Somatosensory System

Editor's Notes

• #2: Readings B&B: Chapter 13, pages 325 - 326, 352 - 357, 370 - 371; Chapter 10, pages 276, 278; Chapter 14, pages 361 & 373 BLKS: Chapter 7
• #3: Cutaneous mechanoreceptors because we already talked about mechanically gated receptors: Hair cells in vestibular and auditory system
• #4: System of sensory receptors and central projections that transduce, encode, and ultimately perceive information generated by stimuli arising from both the external and internal environment Next lecture, muscle proprioceptors and vestibular system Differences between somatosensory system and special senses is that somatosensory receptors are diffusely located thru out the body instead of densely distributed to a specific organ. Special senses convey information to the brain via single nerve bundle whereas somatosensory information arrives via the spinal cord dorsal roots and cranial nerves Enteroreceptive – mechanoreceptors detecting distension of gut and fullness of bladder . Although the basic senses—somatic sensation, vision, audition, vestibular sensation, and the chemical senses—are very different from one another, a few fundamental rules govern the way the nervous system deals with each of these diverse modalities. Highly specialized nerve cells called receptors convert the energy associated with mechanical forces, light, sound waves, odorant molecules, or ingested chemicals into neural signals that convey information about the stimulus to the brain. These afferent sensory signals activate central neurons capable of representing both the qualitative and quantitative aspects of the stimulus (what it is and how strong it is), and in some modalities—somatic sensation, vision, and audition—the location of the stimulus in space (where it is). The clinical evaluation of patients routinely requires an assessment of the sensory systems to infer the nature and location of potential neurological problems. Knowledge of where and how the different sensory modalities are transduced, relayed, represented, and further processed to generate appropriate behavioral responses is therefore essential to understanding and treating a wide variety of diseases. Accordingly, these chapters on the neurobiology of sensation also serve to introduce some of the major structure/function relationships in the sensory components of the nervous system.
• #5: Increased length constant due to decrease axoplasmic resistance
• #6: The local depolarization produced is called a generator or receptor potential. Receptor potential increases with stimulus strength. Site of AP initiation = trigger zone Stimulus intensity encoded in: firing rate of neuron (number of action potentials per second) or number of activated sensory receptors The firing rate of single receptor is frequently an exponential function of stimulus intensity.
• #8: Ruffini's end organs detect tension deep in the skin. Meissner's corpuscles detect changes in texture (vibrations around 50 Hz) and adapt rapidly. Pacinian corpuscles detect rapid vibrations (about 200–300 Hz). Merkel's discs detect sustained touch and pressure. Mechanoreceiving free nerve endings detect touch, pressure and stretching Hair follicle receptors are located in hair follicles and sense position changes of hairs. Krause bulbs – innervate border areas of dry skin and mucous membranes (ie. lips external genitalia) Pacinian Corpuscle – glaborous and hair skin. vibration at 200-300Hz due to specialized capsule
• #9: Repetitive discharge = train of action potentials Slowly adapting mechanoreceptors continue responding to a stimulus, whereas rapidly adapting receptors respond only at the onset (and often the offset) of stimulation. These functional differences allow the mechanoreceptors to provide information about both the static (via slowly adapting receptors) and dynamic (via rapidly adapting receptors) qualities of a stimulus.
• #10: A receptive field is a region that causes activation of a sensory neuron when stimulated. Stimulus may not be perceived unless several sensory receptors are activated (spatial summation) or unless the firing rate is sufficiently high (temporal summation) to bring postsynaptic neurons to threshold. Small receptive fields are characteristic of Meissner’s corpuscles and Merkel’s disks (2-4 mm in diameter). Pacini’s and Ruffini’s corpuscles have large receptive fields. Example: Ruffini’s corpuscles respond to stretch of skin even at some distance from receptor site.
• #11: Meissner’s Corpuscles (FA1 afferents) also rapidly adapting receptors respond best to vibration in 30-40 Hz range Meissner’s corpuscles are located in ridges of the dermis of glabrous skin; they are much smaller than Pacini’s corpuscles. Meissner’s corpuscles receive innervation from 2-5 axons, while one axon innervates about 20 corpuscles; other receptors innervated by single axon. The rapidly adapting receptors (Pacini’s and Meissner’s corpuscles) provide contact information, slip information, &contrast information (e.g. important for edge detection). Merkel’s Disks (SA1 afferents) slowly adapting receptors that respond to touch Merkel’s disks are located at border of dermis and epidermis of glabrous skin. Merkel cells seem to best encode spatial characteristics of textured surface. Pacinian Corpuscles (FA2 Afferents) respond transiently to applied pressure during sustained pressure layers slip reducing deformation of receptor membrane also respond during restitution of membrane when pressure is released respond best to vibration in 200-300 Hz range Ruffini’s Corpuscles (SA2 afferents) slowly adapting receptors respond to indentation of the skin (touch & low frequency vibration) Hair Follicle Receptors innervated by free nerve endings that wrap around or run parallel to follicle bending of hair causes deformation of follicle which deforms nerve endings and modulates their firing frequency
• #12: Meissner’s Corpuscles (FA1 afferents) also rapidly adapting receptors respond best to vibration in 30-40 Hz range Meissner’s corpuscles are located in ridges of the dermis of glabrous skin; they are much smaller than Pacini’s corpuscles. Meissner’s corpuscles receive innervation from 2-5 axons, while one axon innervates about 20 corpuscles; other receptors innervated by single axon. The rapidly adapting receptors (Pacini’s and Meissner’s corpuscles) provide contact information, slip information, &contrast information (e.g. important for edge detection). Merkel’s Disks (SA1 afferents) slowly adapting receptors that respond to touch Merkel’s disks are located at border of dermis and epidermis of glabrous skin. Merkel cells seem to best encode spatial characteristics of textured surface. Pacinian Corpuscles (FA2 Afferents) respond transiently to applied pressure during sustained pressure layers slip reducing deformation of receptor membrane also respond during restitution of membrane when pressure is released respond best to vibration in 200-300 Hz range Ruffini’s Corpuscles (SA2 afferents) slowly adapting receptors respond to indentation of the skin (touch & low frequency vibration) Hair Follicle Receptors innervated by free nerve endings that wrap around or run parallel to follicle bending of hair causes deformation of follicle which deforms nerve endings and modulates their firing frequency
• #13: In accordance with this functional diversity, TRP channels are implicated in a multitude of physiological processes, ranging from Ca2+ and Mg2+ homeostasis and regulation of the vascular tone to bone development, taste perception, temperature sensing and vision. These membrane receptors belong to a class (family) of proteins called the transient receptor potential proteins; these receptors bind to the chemical capsaicin, the molecule in chili peppers that gives them their heat. Other binds different chemicals including camphor, menthol, and mustard oil
• #15: Stimulus-response curve of cold receptors is non-monotonic so temperature cannot be signaled by firing rate alone; perceived temperature is determined by relative activity of cold and warm receptors. Rapid changes in skin temperature evoke dynamic responses (transient increase in firing rate of warm receptors for increase in temperature or transient increase in firing rate of cold receptors for decrease in temperature).
• #16: Neuroactive substances include histamine and bradykinin in sites of infection Adelta: fast, myelinated C: slow, unmyelinated
• #17: Increased pain following sunburn. Secondary hyperalgesia describes pain that occurs in surrounding undamaged tissue.
• #18: large 1 afferent axons bifurcate in dorsal column and branches: rostrally to dorsal column nuclei (DCN) caudally, over a distance of several spinal segments Collateral branches of primary afferents project ventrally into grey matter of spinal cord where they synapse on interneurons and motoneurons. The person is on all fours because this is how dermatomes develop during development, they are distorted during embryological development. Epidural from L3-4 blocks sensation in uterus and lower dermatomes. Inserted into epidural space because this is the location of the dorsal root ganglia
• #19: Carry signals from mechanoreceptors of the skin, joints & muscle  fine, discriminatory touch Large, myelinated fibers with rapid conduction velocities Information has high spatial & temporal resolution 1º afferents terminate & form synaptic connections with 2nd order neurons in the dorsal column nuclei within the medulla 2nd order neurons cross over at the medulla and continue to the thalamus via the medial lemniscus pathway Pain, temperature, crude touch, tickle, itch, sexual sensations Small, myelinated fibers with slower conduction velocities Low spatial or temporal resolution 1 afferents terminate upon entering spinal cord & synapse on 2nd order neurons 2nd order neurons cross to contralateral side, ascend to brain in ventral part of lateral funiculus & project to thalamic nuclei 3rd order neurons project to somatosensory cortex & other cortical areas
• #20: START! Mechanoreceptors of the skin, joints and muscle provide tactile, discriminative, and proprioceptive information. A high spatial and temporal resolution refers to the ability to discriminate details of spatial location and stimulus intensity. Fasciculus gracilis: fibers conveying information from lower limbs are in the medial region Fasciculus cuneatus: fibers from upper limbs, trunk and neck are in the lateral region DCN = gracile and cuneate nuclei
• #21: most prominent ascending pathway for pain and thermal sensations, also carries crude tactile sensation thalamic nuclei include VPL nucleus & intralaminar complex & others cortical projections of 3rd order neurons are involved in affective responses such as the cingulate gyrus and insula, which have limbic system functions
• #22: Somatotopic order in the human primary somatic sensory cortex. (A) Diagram showing the region of the human cortex from which electrical activity is recorded following mechanosensory stimulation of different parts of the body. The patients in the study were undergoing neurosurgical procedures for which such mapping was required. Although modern imaging methods are now refining these classical data, the human somatotopic map first defined in the 1930s has remained generally valid. (B) Diagram along the plane in (A) showing the somatotopic representation of body parts from medial to lateral. (C) Cartoon of the homunculus constructed on the basis of such mapping. Note that the amount of somatic sensory cortex devoted to the hands and face is much larger than the relative amount of body surface in these regions. A similar disproportion is apparent in the primary motor cortex, for much the same reasons (see Chapter 17). (After Penfield et al., 1953, and Corsi, 1991.) after denervation, representation of neighboring skin regions expands to occupy that of denervated region (may enhance sensitivity to stimuli or ability to process) - signals from neighboring regions to compensate for missing input {Fig. 14-13} excessive use of one digit at expense of others results in expansion of its cortical representation
• #25: Allodynia: different form hyperalgesia in that a normally non painful stimulus, such as light touch, is felt as extrememley painful, (A fiber mediated) whereas in hyperalgasia the painful stimulus intensity is heightened (A or C fiber mediated)
• #26: Reduced density not known if it is a cause or a result of CRPS
• #28: Left side of body fine discriminatory touch below T4 Right side of the body, pain temp heat below T4 3. Moderately adpating phasic receptors with small receptive fields.