Pathophysiology/Physiology

• What would an issue with the ventral pathway for vision lead to?

  Object recognition difficulties
• What do ganglion cells act as?

  Output cells

  -> their axons form the optic nerve
• What is a receptor potential?

  Electrical signals generated in response to a stimulus by a sensory receptor

  -> when activated, produces receptor potential that causes a change in cell membrane potential
• What do amacrine & horizontal cells do?

  Enhance contrast & adjust to different levels of light
• What is location coding?

  Encoded by the spinal pattern of neural activity -> allows brain to determine the location of stimulus
• What do bipolar cells do?

  Acts as a 'middle man'

  -> passes info from the PR cell to the ganglion cell
• What is type coding?

  Different types of sensory stimuli are detected by different types of sensory receptor cells
• What is intensity coding?

  Frequency & amplitude of neural activity -> higher intensity stimuli = more frequent & stronger neural responses 
• What would an issue with the dorsal pathway for vision lead to?

  Spatial awareness deficits
• What is duration coding?

  Length of time that neural activity persists
• What is the tectospinal tract for?

  Mediating movements in response to visual stimuli
• What protein is found in cones?

  Opsin
• What protein is found in rods?

  Rhodopsin
• Where are rods & cones found?

  In photoreceptor cells
• What is the cornea?

  Clear, transparent, avascular structure

  Provides 78% of focusing power of the eye
• What are the 5 layers of the cornea?

  Epithelium

  Bowman's layer

  Stroma

  Descemet's membrane

  Endothelium
• What are the endothelial cells of the cornea responsible for?

  Maintaining clarity of cornea by continuously pumping fluid out of the tissue
• What is the sclera?

  Opaque white structure

  Covers 4/5 of the globe, continuous with cornea

  Extraocular muscles are attached to sclera

  Optic nerve perforates sclera posteriorly
• What is the conjunctiva?

  Richly vascularized & innervated mucous membrane

  Covers anterior surface of sclera
• What is the anterior chamber?

  Space between cornea & lens

  Filled with aqueous humour → produced by ciliary body, provides nutrients & O2 to avascular cornea

  Any factor that impedes aqueous humour → raises intra-ocular pressure
• What is the uveal tract?

  Iris

  Ciliary body

  Choroid
• What is the iris?

  Coloured part of eye

  Under transparent cornea

  Muscles of iris regulate size of pupil
• What is the ciliary body?

  Muscles control the accommodation of the lens

  Secretory epithelium produces aqueous humour
• What is the choroid?

  Highly vascular

  Inner aspect of sclera
• What is the lens?

  Responsible for 22% of refractive power of eye

  Changes shape of lens → alters refractive power → allows to focus on nearby objects

  By 40s, ability to change shape of lens declines
• What is the vitreous humour?

  Clear gel-like substance in the eye

  Fills cavity between retina & lens
• What is the retina?

  The retina is the light-sensitive layer of tissue at the back of the eye that contains photoreceptor cells

  Macula -> centre (where cones are most concentrated)
• What is the blood supply to the eye?

  Ophthalmic artery

  Central retinal artery -> supplies inner retitnal layers
• What is the venous drainage of the eye?

  Central retinal & ophthalmic veins
• What is the lympathic drainage of the eye?

  Pre-auricular & submental nodes
• What provides sensory innervation to the eye?

  Trigeminal
• What provides motor innervation to the eye?

  Oculomotor nerve

  Trochlear nerve

  Abducens nerve
• Pupuil dilation

  Sympathetic NS -> stimulates contraction of radial muscles (dilator pupillae) Begins in hypothalamus & descends through midbrain -> synapses in spinal cord (C8-T2) -> preganglionic sympathetic neuron exits spinal cord (through ventral roots) -> projects to superior cervical ganglion & synapses -> long ciliary nerve projects to dilator pupillae (radial muscle) -> pupil dilation
• Pupil constriction

  Parasympathetic NS -> stimulates contraction of circular muscles (sphincter pupillae) Light enters eye -> stimulates retina -> signals travel via optic nerve & synapse with the pre-tectal nucleus -> synapses with Edinger-Westphal (oculomotor) nucleus -> gives rise to bilateral oculomotor outputs to ciliary ganglion -> short ciliary nerve innervates the sphincter pupillae (circular muscle) -> pupil constriction
• Accommodation reflex - what is it? what does it involve?

  Visual response to focusing on nearby objects

  Involves;

  • contraction of medial recti muscles -> convergence
  • constriction of pupils (parasympathetic NS)
  • contractions of ciliary body -> broadens lens & increases refractive power
• What is the role of the optic nerve?

  Carries afferent fibres from retina to visual cortices of the brain -> transmits visual image

  Surrounded by cranial meninges
• Where does the optic nerve leave the skull?

  Optic canal
• What is the function of oculomotor nerve?

  Movement of most extraocular muscles (except LR & SO) + levator palpebrae superiosis

  Parasympathetic division responsible for pupillary constriction -> pupillary reflex
• Where does the oculomotor nerve exit the skull?

  Superior orbital fissure
• What is the function of the trochlear nerve?

  Innervates superior oblique -> assists in depressing & abducting eye