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SENSES Sensory Receptors (Sensors) • Transducer: a Structure That Converts a Stimulus Into a Graded Depolarization

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SENSES Sensory Receptors (Sensors) • Transducer: a Structure That Converts a Stimulus Into a Graded Depolarization Sensors & Transducers SENSES Sensory Receptors (Sensors) • Transducer: a structure that converts a stimulus into a graded depolarization. • Physiological transducers are derived from specialized dendrites, often associated with modified epithelia. 1. Exteroceptors: sense external stimuli a) passive — sense environmental stimuli b) active — sense self-generated stimuli ÿ bat sonar, flashlight fish, knife fish electric fields. 2. Interoceptors: sense internal body condition Types of Sensory Receptors: Types of Sensory Receptors: Mechanisms of Transduction Mechanisms of Transduction 1. Chemoreceptors 1. Chemoreceptors: chemical stimulus binds to 2. Mechanoreceptors specific binding protein on cell surface ‡ open ion gates ‡ depolarization. 3. Thermoreceptors • Interoceptors 4. Photoreceptors – Blood glucose, fatty acids, pH 5. Other Electromagnetic Receptors – Nocioceptors — chemicals released by damaged cells (pain) • Exteroceptors (special senses) – Olfaction (smell) – Gustation (taste) Types of Sensory Receptors: Types of Sensory Receptors: Mechanisms of Transduction Mechanisms of Transduction 1. 1. 2. 2. Mechanoreceptors: physical distortion of cell 3. Thermoreceptors: temperature-induced change membrane ‡ open ion gates ‡ depolarization. of membrane protein ‡ open ion gates ‡ • Interoceptors depolarization. – Propioceptors — muscles, tendons, joints • Interoceptors – Visceral stretch receptors – Body temperature [BT] — hypothalamus – Blood pressure, osmotic pressure • Exteroceptors (cutaneous senses) • Exteroceptors (cutaneous senses) – Hot & cold sensations – Touch & pressure (“Cold” is a perception, not a real physical entity.) • Exteroceptors (special senses) – Auditory “hair cells” – Equillibrium “hair cells” Heyer Sensors & Transducers Types of Sensory Receptors: Types of Sensory Receptors: Mechanisms of Transduction Mechanisms of Transduction 1. 1. 2. 2. 3. 3. 4. 4. Photoreceptors: light absorbed by pigment- 5. Other Electromagnetic Receptors: EM energy protein ‡ close ion gates ‡ hyperpolarization. absorbed by special metallo-protein complexes • Exteroceptors (special senses) ‡ transduction mechanism unclear – Vision — retina rods & cones • Electromagnetic fields • Infrared radiation Generator Potentials afferent transmission of stimulus intensity Gentle pressure • In response to stimulus, sensory nerve endings 1. Frequency modulation produce a local graded • Stronger stimuli result in Sensory change in membrane Low frequency of action potentials receptor More pressure more frequent action potential. potentials • Potential changes are called – Amplitude not changed! receptor or generator (a) Single sensory receptor potential. activated High frequency of action potentials • Must be cummulative to Gentle threshhold before decay pressure 2. Receptor recruitment nPhasic response: Fewer receptors activated • Stronger stimuli may n Generator potential increases with increased Sensory stimulus, then as stimulus continues, generator receptors result in transduction by potential size diminishes. More pressure more sensors nTonic response: – More sensory neurons send n Generator potential proportional to intensity More receptors signals of stimulus. (b) Multiple receptors activated activated Receptive Sensory Convergence Receptive Sensory Convergence Stimuli fields Sensors neurons Stimuli fields Sensors neurons Low convergence: few Low convergence: more sensory receptors converge on a single neurons from a given area connect sensory neuron. to more cells of the sensory High acuity: many small cortex: High acuity. receptive fields per given area distinguish among individual stimuli. But low sensitivity. High convergence: many receptors converge on a single sensory neuron. High sensitivity: many stimuli summed within a single receptive field. Sensory But low acuity. cortex Heyer Sensors & Transducers Perception of Senses Sensations • Law of Specific Nerve Energies: “Any stimulation of a — sensory neuron is perceived according to that nerve’s • Somatesthetic senses (“body feeling”) adequate stimulus.” vertebrates, via spinal nerves • Adequate stimulus: the normal type and intensity of stimulus – Cutaneous senses that would cause stimulation of that neuron’s sensory receptor. – Visceral senses • I.e., “it’s all in your head!” Anything that stimulates the sensor, – Proprioceptors sensory neuron, or sensory cortex will be interpreted by the brain to be the sensation normally transduced or transmitted by those • Special senses — via cranial nerves structures. • Examples of misinterpretation: – Gustatory (taste) – “Seeing stars” from eye trauma – Olfactory (smell) – Tinnitus: “ringing in your ears” – Auditory (hearing) – Phantom pain in amputees – Equilibrium – Referred pain from visceral trauma – Vision – “Hot” spices Somatesthetic Sensory Neurons Ascending Somatesthetic Spinal Tracts Spinothalamic tracts: the three-sensory-neuron trail 1. First order sensory neuron Axon travels from cutaneous receptors, proprioceptors • Spinal sensory neurons: pseudounipolar neurons or visceral receptors to the with cell bodies located in dorsal root ganglia. CNS via spinal dorsal root. Axon either terminates at spinal cord or ascends ipsilateral tract to medulla. 2. Second order sensory neuron crosses to contralateral side and ascends to thalamus. 3. Third order sensory neuron ascends from thalamus to sensory cortex. (Remaining on contralateral side.) Cutaneous Sensations Cutaneous-like Sensations through a cuticle Heat Gentle Pain Cold Hair touch • Mediated by dendritic nerve endings of different sensory Arthropods neurons. • Mechanoreceptors — dendritic nerve endings of • Mostly in the dermis. But some in sensory neurons attached to articulating bristles Epidermis basal stratum of epidermis. • Thermoreceptors: heat and cold – More receptors respond to cold Dermis than warm. • Nociceptors (pain) • Mechanoreceptors — free – Touch Hypodermis – Slow adapting (tonic) • Mechanoreceptors — encapsulated – Touch & pressure Nerve Connective Hair Strong – Rapidly adapting (phasic) tissue movement pressure Fig. 50-3 Heyer Sensors & Transducers Antenna sensilla Proprioceptors Proprioceptors: body positioning sensors • Spindles within muscles Magnified view of – Maintain muscle tension The seven types of sensory one of the pore structures on the antennae: plates on the • Golgi organs in tendons a. Small thick-walled hair worker bee’s b. Thick-walled peg antenna – Inhibitory reflex; prevents c. Slender thin-walled peg overexertion of muscle d. Large thin-walled peg e. Pore plate • Ligament stretch receptors f. Pit organ g. Pit organ – Detect degree of joint extension/flexion Transduction in stretch receptors Chemoreception • Taste & smell food — primal importance – A stretch receptor in a crayfish abdominal muscle – palatibility Weak Strong Muscle – hunting muscle stretch muscle stretch Dendrites –50 Receptor potential –50 • Smell conspecifics –70 –70 Stretch (mV) receptor Action potentials – sexual pheromones Membrane 0 0 Axon potential • elephants & insects use (Z)-7-dodecen-1-yl acetate) –70 –70 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Time (sec) Time (sec) – kin recognition (a)Crayfish stretch receptors have dendrites stretch, producing a receptor potential in the stretch receptor. A stronger stretch produces embedded in abdominal muscles. When the receptor. The receptor potential triggers action a larger receptor potential and higher • wasp nestmates abdomen bends, muscles and dendrites potentials in the axon of the stretch requency of action potentials. • ant trails Figure 49.2a Gustation: Sensation of taste. Taste Taste Myth • Epithelial cell receptors clustered in barrel-shaped taste buds. – Apical microvilli (taste hairs) increase surface area for chemoreception. – Stimulated taste cells release NTs to stimulate sensory neuron. • Each taste bud has taste cells only responding to one class of dissolved chemical stimulus. • 1° sensory neuron from taste bud via cranial nerves to medulla oblongata. 2° neuron to thalamus. Illustration reproduced and artistically enhanced over many 3° neuron to sensory cortex. generations of textbooks. But never actually verified! Heyer Sensors & Transducers Gustatory Sensilla • Bipolar sensory neuron extends Olfaction (Smell) ciliated dendrites into nasal Insect taste chemoreception via hollow epithelium, exposing tips to nasal sensory hairs (sensilla) primarily on legs To brain mucus. • Odorant molecules (odorogens) and mouth-parts. Chemo- receptors Sensillum dissolve in mucus and bind to dendritic receptor proteins. Microelectrode EXPERIMENT Insects taste using gustatory sensilla (hairs) on their feet To voltage • Intracellular amplification: and mouthparts. Each sensillum contains four chemoreceptors with recorder dendrites that extend to a pore at the tip of the sensillum. To study the odorogen bound to a single receptor sensitivity of each chemoreceptor, researchers immobilized a blowfly protein may open many ion gates. (Phormia regina) by attaching it to a rod with wax. They then inserted the tip of a microelectrode into one sensillum to record action potentials in the Pore at tip Very sensitive response! chemoreceptors, while they used a pipette to touch the pore with various Pipette containing test substances. test substance • Very large variety of chemoreceptors. Humans can detect RESULTS Each chemoreceptor is especially sensitive to a Chemoreceptors >10,000 different odors! particular class of substance, but this specificity is relative; each cell can respond to some extent to a broad range of different chemical stimuli.
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