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Receptors

Pacinian Corpuscles

The Eye

Table 16-8-1: The structure and function of the mammalian 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

Transmissive And Refractive Properties Of The Eye In Focusing An Image On The Retina

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

Role of Rod Cells and Cone Cells in Effecting Monochromatic and Trichromatic Vision

Table 16-8-2: 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 16-8-3: Absorption of light by rhodopsin creates a generator potential in rod cells (AP = action potential)


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

Resynthesis of rhodopsin

The Connection Between Sensory Cells and The Neurone of the Optic Nerve