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Module 7 - Sensory Systems.docx

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Physiology 2130
Anita Woods

Module 7: Sensory Systems 7.1 – Objectives By the end of this section, you should be able to:  Define a sensory receptor and its adequate stimulus.  List four characteristics of generator potentials.  List the receptors responsible for touch, vibration, temperature, pain, and proprioception (limb position and movement).  Define receptive field of a neuron. Name the two major ascending sensory pathways and describe their anatomy and the information they carry.  List the somatotopic organization on the postcentral gyrus (somatosensory area), going from medial to lateral on the cortex.  Draw and label a picture of the visual system and the eye.  List the cell types in the retina and draw a diagram of their anatomical arrangement.  List the functional characteristics of the rod and cone systems.  Draw a flow diagram of the sequence of steps in the retina by which light is transduced to action potentials.  List four types of eye movements, describe when they occur, and describe their overall function.  Draw a simple diagram of the auditory system.  List three ways in which the outer and middle ear act to transmit pressure waves from air to fluid.  Describe how different frequencies of sound are transduced into action potentials.  Draw a simple diagram of a single semicircular canal with hair cells and cupula and the utricle and saccule with otoliths.  List the major functions served by the vestibular system.  Name the movement detected by the semicircular canal receptors and the two detected by the otolith organs.  Describe how angular motion of the head is transduced into action potentials. 7.2 – Introduction: Changes to the Sensory System 7.3 – Transduction of Environmental Information  Transduction of Environmental Information: How information from the external environment is turned into language the brain understands (action potentials).  In order for the brain to know what is happening outside the body, Environmental Stimuli (energy) like light, heat, touch, or sound must first be detected by the Sensory Receptors which then convert the information into action potentials. 7.4 – Environmental Stimuli  In order for the brain to consciously perceive an Environmental Stimulus, that stimulus must be detected by a sensory receptor.  Environmental stimulus come in different forms, and therefore, will require different receptors to detect the stimulus and then convert it to action potentials.  For example, a mechanical stimulus (touching skin) will stretch the sensory receptors in the skin and open ion channels, causing a depolarization of the sensory neuron producing an action potential.  A chemical stimulus, like a sour taste on the tongue, or odor in the nose, binds with a receptor, causing a depolarization and then an action potential.  Light Energy is absorbed by Photoreceptors of the eye (rods and cones in the retina) and eventually produces action potentials.  Gravity and Motion can also be detected by hair cells in the vestibular system, which convert this form of external stimulus to action potentials. 7.5 – Adequate Stimulus for the Receptor  Some receptors can detect more than one type of stimulus  An Adequate Stimulus is the particular forms of environmental stimulus to which the sensory receptor is most sensitive.  The adequate stimulus for the rod and cone cells found in the retina of the eye is light.  Sensory receptors do respond do respond to other forms of energy but not in an optimal way.  For example, the rod and cone cells of the eye also respond to pressure on the eyeball. 7.6 – Receptor (Generator) Potentials  Once the sensory receptor is stimulated by an eviornmental stimulus, it will cause a change in ion permeability, leading to a local depolarization.  This local depolarization is called a Generator or Receptor Potential.  Since the receptor does not have voltage-gated ion channels nesscary to fire an action potential, the receptor potential 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 action potential will then be generated and propagated along the axon and into the spinal cord.  In receptors with no axons (like the hair cells in the inner ear), the depolarization has to spread to the synapse to result the release of neurotransmitter.  Receptor potentials are similar to EPSPs and IPSPs and share some of the same characteristics. 7.9 - The Somatosensory System  This system detects and processes the sensations of touch, vibration, temperature, and pain (originate from 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, such as: 1. Hair Follicle: Receptors that are receptive 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 (b/w 30-50 cycles per sec) and touch 4. Ruffini’s Corpuscles: Detect Touch 5. Pacinian Corpuscles: Detect High-Frequency vibrations (250-300 cycles/sec) and touch 7.10 – Receptive Field  The receptive field is the area on the surface of the skin where an adequate stimulus will activate a particular receptor to fire an action potential in the neuron.  Any stimulus applied outside the receptor field 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 rain so that the individual becomes consciously aware of the stimulus.  These action potentials reach the brain via 2 spinal tracts. 7.11 – Somatosensory Pathways from the Periphery to the Brain: Spinothalamic (Anterolateral) Tract  The spinothalamic tract transmits information dealing with very basic sensations like pain, temperature, and crude touch.  The information from the sensor 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. NOTE: It is important to realize that sensory information from the RIGHT side of the body goes to the LEFT side of the brain and vice versa 7.12 Somatosensory Pathways from the Periphery to the Brain: 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 vibrations.  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. NOTE: It is important to realize that sensory information from the RIGHT side of the body goes to the LEFT side of the brain and vice versa 7.13 – 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. 7.14 Primary Somatosensory Cortex – The Somatosensory Homunculus  The primary somatosensory cortex is arranged in a very specific manner.  The sensory information is arriving at this cortex is NOT randomly scattered around on the surface; rather, it is “Geographically Preserved”.  It is as if the entire body were projected onto the surface of the brain like a map.  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).  This topographical representation of the body on the surface of the cortex i s called the Somatosensory Homunculus.  Some areas on the cortex, like the areas dealing with the hands, tongue, and lips, receive more sensory information and require more of the brain to process that information.  The hands, tongue, and lips are the most sensitive parts of the body; they contain many more sensory receptors than any other part. 7.16 – The Visual System  The visual system consists of the eye (which contains photoreceptors that convert the light to action potentials), the visual pathway (Which transmits the action potentials), and the Primary Visual Area in the Occipital Lobe of the brain (processes incoming signals). 7.17 – 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 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 point towards back of the head. The centre of your vision is focused on to a part of the retina called the fovea. This area has the highest concentration of cone cells. 7.18 – The Photoreceptors of the Eye: Rod Cells and Cone Cells  Rods are extremely sensitive to light and, therefore, function best under low light conditions.  They contain one type of photopigment (chemically sensitive to light) and consequently, do not detect color.  Rods are located mostly in the region of the retina outside and around the fovea.  Cones on the other hand, function best under bright light and are ideal for detecting detail.  There are 3 different types of cone cells, each with a different photopigment and each sensitive to one primary color.  The cones are principally lcated in the region of the fovea where they are found in large concentrations. NOTE: The rods and cones do NOT have axons, and therefore, do NOT generate action potentials. However, they do generate receptor potentials that cause the release of an Inhibitory Neurotransmitter (IMPORTANT) from their synaptic ending. 7.19 – 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
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