The Senses PDF

The Senses Sight The retina is formed out of the neural tube 5 cell types in the retina: Photoreceptors Rod: light sensitive receptor for black/white vision. Cone: less sensitive receptor for coloured light Photoreceptors turn over the...

The Senses Sight The retina is formed out of the neural tube 5 cell types in the retina: Photoreceptors Rod: light sensitive receptor for black/white vision. Cone: less sensitive receptor for coloured light Photoreceptors turn over their outer segment discs with 12 days  Discs increase surface area Light hyperpolarises the receptor (they are always depolarised) Light reduced cGMP so reduced sodium influx Opsin receptor inactivates the Na+/Ca2+ channel when exposed to light Retinal accepts a photon and activates Opsin GCPR Opsin activates transducin (special Gα), Transducin activates phosphodiesterase (PDE), which hydrolyses cGMP cGMP level drops, Ca2+/Na+ channel closes Horizontal cells Bipolar cells Amacrine cells Ganglion cells (their axons form the optic nerve) Optogenetics Best colour vision in the fovea. More cones in fovea and less rods Better light detection in the macula lutea. No sight in the optic disc (exit of optic nerve, blind spot) Rods: 15-30 rods project on 1 bipolar cell, providing a large sensitive receptive field. Rods in the fovea project to only one bipolar cell for a smaller, more precise receptive field. Cones also project to one bipolar cell, resulting in a small receptive field but also limited sensitivity Off In the dark, rod fires APs via secretion of glutamate to the bipolar cell, which fires to the ganglion cells On In the light, rods glutamate is less secreted by the mGluR6 and on cells are depolarised then fired to the on ganglion cells Surround suppression by horizontal cells  Hyperpolarization of surrounding cones by light results in hyperpolarization of the connecting horizontal cells. These horizontal cells project on the central cone and reduce their suppressive signals, leading to depolarisation of the central cone. This produces a strongly focussed signal in the centre receptive field. Hyperpolarisation means less glutamate being secreted in the centre so less stimulation and surrounding cones are more depolarised so it is more focused and precise Sound Sound is the oscillation of pressure Audible sound for human ear: ~50 Hz to ~20 kHz Older people loose the higher frequencies The ear The ear The middle ear translate the vibration into mechanical movement and amplifies the amplitude 200x The cochlea The cochlea is tuned, high frequencies resonate at the base while low frequencies resonate at the tip Organ of Corti Translates the basal membrane movement to APs The vertical movement of the basal membrane cause horizontal shearing force of the tectorial membrane on the hair cell tips. The outer hair cells detect small vibration because they are located further away from the tectorial membrane rotation point Hair cells The cochlea is tuned, high frequencies resonate at the base while low frequencies resonate at the tip  High potassium concentration outside the cells Hair cell axons within the auditory nerve are tuned You can tell where sound is coming from (3D detection) Longer path from the first ear Auditory Cortex Auditory cortex is positioned between Wernicke’s area (sensory speech area) and Broca’s area (motor speech control) Smell Humans have relatively few smell sensory cells, olfactory receptor neurons (ORN) Humans can track a scent trail The projection of ORNs to the brain Pyriform cortex = olfactory cortex. Smell is the only sense that does not require the thalamus as relay-station Aromatic substances have a concentration-dependent response Olfactory epithelium The nose cavity is directly accessible for pathogens, hence the olfactory epithelium requires continuous regeneration. ORNs are one of the few neurons that are replenished during adult life by stem cells Receptors G protein-coupled receptors (GPCRs) with a unique G protein: Golf (G ol f ) Receptors are expressed at the cilia of the ORNs  Metabotropic receptors Calcium The expression pattern of olfactory receptors determines the ORNs response to an odor. Pheromones Pheromones are detected in the vomeronasal organ: Pheromones play an important role in mating and aggression behaviour of rodents and carnivores. Specific pheromone receptors: Vomeronasal receptor 1 & 2, detect pheromones. Primates and humans lack a vomeronasal organ and V1R and V2R are not expressed, hence the role of pheromones in humans is largely obscure Taste Taste is projected to the brain through cranial nerves VII, IX and X Projections Microvilli have receptors Salt and acid are ions, depolarisation through ion channel Sweet, bitter and umami are recognised through GPCRs, which activate the TRM5 calcium channel