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Drugs can affect how we perceive the world around us

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Detecting light - the eye

Conjunctiva	Protection of cornea
Sclera	- Protection - Attachment for eye muscles
Cornea	Refracts (→focuses) and allows passage of light
Choroid	Pigment prevents light reflection within the eyeball by absorbing light
Ciliary body	- Accommodation - Secretion of humour
Iris	Regulates passage of light
Lens	Refracts light
Retina	Contains light receptors
Fovea	Contains only cone cells
Blind spot	Optic nerve (sensory nerve fibres) leave the eyeball
Humour	Maintains shape of the eyeball

Light/photons travel through transparent media in a light ray
- Rays reflect at a predictable angle when they strike an object
- Rays passing through mediums of different density refract (change angle)
Accommodation → focus of rays from near/distant objects by changing shape of lens
Light rays form an image in the retina [EXAM]
- Refraction / by lens or cornea / shape of lens changes

	NEAR ACCOMMODATION	DISTANT ACCOMMODATION
CILIARY MUSCLES	CONTRACT	RELAX
TENSION IN SUSPENSORY LIGAMENTS	REDUCED	INCREASED
SHAPE OF LENS	FAT, ROUNDED	THIN, FLAT
RESULT	LIGHT BENDS	LIGHT BENDS LESS
FOCUSES	DIVERGING LIGHT RAYS	PARALLEL LIGHT RAYS

Retina contains 4 layers → synapse between them
- Cone and rod cells (light-sensitive receptor)
  - Inner segment → nucleus, mitochondria, ribosomes, synaptic region
  - Outer segment → membranous disks containing pigments
- Bipolar neurones (relay neurone)
- Ganglion cells (sensory neurones)
- Axon of ganglion cells → optical nerve
  - Send impulses to the brain
Light passes through neurones before it strikes the retina
There are no cone and rod cells where the optic nerves pass through the retina; this point is called the blind spot

Table: Features of rod cells and cone cells

FEATURE	ROD CELLS	CONE CELLS
Number in retina	More	Fewer
Distribution	- Evenly throughout the retina - Absent from the fovea - Only type of light receptor at the periphery of the retina	Present in the fovea
Shape of outer segment	Rod shaped	Cone shaped
Sensitivity to	Dim light	Bright light
Visual acuity	Poorly resolved images	Well-resolved images
Light-sensitive pigments	- Only rhodopsin - Monochromic vision	- Iodopsin - Sensitive to blue, green, blue light - Trichromatic vision (combination)
Synapse with relay cells	Several rod cells synapse with same relay cell	Each cone cell synapses with just one relay cell

Table: Absorption of light by rhodopsin creates a generator potential in rod cells

In the dark (rod cell)	In light (rod cell)
Opsin + Cis-Retinal → Rhodopsin	Rhodopsin → Opsin + Trans-Retinal
Causes sodium channels to open	Causes sodium channels to close
Membrane depolarised	Membrane hyperpolarised
Neurotransmitter released into inhibitory synapse [rod → bipolar cell]	No neurotransmitter released into inhibitory synapse
Bipolar neurone hyperpolarised → no impulse	Bipolar neurone depolarised → AP
No neurotransmitter released into excitatory synapse [bipolar → ganglion cell]	Neurotransmitter released into excitatory synapse
No action potential	Action potential along ganglion neurone

TRANS-RETINAL + OPSIN → RHODOPSIN ATP → ADP + PI
Mitochondria in inner segment synthesis ATP
Slow reaction compared to rhodopsin breakdown by light
Bright light into dim light conditions → poor vision until rhodopsin is resynthesised
Retinal is a derivative of vitamin A

Rod cells are working in dim light conditions
- Several rod cells synapse with one relay cell → retinal convergence
- Impulse by summation, thus, rod cells collectively cause generator potential
- Poor visual acuity but high sensitivity to dim light
Cone cells are working in bright light
- Each cone cell synapses with each individual relay cell
- Several impulses pass along the optic nerve to the brain
- High visual acuity (ability of the brain to resolve images)

Visual pathway
- Binocular vision
  - Overlap of visual fields of both eyes
  - Right visual field focuses on left retina and vice versa
- Impulses from rod and cone cells travel within optic nerve
- Arrive at optic chiasma
  - Half of the fibres cross over to the other side
  - Right brain processes the left visual field of both eyes and vice versa
- Impulses continue in the optic tract and pass to the lateral geniculate nucleus (LGN)
- From there, impulses pass to the visual cortex in the occipital lobe
Binocular vision allows the judgment of distances
Top-down process
- Perception depends on content of the image and is processed
- Knowledge, expectations, or thoughts influence perception
- Evidence from visual illusions
- Constructivism
Bottom-up process
- Perception from physical characteristic of stimulus
- Only information received from eye is used for perception
- Realism