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Equine Special Senses - Horse Anatomy (view source)
Revision as of 16:52, 22 November 2012
, 16:52, 22 November 2012→Vision
===Vision===
===Vision===
[[Image:Aspinall Slide6.JPG|thumb|right|350px|<small>Image from [http://www.elsevierhealth.co.uk/veterinary-nursing/spe-60136/ Aspinall, The Complete Textbook of Veterinary Nursing], Elsevier Health Sciences, ''All rights reserved''</small>]]
[[Image:Aspinall Slide6.JPG|thumb|right|350px|<small>Image from [http://www.elsevierhealth.co.uk/veterinary-nursing/spe-60136/ Aspinall, The Complete Textbook of Veterinary Nursing], Elsevier Health Sciences, ''All rights reserved''</small>]]
====Accommodation====
Accommodation is primarily achieved by the '''lens''', which is stretched by rectus muscle fibres of the ciliary body, causing the optical power of the eye to increase. This allows the eye to maintain a clear focus on an object, especially as the animal moves nearer towards the object.
===Visual Pigments===
'''Rods and cones''' are the photosensitive receptor cells of the retina, that is the light-sensitive parts of the retina.
Rod cells are mainly used at night (scotopic vision), as they function at lower light intensity. Rods are so-named due to their cylindrical shape. They are positioned in higher numbers at the edges of the retina, so are also used in peripheral vision. Rod cells are a lot more sensitive to light than cone cells, meaning that they require less light to function than cones, this being the reason that they are the predominant cells providing visual information at night. However, many rod cells converge into one interneuron, meaning that signals are amplified, but visual acuity is not as great, so the visual information that is collected may be less distinct.
Rod cells contain a pigment called '''rhodopsin''', which itself consists of a protein called '''opsin'''. Bound to opsin is a molecule called '''retinene''' - a derivative of '''vitamin A'''. Retinene exists in the form of '''cis-retinene''', an aldehyde, when in the dark. When stimulated by light, the cis-retinene undergoes structural change to form '''trans-retinene''':
'''Vitamin A (alcohol) → cis-retinene (aldehyde) → trans-retinene'''
This leads to hyperpolarisation of the cell.
When rods and cones are not being activated, they are in a state of depolarisation, and release a neurotransmitter – '''rods''' cells release '''glutamate''', '''cone cells''' release '''acetylcholine'''. This state of depolarisation is achieved in darkness, because the cells have a high concentration of cGMP, which causes the opening of ion channels. Sodium and calcium ions pass through these ion channels, and its their positive charges that cause the depolarisation of the cell, which leads to the release of neurotransmitters. When light hits the rod and cone cells, the cis-retinene undergoes structural change to become trans-retinene. This structural change causes the activation of '''transducin''', which is a regulatory protein. Transducin activates cGMP phosphodiesterase, which causes the break-down of cGMP into GMP. This action allows the sodium channels to close, thereby preventing influx of positive ions, which leads to hyperpolarisation of the cell, and therefore also preventing the release of neurotransmitters.The breakdown of one molecule of rhodopsin leads to signal amplification, so more molecules of rhodopsin are activated, hence rod cells being highly sensitive to small amounts of light. However, when rod cells are exposed to high levels of light intensity for prolonged periods of time, they become desensitised.
Cone cells function better in higher intensities of light (photopic vision). Most cone cells are found concentrated in the '''fovea'''. The fovea is the region of the retina where the retinal layers are parted, allowing light to fall directly onto the cone cells. Cone cells provide greater visual acuity, as each cone synapses with a single interneuron, meaning that the visual signal is not amplified, therefore is more distinct. Cone cells respond to light in the same way as rod cells. The only difference is that the pigment present in cone cells is '''iodopsin''', as opposed to rhodopsin. Retinene that is bound to opsin within the iodopsin is stimulated in the presence of light to undergo structural change to form trans-retinene.
==Ear==
==Ear==