Chapter 15! Chapter 15 Sensory Pathways, Somatic Nervous System! Neural Integration I: Sensory Pathways and the Somatic Nervous System!

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1 Chapter 15! Neural Integration I: Sensory Pathways and the SECTION 15-2! Sensory receptors connect our internal and external environments with the nervous system! 2 Sensation and Receptors! Transduction! Translation of stimulus into action potential! No transduction, no sensation! Perception! Conscious awareness of sensation! Senses! General senses! Temperature, pain, touch, pressure, vibration, proprioception! Receptors distributed throughout body! Receptors relatively simple in structure! 3 1!

2 ! Chapter 15 Sensory Pathways, Sensation! Special senses! Olfaction, vision, gustation, equilibrium, audition! Receptors found in special sense organs! Receptors more complex in structure! Receptor specificity = Law of specific nerve energies or Modality specificity! You can t see with your ears.! Specificity results from:! Structure of receptor cell and/or! Presence of accessory structures! 4 Specificity Examples! 1. Pain receptors are free nerve endings! Respond to tissue damage from:! Pressure, chemicals, heat! i.e., simple structure; somewhat non-specific! 2. Photoreceptors! Respond to light! Complex receptor cells! Surrounded by specialized pigment and CT cells! i.e., complex structure; very specific! 5 Receptive Field! Receptive field = area monitored by a single receptor! Smaller receptive field more precise localization! e.g. on finger tips! Larger receptive field less precise localization! e.g. on back! Two-point discrimination test! 6 2!

3 Receptors and Receptive Fields Figure 15-2! 7 Receptor and Generator Potentials 1! Stimulus transduced into action potentials! Stimulus causes a change in membrane potential of receptor! Change = receptor potential, a graded potential! Stronger stimulus larger change! Can be de- or hyperpolarization! A graded depolarization in a neuronal receptor is called generator potential.! The greater the generator potential, the more frequently action potentials are generated. 8 Receptor (Generator) Potential! Generator (receptor) potential Guyton and Hall, 1996! 9 3!

4 Interpretation of Sensory Information! Labeled line:! Specific sensory info sent from specific receptor in specific part of body to specific area of cortex! Neuronal pathway from receptor to cortex is the labeled line! Cortex interprets:! 1. Type of stimulus based upon labeled line! 2. Location of stimulus based upon where in cortex labeled line arrives (homunculus)! 3. Intensity, duration of stimulus based upon action potential frequency and pattern! 10 General Types of Receptors! Rapidly-adapting (phasic) receptors! Reduce firing rate during constant stimulation Fire rapidly at the beginning and end of stimulation! Conveys information about changes in stimulus intensity! E.g. Pacinian corpuscle, thermoreceptor! Slowly-adapting (tonic) receptors! Fire at some level all the time! Continue firing during constant stimulation Conveys information about the duration of the stimulus! E.g. pain receptors, muscle spindles! 11 Tonic and Phasic Receptors Figure 15-3! Tonic Receptors! Pain receptors! Muscle spindle! Merkel disc! Phasic Receptors! Pacinian corpuscle! Thermoreceptor! Meissner s corpuscle! 12 4!

5 Pacinian Corpuscle Rapid Peripheral Adaptation! 13 Adaptation to a Stimulus! Adaptation = sensitivity to constant stimulus 1. Peripheral adaptation! Receptor sensitivity changes! amount of info reaching CNS! 2. Central adaptation! Occurs within CNS, along sensory pathways! A. Subconscious adaptation - e.g. olfaction! Brain signals inhibitory cells in olfactory bulb! B. Conscious adaptation! e.g., tuning out noise during an exam! 14 SECTION 15-3! General sensory receptors can be classified by the type of stimulus that excites them! 15 5!

6 General Senses Receptors! Exteroceptors! Interoceptors! Proprioceptors! Note: There are no proprioceptors in visceral organs of thoracic and abdominopelvic cavities.! Why is this a good thing?! 16 Nociceptors (Pain Receptors)! Slow-adapting Few are in deep tissues or viscera! Are free nerve endings of sensory neurons! Have large receptive fields! Subtypes based on sensitivity to:! 1. Extreme temperature! 2. Mechanical damage! 3. Chemicals! 4. A strong stimulus can excite all three types! Neurotransmitters: Glutamate, Substance P (for pain)! 17 Nociceptors 2 Fiber Types! 1. Type A fibers! Myelinated - fast pain or prickling pain! Relayed to primary sensory cortex Conscious, localized pain! e.g. deep cut, pin prick! 2. Type C fibers! Unmyelinated - slow pain = burning, aching! Relayed to thalamus and reticular formation Conscious, but not precisely localized! e.g. uterine cramps! 18 6!

7 Nociceptors 3! Central adaptation to pain! May decrease perception of pain! e.g. endorphins, enkephalins prevent release of substance P! Part of the mechanism of pain! 1. Tissue damage! 2. Arachadonic acid released from cell membranes! 3. Converted to prostaglandins by cyclo-oxygenase! 4. Prostaglandins stimulate pain receptors! (Aspirin, Ibuprofen - block cyclo-oxygenase)! 5. Info ascends in cord via lateral spinothalamic tract! 19 Thermoreceptors! Are free nerve endings! Phasic (fast-adapting)! But background firing rate depends on temperature! There are 3 to 4 times more cold than warm receptors! Information ascends in lateral spinothalamic tract (with pain information)! 20 Thermoreceptors 2 Handout! Constant! temperature! Changing temperature! Cold fiber! Warm fiber! Kandel, Schwartz and! Jessell, 2000! Temperature! 21 7!

8 Tactile Receptors in the Skin Figure 15-4! 22 Mechanoreceptors! Have mechanically-gated ion channels! A. Tactile receptors! Touch, pressure (deep touch), vibration! Fine touch and pressure! Precisely localized - small receptive fields! (Posterior column pathway)! Crude touch and pressure! Poorly localized - larger receptive fields! (Anterior spinothalamic tract)! 23 Tactile (Touch and Pressure) Receptors 1! 1. Free nerve endings! Between epithelial cells, in corneal epithelium! Touch and pressure! Tonic (slow-adapting)! Small receptive fields! 2. Root hair plexus! Dendrites surround hair root! Movement of hair! Phasic (fast-adapting)! 24 8!

9 Touch and Pressure Receptors 2! 3. Merkel s (tactile) discs! Dendrites contact Merkel (epidermal) cells! Fine touch and pressure - very sensitive! Tonic (slow adapting)! Small receptive field! 4. Meissner s (tactile) corpuscles! Papillary layer of dermis! Capsule = modified Schwann cells! Fine touch and pressure, low freq. vibration! Phasic (fast adapting)! 25 Touch and Pressure Receptors 3! 5. Pacinian (lamellated) corpuscles! Dermis, joint capsules, fasciae, serous membranes, some viscera! CT capsule! Deep pressure, high freq. vibration! Phasic (fast adapting)! Large size large receptive field! 26 Touch and Pressure Receptors 4! 6. Ruffini corpuscles (end organs)! Reticular layer of skin! Capsule covers dendrites innervating collagen fibers of skin! Senses stretch, distortion of dermis! Tonic (very slowly adapting - if at all)! 27 9!

10 Mechanoreceptors Baroreceptors! B. Baroreceptors! Monitor pressure (actually monitor stretch due to fluid pressure)! Free nerve endings within elastic CT! Stretch/recoil change in AP frequency! Phasic (fast adapting), respond to change! Some locations:! Digestive tract, urinary bladder! Carotid sinus and aortic sinus! Lungs! 28 Proprioceptors! Do not adapt to constant stimulation Muscle spindles (chapter 13)! Golgi tendon organs (chapter 13)! Free nerve endings! Sense tension in tendon! Joint capsule receptors! Free nerve endings! Pressure, tension, movement of joint! 29 Chemoreceptors! Respond to water- and lipid-soluble substances! I.e. must be dissolved to be sensed! Adapt rapidly (seconds)! General chemosensation:! Send info to brain stem (not sensory cortex)! e.g. CNS sensors in pons, medulla! [H + ], [CO 2 ] in CSF! e.g. Peripheral sensors! [H + ], [CO 2 ], [O 2 ] in blood! Info to brain via CN IX and X! 30 10!

11 SECTION 15-4! Separate pathways carry somatic and visceral sensory information! 31 Organization of Sensory Pathways! 1. First-order neuron connected to receptor! Cell body in dorsal root ganglion or cranial nerve ganglion! Synapses with second-order (interneuron) in brain stem or cord! 2. Second-order neuron! Crosses over to contralateral side! Usually synapses with third-order neuron in thalamus! 3. Third-order Synapses in post-central gyrus (primary sensory cortex)! Does not cross over! 32 Sensory/Ascending Tracts/Pathways Fig. 15-5! Labeled line: Neuronal pathway from receptor to cortex! 33 11!

12 Posterior Column Pathway Fine touch! Vibration! Pressure! Proprioception! (from contralateral! side of body)! Midbrain! Spotlight 15-6c! Third-order! Second-order! Medulla! First-order! 34 Posterior Column Medial Lemniscus 1! Sensations! Fine (highly localized) touch, pressure, vibration, conscious proprioception! Tracts! Fasciculus gracilis! Lower half of body - trunk and lower limbs! Fasciculus cuneatus! Upper half of body - neck, arms, chest! 35 Posterior Column Medial Lemniscus - 2! 1. First-order Receptor synapse in nucleus gracilis or cuneatus in medulla! 2. Second-order Cross over to other side in medulla! Enter medial lemniscus ( ribbon )! Synapse in thalamus! 3. Third-order Thalamus somatosensory cortex! Thalamus processes info according to stimulus type and location! 36 12!

13 !! Chapter 15 Sensory Pathways, Anterior Spinothalamic Tract Crude touch and pressure! (from other side of body)!! crude = poorly localized! Thalamus Spotlight 15-6a! Third-order! Midbrain First-order! Medulla Second-order! 37 Lateral Spinothalamic Tract Figure 15-5b! Pain and Temperature! (from other side of body)! Medulla Thalamus Third-order! Second-order! First-order! 38 Anterior and Lateral Spinothalamic Tracts! Sensations! A. Anterior spinothalamic tract! Crude (poorly-localized) touch and pressure!! B. Lateral spinothalamic tract! Pain and temperature! 39 13!

14 Anterolateral Pathways! BOTH Anterior and Lateral Pathways! 1. First-order neurons! Synapse in dorsal horn! 2. Second-order neurons! Cross over in spinal cord! Synapse in thalamus! 3. Third-order neurons! Synapse in primary sensory cortex! 40 The Spinocerebellar Pathway Spotlight 15-6d! Unconscious proprioception!! No third-order neuron First-order! Second-order! 41 Spinocerebellar Pathways! Sensation! Unconscious proprioception (cerebellum!)! Input from Golgi tendon organs, muscle spindles, joint capsules! Pathway! 1. First-order neurons! Synapse in dorsal horn! 2. Second-order neurons! Synapse with Purkinje cells in cerebellum! 3. NO third-order neurons! 42 14!

15 Referred Pain! Mechanisms of Pain! Cleveland Clinic Journal of Medicine, 74(1): 2007:! Pain of visceral origin is referred to somatic regions that are innervated from the same spinal segments as the heart.! The pain is generally referred to proximal, but not distal, somatic structures.! The referred pain is experienced as deep (slow) pain.! Slides 44, 45, and 46 are FYI only 43 Neuroscience at University of Texas! 1. Common dermatome hypothesis! Structures develop from same embryonic segment (dermatome)! 2. Convergence hypothesis! Axons from visceral (heart) and somatic (skin) receptors synapse with same neurons in cord! 3. Facilitation hypothesis! Signals from viscera facilitate perception of pain from skin by higher centers! 4. Learned phenomenon hypothesis! The brain is not used to receiving visceral pain signals so interprets them as being somatic! FYI Only! 44 Phantom Pain! 1. Peripheral mechanism! Damage changes inputs to cord! deafferentation and neuroma growth! Increased Na + channel synthesis! Increased sensitivity pain! 2. Central mechanism! a. Central sensitization! New growth of proximal end of cut New growth of non-pain neurons into pain areas of DRG! New connections - increased receptive field! FYI Only! 45 15!

16 Phantom Pain 2! 2b. Windup effect! Increased (upregulation) of pain pathway receptors! 2c. Rewiring may cause loss of inhibitory signals from higher centers.! 3. Brain changes mechanism! a. Cortical reorganization! Areas formerly responsible for amputated limb taken over by neurons from other areas! b. Changes in body schema! Disconnect between intended and actual movements upsets body schema! FYI Only! 46 Principal Sensory Pathways 1 Table 15-1! 47 Principal Sensory Pathways 2 Table 15-1! 48 16!

17 SECTION 15-5! The somatic nervous system is an efferent division that controls skeletal muscles! 49 Descending (Motor) Tracts Figure 15-8! Note: Names indicate direction.! 50 Motor Pathways! Pathways involve at least 2 neurons! 1. Upper motor neuron (UMN)! Cell body in CNS (e.g. primary motor cortex)! Synapses with lower motor (Excitatory or inhibitory synapse)! Stays in CNS! 2. Lower motor neuron (LMN)! Cell body in brain stem or cord! Innervates skeletal muscle fibers of a motor unit! (Always excitatory synapse)! Called Final Common Pathway! 51 17!

18 Corticospinal Pathways 1! A.K.A. Pyramidal System or Direct Pathways! Conscious control of skeletal muscle! 1. Corticobulbar tract! ( bulbar refers to the brain stem)! UMN cell bodies in primary motor cortex! Synapse in cranial nerve nuclei Conscious control of skeletal muscle! e.g. eye, jaw, facial muscle movements! 52 Corticospinal Pathways 2! 2. Lateral corticospinal tract! (85% of motor corticospinal pathway neurons)! UMN in primary motor cortex! Cross over at decussation of pyramids in medulla! Synapse on LMN in cord! Conscious control of skeletal muscle! 53 Corticospinal Pathways - 3! 3. Anterior corticospinal tract! 15% of corticospinal neurons! UMN in primary motor cortex! Synapses on LMN in cord! Cross over in spinal cord (not medulla)! Conscious control of skeletal muscle! 54 18!

19 The Corticospinal Pathway Figure 15-9! Parts of Corticobulbar tract Decussation of the pyramids Lateral corticospinal tract 85% Final Common Pathway Anterior corticospinal tract 15% 55 Principal Motor Pathways Table 15-2! 56 19!