Receptors
Introduction
The surface of our bodies receive a constant barrage of sensory stimuli from our environment that are sensed by specialized sensory neurons called receptors. A myriad of receptors are embedded in a matrix in the skin and are activated by mechanical energy (mechanoreceptor) which transduce (convert into electrical signal) the type (touch, pressure, pressure, pain) and quality (moderate to severe) of sensation into nerve impulses (electrochemical event). Impulses are packets of information that are transmitted via nervous tissue (nerves) to the dorsal root ganglion. Neurologists categorize somatic sensation into epicritic (fine discrimination) and protopathic (coarse stimuli). Exteroreceptors receive stimuli on the surface of the body, proprioceptors receive stimuli from muscles tendons and joints, and interoreceptors receive stimuli from within the body.
Types of Receptors
There are four main kinds of receptors. Meissner's corpuscle and Merkel disk 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 ver fine spatial resolution allows humans to perform fine tactile discrimination of surface texture and to read Braille.
Pacinian corpuscles and Ruffini endings 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.
There are four major mechanoreceptors that innervate glabrous (smooth) skin and subcutaneous tissue. General receptors of the body surface. Meissner's Corpuscles, Pacinian Corpuscles, Merkel's receptors and Ruffini's Corpuscles. In addition, the muscle spindle is the key receptor for muscle stretch and the Golgi tendon organ for force. Pacinian corpuscles are rapidly-adapting mechanoreceptors sensitive to high-frequency mechanical stimuli. Neuromuscular spindles (Ia System) are less rapidly-adapting stretch receptors embedded in muscle which send data on muscle length toward the brain and spinal cord. The importance of slowly- and rapidly-adapting receptors for neuronal signaling. Slowly-adapting, tonic output, duration sensitive static sensors and rapidly-adapting, phasic output, onset-offset responsive, dynamic sensors. b. receptors
1. pacinian corpuscles are rapidly-adapting mechanoreceptors sensitive to high-frequency mechanical stimuli
2. neuromuscular spindles (1a) are less rapidly-adapting stretch receptors embedded in muscle which send data on muscle length
toward the brain and spinal cord
3. importance of slowly and rapidly adapting receptors for neural signaling
a. slowly adapting; tonic output, duration-sensitive, static sensors
b. rapidly adapting: phasic output, onset-offset responsive, dynamic sensory. receptors
Unencapsulated nerve endings terminate as free branched fibers in the epithelial layer of the skin, wrap around the lower shafts of hair or form flattened discs (Merkle's disc) abutted against modified epithelial cells. Free nerve endings are the simplest type of receptor. As the parent axon reaches the epithelial layers, it loses its Schwann cell sheath and forms numerous branches, which ramify throughout the deep layer. Free nerve endings are receptive to stimuli that may be interpreted as touch, temperature and pain.
Touch a hair on your arm or head and note the tactile sensation. The nerve endings responsible are of the perifollicular type and are generally filamentous, like the free nerve endings. One parent axon approaches a nest of hair, loses its sheath and breaks up into as many as 100 branches, innervating as many hairs.
Certain receptors consist of nerve endings shaped like flattened discs, each of which encloses the base of an epithelial cell. Each of these Merkle's discs lies adjacent to vesicles within the epithelial cell. There is some evidence that light pressure (touch) on the epithelial cell releases from the vesicles a transmitter that stimulates the Merkle's disc.
There is a large family of encapsulated endings, of which Meissner's corpuscles and the Pacinian corpuscles are typical examples.
Meissner's corpuscles are distributed in the hairless parts of the skin, most commonly in the epithelial layer in the fingertips, the palms of the hands and the soles of the feet, the nipples, and the external genitals. Meissner's corpuscle consists of a laminated capsule of connective tissue cells surrounding a spiral-coursed naked nerve ending in the center of the capsule. The sheaths of the nerve blend with the connective tissue capsule, at which point the myelin covering is lost. Meissner's corpuscles are particularly receptive to mechanical shear forces applied to the skin and probably participate in two-point discrimination.
The Pacinian corpuscle is the most widely distributed of the encapsulated receptors. It is also the largest; in fact, it is visible with the naked eye (1 by 2 mm in size). The laminations of the connective tissue capsule resemble a slice of onion with the laminae separate from one another by clear fluid and tiny blood vessels. Known to respond to pressure stimuli, Pacinian corpuscles may also be sensitive to vibratory stimuli in the upper and lower limbs.
The two principal mechanoreceptors int he superficial layer of the skin are the Meissner's corpuscle and the Merkel disk receptor. The Meisner's corpuscle, a rapidly adapting receptor, is coupled mechanically to the edge of the papillary ridge, a relationship that confers fine mechanical sensitivity. The receptor is a globular, fluid-filled structure that encloses a stack of flattened epithelial cells; the sensory nerve terminal is entwined between the various layers of the corpuscle. The Merkel disk receptor, a slowly adapting receptor, is a small epithelial cell that surrounds the nerve terminal. The Merkel cell encloses a semirigid structure that transmits compressing strain from the skin to the sensory nerve ending, evoking sustained, slowly adapting responses. Merkel disk receptors are normally found in clusters at the center of the papillary ridge.
The two mechanoreceptors found in the deep subcutaneous tissue are the Pacinian corpuscle and the Ruffini ending. These receptors are much larger than the Merkel cells and Meissner's corpuscles, and less numerous. The Pacianian corpuscle is physiologically similar to the meissner's corpuscle. It responds to rapid indentations of the skin but not to steady pressure because of the connective tissue lamellae that surround the nerve ending. The large capsule of this receptor is flexibly attached to the skin, allowing the receptor to sense vibration occurring several centimeters away. These receptors are activated selectively by the common neurological test of touching a tuning fork (oscillating at 200-300Hz) to the skin or boy prominence. Ruffini endings are slowly adapting receptors that link the subcutaneous tissue to folds in the skin at the joints and in the palm or to the fingernails. These receptors sense stretch of the skin or bending of the fingernails as these stimuli compress the nerve endings. Mechanical information sensed by Ruffini endings contributes to our perception of the shape of grasped objects.
Mechanoreceptors in the superficial and deep layers of the skin have different receptive fields. 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.
Receptive Field
Receptive fields are localized areas of nervous tissue activation that are organized into patterned representations in superficial and deep layers of the skin that differ with a respective receptor. The difference is size of the receptive field plays an important role in the function of the receptor.
Center-surround organization denotes neural integration. Inhibition, excitation, modulation, of receptive fields. It is the input from the peripheral nerves that defines the dermatome which is made up of many receptive fields. The organization of dermatomes is a reflection of the cortical and spinal representations of muscle. Receptive field organization is a pattern that is reflected in other sensory systems. Maps do not simply recapitulate the body surface but integrate input and expand or compress representations via convergence of receptive fields. The elementary structure of center-surround receptive field organization. d - stimulated area were represented over a larger area
The size and structure of receptive fields differ for receptors in the superficial and deep layers of the skin. A single dorsal root ganglion cell innervating the superficial layers receives input from a cluster of 10-25 Meissner's corpuscles or Merkel disk receptors. The afferent fiber has a receptive field that spans a small circular area with a diameter ranging from 2 to 10 mm. These receptive fields are at least an order of magnitude greater in diameter than that of an individual receptor. Therefore, nerve fibers innervating the superficial layers of the skin sample the activity one particular sort. In contrast, each nerve fiber innervating the deep layers of skin innervates a single Pacinian corpuscle or Ruffini ending. Consequently, the receptive fields of these receptors cover large areas of skin, and their borders are indistinct. Usually, these receptive fields have a single "hot spot" where sensitivity to touch is greatest; this point is located directly above the receptor. The large receptive fields result from the ability of these receptors to sense mechanical displacement at some distance from the end organ.
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.
Receptive field Processing
Parallel processing of touch, pressure, vibration, joint sensibility. Somatosensory cortex has a representation of the body surface. The maps of the body have an added layer of organization of cortical columns/pathways that contain neurons tuned to slowly or rapidly adapting mechanoreceptor populations meaning that a specific cortical column and neuron specifies modality, body surface position and speed of adaptation. The parallel pathways are physiologically manifested in multiple maps of the body surface, in the dorsal column nuclei, sensory thalamus and somatosensory cortex. The sensory maps are not redundant or functional duplicative, they maps are co-participants in serial processing such as the projection of S1 to S2.
2 major sensory pathways, the dorsal column-medial lemniscal system, which mediates touch and limb position sense and the anterolateral system which mediates pain, temperature, itch senses and crude touch. These systems differ in four major ways. 1. the sensitivity of input to the dorsal root ganglion. 2. the location of the first relay site. 3. the level of decussation. and 4. the functional and anatomical homogeneity of the pathway
From the periphery, neurons sensitive to noxious and thermal stimuli have bare nerve endings and small-diameter axons. To mechanical stimuli have encapsulated endings and large-diameter axons.
Dorsal Root Ganglion
The central branches of the dorsal root ganglion neurons collect into the dorsal roots. The area of skin innervated by the axons in a single dorsal root is termed a dermatome. Dermatomes of adjacent dorsal roots overlap extensively with those of their neighbors (figure 5-4). this explains the common clinical observation that, when a physician probes sensory capacity after injury to a single dorsal root, typically no anesthetic area is observed, although patients with such damage sometimes experience tingling or even a diminished sensory capacity. Single dorsal root injury commonly produces radicular pain, which is localized to the dermatome of the injured root. By comparing the location of radicular pain or other sensory disturbances with a dermatomal map, such as in figure 5-4, the clinician can localize the site of damage. The morphology of a dorsal root ganglion cell; 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, where its specialized terminal is sensitive to a particular form of stimulus energy, and one projecting to the central nervous system. all somatosensory information from the limbs and trunk is conveyed by the dorsal root ganglion
Dorsal root ganglion are main site of entry into the spinal cord to mediate these pathways and are composed of pseudo-unipolar neurons. They receive sensory information and transmit it from the periphery to the spinal cord. This distal terminal of the dorsal root ganglion neurons is the sensory receptor. The axons of dorsal root ganglion neurons enter the spinal cord via the dorsal root. A dermatome is the area of skin innervated by a single dorsal root. The afferent information carried by adjacent dorsal roots overlaps nearly completely on the body surface. Once in the spinal cord, dorsal root ganglion neurons have three major branches: segmental, ascending, and descending . The principal branching pattern of large-diameter fibers is to ascend to the brain stem in the dorsal columns. Whereas small-diameter fibers ascend and descend in Lissauer's tract, they eventually terminate in the gray matter of the spinal cord. The majority of the axons in the dorsal columns are central branches of dorsal root ganglion neurons. Damaging a dorsal root has such severe functional consequences.
Subsequent input into the spinal cord and central nervous system from the dorsal root ganglion; the notorious DRG
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.
The surface of our bodies receive a constant barrage of sensory stimuli from our environment that are sensed by specialized sensory neurons called receptors. A myriad of receptors are embedded in a matrix in the skin and are activated by mechanical energy (mechanoreceptor) which transduce (convert into electrical signal) the type (touch, pressure, pressure, pain) and quality (moderate to severe) of sensation into nerve impulses (electrochemical event). Impulses are packets of information that are transmitted via nervous tissue (nerves) to the dorsal root ganglion. Neurologists categorize somatic sensation into epicritic (fine discrimination) and protopathic (coarse stimuli). Exteroreceptors receive stimuli on the surface of the body, proprioceptors receive stimuli from muscles tendons and joints, and interoreceptors receive stimuli from within the body.
Types of Receptors
There are four main kinds of receptors. Meissner's corpuscle and Merkel disk 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 ver fine spatial resolution allows humans to perform fine tactile discrimination of surface texture and to read Braille.
Pacinian corpuscles and Ruffini endings 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.
There are four major mechanoreceptors that innervate glabrous (smooth) skin and subcutaneous tissue. General receptors of the body surface. Meissner's Corpuscles, Pacinian Corpuscles, Merkel's receptors and Ruffini's Corpuscles. In addition, the muscle spindle is the key receptor for muscle stretch and the Golgi tendon organ for force. Pacinian corpuscles are rapidly-adapting mechanoreceptors sensitive to high-frequency mechanical stimuli. Neuromuscular spindles (Ia System) are less rapidly-adapting stretch receptors embedded in muscle which send data on muscle length toward the brain and spinal cord. The importance of slowly- and rapidly-adapting receptors for neuronal signaling. Slowly-adapting, tonic output, duration sensitive static sensors and rapidly-adapting, phasic output, onset-offset responsive, dynamic sensors. b. receptors
1. pacinian corpuscles are rapidly-adapting mechanoreceptors sensitive to high-frequency mechanical stimuli
2. neuromuscular spindles (1a) are less rapidly-adapting stretch receptors embedded in muscle which send data on muscle length
toward the brain and spinal cord
3. importance of slowly and rapidly adapting receptors for neural signaling
a. slowly adapting; tonic output, duration-sensitive, static sensors
b. rapidly adapting: phasic output, onset-offset responsive, dynamic sensory. receptors
Unencapsulated nerve endings terminate as free branched fibers in the epithelial layer of the skin, wrap around the lower shafts of hair or form flattened discs (Merkle's disc) abutted against modified epithelial cells. Free nerve endings are the simplest type of receptor. As the parent axon reaches the epithelial layers, it loses its Schwann cell sheath and forms numerous branches, which ramify throughout the deep layer. Free nerve endings are receptive to stimuli that may be interpreted as touch, temperature and pain.
Touch a hair on your arm or head and note the tactile sensation. The nerve endings responsible are of the perifollicular type and are generally filamentous, like the free nerve endings. One parent axon approaches a nest of hair, loses its sheath and breaks up into as many as 100 branches, innervating as many hairs.
Certain receptors consist of nerve endings shaped like flattened discs, each of which encloses the base of an epithelial cell. Each of these Merkle's discs lies adjacent to vesicles within the epithelial cell. There is some evidence that light pressure (touch) on the epithelial cell releases from the vesicles a transmitter that stimulates the Merkle's disc.
There is a large family of encapsulated endings, of which Meissner's corpuscles and the Pacinian corpuscles are typical examples.
Meissner's corpuscles are distributed in the hairless parts of the skin, most commonly in the epithelial layer in the fingertips, the palms of the hands and the soles of the feet, the nipples, and the external genitals. Meissner's corpuscle consists of a laminated capsule of connective tissue cells surrounding a spiral-coursed naked nerve ending in the center of the capsule. The sheaths of the nerve blend with the connective tissue capsule, at which point the myelin covering is lost. Meissner's corpuscles are particularly receptive to mechanical shear forces applied to the skin and probably participate in two-point discrimination.
The Pacinian corpuscle is the most widely distributed of the encapsulated receptors. It is also the largest; in fact, it is visible with the naked eye (1 by 2 mm in size). The laminations of the connective tissue capsule resemble a slice of onion with the laminae separate from one another by clear fluid and tiny blood vessels. Known to respond to pressure stimuli, Pacinian corpuscles may also be sensitive to vibratory stimuli in the upper and lower limbs.
The two principal mechanoreceptors int he superficial layer of the skin are the Meissner's corpuscle and the Merkel disk receptor. The Meisner's corpuscle, a rapidly adapting receptor, is coupled mechanically to the edge of the papillary ridge, a relationship that confers fine mechanical sensitivity. The receptor is a globular, fluid-filled structure that encloses a stack of flattened epithelial cells; the sensory nerve terminal is entwined between the various layers of the corpuscle. The Merkel disk receptor, a slowly adapting receptor, is a small epithelial cell that surrounds the nerve terminal. The Merkel cell encloses a semirigid structure that transmits compressing strain from the skin to the sensory nerve ending, evoking sustained, slowly adapting responses. Merkel disk receptors are normally found in clusters at the center of the papillary ridge.
The two mechanoreceptors found in the deep subcutaneous tissue are the Pacinian corpuscle and the Ruffini ending. These receptors are much larger than the Merkel cells and Meissner's corpuscles, and less numerous. The Pacianian corpuscle is physiologically similar to the meissner's corpuscle. It responds to rapid indentations of the skin but not to steady pressure because of the connective tissue lamellae that surround the nerve ending. The large capsule of this receptor is flexibly attached to the skin, allowing the receptor to sense vibration occurring several centimeters away. These receptors are activated selectively by the common neurological test of touching a tuning fork (oscillating at 200-300Hz) to the skin or boy prominence. Ruffini endings are slowly adapting receptors that link the subcutaneous tissue to folds in the skin at the joints and in the palm or to the fingernails. These receptors sense stretch of the skin or bending of the fingernails as these stimuli compress the nerve endings. Mechanical information sensed by Ruffini endings contributes to our perception of the shape of grasped objects.
Mechanoreceptors in the superficial and deep layers of the skin have different receptive fields. 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.
Receptive Field
Receptive fields are localized areas of nervous tissue activation that are organized into patterned representations in superficial and deep layers of the skin that differ with a respective receptor. The difference is size of the receptive field plays an important role in the function of the receptor.
Center-surround organization denotes neural integration. Inhibition, excitation, modulation, of receptive fields. It is the input from the peripheral nerves that defines the dermatome which is made up of many receptive fields. The organization of dermatomes is a reflection of the cortical and spinal representations of muscle. Receptive field organization is a pattern that is reflected in other sensory systems. Maps do not simply recapitulate the body surface but integrate input and expand or compress representations via convergence of receptive fields. The elementary structure of center-surround receptive field organization. d - stimulated area were represented over a larger area
The size and structure of receptive fields differ for receptors in the superficial and deep layers of the skin. A single dorsal root ganglion cell innervating the superficial layers receives input from a cluster of 10-25 Meissner's corpuscles or Merkel disk receptors. The afferent fiber has a receptive field that spans a small circular area with a diameter ranging from 2 to 10 mm. These receptive fields are at least an order of magnitude greater in diameter than that of an individual receptor. Therefore, nerve fibers innervating the superficial layers of the skin sample the activity one particular sort. In contrast, each nerve fiber innervating the deep layers of skin innervates a single Pacinian corpuscle or Ruffini ending. Consequently, the receptive fields of these receptors cover large areas of skin, and their borders are indistinct. Usually, these receptive fields have a single "hot spot" where sensitivity to touch is greatest; this point is located directly above the receptor. The large receptive fields result from the ability of these receptors to sense mechanical displacement at some distance from the end organ.
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.
Receptive field Processing
Parallel processing of touch, pressure, vibration, joint sensibility. Somatosensory cortex has a representation of the body surface. The maps of the body have an added layer of organization of cortical columns/pathways that contain neurons tuned to slowly or rapidly adapting mechanoreceptor populations meaning that a specific cortical column and neuron specifies modality, body surface position and speed of adaptation. The parallel pathways are physiologically manifested in multiple maps of the body surface, in the dorsal column nuclei, sensory thalamus and somatosensory cortex. The sensory maps are not redundant or functional duplicative, they maps are co-participants in serial processing such as the projection of S1 to S2.
2 major sensory pathways, the dorsal column-medial lemniscal system, which mediates touch and limb position sense and the anterolateral system which mediates pain, temperature, itch senses and crude touch. These systems differ in four major ways. 1. the sensitivity of input to the dorsal root ganglion. 2. the location of the first relay site. 3. the level of decussation. and 4. the functional and anatomical homogeneity of the pathway
From the periphery, neurons sensitive to noxious and thermal stimuli have bare nerve endings and small-diameter axons. To mechanical stimuli have encapsulated endings and large-diameter axons.
Dorsal Root Ganglion
The central branches of the dorsal root ganglion neurons collect into the dorsal roots. The area of skin innervated by the axons in a single dorsal root is termed a dermatome. Dermatomes of adjacent dorsal roots overlap extensively with those of their neighbors (figure 5-4). this explains the common clinical observation that, when a physician probes sensory capacity after injury to a single dorsal root, typically no anesthetic area is observed, although patients with such damage sometimes experience tingling or even a diminished sensory capacity. Single dorsal root injury commonly produces radicular pain, which is localized to the dermatome of the injured root. By comparing the location of radicular pain or other sensory disturbances with a dermatomal map, such as in figure 5-4, the clinician can localize the site of damage. The morphology of a dorsal root ganglion cell; 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, where its specialized terminal is sensitive to a particular form of stimulus energy, and one projecting to the central nervous system. all somatosensory information from the limbs and trunk is conveyed by the dorsal root ganglion
Dorsal root ganglion are main site of entry into the spinal cord to mediate these pathways and are composed of pseudo-unipolar neurons. They receive sensory information and transmit it from the periphery to the spinal cord. This distal terminal of the dorsal root ganglion neurons is the sensory receptor. The axons of dorsal root ganglion neurons enter the spinal cord via the dorsal root. A dermatome is the area of skin innervated by a single dorsal root. The afferent information carried by adjacent dorsal roots overlaps nearly completely on the body surface. Once in the spinal cord, dorsal root ganglion neurons have three major branches: segmental, ascending, and descending . The principal branching pattern of large-diameter fibers is to ascend to the brain stem in the dorsal columns. Whereas small-diameter fibers ascend and descend in Lissauer's tract, they eventually terminate in the gray matter of the spinal cord. The majority of the axons in the dorsal columns are central branches of dorsal root ganglion neurons. Damaging a dorsal root has such severe functional consequences.
Subsequent input into the spinal cord and central nervous system from the dorsal root ganglion; the notorious DRG
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.