- Stimulus: environmental change of an organism → change in energy
- Transduce (→convert) a stimulus into a nerve impulse
- Stimulus above threshold
- Detected by receptors
- Changes its membrane potential
- Causes generator potential
- Action potential along sensory neurone
- Receptors respond to specific stimuli only
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Pacinian Corpuscles |
- Found in dermis of skin, joints, tendons, external genitalia, internal organs
- Structure
- Layers (maellae) of connective tissue surround
- Myelinated sensory neurone (nerve fibre ending) that have
- Stretch-mediated Na+ channels
- Round pacinian corpuscle has a resting potential
- Stretch-mediated sodium channels restrict movement of ions
- PRESSURE OPENS SODIUM CHANNELS
- Entry of sodium ions
- Causes depolarisation / membrane potential / generator potential
- Threshold potential reached
Slight pressure / small generator potential / no depolarisation Great pressure / more channels open / larger generator potential
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The EyeTable 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 |
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Transmissive And Refractive Properties Of The Eye In Focusing An Image On The Retina |
- 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
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| |
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 |
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Role of Rod Cells and Cone Cells in Effecting Monochromatic and Trichromatic Vision |
- 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
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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 |
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Table 16-8-3: 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 |
| *AP = Action Potential |
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- Resynthesis of rhodopsin
- 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
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The Connection Between Sensory Cells and The Neurone of the Optic Nerve |
- Rod cells are working in dim light conditions
- Several rod cells synapse with one relay cell → retinal convergence
- Impulse by summation \ 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)
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