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Chapter 8&9

Chapter 8&9 B65.docx

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Zachariah Campbell

Chapter 8: Organization of the Sensory Systems General Principles of Sensory-System Function  Sensory receptors= specialized cells that transduce sensory energy  neural activity Receptors As Energy Filters – designed to respond to a narrow band of energy within each modality energy spectrum - Color-blind= lacking receptors for part of the usual visual spectrum - 60% of men have one type of red receptor while the other 40% have another; many women have both - Audition – 20-20,000Hz - Filter sensory world to produce an idiosyncratic representation of reality Receptors Transduce Energy - Vision= light is converted into chemical energy in the form of photoreceptors of the retina and then is converted into action potentials - Auditory system= air-pressure waves are converted into a number of forms of mechanical energy, which eventually activates the auditory receptors, which then produce action potentials - Somatosensory= mechanical energy activated mechanoreceptors which generate action potentials - Taste and olfaction= chemical molecules carried by air or contained in food fit themselves into receptors of various shapes to activate action potentials - Pain sensation= tissue damage releases a chemical that acts like a neurotransmitter to activate pain fibers and produce action potentials - Ex. Displacement of hair – causes stretch-sensitive channels on dendrite to open, allowing an influx of Na+, which causes voltage-sensitive Na+ and K+ channels to open producing a nerve impulse heading to the brain Receptive Fields Locate Sensory Events - Receptive field= specific part of the world to which an organ receptor and cell respond to - Spatial dimensions of sensory information produce cortical maps and patterns of sensory world that forms our reality Receptors ID Change and Constancy - Rapidly adapting receptors= detect whether something is there; easy to activate but stop responding after a short period of time; detect movement of objects - Slowly adapting receptors= adapt more slowly to stimulation; i.e. rods= rapidly adapt ad respond to visible light of any wavelength and have lower thresholds; cones are sensitive to colour and position Receptors Distinguish Self from Other - Exteroceptive= respond to external stimuli - Interoceptive= respond to our own activity; muscles, joints, vestibular system; tell us about position and movement - Optic flow= stimuli stream by when we are running - Auditory flow= change in intensity of sound because of changing locations to let us know how fast we are going - People who experience hallucinations perceive internally generated events as external Receptor Density Determines Sensitivity - Two-point sensitivity is highest on parts of body having most touch receptors - In the fovea the receptors (all cones) are small and densely packed to make sensitive colour discriminations and in the periphery the rods are larger and spread more apart to react to light Neural Relays  Receptors connect to cortex through 3 or 4 intervening neurons  Information can be modified at different stages in the relay, allowing sensory system to mediate different responses Relays Determine the hierarchy of Motor Responses  Some of the 3 or four relays in each sensory system are in the spinal cord, brainstem, and others in neocortex  Pain pathway has relays in the brainstem, specifically in the midbrain periaqueductal gray matter that surrounds the cerebral aqueduct – responsible for behavioral activation and emotional response  Pain relays located in the neocortex not only localize pain but also ID the kind of pain felt, the external causes, and potential remedies Message Modification Takes place at Relays  Messages carried by sensory systems can be modified at relays  Gating, or inhibition of sensory information can be produced by descending signals from the cortex, through the PAG matter, and on to lower sensory relays  Inhibition gates many senses when we are occupied and later when we think about an injury the signal is more amplified Relays Allow Sensory Interactions  When relays take place in sensory pathways, systems interact with one another  Increase the activity in fine touch and pressure pathways, and this can block the transmission of information in spinal cord relays of the pain pathways  McGurke effect – visual modification of sound; ba is played to a listener who sees a person say da, the listener may not hear the actual sound played, but the articulated sound da  Our perception of speech sound is influenced by facial gestures of a speaker  Difficulty for people learning a new language is that they have in blending a speakers movements of articulation with the sounds produced by the speaker Central organization of Sensory Systems Sensory Information is Coded  All sensory information from al sensory systems in encoded by action potentials that travel along peripheral system nerves until they enter the brain or spinal cord and then on tract within the central nervous system  Every bundle carries the same type of signal  Presence of a stimulus= encoded by an increase or decrease in the discharge rate of a neuron & amount of increase or decrease can encode stimulus intensity  Qualitative visual changes can be encoded by activity in different neurons or different levels of discharge in the same neuron  Different sensations are processed in distinct regions of the cortex and we learn through experience to distinguish them  Each sensory system has preferential links with certain kinds of reflex movements, constituting a distinct wiring that helps keep each system distinct at all levels of neural organization  Synesthesia= some people hear in colour or identify smells by how they sound to them; mixture of senses Each Sensory System is Composed of Subsystems  Within each of the five sensory systems are many subsystems which are independent in the function  Fig. 8.3 *** o Superchiasmatic nucleus – daily rhythms o Pretectum – changes in pupil dilation in response to light o Pineal gland – long term circadian rhythms o Superior colliculous – head orientation o Accessory optic nucleus – eye movement to compensate for head movement o Visual cortex – pattern perception, depth perception, colour vision o Frontal eye fields – eye movements  Taste receptors located at the front two-thirds of the tongue send information to the brain through facial nerve while the back 1/3 send it through the glossopharyngeal nerve Sensory Systems have Multiple Representations  Neocortex represents the sensory field of each modality a number of times, the number of times depends on the species  Topographic organization – neural-spatial representation of the body or areas of the sensory world perceived by a sensory organ  All mammals have at least one primary cortical area for each sensory system and additional areas referred to as secondary areas where most of the information that reaches them is relayed through the primary area Vision Photoreceptors  Lights having to pass through the layer of retinal cells poses little obstacle to our visual acuity for two reasons o Cells are transparent and photoreceptors are extremely sensitive o Many of the fibers forming the optic nerve bend away from the fovea so as not to interfere with the passage of light through the retina o Rods – sensitive to dim light and used mainly for night vision o Cones – better to transduce bright light and are used for daytime vision ; three types of cones mediate colour vision – each maximally responsive to a different set of wavelengths  Bipolar cells – simple type of neuron; induce action potentials in ganglion cells  Ganglion cells – send axons into the brain proper (retina is part of the brain) Visual Pathways  Optic chiasm – the two optic nerves meet; half the fibers from each eye corss – right half of each eye crosses to the left hem and the left half of the eyes visual field is represented in the right hemisphere  LGN cells project mainly to level IV of the primary visual cortex – striate cortex because it is striped  The major visual pathway from the retina to the LGN to the striate cortex is the geniculostriate pathway, bridging the thalamus and the striate cortex; the geniculostriate pathway takes part in pattern recognition and conscious visual function  Second main pathways is the tectopulvinar pathway that takes part in detecting and orienting to visual stimulation; Look at FIG 8.8 for more info about the two pathways  Having two pathway lessens the chance that complete destruction of the geniculostriate pathway will render a subject completely blind Hearing  Sound localization – identifying source of pressure waves  Echo localization – detecting pressure waves reflected from objects  Ability to detect the complexity of pressure waves, through which we hear speech and music Auditory Receptors  We hear the frequency (speed) of pressure changes as change sin pitch  We hear amplitude (intensity) of pressure as loudness  Hear the complexity of pressure changes as timbre – perceived uniqueness of a sound  Areas of the temporal lobe interpret action potentials as sound, language, and music  Ear has three major anatomical divisions – inner, middle and outer ear  Outer ear – pinna & external ear canal o Pinna catches waves of air pressure and directs them into external ear canal which amplifies and directs them to the ear drum  Middle ear – eardrum and is connected to the hammer, anvil and stirrup (three little bones; ossicles) o Sound waves strike the eardrum and it vibrates o Transferred to the bones which conveys the vibrations to the oval window and amplifies them  Inner ear – cochlea, which contains the auditory sensory receptors called hair cells o Cochlea – rolled up into a snail shape and is filled with fluid, floating in the middle of the fluid is the basilar membrane o Hair cells are embedded in part of the basilar membrane called the organ of Corti  When the oval window vibrates, the round window, a membrane within the cochlea, bulges, sending waves through the cochlear fluid  basilar membrane bends and stimulates the hair cells to produce action potentials  Frequency of a sound is transduced by the longitudinal structure of the basilar membrane which proves to be a sheet of tissue when the cochlea is unrolled  Higher sound frequencies cause max peaks near the cochlear base; lower sound frequencies cause max peaks near the apex (farthest from oval window)  Single-cell recordings from the primary auditory cortex in the temporal lobes show that dif points in the cortex respond maximally to different frequencies  Tonotopic theory= dif points in the basilar membrane represent different sound frequencies also applies to the auditory cortex  The receptive field of a hair cell is a particular frequency of sound  Auditory system maps sound in space and localizes the sources of sound within the sace around the body Auditory Pathways  Fig 8.12 – midbrain, thalamus, and cerebral cortex  Auditory input cross to the hemisphere opposite the ear in the hindbrain and midbrain, and then recross in the thalamus so that information from each ear reaches both hemispheres  Projections of the visual-system pathways provide both ipsilateral and contralateral inputs to the cortex - There is a bilateral representation of each cochlear nucleus in both hemispheres Body Senses  Interoceptive function – monitors internal bodily events and informs the brain of the position of body segments relative to one another and of the position of the body in space  Composed of several submodalities  Four major somatosensory submodalities: o Nocioception – perception of unpleasant stimuli – pain and temp o Hapsis – perception of objects with the use of fine touch and pressure receptors o Proprioception – body sense/awareness o Balance – mediated by a specialized receptor system in the inner ear Somatosensory Pathways  Two major somatosensory pathways extend from the spinal cord to the brain o Dorsal tract for Hapsis o Proprioception and a ventral tract for nocioception  Fibers relatively large and heavily myelinated  Dendrite and axon of each somatosensory neuron are joined into one continuous nerve fiber  Fibers of the pathways for pain and temperature sensations are smaller and less myelinated  For vision and hearing, we see two somatosensory pathways, each taking a different route to the brain and somatosensory cortex of the opposite hemisphere Somatosensory Cortex  Homunculus – relative sensitivity of body parts are represented by size  Density of somatosensory receptors varies greatly from one place to another on the body surface and the somatic maps manifest the variability  Areas representing the hands and tongue are extremely large, areas representing the trunk and legs are small The Vestibular System: Motion and Balance  Interoceptive receptors in the inner-ear vestibular system informed your visual system that your head, and not you arm is moving  Helps us to distinguish between our behavior and actions of others  Three semi-circular canals are oriented in lanes that correspond to the three- dimensions in which we move and so can respond to movement of the head  Otolith organs detect linear acceleration of the head and are responsive to changes in the position of the head, with respect to gravity as well o Sensitive to static position of the head in space, in contrast to the semi- circular canals’ sensitivity to head movement th  Fibers from the balanced receptors project over the 8 cranial nerve to a number of nuclei in the brainstem – the nuclei interact in the hindbrain to help keep us balanced while we move and aids in the controlling of eye movement at the midbrain level  Info from the vestibular system allows us to record and replay the movement we have made through connections in the cerebellum Taste and Smell The Chemical Receptors Taste Receptors  Bumps on the tongue are called papillae which are there to grasp food  Taste buds buried around them  Chemicals in food dissolve in the saliva that coats the tongue and disperses
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