Somatic Sensory 2023 PDF

SOMATIC SENSORY NG SOOK LUAN, PhD Jabatan Diagnostik Kraniofasial & Biosains Fakulti Pergigian UKM 012-9305208 [email protected]  LEARNING OBJECTIVES 1. Explain organization and features of the sensory nervous system. 2. Explain the classific...

SOMATIC SENSORY NG SOOK LUAN, PhD Jabatan Diagnostik Kraniofasial & Biosains Fakulti Pergigian UKM 012-9305208 [email protected]  LEARNING OBJECTIVES 1. Explain organization and features of the sensory nervous system. 2. Explain the classification, characteristics and functions of various receptors. 3. Explain the characteristics of receptor potential. 4. Discuss the brain perception of intensity and quality of different stimuli. 5. Discuss the sensory homunculus. 6. Discuss the sensory pathways and their functions.  ORGANIZATION OF THE SOMATIC NERVOUS SYSTEM When stimulation, the somatic receptors (nerve cell endings) generate action potentials in the axons of somatic sensory neurons (afferent neurons) Interneurons In the CNS, interneurons interpret and integrate the input delivered by the sensory neurons. Response activities are planned and Sensory Motor relayed to motor neurons. neuron neuron Motor neurons (efferent neurons) conduct impulses out to the skeletal muscles (effectors), which produce movements Somatic receptors  PROCESSING AT THE CIRCUIT LEVEL First order neurons Cell bodies in dorsal root ganglion or cranial nuclei Conduct impulses from receptors/proprioreceptors to the spinal cord or brain stem to synapse with second order neurons Second order neurons Cell bodies in dorsal horn of cord or medullary nuclei Transmit impulses to the thalamus or cerebellum where they synapse Third order neurons None found in the cerebellum Located in the thalamus & conduct impulses to the somatosensory cortex of the cerebrum  CEREBRAL CORTEX Cerebral cortex has 3 types of functional areas: Motor areas – control voluntary motor function Sensory areas – provide conscious awareness of sensation Association areas – act mainly to integrate diverse information for purposeful action Each hemisphere is chiefly concerned with the sensory & motor functions of the opposite (contralateral) side of the body  PRIMARY MOTOR CORTEX The primary motor cortex is located in the precentral gyrus of the frontal lobe of each hemisphere Large neurons (pyramidal cells) in these gyri allow us to consciously control the precise or skill voluntary movements of our skeletal muscles The lateral corticospinal / pyramidal tract consists of the long axons of the pyramidal cells located within the primary motor cortex  PREMOTOR CORTEX Role play of the premotor cortex: controls motor skills of repetitive or patterned nature (typing or piano) Coordinates the movement of several muscle groups to act simultaneously or sequentially Sends activating impulses to the primary motor cortex Influences motor actively more directly by supplying 15% pyramidal tract fibres As a memory bank of skilled motor activities Involved with motor planning Controls voluntary actions that depend on sensory feedback Damage to the premotor cortex: Results in the loss of motor skills in that region Muscle strength and the ability to perform the discrete individual movements are not hindered Neurons relearning the skill would require practice  SOMATOSENSORY CORTEX Sensory signals from all modalities terminate just posterior to the central fissure – somatosensory cortex: Anterior half of the parietal lobe - reception & interpretation of somatosensory signals Posterior half of the parietal lobe – provides still high levels of interpretation LAYERS OF THE SOMATOSENSORY CORTEX & THEIR FUNCTION 6 layers of neurons Incoming sensory signal excites layer IV first; signal spreads toward the surface and deeper layers Layers I & II receive diffuse non-specific input signals Neurons in II & III send axons to related portions of the cerebral cortex and to the opposite hemisphere via the corpus callosum Neurons in V & VI send axons to deeper parts of the nervous system  Sensory cortex is organized in vertical column Each column detects a different sensory spot on the body with a specific sensory modality Functions of somatosensory area I – bilateral excision cause the following types of sensory judgement: Person is unable to localize discretely the different parts of the body; can localize the sensations crudely. Person is unable to judge critical degrees of pressure against the body the weights of objects shapes or forms of objects texture of materials  Somatosensory Association Areas Brodmann’s Area 5 &7 play an important role in deciphering deeper meanings of the sensory information Receives information from somatosensory area I, ventro-basal nuclei of the thalamus, other areas of the thalamus, visual cortex and the auditory cortex  GENERAL SENSES Describe our sensitivity to temperature, pain, touch, pressure, vibration & proprioception Sensation – the arriving information from these senses Perception – conscious awareness of a sensation  CLASSIFICATION OF SOMATIC SENSES Mechanoreceptic somatic include both tactile and position sensations senses stimulated by mechanical displacement (mechanoreceptor) Touch, pressure, vibration, stretch, itch Thermoreceptive senses detect heat and cold (thermoreceptor) Pain sense activated by factors that damage tissues (nociceptor)  OTHER CLASSIFICATION OF SOMATIC SENSES Exteroreceptive from the surface of the body sensation Stimuli arising outside the body (exteroreceptor) Touch, pressure, pain, special senses (smell, sight, taste, balance/equilibrium, hearing) Proprioceptive relating to the physical state of the body (position, sensation tendons, muscles, equilibrium) (proprioreceptor) Internal stimuli Visceral sensation sensation from the internal/visceral organs (visceroreceptors / Chemical changes, stretching of tissues, temperature interoreceptors) Typically unaware of these receptors except for pain, discomfort, hunger & thirst Deep sensation come from the deep tissues (fascia, muscles and bone)  FREE NERVE ENDINGS The simplest of our sensory receptors Branching tips of dendrites Not protected by accessory structures Can be stimulated by many different stimuli  RECEPTIVE FIELD Area is monitored by a single receptor cell The larger the receptive field, the more difficult it is to localize a stimulus  NOCICEPTORS TYPE A & TYPE C FIBERS Also called pain receptors Type A fibers – carry sensations of fast Free nerve endings with large receptive pain, or prickling pain, e.g. an injection fields or a deep cut Sensations reach the CNS quickly and often Are common in the: trigger somatic reflexes Superficial portions of the skin Relayed to the primary sensory cortex and Joint capsules receive conscious attention Within the periostea of bones Around the walls of blood vessels May be sensitive to Extremes of temperature Type C fibers – carry sensations of slow Mechanical damage pain, or burning and arching pain Dissolved chemicals, such as chemicals Aware of the pain but only have a general released by injured cells idea of the area affected  THERMORECEPTORS Also called temperature receptors Are free nerve endings located in the: Dermis Skeletal muscles Liver Hypothalamus Conducted along the same pathways that carry pain sensations  3 CLASSES OF MECHANORECEPTORS Tactile receptors: Baroreceptors: Proprioreceptors: Provide the sensations Detect pressure Monitor the positions of touch, pressure and changes in the walls of of joints and muscles vibration blood vessels and in portions of the digestive, reproductive and urinary tracts Fine Touch & Pressure Receptors: Crude Touch & Pressure Receptors: Extremely sensitive & have a relatively narrow Have a relatively large receptive fields & provide receptive field poor localization Provide detailed information about a source Give little information about the stimulus of stimulation, including its exact location, shape, size, texture & movement  TACTILE RECEPTORS Range in complexity from free nerve endings to specialized sensory complexes with accessory cells and supporting structures DETECTION & TRANSMISSION OF TACTILE SENSATIONS Interrelations among the tactile sensations of touch, pressure and vibration – 3 principle differences: Touch sensation generally results from stimulation of tactile receptors in the skin or subcutaneous tissues Pressure sensation generally results from deformation of deeper tissues Vibration sensation results from rapidly repetitive sensory signals 6 TYPES OF TACTILE RECEPTORS IN THE SKIN 1) Free nerve endings found everywhere in the skin and in many other tissues detect touch & pressure Situated between epidermal cells 2) Root hair plexus / Hair end organ Touch receptor around each hair Monitor distortion and movement across the body surface wherever hairs are located Adapt rapidly, so are best at detecting initial contact and subsequent movement 6 TYPES OF TACTILE RECEPTORS IN THE SKIN 3) Merkel’s / tactile discs Expanded tip tactile receptor Small receptive fields, fine touch and pressure receptors Transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts slowly Found in the hairy parts of the skin Grouped together into “Iggo dome receptor” 4) Meissner’s / tactile corpuscles touch receptor with great sensitivity, small receptive fields (fine touch, pressure & low frequency vibration) Fast-adapting receptors elongated, encapsulated nerve ending of large myelinated nerve fibre present in the non-hairy areas of the skin (e.g. the fingertips, eyelids, lips, nipples & external genitalia) 6 TYPES OF TACTILE RECEPTORS IN THE SKIN 5) Pacinian / lamellated corpuscles Sensitive to deep pressure Fast-adapting receptors Large receptive field 6) Ruffini’s corpuscles / endings Multi-branched, encapsulated Adapt slowly, prolonged touch & pressure sensations Sensitive to pressure and distortion of the skin Large receptive field Located in the reticular (deep) dermis, joint capsules 6 TYPES OF TACTILE RECEPTORS IN THE SKIN  PROPRIOCEPTIVE SENSES A.K.A position senses – 2 subtypes : Static position sense Rate of movement sense: Kinesthesia - cognizance of joint movement Dynamic proprioception – awareness of the joint position Knowledge of position depends on knowing the degrees of angulation of all joints in all planes and their rates of change Multiple different types of receptors are used: Deep receptors Corpuscles Muscle spindles Processing of position sense information – thalamic neurons responding to joint rotation are of 2 types: Those maximally stimulated when the joint is at full rotation Those maximally stimulated when the joint is at minimal rotation  3 MAJOR GROUPS OF PROPRIOCEPTOR Muscle spindles Monitor skeletal muscle length Trigger stretch reflexes Golgi tendon organs Located at the junction between skeletal muscle and its tendon Stimulated by tension in tendon Monitor external tension developed during muscle contraction Receptors in joint capsules Free nerve endings detect pressure, tension and movement at the joint  CHEMORECEPTORS Located in the Carotid bodies: Near the origin of the internal carotid arteries on each side of the neck Aortic bodies: Between the major branches of the aortic arch Receptors monitor pH, carbon dioxide and oxygen levels in arterial blood  CHARACTERISTICS OF RECEPTOR POTENTIAL Stimulus – a change detectable by the body; exist in various energy forms (modalities), such as heat, light, sound, pressure and chemical changes Afferent neurons have sensory receptors at their peripheral endings that respond to stimuli in both external and internal environment. transmit information (stimuli) to the CNS via action potential propagation (electrical signals). Stimuli bring about graded potentials known as receptor potentials in the receptor. Sensory transduction - conversion of stimulus energy into a receptor potential Receptor potentials trigger action potentials in the afferent fibre. CONVERSION OF RECEPTOR POTENTIAL INTO AP  ADAPTATION Reduction in sensitivity of a constant stimulus The receptor “adapts” to the sustained stimulus and diminish the extend of their depolarization and no longer responding to it at the same degree. It is a receptor adjustment in the PNS. Tonic receptors – are always active, do not adapt or adapt slowly called slow-adapting receptors (proprioceptor & nociceptor), maintain info about a stimulus Remind you of an injury long after the initial damage has occurred To maintain posture and balance  ADAPTATION Phasic receptors – are normally inactive & become active for a short time whenever a change occurs Fast-adapting receptors – response characteristic to phasic receptors (smell & taste) E.g. Pacinian corpuscle – respond with a slight depolarization called the off response when the stimulus is remove. Sensory Sensory E.g. Tactile receptors – signal changes in neuron neuron pressure on the skin – adapts rapidly – not continually conscious of wearing your watch; aware of its removal because of the off response  Overall characteristics of signal transmission & analysis in the dorsal column Transmission of a pinpoint stimulus signal to the cerebral cortex Two-Point Discrimination: Transmission of signals to the cortex from 2 adjacent pinpoint stimuli The smaller the receptive field is in a region, the greater its acuity or discriminative ability Small receptive fields – fingertips Large receptive fields - skin  Effect of lateral inhibition – increases the degree of contrast in the perceived spatial pattern Virtually every sensory pathway, when excited, gives rise simultaneously to lateral inhibitory signals Importance of lateral inhibition is that it blocks the lateral spread of excitatory signals and therefore, increases the degree of contrast in the sensory pattern perceived in the cerebral cortex. In the dorsal column lateral inhibition signals occur at each synaptic level The most lateral inhibition – touch & vision – bring about the most accurate localization Transmission of rapidly changing & repetitive sensations Dorsal column can recognize changing stimuli that occur in as little as 1/400 of a second Vibration sensation – rapidly repetitive and can be detected up to 700 cycles/second  SENSORY HOMUNCULUS Homunculus means “little man” Different parts of the body are not equally represented Precisely where on the body a specific stimulus originated depends on the projection of information from the thalamus to the primary sensory cortex Receives sensory input from the opposite side of the body (decussation at spinal cord) Thalamus detects the simple awareness of touch, pressure, temp. or pain, but somatosensory cortex localizes the source of sensory input & perceive the level of intensity of the stimulus.  WHITE MATTER IN THE SPINAL CORD Fibers run in 3 directions: ascending, descending & transversely Divided into 3 funiculi (columns) – posterior, lateral & anterior Each funiculus contains several fiber tracts Fiber tract names reveal their origin and destination Fiber tracts are composed of axons with similar functions Pathways decussate (cross-over) Most consist of 2 or 3 neurons Most exhibit somatotopy (precise spatial relationships) Pathways are paired (one on each side of the spinal cord or brain) 3 MAJOR SOMATIC SENSORY PATHWAYS 1) Posterior column pathway 2) Anterolateral pathway 3) Spinocerebellar pathway 1) POSTERIOR COLUMN PATHWAY Fasciculus gracilis Fasciculus cuneatus Carries sensations of highly localized (fine) touch, pressure, vibration & proprioception POSTERIOR/DORSAL COLUMN – MEDIAL LEMNISCAL SYSTEM Touch sensations requiring high degree of localization Touch sensations requiring transmission of fine degradations of intensity Phasic sensations, e.g. vibratory sensations Sensations that signal movement against the skin Position sensations from the joints Pressure sensations related to fine degrees of judgement of pressure intensity 2) ANTEROLATERAL PATHWAY Transmits sensory signals that do not require highly discrete localization or discrimination of fine gradations of intensity Provides sensations of “crude” touch, pressure, pain, temperature, tickle & itch, sexual sensation Ascend within the anterior or lateral spinothalamic tracts: Anterior spinothalamic tracts carry crude touch & pressure sensations Lateral spinothalamic tracts carry pain & temperature sensations  2) ANTEROLATERAL PATHWAY Transmission of less critical sensory signals Characteristics of transmission Velocity of transmission is 1/3 of that of the dorsal column Degree of spatial localization of signals is poor Gradations of intensities are less accurate Ability to transmit rapidly changing or repetitive signals is poor 3) SPINOCEREBELLAR PATHWAY Cerebellum receives proprioceptive information about position of skeletal muscles, tendons and joints  VISCERAL SENSORY INFORMATION Collected by interoceptors monitoring visceral tissues and organs, primarily within thoracic and abdominopelvic cavities These interoceptors, not as numerous as in somatic tissues, include: Nociceptors Thermoreceptors Tactile receptors Baroreceptors Chemoreceptors  DESCENDING (MOTOR) PATHWAY Upper motor neuron: Cell body lies in a CNS processing centre Synapses on the lower motor neuron Activity in upper motor neuron may facilitate or inhibit lower motor neuron Lower motor neuron: Cell body lies in nucleus of the brain stem or spinal cord Triggers a contraction in innervated muscle Destruction or damage of lower motor neuron eliminates voluntary and reflex control over innervated motor unit  CORTICOSPINAL PATHWAY Sometimes called pyramidal system Provides voluntary control over skeletal muscles: System begins at pyramidal cells of primary motor cortex Axons of these upper motor neurons descend into brain stem and spinal cord to synapse on lower motor neurons that control skeletal muscles  MOTOR HOMUNCULUS Primary motor cortex corresponds point by point with specific regions of the body Cortical area have been mapped out in diagrammatic form SOMATIC MOTOR COMMANDS Several centres in cerebrum, diencephalons and brain stem may issue somatic motor commands as result of processing performed at subconscious level