerio. Progress in Molecular Biology and Translational Science. 100.

fig 4: Kassai and Fukada (2011) 7 – Farnesylation Versus Geranylgeranylation in G-Protein-Mediated Light Signaling. The Enzymes. 29.

fig 5: Modified from Buschbeck & Friedrich (2008). Evolution of Insect Eyes: Tales of Ancient Heritage, Deconstruction, Reconstruction, Remodeling, and Recycling. Evolution: Education and Outreach. 1.

Fig 6: Modified from Armstrong and Cubbidge (2014) Chapter 1 – The Eye and Vision: An Overview. Handbook of Nutrition, Diet and the Eye.

Fig 7: Williams (2018) Chapter 7 – Albinism and the Eye. Albinism in Africa, Historical, Geographic, Medical, Genetic, and Psychosocial Aspects.

Fig 8:Bazroon and Singh. (2019) Anatomy, Head and Neck, Foramen Lacerum. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.

Fig 9: Raven et al (2002). Biology (6th ed). McGraw-Hill, Boston