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Physiology Chapter 7 – Notes.docx

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Department
Physiology
Course
Physiology 2130
Professor
Anita Woods
Semester
Winter

Description
Physiology Chapter 7 – Notes Objectives: - Sensory receptor and adequate stimulus - Generator potentials (4 characteristics) - Receptors o Touch o Vibration o Temperature o Pain o Proprioception (limb position and movement) - Receptive field of neuron - Ascending sensory pathways - Somatic organization on the postcentral gyrus (somatosensory area) going from medial to lateral on the cortex - Visual system (eye) - Retina cell types (arrangements) - Rods and Cones - Light  Action potentials (steps) - Eye movements (4 types) and function - Auditory system - Outer and middle ear transmit pressure waves from air to fluid (3 ways) - Different frequencies of sound  AP’s - Semicircular canal (hair cells, cupula, utricle, saccule with otoliths) - Vestibular system - Movement detection (1 by semicircular canal, 2 by otolith organs) - Angular motion of head  AP’s Intro - Changes to the Sensory System - Homeostasis o Detecting changes in external environment and adapting - Several sensory systems: o Somatosensory (touch) o Visual system o Auditory and vestibular (sense of balance) o Olfactory (smell) o Gustatory (taste) Transduction of Environmental Information - How information from the external environment is turned into language understood by brain - External stimuli (energy) – heat, light, sound, touch are detected by sensory receptors - Convert into Action Potentials Environmental Stimuli - Must be detected by sensory receptor - Example: mechanical stimulus o Touch or vibration causes sensory receptors in the skin to stretch o Open ion channels – causing depolarization – Action Potential - Chemical Stimulus o Sour taste binds with receptor on tongue causing depolarization – AP - Light Energy o Absorbed through photoreceptors (rods and cones) - Gravity and motion stimuli o Detected by hair cells in the vestibular system – AP’s Adequate Stimulus for the receptor - Some receptors detect more than one kind of stimuli - Adequate stimulus o Environmental stimulus to which the sensory receptor is MOST sensitive - Example: for rods and cones, the adequate stimulus is light in the retina - May respond to other forms of energy, but not optimally Receptor (generator) Potentials - Recall that at a chemical synapse an excitatory neurotransmitter first produces an EPSP that, if strong enough, then generates an action potential at the axon hillock. This is similar to the events that take place at a sensory receptor - Sensory receptor is stimulated by environmental stimuli, changes ion permeability, causing local depolarization - Called Generator or Receptor Potential - Doesn’t have voltage gated ion channels, so it must spread to an area on the sensory neuron that does contain these channels - At the first node of ranvier on the axon - The AP is then generated and propagated up the spinal cord - If the receptors do NOT have axons (e.g. hair cells in inner ear), the depolarization has to be spread to the synapse in order to release neurotransmitter - Receptor potentials are similar to EPSPs and IPSPs and share some of the same characteristics 1. Are generally depolarizing but can be hyperpolarizing 2. Caused by increase in permeability to Na+ or K+ 3. LOCAL, do not propagate down neuron like AP,but spread like EPSP decreasing with time and distance from the stimulus 4. Are proportional to the strength of the stimulus – stronger the stimulus, the larger the receptor potential and the more likely to fire an AP Receptor Potential and Neural Coding - Neural coding informs the brain of the weight of an object in your hands for example - Heavier the object, the more action potentials fired per second - Heavier weight causes larger receptor potential, which triggers more AP’s - Burst of high frequency AP’s will eventually reach brain and you will be consciously aware of the heavier object Somatosensory System - Detects and processes: Touch, vibration, temperature and pain - Majority originate in skin - Accomplished through different receptors in skin, referred to as cutaneous receptors 1. Hair follicle receptor – fine touch and vibration 2. Free nerve ending – pain and temperature 3. Meissner’s corpuscle – low frequency vibrations (30-40 cycles/sec) and touch 4. Ruffini’s corpuscles – Touch 5. Pacinian corpuscles – High frequency vibrations (350-300 cycles/sec) and touch Receptive Field - Each receptor only responds to stimulus with certain region on the skin - Receptive field is the area on the surface of the skin where an adequate stimulus will activate a particular receptor to fire an AP in the neuron - If the stimulus is within the area then the AP will be propagated to the brain along one of 2 spinal tracts Somatosensory Pathways from the Periphery to the Brain: The Spinothalmic (Anterolateral) Tract - The spinothalmic (Anterolateral) tract deals with simpler sensations: o Pain, temperature, crude touch - The information from the sensory neuron (first order neuron) enters the spinal cord where it synapses with a second order neuron. - This neuron crosses to the opposite or contralateral side of the spinal cord and ascends to a region of the brain called the thalamus. - The thalamus acts as a relay station for almost all sensory information (except smell). - A second synapse with a third order neuron occurs here and then travels to the somatosensory cortex. - Sensory info from the right side of the body goes to the left side of the brain and vice versa Somatosensory Pathways rom the Periphery to the Brain: Dorsal Column, Medial Lemniscal System - Transmit info associated with the more advanced sensations: o Fine detailed touch, proprioception (muscle sense), and vibration - The information from the sensory neuron (first order neuron) enters the spinal cord and immediately travels up the spinal cord before crossing to the contralateral side (unlike the spinothalamic system). - In the upper spinal cord, the sensory neuron synapses with a second order neuron which then crosses to the opposite side of the spinal cord. - From here it continues to the thalamus where it synapses again onto a third order neuron that then travels to the somatosensory cortex. - Again going from right side of the body to left side of brain and vice versa Primary Somatosensory Cortex - When sensory info reaches the brain, it travels to the primary somatosensory cortex (in the parietal lobe) - On the postcentral gyrus, behind the central sulcus - Arranged by specific manner - The sensory info arriving at this cortex is not randomly scattered, its “geographically preserved” - Each area is associated with a different part of the body - The topographical representation of the body on the surface of the cortex is called the Somatosensory homonuculus - Different areas of the cortex are not proportional - Because areas dealing with certain sensory info require more brain power to analyze (e.g. hands and lips receive more sensory information) Visual System - Detects light, converts to AP’s, sends to primary visual areas for processing - Then we become aware of our visual world - Consists of: o Eye - photoreceptors convert light to AP’s o Visual pathway – transmit AP’s o Primary visual area (in the occipital lobe) – process incoming signals The Eye - Light passes through cornea - Iris – regulates amount of light let through (constricting and dilating) - Lens – flips the light upside down and backwards - Retina – contains photoreceptors (rods and cones) o Rods and cones point towards back of the head o Center of vision focused onto part of retina called the fovea (highest concentration of cone cells) Photoreceptors of the Eye – Rod cells and Cone cells Rods: - Sensitive to light - Function best under low light conditions - Contain 1 type of photopigment (chemical sensitive to light) and therefore do not detect colour - Mostly found in region of retina outside and around the Fovea Cones: - Function best under high light conditions - For detecting detail - 3 types of cones, each with different photopigment sensitive to different primary colour - Located in fovea region in large concentrations Rods and Cones: - Rods and cones do not have axons, and don’t generate Action Potentials - But, they do generate receptor potentials that cause the release of an inhibitory neurotransmitter from their synaptic ending.*** Other Cells of the Retina - Retina contains pigmen
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