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Equine Special Senses - Horse Anatomy (view source)
Revision as of 17:05, 22 November 2012
, 17:05, 22 November 2012→Central Visual Pathways
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.
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.
==Central Visual Pathways==
====Central Visual Pathways====
[[Image:Central Visual Pathway.jpg|thumb|right|200px|Central Visual Pathway - Copyright David Bainbridge]]
[[Image:Central Visual Pathway.jpg|thumb|right|200px|Central Visual Pathway - Copyright David Bainbridge]]
The '''optic nerve''' ([[Equine Cranial Nerves - Horse Anatomy#Optic Nerve (II)|CN II]]) is a paired nerve that carries visual information from the retina to the brain. The ganglion cell axons leave the '''retina''' and information passes through the optic nerve to the '''optic chiasm''', where 85-88% of fibres cross over (decussate). The optic tract (the optic nerve fibres) wraps around the cerebral peduncles of the [[Equine Brain - Horse Anatomy#Midbrain|midbrain]], where it passes into the '''lateral geniculate nucleus'''; which is part of the [[Equine Brain - Horse Anatomy#Forebrain|thalamus]]. Most of the optic tract axons synapse here, and the remaining fibres branch off and synapse in the '''pretectal nuclei''' of the '''superior colliculi'''. The integrated visual information is then passed via nerve fibres to the [[Equine Brain - Horse Anatomy#Forebrain|cerebral cortex]].
The '''optic nerve''' ([[Equine Cranial Nerves - Horse Anatomy#Optic Nerve (II)|CN II]]) is a paired nerve that carries visual information from the retina to the brain. The ganglion cell axons leave the '''retina''' and information passes through the optic nerve to the '''optic chiasm''', where 85-88% of fibres cross over (decussate). The optic tract (the optic nerve fibres) wraps around the cerebral peduncles of the [[Equine Brain - Horse Anatomy#Midbrain|midbrain]], where it passes into the '''lateral geniculate nucleus'''; which is part of the [[Equine Brain - Horse Anatomy#Forebrain|thalamus]]. Most of the optic tract axons synapse here, and the remaining fibres branch off and synapse in the '''pretectal nuclei''' of the '''superior colliculi'''. The integrated visual information is then passed via nerve fibres to the [[Equine Brain - Horse Anatomy#Forebrain|cerebral cortex]].
===Autonomic Innervation of the Eye===
====Autonomic Innervation of the Eye====
'''Parasympathetic innervation''' to the eye is supplied by the oculomotor nerve ([[Equine Cranial Nerves - Horse Anatomy#Oculomotor Nerve (III)|CN III]]). When parasympathetic innervation is predominant, it acts upon the circular muscles of the iris, causing '''constriction of the pupil'''.
'''Parasympathetic innervation''' to the eye is supplied by the oculomotor nerve ([[Equine Cranial Nerves - Horse Anatomy#Oculomotor Nerve (III)|CN III]]). When parasympathetic innervation is predominant, it acts upon the circular muscles of the iris, causing '''constriction of the pupil'''.
'''Sympathetic innervation''' to the eye is via the '''cranial cervical ganglion'''. When sympathetic innervation is predominant, it acts upon the radial muscles of the iris, causing '''dilation of the pupil'''.
'''Sympathetic innervation''' to the eye is via the '''cranial cervical ganglion'''. When sympathetic innervation is predominant, it acts upon the radial muscles of the iris, causing '''dilation of the pupil'''.
====Reflexes with Optic Nerve as the Sensory Arm====
1. '''Pupillary light reflex''': the constriction of a pupil in response to increased light intensity, and a dilatation of a pupil in response to a decreased light intensity. This reflex also involves parasympathetic fibres of the Oculomotor nerve ([[Equine Cranial Nerves - Horse Anatomy#Oculomotor Nerve (III)|CN III]]). The presence of this reflex shows the efficiency of the retina, the optic and oculomotor nerves, and the musculature of the iris.
2. '''Pupil dilation''': also called mydriasis. It is a sympathetic response.
3. '''Menace response''': involves the facial nerve ([[Equine Cranial Nerves - Horse Anatomy#Facial Nerve (VII)|CN VII]]) in the motor arm of the reflex. The response also requires integration from the [[Equine Brain - Horse Anatomy#Forebrain|cerebral cortex]], the [[Equine Brain - Horse Anatomy#Hindbrain|cerebellum]] and the rostral colliculi. However, the menace response is a learned response, so it will not be present in the first few weeks of life.
4. '''Fixating response''': involves oculomotor ([[Equine Cranial Nerves - Horse Anatomy#Oculomotor Nerve (III)|CN III]]), trochlear ([[Equine Cranial Nerves - Horse Anatomy#Trochlear Nerve (IV)|CN IV]]) and abducens ([[Equine Cranial Nerves - Horse Anatomy#Abducens Nerve (VI)|CN VI]]) nerves in the motor arm of the reflex.
==Ear==
==Ear==