V Neuroanatomy and Neurophysiology

• Accessory Structures of the Eye:

  • Eyebrows overlie the supraorbital margins and shade the eye
  • Eyebrows protect the eyes from perspiration
  • Eyelids (Palpebrae) separated by the palpebral fissure
  • Lacrimal caruncle contains sebaceous & sweat glands
  • Eyelash follicles are richly innervated and trigger reflex blinking
  • Tarsal glands lubricate eyelid and eye with an oily secretion
  • Conjunctiva is a transparent mucous membrane that lines the eyelids and folds back over the eye as the bulbar conjunctiva
  • Conjunctiva produces lubricating mucus to prevent drying of eyes
• Lacrimal Apparatus:

  • Lacrimal fluid contains mucus, antibodies, and lysozyme
  • Lots of tears go through this duct causing a runny nose
• Structure of the Eye:

  • The eyeball is a fluid-filled sphere composed of three layers: fibrous, vascular, and inner (retina)
  • Lens divides the eye into anterior and posterior segments
  • Sclera protects and shapes the eyeball; anchoring site for extrinsic eye muscles
  • Cornea is transparent and allows light entry and is important in light refraction
  • Fibrous layer is dense connective tissue that is avascular
  • Vascular Layer includes the Choroid, Ciliary Body, and Iris
  • Inner Layer (Retina) contains millions of photoreceptors, rods, and cones for vision
• Retina:

  • The retina has two layers: outer pigmented layer and inner neural layer
  • Photoreceptors are rods and cones
  • Rods are for dim light & peripheral vision, more numerous, and don't give sharp images
  • Cones are for bright light, high resolution, and color vision
  • Macula lutea is an area with a high concentration of cones for visual acuity
  • Fovea centralis is a central pit in the macula with only cones for sharp vision
• Aqueous and Vitreous Humor:

  • Aqueous humor supplies nutrients and oxygen to the lens and cornea
  • Vitreous humor forms in the embryo and lasts a lifetime, found in the posterior segment
• Light Refraction in the Eye:

  • Light is refracted three times: cornea, entry to lens, leaving the lens
  • Refraction effect of the cornea is constant but that of the lens can be adjusted for distant versus close vision
  • Our eyes are best adapted for distant vision
  • Normal (emmetropic eye) vision far point is 6m
• Near Vision:

  • Accommodation of lens allows the image to be focused on the retina for near vision
  • Ciliary muscles contract (PNS) and lens bulges for close vision
• Three Events of Near Vision:
• Accommodation of lens:

  • Allows the image to be focused on the retina
  • Near point of vision is 10 cm from the eye, closer for children and farther as we age
  • Presbyopia: after age 50, lens cannot accommodate, need reading glasses
• Constriction of pupils:

  • PNS-mediated
  • Involves contraction of sphincter pupillae muscles of iris to prevent blurry vision
• Convergence of eyeballs:

  • Keeps object focused on retinal foveae
  • Involves medial rectus muscle and oculomotor cranial nerve
• Most refractive problems are related to eyeball shape
• Myopia (nearsightedness):

  • Eyeball is long, distant objects focus in front of retina
  • Can focus close objects on the retina
  • Hyperopia (farsightedness):
  • Eyeball is too short, distant objects focus behind the retina
  • Ciliary muscles contract to move focus to the retina for distance, but unable to bring close objects onto the retina
• Correcting Problems with Refraction:
• Concave lenses diverge the light
• Convex lenses converge the light
• Photoreceptors:
• Receptive region embedded in pigmented layer
• Cilium connects outer segment to inner segment
• Rods and cones are vulnerable to damage
• Visual pigments (rhodopsins) change shape as they absorb light
• Rods and cones renew outer segments every 24 hours with new discs
• Light and Dark Adaptation:
• Coming from dark into bright light:

  • Initially, both rods and cones are stimulated
  • Rhodopsin in rods bleaches quickly, rendering rods non-functional
  • Cones gradually take over
• Going from brightness into dark:

  • Initially looks dark because cones are no longer stimulated
  • Rhodopsin starts to accumulate in rods
  • Reflexive changes in pupil diameter adjust light reaching the retina
• Key Points regarding Visual Pathways:
• Axons of retinal ganglion cells form the optic nerve
• Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus
• Optic radiations travel from the thalamus to the primary visual cortex
• From the Retina to the Visual Cortex:
• Medial half of each retina receives light rays from the lateral field of view
• Lateral half of each retina receives light from the central part of the visual field
• Optic tracts contain fibers from the same side and opposite eye, carrying information for the same half of the visual field
• Depth Perception:
• Visual field of each eye is about 170 degrees
• Primary visual cortex fuses images from both eyes for depth perception
• Depth perception is lost if looking with only one eye