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Physiology 2130
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Physiology 2130 Module 7 Online Notes (Sec 7.1 to 7.45) Sensory Systems Introduction  The human body has several sensory systems that allow it to detect external changes rapidly, which include: o Somatosensory (touch) o Visual System o Auditory System o Vestibular System o Olfactory System (Smell) o Gustatory System (Taste) Transduction of Environmental Information  Transduction of environmental information is how information from the external environment is turned into action potentials (brain’s language)  Environmental stimuli (energy) like light, heat, touch, and sound must be detected by sensory receptors which convert info into action potentials Environmental Stimuli  For the brain to consciously perceive environmental stimulus, the stimulus must be detected by a sensory receptor  These stimuli come in different forms, and therefor require different receptors to detect the stimulus and convert it into action potentials  Ex. Mechanical stimulus (touching) will stretch sensory receptors in the skin and open ion channels causing a depolarization of the sensory neuron which will produce an action potential  A chemical stimulus, like a sour taste on the tongue, binds with a receptor and causes a depolarization and then an action potential  Light energy is absorbed by photoreceptors of the eye (rods and cones in retina) and eventually produces action potentials  Gravity and motion is detected by hair cells in vestibular system, which concert this form of external stimulus to action potentials Adequate Stimulus for the Receptor  Some receptors can detect more than one type of stimulus  An adequate stimulus is the particular form of environmental stimulus to which the sensory receptor is most sensitive  The adequate stimulus for the rod and cone cells for example is light  Sensory receptors do respond to other forms of energy, but not optimally  Ex. The rods and cones also respond to pressure on the eyeball Receptor (Generator) Potentials  Remember that at a chemical synapse, an excitatory neurotransmitter produces an EPSP that if strong enough, then generates an action potential at the axon hillock, well the events at the sensory receptor are similar  Once the sensory receptor is stimulated, it causes a change in ion permeability leading to local depolarization  This is called a generator or receptor potential  Since the receptor does not have voltage-gated ion channels necessary to fire an action potential, the receptor potential must spread to an area on the sensory neuron that does contain these channels  This is often at the first node of Ranvier on the axon  The action potential is then generated and propagated along the axon and into the spinal cord  In receptors with no axon, the depolarization has to spread to the synapse to result in the release of a neurotransmitter (ex. Hair cells in inner ear) The Somatosensory System  The somatosensory system detects and processes the sensations of touch, vibration, temperature, and pain – mainly originate in the skin  Detecting each sensation requires various sensory receptors within the skin, each developed to detect its adequate stimulus  These receptors are collectively referred to as cutaneous receptors, including: 1. Hair follicle receptors that are sensitive to fine touch and vibration 2. Free nerve endings that respond to pain and temperature (hot and cold) 3. Meissner's corpuscles that detect low-frequency vibrations (between 30 and 40 cycles/sec) and touch 4. Ruffini's corpuscles that detect touch 5. Pacinian corpuscles that detect high-frequency vibrations (250 to 300 cycles/sec) and touch Receptive Field  Although it may seem that our skin is covered with sensory receptors, each receptor only responds to a stimulus within a certain region on the skin  The receptive field is the area on the surface of the skin where an adequate stimulus activates a particular receptor to fire an action potential in neuron  The receptive field, when touched generates an action potential  When a stimulus is applied outside the receptor field it will not generate an action potential  Now that action potentials have been generated in the sensory nerve, they must be propagated to a specific area of the brain so that the individual becomes consciously aware of the stimulus. These action potentials reach the brain via two spinal tracts Somatosensory Pathways from the Periphery to the Brain The Spinothalamic (Anterolateral) Tract  Spinothalamic tract transmits info dealing with very basic sensations such as pain, temperature, and crude touch  The info from the sensory 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 the thalamus  The thalamus acts as a relay station for most sensory info (except smell)  A second synapse, with a third order neuron occurs here and travels to the somatosensory cortex Dorsal Column, Medial Lemniscal System The dorsal column, medial lemniscal system transmits information associated with the more advanced sensations of fine detailed touch, proprioception (muscle sense), and vibration.  Information from the sensory neuron enters the spinal cord and immediately travels up the spinal cord before crossing to the contralateral side (unlike Spinothalamic system)  In 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 goes to the thalamus where it synapses again onto a third order neuron that then travels to the somatosensory cortex  Sensory info from right side of body, left side of brain, and vice versa Primary Somatosensory Cortex  Once the sensory info has reached the brain, it travels to the primary somatosensory cortex, located in the parietal lobe on the postcentral gyrus behind the central sulcus The Somatosensory Homunculus  Primary somatosensory cortex is arranged in a very specific manner  Sensory info arriving at this cortex is not randomly scattered on the surface but is “geographically preserved”  It is as if the entire body were projected onto the surface of the brain like a map  All sensory info for foot located in one area, that of the leg just next to it, the hip next to the leg, and so on for the entire body  This topographical representation of the body on the surface of the cortex is called the somatosensory homunculus  The picture of the human body represented in the homunculus is somewhat out of scale, some of the representative areas are out of proportion  This is because some areas on the cortex, like areas dealing with the hands, tongue and lips are most sensitive, they contain more sensory receptors than any other part  When you really want to experience how something feels you use your hands The Visual System  The visual system detects light, converts it into action potentials, and sends them to the primary visual areas for processing  After being processes, we become aware of our visual world and are able to distinguish and recognize features in the external environment  The visual system consists of o The eye (contains photoreceptors that convert the light to action potentials o The Visual Pathway (transmits action potentials) o Primary visual area in the occipital lobe of the brain (processes incoming signals The Eye  Eye is a lot like a camera, after passing through the cornea, amount of light is regulated by the iris, which can constrict in bright light or dilate in low light  The lens flips the light (upside down and backwards) and focuses it onto the retina and the back of the eye  The retina contains photoreceptors called rods and cones  Rods and cones actually point toward the back of the head  The center of one’s vision is focused onto a part of the retina called the fovea  This area has the highest concentration of cone cells The Photoreceptors of the Eye - Rod Cells and Cone Cells  Rods are extremely sensitive to light, function best in low light conditions  They contain one photopigment (chemical sensitive to light) and do not detect colour, they are located mostly in the region of the retina outside and around the fovea  Meanwhile, cones function best under bright light and are ideal for detecting detail  There are three different types of cone cells, each with a different photopigment and sensitive to one primary colour  The cones are principally located in the region of the fovea where they are found in large concentrations  Rod and cone cells do not have axons, and do not generate action potentials!  But, they do generate receptor potentials, which cause the release of inhibitory neurotransmitter, from their synaptic ending Other Cells of the Retina  The retina contains a pigment layer at the very back of the eye that absorbs excess light  Other cells in the retina include: o Bipolar cells o Ganglion cells o Horizontal cells o Amacrine cells  These other cells are responsible for the integration of info from the rods and cones and the production of action potentials Transduction of Light to Action Potentials  The visual system words backwards, the light striking the retina is flipped upside down and backwards due to the lens  When depolarized, the rod and cone cells release an inhibitory neurotransmitter, shutting off the bipolar cells  When light strikes the retina it does not excite and depolarize the rod and co
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