The bodily senses

1. The bodily senses From Ch. 22 “Principles of Neural Science”, 4th Ed. Kandel et al

2. Bodily senses • Bodily senses = somatic sensation • Large variety of receptors • Distributed throughout the body • Sensory information • Nerves transmit information from the receptors by frequency modulation of electrical signals (action potentials)

3. Dorsal root ganglion (DRG) • DRG contains the cell bodies of sensory neurons (afferent spinal nerves) • The axons of the DRG neurons are known as afferents • All somatosensory information from limbs and trunk are transmitted via DRG • Stimulus transmission from sensory receptor to CNS • Primary afferent neuron has two branches to • Periphery • Spinal cord

4. The sensory receptor • Peripheral receptor • Located at the terminal of the sensory neuron • Molecular specialization that transforms one type of energy into action potentials • Special transducer molecules

5. Somatic receptor types • Fiber classification • Conduction velocity (skin) or fiber diameter (muscle) • Myelinated or unmyelinated • 4 major modalities (distinct system of receptors and pathways to the brain) • Discriminative touch (size, shape, texture, movement across skin) • Proprioception (joint position) • Nociceptors (tissue damage, inflammation, chemical irritation, pain, itch) • Thermal receptors (warm, cold)

6. Somatic receptor types 1 2 3 4

7. Somatic receptor types 1 2 3 4

8. Somatic receptor types 1 2 3 4

9. Nerve endings and fiber types • Nerve endings • Bare nerve endings • Thermal and painful (nociception) sensations • Encapsulated nerve endings • Touch and proprioception • Deformation of receptive surface • Large diameter myelinated axons (rapid conduction) • Mechanoreceptors (touch, greatest density in glabrous skin [hairless], finger tips, lips) • Proprioceptors (joint position) • Small diameter myelinated and unmyelinated (slow conduction) • Thermal receptors • Nociceptors

10. Location of nerve endings

11. Mechanoreceptors • Specialized organs surrounding the nerve endings • Sensitive to displacement/ deformation • 4 major types in glabrous skin • Superficial location (located below skin ridges) • Meissner corpuscle – in glabrous skin • Rapidly adapting, fluid filled structure, sense deformation of small areas • Merkel disk receptor - in glabrous skin and hairy skin • Slowly adapting, sense sustained pressure, salient bumps, sharp edges • Deep subcutaneous location (less numerous) • Pacinian corpuscle • Similar to Meissner corpuscle, rapid indentation, minute vibration, frictional displacement, small irregularities (edges/ corners) • Ruffini ending • Slowly adapting, links folds in skin at the joints, sense stretch and bending, shape of grasped objects, global properties of objects, wide area of skin

12. Mechanoreceptors • Deep receptors sense deformation of a wider skin area that extends beyond the overlying ridges • Nerve fibers to superficial layers branch off to several nearby sensory receptors • Nerve fibers in subcutaneous layers only innervate one receptor • 4 types of mechanosensitivity • Gentle touch of skin (well-localized) • Vibration (frequency and amplitude) • Texture (discrimination with fine spatial detail, two-point discrimination) • Shape of objects grasped

13. Small, well- localized Large, central zone with max sensitivity Receptive field (RF) Direction- specific stretch • Size and structure of RF vary

14. 2-3 mm diameter Fine spatial differences Small, well- localized 10-25 receptors 10 mm diameter Relative sensitivity to pressure Large, central zone with max sensitivity Direction- specific stretch coarse spatial differences Central zone with large continuous surface Directly above receptor Receptive field (RF) • Size and structure of RF vary

15. Uniform distribution Most numerous receptor types Fine spatial sensitivity Best at finger tips Finger tips are the most densely innervated region of the skin Receptor distribution 300 mechanoreceptive nerve fibers per square centimeter

16. Two-point discrimination • Two-point discrimination • Min distance as which 2 stimuli can be resolved as distinct • Determine if one or more points are stimulated • Spatial resolution depends on the RF size/ receptor density • Spatial resolution of stimuli varies across the body • Smallest receptive fields in fingers, lips, and tounge

17. Vibration sense • Vibration is coded by spike trains • Each AP signals one sinusoidal cycle • Vibration frequency is signalled by the AP frequency • Different receptors have different sensitivity • Merkel: 5-15 Hz • Meissner: 20-50 Hz • Pacinian: 60-400 Hz • Detection depends on size of skin indentation and frequency • Detection threshold = tuning threshold = Lowest stimulus intensity that evokes one AP/ cycle • Intensity of vibration depends on the total number of nerve fibers activated Sensory threshold

18. Adaption and threshold • Slowly adapting (SA) • Constant pressure • Rapidly adapting (RA) (1) Adapting at the beginning and end of stimulus (2) Encode sense of motion of object - Fires when position change (firing rate proportional to speed) - Stops firing when object is at rest • AP/ sec depends on indentation force • Sensory threshold • The minimum stimulus intensity generating an AP • RA’s have lowest touch threshold • Pacinian corpuscles are the most sensitive mechanoreceptor

19. Constant force Shape and size • P= F/a • At constant force (F), the smaller area (a) stimulated results in bigger pressure (P) => higher firing rate • Strong initial response • Firing rate is proportional to the curvature of each probe

20. Smaller Receptive field diameter Bigger Higher Spatial resolution Lower Spatial characteristics • Texture, size, and shape are signalled by population of receptors • Periodic firing of groups of receptors signal the spatial characteristic • Active and inactive receptors contribution • The individual receptor is only stimulated by a part of the pattern • The spatial resolution depends on receptor density and type of receptor • Natural stimuli rarely activates a single receptor alone

21. Example: lifting an object • Lifting and object • Grasp, force increase, object lifted, vertical gravitational pull, force decrease, release • Grasp and release • Meissner c. : contact/ release; increased grasp force • Pacinian c. : transient pressure at start/ stop • Grip force • Merkel disks: continous firing/ proportional with force • Gravitational pull • Ruffini endings: slowly adapting, sense stretch

22. Somatic receptor types 1 2 3 4

23. Peak sensitivity Adaption Silenced Thermal receptors 4 thermal sensations: cold, cool, warm, hot • Constant temperature • Adaption • Tonic discharge/ steady rate • Body temperature • Continuously low rate • Cold fibers more active • Most sensitive to changes in temp than constant temp • Warm fibers: • Range, 29-49 deg • Peak/ preferred, 45 deg • Cold fibers: • Range 5-40 deg • Peak/ preferred, 25 deg Encoding of temperature involves comparing the relative activity of different populations

24. Somatic receptor types 1 2 3 4 Proprioception: sense of position and movement of one’s own limbs wo. Vision (1) static limb position, (2) limb movement (kinesthesia); in muscle and joints

25. Afferent fibers • Different size and conduction velocity of axons • Large fibers conduct faster than small/ thin fibers because the internal resistance to current flow is low and nodes of Ranvier are spaced further apart • Myelination sheets increase conduction velocity Compound AP = sum of all activated nerves Spike amplitude is proportional to fiber diameter

26. Innervations of dorsal roots Dermatomes Important for location of spinal injury

27. Distinct ascending pathways • Dorsal column-medial lemniscal system • Touch and proprioception from limbs and trunk • Somtatotopically organized from spinal to cortical level • Ascends ipsilateral side • Cross over to contralateral side in medulla • Anterolateral system • Spinal lamina I, IV, V, VII, VII • Pain and temperature from limbs and trunks • Cross over to contralateral side in spinal cord • Somtatotopically organized from spinal to cortical level Contralateral