The Bodily Senses Ch. 22, “Principles of Neural Science”, 5th Ed. 陳韋達 MD PhD 臺北榮總神經醫學中心 主治醫師 國立陽明大學醫學系/腦科所 副教授 哈佛大學醫學院麻省總醫院生醫影像中心 進修

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

Dorsal root ganglion neurons 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

The sensory receptors Located at the terminal of the sensory neuron Molecular specialization that transforms one type of energy into action potentials 4 major modalities (distinct system of receptors and pathways to the brain) Discriminative touch (size, shape, texture, movement across skin) Thermal receptors (warm, cold) Nociceptors (tissue damage, inflammation, chemical irritation, pain, itch) Proprioception (joint position) Reference

Classification of sensory fibers C (IV) Aδ (III) Aβ (II)1:6 Aα (I) 1:6

Classification of sensory receptors

Nerve endings and fiber types 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 Reference

Measurement of nerve conduction velocities Reference

Measurement of nerve conduction velocities

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 flutter 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

Location of touch receptors Aα (I) Aβ (II)1:6

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 Reference

Receptive field and adaptation of touch receptors

Activation of mechanoreceptors

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 tongue

Roles of mechanoreceptors in lifting 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: continuous firing/ proportional with force Gravitational pull Ruffini endings: slowly adapting, sense stretch

Nociceptors mediate mechanical pain Aδ (III) Aδ (III)

Itch sensation A distinctive cutaneous sensation Transducers are coupled to histamine receptor Mediated by fibers with very low conduction velocities (0.5 m/s) C (IV)

Thermal receptors normal skin 32C Cold pain < 15 C (C) TR normal skin 32C Cold pain < 15 C (C) cold  cool (Aδ) Heat pain > 45 C (Aδ) warm (C) heat TRP= transient receptor potential A = Ankyrin; M = melastatin; V=vanilloid *TRPV4 also play a role in touch.

Proprioceptors measure joint positions Aα (I) Aβ (II)1:6

Distribution of dermatomes Trigeminal (V1/V2/V3) Cervical C1-C8 Thoracic T1-T12 Lumbar L1-L5 Sacral S1-S5 Coccygeal (single)

Spinal gray matter

Dorsal horn (lamina I-VI) I, II, V I, II IV

Somatotopical organization of spinal cord Light touch vs. deep touch

Somatosensory ascending pathways Dorsal column-medial lemniscal system Touch and proprioception from limbs and trunk Ascends ipsilateral side and cross over to contralateral side in medulla Somtatotopically organized from spinal to cortical level Anterolateral (spinothalamic) system Pain, thermal and visceral sensation from limbs/trunk Cross over to contralateral side in spinal cord Branches: spinoreticular & spinalmesencephalic tracts

Pain Touch Thermal proprioception Anterolateral system 3 neuron Pain Touch Thermal proprioception Anterolateral system Dorsal column system 2 neuron 1 neuron

Homunculus