Neurophysiology Lecture 2a: Somatosensory System PDF

PHSL 410b/511: Neurophysiology Lecture 2a: The Somatosensory System  Previously… Sensory experience is shaped by receptor systems Encode stimuli as electrical signals Each receptor is sensitive to a broad range of modality specific stimuli – rece...

PHSL 410b/511: Neurophysiology Lecture 2a: The Somatosensory System  Previously… Sensory experience is shaped by receptor systems Encode stimuli as electrical signals Each receptor is sensitive to a broad range of modality specific stimuli – receptive field Overlapping spectra produce a diversity of sensory perceptions The range of combinations defines our sensory experience  Today… Return to somatosensation Soma = body (Greek) Sense of one’s body Both to one’s self and in relation to the world around it Beautifully organized and conserved neurocircuit Can be studied in humans  Somatosensation has 3 primary functions 1. Interoception Stimulus sensitivity increases as one moves inwards! Sense of one’s organs Useful in monitoring organ function and potential damage Mostly unconscious 2. Proprioception Sense of one’s body in time and space Static and moving 3. Exteroception Relationship between the body and the external world E.g. The hood of the car is hot  Sensory Receptors Sensory modalities are defined by the receptors that encode a stimulus 4 major receptor types 1. Photoreceptors 2. Chemoreceptors 3. Thermoreceptors 4. Mechanoreceptors Somatosensation uses all but photoreceptors  Sensory Receptors Sensory modalities are defined by the receptors that encode a stimulus 4 major receptor types 1. Photoreceptors 2. Chemoreceptors 3. Thermoreceptors 4. Mechanoreceptors Somatosensation uses all but photoreceptors  Somatosensory receptors transmit stimuli electrically Sensory receptors have two primary functions: 1. Encode stimuli using electrical signals E.g. changes in pressure into trains or bursts of action potentials 2. Transmit to Brain Spinal cord, brain stem, and CNS Generates thought and motivates behavior To brain  Types of somatosensory receptors Thermo- and Characterized by submodality Mechanoreceptors Mechanoreceptors Nociceptors (e.g. high frequency vibration) (A and C) (A) (A and C) AND fiber group Mechanoreceptors are most diverse “Sheathed” and “Unsheathed” Depth from surface is a major factor Thermoreceptors and nociceptors have exposed nerve endings Unsheathed  Mechanoreceptors are ion channels Mechanoreceptors have been Top crystolographically resolved Cation Each has 6 subunits that make a barrel- like structure Side Ions flow through pore when opened by mechanical deformation Highly selective for the type of stimulus  3 Types of Mechanoreceptors Pulled open by the membrane 1. Opened through tension in the plasma membrane E.g. cellular swelling 2. Opened by proteins that link to the ion channel E.g. hair cells of inner ear 3. Opened by second messenger systems that activate the channel with deformation E.g. pain receptors  3 Types of Mechanoreceptors Pulled open by the membrane 1. Opened through tension in the plasma membrane E.g. cellular swelling 2. Opened by proteins that link to the ion channel E.g. hair cells of inner ear Tethers at the membrane 3. Opened by second messenger systems that activate the channel with deformation E.g. pain receptors Second messenger  3 Types of Mechanoreceptors Pulled open by the membrane 1. Opened through tension in the plasma membrane E.g. cellular swelling 2. Opened by proteins that link to the ion channel E.g. hair cells of inner ear Tethers at the membrane 3. Opened by second messenger proteins that are activated by mechanical forces E.g. pain receptors Second messenger  3 Types of Mechanoreceptors Fast Pulled open by the membrane 1. Fastest: direct activation through tension in the plasma membrane 2. Medium: proteins that link to the ion channel Tethers at the membrane 3. Slowest: second messenger systems that activate the Slow channel enzymatically Second messenger  Touch Mechanoreceptors Best characterized Differ based on: Morphology Location Receptive field Conduction velocity Adaptation  4 Types of Touch Mechanoreceptors 1. Meissner corpuscles Detect initial contact and motion Wrapped nerve endings Near skin surface Small receptive fields Fast conduction velocities Rapidly adapting (RA1)  4 Types of Touch Mechanoreceptors 2. Merkel cells Detect pressure and edges Unwrapped Near skin surface Medium conduction Smallest receptive fields Slowly adapting (SA1) Ideal for reading Braille  4 Types of Touch Mechanoreceptors 3. Pacinian corpuscles Detect very fast vibration Wrapped like an onion Deeper from skin surface Large receptive field BUT most receptive in center Fast conduction Rapidly adapting (RA2)  4 Types of Touch Mechanoreceptors 4. Ruffini endings Detect skin stretch and are essential for sensing the shape of objects Wrapped Deeper from skin surface Large, non-concentrated receptive field Slow conduction Slowly adapting (SA2)  “Proprioceptors” Mechanoreceptors that detect movement of the muscles Found on muscle spindles Wraps around them to efficiently open and close during stretch Fires in response to the velocity of stretch Adapts to constant stretch AND constant rate of change of stretch  Nociceptors Pain receptors Necessary for detecting current pain and past damage Firing rate depends on intensity, but adapt to constant force Activated by mechanical, thermal, and chemical signals  Nociceptors: 2 classes 1. A fibers Detect sharp, pricking, burning or pinching stimuli Short-latency Highly myelinated Small receptive fields Rapidly adapting  Nociceptors: 2 classes 2. C fibers Diffuse burning Slow conducting Large receptive fields so poorly localized Slow adapting  Thermoreceptors Ion channels that open in response to thermal sensations Cold, cool, warm, and hot Also open to certain chemicals Why certain foods like chili pepper can appear to have “temperature” Some are dedicated to the polar extremes, while others are mixed (large spectra) Allows for temperature complexity Highly sensitive but slower responding than the mechanoreceptors  Peripheral Nerves Bundled sensory axons of several modalities Classified by the diameter and amount of myelin surrounding each nerve Bundling multiple axons profoundly increases the conduction velocity of an AP  Classification of Peripheral Nerves Classified by Charles Sherrington 3 Myelinated A fibers A − fast, mechanoreceptors A − slow, mostly thermo/chemo/nociceptors 1 Unmyelinated C fiber Slowest, mostly thermo/chemo/nociceptors 1x 1/2x 1/4x 1/60x  Classification of Peripheral Nerves Classified by Charles Sherrington 3 Myelinated A fibers A − fast, mechanoreceptors A − slow, mostly thermo/chemo/nociceptors 1 Unmyelinated C fiber Slowest, mostly thermo/chemo/nociceptors  Different organs have different conduction demands Skin has greatest surface area and so requires more nerve endings Thus less myelin Conduction velocity No A alphas Muscles undergo large, rapid changes and so require faster conduction More myelin Fewer nerve endings Many types of peripheral nerves innervate the same tissue and overlap  Different organs have different conduction demands Skin has greatest surface area and so requires more nerve endings Thus less myelin Conduction velocity No A alphas Muscles undergo large, rapid changes and so require faster conduction More myelin Fewer nerve endings Conduction velocity Many types of peripheral nerves innervate the same tissue and overlap  Different organs have different conduction demands Skin has greatest surface area and so requires more nerve endings Thus less myelin Conduction velocity No A alphas Muscles undergo large, rapid changes and so require faster conduction More myelin Fewer nerve endings Conduction velocity Many types of peripheral nerves innervate the same tissue and overlap Allows for detection of multiple modalities  The Compound Action Potential Space demands limit the amount of myelin that can surround a nerve Fine discrimination requires exposed nerve endings Thus action potentials are only so fast and can travel only so far BUT, if peripheral nerves are close enough, their action potentials can sum  The Compound Action Potential Space demands limit the amount of myelin that can surround a nerve Fine discrimination requires exposed nerve endings Thus action potentials are only so fast and can travel only so far BUT, if nerves are close enough to one another, their action potentials can sum  The Compound Action Potential Larger axons fire sooner and are more sensitive than smaller axons The greater the stimulus, the more peripheral nerves are recruited Peripheral nerves have broad ranges of conduction velocity Activity persists after AP peak Thus, if APs overlap in time, they can amplify each others response Thus APs travel faster and farther  The Compound Action Potential Larger axons fire sooner and are more sensitive than smaller axons But peripheral nerves have broad ranges of conduction velocity Thus APs can overlap in time C fibers are too slow to add to the compound AP Overlapping APs amplify each others response Thus APs travel faster and farther  Spinal nerves Peripheral nerves combine into spinal nerves for faster signal transduction 31 pairs (2 for each side) Named for the vertebrae where they enter the spinal cord Lumbar 4, Thoracic 6, etc. Function based on the organs they innervate  Spinal nerves Peripheral nerves combine into spinal nerves for faster signal transduction 31 pairs (2 for each side) Named for the vertebrae where they enter the spinal cord Lumbar 4, Thoracic 6, etc. Function based on the organs they innervate  Dermatomes Overlapping receptive fields bundled into a single spinal nerve Thus receptive fields are large Less spatial resolution So spinal nerves can only detect a large area of tissue  Dermatomes are used clinically Dermatome maps can be used to locate spinal cord damage E.g. Thumb numbness = damage to C6 But spinal nerve damage can cause referred pain Pain in unaffected area that is part of the spinal nerve Some spinal nerves can innervate overlapping areas Thus damage to a particular spinal nerve can be difficult to locate  Dorsal root ganglion (DRG) Spinal nerve separates into sensory and motor fibers at DRG Cluster of cell bodies from sensory neurons Swelling outside the spinal column containing multiple cell bodies Further separate into modality specific fibers before entering the spinal cord  Dorsal root ganglion (DRG) Spinal nerve separates into sensory and motor fibers at DRG Swelling outside the spinal column containing multiple cell bodies Further separate into modality specific fibers before entering the spinal cord  Separate into submodalities in dorsal horn Fibers innervate specific anatomical regions in dorsal horn, representing modalities/submodalities 1. Dorsomedial columns Fine touch, proprioception, vibration 2. Lateral spinothalamic tract Pain and temperature 3. Anterior spinothalamic tract Light touch  Separate into submodalities in dorsal horn Fibers innervate specific anatomical regions in dorsal horn, representing modalities/submodalities 1. Dorsomedial columns Fine touch, proprioception, vibration 2. Lateral spinothalamic tract Pain and temperature 3. Anterior spinothalamic tract Light touch  Separate into submodalities in dorsal horn Fibers innervate specific anatomical regions in dorsal horn, representing modalities/submodalities 1. Dorsomedial columns Fine touch, proprioception, vibration 2. Lateral spinothalamic tract Pain and temperature 3. Anterior spinothalamic tract Light touch  Separate into submodalities in dorsal horn Fibers innervate specific anatomical regions in dorsal horn, representing modalities/submodalities 1. Dorsomedial columns Fine touch, proprioception, vibration 2. Lateral spinothalamic tract Pain and temperature 3. Anterior spinothalamic tract Light touch  Local vs Distal Terminals To CNS Neurons terminate locally and distally 1. Local branches Dorsal column medial lemniscal Antero- lateral Detecting pain and temperature through system system anterolateral system Involuntary movement (spinal reflex) A delta and C fibers Anterolateral pathway 2. Distal branches Voluntary and complex involuntary movement Composed of A fibers Dorsal column medial lemniscal pathway  Stretch Reflex Responsible for simple involuntary movement Discovered John Eccles in the 1950s (1) Proprioceptive sensory neurons from (2) muscle spindles connect to excitatory motor neurons and inhibitory interneurons in the spinal cord (3) Excitatory motor neurons excite extensors (4) Interneurons inhibit flexors Causes “kick out”  Ascending Pathways Modality-specific fibers project to midbrain through 2 tracts 1. Dorsal column medial lemniscal system Convey proprioceptive and tactile sensations Synapse and decussate in medulla Mostly fast A fibers 2. Anterolateral system Convey thermal and painful sensations Synapse and decusate in spinal cord Mostly slow C fibers  Ascending Pathways Modality-specific fibers project to midbrain through 2 tracts 1. Dorsal column medial lemniscal system Convey proprioceptive and tactile sensations Synapse and decussate in medulla Mostly fast A fibers 2. Anterolateral system Convey thermal and painful sensations Synapse and decusate in spinal cord Mostly slow C fibers  Ascending Pathways Modality-specific fibers project to midbrain through 2 tracts 1. Dorsal column medial lemniscal system Convey proprioceptive and tactile sensations Synapse and decussate in medulla Mostly fast A fibers 2. Anterolateral system Convey thermal and painful sensations Synapse and decusate in spinal cord Mostly slow C fibers  Decussations To CNS To CNS Dorsal column medial leminiscal system crosses in the medulla Anterolateral system crosses in Mediulla the spinal cord Reason why one side of body is represented and controlled by Spinal cord opposite side of brain Somatic twist hypothesis – moves organs to more protected side of body  Ascending Pathways Fibers are added successively Outer (caudal) to inner (rostral) When entering the brain stem, each system has over 1 million fibers Fibers begin to separate in the brain stem and thalamus to create crude map of the body “Somatotopically organized” Fully segregated in the cortex for full somatotopic map of the body  Conclusions Somatosensation informs the brain about the body in space and time Accomplished through incredibly sensitive receptors Organized into peripheral nerves and then spinal nerves with different conduction velocities for different demands Separated in the spinal cord and finally the brain to create somatotopic map of the body  Learning Objectives Understand the Functions of Somatosensation: Describe the Structure and Role of the Dorsal Root Explain interoception, proprioception, and exteroception, Ganglion (DRG): including their roles in sensory perception and body Understand the organization of sensory and motor fibers awareness. within the DRG and their projection patterns. Identify the Types of Sensory Receptors: Analyze Ascending Sensory Pathways: Differentiate among mechanoreceptors, chemoreceptors, Compare the dorsal column-medial lemniscal system and thermoreceptors, and nociceptors, and describe their the anterolateral system in terms of function, fiber types, specific roles in somatosensation. and decussation points. Classify Peripheral Nerves: Explore the Concept of Somatotopic Organization: Understand the classification system of peripheral nerves Explain how somatosensory information is mapped in the (Aα, Aβ, Aδ, and C fibers) and their conduction velocities. brain and spinal cord to represent different body parts. Explain Mechanisms of Sensory Signal Transmission: Apply Knowledge of Dermatome Mapping: Describe how sensory receptors encode stimuli into Utilize dermatome maps to predict spinal cord or nerve electrical signals and transmit them to the brain. damage based on sensory deficits. Examine Mechanoreceptor Functionality: Identify the subtypes of mechanoreceptors (e.g., Meissner corpuscles, Merkel cells, Pacinian corpuscles, Ruffini endings) and their adaptation and conduction properties.