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Chapter 3

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PSYC 212
Evan Balaban

PSYC CHAPTER 3 The Somatosensory System: Touch, Feeling, and Pain · Experience of touch can be broken down into two fundamental features: 1. We are aware of the point on our body where the interaction occurred with an object in the physical world –this localization feature forms the objective aspect of touch –asks “where was I touched” 2. The subjective feeling of touch that allows for the classification and identification of the object accompanies this –asks ”what did I touch and how did it feel”? o Subjective experience of touch = more highly developed than objective · Another distinctive feature of touch is that the sensations arise from receptors that are distributed throughout the entire body rather than in discrete sensory organs · Although somatosensory perceptions are associated with touch and pain, there are specialized receptors within muscles and joints that provide a further sensory dimension that is known as proprioception o Proprioception mechanisms allow us to be aware of the location of our limbs in space The Neural Basis of Somatosensory Perception · Two main ideas have been proposed as to how touch is processed: 1. The theory of receptor specificity –individual receptors and nerve endings are selectively sensitive to a particular form of energy impinging on the skin –each type of receptor not only produces a particular touch sensation, but also indicates where on the skin surface the stimulation occurred –Max von Frey furthered this concept by stating that there are specific receptors for heat, cold, pressure, and pain. 2. The Pattern Theory –the specificity of touch sensations believe to arise from the overall pattern of activity across a broad spectrum of receptors –different types of touch generate different patterns of activity and somehow the brain is able to distinguish those patterns to produce the different touch perceptions · Touch signals are highly specific and selectively generated by a particular type of receptor –these receptors are known as mechanoreceptors and are present in the skin throughout the body · Most forms of touch usually involve stimulation of multiple types of receptors The Skin and its Receptors · Skin covers the entire body, has surface area of about 15 square feet, and weighs about 10 pounds · Highly complex organ composed of tiny glands, a rich network of blood vessels, and sensory receptors all interwoven within a matrix of epithelial cells and connective tissue · Skin tissue can be anatomically divided into two divisions: 1. The epidermis, which is the outermost layer –serves as a protective shield and is composed of several sub layers that are constantly being replenished 2. The dermis is the underlying layer which makes up the bulk of skin tissue and contains most of the mechanoreceptors and nerve endings that generate touch sensations · There are two types of skin tissue, which each having its own structural makeup and mechanoreceptor composition –the structural differences impart different mechanical properties that affect the elasticity and resilience of skin that in turn affects the sensation of touch 1. Hairy skin –contains hairs and covers most of the body 2. Hairless skin –found only in certain distinct body parts i.e. palms of hands, lips, etc. Type of Mechanoreceptors · The human skin contains a variety of mechanoreceptors, which can be classified into three types: 1. Encapsulated receptors –specialized capsule that surrounds a nerve ending –the Pacinian, Meissner, and Ruffini corpuscles are examples of this type of receptor –the capsule itself serves specific functions that are ideally suited to the kinds of mechanical stimulation captured by the particular receptor a. The Pacinian – acts a mechanical filter, which aids in the transmission of vibrational stimuli b. Meissner –transmitting light touch c. Ruffini –transmitting steady pressure 2. Receptors with accessory structures –composed of a sensory nerve fiber in conjunction with a separate accessory structure a. Merkel disc –type of mechanoreceptor where the sensory nerve ending is associated with a type of epithelial cell –responsible for the detection of light touch to the skin b. Lanceolate endings –found in hairy skin where they run along the hair shaft – therefore triggered by the movement of the hair follicles 3. Free Nerve ending receptors –made of a various types of free nerve endings that do not have any specialized terminal structures or other associations –responsible for detecting thermal changes, such as warmth and cold, as well as pain –the pain receptors, also called nociceptors, can be further subdivided on the types of pain that are detected · The distribution of these different types of mechanoreceptors can vary with skin type o The receptors are found either in superficial skin, near the interface of the dermis and epidermis, or more deeply in the dermis and even below it o Superficial receptors found in hairy skin include: Merkel discs, hair receptors, and free nerve endings o Both Pacinian and Ruffini corpuscles are found in the deep zone of hairy and hairless skin Signal transduction in mechanoreceptors · When a mechanical stimulus is applied to the skin, the pressure is ultimately transmitted to the receptor itself where the strain produces the openings of Na channels o The resulting movement of Na into the mechanoreceptors will result in a depolarization of the receptor potential –if this potential is large enough, it will cause action potentials to be generated Mechanoreceptors are terminals of modified bipolar neurons · The vast majority of neurons in the brain have a multipolar structure –Mechanoreceptors, however, have a bipolar structure, in which a single axon and a single dendrite are connected on opposite ends of the soma –at some point, the two processes fuse and produce a single emergent fiber that quickly spits into two just outside the cell body o The cell body, which is located in the dorsal route ganglion, gives off two processes: one that carries signals from the skin mechanoreceptor and another that transmits signals to the spinal cord · One of the fibers is called the peripheral branch and proceeds to the skin where it ends up either as free nerve endings or in some kind of encapsulated form · The other fiber is called the central branch and carries touch signals to the spinal cord · The cell body is actually situated quite far away from the skin in a structure called the dorsal root ganglion (DRG), which is located right next to the spinal cord · The flow of sensory signals that arise from tactile stimulation is as follows: o Touch stimulation produces a depolarizing receptor potential in the terminal end of the peripheral branch (i.e. the mechanoreceptor), the only site in the dorsal root ganglion neuron that is sensitive to stimulus energy · What makes the dorsal route ganglion neuron unusual is that once the receptor potential reaches threshold, an action potential is generated in the immediate vicinity of the end organ itself –thus, DGR neurons are capable of producing action potentials right near the terminal end of the peripheral branch –these action potentials then flow along the peripheral and central branch to reach the spinal cord Afferent Fibers · The nature of the sensory response to tactile stimulus and the speech which it is transmitted are determined by the afferent fibers · DRG neurons come in many varieties, each suited to a particular role in tactile sensation · DRG neurons can be broadly classified into two groups based on the size of the cell body and the diameter of its fibers · The afferent fibers can be either myelinated or unmyelinated · These anatomical features, along with the precise type of mechanoreceptor residing at the terminal end, determine the nature of the physiological response to touch Transmission Speed · The speed at which afferent fibers conduct action potentials is determined by the diameter of the fiber and the degree of myelination · The larger the diameter, and the greater the degree of myelination, the faster the nerve signal transmission · Sensory and motor nerve fibers have been designated by a lettering system –Aα, Aβ, Aδ, and C. o The latter three are associated with mechanoreceptors in the skin, the bulk being of the Aβ type –this fiber type is moderately myelinated and has a relatively large diameter compared to others, thus making it a fast-conducting fiber o Both the Aδ and C fibers are thinner and contain either little or no myelination thus the transmission speed is much less · The vast majority of skin mechanoreceptors are linked to fact-conducting afferent fibers · All of the encapsulated receptors are linked to Aβ Response Adaptation · Responses of tactile afferent fibers can be placed in one of two classes in terms of how they adapt to ongoing or continuous stimulation · Touch signals can be either sustained or transient in nature –terms applied to this finding are “slowly adapting (SA)” and “fast adapting (FA)” o SA-type fibers continue to generate signals as long as the stimulus is present where as FA fibers only produce a neural response at the beginning and end of skin indentation · Response adaptation refers to how the fibers respond to continuous touch stimulation · Afferent fibers associated with Merkel and Ruffini receptors show SA-type responses, whereas those linked to Meissner and Pacinian receptors show FA responses –these are further subdivided on the basis of the receptors’ location in the skin o Those receptors located in superficial areas are of type I (SA-I or FA-I), whereas receptors located in deep skin are of type II (SA-II or FA-II) · The transient and sustained nature of the sensory response in tactile afferent fibers plays a crucial role in the kinds of touch stimuli that they can effectively transmit to the CNS · The receptors with SA-type responses have low temporal resolution and thus are best able to transmit tactile information that does not change with time (e.g., steady pressure) · The receptors with FA-type responses have much higher temporal resolution and are optimal detectors of physical stimulation that varies with time, such as vibration, motion, and flutter Receptive Field Size · The receptive field of a mechanoreceptor is determined by recording from its afferent fiber and surveying the area of skin that will generate electrical signals · Receptors located in the superficial part of the skin have small receptive fields whereas receptors in the deeper part generally have large fields o Both Pacinian and Ruffini receptors have large receptive fields whereas Meissner and Merkel receptors have small receptive fields with sharp borders · Small field receptors in the superficial skin are denoted as type I, where as the large field receptors are type II · Our ability to resolve the spatial details of tactile stimuli is determined by the size of the receptive field · The smaller the receptive field size, the greater our capacity for spatial resolution · The Markel and Meissner receptors have small receptive fields and are used by the somatosensory system to resolve fine spatial differences among various tactile stimuli · The Ruffini and Pacinian receptors have large receptive fields and thus can only detect coarse spatial differences Spinal Mechanisms and Signal Transfer · Mechanoreceptors are actually terminal end organs of sensory axons that arise from DRG neurons · The cell bodies of these neurons reside in structures that are located just outside the spinal cord · Each dorsal root ganglion gives off a fiber bundle that branches out to collect somatosensory information from that side of the body · Action potentials generated by tactile stimulations slow along the afferent fiber, cross over from the peripheral to the central branch, and thereafter enter the spinal chords Spinal Nerves and Ganglia · The spinal cord is the only channel for the transmission of somatosensory and motor information to and from the brain · The spinal cord in turn receives and distributes these signals thru a total of 31 pairs of spinal nerves that emerge out of the cord from top to bottom · Each spinal nerve is made up of two roots: the dorsal and ventral · Dorsal root fibers carry somatosensory signals from the periphery into the spinal cord, whereas ventral root fibers carry motor signals from the spinal cord to the muscles · The spinal cord is composed of white matter along the outer margins and a central, butterfly core of grey matter · The cell bodies of motor neurons are located in the ventral horn, whereas those of the somatosensory neurons are located in the dorsal root ganglion · Two fundamentally different types of signals are coordinated by the nervous system and transmitted by the same fiber bundle to and from the periphery –this basic principle regarding the separation of function in the dorsal and ventral roots is called the Bell- Megandie Law · Another important difference between the two roots of spinal nerves is the presence of the dorsal root ganglion and the absence of a similar collection of neurons in the ventral root –the dorsal root fibers enter the spinal cords, proceed thru the white matter, and terminate in the grey matter · Since there is no ventral route system, these neurons are located within the grey matter of the spinal corn itself in a part that is called the ventral horn Dermatomal Map · All somatosensory information is sent to the CNS thru spinal nerves and –in the case of the head, neck, and face –through a set of cranial nerves that emerge out of the brain stem · Entire body surface can be divided into discrete areas that are represent by a single nerve –each of these areas is called a dermatome · Dermatomal maps are valuable as a clinical tool in the event of injury or infection to a particular dorsal root Signal transmission thru the spinal cord · There is a serial relay of signals from site to site as the signals ascend the hierarchy of neural structures to ultimately reach the cerebral cortex · The different qualities of touch on are transmitted independently thru separate pathways · The route taken by afferent fibers after entering the spinal cord is largely determined by the type of fiber concerned and the kind of somatosensory information that is being transmitted · The large-diameter, myelinated fibers (Aα and Aβ) which carry tactile and proprioceptive signals, branch out soon after entering the spinal cord o Some branches proceed into the dorsal horn and synapse onto neurons there, whereas other branches take a sharp turn and proceed vertically up with the white matter of the spinal cord within fiber tracts contained in the dorsal column · Second order neurons within the dorsal horn either synapse onto other neurons locally, such as motor neurons in the ventral horn, or alternatively can send their projections up the dorsal columns as well · Almost half the dorsal column axons originate from these neurons with the remainder being branches of DRG neurons · The route taken by the small-diameter poorly or non myelinated fibers (Aδ or C) that carry pain or temperature signals is quite different –these fibers first synapse in the dorsal horn upon neurons that cross over to the other side of the spinal cord –once there, they enter in fiber tracts that are part of the anterolateral system, which transmits the signals to higher levels in the CNS Serial arrangement of relay nuclei · The somatosensory systems transmits signals to the cerebral cortex through a series of relay sites where synaptic transfer occurs onto neurons that in turn project to higher levels · The dorsal horn of the spinal cord itself is one such site of signal relay o The fibers of these neurons, as well as afferent fibers of DGR neurons, course upwards within the dorsal column to terminate in the nuclei located in the lower margin of the brains stem (medulla) o These so called dorsal column nuclei represent the second relay site · The axons of these neurons then arch over the midline to the other side of the brain and ascent to the thalamus in a fiber called the medial lemniscus · The fibers terminate in specific nuclei within the thalamus that serve as the next relay · The thalamic relay neurons project directly to the cerebral cortex through a fiber bundle known as the internal capsule · Signal relay in the anterolateral pathways that carry pain and temperature information is more complicated o The first relay here occurs in the dorsal horn from where the projections cross the midline and proceed up thru the anterolateral pathways on the opposite side of the spinals cord o Most of the axons terminate in relay nuclei that are distributed in three subcortical regions: the medulla, midbrain, and thalamus o The relay nuclei in the thalamus, which are different from the ones involved in the dorsal column-medial lemniscus pathway, project to the cortex in a diffuse manner Parallel processing and modality segregation · Two pathways –the dorsal column-medial lemniscus and anterolateral pathways · The dorsal column-medial lemniscus pathway transmits tactile signals that are gathered from mechanoreceptors linked to Aβ fibers o The signals present in these fibers can be either of SA or FA type and are triggered by steady or dynamic forms of touch · The anterolateral pathways transmits signals from the Aδ and C-type afferent fibers, carrying info on warmth, cold, and pain · This division of labor in signal transmission is not absolute · The presence of two parallel pathways permits independent transmission of different types of touch signals aka modality segregation Somatotopic organization · The projection fibers in both the dorsal column-medial lemniscus and anterolateral systems are precisely organized by function and body location –axons that enter at the lowermost levels of the spinal cord are found near the midline of the dorsal columns, with axons at successively higher levels being displaced more laterally · An orderly representation of the body surface is contained in the fiber tracts that transmit somatosensory information to the brain –this so-called somatotopic representation is maintained in all structures along the way · The neural representation in these structures is such that neighboring neurons produce tactile signals from adjacent locations on the body surface The Somatosensory Cortex · Different aspects of sensation are coordinated thru different pathways that are arranged in a parallel fashion and that display different anatomical and physiological properties · The ascending fibers cross over to opposite side so that ultimately the left side of brain receives somatosensory signals from right side of body and vice versa · The afferent fibers are organized in a precise way within the ascending structures to produce an orderly representation of the body surface · Several cortical areas are involved in the processing of somatosensory signals · Cortical areas that process these signals are located in the parietal lobe and are known as the somatosensory cortex Cortical processing begins in area S-I · The primary cortical area that receive inputs from the thalamus thru the internal capsule are located posterior to the central sulcus · Primary somatosensory cortex begins in the floor of the central sulcus and extends up the posterior wall of the sulcus, onto the mound of brain tissue located adjacent to the sulcus (postcentral gyrus) o This area of the brain serves as the starting point for cortical processing of somatosensory information –referred to as the primary somatosensory cortex (area S-I) · Area S-I is actually a composed of four smaller areas that are distinguishable on anatomical grounds · The cortical region posterior to the central sulcus, according to Korbinina Brodmann’s criteria, was composed of three distinct subdivisions, which he named areas 1, 2, and 3 – area 3 was later divided into 3a and 3b Serial and parallel processing in the somatosensory cortex · The heaviest input occurs upon areas 3a and 3b –neurons in both these areas then project in a serial fashion to areas 1 and 2, thus forming a hierarchy of cortical modules within area S-I itself · The four anatomical subdivisions of area S-I are involved in basic processing of tactile and proprioceptive signals o Area 3a acts on proprioceptive signals arising from muscles and joints whereas area 3b processes tactile signals from the skin · Neurons in area 1 process signals from FA and SA-type mechanoreceptors in a separate manner · The serial arrangement of cortical area extends beyond area S-I as well –two other cortical areas that process somatosensory signals are area S-II, a small area located just below S-I, and the posterior parietal cortex · Although area S-11 receives a small projection from the thalamus, its function is largely dependent on the heavy projection it receives from area S-I · There are two areas in the region known as the posterior partietal cortex that are in turn dependent on signal processing in earlier somatosensory areas · Areas 5 and 7, which lie adjacent to area S-I represent modules that are next in the hierarchy of cortical areas · These areas represent higher-order sensory areas that integrate various aspects of somatosensory function The Somatotopic Map · Orderly representation of the body surface · Shows that the postcentral gyrus (area 1) contains a map of the entire body · Somatotopic map · Each of the anatomical subdivisions of S-1 actually has a separate and total body representation, often referred to as homunculus –found in each of areas 3a, 3b, 1 and 2. · The map is an orderly representation of the opposite half of the body –contralateral mapping · There is non-linearity in the representation Receptive Fields · Each neuron in the somatosensory cortex has a specific receptive field such that stimulation applied only in that location will activate the neuron · Receptive fields are small and simple in early cortical areas (3a and 3b), and become larger and more complex as we proceed up hierarchy to areas 1 and 2 · Receptive field size within each of these areas is highly variable and determined by its location in the somatotopic map · Cortical neurons that represent the extremities have among the largest receptive fields Columnar Organization · Vernon Mountcastle –showed how neurons in all six layers of somatosensory cortex actually respond to the same modality in any given area of S-I · The responses of different types of receptors are organized in columns that run vertically from the cortical surface to the white matter · Some columns are activated by FA-type fibers while others are activated by SA-type · Columnar organization is a basic structural principle of several cortical compartments Somatomotor Circuits · Two key sites of somatosensory and motor interaction that are critical for variety of functions: 1. The Spinal cord · The spinal cord represents the lowest level in the hierarchy of Somatomotor control · Neuronal circuits produce patterned output thru the motor neurons in the spinal ventral horn that results in a number of stereotypes reflex movements · Motor signals emerge out of spinal cord thru ventral routes and proceed thru the spinal nerves to innervate the muscles · The activity in the ventral horn motor neurons is modulated by interneurons, which in turn are controlled by two sources: (1) the descending fibers within the spinal cord that carry motor commands issued by the motor cortex and thus executed by same network of spinal neurons involved in reflex behavior; (2) somatosensory afferents from the dorsal roots –neural circuits are responsible for the reflex withdrawal when that we are familiar with when we come into contact with painful stimulus –even before we are aware, we withdraw body –speed at which we are able to do this suggests that there is little time for the signals to proceed to the cerebral cortex and make us aware that damage is being done before initiating the appropriate motor commands for withdrawal 2. The cortex · The primary motor cortex, which is located in the precentral gyrus and associated with areas located nearby, represents the highest level of motor control · Planning and coordination of complex voluntary movements · Signals passed thru relay sites where they are fine-tuned · Primary motor cortex receives large projection from all of somatosensory processing areas · Those somatosensory signals are represented as an organized, somatotopic map · The precentral gyrus contains a similar map, but one representing motor output to muscles over the entire body –this motor map shares many similarities with the somatotopic map · Advantage of having motor and somatotopic maps close together is that it permits a more precise registration in the connectivity –the somatosensory projections to the motor cortex organized in a homotopic fashion Perceptual Aspects of Tactile Sensation Intensity and Sensation · The area of the skin affected, the depth of skin indentation, and the rate at which the indentation occurs are important factors that affect the touch sensation · Threshold values can be different depending on the velocity of indentation Esthesiometers –past and present · Max von Frey –discovered that horse hairs tend to apply a single downward force that depends on the thickness and stiffness of the hair –created the first esthesiometer · Human skin is very sensitive to touch and small forces are sufficient to elicit sensation · More recent work uses nylon filaments –the Semmes-Weinstein esthesiometer is a set of 20 nylon fibers of different diameters attached to Plexiglas handles o Nylon fibers can be used to determine the minimal force needed to elicit sensation and thereby determine the absolute threshold of touch over various parts of the body Absolute thresholds · Absolute threshold of touch is very small · Threshold value can depend on a number of different factors · One of the most important factors that affects touch threshold is the actual site on the body where it is measured · Sidney Weinstein studied how absolute thresholds varied across the body using von Frey hairs o Found that facial regions around the mouth and nose are among the most sensitive to pressure whereas the extremities at the lower end of the body require relatively large forces to produce a touch sensation · The absolute threshold for pressure detection can also be influenced by age, gender, and condition of the skin o A reduction in skin elasticity will reduce sensitivity and therefore raise threshold o A more gradual loss of skin elasticity occurs with age, which in turn causes progressive increments in detection threshold o Men and women differ in how the rate of detections threshold changes with age  Women are generally more sensitive than men except tongue Difference Thresholds · Weber’s law actually holds true for only a limited range of tactile intensities · Site on the body where the measurements are made affects difference threshold –areas that have low touch thresholds are more sensitive to stimulus change and therefore have lower Weber constants · The actual way in which difference thresholds are measured can also have a bearing on the outcome Sensory Magnitudes · Differing results of sensory magnitude studies arise from whether the stimulus is a single indentation or a vibrating one · The response characteristics of two types of skin are different and yield different sensory magnitudes as stimulus intensity is increased Spatial Factors · Our ability to resolve two different poi
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