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CHAPTER 12.docx

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Connell Mc Cluskey

INTRODUCTION Somatic sensation enables our body to feel, to ache, to chill, and to know what its parts are doing. It is sensitive to many kinds of stimuli. The somatic sensory system is different from other sensory systems in two interesting ways:  its receptors are distributed throughout the body rather than being concentrated at small, specialized locations  it responds to many different kids of stimuli (grouped into four senses – touch, temperature, pain, and body position) A single stimulus usually activates many receptors. The central nervous system interprets the activity of the vast receptor array and uses its to generate coherent perceptions. TOUCH Touch begins at the skin. There are two major types of skin: hairy and glabrous (hairless). Skin has an outer layer, the epidermis, and an inner layer, the dermis. Skin performs an essential protective function, and it prevents the evaporation of body fluids into the dry environment we live in. Mechanoreceptors of the Skin Most of the sensory receptors in the somatic sensory system are mechanoreceptors, which are sensitive to physical distortion such as bending or stretching. These receptors monitor:  contact with the skin  pressure in the heart and blood vessels  stretching of the digestive organs and urinary bladder  force against the teeth Each mechanoreceptor contains unmyelinated axon branches which have mechanosensitive ion channels whose gating depends on stretching or changes in tension of the surrounding membrane. See Fig. 12.1. The largest and best studied receptor is the Pacinian corpuscle which lies deep in the dermis. Ruffini’s endings are found in both hairy and glabrous skin (slightly smaller than the Pacinian corpuscles). Meissner’s corpuscles are located in the ridges of glabrous skin. Merkel’s disks are located within the epidermis and consist of a nerve terminal and flattened, non-neural epithelial cells. In Krause end bulbs borders regions of dry skin and mucous membrane. Skin can be vibrated, pressed, pricked, and stroked, and its hair can be bent or pulled. The skin is able to tell these different stimuli apart. We have mechanoreceptors that vary in their preferred stimulus frequencies, pressures, and receptive field sizes. Mechanoreceptors also vary in the persistence of their responses to long lasting stimuli. For some animals, hair is a major sensory system. Hairs grow from follicles embedded in the skin. Each follicle is richly innervated by free nerve endings that either wrap around it or run parallel to it. There are several types of hair follicles. In all types, the bending of the hair causes:  deformation of the follicle and surrounding skin tissues  This stretches, bends, or flattens the nearby nerve endings which increases or decreases their action potential firing frequency.  Mechanoreceptors of hair follicles may be either slowly adapting or rapidly adapting. The different mechanical sensitivities of mechanoreceptors mediate different sensations.  Pacinian corpuscles: 200 – 300 Hz  Meissner’s corpuscles: 50 Hz Vibration and the Pacinian Corpuscles The selectivity of a mechanoreceptive axon depends primarily on the structure of its special ending. Pacinian corpuscle has a football-shaped capsule with 20-70 concentric layers of connective tissue with a nerve terminal in the middle. When the capsule is compressed, energy is transferred to the nerve terminal, its membrane is deformed, and mechanosensitive channels open. Current flowing through the channels generates a depolarizing receptor potential (see Fig. 12.5). If the depolarization is large enough, the axon will fire an action potential.  These capsule layers are slick and if the stimulus pressure is maintained, the layers slip past one another and transfer the stimulus energy in such a way that the axon terminal is no longer deformed and the receptor potential dissipates.  When the pressure is released, the events reverse themselves. The terminal depolarizes again and may fire another action potential. Two-Point Discrimination Two-point discrimination varies at least twentyfold across the body. Fingertips have the highest resolutions. Several reasons explain why the fingertip is so much better than, say, the elbow for Braille reading:  There is a much higher density of mechanoreceptors in the skin of the fingertips than elsewhere  The fingertips are enriched in receptor types that have small receptive fields  There is more brain tissue devoted to the sensory information of each square millimetre of fingertip than elsewhere  There may be special neural mechanisms devoted to high-resolution discriminations Primary Afferent Axons The skin is richly innervated by axons that course through the vast network of peripheral nerves on their way to the CNS. Axons bringing information from the somatic sensory receptors to the spinal cord or brain stem are the primary afferent axons. The primary afferent axons enter the spinal cord through the dorsal roots which lie in the dorsal root ganglia (see Fig. 12.8). Primary afferent axons have widely varying diameters and the size correlates with the type of sensory receptor to which they are attached (see Fig. 12.9). The diameter of an axon (with its myelin) determines its speed of action potential conduction. The smallest axons (C fibers) have no myelin and mediate pain and temperature sensation. They are the slowest of axons.  Touch sensation, mediated by the cutaneous mechanoreceptors, are conveyed by the relatively large Aβ axons. The Spinal Cord Segmental Organization of the Spinal Cord The arrangement of paired dorsal and ventral roots (see Fig. 12.8) is repeated 30 times down the length of the human spinal cord. Each spinal nerve, consisting of dorsal root and ventral root axons, passes through a notch between the vertebrae of the spinal column. See Fig. 12.10, the 30 spinal segments are divided into four groups: cervical (C) 1-8, thoracic (T) 1-12, lumbar (L) 1-5, and sacral (S) 1-5. The area of skin innervated by the right and left dorsal roots of a single spinal segment is called a dermatome. There is a one-to-one correspondence between dermatomes and spinal segments. When mapped, the dermatomes delineate a set of bands on the body surface (see Fig. 12.11 & 12.12). When a dorsal root is cut, the corresponding dermatome on that side of the body does not lose all sensation. The residual somatic sensation is explained by the fact that the adjacent dorsal roots innervate overlapping areas. To lose all sensation, three adjacent dorsal roots must be cut. The spinal cord in the adult ends at about the level of the third lumbar vertebra (see Fig. 12.10). The bundles of spinal nerves streaming down within the lumbar and sacral vertebral column are called the cauda equina. It is filled with cerebral spinal fluid (CSF). Sensory Organization of the Spinal Cord The spinal cord is composed of an inner core of gray matter, surrounded by a thick covering of white matter tracts called columns. Each half of the spinal gray matter is divided into a dorsal horn, an intermediate zone, and a ventral horn (see Fig. 12.3). The neurons that receive sensory input from primary afferents are called second-order sensory neurons. Most of these lie within the dorsal horns. The large, myelinated Aβ axons conveying information about a touch to the skin enter the dorsal horn and branch. One branch synapses in the deep part of the dorsal horn on second-order sensory neurons. These connections can initiate or modify a variety of rapid and unconscious reflexes. The other branch of the Aβ primary afferent axon ascends straight to the brain. This branch is responsible for perception. The Dorsal Column-Medial Lemniscal Pathway Information about touch or vibration of the skin takes a path to the brain that is entirely distinct from that take by information about pain and temperature. The pathway serving touch is called the dorsal column-medial lemnsical pathway (see Fig. 12.14). The ascending branch of the large sensory axons (Aβ) enters the ipsilateral dorsal column of the spinal cord which carries information about tactile sensation toward the brain. The dorsal columns are also composed of primary sensory axons, as well as second-order axon from neurons. The axons of the dorsal column terminate in the dorsal column nuclei which lie at the junction of the spinal cord and medulla. This is a fast, direct path that brings information from the skin to the brain without an intervening synapse. At this point, information is still represented ipsilaterally. This means that information from the left side of the body is represented in the activity of cells in the left dorsal column nuclei. From the dorsal column nuclei the somatic sensory system of one side of the brain is concerned with sensations deriving from the other side of the body. Axons of the dorsal column nuclei ascend the medial lemiscus which rises through the medulla, pons, and midbrain, and its axons synapse upon neurons of the ventral posterior (VP) nucleus. The sensory information must synapse in the thalamus. Thalamic neurons of the VP nucleus then project to specific of primary somatosensory cortex (S1). In both dorsal column and thalamic nuclei, considerable transformation of information takes place. As a general rule, information is altered every time it passes through a set of synapses in the brain. The Trigeminal Touch Pathway Somatic sensation of the face is supplied mostly by the large trigeminal nerves (cranial nerve V) which enters the brain at the pons. There are twin trigeminal nerves (one on each side) which break up into three peripheral nerves that innervate the face, mouth areas, the outer two-thirds of the tongue, and the dura mater covering the brain. The large-diameter sensory axons of the trigeminal nerve carry tactile information from skin mechanoreceptors and synapse onto second-order neurons in the ipsilateral trigeminal nucleus (see Fig. 12.15). This nucleus then projects into the medial part of the VP nucleus of the thalamus. Then then information is relayed to the somatosensory cortex. Somatosensory Cortex The most complex levels of somatosensory processing occur in the cerebral cortex and mainly in the parietal lobe (see Fig. 12.16).  Area 3b is the primary somatic sensory cortex because: o It receives dense inputs from the VP nucleus of the thalamus o Its neurons are very responsive to somatosensory stimuli (not other stimuli) o Lesions here impair somatic sensation o Electrically stimulated, it evokes somatic sensory experiences  Area 3a is concerned with body position  Areas 1 and 2 receive dense input from area 3b o Area 1 is mainly concerned with texture information o Area 2 concerns size and shape Somatosensory cortex, like other areas of neocortex, is a layered structure. Thalamic inputs to S1 terminate mainly in layer IV. The neurons of layer IV project to cells in other layers. Another important similiarty is that S1 neruosn with similar inputs and responses are stacked vertically into columns that extend across the cortical layers (see Fig. 12.17). Cortical Somatotropy What happens to the somatotopic map in cortex when an input, such as the finger, is removed? Such question was experimented on owl monkeys (see Fig. 12.22):  The fingers of the hand of the owl monkey are mapped onto the surface of S1 cortex  If digit 3 is removed, over time the cortex reorganizes so that the respresentations of digits 2 and 4 expand Therefore, the cortex orginally devoted to the amputated digit now responded to stimulation of adjacent digits. There had been a major rearrangment of the circuity underlying cortical somatotropy. What happens when the input activity from a digit is increased?  If digits 2 and 3 are selectively stimulated, the cortical representations also expand Therefore, cortical maps are dynamic and adjust depending on the amount of sensory experience. The Posterior Parietal Cortex The segregation of different types of information is a general rule for the sensory systems. However, information of different sensory types cannot remain separate forever.
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