Sensory System PDF

Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Proprioceptive and Tactile TOPIC OVERVIEW...

Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Proprioceptive and Tactile TOPIC OVERVIEW Discriminative Function v. Effects of Lesions b. Ventrolateral Quadrant System A. Introduction i. Primary Afferents B. Overview ii. Dorsal Horn Neurons a. Somatosensory Pathways iii. Spinothalamic Tract i. Direct (Discriminative) iv. Pathways for the Affective- Pathways Arousal Components of ii. Indirect Pathways Pain iii. Clinical Significance v. Effect of Lesion of the C. Receptors: General Organization and Spinothalamic System Mechanisms c. Pathways for Transmission of a. Receptor Specificity Simple Touch b. Impulse Initiation in Sensory G. Mechanisms of Pain and Algesia Receptors a. Nociceptive Afferents i. Direct (Discriminative) b. Dorsal Horn Pathways i. Interneurons in the Dorsal ii. Indirect Pathways Horn c. Encoding of Sensory Information c. Segmental Mechanisms d. Receptor Adaptation H. Somatosensory Pathways and Control of D. Functional Organization of the Sensory Motor Function Pathways a. Motor Function of the Dorsal a. Serial and Parallel Processing of Column-Lemniscal System Sensory Input b. Spinocerebellar Tracts b. Sensory Unit and Receptive Field i. Dorsal Spinocerebellar c. Thalamic Station Tract d. Primary Sensory Areas of the ii. Ventral Spinocerebellar Cerebral Cortex Tract E. General Organization of the c. Effect of Lesions Somatosensory System I. Differential Diagnosis of Ataxia a. Somatosensory Receptors a. Ataxia b. Dorsal Root Ganglions i. Types of Ataxia i. Clinical Significance b. Other Sensory Systes c. Dorsal Root Entry Zone and i. Visual Pathway Termination in the Spinal Cord d. Spinal Somatosensory Neurons e. Somatosensory Pathways ii. i. Direct (Lemniscal) Pathways ii. Indirect Pathways INTRODUCTION iii. Spinocerebellar Pathways F. Specific Somatosensory Pathways a. Direct Dorsal-Column-Lemniscal The sensory system provides information to the Pathway central nervous system (CNS) about the i. Primary Afferents external world, the internal environment, and ii. Dorsal-Column-Lemniscal- the position of the body in space System Afferent impulses- impulses traveling toward iii. Mechanisms of Sensory the CNS Discrimination Afferent impulses can be transmitted in three iv. Parallel Pathways for ways: PAGE 1 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Conscious data- perceived by the transmitted by different ascending organism and used to modify behavior. pathways ○ Unconscious data- modify behavior but remain unperceived by the organism. A SOMATOSENSORY PATHWAYS ○ Combination of both conscious and unconscious data. Somatosensory pathways from the trunk Afferent impulses are functionally subdivided and extremities travel through the spinal cord into the following categories: Pathways transmitting information from the ○ General somatic afferent- sensory face form the trigeminal system. information from the skin, striated Somatosensory Pathways Functions: muscles, and joints ○ transmission of precise information ○ General visceral afferent- largely about the type, intensity, and unconscious sensory information from localization of a sensory stimulus serosal and mucosal surfaces, smooth ○ initiation of arousal, affective, and muscle of the viscera, and adaptive responses to the stimulus baroreceptors ○ continuous unconscious monitoring ○ Special somatic afferent- sensory and control of motor performance information relating to vision, audition, and equilibrium ○ Special visceral afferent- sensory A1 DIRECT (DISCRIMINATIVE) PATHWAYS information relating to taste and smell Direct (Discriminative) Pathways: Important OVERVIEW for localization of lesions. Two Main Pathways: ○ Direct Dorsal Column Pathway: Receptor organs Transmits tactile-discriminative and ○ translate information from the conscious proprioceptive information environment, serving as specialized ○ Spinothalamic Tract: Transmits pain parts of the peripheral nervous and temperature sensations system ○ convert mechanical, chemical, photic, and other forms of energy into electrical potentials Action potentials are then transmitted through specific sensory pathways to regions in the central nervous system where the information is integrated, and perception occurs Receptive field- peripheral region from which a stimulus affects a central sensory neuron Conscious perception pathways have both: ○ Hierarchical organization- sensory information is transmitted sequentially by multiple neurons in relay nuclei and processed at each level, under the Three Orders of Neurons: control of higher stations ○ First-Order Neurons: Receptor ○ Parallel organization- different neurons in dorsal root or sensory submodalities within sensations (e.g., ganglia; peripheral branch innervates tactile, visual) are transmitted by receptors, central branch enters spinal separate, parallel channels, and a cord/brainstem sensory modality (like touch) is PAGE 2 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Second-Order Neurons: Located in A3 CLINICAL SIGNIFICANCE gray matter of spinal cord or relay nuclei; axons decussate and ascend ○ Third-Order Neurons: Located in Understanding pathways aids in localizing thalamic sensory nuclei; project to neurological disorders primary somatosensory cortex Lesions at different levels affect sensory Primary Somatosensory Cortex: functions differently ○ Located in the postcentral gyrus of Lesions in direct pathways impact specific the parietal lobe modalities (e.g., tactile discrimination in dorsal ○ Discriminates sensory impulses; column pathway, pain/temperature in organized somatotopically spinothalamic system). ○ Lower extremity on medial surface, Overlap in pathways means some modalities arm/hand on lateral, (e.g., touch) can still be perceived if one face/mouth/tongue in suprasylvian pathway is interrupted region. Visual pathways also have topographic ○ Pain representation includes parietal (retinotopy), hierarchical, and parallel cortex, insular cortex, and cingulate organization for information transmission gyrus ○ Dynamic representation with use- RECEPTORS: GENERAL ORGANIZATION AND dependent cortical plasticity MECHANISMS A2 INDIRECT PATHWAYS Sensory receptors ○ highly specialized structures that respond to environmental changes by Mediate arousal-affective aspects and visceral producing action potentials that are sensation. transmitted to the central nervous Key Pathways: Paleospinothalamic, system (transduction) spinoreticular, spinomesencephalic tracts, and propriospinal system Ascend bilaterally and terminate diffusely; A RECEPTOR SPECIFICITY important for pain mechanisms and visceral/sensory functions Each receptor type is specialized and more Dorsal and Ventral Spinocerebellar Tracts: sensitive (i.e., has the lowest threshold) to one ○ Transmit unconscious posture and particular kind of stimulus movement control information Receptors are classified by sensory modality: ○ Two-neuron pathways terminating in ○ Mechanoreceptors ipsilateral cerebellum ○ Chemoreceptors ○ Thermoreceptors ○ Photoreceptors Receptors can also be classified by the origin of the stimulus: ○ Exteroceptors: skin mechanoreceptors (for touch and pain), skin thermoreceptors, labyrinthine mechanoreceptors (for hearing), retinal photoreceptors, and chemoreceptors (for taste and smell). ○ Proprioceptors: mechanoreceptors in muscles, tendons, and joints, as well as vestibular mechanoreceptors. PAGE 3 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Visceral receptors: include ○ Olfactory receptors are the first neuron mechanoreceptors and in the olfactory pathway chemoreceptors that encode signals ○ Photoreceptors, hair cells, and taste related to internal body functions receptors release the excitatory transmitter L-glutamate, maintaining basal activity in the first-order neuron B IMPULSE INITIATION IN SENSORY In response to a stimulus, the receptor cell may RECEPTORS generate a depolarizing receptor potential, increasing glutamate release and thus activity The mechanism of receptor potential in the primary afferent production varies with the receptor organ, but certain principles are common The process starts with a specific stimulus (the stimulus with the lowest threshold for the receptor) The threshold is the minimal intensity of stimulus needed to excite the first-order neuron Major steps in sensory processing: ○ Transduction ○ Receptor potential generation ○ Electrotonic spread ○ Impulse generation Transduction typically occurs in a specialized site in the receptor cell membrane, leading to Exception: Photoreceptors are depolarized at the gating of ion currents. rest and undergo transient hyperpolarization in ○ Mechanoreceptors in skin, muscles, response to light, reducing glutamate release and inner ear hair cells have Receptor potentials do not directly cause mechanically sensitive cation impulse discharge and are often generated at a channels. different site from where impulses are ○ Photoreceptors, odor receptors, and generated some taste receptors use cyclic ○ In somatosensory receptors, both nucleotide-gated channels (either receptor potentials and impulses are nonselective cation or selective K+ generated on axons channels) ○ In other systems (e.g., vision, hearing, The common result of transduction is a change and taste), these sites are on different in conductance of a membrane ion channel, cells, requiring a synaptic relay producing a receptor potential (or generator The spread of receptor potentials occurs potential) through electrotonic potentials, similar to ○ In most receptors, this potential is synaptic potentials depolarizing, often involving the opening of Na+ or Ca2+ channels or C ENCODING OF SENSORY INFORMATION the closing of K+ channels ○ In photoreceptors, the receptor potential is hyperpolarizing because Encoding of receptor impulse into an impulse light causes cation channels to close discharge for the central nervous system Receptor potentials can affect the primary Receptor potential is graded based on sensory neuron directly or indirectly: stimulus intensity ○ Skin and muscle mechanoreceptors Sensory reception involves transforming this are innervated by the first neuron’s graded response into a pattern of all-or-none axon impulses ○ Nociceptors and thermoreceptors Discharge frequency varies with receptor consist of free nerve endings potential depolarization and rate of change PAGE 4 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Cell ensembles encode spatial and temporal Keep the nervous system stimulus information informed about the body's Stimulus location is encoded by neuron status and its surroundings populations in relay nuclei Stimulus intensity in the somatosensory system is encoded through: ○ Frequency coding (or temporal summation): the frequency of firing of specific neuronal populations ○ Population coding (or spatial summation): the size of the active neuronal population D RECEPTOR ADAPTATION Receptor adaptation is an intrinsic property of Receptor fatigue occurs with repetitive the receptor stimulation: It causes the amplitude of the generator ○ Generator potentials decrease in potential and firing of action potentials to amplitude progressively decrease with a continuous ○ The receptor eventually stops stimulus responding to the stimulus or changes Receptors are classified into: in the stimulus ○ Rapidly adapting (or phasic) receptors: FUNCTIONAL ORGANIZATION OF THE Detect transient and rapidly SENSORY PATHWAYS changing stimuli Fire a few impulses with a sustained stimulus but are A SERIAL AND PARALLEL PROCESSING OF silent during steady SENSORY INPUT continuation May discharge again when the stimulus is removed Stimulus transformed into a frequency code The number of action and transmitted to the central nervous system potentials is related to the rate by a primary afferent neuron of change of the stimulus Sensory information relayed through relay Alert the nervous system to centers where it is processed and integrated changes and are suited for Sensory pathways consist of modality-specific spatiotemporal discrimination neurons connected by synapses ○ Slowly adapting (or tonic) receptors: Pathways have: Respond with sustained firing ○ Series connections for temporal to a sustained stimulus sequence Reflect the final intensity and ○ Parallel organization for simultaneous rate of application of the information transfer stimulus Divergence: primary afferent axon synapses on multiple central neurons Convergence: single central neuron receives input from multiple axons Specific pathways: precise sensory information transmission Multimodal pathways: sensory integration and behavioral adjustments PAGE 5 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep B SENSORY UNIT AND RECEPTIVE FIELD The receptive field of a neuron includes all sensory receptors that can influence its activity Connections with a neuron may be: ○ Excitatory (through projection relay neurons) ○ Inhibitory (through interneurons) Receptive fields are organized topographically: ○ Somatotopy: body surface is represented at each level of the sensory pathway ○ Retinotopy: visual field of each eye is represented at each relay station Somatosensory and visual maps are primarily contralateral (contralateral hemibody or visual field) Auditory system representation is mainly contralateral but also has prominent bilateral representation Maps are distorted: ○ Size of central neuron population is proportional to receptor density ○ Areas of high sensory discrimination (e.g., fingertips, macula) have a large number of receptors and neurons with small receptive fields C THALAMIC STATION Receptive field size can vary in response to factors like denervation All sensory modalities, except olfaction, relay in Receptive fields have a center-surround specific relay nuclei of the thalamus organization: The thalamus serves as: ○ Greatest discharge when stimulus is ○ A relay station for most sensory applied to the center channels. ○ Discharge decreases as stimulus ○ An important gatekeeper for sensory moves to the periphery transmission to the cerebral cortex ○ Stimulation of the surrounding area Each thalamic relay nucleus contains excitatory may inhibit the central neuron neurons that project to a specific area of the (inhibitory surround) cerebral cortex The activity of thalamocortical neurons is controlled by interneurons within the relay nucleus D PRIMARY SENSORY AREAS OF THE CEREBRAL CORTEX Axons from each specific relay nucleus in the thalamus terminate in a primary sensory area of the cerebral cortex Each primary sensory area has neurons that respond selectively to specific characteristics of PAGE 6 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep stimuli (e.g., texture in primary somatosensory ○ Free nerve endings: respond to cortex, color in primary visual cortex) muscle pressure and pain Primary sensory areas project to association areas of the cerebral cortex Neurons in association areas respond selectively to specific combinations of features (e.g., faces) The mature brain can undergo reorganization, allowing changes in sensory maps in response to injury or experience (plasticity of the cortical sensory field) GENERAL ORGANIZATION OF THE SOMATOSENSORY SYSTEMS A SOMATOSENSORY RECEPTORS B DORSAL ROOT GANGLION NEURONS Somatosensory receptors include: ○ Cutaneous receptors Information from somatic receptors is ○ Joint receptors transmitted to the spinal cord by first-order ○ Muscle receptors neurons Cutaneous receptors consist of: Cell bodies of these neurons are in the dorsal ○ Low-threshold mechanoreceptors: root ganglia (spinal ganglia) innervated by large myelinated fibers, Each neuron has: transmit touch sensation ○ Distal (peripheral) branch: innervates ○ High-threshold mechanoreceptors, the receptor chemoreceptors, and ○ Proximal (central) branch: enters the thermoreceptors: innervated by small spinal cord through the dorsal root myelinated or unmyelinated fibers, The skin area innervated by a single dorsal root mediate pain and temperature is a dermatome sensation Dermatomes are highly ordered and form Joint and muscle receptors are innervated strips perpendicular to the spinal cord mainly by large, rapidly conducting myelinated Spinal nerves receive afferents from multiple fibers peripheral nerves, making individual dorsal root Muscle receptors include: areas less defined and overlapping ○ Muscle spindles: signal muscle length Damage to a peripheral nerve causes and rate of change in length circumscribed sensory loss; damage to a spinal ○ Golgi tendon organs: respond to nerve or dorsal root causes moderate sensory changes in muscle tension deficit Dorsal root ganglion neurons: ○ Large neurons: large myelinated axons, innervate low-threshold mechanoreceptors (touch) and proprioceptors ○ Small neurons: small myelinated or unmyelinated axons, innervate PAGE 7 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep nociceptors, thermoreceptors, and Medial: largest, most heavily myelinated fibers visceral receptors mediating proprioception Central: medium-sized myelinated fibers mediating touch Lateral: finely myelinated fibers carrying pain and temperature sensation In this zone: ○ Medial division: large, heavily myelinated proprioceptive and tactile fibers. ○ Lateral division: finely myelinated and unmyelinated fibers for pain and temperature. Dorsal root fibers: 1. Ascend and descend in the white matter 2. Arborize in the gray matter 3. Diverge as they ascend to higher B1 CLINICAL SIGNIFICANCE centers Medial fibers: Diseases affecting large sensory fibers or 1. Ascend directly in ipsilateral dorsal large dorsal root ganglion neurons cause columns (direct dorsal column severe loss of tactile modalities and pathway) proprioception but spare pain and 2. Synapse on dorsal horn neurons temperature sensations contributing to the dorsal column, Diseases affecting small sensory fibers or dorsolateral funiculus, and small dorsal root ganglion neurons affect pain spinothalamic tract (postsynaptic and temperature but spare touch and dorsal column pathway) proprioceptive sensations 3. Synapse in intermediate gray matter for the spinocerebellar tract 4. Synapse on interneurons and motor C DORSAL ROOT ENTRY ZONE AND neurons in the ventral horn for TERMINATION IN THE SPINAL CORD segmental reflexes 5. Synapse in dorsal horn for segmental Primary afferent fibers from dorsal root ganglion modulation of pain cells enter the spinal cord in the posterolateral Lateral fibers: sulcus at the dorsal root entry zone ○ Run in Lissauer tract (dorsolateral funiculus) ○ Enter dorsal horn and intermediate gray matter, then: 1. Synapse on neurons forming spinothalamic and other tracts ascending in the contralateral ventrolateral quadrant 2. Synapse on dorsal horn interneurons for pain modulation and intersegmental pathways 3. Synapse on interneurons and motor neurons for segmental visceral and somatic reflexes Dorsal root entry zone PAGE 8 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Dermatome Area of the skin innervated by a single dorsal Cutaneous distribution of the major peripheral root nerves Arranged in a highly ordered on the body Distal/Peripheral branch: sensory afferent that surface innervates the receptor – well defined area Areas innervated may overlap – less well Damage: circumscribed area of sensory loss in defined the skin Damage: moderate sensory deficit Additional Information Direct dorsal column pathway Postsynaptic dorsal column pathway: dorsal horn neurons – contribute axons to the dorsal column ○ Dorsolateral funiculus, Spinothalamic tract Spinocerebellar tract: synaps in intermediate gray matter Ventral horn: segmental/myotatic reflexes ○ Interneurons, Motor neurons Dorsal horn: provide segmental modulation of pain transmission Spinothalamic and tracts ascending in contralateral ventrolateral quadrant: different groups of dorsal horn and intermediate gray matter neurons Intersegmental pathways: synapse in dorsal horn interneurons involved in PAGE 9 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep segmental modulation of pain and in intrinsic Contralateral, somatotopically organized, and propriospinal synapse in the ventral posterior thalamus, Segmental visceral and somatic reflexes: and send axons to the primary sensory synapse on interneurons cortex. ○ Activate somatic and preganglionic Involved in sensory discrimination, helpful for autonomic motor neurons localizing central lesions, located in the outer tube of the neuraxis. Tactile Discrimination & Conscious D SPINAL SOMATOSENSORY NEURONS Proprioception: ○ Direct dorsal column–lemniscal Second-order spinal somatosensory neurons pathway and parallel pathways in the are found in the dorsal horn and intermediate dorsal column and dorsolateral gray matter of the spinal cord. quadrant. Contribute to all somatosensory pathways Discriminative Pain & Temperature except the direct dorsal column pathway. Sensation: Neuron Types: These neurons can be ○ Direct spinothalamic classified based on their response (neospinothalamic) tract. characteristics: Note: Simple touch and spatial ○ Nociceptive-specific neurons discrimination transmitted by dorsal column, ○ Low-threshold mechanoreceptive neospinothalamic tract, and parallel pathways, neurons making abnormalities in these less helpful for ○ Wide dynamic range neurons lesion localization. Most common and contribute to all ascending pathways that E2 INDIRECT PATHWAYS relay in the spinal cord. The response characteristics of these neurons can change based on segmental and Poor somatotopy, ascend bilaterally, have suprasegmental control mechanisms, which is multiple interconnections, and relay in midline significant for understanding central pain thalamic nuclei. mechanisms. Transmit affective-arousal components of pain, visceral sensation, and initiate reflex somatic, autonomic, and hormonal responses. Located in the inner tube of the neuraxis. Types: ○ Paleospinothalamic, spinoreticular, and spinomesencephalic tracts: anterolateral quadrant of the spinal cord. ○ Propriospinal multisynaptic pathway. E3 SPINOCEREBELLAR PATHWAYS E SOMATOSENSORY PATHWAYS Two-neuron pathways transmitting unconscious proprioceptive information. Destination: Ipsilateral cerebellum. Divided into three groups based on anatomical organization: Additional Information Dorsal and ventral spinocerebellar tracts E1 DIRECT (LEMINISCAL) PATHWAYS PAGE 10 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Pathway: Contributes to the medial lemniscus ○ Transmit information for in the brainstem for further processing. unconscious control of posture and movement Information from somatosensory receptors A1 PRIMARY AFFERENTS are transmitted to the spinal cord by first- order neurons with cell bodies in a dorsal root ganglion. Tactile Discrimination: Involves an active Large-fiber neurons typically carry process integrating low-threshold information about conscious and mechanoreceptive cutaneous and unconscious proprioception, touch, and proprioceptive information. vibration. ○ Mechanoreceptors: The skin contains Small-fiber neurons typically carry pain, four main types of low-threshold temperature, and visceral sensory mechanoreceptors. information. ○ Proprioception: Involves low- Sensory information enters the spinal cord in threshold mechanoreceptors in joints, the posterolateral sulcus at the dorsal root tendons, and muscles. entry zone. Muscle Spindles: Crucial for Second-order spinal somatosensory finger position sense, essential neurons are in the dorsal horn and for recognizing object forms. intermediate gray matter of the spinal cord. Hand and Fingertips: Somatosensory pathways can be divided into ○ Innervation: Highest density and tactile the direct (dorsal column and spinothalamic acuity of any body surface. tracts), indirect (paleospinothalamic, ○ Most important tactile organ for object spinoreticular, spinomesencephalic, and identification through active propiospinal tracts), and spinocerebellar exploration. pathways. Transmission of Tactile and Proprioceptive Information: ○ Afferents: Large myelinated, fast- SPECIFIC SOMATOSENSORY PATHWAYS conducting axons from large dorsal horn neurons. ○ Pathways: Large primary afferents A DIRECT DORSAL COLUMN-LEMNISCAL ascend directly in the PATHWAY ipsilateral dorsal column and synapse on second-order neurons in the medulla (direct Pathways for tactile discrimination and dorsal column pathway). conscious proprioception Some primary afferents Critical for highly discriminative tactile synapse on second-order sensation (stereognosis) and fine motor neurons in the dorsal horn or control. intermediate gray matter, with Functions: axons ascending ipsilaterally ○ Stereognosis: Ability to recognize in the dorsal columns and objects by touch. dorsolateral funiculus. ○ Conscious Proprioception: Senses ○ Relay Point: All pathways relay in the like joint position. lower medulla and decussate to ○ Static Tactile Discrimination: ascend with the contralateral medial Includes two-point discrimination. lemniscus. ○ Vibration Sensation. These modalities are also transmitted through parallel pathways. PAGE 11 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep A2 DORSAL-COLUMN-LEMNISCAL SYSTEM spinal cord and ascend ipsilaterally to the medulla. Fibers carrying lemniscal information from The direct dorsal column pathway, composed the lower extremities and lower trunk of large myelinated dorsal root axons, is (spinal cord segment T7 and below) travel to critical for spatiotemporal tactile discrimination the medulla in the fasciculus gracilis and fine motor control. Fibers carrying information from the upper Anatomical Divisions: extremities and upper trunk (spinal cord ○ Fasciculus Gracilis: Medial, transmits segment T6 and above) travel to the medulla information from lower extremities and in the fasciculus cuneatus. trunk (T7 and below). Second-order neurons for the upper ○ Fasciculus Cuneatus: Lateral, carries extremities and lower extremities are located input from upper extremities and trunk in nucleus cuneatus and nucleus gracilis, (T6 and above). respectively. The medial lemniscus ascends in the mediolateral aspect of the pons and midbrain to synapse in the ventral posterolateral nucleus of thalamus. Third-order neurons from the thalamus project to the primary somatosensory cortex. Input: ○ The dorsal columns carry mostly cutaneous and some proprioceptive input to the dorsal column nuclei. ○ Also relay proprioceptive input to cerebellar relay nuclei (via the spinocerebellar pathway). Second-Order Neurons: ○ Located in the nucleus gracilis (lower extremity input) and nucleus cuneatus (upper extremity input) in the lower medulla. ○ Sensory transmission is modulated at these nuclei for sensory discrimination. Decussation: ○ Second-order axons cross in the lower medulla (internal arcuate fibers) to form the medial lemniscus, which ascends Additional Information to the thalamus. Conscious proprioception, static tactile Somatotopic Organization of the Medial discrimination and vibration sense Lemniscus: Receptors for this pathway include ○ Upper Medulla: Organized cutaneous and joint mechanoreceptors. dorsoventrally (cervical segments Fibers from nuclei cuneatus and gracilis dorsally, sacral segments ventrally). sweep ventrally as the internal arcuate fibers ○ Pons: Arranged mediolaterally and cross he midline to form the medial (cervical medially, sacral laterally). lemniscus. First-order neurons are located in dorsal root ganglia and information is carried by large-fiber axons to the dorsal horn of the PAGE 12 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Neurons are organized into submodality-specific columns. Further Processing: ○ Occurs in secondary and supplementary sensory cortices, and in the somatosensory association cortex in the posterior parietal lobe. A3 MECHANISMS OF SENSORY DISCRIMINATION Key Features of Sensory Discrimination in the dorsal column–lemniscal system: ○ Small Receptive Fields: Enable precise spatial discrimination, especially in areas like the fingertips. ○ Contrast Sharpening: Enhances sensory discrimination through lateral inhibition. ○ Parallel Modality-Specific Channels: Sensory information is segregated by modality (e.g., touch, proprioception). Fingertips: ○ Receptive Fields: Smallest receptive fields with the largest cortical representation. Thalamic Termination: ○ Innervation Density: Four times higher ○ Terminates in the ventral than the palm. posterolateral nucleus of the ○ Two-Point Discrimination Threshold: 1 thalamus, which receives cutaneous mm in fingertips, 10 mm in the palm. input from limbs. Processing Sites: ○ This region forms a unit with the ventral ○ Dorsal Column Nuclei and Ventral posteromedial nucleus, which receives Posterolateral Nucleus of the input from the face (via the trigeminal Thalamus: Not just relay stations, but system). sites of information processing for Thalamic Organization: spatial and temporal discrimination. ○ Somatotopically organized maps in the Contrast Sharpening: thalamus, with the head medially, hand ○ Depends on lateral inhibition centrally, and leg laterally represented. involving inhibitory interneurons that Projection to Cortex: use GABA. ○ Thalamic neurons project through the ○ L-glutamate: Excites relay cells and posterior limb of the internal capsule to interneurons. the primary somatosensory cortex ○ Lateral Inhibition: Helps prevent (postcentral gyrus of the parietal lobe). fusion of excitatory zones when stimuli Primary Somatosensory Cortex: are close together, allowing for precise ○ Contains four distinct areas, each with spatial discrimination. separate somatotopic representations Renshaw: forward input from of cutaneous or proprioceptive inputs. the axon hillock to axon ○ Lower extremity represented medially; collaterals (red) that synapse upper extremity and head represented on an interneuron and this laterally. interneuron inhibits (green) PAGE 13 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep other neurons through lateral ○ Third-order axons from both of these axon connections. tracts, as well as from the postsynaptic Dendrodentritic: back dorsal column pathway, decussate and propagation of excitatory input join the contralateral medial (red) from axon hillock to the lemniscus. dendrites activate the Spinothalamic Tract: inhibitory (black) interneuron. ○ Joins the medial lemniscus before reaching the thalamus. Pathway Composition: ○ All pathways consist of second-order axons from low-threshold mechanoreceptive or wide dynamic range neurons in the dorsal horn. A5 EFFECTS OF LESIONS Modality-Specific Sensory Channels: Lesions in the Dorsal Column–Lemniscal ○ Segregation: Sensory channels for System: different submodalities are spatially ○ Site-Dependent Effects: Clinical signs segregated in the dorsal column nuclei, vary based on the location of the lesion thalamus, and primary somatosensory in the pathway. cortex. Lesions of Large Dorsal Root Ganglion ○ Neuron Specificity: A single neuron Neurons or Large Myelinated Fibers: responds to one sensory submodality ○ Loss of Tactile Discrimination: (e.g., touch or muscle spindles). Inability to detect joint position and Cerebral Cortex: vibration. ○ Columnar Organization: Neurons in ○ Pseudoathetosis: Erratic finger vertical columns respond to the same movements without visual guidance, sensory submodality, forming making object manipulation difficult. topographic and modality-specific ○ Sensory Ataxia: Loss of muscle units. coordination and severe gait disturbance due to lack of proprioceptive feedback, leading to A4 PARALLEL PATHWAYS FOR stumbling and falling unless PROPRIOCEPTIVE AND TACTILE movements are visually monitored. DISCRIMINATIVE FUNCTION Central Lesions: ○ Cause similar but less severe or partial abnormalities, as other ascending In addition to the direct dorsal column– pathways can still transmit tactile and lemniscal pathway, there are several parallel proprioceptive information, reducing pathways that also transmit tactile and the severity of deficits. proprioceptive information. Lesions of the Dorsal Columns: Postsynaptic Dorsal Column Pathway: ○ Vibration Sense and Stereognosis: ○ Contributes to the ipsilateral gracile Significant deficits in detecting and cuneate fasciculi. vibration and spatiotemporal Dorsolateral Funiculus Pathways: discrimination (stereognosis). ○ Spinocervical Tract: Relays in the ○ Astereognosia: Inability to recognize lateral cervical nucleus in the upper objects through active touch, which cervical cord. includes graphesthesia (recognizing ○ Spinomedullary Tract: Relays in a numbers drawn on the skin), and nucleus in the lower medulla. PAGE 14 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep difficulty detecting shapes, patterns, ○ The sensory-discriminative speed, and direction of moving stimuli. component is transmitted via a direct ○ Lesions in the primary somatosensory pathway, while the arousal-affective cortex, thalamus, medial lemniscus, component is carried by indirect and dorsal column nuclei can also pathways. affect these discriminative functions. Spinothalamic Tract: Fasciculus Cuneatus Lesions: ○ These pain and temperature signals ○ Can cause deficits similar to are transmitted primarily through the corticospinal lesions, including loss spinothalamic tract, which ascends in of finger dexterity and disruption of the anterolateral quadrant of the spinal motor precision in space and time. cord. Parallel Channels for Spatial Discrimination: ○ Static Stimuli Discrimination: B1 PRIMARY AFFERENTS Functions like two-point discrimination and joint position sense are transmitted not only by the dorsal column– Thermoreceptive Fibers: lemniscal pathway but also through ○ Respond to warming or cooling but not other parallel pathways. to tactile stimulation. ○ These functions may not be Peripheral Receptors for Pain: permanently impaired by isolated ○ Free nerve endings serve as receptors dorsal column lesions. for pain. Touch-Pressure Sensation: Types of Nociceptive Units: ○ May be transmitted via parallel ○ High-threshold mechanoreceptive pathways, such as the spinothalamic units: tract. Respond to noxious Severe Proprioceptive Deficits: mechanical stimuli (e.g., ○ Occur when lesions affect both the pressure). dorsal column and the dorsolateral Innervated by small funiculus. myelinated axons. ○ Sensory Ataxia: May result from Mediate fast pain (sharp, well- additional involvement of the localized pain, such as from a spinocerebellar pathways. pinprick). ○ Polymodal nociceptive units: B VENTROLATERAL QUADRANT SYSTEM Respond to noxious mechanical, thermal, and chemical stimuli. Pain and Temperature Pathways: Innervated by unmyelinated ○ Shared Pathways: Pain and axons. temperature sensations likely follow Mediate slow pain (diffuse, similar pathways through the nervous dull-aching, or burning pain). system. ○ Pain Components: Sensory-Discriminative Component: Conveys information about the quality, intensity, and location of the painful stimulus. Arousal-Affective Component: Involves emotional, behavioral, and autonomic responses to pain. First-order Nociceptive Neurons: Direct and Indirect Pathways: PAGE 15 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Small myelinated fibers: Located superficially in the Found in medium-sized dorsal dorsal horn. root ganglion neurons. Receive input exclusively from Use L-glutamate as a small myelinated and neurotransmitter. unmyelinated fibers. Correspond to high-threshold ○ Wide Dynamic Range (WDR) mechanoreceptors. Neurons: ○ Unmyelinated fibers: Located deep in the dorsal Found in small dorsal root horn and in the intermediate ganglion neurons. gray matter. Use glutamate, substance P, Receive input from small and calcitonin gene-related myelinated and unmyelinated peptide (CGRP) as fibers, visceral afferents, and neurotransmitters. large myelinated fibers. Correspond to polymodal Functionally important for nociceptors. several reasons: Branching and Projections: Contribute most axons ○ Small nociceptive dorsal root neurons to the spinothalamic extensively branch and innervate system. several sensory fields. Transmit both ○ Some projections enter the spinal cord nociceptive and non- via ventral roots (ventral root afferents). nociceptive Axon Reflex and Neurogenic Inflammation: information. ○ Antidromic release of Serve as sites of neurotransmitters (e.g., substance P) viscerosomatic occurs at the peripheral branches convergence, which during stimulation. is important for ○ This process forms the basis of referred visceral pain. neurogenic inflammation (flare Their functional response) with vasodilation and properties can change increased vascular permeability. based on local Entry of Pain Fibers: modulatory influences ○ Pain fibers, along with those for from central pain- temperature and visceral sensation, modulation enter the spinal cord via the lateral mechanisms. division of the dorsal root entry Axonal Pathways: zone. ○ Axons from both nociceptive-specific ○ These fibers branch into short and WDR neurons, along with those ascending and descending projections from thermoreceptive neurons: in the Lissauer tract before synapsing ○ Cross to the opposite side of the with second-order neurons in the spinal cord. dorsal horn. ○ Continue rostrally in the anterolateral Neurotransmitters in Dorsal Horn Synapses: quadrant of the spinal cord. ○ L-glutamate and substance P are the ○ Primarily form part of the primary excitatory neurotransmitters. spinothalamic pathways. B2 DORSAL HORN NEURONS B3 SPINOTHALAMIC TRACT Second-Order Nociceptive Neurons: Transmits: Pain and temperature sensations. ○ Nociceptive-Specific Neurons: Mediates the discriminative and arousal emotional components of pain sensation as PAGE 16 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep well as thermal, visceral, and simple tactile NEOSPINOTHALAMIC TRACT information. ○ In the Spinal Cord: Ascends: In the ventrolateral quadrant of the Axons cross the midline spinal cord, contralateral to the side of entry of through the ventral white primary afferents. commissure. Ascend contralaterally in the anterolateral quadrant. Somatotopic Organization: Sacral component is dorsolateral, cervical component is ventromedial. ○ In the Brainstem: Medulla: Dorsal to the lateral aspect of the inferior olivary nucleus. Pons and Midbrain: Lateral to the medial lemniscus. Mesodiencephalic Junction: Joins with the medial lemniscus. ○ Synapses on third-order neurons in several thalamic nuclei: Ventral Posterolateral Nucleus: Projects to the primary sensory cortex in the postcentral gyrus. Other axons terminate in thalamic nuclei that project to the insular cortex or anterior cingulate gyrus. Clinical Significance: ○ Rapid transmission of nociceptive and thermal information for localization and intensity of pain and temperature sensations. ○ Localization: Important for localizing lesions in the central nervous system. Components: ○ Parallel Channel: Transmits tactile ○ Neospinothalamic Pathway: information including simple touch and Mediates discriminative static discriminative touch modalities. aspects of pain and Spinothalamic pathway transmits information temperature. about pain and temperature sensations from Important for localization. the contralateral upper and lower extremities ○ Indirect Pathways: and trunk. Paleospinothalamic Tract: Trigeminal System: pain and temperature For affective-arousal sensations from the face and cranium components of pain. ○ Pain Fibers: Spinoreticular Tract: For Travel in the trigeminal nerve arousal and emotional (cranial nerve V). responses. Cell bodies in the gasserian Spinomesencephalic Tract: (semilunar) ganglion. Involved in pain modulation. ○ Brainstem Pathway: PAGE 17 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep Enter at the pons, descend in Receive convergent input from the spinal tract of the trigeminal cutaneous, visceral, and other nerve. receptors. Synapse with second-order ○ These pathways initiate arousal, neurons in the nucleus of the autonomic, endocrine, and motor spinal tract of the trigeminal responses to pain stimulation. nerve. Two main groups of these pathways: Second-order neurons cross to ○ VENTROLATERAL QUADRANT the opposite side and ascend PATHWAYS to the ventral posteromedial Paleospinothalamic tract: nucleus of the thalamus. provides multiple input Third-order neurons project to to the reticular the parietal lobe through the formation posterior limb of the internal terminates in the capsule. midline and intralaminar thalamic nuclei, which project diffusely to the cerebral cortex, particularly to the anterior cingulate gyrus. Spinoreticular tract: terminates in sensory, motor, autonomic, and endocrine relay areas of the medullary and pontine reticular formation. Spinomesencephalic tract: synapses in the B4 PATHWAYS FOR THE AFFECTIVE- periaqueductal gray AROUSAL COMPONENTS OF PAIN matter. Indirect pathways for affective and arousal ○ PROPRIOSPINAL MULTISYNAPTIC components of pain originate from wide ASCENDING SYSTEM dynamic range neurons in the deep dorsal horn originates from neurons in the and intermediate gray matter. substantia gelatinosa of the ○ Second-order axons: dorsal horn and in the Ascend bilaterally in the spinal intermediate gray matter. cord. forms a functional continuum Have poor somatotopy with the reticular formation of Make multiple synapses in the the brainstem. reticular formation ○ Collaterals from these pathways reach the hypothalamus and parts of the limbic system. ○ Neurons in these pathways: Have large bilateral receptive fields. PAGE 18 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ The separate pathways for body and limb pain and for facial pain are the neuroanatomical basis for this clinical observation. ○ Thus, lesions at the level of the medulla produce crossed anesthesia, whereas those rostral to the medulla produce complete contralateral hemianesthesia, which includes the face. Lesions at the Supratentorial level ○ produce contralateral loss of pain and temperature sensations. Suprathalamic lesions ○ crude pain perception may remain intact, but precise localization of painful stimuli is impaired. Additional Information Receptors that transmit: B5 EFFECT OF LESION OF THE ○ Temperature: Thermoreceptors SPINOTHALAMIC SYSTEM ○ Pain: Nociceptors Lesions of the Peripheral Level ○ may cause either the sensation of pain DIRECT SPINOTHALAMIC PATHWAY or some loss of pain and temperature ○ transmits pain and temperature in the distribution of the affected information to the cortex over a nerves. three-neuron pathway. Lesions of the Central nervous system ○ First-order neurons: ○ seldom produce pain unless pain- are located in dorsal root sensitive structures are involved or ganglion central pain-controlling pathways are small-fiber neurons. interrupted. ○ Second-order neurons: Central lesion that interrupts the are located in the dorsal Spinothalamic tracts horn of the spinal cord. ○ results in the inability to perceive their axons cross the midline painful stimuli and to discriminate in the ventral white between hot and cold in areas below commissure and ascend the level of lesion contralaterally in the Lesion at the Spinal level involving the anterolateral quadrant of the spinothalamic tract spinal cord and lateral ○ results in contralateral loss of pain and portion of the brainstem. temperature sensations below the level ○ Third-order neurons: of the lesion. Spinothalamic tract axons Lesion at the posterior fossa level synapse on third-order ○ results in contralateral loss of pain and neurons in the ventral temperature sensations in the trunk posterolateral nucleus of the and extremities, but if the same lesion thalamus, which projects to also involves the pain fibers in the the primary somatosensory descending tract of the trigeminal cortex. nerve, there is ipsilateral loss of pain INDIRECT PATHWAYS and temperature sensation of the face. PAGE 19 BATCH TALAGHAY  Sensory System NEUROSCIENCE Professor Trans by: Bulaqui, Mamenting, Orata, Paltep ○ Neospinothalamic Tract: ○ involved in the affective and arousal aspects of pain sensation are discriminative pain and temperature Touch is not very useful clinically for localizing complex, multi- synaptic pathways lesions in the central nervous system. ○ include two main groups: Ventrolateral quadrant pathways MECHANISMS OF PAIN AND ALGESIA Propriospinal multisynaptic ascending system Pain ○ is a frequent manifestation of neurologic and non- neurologic disease. C PATHWAYS FOR TRANSMISSION OF Organic pain: SIMPLE TOUCH ○ Nociceptive pain is related to the activation of Tactile Sensation normal pain mechanisms in ○ initiated by the stimulation of low- response to tissue injury or threshold mechanoreceptors in the inflammation; skin. ○ Neurogenic pain receptors vary in degree of is due to peripheral or central adaptation and size of nervous system lesions that receptive field. affect processing of Axons of primary, large dorsal root ganglion information in the pain neurons: transmission pathway. ○ are in the medial division of the dorsal Endogenous antinociceptive mechanisms root entry zone. are activated by: ○ Large myelinated fibers: ○ Stress ascend directly in the dorsal ○ Exercise colu