Changes

====Optic Nerve (II)====

====Optic Nerve (II)====

The optic nerve represents the connection between the receptor cells of the [[Eye - Anatomy & Physiology#The Wall (retina, uvea and sclera)|retina]] and the [[Forebrain - Anatomy & Physiology|forebrain]]. The optic nerve is a sensory nerve and is composed of '''Special Somatic Afferent fibres'''. The optic nerve passes through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic chiasm''']] which is an area of the ventral brain where both optic nerves run in a medial direction and eventually decussate. In the horse, approximately 85-88% of fibres decussate. The optic nerve runs through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic canal''']]. Fibres then go to geniculate nucleus then impulses travel via the optic radiations to occipital lobe for visual processing. The ganglion cells also project to pretectal nuclei for modulation of parasympathetic tone to the iris and rostral colliculi for the startle response.

The optic nerve represents the connection between the receptor cells of the [[Eye - Anatomy & Physiology#The Wall (retina, uvea and sclera)|retina]] and the [[Forebrain - Anatomy & Physiology|forebrain]]. It is not a true nerve, but represents an extension of the brain.  The optic nerve is sesory, and is composed of '''Special Somatic Afferent fibres'''.  

 

*The second order neuron is the ganglion cells of the retina and axons within the optic nerve.  The optic nerve passes through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic chiasm''']], which is an area of the ventral brain where both optic nerves run in a medial direction and eventually decussate (cross). In the horse, approximately 85-88% of fibres decussate. The optic nerve then runs through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic canal''']].  

*The third order neuron has its cell body in the lateral geniculate nucleus in the diencephalon. Its axon projects to the visual cortex, which is mostly the contralateral occipital cortex, in the '''optic radiation'''. The occipital lobe is where visual processing takes place at a conscious level.

 

The nerve is also involved in modulation of '''parasympathetic tone to the iris'''. The first and second order neuron pathways are the same as those responsible for vision, however after synapsing with the lateral geniculate nucleus axons involved in modulation of parasympathetic tone synapse with a third order neuron in the '''pretectal nucleus'''.  Most axons from the pretectal nucleus then decussate back to synapse in the parasympathetic component of the '''Occulomotor nerve (III)''' in the ipsilateral eye (because it has crossed once at the optic chiasm and then again at the pretectal nucleus).

====Oculomotor nerve (III)====

====Oculomotor nerve (III)====