Somatosensory System MBBS1 GBE PDF

Somatosensory System MBBS1 GBE Isabella Gavazzi ( [email protected]) Wolfson SPaRC – Neuroscience Education  the classification of the senses [GBE 8] Describe the transduction of somatosensory signals [GBE 8] Describe...

Somatosensory System MBBS1 GBE Isabella Gavazzi ( [email protected]) Wolfson SPaRC – Neuroscience Education  the classification of the senses [GBE 8] Describe the transduction of somatosensory signals [GBE 8] Describe the different types of cutaneous and proprioceptive somatosensoy receptors Learning Describe and their function [GBE 8] the four attribute of sensory stimuli, modality, intensity, duration and location Describe outcomes [GBE 8] the terms receptor sensitivity, receptor specificity, and receptive field [GBE 8] Define : Describe the factors that contribute to the high somatic sensory acuity of the hands and face. [GBE 8] the submodalities of somatic sensation subserved respectively by the Dorsal Describe Column-Medial Lemniscus system and by the anterolateral system [GBE 8] the topographic representation of the body at the level of the somatic sensory Describe cortex. [GBE 8] the plasticity of cortical maps [GBE 8] Explain In order to control behaviour, the brain senses: The surrounding environment The internal milieu of the body The Five Senses hearing vision through which the brain senses the surrounding taste smell environment But this is not all the brain touch senses  Somatosensation is not touch only… Different sensory modalities are mediated by different receptors Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill Physiological mechanisms of sensation TRANSDUCTION TRANSMISSION PERCEPTION MODULATION F. M. Ferrante, T. R. VadeBoncouer Churchill Livingstone, 1993 Sensory receptors are specialized to transduce a particular type of stimulus energy into electrical signals (receptor specificity). Different sensory neurons have different transduction molecules, which respond to different stimuli  Primary sensory Anatomy of DRG neurons form two principal subgroups, Neurons A large and C small DRG Touch is mediated by 4 types of mechanoreceptors The receptors differ in morphology, innervation patterns, location in the skin, receptive field size, and physiological responses to Kandel, Schwartz, Jessel. 2000 Principles of touch. neural sciences. McGraw-Hill  A Sensory receptors in the joint Proprioceptors: to report on limb position B Joint capsule receptors transduce tension in the joint 1. Joint receptors capsule while perception of angle achieved via afferent signals from muscle spindles and efferent motor  Muscle spindles lie in parallel with main muscle fibres (sensitive to small changes in length with range set by special motor input) regulate motor control of muscle length Proprioceptors contribute to sense of limb position (continued) 2. Muscle receptors Golgi Tendon Organs lie in series with main muscle fibres and are sensitive to changes in muscle tension  Thermoreceptors respond to warm or cold stimuli in the non-noxious range ‘hot’ and ‘cold’ are signalled by different neuronal populations most cold-sensitive fibres in humans are small myelinated Aδ axons most warm-sensitive fibres in humans are small unmyelinated C axons Thermoreceptors mainly detect temperature changes  Nociceptors respond to damaging or potentially damaging stimuli Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill There are 4 main modalities of Nociceptors Nociceptors Receptors Modality: different receptors Intensity: encoded by are activated by different type firing frequency (and respond to of stimuli (‘adequate stimulus’); the concept of number of receptors activated; also different 4 receptor specificity; depends on the presence of different receptors have different thresholds of activation: transduction molecules receptor sensitivity) properties of a stimulus: Duration: encoded by Location: related duration of firing (and to receptive fields changes in spike trains) Somatosensory receptors for different stimulus modality are associated with different types of axons which show different conduction velocities  Information on intensity and duration of stimuli is conveyed by changes in the firing rates of sensory neurons Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill  The receptive field of a sensory neuron is the spatial domain where stimulation excites (or inhibits) the neuron Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill  The two-point perceptual thresholds of different parts of the body match the diameters of the receptive fields of receptors Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill  Map of dermatomes in a typical adult: A dermatome is the area of skin and deeper tissues innervated by a single dorsal root ganglion Important to determine the location of suspected spinal lesions Kandel, Schwartz, Jessel. 2000 Principles of neural sciences. McGraw-Hill The dorsal columns/medial lemniscal system 3rd order neuron (red) ascends ipsilaterally in the spinal cord, and conveys mechanosensory information from the limbs and trunk to the cortex via to the VPL thalamus. 2nd order neuron 1st order neurons  The contralateral anterolateral system 3rd order neuron (in brown; spinoreticular, spinomesencephalic and spinothalamic tracts) crosses the midline in the spinal cord and transmits itch, 2nd order temperature, and visceral neuron information to the cortex via the brain stem and thalamus. 1st order neurons Somatosensory Cortex The somatic sensory cortex has 3 major divisions: the primary (S1) and secondary (S2) somatosensory cortices and the posterior parietal cortex S1 is subdivided into 4 cytoarchitectonic regions, Brodmann’s areas 3a, 3b, 1 and 2  Somatotopic map of the body surface onto the primary sensory cortex NB: The map is discontinuous and distorted Bear, Connors and Paradiso. Neuroscience: Exploring the Brain. Williams and Wilkins  Postcentral Central gyrus sulcus Postcentral sulcus Inputs of individual Skin (RA receptors) modalities to the Deep tissue (pressure and joint position) somatic sensory Skin (SA and RA receptors) cortex are Deep tissue (muscle stretch receptors) organised in To ipsilateral S2 Contralateral S1 columns. posterior parietal cortex and motor cortex From thalamus To basal ganglia, brain stem and spinal cord Fourth digit Third digit Second digit  Size and shape Texture Projections to Brodmann’s areas from the thalamus Inputs are processed serially Texture, size, shape Schwartz and Jessel. Principles of Neurosciences. Elsevier How is sensory information integrated to give rise to perception?  Area 3b Area 1 Area 2 Neurons in S1 have receptive fields of different sizes Neurons in Brodmann’s area 1 and 2 have larger receptive fields than neurons in area 3b and different modalities converge 3a 3b 1 2 Connections in S1 Kandel, Schwartz and Jessel. Principles of Neurosciences. Elsevier Can we identify a specific brain area as responsible for pain perception? A collection of brain regions is accessed during nociceptive processing: the so-callled ‘Pain Matrix’. However, it is extremely unlikely that the ‘Pain Matrix’ is indeed nociceptive-specific. It is likely to be a multimodal network related to the detection of salient sensory input. Tracey and Mantyh (2007) Neuron 55(3): 377-391 Somatotopic maps are plastic  Use-dependent plasticity: Assessment of sensory cortical representation asymmetries in violin players Violin player with somatosensory evoked potential (SEP) mapping stimulation of the median nerve (red) and ulnar nerve (blue) Control Schwenkreis, P, El Tom, Susan, Ragert, P, Pleger, B, Tegenthoff, M, Dinse, RH (2007) Eur J of Neurosci 26(11): 3291-3302  Clinical relevance of somatosensory cortex plasticity  Map plasticity (maladaptive) may underlie pain following nerve injury (phantom limb pain, pain following Peripheral Nerve Injury)  therapies targeting the maladaptive plasticity, i.e. mirror therapy, virtual reality interventions  Map plasticity can occur as a consequence of injury e.g. stroke  intervention can exploit plasticity for functional recovery  Map plasticity underlies learning, but excessive synchronicity of stimuli can lead to pathology, e.g. focal hand dystonia in musicians  therapies target the maladaptive plasticity  Overview Information about the environment and the Information is processed serially The brain constructs a neural body is conveyed to consciousness from along these pathways, with representation of external mechano- and other sensory receptors by the different modalities initially objects and of our own bodies dorsal columns tracts and the anterolateral processed in parallel. The by analyzing patterns of activity system of the spinal cord, relay nuclei in the pathways are topographically across the entire population of brain stem and thalamus, and a hierarchy of organised (somatotopy), but sensory neurons, progressively intracortical pathways. somatotopic maps can be integrating information from altered by training, injury or larger receptive fields and other pathologies. different modalities.  Reading Neuroscience – Exploring the Brain (3rd edition) Bear, Connors and Paradiso (2007) Chapter 12 The Somatic Sensory System Neuroscience (5th edition) Purves et al. (2012) Chapter 9 The Somatic Sensory System: Touch and Proprioception Chapter 10 Principles of Neural Science (5th edition) Kandel, Schwartz, Jessell, Siegelbaum and Hudspeth Chapter 21 Sensory Coding Chapter 22 The Somatosensory System: Receptors and Central Pathways Chapter 23 Touch Chapter 24 Pain Thank you for listening! For any question contact: [email protected]