Receptors
- 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
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
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
- 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 |
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
- Cone and rod cells (light-sensitive receptor)
- 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 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
- 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
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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