Visual system Flashcards
What is the anatomy of the eyeball?
- Optic disc : exit point for the optic nerve
- Central retinal artery : supplies the retine from the opthalamic artery (internal carotid artery)
- Macula lutea : contains a high concentration of cone cells
- Fovea : densest concentration of cone cells at centre of macula - high acuity vision
What is the retina?
- the innermost layer of the eyeball
- The retina contains photoreceptor cells which are sensitive to light waves
- Cones - responsible for high acuity, daylight and colour vision whereas rods are specialised to detect dim light and night vision (but not colour)
- Generates action potentials along the optic nerve in response to light waves (photons)
What are the cells of the retina?
- Pigmented epithelium : light absorping cells next to the choroid
- Neural retina : photoreceptors (rods and cones) and glial cellls
- Bipolar cells (interneurones)
- Ganglion cells : axons form the optic nerve
- Light has to physically travel through the ganglion cell and bipolar cell layers to reach the photoreceptors
How is an image formed on the retina?
- Light refracted by the cornea and lens
- Passes through the aqueous an and virteous humours
- Image is projected onto the retina upside down (inverted) and reversed
- The optic nerve exits the orbit at the optic canal
- Visual cortex of occipital lobe processes the image and corrects it, combining info from both eyes to form a single image (binocular vision)
What is the optic chiasm?
Optic chiasm
* The left and right optic nerves (CN Il) converge in the midline at the optic chiasm (crossing)
- The optic chiasm lies just superior to the pituitary gland and midbrain
What are the visual fields?
1
Temporal retina
Fovea
*
*
Visual fields
Nasal retina perceives the
tem poral visual field (outer half)
Nerve fibres decussate at optic
chiasm
Temporal retina perceives the
nasal visual field (inner half)
* Nerve fibres do not decussate
at optic chiasm
Lateral geniculate nucleus of
thalamus — important relay point
Projections to visual cortex via the
optic radiations
What is meyer and baum’s loop?
M loop : pathway from inferior retina carrying superior visual field information from opposite side (contralateral superior quadrant)
Pass through temporal lobe to lower bank of calcerine sulcus
B loop : pathway from superior retina carrying inferior visual field information from opposite side (contralateral inferior quadrant)
- Pass through parietal lobe to upper bank of calcerine sulcus
What is the primary visual cortex?
- Locared on either side of the calcarine sulcus
- Around 50% of this cortex is taken up by neurons receiving information from the macyla ad fovea
- Projects to visual asoociation area
- Areas enable us to :
- see objects in colour , 3D and dim light
- Recognise faces and objects
- Perceive distance and speed of objects
What are the different types of visual pathway lesions?
Key terms
Monocular = one eye
Binocular = two eyes
Homonymous = affecting the same part of the visual field in each eye
* Heteronymous = affecting different parts of the visual field in each eye
* Anopia = a defect in the visual field
* Hemianopia = an anopia affecting half of the visual field of one eye
* Quadrantanopia = an anopia affecting a quarter of the visual field of one eye
What are visual field defects?
Right monocular vision loss : lesion of right
optic nerve e.g. due to optic neuritis
Bitemporal hemianopia: loss of temporal
visual fields due to optic chiasm lesion e.g.
pituitary adenoma
Left homonymous hemianopia: complete
lesion of optic tract/radiation/primary visual
cortex
Left inferior quadrantanopia: partial lesion of
optic tract/radiation i.e. Baum’s loop (‘pie on the floor’ defect)
Describe the iris and the pupil
Light enters the eye through the transparent cornea
The iris has two layers of smooth muscle:
Sphincter pupillae: constricts the pupil, parasympathetic
innervation
Dilator pupillae: dilates the iris, sympathetic innervation
What are the sphincter pupillae and dialator pupillae?
- Parasympathetic neurons cause contraction of the pupil - oculomotor nerve (CN III)
- Sympathetic neurons cause dilation of the pupil
- Pre- ganglionic neurons from T1 spinal cord segment
- Post-ganglionic neurons from the superior cervical ganglion travel along internal carotid artery and branches to reach iris
What is the pupillary light reflex?
- Pupillary light reflex
Involuntary parasympathetic constriction of pupil when bright
light shone into one or both eyes
Afferent (sensory) neurons in the optic nerve transmit the
signals to the pretectal nucleus of the midbrain
Neurons from the pretectal area transmit signals to both
Edinger-Westphal nuclei of the midbrain
Pre-ganglionic parasympathetic fibres travel in
oculomotor nerves (CN Ill) to ciliary ganglia
Post-ganglionic parasympathetic neurons from the
ciliary ganglia run to sphincter pupillae - Both pupils constrict
Direct pupillary light reflex — effect of light in ipsilateral eye
Consensual pupillary light reflex — effect of light in contralateral (opposite) eye
What is the ciliary muscle?
- Ring shaped layer of muscle attached to the lens via suspensory ligaments (zonular fibres)
- Cilliary muscle contracts : lens relaxes and becomes more convex in shape
- Near vision
- Ciliary muscle relaxes : lens stretches and becomes less convex
- Distant vision
What happens when light is focused on the retina?
Distant objects : Light rays are hitting the eye in a parallel fashion and don’t need to be refracted much
Close objects : Light rays diverge as they hit the eye and need to be refracted more
- The cornea also helps to regract light, but the lens is able to change its shape to help with focusing
- Light rays are converged onto a focal point , ideally right on the fovea
