Unit 6 Notes - Sensory Physiology PDF

UNIT 6: SENSATION (AFFERENT NERVOUS SYSTEM) BIOL2410 D01 Lecture Notes A) Overview In order to maintain homeostasis the nervous system must be able to gather information about the state of the internal and external environment. It does this using a number of different type...

UNIT 6: SENSATION (AFFERENT NERVOUS SYSTEM) BIOL2410 D01 Lecture Notes A) Overview In order to maintain homeostasis the nervous system must be able to gather information about the state of the internal and external environment. It does this using a number of different types of sensory receptors in the sensory division of the peripheral nervous system. We are often told as children that there are five senses (Vision, Hearing, Smell, Taste, and Touch). But the reality is that humans have many more senses than this, with most neuroscientists agreeing that there are approximately 9 up to 21 senses. In this unit we will continue our examination of the physiology of the nervous system by looking at how the “Visceral Sensation” (Izumi Kato) sensory (afferent) nervous system detects and interprets stimuli in the internal and external environment using somatic, visceral and special senses. A) Overview 1. Types of Senses 2. General Functions of the Senses 3. General properties of Sensory Systems 4. Types of Sensory Receptors 5. Signal transduction by sensory receptors. 6. Receptive Fields of sensory receptors 7. Integration of sensory information by CNS a. Where is sensory information processed? b. How are the properties of a stimulus determined? a. Modality b. Location c. Intensity “Visceral Sensation” (Izumi Kato) d. Duration 8. Somatic Senses & Pathways A) Overview 9. Special Senses – Olfaction 10. Special Senses – Gustation 11. Special Senses – Hearing 12. Special Senses – Equilibrium (Balance) 13. Special Senses – Vision “Visceral Sensation” (Izumi Kato) B) Types of Senses Ø Senses can be categorized based on where the sensory signal originates and whether of not the sensation is consciously or unconsciously perceived by the brain. Ø Conscious senses: these are sensations that are perceived and processed in the conscious areas of the brain (i.e. in the sensory areas of the cerebral cortex). They include: 1. Somatic Senses – also known as the “general” senses, as they are more generally distributed throughout the body. Many (but not all) are associated with sensory receptors in the skin. a. Touch b. Temperature c. Pain d. Itch Table 10.1 e. Proprioception – position of body B) Types of Senses Ø Senses can be categorized based on where the sensory signal originates and whether of not the sensation is consciously or unconsciously perceived by the brain. Ø Conscious senses: these are sensations that are perceived and processed in the conscious areas of the brain (i.e. in the sensory areas of the cerebral cortex). They include: 2. Special Senses a. Vision b. Hearing c. Taste (Gustation) d. Smell (Olfaction) e. Balance (Equilibrium) Table 10.1 B) Types of Senses Ø Unconscious senses: areas of brain that process information from unconscious senses are not located in the cerebral cortex. The stimuli associated with these senses include: 1. Somatic stimuli, including: a. Muscle length and tension b. Proprioception 2. Visceral stimuli, including: a. Blood pressure b. Osmolarity of extracellular fluids c. Blood glucose concentration d. Internal (core) body temperature e. Lung inflation f. pH of plasma and CSF Table 10.1 g. O2 and CO2 content of blood. C) General Functions of Senses ØSomatic, visceral and special senses provide us with information about: 1. The external environment Ø They help us to identify food sources, temperature changes, weather changes; potential mates, potential hazards, shelter, etc. 2. Ourselves. Ø Energy stores, body temperature; water/ion balance (thirst/hunger); the position of our body, etc. D) General Properties of Sensory Systems 1. Sensory receptors are most sensitive to a certain stimulus or form of energy. The specific form of the stimulus (e.g. chemicals, mechanical energy, temperature, light, etc.) is called the sensory modality. 2. Signal transduction in sensory receptors converts a stimulus into graded potentials. The receptor acts as the transducer - converting the energy of the stimulus into electrical energy (a change in membrane potential). ØStimulus causes gated ion channels to open or close on receptor cells. Whether or not the channel opens or closes depends on the receptor type and modality. ØClosing or opening of gated channels results in movement of ions down their electrochemical gradients producing a depolarizing or hyperpolarizing graded potential. 3. Sensory neurons have receptive fields. 4. The CNS integrates sensory information. 5. Coding and processing distinguish stimulus properties. E) Types of Receptors Ø Sensory receptors are either: 1. The dendrites of unipolar sensory neurons, which can be: a. free nerve endings Ø E.g. pain receptors (nociceptors) b. wrapped/encapsulated in connective tissue Ø E.g. some touch receptors (like Meissner’s (tactile) corpuscles that detect light touch Ø In each case, the axons of these neurons may be myelinated or unmyelinated 2. Non-neuronal transducer cells (often receptors for special senses) Ø cells that are not neurons themselves, but that are separated from the dendrites of a sensory neuron by a synapse. When stimulated, these cells release Fig. 10.1 neurotransmitter onto the associated unipolar neuron. Ø E.g. hair cells of inner ear; rods and cones of the eye. E) Types of Receptors Ø Receptors can be classified in several ways, and one receptor may fall under more than one category. Ø Receptors may be classified by: 1. Structure of the receptor: (general senses only) a. Free nerve endings – dendrites of unipolar senosry neurons. E.g. for pain b. Encapsulated receptors – terminal dendrites wrapped in connective tissue. 2. Location of receptor: a. Interoceptors – detect stimuli in internal environment (in viscera, blood vessels, etc). b. Exteroceptors – detect stimuli in external environment so receptors are at the body’s surface (e.g. touch receptors, special senses, etc.) Fig. 10.1 E) Types of Receptors 3. Type of Adequate stimulus for the receptor Ø Adequate stimulus = modality or form of energy to which the receptor is most responsive Ø Receptors can sometimes respond to other modalities but only if the stimulus strength has a high enough intensity Ø For example temperature receptors can respond to the chemical found in hot peppers (capsaicin) and cause you to feel heat. Ø Includes: a. Mechanoreceptors – detect physical forces/stimuli (touch, pressure, stretch, vibrations, gravity, soundwaves, etc.). Includes baroreceptors (detect pressure/stretch in blood vessels, lungs, etc.) and some transducer cells like the hair cells in the inner ear. b. Thermoreceptors – detect hot and cold temperatures. c. Chemoreceptors – detect chemicals – e.g. olfactory neurons; interoceptors for O2 or pH. d. Photoreceptors – detect light. Includes the rods and cones (transducer cells) in the retina of the eye. e. Nociceptors – detect pain. f. Proprioceptors – detect body position and movement. E) Types of Receptors 4. Rate of Adaptation of the receptor Ø In the presence of a continuous (sustained) stimulus, some receptors show adaptation = a reduction in the frequency of action potentials sent though sensory neurons to the brain. Ø Types of receptors: a. Tonic receptors – slowly adapting receptors. Fire rapidly upon stimulus, then decrease firing rate, but maintain firing rate under a constant stimulus. Fig. 10.7 Ø Often used with sensations that are continuously monitored, like proprioceptors, baroreceptors in blood vessels, etc.). b. Phasic receptors – rapidly adapting receptors. Fire at onset of stimulus, then stop if stimulus intensity remains constant (adaptation). They do not fire again until there is a change in the intensity of the stimulus. Ø E.g. many touch receptors. Ø For example, putting glasses on in the morning – produces a burst of APs in mechanoreceptors for touch (and you feel the glasses) ⇒ over time the receptors adapt and action potentials to CNS stop (you no longer feel the glasses even though the stimulus is still there/constant). F) Signal Transduction by Sensory Receptors Ø Stimulation leading to activation of the receptor triggers the signal transduction process in the receptor cell. The minimum stimulus required to activate a receptor is the receptor threshold. Ø Signal transduction involves changes in membrane potentials of sensory neurons or non-neuronal transducer receptor cells. Ion channel Ion channels acts on receptors open (activate) Change in membrane Stimulus potential = graded or potential ac ts GPCR (depolarization or on Second (or other Ion channels hyperpolarization) messenger protein close (inactivate) pathways receptor) Ø The graded potential produced on the receptor cell is called a receptor potential. It is triggered by an adequate stimulus (the preferred stimulus of the receptor cell type). Ø The intensity of the stimulus is encoded by the magnitude (amplitude) of the graded potential. Ø The amplitude of graded potential is translated into the frequency of action potentials in the associated neurons. Higher graded potential amplitude = higher frequency of action potentials F) Signal Transduction by Sensory Receptors Ø Transduced signals then travel along sensory neurons to the brain. Ø Depending on the sense involved, the pathway may involve 2-3+ neurons. Ø For most general senses, except some proprioceptive pathways, the pathway is always 3 neurons: 1. Primary (first order) sensory neurons Ø unipolar neurons Ø first neuron in the pathway Ø are the receptor in which transduction occurs (in the dendrites) OR they are the unipolar neurons that receive the transduced signal from the non-neuronal receptor cell. 2. Secondary (second order) sensory neurons Ø Multipolar neurons Ø Cell bodies in dorsal horn of spinal cord or in medulla depending on the sense involved 3. Tertiary (third order) sensory neurons Ø Multipolar neurons Ø Cell bodies in thalamus with axons relaying to cortex. After Marieb and Hoehn 2019. G) Receptive Fields of Sensory Receptors Ø Receptive field = physical area in which the presence of a stimulus will activate the receptor and alter the firing of a sensory neuron. Ø Often associated with cutaneous (skin) receptors and the retina (so would represent the area in a patch of skin that would activate the receptor in question or the area of the retina that would need to be stimulated by light to activate specific rods/cones). Ø Primary sensory neurons converge onto secondary neurons, allowing for spatial summation of signals on the secondary neuron. Fig. 10.2 G) Receptive Fields of Sensory Receptors Ø The more convergence of primary sensory neurons onto secondary sensory neurons, the larger the receptive field and the lower the sensory acuity. Ø Sensory Acuity = how accurately a stimulus can be located. Ø When there is a lot of convergence, the receptive fields associated with each individual primary sensory neuron (primary receptive fields) overlap more and form one large secondary receptive field. So instead of having sensation originating from each of the individual receptive fields, the sensation is assigned to the larger secondary receptive field, which makes it more difficult to determine the precise location of the stimulus. Ø Multiple stimuli occurring within the same secondary Similar to Fig. 10.2 receptive field will be interpreted as one stimulus. G) Receptive Fields of Sensory Receptors Ø The less convergence of primary sensory neurons onto secondary sensory neurons, the smaller the receptive fields and the higher the sensory acuity. Ø When there is less convergence the receptive fields associated with each individual primary sensory neuron do not overlap, and so each primary neuron/receptor can send signals to the brain, which makes it easier to determine the precise location of the stimulus (higher sensitivity). Fig. 10.2 G) Receptive Fields of Sensory Receptors Ø Two point discrimination tests Ø determine the sensory acuity of touch receptors on the surface of the skin. Ø Tested using an instrument called an aesthesiometer. Ø Demonstration: https://www.youtube.com/watch?v=QplcT2MjuEc Ø In areas of the skin where there is less convergence and the secondary receptive fields are small (such as the skin. on fingertips), the two points of the aesthesiometer activate separate pathways to the brain and both points are felt even if they are close together (positive two point discrimination test) = High acuity. Ø In areas where there is more convergence and secondary Fig. 10.2 receptive fields are large (such as the skin on your back) the two points of the aesthesiometer activate only a single pathway to the brain and are perceived by the brain as a single point, even if the two points are far apart (i.e. negative two point discrimination test) = Low acuity. G) Receptive Fields of Sensory Receptors Ø Each part of the body can be mapped to a specific area of the primary somatosensory cortex (the the part of the cortex that that perceives general senses). Ø These maps were developed by electrically stimulating the different regions of the cortex in people undergoing brain surgery. Ø Patients were conscious - local anesthetics can be used to remove the cranium (skull cap) and the brain itself does not have any pain receptors. it can perceive pain from other areas, but has no ability to detect its own pain. Ø Canadian doctor Wilder Penfield was instrumental in developing these maps Ø https://citeseerx.ist.psu.edu/viewdoc/download?doi Marieb and Hoehn 2019. =10.1.1.873.4232&rep=rep1&type=pdf Ø https://www.youtube.com/watch?v=pUOG2g4hj8s Fig. 10.9 G) Receptive Fields of Sensory Receptors Ø Areas of the body that appear larger on the map, have greater acuity/sensitivity (less convergence). Ø Areas of the body that appear smaller on the map have lower acuity/sensitivity (more convergence) Marieb and Hoehn 2019. Fig. 10.9 H) Integration of Sensory Signals by CNS Ø Sensation vs perception Ø Sensation occurs when sensory information is detected by receptors in the peripheral nervous system. Ø E.g. auditory receptors detect a loud, high pitched ringing noise Ø Perception occurs in the brain and involves awareness, organization and interpretation (assigning of meaning) of the sensation. Ø E.g. the high pitched ringing noise is a fire alarm and you need to gather your things and leave the building. Ø In the brain, perception is built from sensation – but not all sensations result in perception H) Integration of Sensory Signals by CNS Ø Sensory information travels 1. From spinal cord to brain via ascending (sensory) pathways comprised of primary, and secondary neurons and in many cases tertiary neurons. 2. Directly to brainstem via cranial nerves (exception – pathway for olfaction – olfactory nerves) H) Integration of Sensory Signals by CNS 1. Where in the brain is sensory information processed? a. Some sensory information may be processed at the level of the spinal cord or brainstem Ø Because they do not reach the cortex of the brain, this sensory information is unconscious. Ø E.g.1: most visceral reflexes, like changes in blood pressure, are not perceived by the brain because they are integrated in the brain stem or spinal cord. Ø E.g.2: Some of the sensory pathways for proprioception and equilibrium project primarily to the cerebellum. As a result, they do not synapse in the thalamus and therefore do not reach the cortex – as such, there is no perception of these sensations. H) Integration of Sensory Signals by CNS 1. Where in the brain is sensory information processed? b. Some sensory information is processed in the cortex. Ø Recall that the cortex is the conscious area of the brain, so conscious sensation and perception occur in the sensory areas of the cortex. If you can physically feel a stimulus, then the sensory signal has ended up in the cortex. Ø Refer to Unit 5 for the list of sensory areas of the cortex (also visible in Fig. 10.3). Ø Many pathways for sensory information synapse with neurons in the thalamus. The thalamus then relays those signals to the appropriate area of the cortex. H) Integration of Sensory Signals by CNS 1. Where in the brain is sensory information processed? b. Some sensory information is processed in the cortex. Ø A stimulus may activate a primary sensory neuron (sensation), but may not have enough intensity to surpass perceptual threshold. Ø Perceptual threshold = level of stimulus necessary for the conscious brain to become aware of the sensation. H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? Ø Recall: all sensory stimuli are converted into action potentials (APs) via transduction. So the properties associated with each stimulus must be encoded in the pathway of the APs or in the APs themselves. ØThe four main properties of a stimulus are: a. Modality – interpretation of the type of stimulus. E.g. Touch, temperature, pain, sound, light, taste etc. b. Location – interpretation of where on the body a stimulus has occurred. c. Intensity – interpretation of the strength of a stimulus (e.g. how to tell if a feather has dropped on your foot vs. a brick). d. Duration – interpretation of how long the stimulus is applied/present. H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? a. Modality: Ø Determined by the neuronal pathway that brings the sensory signals to the CNS = labeled line coding. Ø Each receptor has a specific adequate stimulus. Ø When stimulated by an adequate stimulus, the receptor initiates signaling in a hardwired pathway to the higher processing centres in the brain. Ø The processing centres in the brain (cortex) contain specific groups of neurons that are associated with specific modalities. Ø Any signals coming through this pathway will always be interpreted as the modality associated with the region of the brain stimulated. Ø Even if the receptor is removed, any signal in the pathway will still be perceived by the brain in the correct modality. As a result, it is the pathway that determines the modality, not the receptor. Marieb and Hoehn 2019. H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? a. Modality: Ø E.g. activating a touch receptor in your right foot creates action potentials in the first order, second order and third order neurons leading up to the cortex of the brain. The axon terminals of the 3rd order neuron will terminate in the specific area of the somatosensory cortex on the left side of the brain that contains groups of neurons that interpret the sense of touch on the foot. Ø Analogy: When you flip the switch (receptor) of the light in your kitchen, electricity travels through the wires (pathway) from the switch to the light and the light in the kitchen turns on (perception). Flipping the kitchen light switch will never turn on the light in the bathroom, since the bathroom light requires a different pathway of wires (neurons) that are connected to a different switch (receptor). Marieb and Hoehn 2019. H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? b. Location: Ø Determined by which receptive fields are activated. Ø The receptive fields for any part of the body are mapped onto the cortex of the brain (somatotopic maps) and stimuli from adjacent areas of the body are processed in adjacent areas of the brain. Ø Labeled line coding ensures that when a particular receptive field is simulated, the pathway of neurons to the brain will connect to the specific region of the brain associated with the area stimulated. Ø E.g. activating a touch receptor in the heel of the right foot creates action potentials in a pathway that terminates in the specific area of the somatosensory cortex on the left side of the brain that is associated with the heel of the right. Marieb and Hoehn 2019. H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? b. Location: Ø Can also be determined by Lateral Inhibition: Ø Mechanism to increase contrast between activated receptive fields and non-activated receptive fields. Ø Steps: i. Stimulus activates receptive fields of primary neurons. ii. Primary neuron closest to stimulus has a higher frequency of APs and releases more neurotransmitter. Adjacent primary neurons that are further from the stimulus produce fewer APs. iii. Collaterals of the secondary neuron closest to the stimulus inhibit adjacent secondary neurons (= lateral inhibition, which is an example of presynaptic inhibition – see Unit 4) Fig. 10.5 H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? b. Location: Ø Can also be determined by Lateral Inhibition: Ø Mechanism to increase contrast between activated receptive fields and non-activated receptive fields. Ø Steps: iv. Lateral inhibition turns off signaling in pathways that are further from the stimulus, which increases the contrast in AP frequency between the different receptive fields. v. Increased contrast between the center and the sides of the receptive fields by the time the signals reach the brain - makes the stimulus easier to localize and as a result enhances perception of the stimulus. Fig. 10.5 H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? b. Location: Ø Can also be determined by Lateral Inhibition: Ø Lateral inhibition is also an example of population coding in which sensory information from several neurons/pathways are combined and compared to provide the CNS with more information than it could have received from an individual neuron/receptor. Fig. 10.5 H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? c. Intensity: Ø Determined by: i. The number of receptors activated (population coding) Ø The greater the number of receptors that are activated the greater the intensity. I.e. activating more than one receptor of the same type or activating multiple types of receptors (e.g. touch, pressure, and pain at the same time). ii. The frequency of action potentials reaching the brain. Ø Graded potentials in receptors vary in strength with the stimulus. So… Stronger stimulus à stronger graded potential à increase in action potential Fig. 10.6 frequency à releases more neurotransmitter in the brain (alerts brain to increased intensity) H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? c. Intensity: Ø Example: A feather touching the bottom of your foot may only activate a few touch receptors. A brick falling on your foot, would activate, touch, pressure, and possibly pain receptors. With more receptors activated, there would be an increase in the frequency. of APs reaching the brain. Fig. 10.5 H) Integration of Sensory Signals by CNS 2. How are the properties of a stimulus determined? d. Duration: Ø Determined (coded for) by duration of action potentials in neurons. Ø The longer a stimulus is present, the longer action potentials will continue to be sent to the brain Ø Exception is phasic receptors, which demonstrate adaptation to a stimulus. Fig. 10.6 I) Somatosensory Pathways - Overview Know this diagram ØPathways for general senses (touch, temperature, and Table! nociception, pressure, proprioception) project to cortex or to cerebellum. ØPathways to cortex have primary, secondary and tertiary neurons. ØSecondary neurons decussate – cross over to the opposite side of body in either the 1. spinal cord - for nociception, temperature and coarse touch 2. medulla - for fine touch, proprioception and vibration. ØAll secondary neurons connect with tertiary neurons in the thalamus. ØSensations are perceived in the primary somatosensory cortex. Fig. 10.8 I) Somatosensory Pathways - Overview Know this diagram ØSomatosensory pathways use three types of neuronal and Table! axons (nerve fibers) 1. Type A𝛽 (A-beta) fibers: ØLarge diameter myelinated neurons (fast conductance) ØCarry sensory information for cutaneous mechanical stimuli. 2. Type A𝛿 (A-delta) fibers: ØSmall myelinated neurons ØCarry sensory information for cold, fast pain (nociceptors) 3. Type C fibers: ØSmall diameter, unmyelinated neurons (slow conductance) ØCarry sensory information for slow pain (nociceptors) ØMost receptors for the general senses are located in the skin. Fig. 10.8 I) Somatosensory Pathways - Senses 1. Mechanoreception – touch, pressure/stretch Øtouch receptors (mechanoreceptors) = most abundant type of receptor in body ØMostly found in skin Ø E.g.1: Pacinian corpuscles that detect generalized pressure and vibrations – deep in the dermis of the skin. ØAlso found in some internal organs: Ø E.g.1: walls of blood vessels have baroreceptors (pressure- stretch receptors) that respond to changes in blood pressure. Ø E.g.2: walls of the lungs have baroreceptors (pressure-stretch Description of receptor receptors) that respond to overinflation of the lungs. ØStimulated by different types of physical contact/deformation Ø Stretch Ø Steady pressure Ø Fluttering/stroking movements Fig. 10.10 Ø Vibrations I) Somatosensory Pathways - Senses 1. Mechanoreception – touch, pressure/stretch ØSignal transduction occurs through opening of mechanically gated ion channels. ØDeformation of the receptor membrane in response to the stimulus opens the ion channel and allows ions (usually Na+) to move into or out of the receptor cell causing a graded potential. Description of receptor Fig. 10.10 I) Somatosensory Pathways - Senses 2. Thermoreception – temperature Fig. 10.10 ØThermoreceptors are free nerve endings in the skin, liver, skeletal muscles, and hypothalamus. ØThe membrane of the free nerve ending contains temperature receptor proteins that are cation channels called transient receptor potential (TRP) channels. These can be classified as cold receptors and warm/heat receptors: ØCold receptors – are activated at temperatures below body temperature. ØWarm receptors are activated at temperatures between 37 and 45℃. Ø At temperatures higher than 45℃, nociceptors (pain) are activated. I) Somatosensory Pathways - Senses 2. Thermoreception – temperature Fig. 10.10 Ø Signal transduction in thermoreceptors is complex and depends on the exact TRP channels involved. ØFor example: Stimulus of cold receptors opens TRP channels sensitive to cold, which allows Na+ and Ca++ to pass into the cell causing a graded potential (receptor potential). Cold temperatures may also close K+ leakage channels, which would also lead to a depolarizing graded potential. Ø Thermoreceptors can also be stimulated by chemicals ØE.g.1: Menthol in the mint plant activates cold receptors (meaning that it opens the ion channels that are normally opened by cold temperatures). ØE.g.2: Capsaicin from chili peppers activates warm receptors (meaning that it opens the ion channels that are normally opened by warm/hot receptors). ØIn these cases, signal transduction is likely to occur via GPCR second messenger pathways. I) Somatosensory Pathways - Senses 3. Nociception – pain/itch Fig. 10.10 ØFree nerve ending in the skin, muscles, joints, bones and some internal organs (viscera). ØThe CNS lacks nociceptors, so the brain can perceive pain from almost anywhere in the body except for itself. ØHeadaches result from pain originating in structures surrounding the brain – blood vessels, the meninges, muscle fibers, cranial nerves, etc). ØActivated by strong noxious (harmful) stimuli that may damage tissue. ØE.g.1: extreme hot or cold temperatures ØE.g.2: extreme pressure (extreme mechanical stimulus) ØE.g.3: Chemical stimuli I) Somatosensory Pathways - Senses 3. Nociception – pain/itch ØTwo types of pain: 1. Fast pain Ø transmits rapidly to CNS through type A𝛿 fibers. ØE.g. acute (sharp) pain that occurs at the moment you stub your toe. 2. Slow pain Ø transmits more slowly to CNS through type C fibers. ØE.g. dull throbbing pain that persists in the moments after you stub Fig. 10.12 your toe. ØPain can be modulated by input from other senses (e.g. mechanoreceptors), or from the brain itself (descending pain control pathways), by stimulating inhibitory interneurons in the spinal cord/brain where neurons in the nociceptive pathway synapse with one another. ØInhibitory interneurons act like gates that open/close the nociceptive pathway to varying degrees = gate-control theory of pain modulation. J) Special Senses – Olfaction ØForm of chemoreception ØMolecules from all types of matter produce millions of odors that are detected by olfactory receptors ØOne of the oldest senses – even bacteria are capable of detecting and interpreting chemical signals in their environment so that they can move in the direction of the optimal environment (chemotaxis) ØOlfactory receptors = bipolar neurons in the roof of the nasal cavity. ØThese synapse with secondary neurons in the olfactory bulb Fig. 10.13 J) Special Senses – Olfaction ØSignal transduction steps: 1. Odorant molecule dissolves in mucus of the olfactory epithelium 2. Odorant molecule binds to a GPCR 3. The olfactory G-protein (Golf) is activated Golf 4. Golf activates the cAMP second messenger pathway (see Unit 3 notes). 5. Increasing [cAMP] in cytosol triggers cAMP gated ion channels to open 6. Na+ and Ca++ ions move into the cell through the Modified from Purves et al. 2001. Neuroscience 2nd edition. Sinauer and Associates. open channel causing depolarization (a graded receptor potential in the olfactory receptor). 7. Ca++ entering the cell binds to Ca++ gated Cl- channels – which open and allow Cl- to move down its electrical gradient (from ICF to ECF). Loss of Cl- further depolarizes the cell. J) Special Senses – Olfaction ØIf the stimulus is strong enough and the graded potential large enough – an action potential occurs on the axon of the olfactory receptor, which will release neurotransmitter onto the secondary sensory neuron in the olfactory bulb. Ø Axons of olfactory receptor neurons form cranial nerve 1 (the olfactory nerve). ØSignals are relayed from olfactory bulb à olfactory tract à olfactory cortex ØOlfactory cortex perceives the odor and signals to other areas of the cortex like the limbic system (smell can be linked to emotions/memory). Fig. 10.13 J) Special Senses – Olfaction ØLoss of olfactory senses (anosmia) as a result of COVID-19 ØCurrently thought to occur as a result of infection of the olfactory supporting cells by SARS-CoV-2. These cells have the ACE2 receptors in their membranes that SARS-CoV-2 uses to infect cells (the primary olfactory neurons themselves do not have ACE2). Ø One of the roles of supporting cells is to clear the odorant (ligand) after binding and signal transduction has occurred – thus Fig. 10.13 making the receptor available to bind to the next molecule. Ø Impaired function of supporting cells in response to infection could prevent the clearing of odorant ligands. As a result, the olfactory receptor sites would essentially be blocked (no new ligands can bind if the old one cannot be removed). Ø Because olfactory receptor cells are phasic receptors and undergo adaptation in the presence of a constant stimulus, there would be a loss off sensation/perception for as long as receptor cells are blocked. K) Special Senses – Gustation (Taste) ØAlso a form of chemoreception. ØFlavour of food is also linked to olfaction. ØTaste is a combination of 5 different sensations 1. Sweet 2. Bitter 3. Umami (taste associated with the amino acids) 4. Sour 5. Salt ØTaste buds are composed of up to 150 taste receptor cells TRCs), in addition to support cells and basal cells (basal cells develop into Fig. 10.14 new TRCs. TRCs regenerate themselves ~ every 10-14 days. K) Special Senses – Gustation (Taste) ØEach taste receptor cell detects only one of the 5 taste sensations. ØThe different taste sensations use different signal transduction pathways. ØSweet, bitter, and umami use GPCR second messenger pathways that activate a G-protein called gustducin. ØSalt and sour tastes use ion channels for Na+ and H+ respectively. Fig. 10.14 K) Special Senses – Gustation (Taste) ØSignal Transduction in Type II taste receptor cells (for sweet, umami, and bitter) 1. Sweet, umami, or bitter ligand binds to GPCR on taste receptor cell. 2. GPCR activates a G-protein called gustducin. 3. Gustducin activates different second messenger pathways depending on ligand (including PLC-IP3 and adenylyl cyclase-cAMP). 4. Second messengers (IP3, cAMP, etc.) open Ca++ channels in the endoplasmic reticulum and/or cell membrane à increases [Ca++] in the cytosol. 5. Ca++ signals stimulate release of ATP from the receptor cell. 6. ATP acts as a paracrine molecule that stimulates a graded potential in the primary gustatory neuron Fig. 10.14 K) Special Senses – Gustation (Taste) ØSignal Transduction in taste receptor cells for Sour/Salt: 1. H+ ions associated with sour taste (e.g. vinegar) or Na+ ions associated with salt, enter the taste receptor cell through ion channels. 2. Intracellular signaling pathways are activated (the exact pathways are still not known). 3. Signaling pathways result in an increase in intracellular calcium. 4. Ca++ triggers exocytosis of a neurotransmitter (serotonin). 5. Neurotransmitter stimulates graded Fig. 10.14 potentials and action potentials in the primary sensory neuron. 6. Signals travel to gustatory cortex of insula. L) Special Senses – Hearing Fig. 10.16 ØHearing is body’s ability to perceive energy in the form of soundwaves. ØSoundwaves have two components: 1. Frequency (number of waves over time) – perceived as pitch Øhigh pitched sounds, like a referee’s whistle, have a high frequency Ølow pitched sounds, like the bass in a song or a very deep voice, have a low frequency. 2. Amplitude (height of sound wave) – perceived as loudness. ØHigh amplitude sounds (like shouting) are louder. ØLow amplitude sounds (like whispering) are quieter. L) Special Senses – Hearing ØStructure of the ear: 1. Pinna – directs soundwaves into the ear canal. 2. Ear canal – carries soundwaves to the tympanica membrane (ear drum). Tympanic membrane vibrates with the same frequency and amplitude as the sound you hear. Fig. 10.16 3. Middle ear ossicles: transmit vibrations from the tympanic membrane to the inner ear a. Malleus – connected to tympanic membrane b. Incus c. Stapes – connected to oval window (thin membrane that separates inner ear from middle ear). Ø Vibrations pass from malleus to incus to stapes – and stapes vibrates against the oval window. Fig. 10.17 L) Special Senses – Hearing Fig. 10.17 ØStructure of the ear: 4. Inner ear: a. Vestibular apparatus – for equilibrium b. Cochlea – for hearing Ø Has 3 fluid filled channels: i. vestibular duct filled with perilymph – similar ii. tympanic duct composition to extracellular fluids) iii. cochlear duct – filled with endolymph = similar to intracellular fluid with high [K+], low Na+. Ø Vibration of stapes on oval window creates waves in the fluids inside the 3 ducts. Fig. 10.18 Ø Cochlear duct and tympanic duct are separated by basilar membrane which plays a role in the coding of pitch. L) Special Senses – Hearing Fig. 10.17 ØStructure of the ear: 4. Inner ear: a. Vestibular apparatus – for equilibrium b. Cochlea – for hearing Ø Cochlear duct contains: i. Tectorial membrane – waves in perilymph of vestibular duct cause this membrane to move and bend hair cells. ii. Hair cells = non-neural mechanoreceptor cells for hearing. Ø Have stereocilia (hair-like structures) that are embedded in tectorial membrane. Fig. 10.18 Ø Stereocilia range in height from short to tall, and tallest stereocilium is called the klinocilium. Ø When stereocilia are physically bent in the direction of the klinocilium by movement of tectorial membrane or basilar membrane, signal transduction occurs and they release neurotransmitter onto the primary auditory neurons. L) Special Senses – Hearing ØSignal transduction by hair cells: 1. Waves in perilymph vibrate the tectorial and basilar membranes. 2. Movement of membranes bends stereocilia on hair cells towards the klinocilium. 3. Physical change causes mechanically gated ion channels to open. 4. K+ and Ca++ enter the hair cell causing a depolarizing graded potential (=receptor potential). 5. Graded potential triggers the opening of voltage gated Ca++ channels in the hair cell membrane. 6. Ca++ enters hair cell and causes exocytosis of neurotransmitter (thought to be glutamate). Fig. 10.19 7. Neurotransmitter stimulates depolarizing graded potentials Purves et al. 2012. Neuroscience 6th edition. Sinauer and Associates. (EPSPs) in the primary auditory neurons that lead to action potentials. (Exact process not known, may involve AMPA receptors). L) Special Senses – Hearing ØSignal coding: 1. The frequency of action potentials to the brain, determines sound intensity (loudness). Ø Loud noises result in a larger graded potential on the hair cell. which increases the amount of neurotransmitter released. The opposite is true for quiet noises. Ø Increased release of neurotransmitter increases the action potential frequency on the primary auditory neuron. 2. Labelled line coding likely determines pitch. Ø Changes in pitch stimulate different areas of the basilar membrane which then activates particular sets of hair cells. Ø These hair cells will create action potentials in specific pathways associated with the pitch detected and will be transmitted to the area of the brain responsible for perceiving that pitch (So pitch Fig. 10.19 is hardwired much like the somatic senses). Purves et al. 2012. Neuroscience 6th edition. Sinauer and Associates. L) Special Senses – Hearing ØAuditory pathways from primary neuron to cortex : 1. Auditory pathways involve 4 different neurons from the receptor to the brain. a. Axons of the primary auditory neurons form the cochlear branch of Cranial Nerve VIII (vestibulocochlear nerve). b. Primary auditory neurons synapse with secondary auditory neurons in the cochlear nucleus in the medulla. c. Secondary neurons transmit signals to the midbrain (inferior colliculus for auditory reflexes) where they synapse with tertiary neurons. d. Tertiary neurons transmit signals to the thalamus (medial geniculate nucleus) where they synapse with quaternary (4th order) neurons. e. Quaternary auditory neurons relay the signals to the auditory cortex in the temporal lobe, where perception Fig. 10.21 takes place. L) Special Senses – Hearing ØAuditory pathways from primary neuron to cortex : 2. Some secondary neurons decussate in the medulla. Ø I.e. some signals from the right ear are carried to and processed in the left auditory cortex, and some from the left ear are carried to and processed in the right auditory cortex. Fig. 10.21 L) Special Senses – Hearing ØHearing Loss: 1. Conductive hearing loss Ø Sounds cannot be transmitted (conducted) through the middle ear. Ø Results from: a. Ear canal plugged with earwax or foreign object b. Fluid in or damage to middle ear that prevents middle ear ossicles from vibrating. Ø Makes sounds louder in the the affected ear. Ø Hair cell receptors and cochlear nerve still intact and and can detect soundwaves through vibration of the bones of the skull in which the cochlea is embedded. Ø Sounds are not dulled by other ambient sounds, as they would be if sound was being conducted through the middle ear. Fig. 10.21 Ø Try covering one of your ears while talking – this simulates conductive hearing loss. L) Special Senses – Hearing ØHearing Loss: 2. Central hearing loss Ø Results from damage to nerve pathways leading to the auditory cortex (rare). Ø Makes sounds louder in the the unaffected ear since there is no transmission of signals into the cortex. 3. Sensorineural hearing loss Ø Results from damage/death to hair cells. Ø Causes: a. Aging b. Chronic exposure to loud noises. Ø Makes sounds louder in the the unaffected ear since there is impaired transmission of signals from receptors to the neurons they stimulate. Fig. 10.21 M) Special Senses – Equilibrium (Balance) ØStructures of the ear involved in equilibrium: 1. Vestibular apparatus of inner ear provides information about movement and position of the body. It consists of: a. Saccule Sense linear movements and head position b. Utricle c. Semicircular canals – sense rotational movements (help you keep balance when spinning). Ø Jean Pierre Flourens, a French physician and physiologist, discovered the functions of the semicircular canals. In his research he destroyed the semicircular canals in pigeons which caused them to fly in circles because they were not actually able to sense that they were going in circles. 2. Each structure in the vestibular apparatus contains hair cells (similar to those in the cochlea) that are activated as a result of change in position and/or movement M) Special Senses – Equilibrium (Balance) ØSignal transduction: Ø Similar to that observed for hair cells in the cochlea 1. Mechanical stimulus bends hair cells 2. Causes mechanically gated ion channels in the cell membrane to open. 3. K+ and Ca++ moves into the cell, causing a depolarizing graded potential 4. Etc. M) Special Senses – Equilibrium (Balance) ØEquilibrium pathways: 1. There are several Equilibrium pathways. all of which begin with the primary neurons in the vestibular branch of Cranial Nerve VIII. a. Vestibulo-cerebellar pathway (most of the pathways go to the cerebellum) Ø Unconscious signaling about balance for coordination of movements i. Primary neuron relays directly to cerebellum or ii. Primary neuron synapses with a secondary neuron in the vestibular nuclei in the medulla and the secondary neuron relays to the cerebellum. b. Vestibulo-oculomotor pathway Ø Primary motor neuron relays to oculomotor nucleus in midbrain Ø Primary neuron synapses with somatic motor neurons that control eye movements. Fig. 10.23 M) Special Senses – Equilibrium (Balance) c. Vestibular pathways to reticular formation, thalamus and cortex i. Primary neurons relay to medulla where they synapse with secondary neurons ii. Secondary neurons relay signals to higher brain regions including the reticular formation, thalamus, and cortex. Ø Pathways to cortex allow for conscious sensation of changes in body position, dizziness, etc. Fig. 10.23 M) Special Senses – Vision ØVision is the body’s ability to detect and perceive light energy (photons) ØThe eye is the sensory organ that contains the receptors necessary to transduce light energy into electrical energy (graded potentials and action potentials) ØThe eye is protected by: 1. Bones of the skull that make up the orbit (eye socket) 2. Eyelids and Eyelashes – keep debris out of the eye 3. Tears – secreted from the lacrimal gland Ø Cleanse the surface of the eye Ø Prevent the surface of the eye from drying out Ø Have antibacterial properties Fig 10.24 M) Special Senses – Vision ØStructure of the eye: Ø3 layered sphere filled with fluid 1. Sclera/Cornea – outer layer a. Sclera = connective tissue surrounding the eye. Forms the whites of the eye and is continuous with the… b. Cornea = most anterior part of the eye. Clear transparent layer that light can pass through. If you wear contact lenses, they sit on the cornea. Choroid 2. Choroid/Ciliary body/Iris – middle layer Fig. 10.25 a. Choroid = pigmented layer between sclera and retina that contains blood vessels (provide nutrients/O2 to retinal cells). Is continuous with the… b. Ciliary body = connects choroid to iris. Has suspensory ligaments and smooth muscle that control the shape of the lens. c. Iris = visible pigmented portion of the eye (eye colour). Contains smooth muscles for pupil constriction and dilation. These muscles are controlled by the autonomic nervous system. M) Special Senses – Vision ØStructure of the eye: Ø3 layered sphere filled with fluid 3. Retina – inner layer a. Outer pigment cell layer – dark green colour. Absorbs excess light so that it does not reflect back into eye. b. Photoreceptor layer – 2 types of photoreceptors that transduce light energy into electrical and chemical signals. Choroid i. Rods – More abundant photoreceptor type Fig. 10.25 Ø Grayscale/Night vision Ø Low acuity (detects sizes, shapes, brightness) as a result of more convergence onto ganglion cells Ø Mainly located in peripheral retina. ii. Cones – less abundant Ø Colour/Day vision Ø High acuity (detects fine details) as a result of less convergence onto ganglion cells. Ø Mainly located in the fovea. M) Special Senses – Vision Fig. 10.29 ØStructure of the eye: Ø3 layered sphere filled with fluid 3. Retina – inner layer c. Bipolar cell layer – integrate signals from rods and cones and relay them to ganglion cells. d. Ganglion cell layer – axons of these neurons form the optic nerve (Cranial Nerve II) – carry signal into the brain. e. Amacrine cells –modulate signaling between ganglion cells and bipolar cells. f. Horizontal cells – modulate signaling between rods/cones and bipolar cells. M) Special Senses – Vision ØStructure of the eye: ØThe eye is divided into two fluid filled compartments: 1. Anterior compartment – between cornea and lens Ø filled with aqueous humor (has similar composition to extracellular fluids) 2. Posterior compartment – between lens and retina Ø filled with vitreous humor (gelatinous substance Choroid produced by the ciliary body) Fig. 10.25 M) Special Senses – Vision ØStructure of the eye: ØOther important structures: 1. Lens – bends light to focus it on the retina 2. Pupil – opening in the iris that allow light to enter the posterior compartment of the eye. 3. Fovea – area of the retina onto which most light entering the eye is focused 4. Canal of Schlemm – drains aqueous humor to the Choroid veins. In certain forms of glaucoma, this canal is blocked, which causes a build up of aqueous humor that increases the pressure in the eye and leads to damage of the retina. 5. Optic disk (blind spot) – area of where ganglion cells exit the eye as the optic nerve (Cranial Nerve II) and where blood vessels enter/exit. Lacks photoreceptors. Fig. 10.25 M) Special Senses – Vision ØOverview of Vision mechanism: 1. Light enters eye through cornea Fig. 10.27 2. Smooth muscles of the iris adjust pupil size based on intensity of the light a. Pupil dilator muscles open the pupil in low light b. Pupil constrictor muscles close the pupil in bright light 3. The light passes through the pupil and lens a. Lens bends light to focus it on retina. Shape of lens changes to ensure light is focused on the right spot. Ø Contraction and relaxation of ciliary body muscles changes lens shape. Ø When viewing objects at a distance (like while driving), ciliary muscles are relaxed and the lens is flat Ø When viewing objects close up (like while reading), ciliary muscles are contracted and the lens is rounded. Ø Changing lens shape in response to viewing distant or near objects = accommodation M) Special Senses – Vision ØOverview of Vision mechanism: b. Lens flips image so that it is projected upside down and Fig. 10.27 inverted left to right on the retina. Processing in the visual cortex reverses the image so that you see it as it appears. 4. Light stimulates the photoreceptors (rods and cones) – signal transduction occurs. 5. Action potentials are carried through the optic nerve (Cranial Nerve II) to the brain 6. Neural pathways in the brain process and integrate sensory signals into perceived visual images. M) Special Senses – Vision Fig. 10.30 ØSignal Transduction: Ø Rods and cones have membranous disks in their outer segments that are embedded in the outer pigment cell layer of the retina Ø Disks have photopigment proteins in their membranes that facilitate transduction. 1. Rods have rhodopsin = made up of retinal (derived from vitamin A) and opsin (a modified GPCR) 2. Cones have 3 different photopigments, that are stimulated by either green, red, or blue light. Ø Rods and cones are tonically active – continuously release neurotransmitter (nt) glutamate unless they are inhibited. Ø In darkness – there is no signal transduction and rods and cones tonically release nt onto bipolar cells Ø In the presence of light – signal transduction in rods and cones varies the release of nt in proportion to the amount of light through inhibition of nt release. M) Special Senses – Vision Fig. 10.31 ØIn Darkness: 1. Rhodpsin is inactive 2. Levels of cGMP (cyclic guanosine monophosphate) in the rod cytosol are high Ø cGMP is a second messenger (like cAMP) 3. Cyclic nucleotide-gated (CNG) Na+/Ca++ channels and K+ channels are open. Ø cGMP binds to CNG channels and opens them Ø Therefore when cGMP levels are high, Na+/Ca++ channels are open and the cell is depolarized. Ø Depolarization of the cell opens voltage-gated Ca++ channels à Ca++ enters rod and causes release of neurotransmitter. Ø Photoreceptors (rods and cones) tonically release neurotransmitter onto bipolar cells until they receive a signal to reduce nt release – that signal is light. M) Special Senses – Vision Similar to Fig. 10.31 ØSignal Transduction in Light: 1. Light is focused on the retina 2. Photons of light cause a conformational (shape) change in the retinal portion of rhodopsin (from cis-retinal to trans-retinal). 5 Ø Conformation change causes retinal to detach from 1&2 opsin – referred to as “bleaching” of rhodopsin. 6 3. Opsin activates a G-protein called transducin. 4. Activated transducin activates phosphodiesterase (amplifier enzyme). 5. Phosphodiesterase hydrolyzes cGMP to GMP. 4 3 Ø So unlike other GPCR signal transduction Purves et al. 2012. Neuroscience 6th edition. Sinauer and Associates. mechanisms we have looked at, where signal transduction triggers production of the second messenger, in photoreceptors, it triggers the breakdown of the second messenger. M) Special Senses – Vision Fig. 10.31 ØSignal Transduction in Light: 6. Decreased levels of cGMP (second messenger) closes cyclic nucleotide-gated (CNG) channels (Na+/Ca++ channels). 7. Cations (Na++/Ca++) can no longer enter cell, so membrane hyperpolarizes due to K+ efflux through leakage channels. 8. Hyperpolarization of membrane reduces release of neurotransmitter onto bipolar cells. 9. Decreasing levels of nt (glutamate) release onto bipolar cells: a. activates some bipolar cells (glutamate is an inhibitory nt for these cells) b. inhibits some bipolar cells (glutamate is an excitatory nt for these cells). M) Special Senses – Vision Fig. 10.26 ØVision Pathways: 1. Photoreceptors relay signals to bipolar cells 2. Activated bipolar cells relay signals to ganglion cells 3. Ganglion cells form the optic nerve which enters the brain 4. Optic nerve relays to optic chiasm where axons of ganglion cells from left visual field of the right eye and the right visual field of the left eye cross over (important for binocular vision). 5. Optic chiasm relays to optic tract 6. Optic tract relays to lateral geniculate nucleus in thalamus, where the axon terminals of the ganglion cells will synapse with a. Interneurons that relay to the oculomotor nucleus in the midbrain à to oculomotor nerve for pupillary reflexes. b. neurons that radiate to visual cortex in occipital lobe. M) Special Senses – Vision Fig. 10.26 ØVision Pathways: 1. Photoreceptors relay signals to bipolar cells 2. Activated bipolar cells relay signals to ganglion cells 3. Ganglion cells form the optic nerve which enters the brain 4. Optic nerve relays to optic chiasm where axons of ganglion cells from left visual field of the right eye and the right visual field of the left eye cross over (important for binocular vision). 5. Optic chiasm relays to optic tract 6. Optic tract relays to lateral geniculate nucleus in thalamus, where the axon terminals of the ganglion cells will synapse with a. Interneurons that relay to the oculomotor nucleus in the midbrain à to oculomotor nerve for pupillary reflexes. b. neurons that radiate to visual cortex in occipital lobe. Fig. 10.34  Fig. 10.27 M) Special Senses – Vision ØCauses of impaired vision: 1. Myopia – near-sightedness Ø Difficulty seeing distant objects, but near objects are clear. Ø Focal point falls in front of the retina instead of directly on the retina because eye is longer than normal or the cornea is curved too much. Ø Corrected with a concave lens 2. Hyperopia – far-sightedness Ø Difficulty seeing near objects, but distant objects are clear. Ø Focal point falls in behind the retina instead of directly on the retina because eye is shorter than normal or cornea is curved too little. Ø Corrected with a convex lens. 3. Astigmatism – flaws in curvature of cornea or lens causes some rays of light to scatter and not hit the fovea , resulting in blurred vision. Ø Causes some areas of an image to be blurry and others clear. Ø When cornea and lens are perfectly round, all rays of light are directed to the fovea and vision is clear. Astigmatism “Normal” M) Special Senses – Vision ØPerception: 1. Much of what we see relies upon the visual processing and perception that occurs in the brain. 2. The brain can often perceive things that are not there – like in these optical illusions. Ø In Fig. 9.15 a) the brain fills in the edges of the square shape, even though there is no square present (the brain sees shapes that it is familiar with, even if those shapes are not actually there). Peripheral drift illusion – see static images as moving one Fig 9.15