Chaspter 48.pptx
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assification of somatic sensations Mechanoreceptive - stimulated by mechanical displacement. Tactile: touch, pressure, vibration, tickle, itch Proprioceptive: static position, rate of change Thermoreceptive. detect heat and cold. Nociceptive. detect pain and any factor that damages tissue. Tactile...
assification of somatic sensations Mechanoreceptive - stimulated by mechanical displacement. Tactile: touch, pressure, vibration, tickle, itch Proprioceptive: static position, rate of change Thermoreceptive. detect heat and cold. Nociceptive. detect pain and any factor that damages tissue. Tactile receptors – small field Meissner corpuscles • Location: non-hairy skin close to surface (fingertips, lips, eyelids, nipples and external genitalia). • Function: motion detection, grip control Stimuli: skin motion, low frequency Meissner corpuscles vibration Merkel discs • Adaptation: rapid adaptation • Location: tip of epidermal ridges • receptive field: 22 mm2 • Function: form and texture perception • type Aβ nerve fibers • Stimuli: edges, points, corners, curvature • Adaptation: slow adaptation • receptive field: 9 mm2 • type Aβ nerve fibers • Merkel discs Purves. Neuroscience. 5th ed, 2012. Figure 9.5 Activity patterns recorded from mechanosensory afferents in fingertip Each dot is an action potential Small field, slow adaptation Larger field, fast adaptation Very large field, slow adaptation Huge field, very fast adaptation Purves. Neuroscience. 5th ed, 2012. Figure 9.6 Tactile receptors – large field Pacinian corpuscle (~ 1 mm) • Location: dermis and deeper tissues • Function: perception of distant events through transmitted vibrations; tool use • Stimuli: vibration (250 Hz is optimal) Ruffini corpusclevery rapid • Adaptation: • adaptation Location: dermis force; hand •• Function: Receptive tangential field: entire finger or shape; hand motion detection • type Aβ nerve fibers • Stimuli: skin stretch • Adaptation: slow adaptation • receptive field: 60 mm2 • type Aβ nerve fibers Pacinian corpuscles Purves. Neuroscience. 5th ed, 2012. Figure 9.5 Free nerve endings Free nerve endings (myelinated). • Location: surface of body and elsewhere • Function/stimuli: pain, temperature • Adaptation: slow adaptation • type Aδ nerve fibers, i.e., A-delta Free nerve endings (unmyelinated). • Location: surface of body and elsewhere • Function/stimuli: pain, temperature, itch • Adaptation: slow adaptation • type C Purves. Neuroscience. 5th ed, 2012. Figure 9.5 Pathways for the Transmission of Sensory Information Anterolateral system Dorsal columnmedial lemniscal system Purves. Neuroscience. 5th ed, 2012. Figure 9.1B Almost all sensory information enters spinal cord through dorsal roots of spinal nerves. Two pathways for sensory afferents. Anterolateral system Dorsal column-medial lemniscal system Dorsal Column-medial lemniscal System • Contains large myelinated nerve fibers (30-110 m/sec). A-beta • Three neurons to sensory cortex / decussates in medulla oblongata • High degree of spatial orientation maintained throughout the tract. • Transmits information rapidly and with a high degree of spatial fidelity (i.e., discrete types of mechanoreceptor information). • Transmits touch, vibration, Figure 48-3 The Anterolateral System • Contains smaller myelinated and unmyelinated fibers for slow transmission (0.5-40 m/sec). (A-delta, C) • three neurons to sensory cortex / decussates in spinal cord • low degree of spatial orientation. • transmits a broad spectrum of modalities. • pain, thermal sensations, crude touch and pressure, tickle and Figure 47-13 Dermatomes Dermatome – area of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Clinical significance • Localizing cord lesion • Viruses such as varicella zoster hibernate in ganglia causing rash in associated dermatome. • Referred pain (discussed later) Figure 48-14 Shingles follows the dermatome Years or decades after a chickenpox infection, the virus (varicella zoster virus) may break out of nerve cell bodies and travel down nerve axons to cause viral infection of skin associated with nerve. The rash occurs in the dermatome of the infected nerve cell. The Somatosensory Cortex • Located in the postcentral gyrus. • Highly organized distinct spatial orientation. • Each side of cortex receives information from opposite side of body. • Unequal representation of the body. – lips have greatest area of representation followed by face and thumb. – trunk and lower body have least area of representation. Figure 48-6 Sensory homunculus Homunculus – (latin) little human Figure 47-7 Brodmann areas Brodmann areas show cytoarchitectural organization of neurons (cell size, packing density, lamination) that he observed in cerebral cortex using Nissl stain (1909). Figure 48-5 Figure 48-6 Somatosensory area 1 (primary somatosensory area) Brodmann’s areas: 1, 2, 3 Somatosensory association area Brodmann’s areas: 5,7 Lesions of somatosensory cortex Destruction of somatosensory area I causes: loss of vibration, fine touch, and proprioception. discrete localization ability. inability to judge the degree of pressure. inability to determine the weight of an object. inability to judge texture. Also: Hemineglect (unilateral neglect, hemispatial neglect or spatial neglect): patients are unaware of items to one side of space. Astereognosis: inability to recognize objects by touch Agraphesthesia: a disorientation of the skin’s sensation across its space (e.g., hard to identify a number or letter traced on the hand) Figure 47-7 Somatosensory association area • Areas 5 and 7 in parietal area. • Receives input from somatosensory cortex, ventrobasal nuclei of thalamus, visual and auditory cortex. • Function is to decipher complex sensory associations. • Loss of these areas • inability to recognize complex objects • neglect of contralateral world and even refusal to acknowledge ownership of contralateral body. Figure 48-5 Figure 47-6 Structure of Cerebral Cortex Diffuse lower input Related brain areas Incoming signals To brainstem and cord To thalamus Figure 48-8 Cellular Organization of the Cortex • Six separate layers of neurons with layer I near the surface of the cortex and layer VI deep within the cortex. • Incoming signals enter layer IV and spread both up and down. • Layers I and II receive diffuse input from lower brain centers. • Layers II and III neurons send axons to closely related portion of the cortex presumably for communicating between similar areas. • Layer V and VI send axons to more distant parts of the nervous system, layer V to the brainstem and spinal cord, layer VI to the thalamus. Cellular Organization of the Cortex (cont’d) • Within the layers the neurons are also arranged in columns. • Each column serves a specific sensory modality (i.e., stretch, pressure, touch). • Different columns interspersed among each other. – interaction of the columns occurs at different cortical levels which allows the beginning of the analysis of the meaning of the sensory signals. 6 Layers of cerebral cortex Vertical columns detect a different sensory spot on body with a specific sensory modality Incoming sensory signal excites layer IV I molecular layer II external granular layer III layer of small pyramidal cells IV internal granular layer V large pyramidal cell layer VI layer of fusiform or polymorphic cells Two-point discrimination • • The two-point discrimination threshold measures the minimum distance at which two stimuli are resolved as distinct; it reflects how finely innervated an area of skin is. The spatial resolution of stimuli on the skin varies throughout the body because the density of mechanoreceptors varies. Lateral inhibition improves twopoint discrimination Lateral inhibitio n present Figure 48-10 • Lateral inhibition is the capacity of an excited neuron to reduce activity of neighboring neurons; it improves degree of contrast – Occurs at every synaptic level (for dorsal column system): dorsal column nuclei, ventrobasal nuclei of thalamus, cortex
