Hearing and Vision: Sensory Anatomy and Physiology PDF

Sensory anatomy and physiology Somatosensory System Taste Olfaction Hearing Vision Olfactory cortex  Sound Signals (Impulse responses) Many...

Sensory anatomy and physiology Somatosensory System Taste Olfaction Hearing Vision Olfactory cortex  Sound Signals (Impulse responses) Many physical objects emit sounds when they are “excited” (e.g. hit or rubbed). Sounds are just pressure waves rippling through the air, but they carry a lot of information about the objects that emitted them. (Example: what are these two objects? Which one is heavier, object A or object B ?) The sound (or signal) emitted by an object (or system) when hit is known as the impulse response. Impulse responses of everyday objects can be quite complex, but the sine wave is a fundamental ingredient of these (or any) complex sounds.  A sine wave by air pressure Diagram of the periodic condensation and rarefaction of air molecules produced by the vibrating tines of a tuning fork A plot of the air pressure versus distance from the fork. Note its sinusoidal quality. Sound wave propagation  The Ear as a Frequency Analyser Physical properties of objects, such as size, mass, stiffness, are reflected in the frequency spectra they emit when they make sounds. An important job of the ear is to perform a time- frequency analysis of incoming sounds. This results in: A place code for frequency (tonotopy) A rate code for intensity A time code for temporal structure (including “fine structure”) Examples of different sounds In each case, the top panel is a spectrogram (frequency versus time plot with increasing intensity represented by hotter colors) and the bottom panel is an oscillogram (amplitude versus time plot). Note that animal vocalizations, speech, and music can contain highly periodic (tonal and harmonic) elements, whereas environmental sounds such as wind lack such periodic structure.  The frequency, amplitude, and temporal structure of sound are critical for sound perception & discrimination Leopard frog Bull frog Frequency ( 7.7 kHz) Southern ground cricket Allard’s ground cricket Tinkling ground cricket  Anatomy of the Ear Outer ear: entry of sound waves – Pinna – Ear canal Middle ear: amplification of sound waves – Tympanic membrane – Ossicles: malleus, incus, stapes – Oval window – Round window Inner ear: Cochlear  The human ear The lever action of the stapes facilitates transmission of airborne sounds to the fluid- filled cochlea Cochlea in the human ear  Hair cells in the cochlea Blowup of the organ of Corti shows that the hair cells are located between the basilar and tectorial membranes; the latter is rendered transparent in the line drawing and removed in the scanning electron micrograph. The hair cells are named for their tufts of stereocilia; inner hair cells send afferent innervation, whereas outer hair cells receive mostly efferent innervation.  Hair Cells Hair cells in the Organ of Corti transduce the mechanical vibration of the basilar membrane into electrical signals. Inner hair cells transmit these signals to auditory nerve fibres. Outer hair cells are mechanical feedback devices which amplify the signal on a tuneable manner. Traveling waves initiate auditory transduction Vertical movement of the basilar membrane is translated into a shearing force that bends the stereocilia of the hair cells. The pivot point of the basilar membrane is offset from the pivot point of the tectorial membrane so that when the basilar membrane is displaced, the tectorial membrane moves across the tops of the hair cells, bending the stereocilia.  Mechanoelectrical transduction mediated by hair cells When the hair bundle is deflected toward the tallest stereocilium (Kinocilium), cation-selective potassium channels open near the tips of the stereocilia, allowing K+ (high K+ but low Na+ in Scala media) to flow into the hair cell down their electrochemical gradient. The resulting depolarization of the hair cell opens voltage-gated Ca2+ channels in the cell soma, allowing calcium entry and release of neurotransmitter onto the nerve endings of the auditory nerve.  The structure of the hair bundle in cochlear hair cells Scanning electron micrograph of a cochlear outer hair cell bundle. Tip links that connect adjacent stereocilia are believed to be mechanical linkages that open and close transduction channels.  Basilar membrane mechanics: a trade-off between stiffness and flexibility The basilar membrane is stiff at the base and floppy at the apex. Vibrations travelling from the stapes to the round window must either take a short route through stiff membrane or a longer route through floppy membrane.  Traveling waves along the cochlea A traveling wave is shown at a given instant along the cochlea, which has been uncoiled for clarity. The graphs on the right profile the amplitude of the traveling wave along the basilar membrane for different frequencies. The position (labeled 1–7 in the figure) at which the traveling wave reaches its maximum amplitude varies directly with the frequency of stimulation: Higher frequencies map to the base, and lower frequencies map to the apex Basilar Membrane Tuning Animation See auditoryneuroscience.com | The Ear  Signal coding for Sound Coding for the qualities of sound – Intensity (rate) coding = loudness (amplitude, decibel) Coded based on the degree of deflection and opening of ion channels in stereocilia – Frequency (place) coding = pitch (Hz) Coded based on the hair cell location on the basilar membrane where the deflection occurs – Time coding = temporal discharge pattern of signals Phase locking enables a timing code in the local population  Auditory Pathways Hair cells (in the Cochlea) synapse on afferent axons (spiral ganglia) of cochlear nerve VIII Cochlear nerve enters brainstem (cochlear nucleus - superior olivary nucleus - inferior colliculus) – thalamus (medial geniculate nucleus) – auditory cortex Auditory cortex (temporal cortex) has a frequency (tonotopic) map which is inherited from the cochlea Cortical map disruption in hearing loss Cortical Map Hair cells in cochlear D Noise trauma  Cochlear implant: Engineering for Compensation Sound is picked up by a small microphone (1) located behind the ear and converted into an electrical signal. An external processor (2) converts the signals into complex digital representations of the sound, which travel by wire (3) to an external transmitter (4), which transmits them as radio waves to the internal processor (5). Here they are reconverted to electrical signals that travel by wire to the cochlea (6), where 22 electrodes are placed. The 22 electrodes stimulate the auditory nerve (7).  Sensory anatomy and physiology Somatosensory System Taste Olfaction Hearing Vision Olfactory cortex  The human eye Some interesting facts about the human eyes: Only 1/6 of the human eyeball is exposed. Corneas are the only tissues that don’t have blood. The eye muscles are the most active muscles in the human body. A fingerprint has 40 unique characteristics, but an iris has 256, a reason retina scans are increasingly being used for security purposes.  Anatomy of the Retina  The retina is a circular disc of between 30 and 40 mm in diameter. Area of the human retina is about 10 square cm.  The retina is a thin sheet of neurons, about ~200 µm thick. The retinal thickness shows greatest variations in the center. The retina is thinnest at the foveal floor and thickest at the foveal rim.  The cones are highly concentrated at the fovea, an area of just 1 square millimeter. This region is responsible for our high-acuity color vision  Anatomy of the retina Reflection – We perceive light waves reflected off objects Cells of the retina – Rods and cones are photoreceptor cells – Rods and cones communicate with bipolar cells – Bipolar cells communicate with ganglion cells – Axons of ganglion cells form the optic nerve – Horizontal and amacrine cells provide lateral inhibition  The blind spot Fovea Blind spot Blind spot The optic nerve is a photoreceptor-free zone. The optic disc is positioned approximately 15° temporal to the fovea. Why does not the consequence of this receptor gap present a conscious visual problem? Hint: binocular vision and “filling-in” The blind spot + Structure of the retina cells Structural differences between rods and cones. Although generally similar in structure, rods and cones differ in size and shape, as well as in the arrangement of the membranous disks in their outer segments. Rods: Dim light, sensitive to low light Cones: Day light, color and high acuity Phototransduction in rod photoreceptors (A) Rhodopsin resides in the disk membrane of the photoreceptor outer segment. The seven transmembrane domains of the opsin molecule enclose the light-sensitive retinal molecule. (B) Absorption of a photon of light by retinal leads to a change in configuration from the 11-cis to the all-trans isomer. (C) The second messenger cascade of phototransduction. The change in the retinal isomer activates transducin, which in turn activates a phosphodiesterase (PDE). The PDE then hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of channels in the outer segment membrane (light-induced hyperpolarization).  Opsin proteins for trichromatic perception Three different types of cones and the rods have slightly different opsins which are sensitive to different wavelengths. Trichromatic vision: L-cones (red), M-cones (green), and S-cones (blue)  Dogs are dichromatic The trick to seeing color is not just having cones, but having several different types of cones, each tuned to different wavelengths of light. Human beings have three different kinds of cones and the combined activity of these gives humans their full range of color vision. Dogs have fewer cones (yellow and blue) than humans which suggests that their color vision won't be as rich or intense as ours. What color of dog toy should you buy for your dog? Why are some people colour blind? Red-green colour blindness (lack red or green opsin) Blue-yellow colour blindness (lack blue opsin)  Why are some people colour blind? Color blindness is caused by mutations in the opsin genes. Red-green color blindness is the most common form, followed by blue-yellow color blindness and total color blindness. Chromosome X Red opsin gene Green opsin gene What is red-green color blindness most common in males? Chromosome 7 Blue opsin gene Ganglion cells emphasize on moving objects and temporal changes in the visual input The shape/outline and movement of an object are important information. When the image is stabilized on the retina with an eye-tracking device, it fades from view within seconds. Fortunately this never happens in normal vision, even when we attempts to fix our gaze, small automatic eye movements continually (microsaccade) scan the image across the retina and prevent the world from disappearing.  Troxler effect If there is no microsaccade, the image across the retina will disappear in a few seconds. The Retinotopic Map in the visual striate cortex Depth perception modulated by the binocular vision and memory Binocular vision (3D viewing glasses) Three-dimensional vision with one eye?  Neural Pathways for Vision Ganglionic cells – Optic nerve (cranial nerve II) – Optic chiasm – Optic tract Lateral geniculate body of thalamus synapses Optic radiations Visual cortex synapses Right visual field to left cortex, and vice versa  Beyond the primary visual cortex (VI) Parallel pathways transfer different types of visual information Dorsal stream – Analysis of visual motion and the visual control of action (Movement & Depth) Ventral stream – Perception of the visual world and the recognition of objects (Color & shape)