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Chapter 21 Somatic Senses

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  1. Chapter 21Somatic Senses

  2. Sensory Modalities • Different types of sensations • touch, pain, temperature, vibration, hearing, vision • Each type of sensory neuron can respond to only one type of stimuli • Two classes of sensory modalities • general senses • somatic are sensations from body walls • visceral are sensations from internal organs • special senses • smell, taste, hearing, vision, and balance

  3. Process of Sensation • Sensory receptors demonstrate selectivity • respond to only one type of stimuli • Events occurring within a sensation • stimulation of the receptor • transduction (conversion) of stimulus into a graded potential • vary in amplitude and are not propagated • generation of impulses when graded potential reaches threshold • integration of sensory input by the CNS

  4. Sensory Receptors • Selectively respond to only one kind of stimuli • Have simple or complex structures • General Sensory Receptors (Somatic Receptors) • no structural specializations in free nerve endings that provide us with pain, tickle, itch, temperatures • some structural specializations in receptors for touch, pressure & vibration • Special Sensory Receptors (Special Sense Receptors) • very complex structures---vision, hearing, taste, & smell

  5. Classification of Sensory Receptors • Structural classification • Type of response to a stimulus • Location of receptors & origin of stimuli • Type of stimuli they detect


  6. Structural Classification of Receptors • Free nerve endings • bare dendrites • pain, temperature, tickle, itch & light touch • Encapsulated nerve endings • dendrites enclosed in connective tissue capsule • pressure, vibration & deep touch • Separate sensory cells • specialized cells that respond to stimuli • vision, taste, hearing, balance

  7. Structural Classification • Compare free nerve ending, encapsulated nerve ending and sensory receptor cell

  8. Classification by Location • Exteroceptors • near surface of body • receive external stimuli • hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature • Interoceptors • monitors internal environment (BV or viscera) • not conscious except for pain or pressure • Proprioceptors • muscle, tendon, joint & internal ear • senses body position & movement

  9. Classification by Stimuli Detected • Mechanoreceptors • detect pressure or stretch • touch, pressure, vibration, hearing, proprioception, equilibrium & blood pressure • Thermoreceptors detect temperature • Nociceptors detect damage to tissues • Photoreceptors detect light • Chemoreceptors detect molecules • taste, smell & changes in body fluid chemistry

  10. Adaptation of Sensory Receptors • Change in sensitivity to long-lasting stimuli • decrease in responsiveness of a receptor • bad smells disappear • very hot water starts to feel only warm • potential amplitudes decrease during a maintained, constant stimulus • Receptors vary in their ability to adapt • Rapidly adapting receptors (smell, pressure, touch) • adapt quickly; specialized for signaling stimulus changes • Slowly adapting receptors (pain, body position) • continuation of nerve impulses as long as stimulus persists

  11. Somatic Tactile Sensations • Touch • crude touch is ability to perceive something has touched the skin • discriminative touch provides location and texture of source • Pressure is sustained sensation over a large area • Vibration is rapidly repetitive sensory signals • Itching is chemical stimulation of free nerve endings • Tickle is stimulation of free nerve endings only by someone else

  12. Meissner’s Corpuscle • Dendrites enclosed in CT in dermal papillae of hairless skin • Discriminative touch & vibration-- rapidly adapting • Generate impulses mainly at onset of a touch

  13. Hair Root Plexus • Free nerve endings found around follicles, detects movement of hair

  14. Merkel’s Disc • Flattened dendrites touching cells of stratum basale • Used in discriminative touch (25% of receptors in hands)

  15. Ruffini Corpuscle • Found deep in dermis of skin • Detect heavy touch, continuous touch, & pressure

  16. Pacinian Corpuscle • Onion-like connective tissue capsule enclosing a dendrite • Found in subcutaneous tissues & certain viscera • Sensations of pressure or high-frequency vibration

  17. Thermal Sensations • Free nerve endings with 1mm diameter receptive fields on the skin surface • Cold receptors in the stratum basale respond to temperatures between 50-105 degrees F • Warm receptors in the dermis respond to temperatures between 90-118 degrees F • Both adapt rapidly at first, but continue to generate impulses at a low frequency • Pain is produced below 50 and over 118 degrees F.

  18. Pain Sensations • Nociceptors = pain receptors • Free nerve endings found in every tissue of body except the brain • Stimulated by excessive distension, muscle spasm, & inadequate blood flow • Tissue injury releases chemicals such as K+, kinins or prostaglandins that stimulate nociceptors • Little adaptation occurs

  19. Types of Pain • Fast pain (acute) • occurs rapidly after stimuli (.1 second) • sharp pain like needle puncture or cut • not felt in deeper tissues • larger A nerve fibers • Slow pain (chronic) • begins more slowly & increases in intensity • aching or throbbing pain of toothache • in both superficial and deeper tissues • smaller C nerve fibers

  20. Localization of Pain • Superficial Somatic Pain arises from skin areas • Deep Somatic Pain arises from muscle, joints, tendons & fascia • Visceral Pain arises from receptors in visceral organs • localized damage (cutting) intestines causes no pain • diffuse visceral stimulation can be severe • distension of a bile duct from a gallstone • distension of the ureter from a kidney stone • Phantom limb sensations -- cells in cortex still

  21. Referred Pain • Visceral pain that is felt just deep to the skin overlying the stimulated organ or in a surface area far from the organ. • Skin area & organ are served by the same segment of the spinal cord. • Heart attack is felt in skin along left arm since both are supplied by spinal cord segment T1-T5

  22. Pain Relief • Aspirin and ibuprofen block formation of prostaglandins that stimulate nociceptors • Novocaine blocks conduction of nerve impulses along pain fibers • Morphine lessen the perception of pain in the brain.

  23. Proprioceptive or Kinesthetic Sense • Awareness of body position & movement • walk or type without looking • estimate weight of objects • Proprioceptors adapt only slightly • Sensory information is sent to cerebellum & cerebral cortex • from muscle, tendon, joint capsules & hair cells in the vestibular apparatus

  24. Muscle Spindles • Specialized intrafusal muscle fibers enclosed in a CT capsule and innervated by gamma motor neurons • Stretching of the muscle stretches the muscle spindles sending sensory information back to the CNS • Spindle sensory fiber monitor changes in muscle length • Brain regulates muscle tone by controlling gamma fibers

  25. Golgi Tendon Organs • Found at junction of tendon & muscle • Consists of an encapsulated bundle of collagen fibers laced with sensory fibers • When the tendon is overly stretched, sensory signals head for the CNS & resulting in the muscle’s relaxation

  26. Somatic Sensory Pathways • First-order neuron conduct impulses to brainstem or spinal cord • either spinal or cranial nerves • Second-order neurons conducts impulses from spinal cord or brainstem to thalamus--cross over to opposite side before reaching thalamus • Third-order neuron conducts impulses from thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)

  27. Posterior Column-Medial Lemniscus Pathway of CNS • Proprioception, vibration, discriminative touch, weight discrimination & stereognosis • Signals travel up spinal cord in posterior column • Fibers cross-over in medulla to become the medial lemniscus pathway ending in thalamus • Thalamic fibers reach cortex

  28. Spinothalamic Pathways • Lateral spinothalamic tract carries pain & temperature • Anterior tract carries tickle, itch, crude touch & pressure • First cell body in DRG with synapses in cord • 2nd cell body in gray matter of cord, sends fibers to other side of cord & up through white matter to synapse in thalamus • 3rd cell body in thalamus projects to cerebral cortex

  29. Somatosensory Map of Postcentral Gyrus • Relative sizes of cortical areas • proportional to number of sensory receptors • proportional to the sensitivity of each part of the body • Can be modified with learning • learn to read Braille & will have larger area representing fingertips

  30. Sensory Pathways to the Cerebellum • Major routes for proprioceptive signals to reach the cerebellum • anterior spinocerebellar tract • posterior spinocerebellar tract • Subconscious information used by cerebellum for adjusting posture, balance & skilled movements • Signal travels up to same side inferior cerebellar peduncle

  31. Somatic Motor Pathways • Control of body movement • motor portions of cerebral cortex • initiate & control precise movements • basal ganglia help establish muscle tone & integrate semivoluntary automatic movements • cerebellum helps make movements smooth & helps maintain posture & balance • Somatic motor pathways • direct pathway from cerebral cortex to spinal cord & out to muscles • indirect pathway includes synapses in basal ganglia, thalamus, reticular formation & cerebellum

  32. Primary Motor Cortex • Precentral gyrus initiates voluntary movement • Cells are called upper motor neurons • Muscles represented unequally (according to the number of motor units) • More cortical area is needed if number of motor units in a muscle is high • vocal cords, tongue, lips, fingers & thumb

  33. Direct Pathway (Pyramidal Pathway) • 1 million upper motor neurons in cerebral cortex • 60% in precentral gyrus & 40% in postcentral gyrus • Axons form internal capsule in cerebrum and pyramids in the medulla oblongata • 90% of fibers decussate(cross over) in the medulla • right side of brain controls left side muscles • Terminate on interneurons which synapse on lower motor neurons in either: • nuclei of cranial nerves or anterior horns of spinal cord • Integrate excitatory & inhibitory input to become final common pathway

  34. Details of Motor Pathways • Lateral corticospinal tracts • cortex, cerebral peduncles, 90% decussation of axons in medulla, tract formed in lateral column. • skilled movements hands & feet • Anterior corticospinal tracts • the 10% of axons that do not cross • controls neck & trunk muscles • Corticobulbar tracts • cortex to nuclei of CNs ---III, IV, V, VI, VII, IX, X, XI & XII • movements of eyes, tongue, chewing, expressions & speech

  35. Location of Direct Pathways • Lateral corticospinal tract • Anterior corticospinal tract • Corticobulbar tract

  36. Paralysis • Flaccid paralysis = damage lower motor neurons • no voluntary movement on same side as damage • no reflex actions • muscle limp & flaccid • decreased muscle tone • Spastic paralysis = damage upper motor neurons • paralysis on opposite side from injury • increased muscle tone • exaggerated reflexes

  37. Final Common Pathway • Lower motor neurons receive signals from both direct & indirect upper motor neurons • Sum total of all inhibitory & excitatory signals determines the final response of the lower motor neuron & the skeletal muscles

  38. Basal Ganglia • Helps to program automatic movement sequences • walking and arm swinging or laughing at a joke • Set muscle tone by inhibiting other motor circuits • Damage is characterized by tremors or twitches

  39. Basal Ganglia Connections • Circuit of connections • cortex to basal ganglia to thalamus to cortex • planning movements • Output from basal ganglia to reticular formation • reduces muscle tone • damage produces rigidity of Parkinson’s disease

  40. Cerebellar Function Aspects of Function • learning • coordinated & skilled movements • posture & equilibrium 1. Monitors intentions for movements -- input from cerebral cortex 2. Monitors actual movements with feedback from proprioceptors 3. Compares intentions with actual movements 4. Sends out corrective signals to motor cortex

  41. Spinal Cord Injury • Damaged by tumor, herniated disc, clot or trauma • Complete transection is cord severed resulting loss of both sensation & movement below the injury • Paralysis • monoplegia is paralysis of one limb only • diplegia is paralysis of both upper or both lower • hemiplegia is paralysis of one side • quadriplegia is paralysis of all four limbs • Spinal shock is loss of reflex function (areflexia) • slow heart rate, low blood pressure, bladder problem • reflexes gradually return