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Perception: Conscious interpretation of the environment around you Prior experiences Senses Psychological being Sensory receptors detect information in the periphery to the CNS Visceral receptors Detect and relay information from the internal environment Not conscious Examples: baroreceptors, chemoreceptors, mechanoreceptors Visceral receptors transmit info the CNS by visceral affarents External sensory receptors Detect and relay information from the external environment Information reaches consciousness and is perceived Ex: nociceptors, thermoreceptors Law of specific nerve energies: a given sensory receptor is specific for a particular modality (the energy form of a stimulus). The modality to which a receptor responds best to is an adequate stimulus Sensory Transduction The conversion of stimulus energy into electric energy Receptor or generator potentials Graded potentials I/o being due to neurotransmitters causing post synaptic potentials, it's caused by opening or closing of ion channels Triggered by sensory stimuli If the receptor potential exceeds threshold, it can generate an action potential Sensory receptors: two types Specialized structure at peripheral end of afferent neurons Opening/ closing ion channels Sensory receptor as a separate cell communicating with afferent neuron through chemical synapses Stimulus opening a calcium channel, causing neurotransmitter to be released in cleft, action potential Changes in the receptor cell's membrane potential cause release of a chem messenger or a transmitter, the greater the excitatory stimulus, the more transmitter is released. The transmitter then binds to receptors on the afferent neuron & causes change sin the membrane potential of that cell. If the afferent neuron is depolarized to threshold, an action potential is generated and transmitted by that cell to the CNS Receptor adaptation Receptors become adapted to the stimulus if you were to continuously stimulate that receptor Slowly adapting receptors (tonic receptors) : Receptor potential that is initially created decreases even though the stimulus remains the same Ex: stretch receptors in the muscles & merkel's disks which detect pressure on skin Rapidly adapting receptors: very rapidly adapts and becomes insensitive to continuous stimulus Off response occurs when you remove the stimulus, brief hyperpolarization Ex: olfactory receptors & pacinian corpuscles which detect vibration Sensory pathways Labeled lines : specific neural pathways transmitting information of a specific modality Activation of a specific pathway causes perception of the associated modality regardless of which stimulus actually activated the pathway Sensory unit Single afferent neuron plus all receptors associated with it All receptors are of same type Receptive field: if you have stimulus anywhere in this area you will stimulate this sensory unit Helps to localize where the stimulus is coming form Generalized pathway: receptive fields apply to first, second, and third order neurons 1st order neuron in PNS, the rest are in CNS 2nd order neurons transmit to thalamus 3rd order neurons transmit to cerebral cortex Sensory areas: visual cortex, somatosensory cortex, gustatory cortex, visual cortex, olfactory cortex, auditory cortex (don't really need to memorize but have overlap with certain lobes) Sensory Coding Coding for stimulus type Receptor activated by stimulus and brain often integrates information from different sensory systems Ex: wetness Ex: latex glove and stick it in a bucket of water, mechanoreceptors perceiving the touch and temperature receptors in the hot or cold water and you may think that your hand is wet Coding for stimulus intensity Frequency of action potentials (frequency coding) Stronger stimulus, larger receptor potential As long as the graded potential/receptor potential exceeds threshold for an action potential, stronger depolarizations can overcome the relative refractory period of an action potential and produce a higher frequency of action potentials Stronger stimulus, increase in action potentials What is the significance of the downward slope? Number of receptors activated (population coding) Stronger stimulus activates more receptors (recruitment) Can be from same afferent neuron or can be from other afferent neurons More receptors activated, the receptor potentials generated can sum, greater frequency of action potentials Coding of stimulus location Based on receptive fields (tactile, proprioceptive, & visual) Acuity depends on size of receptive field, degree of overlap, and lateral inhibition Afferent neurons: first order neurons Second order neurons Y1 is stimulated and as it's stimulated it's synapting with inhibitory interneurons which are inhibiting X2 and Z2 (inhibiting action potentials in the second order neurons). Neurons X and Z aren't as close. Helps us figure out where the stimulus is srcinally from. The synapse is occuring at two axons (axoaxonic) Acuity is improved by the overlapping of the receptive fields of different afferent neurons bc overlapping improves the localization via the fact that nay stimulus that occurs within the region of overlap b/w the receptive fields of two afferent neurons activates both neurons and lateral inhibition Two point discrimination test Ability to perceive two points on the skin Smaller receptive fields have more neurons & have greater overlap Areas with larger receptive neurons with larger receptive fields won't be able to distinguish, will interpret it as one pin prick Two separate neurons, two separate fields, perceived as two points Humonculi Two point discrimination threshold: small receptive field, neurons closer together, take up greater space in brain In some sensory systems, localization is unrelated to coding by receptive fields These sensory systems code for quality and intensity Ex: sound, smell Localization is accomplished by time of arrival to receptor The systems also rely on behavioral actions Ex: smell a firecracker, turn her head towards it to figure out where it's coming from Auditory is more acute than olfactory The Somatosensory system Somatosensory receptors Proprioceptors, mechanoreceptors, thermoreceptors, nociceptors Most are specialized structures at nerve endings Free nerve endings lack specialized structures Mechanoreceptors: rapidly adapting (vibration) & slowly adapting (pressure) Rapidly adapting: hair follicle receptor, meissner's corpuscle, pacinian corpuscle (ex: vibration) Slowly adapting: pressure Receptive fields vary greatly Thermoreceptors Warm receptors: respond to temperature in the range of 30-45 C Cold receptors: respond to 5-35C with peak around 20, and after 45C Several different types and the channels are also associated with nociceptors as well and chemical sensitivity Free nerve endings Transient receptor potential ion channels (TRP) Temperature sensitive ion channels Can respond to chemical stimuli, and can transduce pain Involved in taste, olfaction, vision, visceral responses Both warm and cold receptors are rapidly adapting. When temp decreases, freq of action potentials in axons associated with cold receptors increases sharply then decliens gradually. Axons associated with warm receptors show freqency of action potentials decreasing sharply when the temp is dropping and increasing rapidly when the temp returns to its relative resting state Nociceptors Free nerve endings Three types: mechanical, thermal and polymodal (histamine, prostaglandings) Somatosensory Cortex Perception of somatic sensations begin in the primary somatosensory cortex Columnar organization: one column-one modality (pressure, vibration, cold etc) Somatosensory Pathways Spinothalamic tract: pain & temp Receptors going into the dorsal horn into the CNS and synapsing with the 2nd order neuron which then travels up to the thalamus to synapse with the third order neuron Dorsal column-medial lemniscal pathway: proprioceptors & mechanoceptors First order neuron travels to the dorsal horn, there's a reflex and a collateral that synapses with the second order neuron in the medulla (crosses to the other side) and a third order neuron in the thalamus Main difference is where the 2nd order neurons are. Pain Produced by tissue damaging or potentially tissue damaging stimulus What type of responses does the pain response elicit? Reflexes Autonomic responses- inc in bp, hr, blood epiephrine, glucose, dilation of pupils Fast pain: A fibers, sharp, pricking sensation, easily localized More myelinated
