OS 202 Neuroanatomy of the Somatosensory Pathway PDF

OS 202: HUMAN BODY AND MIND 1: INTEGRATION AND CONTROL SYSTEMS NEUROANATOMY OF THE SOMATOSENSORY PATHWAY UPCM 2029 | Dr. Kenny S. Seng | LUIII A.Y. 2024-2025 ​ Sensor...

OS 202: HUMAN BODY AND MIND 1: INTEGRATION AND CONTROL SYSTEMS NEUROANATOMY OF THE SOMATOSENSORY PATHWAY UPCM 2029 | Dr. Kenny S. Seng | LUIII A.Y. 2024-2025 ​ Sensory receptors are classified according to: OUTLINE ○​ Stimuli detected I.​ Introduction V.​ Tracts of the Face ○​ Adaptation A.​ Definitions A.​ Gasserian Ganglion ○​ Structure B.​ Somatosensation B.​ Mesencephalic, Chief STIMULI DETECTED II.​ Somatosensory System Sensory, and Spinal A.​ Receptor Nucleus ​ Mechanoreceptors Classifications C.​ Ventral Posteromedial ○​ Stimulated by pressure or stretching B.​ Sensory Modalities Nucleus ○​ Touch, pressure, vibration, hearing, proprioception, equilibrium, III.​ Somatosensory Tracts VI.​ Visceral Afferent and blood pressure A.​ Naming the Tracts Pathway ​ Thermoreceptors B.​ Neurons of the A.​ Visceral Pain ○​ Detect temperature Sensory Pathway B.​ Somatovisceral ​ Nociceptors IV.​ Ascending Tracts Convergence ○​ Detect tissue damage A.​ Dorsal Column Medial C.​ Referred Pain ​ Photoreceptors Lemniscal Tract VII.​ Pain ○​ Detect light B.​ Spinocerebellar Tract A.​ Chemicals Involved in ​ Chemoreceptors C.​ Spinothalamic Tract Pain ○​ Detect molecules D.​ Spinal Cord Pathways B.​ Anterolateral Systems ○​ Taste, smell, and changes in body fluid chemistry E.​ Other Ascending C.​ Control of Pain ​ Ex. taste buds, carotid sinuses (blood pressure), carotid Tracts Perception bodies (pH) VIII.​ References Table 2. TYPE OF RECEPTORS AND THEIR FUNCTIONS Type Stimulus Function I.​ INTRODUCTION Mechanoreceptors Pressure, touch Respond to A.​ DEFINITIONS mechanical Table 1. DEFINITIONS pressure or distortion DEFINITION CNS PNS Collection of axons Tract Nerve Photoreceptors Light - Collection of neuron cell bodies Nucleus Ganglion Nociceptors Tissue damage Respond to damaging PERCEPTION VS. SENSATION stimuli ​ Perception ○​ Conscious awareness and interpretation of sensation Thermoreceptors Temperature Respond to ○​ Precise localization and identification changes in ○​ Memories of one’s perceptions are stored in the cortex blood ​ Sensation concentrations ○​ Stimuli the body is aware of ○​ Involve chemo-, thermo-, noci-, and baroreceptors Chemoreceptors Molecules Respond to ​ Action potential is generated via influx of cations changes in ​ Permeability of channels may change depending on the stimuli blood ○​ No sensory receptors for x-rays, ultra high frequency sound concentrations waves, and UV light to ​ We are unaware of their effects on us due to the lack of receptors maintain ​ E.g., High x-ray dose for tumor prevention, high frequency homeostasis ultrasound waves for tremor improvement (cause lesions in the thalamus) Proprioceptors (found in Changes in Receive stimuli ASCENDING VS. DESCENDING PATHWAYS AND TRACTS structures that are stretched length from joints, ​ Pathways accomplish primary functions of the nervous system [e.g., muscle, tendons]) tendons, ○​ Ascending pathways muscles in ​ Neural pathways to transmit impulses from receptors to the relation to brain circuitry position ○​ Descending pathways ​ Brain manipulates the circuitry to form directives that are ADAPTATION transmitted via neural pathways to effectors ​ Example: you want to take a shower but you forgot to turn on the ​ Tracts communicate to and from the brain heater ○​ Ascending tracts = Sensory ○​ Sudden change in temperature (room temperature → cold) will ​ Deliver information to the brain produce a shock response ○​ Descending tracts = Motor ○​ After the initial unpleasant sensation, your body adjusts to the ​ Deliver information to the periphery sensation ​ Thermoreceptors within that temperature range would B.​ SOMATOSENSATION eventually stop firing because they adapt rapidly [2025 Trans] ​ Somatosensory system serves as the conduit between the brain ​ Does not mean that your body does not detect the stimulus, and the external environment but there is just a downregulation of the firing of the ​ Millions of sensory neurons deliver information to the CNS at all times receptors ​ Millions of motor neurons cause the body to respond in varied ways Classifications ○​ Motor neurons deliver CNS instructions to the body for a response ​ Sensory and motor neurons travel by different spinal cord tracts ​ Slow Adaptation ○​ There is a neuron residing in the dorsal root ganglia ○​ Slow to give a response to a stimulus ○​ The axons stretch all the way out to the fingertips ​ When stimulus is removed, response falls slowly ○​ Because it is a pseudounipolar neuron, the other end of the ○​ Merkel Discs axon ends at the dorsal gray horn to make synapses ​ Used more for discriminative touch (stereognosis) ​ Carries the impulse in the spinal cord to go up to the brain ​ Fast Adaptation ○​ Rapid but unsustained response ​ Ex. When placing your hand on a surface, after some time, you forget that your hand is on the surface ○​ Meissner’s Corpuscles ​ Fastest adaptation due to its involvement in the sensation of touch (Seng, 2022) ​ Online literature states that Pacinian corpuscles adapt more rapidly than Meissner corpuscles (Purves et al., 2001) Figure 1. Receptor in the dorsal gray horn II. SOMATOSENSORY SYSTEM Table 3. ADAPTATION OF SOMATOSENSORY RECEPTORS A. RECEPTOR CLASSIFICATIONS RECEPTOR STIMULUS ADAPTATION Trans 5 TG1 Abacan, Abellaneda, Adviento, Aggabao, Agustin, Alandy-dy, Alano, Alcantara TH: Narvasa 1 of 9  ​ Tongue and fingertips can detect shorter two-point distances Pacinian corpuscles Vibration Very rapid due to the relative abundance of nerve endings compared to the back Hair follicles Touch Rapid ​ Not the same for everyone, but: ​Tongue can detect 2 mm distance Meissner corpuscles Touch Rapid ​Fingertips can detect 4 mm distance ​Skin of the back can reliably detect about 6 cm distance Merkel’s discs Pressure Slow ○​ Stereognosis ​ Figure out what is being touched without looking at it Ruffini endings Pressure, Slow stretch STRUCTURE ​ Free nerve endings ○​ Bare dendrites ○​ Pain, temperature, tickle, itch, light touch ​ Encapsulated nerve endings ○​ Dendrites enclosed in connective tissue capsule ○​ Pressure, vibration, and deep touch ​ Ex. Meissner corpuscles, Merkel’s discs, Pacinian corpuscles ​ Separate sensory cells ○​ Specialized cells that respond to stimuli Figure 4. Discriminative touch. ○​ Vision, taste, hearing, balance Pressure and Vibration B. SENSORY MODALITIES ​ Pressure: sustained sensation felt over a long period of time and ​ Property by which one sensation is distinguished from another over a large area ○​ Different receptors are needed for these ○​ Wider type of sensation compared to touching ​ Two classes of sensory modalities: ○​ Component of some weight to the touch ○​ General senses ​ Vibration: rapidly repetitive sensory signals from tactile receptors ○​ Special senses ○​ Corpuscles of touch and lamellated corpuscles ​ Different types of sensations: touch, pain, temperature, vibration, ​ Type II cutaneous mechanoreceptors (Ruffini) and lamellated hearing, vision (Pacinian) corpuscles [2025 Trans] TACTILE SENSATIONS ○​ Receptors for pressure ​ Meissner corpuscles ​ Touch, pressure, vibration, itch, and tickle ○​ Detect gentle touch ​ Free nerve endings [2026 Trans] ​ Ruffini endings ○​ Dendrites exposed to stimuli ○​ Detect firm massage (pressure) ​ Pacinian corpuscles, Merkel’s discs [2026 Trans] ​ Pacinian corpuscles ○​ Encapsulated mechanoreceptors ○​ Detect vibration Figure 5. Pressure and vibration receptors and sensations on the skin. Itch and Tickle Figure 2. Tactile receptors and corresponding stimuli on the skin. ​ Free nerve endings Temperature and Touch ○​ Receptors for itch and tickle ​ Root hair plexus ​ Tickle: only sensation you cannot elicit on yourself (gargalesis) ○​ Light breeze ○​ Stimulation of free nerve endings by someone else ○​ Ex. a crawling ant disturbs your hair follicle ​ Itch: chemical stimulation (irritating chemicals) ​ Free nerve endings THERMAL SENSATIONS ○​ Temperature change ○​ Pain and temperature share receptors: extreme temperatures ​ Free nerve endings with 1 mm diameter receptive fields on the skin can be damaging to the body → interpreted as pain surface ​ Merkel’s Discs ​ Both adapt rapidly at first, but continue to generate impulses at a ○​ Brush of a feather low frequency ○​ Very sensitive ​ Pain is produced below 10°C and over 48°C because tissue damage ○​ Very light touch that doesn’t touch the hair is possible already (which will trigger the nociceptors) ​ Very responsive to changes due to adaptive property Nociceptors ​ Sensory receptors specialized in informing the CNS about the presence of a tissue-threatening stimulus ​ Free nerve endings that respond to stimuli that can cause tissue damage or when tissue damage has taken place ​ Receptors (protein molecules) present in the membrane of free nerve endings ​ Changes activity in the presence of a wide variety of stimuli (e.g., heat, pressure, chemicals) ​ Senses tissue damage ​ Transduction ​ Process of converting the stimuli into a nerve impulse Figure 3. Temperature and touch receptors and sensations on the skin. ​ Mechanical force → nerve impulse ○​ Requires change in flow of ions across nerve membrane ​ Crude touch: ability to perceive that something has simply ​ Allows entry of Na+ or Ca2+ ions to cause depolarization touched the skin ○​ Involves a receptor molecule that either directly or indirectly ​ Discriminative touch (fine touch): provides specific information opens ion channels about a touch sensation ○​ Location, shape, size, and texture of stimulus source ○​ Requires a more specialized type of receptor Two-point Discrimination OS 202 Neuroanatomy of the Somatosensory Pathway 2 of 9  Conduction > 40 ms-1 5-15 ms-1 < 2 ms-1 Velocity Receptor Activation Low High and Low High Thresholds Light touch, Sensation on Rapid, sharp, Slow, diffuse, non-noxiou Stimulation localized pain dull pain s Figure 6. Process of transduction. Sensory Neuron Types ​ A-alpha (Aα) fibers Figure 8. Proprioceptors (Left to right: joint receptors, ○​ Related to muscle sense tendon receptors, muscle receptors). ○​ Proprioception III. SOMATOSENSORY TRACTS ​ A-Beta (Aβ) fibers ○​ Related to discriminative touch A.​ NAMING THE TRACTS ​ A-Delta (Aδ) fibers ○​ Related to pain and temperature ​ The name states where it starts and ends ○​ Fast Pain ○​ E.g., Spinocerebellar (starts at spinal cord, ends at cerebellum) ​ Damaged cells produce prostaglandins that cause change in ​ If the name begins with “spino” (e.g., Spinocerebellar) membrane potential of nerve endings → pain receptors are ○​ It is a sensory tract activated ○​ Delivers information from the spinal cord to the cerebellum ​ C fibers ​ If the name ends with “spinal” (e.g., Vestibulospinal) ○​ Related to pain, temperature, and itch ○​ It is a descending and motor tract ○​ Slow pain ○​ Delivers information from the vestibular apparatus to spinal cord ○​ Only type that is unmyelinated B.​ NEURONS OF THE SENSORY PATHWAY ​ Involves three types of neurons ​ 1st order ○​ ALWAYS the dorsal root ganglion (DRG) ○​ Enters spinal cord from periphery ​ 2nd order ○​ ALWAYS the one that decussates (crosses over to other side) ○​ Right side of the brain detects sensation from left side of the body ○​ Ascends in the spinal cord or brainstem to the thalamus ○​ Decussation (e.g., at the spinal cord or brain stem) depends on the tract (e.g., spinothalamic or dorsal column medial lemniscus) ​ 3rd order ○​ ALWAYS the thalamus ​ Primarily consists of nucleus ○​ Cell bodies from the thalamus project an axon going to the somatosensory cortex for the brain to detect the type of sensation ○​ Final destination will ALWAYS be the somatosensory cortex Figure 7. Primary afferent axons. Table 4. TYPES OF SENSORY NEURONS. SENSORY NAME DIAMETER RECEPTORS MODALITY Muscle spindle A-ɑ (I) 13-20 μm Golgi tendon Proprioception organ Muscle spindle Meissner’s Proprioception Figure 9. Neurons of the sensory pathway corpuscle Superficial Merkel’s touch Example A-β ​ How many neurons do you need to feel someone tickling you? 3 6-12 μm receptors Deep touch (II) ​ 1st neuron will always be the DRG; 2nd neuron will decussate, Pacinian Vibration corpuscle Touch signal will cross at either the spinal cord or brain stem (since 2 Ruffini ending tracts: spinothalamic or dorsal column), once crossed, will always Hair receptor go up to the 3rd neuron, the thalamus ​ Sensation of being touched is part of the spinothalamic tract, but it Pain is carried into the anterior spinothalamic area A-δ (III) 1-5 μm Temperature ​ All these sensations come from your limbs and go to the Ventral (cool) Posterolateral (VPL) nucleus of the thalamus Bare nerve ending Pain ​ VPL = From Limbs 0.2-1.5 Temperature ​ VPM = From Mukha (face) C (IV) μm (warm) Itch IV. ASCENDING TRACTS Muscle spindle A.​ DORSAL COLUMN MEDIAL LEMNISCAL TRACT A-ɑ (I) 13-20 μm Golgi tendon ​ Proprioception, vibration, discriminative touch, light touch Proprioception organ ​ Knowing the body’s position in space (proprioception) requires a combination of: Table 5. CHARACTERISTICS OF PRIMARY AFFERENT FIBERS ○​ Sense of force A-β FIBERS A-δ FIBERS C FIBERS ○​ Sense of weight ○​ Sight Small Smallest ○​ Equilibrium from inner ear Diameter Large (2-5 μm) (