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Module 7 - Sensory

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Physiology 2130
Paul Gillespie

Sensory System For homeostasis to function properly it is necessary for the body to detect changes in the external environment so it can react appropriately in order to maintain its internal environment. The human body has several sensory systems that allow it to detect these external changes rapidly. These systems include:  The somatosensory (touch) system  The visual system  The auditory and vestibular system  The olfactory (smell) system  The gustatory (taste) system Transduction of Environmental Information It is how information from the external environment is turned into language the brain understands  action potentials Environmental stimuli (ES) - It is energy like light, heat, touch, or sound - This energy must be detected by sensory receptors which then convert the information into action potentials - The stimuli comes in different forms and, therefore, will require different receptors to detect the stimulus and then convert it to action potentials - Example: o A Mechanical stimulus will stretch the sensory receptors in the skin and open ion channels, causing a depolarization of the sensory neuron producing an action potential o A Chemical stimulus binds with a receptor, causing depolarization and then an action potential o A Light energy is absorbed by photoreceptors of the eye (rods and cones in the retina) and eventually produces action potentials o A Gravity and motion type of stimuli can also be detected by hair cells in the vestibular system, which convert this form of external stimulus to action potentials - Some receptors can detect more than one type of stimulus - Adequate stimulus – ES and most sensitive sensory receptor o The adequate stimulus for the rod and cone cells found in the retina of the eye is light o Example: If a pencil tip touches a point on your hand, it provides adequate stimulus for the activation of the cold receptor. - Sensory receptors do respond to other forms of energy but not in an optimal way - Example: the rod and cone cells of the eye also respond to pressure on the eyeball Receptors (Generator) Potentials Once the sensory receptor is stimulated by an environmental stimulus, it will cause a change in ion permeability, leading to a local depolarization This local depolarization is called a generator or receptor potential (RP) - Since the receptor does not have voltage-gated ion channels necessary to fire an AP, the RP must spread to an area on the sensory neuron that does contain these channels - This is usually at the first node of Ranvier on the axon - The AP will then be generated and propagated along the axon and into the spinal cord - In receptors with no axons (like hear cells in the inner ear), the depolarization has to spread to the synapse to result in the release of a neurotransmitter - RPs are similar to EPSPs and IPSPs and share some of the same characteristic: o They are generally depolarizing but can by hyperpolarizing as well o They are caused by an increase in permeability to sodium ions or potassium ions in the cause of a hyperpolarizing stimulus o They are local and do not prorogate down the neuron like an AP but spread like an EPSP, decreasing with time and distance from the stimulus o They are proportional to the strength of the stimulus – the stronger the stimulus, the large the RP and the more likely to fire an AP - Receptor Potential and Neural Coding o Neural coding informed the brain of the weight of an object in your hand o The heavier weight will trigger the receptors to produce a large RP o This large RP will trigger many APs on the sensory neurons axon o This burst of high-frequency AP will eventually reach the brain where you become consciously aware of the heavier weight in your hand The Somatosensory System The Somatosensory system detects and processes the sensations of touch, vibration, temperature, and pain (majority of which originate in the skin) Detecting each sensation requires several different sensory receptors within the skin, each developed to detect its adequate stimulus The receptors in the skin are collectively referred to as cutaneous receptors. They include the following: 1. Hair Follicle Receptors – sensitive to fine touch and vibration 2. Free Nerve Endings – respond to pain and temperature (hot and cold) 3. Meissner’s corpuscles – detect low-frequency vibrations (between 30 and 40 cycles/sec) and touch 4. Ruffini’s corpuscles – detect touch 5. Pacinian corpuscle – detect high-frequency vibrations (250 to 300 cycles/sec) and touch Receptive Field This is an area on the surface of the skin where an adequate stimulus will activate a particular receptor to fire an AP in the neuron Any stimulus applied outside the receptor field will not generate an AP After AP 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 APs reach the brain via two spinal tracts:  Spinothalamic (Anterolateral) Tract o Tranmits information dealing with very basic sensations like pain, temperature and crude touch o The information from the sensory neuron (first order neuron) enters the spinal cord where is synapses with a second order neuron o The second order neuron crosses to the opposite or contralateral side of the spinal cord and ascends to a region of the brain called the thalamus o The thalamus acts as a relay station for almost all sensory information (except smell) o A second synapse with a third order neuron occurs here and then travels to the somatosensory cortex o Note: Important to realize that sensory information from the right side of the body goes to the left side of the brain  Dorsal Column, Medial Lemniscal System o Transmit information associated with the more advanced sensations of fine detailed touch, proprioception (muscle sense), and vibration o The information from the sensory neuron (first order neuron) enters the spinal cord and immediately travels up the spinal cord because crossing to the contralateral side (unlike the spinothalamic system) o 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 o From here it continues to the thalamus where it synapses again onto a third order neuron that then travels to the somatosensory cortex Primary Somatosensory Cortex Once the sensory information has reached the brain, it travels to the primary somatosensory cortex, which is located in the parietal lobe on the postcentral gyrus behind the central sulcus  The Somatosenory Homunculus o Arranged in a very specific manner o The sensory information arriving at this cortex is not randomly scattered around on the surface; rather, it is geographically preserved o It is as if the entire body were projected onto the surface of the brain like a map o All the sensory information for the foot is located in one area-that of the leg just next to it and the hip next to the leg, and so on-for the entire body o This topographical representation of the body on the surface of the cortex is called the somatosensory homunculus o Some of the areas of the human body take up more space in the somatosensory homunculus o Areas dealing with the hand, tongue, and lips, receive more sensory information and require more of the brain to process that information o The hands, tongue, and lips are the most sensitive parts of the body; they contain many more sensory receptors than any other part The Visual System The visual system detects light, converts it into APs, and sends these to the primary visual areas for processing Once processed, we become aware of our visual world and are able to distinguish and recognize features in our external environment The visual system consists of: - Eye  contains photoreceptors that convert the light to APs - Visual Pathway  transmits the APs - Primary Visual Area  in the occipital lobe of the brain (which processes the incoming signals)  The Eye - After passing through the cornea, the amount of light is regulated by the iris, which can constrict with bright light or dilate in low light - The lens flips the light (upside down and backwards) and focuses it onto the retina at the back of the eye - The retina contains photoreceptors called rods and cones - The rods and cones actually point toward the back of the head - The center of your vision is focused onto a part of the retina called the fovea. This area has the highest [ ] of cone cells  The Photoreceptors of the Eye – Rod Cells and Cone Cells - Rods o Are extremely sensitive to light and therefore functions best under low light conditions o They contain one type of photopigment (a chemical sensitive to light), and do not detect color o Rods are located mostly in the region of the retina outside and around the fovea - Cones o Functions best under bright light and are ideal for detecting detail o There are three different types of cone cells, each with a different photopigment and each sensitive to one primary color o The cones are principally located in the region of the fovea where they are found in large [ ] - Rods and cones do not have axons and therefore do not generate APs - Generate RPs that cause the release of an 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 retine include: o Bipolar cells o Ganglion cells o Horizontal cells o Amacrine cells - These other cells are responsible for the integration of information from the rods and cones and the production of APs  Transduction of Light to Action Potentials - The visual system works “backwards” - When depolarized, the rod and cone cells release an inhibitory neurotra
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