Somatosensory System PDF Fall 2024

Somatosensory System DPT 542– Functional Neuroscience Fall 2024 Objectives Understand the structure and function of somatosensory peripheral neurons, cutaneous receptors. Understand and be able to trace the main pathways for sensation. Identify the main historical theories...

Somatosensory System DPT 542– Functional Neuroscience Fall 2024 Objectives Understand the structure and function of somatosensory peripheral neurons, cutaneous receptors. Understand and be able to trace the main pathways for sensation. Identify the main historical theories of pain processing. Review of Terms Afferent refers to sensory neurons Efferent refers to motor neurons Somatotopic refers to the organization of areas within the CNS (SC and Brain) Review: Somatosensory Peripheral Neurons Cell bodies of most peripheral sensory neurons lie outside spinal cord in dorsal root ganglia or outside of brain in cranial nerve ganglia Peripheral Sensory neurons have 2 axons: Distal axons conduct messages from receptor to cell body Proximal axons project from cell body into spinal cord or brainstem Term: Unipolar or Psuedo- unipolar Classification of sensory neurons Axon classification: diameter, myelination and speed of conduction Review from week 2 Modality Specific : Receptor type (function) Location: cutaneous, muscle, joint  Conduction Velocity of Axons by Type Peripheral sensory neurons classified according to axon diameter Larger diameter transmit faster than smaller diameter (resistance to current flow lower in large diameter axons; larger diameter axons are myelinated) Sensory neuron fiber types Cutaneous Innervation Receptor fields also apply to other sensations such as vision. Receptive Field Area of skin innervated by single afferent neuron Distally smaller and greater density Proximal larger and less density Allows greater ability to distinguish between two closely applied stimuli on fingertips – same not true on trunk Three types of skin sensation Touch Pain Temperature Somatic Receptors In general, the type of environmental energy that a specific receptor responds to is unique and unimodal; some receptors are polymodal Classification of receptors By structure By the source of the stimulus By the type of stimulus energy By the rate of adaption Primary source of information to Spinal Cord or Brainstem Determine activity and output of the CNS Classification of Somatic Receptors By structure Free and diffuse nerve endings Encapsulated receptors By the source of stimulus Exteroreceptors Interoreceptors Proprioceptors Classification of Somatic Receptors By rate of adaptation Slowly adapting Rapidly adapting Classification of Receptors Chemoreceptors Smell, taste, pH, metabolites Photoreceptors Visual receptors Thermoreceptors Temperature (Hot or Cold) Mechanoreceptors Physical Deformation (Touch, Pressure, Stretch, or Vibration) Nociceptors Noxious – sensitive to stimuli that damage or threaten to damage tissues. Stimulation of these receptors leads to pain Cutaneous Innervation Superficial fine touch receptors Meissner’s Corpuscles (light touch/vibration) Merkel’s Disks (Pressure) Hair Follicle Receptors (displacement of hair) Subcutaneous fine touch receptors Large Receptive Fields (less localization) Pacinian Corpuscles (touch and vibration) Ruffini Endings (stretch of skin) Free Nerve Endings (crude localized touch/ pressure, tickle, and itch) Coarse Touch Nociceptors (respond to stimuli that threaten tissue) Thermal Receptors (respond to warmth or cold that does not threaten tissue) Cutaneous innervation Meissner’s corpuscle Light Touch/Vibration Merkel’s Disk’s (Cell) Pressure Pacinian Corpuscle Pressure and Vibration Ruffini Endings Respond to stretch of skin Adaptation speed Slow adapting fire continuously Fast adapting fire at onset and offset of stimulus Cutaneous innervation While each receptor type responds to specific types of stimulation, naturally occurring stimuli will affect more than one receptor at any given time. Musculoskeletal innervation Musculoskeletal innervation Muscle Spindle Detect when there is a stretch on Sensory organ in muscle the muscle and initiate reflex to Contains muscle fibers, sensory resist that stretch endings, and motor endings. Greater the density of muscle Sensory endings respond to changes spindles in a muscle the more in muscle length and velocity of precise the muscle can be length change (stretch) Fusiform shaped (tapered at both Muscle of the upper extremity digits ends) Proprioceptors Transmit information regarding: Muscle Length Muscle Tension Muscle Load  Muscle Spindle Intrafusal Fibers Specialized fibers inside muscle spindle Ends connect to extrafusal fibers and are contractile Stretching muscle stretches these fibers Two Types of Intrafusal Fibers Nuclear Bag Fibers (clump of nuclei in central region) Nuclear Chain Fibers (nuclei arranged single file) Extrafusal Fibers Ordinary skeletal fibers outside the spindle  Muscle Spindle Primary Endings (annulospiral endings) Type Ia Large myelinated (FAST) Wrap around central region of each intrafusal fiber Respond to Rate of Muscle Stretch and changes in muscle length  Muscle Spindle Secondary Endings (flower- spray endings) Type II Medium - Slower End on nuclear chain fibers Respond to changes in length of the muscle no matter the rate of stretch Function of Muscle Spindle Primary endings discharge phasic and tonic Phasic discharge maximal during quick stretch and fades quickly (reflex hammer) Tonic discharge sustained during constant stretch Rate is proportional to stretch of spindle fibers Secondary Endings are Tonic only Function of muscle spindle Muscle passively stretched – extrafusal fiber Causes intrafusal fiber to stretch Spindles elongated activating sensory receptors (Ia) in spindle – will fire no matter the rate of stretch  Normal muscle contraction Alpha (α) and gamma (γ) motor neurons active simultaneously Gamma (γ) firing causes intrafusal fibers to contract – maintains stretch on intrafusal fiber central region Abnormal condition Golgi tendon organs Encapsulated nerve endings woven among collagen strands of the tendon near the musculotendinous junction Structures that relay tension in tendons Sensitive to