Chapters 3-6 + Appendix
Chapters 3-6 + Appendix PSYCH 50
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Emily Wu firstname.lastname@example.org Chapter 3: Sensory Systems and Perceptions: Vision The Initiation of Vision ● begins in nonneural elements: cornea, lens, ocular media → focus and filter light ● next step: sensory processing, transform light energy to neural signals via rods/cones of retina ○ rods: perceive low levels of light ○ cones: perceive higher levels of light, detail and color ● sensory adaptation resetting of sensitivity according to surrounding conditions ○ ensures max efficiency of sensory processing over range of environmental conditions ● acuity degree of precision ○ depends on distribution of receptor cells across retina ○ lessened acuity outside of central retina ○ fovea has highest acuity ● fovea: central region of retina, most density of cones here ○ rods absent in fovea ● saccades: movement of eyes to focus on different positions ○ is a result of having lessened acuity outside of fovea Subcortical Visual Processing ● primary visual pathwaprocessing of stimuli in retina → info converges into retinal ganglion cells → leaves retina via optic nerateral geniculate nuclein the thalamus → primary visual cortex of occipital lobe Cortical Visual Processing ● primary visual cor: also known as V1 or striate cortex; projeextrastriate visual cortical are, which are a part of cortical association areas ● cortical association arecortical regions not sensory or motor, deal with higherorder processing ● V4 areaimportant in processing color vision ● MT and MST: middle temporal and middle superior temporal lobes; important for perception of motion ● pathways of the extrastriate cortical area: ○ ventral stream (“what” pathwayisual cortex → inferior temporal lobe ■ analysis of form and color ○ dorsal stream (“where” pathwaystriate cortex → parietal lobe ■ analysis of motion and spatial relations Other Characteristics of the Visual Cortex 1. Topography ○ organization of retinal receptors is reflected in corresponding regions of thalamus and visual cortex ○ ex: stimulate area of retina → area of visual cortex activated; stimulate area adjacent to first area → area adjacent to first area of visual cortex activated 2. Cortical magnification ○ size of a unit area in fovea is disproportionately represented by a lot more area in visual cortex than area of peripheral retinal ○ perceiving visual detail from fovea requires more neural activity than peripheral areas 3. Cortical modularity ○ neurons of similar functions are arranged in iterated groups ortical columns 4. Visual receptive fields ○ receptive fiel region of retina that, when stimulated, elicits a response in neuron being examined ○ data obtained by singleunit recordings Visual Perception 1. Lightness/brightness ○ lightness appearance of a surface ○ brightness: appearance of a light source (e.g., sun, lightbulb) ○ luminance: measure of light by photometer (units=candelas) ○ luminance and perception of lightness/brightness are not directly proportional ○ simultaneous lightness/brightness contrapatch on low luminance background appears lighter than same patch on high luminance background (picture above) ○ retinal luminance determined by three aspectlluminatioof objectsreflectance of object surfacetransmittance of space between objects and observer ○ same value of luminance can be produced by different values of the three aspects 2. Color ○ created by distribution of light across the visible spectrumwavelengths ○ hue: relative red, blue, green, or yellowness ○ saturationhow close color is to neutral gray ○ color brightnesbrightness but applied to a hue ○ all three contributecolor space ○ humans are trichromat color vision based on three cone types, each sensitive to specific wavelengths ○ color perception is not needed as much for survival (like brightness/lightness perception is), but helps us discern things more easily ○ color perception is influenced by surrounding scene: i. color contras same light energy perceived as different colors ii. color constancy different light energy perceived as same colors ○ cerebral achromatopsia loss of ability to see color; V4 area seems important to color perception 3. Form ○ the geometrical characteristics of objects ○ perception of form also influenced by surroundings 4. Depth ○ the perception of 3D world from 2D images ○ monocular cues:only need one eye to perceive 3D i. occlusion if object A covers object B, we know object B is farther from us ii. relating size and distance: smaller objects perceived as farther away iii. motion paralla when observer moves, background position in reference to a nearby object changes more than that of farther objects ○ binocular cues needs two eyes to perceive 3D i. retinal dispar each eye perceives slightly different image of same object ii. cyclopean fusionperceived image of both eyes is unified; possibly explained by fact that inputs of both eyes converge onto same neurons in primary visual cortex 5. Motion ○ perception of speed and direction (associated with MT and MST) ○ apparent motion:static images presented quickly in sequence appear to be in motion (basis of movies and videos) ○ motion aftereffec staring at motion going in one direction then perceiving motion in opposite direction when looking away → “waterfall effect” i. possible explanation: neurons adapt to motion of one direction, so other neurons detecting other motion directions become more active and cause one to perceive the other direction of motion after looking away ○ Newsome experiment with rhesus monkeys: found neurons in MT with selective activity for a specific direction of motion Object recognition ○ depends on associating the previous fundamental qualities of vision to identify the stimulus ○ some neurons in temporal lobe specifically responsive to faces (fusiform face area) ○ some neurons respond to face and voice of face; suggests they are part of network that integrate multiple modalities ○ humans are better at recognizing faces with extreme features; suggests that faces are identified in comparison to a norm/standard ○ prosopagnosia: inability to recognize faces (related to damage in fusiform gyrus) Remembering images ● many of the same neurons in visual cortex used for visual perception activate when remembering visual scenes Emily Wu email@example.com Chapter 4: Sensory Systems and Perception: Auditory, Mechanical, and Chemical Senses The Auditory System Sound stimuli ● form from changes in local air pressures due to displacement in air molecules ● pressure changes that fall within range of perception trigger receptor cells of inner ear ● auditory stimuli (pressure changes) → mechanical changes in ear → neural signals in brain ● resonance:tendency of object to vibrate in ongoing manner ○ producestoneif vibrations are periodic ○ producesnoiseif aperiodic ● resulting compression of air molecules proound wave ● most natural stimuli don’t generate tones, most are just noise ● harmonic serie characterizes stimuli that systematically resonate ○ ex: when plucking a guitar string, standing waves will be produced ○ fundamental frequenc greatest updown movement is across entire length of string ○ next mode is at half length, ⅓ length, ¼ length, etc. (picture below) Peripheral auditory system ● preneural effects: local pressure change → mechanical changes in ear ● begins witexternal eaand middle ear ○ collect sound energy and amplify local pressure ○ external ear: concha and pinna focus sound energy ○ 3 bones of middle ear: link deflections of souympanic membrane (eardrum) to inner ear ○ enhanced sound energy sent tval window ● oval window enters inochlea ○ neural effects here: mechanical changes in ear → neural signals ○ houses basilar membran and its receptor ceair cells ○ movement from oval window moves fluid → moves tips of hair cells (stereocilia) ○ movement of stereocilia depolarizes hair cells → releases transmitter molecules → elicits action potentials → travels to auditory nerve ○ tonotopic organization: ■ basilar part near oval window: detects higher frequencies ■ near cochlear apex: detects lower frequencies ■ rest of auditory system also shows tonotopic organization ● primary auditory pathway of the brain: ○ auditory nerve → cochlear nucleus in rostral medulla→ diverge to several places: ■ inferior colliculus in midbrain (integrates auditory info with motor system) ■ superior olivary complex (integrates info from both ears) ■ nucleus of the lateral lemniscus in midbrain (localization of sound source) ○ all this info goemedial geniculate nuclin thalamus → relayed rimary auditory cortex (A1) TO RECAP: sound stimulus → external ear → middle ear → oval window → cochlea of the inner ear → basilar membrane cells stimulated → auditory nerve → cochlear nucleus → medial geniculate nucleus of thalamus → primary auditory cortex The auditory cortices ● located in superior temporal lobe + adjacent areas in parietal lobe ● includes primary (main recipient of auditory info) and secondary auditory cortices (also called A2, does higherorder processing like speech comprehension) The perception of sound 1. Loudness ○ perception of sound intensity ○ measured as sound pressure level (P=F/A) ○ units in decibels 2. Pitch ○ perception of tones related to frequencies of periodic wave stimuli ○ hearing the missing fundamental: perception of hearing a fundamental frequency even when the stimulus isn’t actually producing energy at the fundamental frequency 3. Timbre ○ perception of differences between sound stimuli when loudness and pitch are the same ○ ex: telling the difference between a flute and clarinet 4. Auditory scenes ○ group of stimuli usually naturally present in the environment, like background noise ○ we can focus on one auditory stimuli and tune out the background noise (ex: focusing on the person you’re talking to in a busy crowd) Perceiving the location of sound sources ● humans can locate the source of a sound in horizontal/lefttoright direction, but less sensitive in vertical (updown) and frontback directions ● to locate sound, we use interaural time differenc (for frequencies <3 kHz) and interaural intensity differen (freq >3 kHz) ● interaural time differenc: arise because of distance between ears; auditory input arrives at same spot in brain at the same time, but take different distances to get there, so analysis of the difference in distances helps us locate where the sound came from (if the left ear input took a shorter distance to reach the spot, source is most likely coming from left) ● interaural intensity differencuses the fact that sound intensities at the two ears depends on location of the sound source The Mechanosensory Systems 1. The cutaneous/subcutaneous system ○ deals with perception of touch, pressure, vibration, and cutaneous tension ○ some areas have more touch receptors than others (ex: more dense in fingers than forearm) ○ somatosensory system: receptors receive info → dorsal root ganglia → spinal cord → ventral posterior nuclear complex of thalamus → primary somatosensory cortex (S1) in parietal lobe ○ secondary somatosensory cortex (S2): “higherorder” processing, receives info from S1 and sends it to limbic structures (which have emotional properties) 2. The pain system ○ perceives sensations harmful or potentially harmful to the body ○ perception begins by free nerve endings in skin calleociceptors ○ pain information from nociceptors → dorsal root ganglia → spinal cord → thalamus → primary somatosensory cortex ○ pain pathway is similar to the cutaneous pathway i. somatosensory percepts are mental constructs → we don’t simply translate physical stimuli into our brain, we construct our perception of it ○ placebo effect physiological response after receiving inert medical treatment The Chemosensory Modalities 1. The olfactory system ○ olfactory epitheliumsheet of receptor cells that lines nasal cavity ○ cilia extend from receptor cells and detect odors ○ receptor neurons only express one receptor protein on its surface ○ neurons with same receptor neurons arranged together; their axons converge together int lomerulclusters in thlfactory bulb ○ pathway: odor → receptor cells in olfactory epithelium → glomeruli in olfactory bulb → olfactory tract → pyriform cortex in temporal lobe → thalamus, hippocampus, amygdala, orbitofrontal cortex i. info about odors influence aspects of emotion, memory, homeostasis, etc. ○ small changes in odor molecular structure can lead to big changes in perceived odor ○ most natural smells are made up of a combo of odorant molecules even though they’re experienced as a single smell ○ pheromones: biochemical signals as a means of social communication 2. The taste system ○ pathway: taste buds → cranial nerve ganglia → nucleus of the solitary tract in the brainstem → ventral posterior complex of thalamus → anterior insula in temporal and frontal lobes ○ interneurons link gustatory and visceral regions i. eating something gross makes you gag/spit it out ○ secondary taste area for higherorder processing; tells when certain food is consumed to satiety, orbitofrontal cortex involved with motivation to eat certain foods 3. Trigeminal chemosensation ○ system that detects irritants, which alert us to potentially harmful stimuli (like air pollutants) ○ neurons in mouth, nasal cavity, and lips Final Points about Sensory Systems Coding and labeled lines ● unlikely that systems follow a labeled line theory, which says that specific receptors receive specific stimuli information for specific pathways through central nervous system Plasticity ● greater use of certain body parts results in enlarged corresponding brain area ● if an area loses function, another area can take over its function to some degree ● plasticity happens more in early life but lessens as one grows older Awareness of sensory stimuli ● we don’t perceive many stimuli consciously Representation of sensory percepts ● what are sensory percepts? (represented in a neurobiological perspective) ● percept arises from activity in neurons of relevant regions of primary sensory cortices ● individual neurons respond to many different stimuli; can’t say that one neuron is responds to one specific stimulus ● perception of stimulus qualities is nonlinear to the stimulus’s actual properties; perception is not simply a translation of the physical world Note: I don’t think it’s that important to remember the specific details of the sensory pathways and memorize all the names. The big takeaway is that the pathways begin with receptors that receive sensory information, which gets processed in subthalamic nuclei, gets sent to the thalamus, and relayed to the primary somatosensory cortex. Emily Wu firstname.lastname@example.org Chapter 5: Motor Systems: The Organization of Action Motor Control is Hierarchical ● upper motor neurons in cerebral cortex govern lower motor neurons ● interneurons coordinate lower motor neurons ● motor programs: the highest level of motor control ○ independent of actual muscle groups used to carry them out ○ do not only depend on incoming sensory info ○ originate in central nervous system than from sensory signals ● elementary behavior units: lowest level of motor control ○ directly activate muscles Anatomical organization of motor systems (organized from lowest to highest level) ● lower motor neurons and local circuit neurin spinal cord and brainstem ○ lower motor neurons: directly innervate skeletal muscles (also synonymous with elementary behavior units) ○ local circuit neurons: provide sensory inputs to lower motor neurons ○ fine motor control involves innervation of less muscle fibers ● upper motor neuronsin cerebral cortex and brainstem ○ provide descending control of local circuitry in spinal cord and brainstem ● cerebellum: sensorimotor coordination ● basal ganglia: gating proper initiation of movement Cortical Pathways for Motor Control ● Primary motor cortex projections originate in upper motor neurons here which descend down spinal cord to lower motor neurons ○ also originates from premotor cortical areas, premotor cortex, and supplementary motor cortex ● upper motor neurons in primary motor cortex branch off at different levels ○ those going down the spinal cord go throuedullary pyramids Organization of primary motor cortex ● topographical and contralateral ○ discovered by Sherrington and mapped by Penfield ● amount of cortical space for a specific ability depends on how much fine motor control is necessary Movement maps in the primary motor cortex ● stimulating certain areas elicit multijoint movements ● movements and not muscles are mapped in the motor cortex ● higher motor centers provide motor command signals → engage lower level circuits ○ also signal how forceful movements should be ● frontal eye fie in cortex, which project to brainstem and activate eye muscles Coding movements by the Activity of Neuronal Populations ● direction and amplitude of movement can’t be studied by observing activity of single neurons ● studying eye movement to test idea that activity of large populations of neurons is averaged to produce a single movement ● superior ollicul in midbrain, controls eye movement through local circuits in brainstem ○ stimulation in this area produaccades:coordinated gaze shifts ● each neuron casts a “vote” for direction of planned movement, and weight of vote depends on how strong the neuron fires ○ the votes are averaged and the result is the desired movement Planning Movements ● many movements are automatic in response to stimuli, but others and planned and then held until proper situation for execution ● premotor areas responsible for abstract planning information (with regards to movement) → translated to primary cortex, intent to perform the movement → translated downwards through spinal cord to local neural circuits to accomplish the movement ● readiness potentia an electrical wave measured in neurons that begins seconds before an actual movement ○ begins in premotor cortex then becomes enhanced over primary motor cortex ○ anosognosia: unaware of inability to move, result of damaged premotor and primary motor cortices ● conscious awareness of movement comes after the brain has already intended to move Selecting goals for action ● Newsome and monkeys study: monkeys trained to move eyes to the direction they believed the net direction the dots were moving ○ mostly accurate when 50% moved in one direction, but less accurate as percent coherence lowered ○ speed of monkey’s eyes correlated with increased activity in parietal cortex when coherence was higher/easier to tell which direction dots were moving ○ conclusion: the intention to move involves accumulating sensory evidence; the harder it is to accumulate evidence, the longer it takes to make a move/make accurate perceptual judgment Motivational control of goal selection ● behavior oriented towards receiving rewards and avoiding punishments ● neurons in posterior parietal cortex sensitieward value ● assumption: neurons accumulate evidence of which movement would bring the greatest reward value or avoid punishment, then guide behavior towards that goal Sequential Movements and the Supplementary Motor Area ● supplementary motor area (SMA): generates movements without sensory cues ○ premotor cortex important for cued movements ○ lesions in SMA cause monkeys to forget welllearned movements, and must reproduce them with help of external cues ● different neurons in SMA activated throughout a sequence of movements ● SMA plays role in guiding production of sequences ○ prefrontal cortex plays role in starting and stopping a sequence of movements SensoryMotor Coordination ● neural mechanisms must use sensory information to understand spatial coordinates and guide our movements ● parietal cortex is important for sensorymotor coordination ● optic ataxi damage to parietal cortex that disrupts saccades and reaching for objects; failure to compute spatial location of eye, hand, and object Initiation of Movement by the Basal Ganglia ● basal ganglia work agating mechanism: inhibit movements until circumstances are appropriate to execute them ○ helps coordinate movement timing ● made of 3 principle nuclaudate,putamen, and globus pallidus ○ caudate and putamen known together as triatum ● striatum collects input from cortical areas ● globus pallidus sends output and relays info from cortical areas to thalamus ● activity of basal ganglia (excitatory + inhibitory) balances and coordinates movements by inhibiting undesired movement and permitting desired movement ● Parkinson’s disease death of neurons that transmit dopamine → lack of excitatory activity in the pathways in basal ganglia → patients can’t generate purposeful movement ● Huntington’s disease atrophy of caudate → unable to control movements due to lack of inhibitory activity in basal ganglia Basal Ganglia and Cognition ● nonmotor pathways also pass through basal ganglia (e.g., limbic/emotional channel and associate/cognitive channel) ● each channel creates a feedback loop: cortex → basal ganglia → limbic or associative striatum → thalamus → cortex ● in some studies, animals w/ lesions in basal ganglia can perform movements but can’t learn new movements Error Correction and Motor Coordination by the Cerebellum ● cerebellum:sits atop the pons in brainstem; corrects errors to help produce smooth and coordinated movements ● computes net error between motor signals being issued by the motor cortex and actual movements being carried out → relays error corrections to frontal and parietal cortices ● appendicular ataxi damage to lateral cerebellum; disrupts coordination of limbs ● intention tremo uncoordinated, shaky movements of hand and arm only evident during voluntary movementdue to damage to ipsilateral cerebellum Emily Wu email@example.com Chapter 6: Attention and its Effects on Stimulus Processing The Concept of Attention Global states, arousal, and attention ● arousal: describes global state of the brain; continuum of whether someone is asleep to awake Selective nature of attention ● attention is different from arousal; attention can be selectively focused ● selective attention: allocation of neural resources towards focusing on specific stimuli at the expense of processing other stimuli ○ in audition: cocktail party effectfocusing on one conversation and tuning out others ○ in visionvisual spatial attent subjects asked to stare at a point of fixation and shown an array of letters while directovert attenti to a specific area→ subjects could recall letters from attended areas, but not from unattended areas ○ covert vs. overt attention: covert attention does not involve eye movement (like seeing something from the corner of your eye); overt attention involves moving one’s gaze towards the stimulus Behavioral Studies of Attention Capacity and Selection The level at which selection occurs ● main goal of these studies: find out at what stage attention affects sensory processing ● two main models: ○ early selectio a lowlevel gating mechanism filters out unnecessary info before sensory processing begins ○ late selectio all stimuli are processed before any selection occurs ● later models modified the main two models ○ most models: more basic sensory processing (e.g., physical characteristics of stimuli) done earlier (“early selection”) and higher order processing (e.g., stimulus meaning) done later (“late selection”) Endogenously vs. exogenously driven selective attention ● endogenous attention: voluntarily directing attention towards stimulus ● exogenous attention: automatically directing attention to a stimulus (reflexive) ● in studies, participants are shown a cue that indicates where the stimulus will appear → after stimulus is shown briefly, participants are asked to recall what the stimulus was → sometimes stimulus does not appear in the cued area → results: recall was better for cued rather than uncued stimuli because focusing attention on an area facilitates sensory processing ● endogenous processing occurs later but lasts longer, exogenous processing occurs earlier but is shorter ● inhibition of ret in exogenous cuing, as the interval between cue and stimulus presentation increases, reaction time slows for identifying targets in the cued area Neuroscience Approaches to Studying Attention Studying the control of attention in the brain ● goals of studies: assess effects of attention on stimulus processing, how attentional modulations of stimuli work in the brain, find what regions work together to form the attention system Neural Effects of Attention on Stimulus Processing: Auditory Spatial Attention Electrophysiological studies ● uses eventrelated potentials (ERPs) from EEG recordings, which occur in three phases ○ a tone is sounded → small waves ofbrainstem evoked responses(BERs) from activity in auditory brainstem → waves showing activity in auditory cortex → longer waves reflecting extended activity in secondary auditory cortices ○ using this approach when subjects do other tasks lets us know when and where stimulus is processed when attention is diverted ● attentional stream paradigm subject wears headphones, asked to focus on sounds coming from one ear → increase in ERPs for stimuli that were attended to about 100ms after a sound → indicates early selection model of attention ● attention affects sensory processing in lowlevel auditory cortex ● auditory N1 ERP wave that becomes larger when tone stimulus is attended to ● P300: longer, larger wave in response to detecting deviant stimuli Neuroimaging studies ● fMRI studies show attention modulates activity in specific areas of auditory cortex The effects of auditory spatial attention on auditory feature processing ● mismatch negativity (MMN): a negative wave in ERP activity when a deviant stimuli is detected among a constant stream of same stimuli (e.g., a higher pitched tone amongst a stream of same pitched tones) ○ MMN amplitude higher in attended channels → analyzing auditory features is influenced by attention ○ MMN also occurs in unattended channels → feature analysis is performed for all auditory input, but not as strong in stimuli that is ignored Neural Effects of Attention on Stimulus Processing: Visual Spatial Attention Electrophysiological studies ● ERPs also used to study brain response to visual stimuli/attention ○ usual activity curve dips and then rises then falls ● studies conducted similarly to those of auditory attention: participants asked to focus on specific area, then shown a target stimulus and report what they saw ● overall, results show relatively early processing of sensory stimuli Neuroimaging studies ● visual spatial attention enhances stimulus processing in lowlevel visual areas (extrastriate visual cortical areas, lateral geniculate nucleus) but not much in primary visual cortex ● when multiple stimuli shown at same time, several areas that process object recognition inhibit activation of each other, but attention to one stimulus counteracts the inhibition of one of those areas, allowing person to fully process that one stimulus biased competition ● reentrant process: attentionrelated activity returns to lowlevel areas in brain after being processed to higherlevel areas → possibly indicates that there is enhanced processing of stimulus information that happens later Animal studies ● generally, a neuron only responds very selectively to stimuli within its receptive field ● studies used this fact to measure neuronal activity as a function of attention ● findings: ○ amplitudes of tuning curves for stimuli in all orientations increased when attention was given to the stimuli ○ attention increases contrast sensitivityneuron firing rates increased when looking at low contrast to help amplify perception of contrast The effects of visual spatial attention on visual feature processing ● when attending to easier tasks in one location, more processing ability is available for objects in unattended locations ● when attending to harder tasks in one location, less processing ability for objects in unattended locations Neural Effects of Attending to Nonspatial Stimulus Attributes The neural effects of attention to nonspatial auditory features ● studies: subject listens to stream of tones of specific pitch then try to detect a deviant sound → ERPs measured and compared ● findings:processing negativity prolonged negative wave that starts after 100 milliseconds → indicates that featurebased attention is related to later stimulus processing than spatialbased attention The neural effects of attention to nonspatial visual features ● studies: subject were presented stream of samefeatured stimuli (e.g., red dots), asked to detect occasional deviant stimuli (e.g., blue dots) → ERPs elicited when seeing red vs. blue dots measured then compared ● findings:election negativitsustained negative wave that starts after 150 milliseconds → indicates that featurebased attention is processed later than spatialbased ● feature similarity gain modelthe amplitude of a neuron’s response to a specific stimulus depends on the neuron’s feature preference and how similar the target stimulus is to that feature ○ ex: searching for someone in a green sweater heightens sensitivity of a neuron that responds to green The effects of visual attention to objects ● study: images of faces and houses were overlapped and subjects were asked to attend to either a face or house → activity in brain area related to face detection increased when attending to face, likewise in housedetecting area when attending to house ● attending to faces → increased activity in fusiform face area (FFA); ignoring faces to attend to other stimuli → decreased activity in FFA to level below passive viewing ● activity enhancement for attended stimuli, suppression for ignored stimuli ● attention increases neuron selectivity Neural Effects of Attention across Sensory Modalities ● supramodal attention: attention that invokes activity across different modalities ○ ex: when visually attending to a stimulus, sensory activity in the brain is also enhanced even though subject was not asked specifically to attend to stimulus aurally ● attention spreads across modalities ○ when two stimuli of different modalities presented together, attention to one modality will spread to the other modality ● multisensory integration: senses of different modalities are integrated and perceived as wholes Emily Wu firstname.lastname@example.org Appendix: Neural Signaling Electrical Potentials across Nerve Cell Membranes ● resting membrane potentia neurons have constant voltage/charge difference across their membranes (about 70 mV) ● neural electrical signals produced as a response to stimuli which change resting membrane potential ○ receptor potentialactivate sensory neurons in response to external stimuli ○ synaptic potentia activate synapses, allow exchange of information between neurons ○ both trigger action potentials along axons How Ion Movements Produce Electrical Signals ● action potentials generated by 1) difference in concentrations of ions across cell membranes 2) membranes being selectively permeable to these ions ● differences in concentration gradient are ductive transporproteins in cell membrane → transport ions against their concentration gradient ○ ATPase pumps like Na+/K+ pumps ○ ion exchangers: takes an ion “up” its gradient in exchange for taking another ion “down” its gradient ● selective permeability of membranes is dueion channels ○ only allows certain ions flow across membrane down their concentration gradients ● active transport stores energy in form of gradients, opening ion channels uses up this energy ● membrane is selectively permeable to K+ and flow out of the cell down their concentration gradient negative resting membrane potential is due to constant efflux of K+ ● electrochemical equilibriwhen the concentration gradient in one direction is equally opposed by the electrical gradient in the other direction Ionic Basis of Action Potentials ● at rest, membrane is most permeable to K+ ● depolarizationmembrane potential becomes more positive and more permeable to Na+ ● voltagegated ion channels open due to change in voltage and Na+ rushes in, increasing voltage in cell to about +58 mV → rising phase ● Na+ channels quickly inactivate → K+ channels open, but more slowly, and K+ flows out → falling phase ● hyperpolarization/undershoo more K+ flows out and generates more negative membrane potential than at rest → then causes K+ gates to close → returns to resting state again ● action potentials occur when membrane is depolarized past threshold potential (~55mV) ○ therefore action potentials are allornone ○ intensity of stimulus increases frequency of action potentials, not amplitude ● refractory peri period in which neuron cannot make another action potential due to inactivation of Na+ channels ○ also prevents action potentials from traveling backwards Long Distance Signaling by means of Action Potentials ● passive current fl conduction by neurons in absence of action potentials ○ however axon membranes are leaky and lose much of the electrical signal as it travels down length of axon ● to improve conduction of axons: increase diameter of axon ○ myelination of axons: wrap axon with layers of glial membranes to insulate it ■ gap in myelinationodes of Ranvier ■ saltatory conductioaction potential jumps from node to node rather than along entire length of axon
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