Special Senses (Vision (Optical Defects (Emmetropia
Normal sight, Myopia…
- Visible spectrum is wavelengths of 400-750nm
- 400 = Blue
- 750 = Red
- Optical component = cornea and lens
- Neural component = retina
- Rods and Cones
- Interneurons: Bipolar, Horizontal, Autocrine
- Retinal Ganglion Cells
- 120 million
- Few in fovea
- Night vision
- Not colour sensitive
- Rhodopsin protein
- 8 million
- Many in fovea
- High light levels
- Sensitive to red, green, blue light
- S, M , or L Photopsin
- Occurs when light travels from the medium of one refractive index to a medium of a different refractive index.
- Results in a change of direction of light.
- The difference between the two RI's and surface curvature.
- Refractive power: ability of the lens to bend light
- Measured in diopters
- The reciprocal of focal length
- 1M convergence = 1 diopter lens
- Distant = ciliary muscle relaxed due to low PNS activity. Zonular fibres are taut and the lens is flat.
- Near = PNS activity increases and ciliary muscle contract. Zonular fibres relax and lens becomes more spherical.
Constriction of Pupil:
- Improves depth of focus, fewer optical aberrations by excluding edges of lens. Pupil gets smaller as image gets closer
Convergence of Eyes:
- Eyes start to turn inwards as image gets closer so focus remains on the fovea
- Eyeball too long
- Image converges in front of retinal plane
- Diverging lens used to correct
- Eyeball too short
- Image converges beyond retinal plane
- Converging lens used to correct by bring ing image forward
- Cornea/lens is aspherical in shape
- Smaller refraction in the shorter planes
- Cylindrical lens used to increase radii of shorter lengths
- Loss of elasticity of lens, stiffens
- Accommodation decreases
- Trouble viewing close images
- Convex lens used to restore
- Lens loses transparency
- Corrected with surgery + plastic lens
- Ability to accommodate is lost
- Changing pressure of the eyes due to inadequate or over production of aqueous/vitreous humour
- Retinal is non-activated
- Intracellular cGMP is high
- cGMP channels open, much Na influx
- Photoreceptor depolarised
- Lots of glutamate released onto bipolar cells
- Retinal changed to active trans form
- Activates G protein transducin
- Transducin activates cGMP phosphodiesterase
- Less cGMP, gated Na+ channels
- Photoreceptor hyperpolarised
- Less glutamate released
- Photoreceptors communicate with interneurons via graded potentials.
- Interneurons can be E or I
- Superior colliculus
- Lateral Geniculate Nucleus
- Suprachiasmatic Nucleus
- 0 is the lowest decibel for human hearing
- Hz determines amount of decibels needed to be able to detect the sound e.g. 1000Hz = 0dB
- Pitch: Determined by activity in hair cells at specific points on basilar membrane
- Intensity: Encoded in the number of impulses per second in auditory nerve fibres
- Duration: Signalled by duration of the afferent discharge caused by the stimulus
- Direction: Indicated by time difference in activation of receptors in each ear + intensity difference
- Broken into outer, middle and inner components
- Inner ear contains the sound transduction apparatus
- Inner ear
- Sensation of hearing
- Organ of Corti = sits in endolymph (K+)
- Scala Vestibuli and Tympani = perilymph = NaCl
The Basilar Membrane:
- Base = narrow and stiff, high frequency
- Apex = wide and floppy = low frequency
- Hair cells lie on basilar membrane and are moved by pressure waves that disrupt the Organ of Corti
- Causes opening of mechanically gated ion channels.
- Once ion channels are open, K+ ions from the endolymph enter the cell and cause depolarisation. Opens VG Ca channels which cause release of neurotransmitter onto the cochlea nerve.
- Extremely fast transduction, no 2nd messengers
- Tip links: protein attachments between tips of stereocilia, enables unitary movement
- Auditory receptors in cochlea: nerve cell bodies in cochlea form a ganglion
- Brain stem neurons
- Medial Geniculate Nucleus: First major auditory structure
- Auditory Cortex: receives info from both ears.
- Different pitches/tones mapped on specific areas of the P.A.C
- Certain tone detected in cochlea and corresponding nerve fibre fires an AP. Nerve runs to certain area of P.A.C to indicate a particular pitch of sound.
- Areas around P.A.C turn sound into meaningful and understandable noise e.g. Wernicke's and Broca's Areas.
- Raised threshold to sound stimuli.
- Can be to either impaired sound transmission through outer or middle ear e.g. infection
- Or can be due to damage to receptors or neural pathways e.g. exposure to loud noises, some antibiotics, tumours, meningitis.
- Generates reflexes to compensate for head movement and perception of motion in space. Reflexes triggered promote stable images on the retina during head movements.
- Maintains posture
- Conscious awareness of position
- 3 loops of membrane fluid filled tubes.
- Bulges at the base of the canals are the otolith organs
- Direction of plane = direction of acceleration stimulation.
- Contains cupula
- Sits in endolymph
- Inertia of fluid during rotation displaces the cupula and in turn displaces the attached hair cells.
- Ion channels are opened and K+ flux leads to hair cell depolarisation.
- Hair cells fare leaky at rest so have a resting firing rate which means acceleration or deceleration can be detected
- Different activity levels or relative pairs determine direction of movement
- Approximately vertical when standing (hair cells are oriented horizontally)
- Vertical acceleration
- Approximately horizontal when standing (hair cells are oriented vertically)
- Horizontal acceleration
- Contain CaCo3 stones called otoliths that provide a weight to sense displacement of.
- Gelatinous capsule attached to hair cells.
- Axons project to vestibular nuclei in the brainstem.
- Information use to stabilise the eyes, the tea and to maintain balance.
- Vertigo: caused by diseases affecting the vestibule or its afferent fibres
- Motion Sickness: caused by mismatch between visual and vestibular information
- Bedspins: caused by alcohol as ethanol causes cupula to float
- Ototoxic drugs: decimated the hair cells, can lead to permanent or temporary hearing loss and disorders of balance.
Taste and Smell
- Allows the detection of chemicals to help us avoid poisons/tainted food and to find a compatible mate
- 1 surface cell has 10-12 cilia
- Olfactory nerves attached to olfactory bulb.
- Branch into receptor cells supported by supporting cells, and then further into cilia.
- Chemicals in the air are dissolved in mucus.
- Travel to odourant receptors on the cilia
- GPCR receptor, converts ATP to cAMP on binding.
- Opens Na+ coupled Ca channels, positive flux.
- Opens chloride channels to further depolarisation.
- Increases Ca concentration, neurotransmitter release and signal sent.
- Olfactory axons
- Olfactory bulb
- Olfactory cortex
- Odorants activate more than one receptor type so that 50000 smells can be interpreted from only 1000 receptor types: combinations = smells.
- On tongue, soft palate, and epiglottis.
- Modified epithelial cells that have a constant turnover
- ENaC channel allows diffusion of Na+ stimulus down the electrochemical gradient. Depolarisation leads to opening of VG Ca channels. Calcium flux = neurotransmitter release
- Protons enter through ENaC, block K+ channel.
- Depolarisation, VG Ca channels open, neurotransmitter release
- GPCR's on apical side of membrane, glucose binds and triggers second messenger pathway.
- AC converts ATP to cAMP which activates PKA.
- PKA closes K+ channels which leads to accumulation of positive charge.
- VG Ca channels opened and calcium flux leads to neurotransmitter release.
- Bitter = poison protection, 3 different mechanisms.
- Blocks K+ channels directly
- GPCR phosphodiesterase breaks down cAMP which leads to depolarisation.
- GPCR phospholipase C mobilises IP3 which opens Ca intracellular stores, calcium flux triggered.
- Ionotropic glutamate-gated channel allows entry of Ca and Na upon binding of Umami stimulus 'glutamate'
- Leads to depolarisation and opening of VG Ca channels to allow neurotransmitter release
- Taste buds innervated by chorda tympani, lingual, and glossopharyngeal nerves.
- Afferent nerves synapse in the medulla
- Info relayed to the thalamus then to cortex.
- Taste cortex near gum, jaw, tongue somatosensory cortex for palatability discrimination.
- Motor nerve supply to tongue muscles mostly by hypolossal CN XII nerve.