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Unit 3 - Sensory Systems - Full Textbook Notes

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BIOL 373
Heidi Engelhardt

Unit 3: Sensory Systems Chapter 10  Special Senses: visions, hearing, taste, smell and equilibrium  Somatic Senses: touch, temperature, pain, itch, and proprioception  Proprioception: the awareness of body movement and position in space; mediated by muscle and joint sensory receptors (proprioceptors) and may be either unconscious or conscious  Sensory Systems: o The simplest systems are single sensory neurons with branched dendrites that function as receptors  Ex. Pain and itch receptors  Free (“naked”) nerve endings  They may have myelinated or unmyelinated axons o The most complex systems include multicellular sense organs  Ex. The eye and the ear  Nerve endings are encased in connective tissue capsules  May be myelinated or unmyelinated o Most special sense receptors are cells that release neurotransmitter onto sensory neurons, initiating an action potential  The receptors for smell are neurons, but the other four special senses use non-neural receptor cells that synapse onto sensory neurons  Both neural and non-neural receptors develop from the same embryonic tissue  Receptors 1. Chemoreceptors: respond to chemical ligands that bind to the receptor (oxygen, pH, organic molecules, etc.)  Ex. taste and smell 2. Mechanoreceptors: respond to various forms of mechanical energy, including pressure (barorecptors), vibration, gravity, acceleration, cell stretch (osmoreceptors), and sound  Ex. hearing 3. Photoreceptors: respond to light  Ex. vision 4. Thermoreceptors: respond to temperature Converting physical stimuli into electrical signal  Transduction: the conversion of stimulus energy into information that can be processed by the nervous system o Ex. Opening/closing of ion channels creates a membrane potential  Adequate Stimulus: a particular form of energy to which a sensory system is most responsive o Receptors can respond to other forms of energy if it is intense enough o Sensory receptors can be incredibly sensitive to their preferred form of stimulus  Threshold: the minimum stimulus required to activate a receptor  Receptor potential: the change in sensory receptor membrane potential; is a graded potential  Receptive field: the region within which a sensory neuron can sense a stimulus o Simplest case: one receptive field = one sensory neuron (primary sensory neuron) which synapses on one CNS neuron (secondary sensory neuron) o They frequently overlap with neighbouring receptive fields o Convergence: multiple synaptic neurons provide input to a smaller number of post-synaptic neurons (ie. 3 primary to 1 secondary)  Allows multiple simultaneous sub-threshold stimuli to sum at the postsynaptic (secondary) neuron  The individual receptive fields of the neurons merge into a single, large secondary receptive field o The size of secondary receptive fields determines how sensitive a given area is to a stimulus  Two-point discrimination test: demonstrates sensitivity to touch; two pins are interpreted as one  Desensitized areas: many primary neurons converge on a single secondary neuron, so the secondary receptive field is very large  Sensitized areas: smaller receptive fields, with as little as a 1:1 relationship between primary and secondary sensory neurons CNS integration  Sensory information that initiates visceral reflexes is integrated in the brain stem or spinal cord and usually does not reach conscious perception o Ie. Blood pressure  The following pathways carry information to the thalamus o The midbrain receives visual information, and the medulla oblongata receives input for sound and taste o Balance and equilibrium is processed primarily in the cerebellum  The thalamus acts as a relay and processing station before passing the information on to the cerebrum  Olfactory information is not routed through the thalamus o The sense of smell, a type of chemoreception, is one of the oldest senses o Nose  first cranial nerve and olfactory bulb  olfactory cortex in the cerebrum o Odours are closely linked to memory and emotion  Perceptual threshold: the level of stimulus intensity necessary for you to be aware of a particular sensation  Habituation = decreased perception of a stimulus : accomplished by inhibitory modulation which diminishes a suprathreshold stimulus until it is below the perceptual threshold Distinguishing Stimuli  4 properties of a stimulus: o Its nature or modality  Indicated by which sensory neurons are activated and by where the pathways of the activated neurons terminate in the brain  Subdivided into qualities (colours, temp, etc.)  The brain associates a signal coming from a specific group of receptors with a specific modality  Labeled line coding: the 1:1 association of a sensory receptor with a sensation o Its location  Coded according to which receptive fields are activated  The cerebrum is highly organized with respect to incoming signals and input from adjacent sensory receptors  Phantom limb pain: occurs when secondary sensory neurons in the spinal cord become hyperactive, resulting in the sensation of pain in a limb that is no longer there  Hearing is different> the brain uses the timing of receptor activation to compute a location  Lateral inhibition: increases the contrast between activated receptive fields and their inactive neighbours  Another way of isolating the location of a stimulus  Primary neuron response is proportional to stimulus strength  pathway closest to the stimulus inhibits neighbours  inhibition of lateral neurons enhances perception of stimulus  The inhibition of neurons farther away from the stimulus enhances the contrast between the center and the sides of the field  Population coding: the way multiple receptors function together to send the CNS more information than would be possible from a single receptor o Its intensity and Its duration  Cannot be directly calculated from a single sensory neuron action potential because a single action potential is “all-or-none”  Coded in two types of information:  The number of receptors activated (population coding) o Occurs because the threshold for the preferred stimulus is not the same for all receptors o Only the most sensitive receptors respond to a low-intensity stimulus o As a stimulus increases in intensity, additional receptors are activated  The frequency of action potentials coming from those receptors (frequency coding)  Receptor potential strength and duration vary with the stimulus  Receptor potential is integrated at the trigger zone  Frequency of action potentials is proportional to stimulus intensity; duration of a series of action potentials is proportional to stimulus duration  Neurotransmitter release varies with the pattern of action potentials arriving at the axon terminal  A longer stimulus generates a longer series of action potentials in the primary sensory neuron  Is a stimulus persists, some receptors adapt, or cease to respond  Tonic receptors: slowly adapting receptors that fire rapidly when first activated, then slow and maintain their firing as long as the stimulus is present; the stimuli are parameters that must be monitored continuously by the body  Ie: baroreceptors, irritant receptors, tactile receptors, proprioceptors etc.  Phasic receptors: rapidly adapting receptors that fire when they first receive a stimulus but cease firing if the strength of the stimulus remains constant; attuned specifically to changes in a parameter  Ie. Sense of smell  The only way to create a new signal is to either increase the intensity of the excitatory stimulus or remove the stimulus entirely and allow the receptor to reset  A stimulus above threshold initiates action potentials in a sensory neuron that projects to the CNS  Stimulus intensity and duration are coded in the pattern of action potentials reaching the CNS Somatic Senses  There are 4 somatosensory modalities: o Touch o Proprioception o Temperature o Nociception ( includes pain and itch)  Neurons associated with receptors for nociception, temperature, and coarse touch synapse onto their secondary neurons shortly after entering the spinal cord o Their secondary neurons cross the midline in the spinal cord, then ascend to the brain  Most fine touch, vibration, and proprioceptive neurons have very long neurons that project up the spinal cord all the way to the medulla o Cross the midline in the medulla  Sensations from the left side of the body are processed in the right hemisphere of the brain and vice versa  In the thalamus, all secondary sensory neurons synapse into tertiary sensory neurons, which in turn project to the somatosensory region of the cerebral cortex o Many sensory pathways send branches to the cerebellum so that it can use the information to coordinate balance and movement  Somatosensory cortex: the part of the brain that recognizes where ascending sensory tracts originate o Each sensory tract has a corresponding region of the cortex, so that all sensory pathways for the left hand terminate in one area, all pathways for the left foot terminate in another area, and so on o The amount of space devoted to each body part is proportional to the sensitivity of that part o Is a particular body part is used more extensively, its topographical region in the cortex will expand  fMRI and PET scans measure the metabolic activity of neurons, so that more active areas of neuronal activity become highlighted and can be associated with their location  Touch receptors: o Free nerve endings: respond to noxious stimuli, temperature, and hair movement; found around the hair roots and under the surface of skin; unmyelinated nerve endings; variable adaptability  Cold receptors: sensitive primarily to temperature lower than body temperature  Warm receptors: stimulated by temperature in the range extending from normal body temperature (37°C) to about 45°C  Above 45°C pain receptors are activated  Temperature receptors slowly adapt between 20-40°C > their initial response tells us that the temperature is changing, and their sustained response tells us about the ambient temperature o Pacinian Corpuscles: respond to vibration, are some of the largest receptors in the body, and much of what we know about somatosensory receptors comes from studies on these structures  Composed of nerve endings encapsulated in layers of connective tissue  Found in the subcutaneous layers of skin and in muscles, joints and internal organs; deep layers of skin  Respond best to high-frequency vibrations whose energy is transferred through the connective tissue capsule to the nerve ending, where the energy opens mechanically gated ion channels  Rapidly adapting phasic receptors> allows them to respond to a change in touch but then ignore it o Ruffini corpuscles: stimulated by stretched skin; located in deep layers of skin; enlarged nerve endings; slow adaptation o Meissner’s Corpuscles: stimulated by flutter or stroking; located in superficial layers of skin; encapsulated in connective tissue; rapid adaptation o Merkel receptors: stimulated by steady pressure and texture; located in superficial layers of skin; enlarged nerve endings; slow adaptation  Nociceptors: receptors that respond to a variety of strong noxious stimuli (chemical, mechanical, or thermal) that cause or have the potential to cause tissue damage o Activation of nociceptors initiates adaptive, protective responses, such as reflexive withdrawal o Not limited to the skin o Two sensations may be perceived when they are activated: Pain and Itch o Sometimes called pain receptors o Nociceptive pain: is mediated by free nerve endings whose ion channels are sensitive to a variety of chemical, mechanical, and thermal stimuli o Activation is modulated by local chemicals that are released upon tissue injury, including K+, histamine, and prostaglandins released from damaged cells; serotonin released from platelets activated by tissue damage; and the peptide substance P, which is secreted by primary sensory neurons > these chemicals either activate nociceptors or sensitize them by lowering their activation threshold o Inflammatory Pain: increased sensitivity to pain at sites of tissue damage o May activate two pathways:  (1) reflexive protective responses that are integrated at the level of the spinal cord  (2) ascending pathways to the cerebral cortex that become conscious sensation (pain or itch)  They then synapse onto secondary sensory neurons that project to the brain or onto interneurons for local circuits  Withdrawal reflex = spinal reflex  Classes of Somatosensory Nerve Fibers: Fiber Type Fiber Characteristic Speed of Conduction Associated with Aβ Large, myelinated 30-70 m/sec Mechanical stimuli Aδ Small, myelinated 12-30 m/sec Cold, fast pain, mechanical stimuli C Small, unmyelinated 0.5-2 m/sec Slow pain, heat, cold, mechanical stimuli  Itches: when histamine or some other stimulus activates a subtype of C fiber; comes only from nociceptors in the skin and is characteristic of many rashes and other skin conditions; it can also be a symptom of a number of systemic diseases, including multiple sclerosis, hyperparathyroidism, and diabetes mellitus  Pain: a subjective perception, the brain’s interpretation of sensory information transmitted along pathways that begin at nociceptors; it is highly individual and can vary with a person’s emotional state o Fast Pain: sharp and localized; is rapidly transmitted to the DNA by small, myelinated Aδ fibers o Slow Pain: duller or more diffuse; carried on small, unmyelinated C fibers o Nociception pathways cross the body’s midline in the spinal cord  ascend to the thalamus and sensory areas of the cortex  send out branches to the limbic system and hypothalamus pain may be accompanied by emotional distress and a variety of autonomic reactions  Gate control theory: explains why rubbing a bumped elbow or shin lessens your pain: the tactile stimulus of rubbing activates Aβ fibers and helps decreases the sensation of pain  Ischemia: lack of adequate blood flow that reduces oxygen supply; can occur during myocardial infarction (heart attack)  Referred pain: pain in internal organs is often sensed on the surface of the body; it occurs because multiple primary sensory neurons converge on a single ascending tract, giving you pain in a far removed area from the site of the stimulus o Ie. Arm pain during a stroke o When painful stimuli arise in visceral receptors, the brain is unable to distinguish visceral signals from the more common signals arising from somatic receptors> it interprets the pain as coming from the somatic regions rather than the viscera  Pathological pain: chronic pain, aka neuropathic pain  Analgesic drugs: range from aspirin to potent opioids such as morphine o Aspirin: inhibits prostaglandins, decreases inflammation, and presumably slows the transmission of pain signals from the site of injury o Potent opioids: act directly on DNA opioid receptors that are part of an analgesic system that responds to endogenous opioid molecules; activation of opioid receptors blocks pain perception by decreasing neurotransmitter release from primary sensory neurons and by postsynaptic inhibition of the secondary sensory neurons  Endogenous opioids include:  β-Endorphin: produced from the same prohormone as ACTH (adrenocorticotropin) in neuroendocrine cells of the hypothalamus  Enkephalins and Dynorphins: secreted by neurons associated with pain pathways Chemoreception: Smell and Taste [Fig. 10.13  Smell and taste are both forms of chemoreception, one of the oldest senses from an evolutionary perspective  Olfaction: allows us to discriminate among thousands of different odours  Olfactory bulb: the extension of the forebrain that receives input from the primary olfactory neurons; much better developed in vertebrate whose survival is more closely linked to chemical monitoring of their environment  The human olfactory system: o Consists of primary sensory neurons (olfactory sensory neurons) whose axons form the olfactory nerve (cranial nerve I) o The olfactory nerve synapses with secondary sensory neurons in the olfactory bulb, which then processes the incoming information o Secondary and higher-order neurons project from the olfactory bulb through the olfactory tract to the olfactory cortex o The olfactory pathway bypasses the thalamus o Pathways lead to the amygdala and hippocampus> parts of the limbic system involved with emotion and memory Stem Cells↓  Smell, memory and emotion are all linked; the processing of odours through the limbic system creates deeply buried olfactory memories o Particular combinations of olfactory receptors become linked to other patterns of sensory experience so that stimulating one pathway stimulates them all  Vomeronasal organ (VNO): an accessory olfactory structure in the nasal cavity of rodents; known to be involved in behavioural responses to sex pheromones o No evidence for/against the existence of VNO in humans  Olfactory sensory neurons are concentrated in a 3cm patch of olfactory epithelium high in the nasal cavity o They have a single dendrite that extends down from the cell body to the surface of the olfactory epithelium, and a single axon that extends up to the olfactory bulb, located on the underside of the frontal lobe o Have very short lives, with a turnover time of about 2 months o Stem cells in the basal layer of the olfactory epithelium are continuously dividing to create new neurons  The axons must then connect with the olfactory bulb and make proper synaptic connections o The surface of olfactory epithelium is composed of the knobby terminals of the olfactory sensory neurons, each knob sprouting multiple nonmobile cilia that function as dendrites  The cilia are embedded in a layer of mucus, and odorant molecules must first dissolve in and penetrate the mucus before they can bind to an odorant receptor protein  Odorant receptors are sensitive to a variety of substances  G-protein linked membrane receptors  ~400 odorant receptor proteins are expressed in humans  The combination of an odorant molecule with its odorant receptor activates a special G protein, Golfwhich in turn increases intracellular cAMP  opens cAMP-gated cation channels, depolarizing the cell and triggering a signal that travels along the olfactory sensory neuron axon to the olfactory bulb o Each olfactory sensory neuron contains a single type of odorant receptor  Axons of cells with the same receptors converge on a few secondary neurons in the olfactory bulb, which then modifies the information before sending it to the olfactory cortex  Gustation: our sense of taste; closely linked to olfaction  Taste is a combination of 5 sensations: Sweet, Sour, Salty, Bitter and Umami o Umami: a taste associated with the amino acid glutamate and some nucleotides; derived from “Deliciousness” in Japanese; it is the reason that monosodium glutamate (MSG) is used as a food additive in some countries  Sour taste is triggered by the presence of H+  Salty taste is triggered by the presence of NA+  Sweet and Umami tastes are associated with nutritious food  Bitter Taste is recognized by the body as a warning of possibly toxic components; our first reaction is to spit  Taste buds: is the primary location of the receptors for taste o they are clustered together on the surface of the tongue o One taste bud is composed of 50-150 taste cells, along with support cells and regenerative basal cells  Taste cells: non-neural polarized epithelial cell tucked down into the epithelium so that only a tiny tip protrudes into the oral cavity through a taste pore  Substance is dissolved in the saliva and mucus of the mouth  taste ligands interact with an apical membrane protein (receptor or channel) on a taste cell initiates a signal transduction cascade that ends with a series of action potentials in the primary sensory neuron  Each taste cell is sensitive to only one taste> there are at least two different types of taste cells  Taste buds contain 4 morphologically distinct cell types designated I, II, and III, plus basal cells that may be the taste stem cells o Only type III taste cells (presynaptic cells) synapse with sensory neurons> they release the neurotransmitter serotonin by exocytosis> respond to sour tastes  Mediated by ion channels  Complicated due to the fact that by increasing H+ you also change pH  H+ may act on ion channels of the presynaptic cell from both extracellular and intracellular sides of the membrane  H+-mediated depolarization of the presynaptic cell results in serotonin release  excites the primary sensory neuron o Type II taste cells (receptor cells) respond to sweet, bitter and umami sensations> do not form traditional synapses, instead they release ATP through gap junction-like channels, and the ATP acts both on sensory neurons and on neighbouring presynatptic cells  Express multiple G protein-coupled receptors  Gustducin (G protein): appears to activate multiple signal transduction pathways  Some pathways release Ca2+ from intracellular stores, while other open cation channels and allow Ca2+ to enter the cell  Calcium signals then initaite ATP release from the type II taste cells  Sweet and umami tastes are associated with T1R receptors  Bitter taste uses about 30 variants of T2R receptors o It is not clear what cell type responds to salt  Some evidence suggests that the glia-like Type I (support cells) may be the salt sensors  Mediated by ion channels  Na+ enters the presynaptic cell through an apical channel, such as the epithlial Na+ channel (ENaC)  Sodium entry depolarizes the taste cell  Neurotransmitters (ATP and serotonin) from taste cells activate primary gustatory neurons whose axons run through cranial nerves VII, IX, and X to the medulla, where they synapse sensory information then passes through the thalamus to the gustatory cortex th  CD36 in rodents is a membrane receptor that lines taste pores and binds fats> “fatty” may turn out to be the 6 taste sensation  Nerve endings in the mouth have TRP receptors and carry spicy sensations through the Trigeminal nerve (cranial nerve V)  Gut chemoreception is being mediated by the same receptors and signal transduction mechanisms that occur in taste buds on the tongue o They’ve found T1R receptors proteins for sweet and umami tastes as well as the G protein gustducin in varrious cells in rodent and human intestines  Specific Hunger: a craving for a particular substance such as salt; humans and animals that are lacking a particular nutrient may develop a craving for that substance  Salt Appetite: representing a lack of Na+ in the body; directly related to Na+ concentration in the body and cannot be assuaged by ingestion of other cations, such as Ca2+ or K+ The Ear: Hearing [Fig. 10.15]  The ear is specialized for hearing and equilibrium  It can be divided into: o External Ear: consists of the outer ear (pinna) and the ear canal  Pinna: important accessory structure to a sensory system; shape and location varies from specie to specie  Ear Canal: is sealed at its internal end by a thin membranous sheet of tissues called the tympanic membrane (eardrum)  Tympanic membrane: separates the external ear from the middle ear; it’s an air-filled cavity that connects with the pharynx through the eustachian tube o Middle  Eustachain Tube: normally collapsed, sealing off the middle ear, but it opens transiently to allow middle ear pressure to equilibrate with atmospheric pressure during chewing, swallowing, and yawning; colds/infections can block the tube and will result in fluid buildup; if bacteria are trapped in the middle ear fluid, the ear infection known as otitis media results  3 small bones of the middle ear conduct sounds from the external environment to the inner ear:  (1) Malleus (hammer) > attached to the tympanic membrane  (2) Incus (anvil)  (3) Stapes (stirrup) > attached to a thin membrane that separates the middle and inner ears  They are connected to one another with the biological equivalent of hinges o Inner  The vestibular apparatus with its semicircular canals is the sensory transducer for our sense of equilibrium  Cochlea: contains sensory receptors for hearing  externally it is a membranous tube that lies coiled like a snail shell within a boney cavity  Two membranous disks: o Oval Window: the stapes is attached to it o Round Window: separate the liquid-filled cochlea from the air-filled middle ear  Branches of cranial nerve VIII, the vestibulocochlear nerve, lead from the inner ear to the brain  The vestibular complex of the inner ear is the primary sensor for equilibrium; the remainder of the ear is used for hearing  Hearing: our perception of the energy carried by sound waves, which are pressure waves with alternating peaks of compressed air and valleys in which the air molecules are farther apart/ less compressed  Noise is a perception that results from the brain’s processing of sensory information; a falling tree emits sounds waves, but there is no noise unless someone or something is present to process and perceive the wave energy as sound  Sound is the brain’s interpretation of the frequency, amplitude, and duration of sound waves that reach our ear o Frequency (the number of wave peaks that pass a given point each second) is translated into pitch  Low-frequency = low-pitch ; high-frequency = high pitch  Measured in waves per second, or hertz (Hz)  The average human ear can hear sounds over the frequency range of 20-20,000 Hz with the most acute hearing between 1000-3000 Hz  Loudness: our interpretation of sound intensity and is influenced by the sensitivity of an individual’s ear o The intensity of a sound wave is a function of the wave amplitude o
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