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Chatper 7 summary .docx

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Junchul Kim

From Ear to the Primary Auditory Cortex - Network of auditory pathways  The axons of each auditory nerve synapse in the ipsilateral cochlear nuclei, from which many projections lead to the superior olives on both sides of the hindbrain stem at the same level. (Superior olives: Medullary nuclei that play a role in sound localization)  The axons of the olivary neurons project via the lateral lemniscus (tract of axons) to the inferior colliculi (tectum) of the midbrain  Inferior colliculi synapse on neurons that project to the medial geniculate nuclei (MGN) of the thalamus, which in turn project to the primary auditory cortex.  They are all transmitted to both ipsilateral and contralateral auditory cortex Subcortical mechanisms of Sound localization - One function of the subcortical mechanism that is well understood localization of the sounds in space - Localization of sounds in space mediated by the lateral and medial superior olives in different ways - Some neurons in the medial superior olives respond to slight differences in the time of arrival of signals from the two ears - Some neurons in the lateral superior olives respond to slight differences in the amplitude of sounds from the two ears - Medial and lateral superior olives project to the superior collicus, as well as the inferior collicus. Primary and secondary auditory cortex - Primary auditory cortex (far from being well understood)  Receives the majority of its input from the medial geniculate nucleus, located in temporal lobe. - primary auditory cortex is organized in functional columns: all of the neurons encountered during a vertical microelectrode penetration of primary auditory cortex tend to respond optimally to sounds in the same frequency range - Both primary and secondary auditory cortex are organized tonotopically (like the cochlea), meaning they are organized on the basis of frequency - Adjacent to the primary auditoria area, there are two other areas referred to the core region. - Belt: Band that surrounds the core region  areas of the secondary cortex - Parabelt areas: areas of secondary auditory cortex outside the belt - There are about 10 separate areas of secondary auditory cortex in primates - Secondary areas do not respond well to pure tones and have not been well- researched Two streams of auditory cortex - Researchers suggested Auditory signals are ultimately conducted to two large areas of association cortex: prefrontal cortex and posterior parietal cortex - Hypothesized anterior auditory pathway more involved in identifying sounds( what), posterior auditory pathway more involved in locating sounds (where) Auditory- visual interactions - One study of monkeys some posterior parietal neurons were found to have visual receptive fields, some were found to have auditory receptive fields, some were found to have both - Functional brain imaging studies (imaging that records activity throughout the brain) have confirmed that sensory interactions do occur in association cortex. Also found that sensory interactions at the lowest level of the sensory cortex hierarchy, in areas of primary sensory cortex. Perception of Pitch - Studies have shown that it occurs in one small area just anterior to primary auditory cortex  contained my neurons that responded to pitch rather than frequency, regardless to the quality of the sound Auditory cortex damage - The major permanent effects of lesions are loss of the ability to localize sounds and impairments of the ability to discriminate frequencies. - Unilateral lesion  disrupts the ability to localize sounds in space contralateral, but not ipsilateral to the lesion. Deafness in Humans - total deafness only occurs to 1% - parallel network of auditory pathways usually allows people to have at least some hearing, since if one auditory brain structure is destroyed, alternative pathways over which auditory information can flow remain. - Parallel organization of the auditory system  leads damage to the inner or the middle ear, or the nerves leading from them, rather than from more central damage. 2 common classes of hearing impairments: 1. Conductive deafness: associated with damage to the ossicles 2. Nerve deafness: associated with damage to the cochlea or auditory nerve. Major cause  loss of hair cell receptors - Hearing loss may be associated with tinnitus: the ringing of the ears. Changes to the central auditory system that were caused by the deafness are the cause of tinnitus. - Cochlear implants: bypass damage to the auditory hair cells by conversing sounds picked up by a microphone on the patient’s ear to electrical signals. The signals excite the auditory nerves  ppl with nerve deafness benefit from cochlear implants Somatosensory system Somatosensations: sensations from your body (bodily sensation) Somatosensory system: system that mediates the bodily sensations 3 separate but interacting systems 1. exteroceptive system: senses external stimuli that are applied to the skin 2. proprioceptive system: monitors information about the position of the body that comes from receptors in the muscles, joints and organs of balance 3. interoceptive system: provides general information about conditions within the body, such as temperature, blood pressure - Each sensation depends on distinct sensory receptor cells that are called mechanosensory receptors Different types of mechanosensory receptors - they have five different types 1. Free nerve endings: Neuron endings that lack specialized structures on them and that detect cutaneous pain and changes in temperature (are the simplest cutaneous receptors) (pain, temperature) 2. Pacinian corpuscles: The largest and most deeply positioned cutaneous receptors, which are sensitive to sudden displacements of the skin. (vibration) 3. Meissner’s corpuscles: light touch 4. Merkel’s disks: detect gradual skin indentation (touch) 5. Ruffini endings: detect gradual skin stretch (stretch) Stereognosis: the identification of objects by touch  when you try to identify objects by touch, you manipulate them in your hands so that the pattern of stimulation continually changes. - somato sensory receptors function in the same way, but each of them are sensitive/ specialized to a particular type of tactual stimulation Transduction at Mechanoreceptors Q. How are mechanical stimuli captured by receptors? (Ex. Transduction at Pacinian corpuscles) - mechanical stimulus ( e.g. vibration) deforms corpuscle, leading to mechanical stretch of the axon membrane opens ion channels - Membrane potential changes (called a generator potential)  If generator potential reaches threshold (i.e. strong stimulus or summation), action potential is generated and propagated to spinal cord. Receptor cells transmitting pain information vs. touch information= different - nociceptors: receptors sensitive to painful stimuli  peripheral nociceptors exist in free nerve endings ( neuron endings with no specialized structures on them)  nociceptors are located throughout the body (ex. Skin, muscles, bones, blood vessels, organs and etc.) - Initial stimulus for pain is the destruction or injury of tissue adjacent to free nerve endings - Damaged tissues release chemicals ( neuropeptides, serotonin, histamine, etc.), that bind to specialized receptor proteins and triggers AP in free nerve endings - Different nerve endings express different receptor proteins, so they report different stimuli - Substance P receptor (pain)  TRPV1 receptor (heat) The way that the somatosensory signals travel to the CNS Neural signals are passed to the CNS (spinal cord and brain) Anatomy of spinal cord - gray matter: spinal horns - white matter: spinal columns - Nerve signals enter and exit spinal cord via the spinal nerves  spinal nerves are attached to spinal cord via dorsal (sensor) and ventral (motor) roots  dorsal roots bring sensory information into spinal cord (into the CNS)  ventral roots send motor information to the body (from the CNS) - Somatosensory signals enter the spinal cord via dorsal roots  somatosensory receptors send AP along axons that enter dorsal roots of the spinal cord  Somatosensory nerve belong to the neurons in the dorsal root ganglia (a cluster of neurons next to dorsal root) Q. Can you tell whether this neuron is unipolar, dipolar or multipolar neurons? **ASK TA ( they’re unipolar neurons) Somatosensory (bodily) signals from different body parts enter spinal cords at different levels - surface area of the body can be divided into distinct strips based on their connectivity to spinal cord - Sensory signals originating from surface areas w/I a particular strip enters the same level of the spinal cord - Dermatome: A strip of skin innervated by the left and right dorsal roots of a given segment of the spinal cord. Cervical back of the head, neck, arms Thoratic-> chest, belly, back (the truck) of the bocy Lumbar lower hip, front side of the legs Sacral the backside of the legs including the bumb and penis How somatosensory signals travel to the brain - There are two major pathways 1. Dorsal- column medial-lemniscus system: “tends to” carry information about touch and proprioception (사지의 위치를 감지하는것 )  it’s “tends to” because lesions of the dorsal- column medial-lemniscus system do not eliminate touch perception or proprioception Dorsal-column medial- lemniscus system’s pathway of carrying touch sensation (pg.176 for diagram) o Sensory neurons enter the spinal cord via a dorsal root; it joins the dorsal columns and ascends ipsilaterally to the brain o In the medulla, the axon makes (first) synapse in the dorsal column neuclei. o The axons of dorsal column neclei neurons decussate (cross over to the other side of the brain) and then ascend in the medial lemniscus to the contralateral ventral posterior nucleus of the thalamus. o Ventral posterior nuclei receive input via the three branches of the trigeminal nerve, which carries somatosensory information from the contralateral areas of the face. o Neurons from ventral posterior nucleus project to 1) Primary somatosensory cortex and 2) secondary somatosensory cortex or 3) the posterior parietal cortex 2. Anterolateral system: carries information about pain and temperature  Lesions of the anterolateral system do not eliminate perception of pain or temperature Anterolateral- system pathway mainly carries pain and temperature sensation o Dorsal roots neurons of the anterolateral system synapse as soon as they enter the spinal cord o Second order neuron axons decussate (cross over to the other side of the brain) but then ascend to the brain in the contralateral anterolateral portion of the spinal cord  Some do not decussate but ascend ipsilaterally o Anterolateral system comprises of three different tracts: 1) spinothalamic tract: projects to the ventral posterior nucleus of the thalamus 2) spinoreticular tract: projects to reticular formation 3) spinotectal tract: projects to the tectum o The pain and temperature information that reaches the thalamus is then distributed to the somatosensory cortex (primary somatosensory cortex, secondary somatosensory cortex
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