Lecture 16 Notes PDF

Somatosensation: The bodily sense Irrespective of modality, all somatosensory information from the limbs and trunk is conveyed by dorsal root ganglion neurons. Somatosensory information from cranial structures (the face, lips, oral cavity, conjunctiva, and dura mater) is transmitted by the trigemina...

Somatosensation: The bodily sense Irrespective of modality, all somatosensory information from the limbs and trunk is conveyed by dorsal root ganglion neurons. Somatosensory information from cranial structures (the face, lips, oral cavity, conjunctiva, and dura mater) is transmitted by the trigeminal sensory neurons, which are functionally and morphologically homologous to dorsal root ganglion neurons. The cell body lies in a ganglion on the dorsal root of a spinal nerve. The axon has two branches, one projecting to the periphery and one projecting to the central nervous system. The terminal of the peripheral branch of the axon is the only portion of the dorsal root ganglion cell that is sensitive to natural stimuli. The properties of the nerve terminal determine the sensory function of each dorsal root ganglion neuron. The remainder of the peripheral branch, together with the central branch, is called the primary afferent fiber; it transmits the encoded stimulus information to the spinal cord or brain stem. Touch Is Mediated by Mechanoreceptors in the Skin Tactile sensitivity is greatest on the hairless (glabrous) skin on the fingers, the palmar surface of the hand, the sole of the foot, and the lips. Glabrous skin contains a dense matrix of mechanoreceptors. These receptors mediate the sense of touch; they are excited by indentation of the skin or by motion across its surface. When an object presses against the hand, the skin conforms to its contours. The depth of indentation depends on the force exerted by the object on the skin as well as its geometry. All mechanoreceptors sense these changes in skin contour but differ morphologically in important ways that affect their physiological function. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Virtually all mechanoreceptors have specialized end organs surrounding the nerve terminal. Although the sensitivity of these receptors to mechanical displacement is a property of the nerve terminal membrane, their dynamic response to stimulation is shaped by the specialized capsule. These nonneural structures must be deformed in particular ways in order to excite the sensory nerve. Histological and physiological studies have identified four major types of mechanoreceptors in glabrous skin. Two of these receptors are located in the superficial layers of the skin, and two are situated in the subcutaneous tissue. The small superficial receptors sense deformation of the papillary ridges in which they reside. The larger subcutaneous receptors sense deformation of a wider area of skin that extends beyond the overlying ridges. Each individual dorsal root ganglion neuron conveys sensory information from a limited area of skin determined by the location of its receptive endings. The region of skin from which a sensory neuron is excited is called its receptive field. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. The difference in size of the receptive fields of receptors in the superficial and deep layers of the skin plays an important role in the functions of the receptors. Receptors in the superficial layers resolve fine spatial differences because they transmit information from a restricted area of skin. As these receptors are smaller in diameter than the fingerprint ridges of glabrous skin, individual receptors can be stimulated by very small bumps on a surface. This very fine spatial resolution allows humans to perform fine tactile discrimination of surface texture and to read Braille. Receptors in the deep layers resolve only coarse spatial differences. They are poorly suited for accurate spatial localization or for resolution of fine spatial detail. Mechanoreceptors in the deep layers of the skin sense more global properties of objects and detect displacements from a wide area of skin. Two-point discrimination varies throughout the body surface. The two-point threshold measures the minimum distance at which two stimuli are resolved as distinct. At smaller separations the stimuli are blurred into a single continuous sensation spanning the distance between the points. Two-point thresholds are measured clinically using a calibrated compass in which the separation of the tips is accurately scaled. The two-point threshold varies for different body regions; it is about 2 mm on the fingertip but increases to 10 mm on the palm and 40 mm on the arm. The two-point thresholds highlighted in pink match the diameter of the corresponding receptive fields shown in pink on the body. The greatest discriminative capacity is afforded in the finger tips, lips, and tongue, which have the smallest receptive fields. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Warmth and Cold Are Mediated by Thermal Receptors Although the size, shape, and texture of objects are also sensed by vision, the thermal qualities of objects are uniquely somatosensory. Humans recognize four distinct types of thermal sensation: cold, cool, warm, and hot. These thermal sensations result from differences between the external temperature of the air or of objects contacting the body and the normal skin temperature of 34°C. Thermal receptors modulate their firing as a function of temperature. At constant temperatures they have tonic discharges, firing action potentials at a steady rate governed by the actual temperature sensed. Unlike mechanoreceptors, which are silent in the absence of tactile stimuli, cold receptors and warmth receptors fire action potentials continuously at low rates (2© Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 5 spikes per second) when the skin temperature is set at its normal value of 34°C. The steadystate firing rate does not increase or decrease monotonically if the skin is slowly warmed or cooled. Instead, each class of thermal receptor shows peak firing at a preferred skin temperature. Cold receptors fire most vigorously at skin temperatures of 25°C, whereas warmth receptors are most active at 45°C. Temperatures above or below these values evoke progressively weaker responses. Therefore, individual cold and warmth receptors do not give a precise reading of the skin temperature, as the same firing rate can be evoked by stimuli greater than or less than the preferred value. Rather, the code for skin temperature involves comparing the relative activity of the different populations of thermal receptors and nociceptors. Spatial Resolution in the Cortex Is Correlated With the Innervation Density of the Skin The somatotopic arrangement of somatosensory inputs in the human cortex is called a homunculus. The internal representation of the body within the homunculus does not duplicate the spatial topography of the skin exactly. Rather, the image of the body in the brain exaggerates certain body regions, particularly the hand, foot, and mouth and compresses more proximal body parts. Each part of the body is represented in the brain in proportion to its relative importance to sensory perception. The map represents the innervation density of the skin rather than its total surface area. In humans a large number of cortical columns receive input from the hands, particularly from the fingers. About 100 times as much cortical tissue is devoted to a square centimeter of skin on the fingers as to a square centimeter of skin on the abdomen. Similarly, large numbers of cortical neurons receive input from the foot and face. More than any other part of the body, the hands, face, and feet are important sensors of the properties of objects and thus have the highest density of touch receptors. The proximal portions of the limbs and trunk are much less densely innervated; correspondingly, fewer cortical neurons receive inputs from these regions. Each region of the somatic sensory cortex receives inputs from primarily one type of receptor. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Look at the figure below: A - In each of the four regions of the somatic sensory cortex—Brodmann's areas 3a, 3b, 1, and 2—inputs from one type of receptor in specific parts of the body are organized in columns of neurons that run from the surface to the white matter. B - Detail of the columnar organization of inputs from digits 2, 3, 4, and 5 in a portion of Brodmann's area 3b. Alternating columns of neurons receive inputs from rapidly adapting (RA) and slowly adapting (SA) receptors in the superficial layers of skin. C - Overlapping receptive fields from RA and SA receptors project to distinct columns of neurons in area 3b. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed.