Sensory information processing, somato-sensory systems

Transcription

1 mm? Sensory information processing, somato-sensory systems

2 Recommended literature 1. Kandel ER, Schwartz JH, Jessel TM (2000) Principles of Neural Science, McGraw-Hill, Ch. xx. 2. Berne EM, Levy MN, Koeppen BM, Stanton BA (2004) Physiology, Mosby, Ch. 6,7. 3. Schmidt RF, Lang F, Thews G (2005) Physiologie des Menschen, Springer, Ch. 13,14.

3 Q1 Which types of receptors are available for sensory transduction?

4 Receptor types, transduction mechanisms Classification according to... Localization - skin - muscle - joint -viscera - exteroceptors - interoceptors - proprioceptors Regulated parameter - blood pressure - blood volume -pco 2 -ph etc. Modality of sensation - touch -temperature - pain - vision - hearing - equlibrium - olfaction -taste Quality of sensation (touch) - pressure - touch -vibration Quality of transduction mechanism - chemoreceptors - mechanoreceptors - photoreceptors Temporal characteristics of transduction and information processing -P -D -PD -SA -RA

5 Receptor types, transduction mechanisms BLKS_2004_6.8

6 Receptor types, transduction mechanisms Transduction = transformation of the specific energy of a stimulus into an electrical signal BLKS_2004_6.9 'The law of the specific sensory energy' Johannes Müller ( ) 1828

7 Q2 How are the properties of cutaneous mechanoreceptors reflected in the electrical signals of first order neurons?

8 Endoding of stimulus properties Initial segment (spike trigger zone) Axon Receptor potential Myelin sheath Connective lamina - Deformation - Opening of Na/K channels - Depolarization of receptor membrane - Summation of receptor potentials - if threshold is surpassed generation of action potential (AP) - If stimulus persists and receptor maintains sensitivity - repetitive AP generation

9 Endoding of stimulus properties Encoding of stimulus intensity (amplitude of deformation) Axon Frequency code Receptor Amplitude code Stimulus AP frequency (AP/s) Please note: The frequency increment decreases with increasing level of stimulus intensity Sensory threshold Stimulus intensity (pressure, N/cm 2 )

10 Endoding of stimulus properties Weber, Fechner, Helmholtz, von Frey described the relationship between stimulus intensity and sensation The law of Weber and Fechner E = k x logi/i 0

11 Endoding of stimulus properties Encoding of stimulus kinetics (temporal characteristics of deformation) Axon Receptor Stimulus AP frequ. Adaptation t (s) Why? What for? - Mechancal properties of receptors - Properties of ion channels - Information on speed and duration of the stimulation

12 Endoding of stimulus properties BLKS_2004_6.10 Responses of slowly and rapidly adapting mechanoreceptors to displacement of the skin

13 Endoding of stimulus properties BLKS_2004_6.12 Encoding for the frequency of stimulation: AP generation in phase with stimulus

14 Endoding of stimulus properties Merkel cell endings Ruffini endings Pacinian corpuscules Hair follicle receptor Meissner's corpuscules SA SA (v)ra RA hypodermis RA: rapidly adapting SA: slowly adapting

15 Endoding of stimulus properties Encoding of stimulus history (number of repetitions, relevance for behavior) Axon Receptor Stimulus AP frequ. Habituation 123 Number of repetitions The efficacy of irrelevant stimuli is decreased by habituation

16 Endoding of stimulus properties Encoding of stimulus position (spatial characteristics of deformation) Meissner Pacini The sensory neurons have receptive fields. Their size increases with the depth of the receptor in the skin The receptive field is a section of the sensory surface where a stimulus is able to change the activity of a given sensory neuron The receptive fields of neighbouring fibers/neurons overlap

17 Q3 How does the brain asign a specific sensation to a given mechanical stimulus of the skin? (Perception of pressure, touch, vibration)

18 Stimulus perception hypodermis Surface (small RFs) Depth (large RFs) Hair follicle receptor Meissner's corpuscules (detection of structures) Merkel cell endings (detection of fine edges) Pacinian corpuscules (detection of vibration) Ruffini endings (detection of diffuse stretch) Importance of sensory experience in shaping the correlation between perception and spatio-temporal stimulus characteristics

19 Stimulus perception BLKS_2004_7.2

20 Q4 How are somato-sensory signals transmitted to the cortex?

21 Somato-sensory pathways BLKS_2004_Tab6.2 The fiber spectrum in the peripheral nerves

22 Somato-sensory pathways Elements of a reflex to activation of skin receptors BLKS_2004_6.7

23 Somato-sensory pathways BLKS_2004_7.1

24 Somato-sensory pathways Please note: - Many collateral connections at all levels - neighbourhip relationsships are maintained throughout the brain Somatosensory cortex (Gyrus postcentralis, SI, SII) Arm representation Thalamus (Nucl. ventralis posterior, VP) Lemniscus medialis Nuclei of dorsal columns (Nucl. gracilis, cuneatus) Skin on arm Medulla Dorsal column Arm Trunk Somatotopic organization of connections Spinal cord Leg

25 Somato-sensory pathways BLKS_2004_7.7 The dermatomes

26 Q5 How are somato-sensory signals modified on their way to the cortex?

27 Central information processing Please note: - Weak convergence means high spatial resolution (skin on finger tips) - Strong convergence means low spatial resolution (skin on back) Many neurons in target area Functional consequences of divergent vs. convergent connectivity Few neurons in target area Receptive Fields remain small (1:1) Receptive Fields become larger (>>3:1)

28 Central information processing BLKS_2004_6.11 In the CNS neurons have reciprocally organized receptive field (inhibitory surround)

29 Central information processing The inhibitory surround is produced by lateral inhibition Neurons in the dorsal column nuclei Inhibitory interneuron Receptive field Lateral inhibition through collateral from neighboring neurons

30 Central information processing Descending pathways from the cortex produce a selection of behaviorally relevant sensory input (>> habituation) Signals from cerebral cortex Afferent signals from skin Inhibitory interneuron Neuron in nucl. of dorsal column Example: we are not continuously aware of cloths

31 Q6 How are somatosensory signals represented in the cerebral cortex?

32 Cortical representation BLKS_2004_7.8 'Homunculus': Cortical map of the skin surface

33 Cortical representation Somatotopic organization of skin afferents Please note: - larger area of representation for skin areas with higher spatial resolution SI Sulcus centralis caudal SII Face rostral This ordered representation requires organizing signals during ontogeny - chemical cues - experience-dependent adjustment

34 Cortical representation The SI cortex is subdivided into 4 parallel stripes 1 +3b: Surface sensibility 3a +2: Proprioception Hand representation Motor signals Parietal associative field (3-D recognition) Proprioception Surface sensibility Multiple representations (4 homunculi) ensure that for each localization/object several features are extracted at the same time

35 Cortical representation Each individual area contains neuron columns specifying the different features of the same object/at the same position Gray matter RA Surface RA Depth SA Surface SA Depth White matter 3b