Audition

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					              Audition
 Likevision, audition is exteroceptive
 Responds to variations in air or water
  pressure on the eardrum/ tympanic
  membrane
 Responds to range from 20 - 20,000 Hz
 Detects loudness, pitch, and timbre
            Sound
Amplitude           Loudness


Frequency           Pitch


Mixture             Timbre
          Auditory transduction
 Organ    of Corti
  – Basilar membrane
  – Hair cells/cilia bundles
  – Reticular membrane
  – Tectorial membrane
 Inner   and outer hair cells
  – Arranged in groups by height
  – Outer hair cells attach to tectorial
    membrane
         Ion channel control
 Tip  links
 Potassium and calcium ion channels
 Influx of cations triggers release of
  neurotransmitter from cilia
 Magnitude and direction sensitive
 Myosin motors
 Bipolar cells
          Auditory Projection
 Innerhair cells connect to 20 bipolar cells.
 30 outer hair cells connect to 1 bipolar cell.
  – Outer h.c. receive efferent input from s. olive
  – Bilateral projection: Bipolar neurons of the
    cochlear branch of the auditory nerve VIII
  – To cochlear nuclei and superior olives in
    brainstem, then through lateral lemniscus
  – To inferior colliculi and deep SC layers
  – Then to medial geniculate nuclei
      Primary auditory cortex
 MGN   projects to primary auditory cortex
 Input from both ears to each
  hemisphere of cortex
 Located in lateral or Sylvian fissure
 Surrounded by about 6 areas of
  secondary cortex
 In columns tonotopically, higher pitches
  to the front and lower to the rear
     Space mapping and sound
           localization
 Superior   colliculus is not laid out
  tonotopically, but spatially for both
  vision (superficial SC) and hearing
  (deep SC).
 Input to SC from medial superior olives
  tells different time of arrival at two ears
 Input from lateral superior olives tells
  different loudness at the two ears
     Adaptive value of sound
          localization
 Locating threats and opportunities at a
  distance: The primary distant warning
  system for mammals.
 Searching for family members:
  kangaroo mothers
 Directing movement in blind situations
  – Cats blind from birth
  – Echolocation and cane tapping
         Auditory agnosias
 Connected  to communication disorders
 Wernicke’s aphasia is a receptive
  auditory agnosia, caused by damage to
  Wernicke’s area, the posterior superior
  temporal gyrus: loss of recognition of
  sounds of words, disrupted thought and
  speech production
 Pure word deafness: Primary cortex
           Vestibular senses
 The   adequate stimulus
  – Angular acceleration
  – Position change
  – Linear acceleration
  – Low frequency stimulation
 The   vestibular apparatus
  – Semicircular canals
  – Vestibular sacs: Utricle and saccule
      Vestibular transduction
 Ampulla    and cupula
 Endolymph moves the cupula, bends
  cilia
 Receptors on wall of saccule and floor
  of utricle
 Cilia of vestibular sacs are embedded in
  a gelatinous mass containing otoconia,
  crystals of calcium carbonate
          Neural pathways
 Vestibular nerve is part of nerve VIII
 Soma of bipolar cells in vestibular
  ganglion
 Project to vestibular nuclei in medulla
 Some project to cerebellum
 Vestibular nuclei to cerebellum, spinal
  cord, medulla, pons, temporal cortex
 Vestibulo-ocular reflex: III, IV, VI
             Somatosensation
 Exteroceptive   senses
  – Pressure: Mechanical stimulation
  – Temperature: Thermal stimulation
     Hot
     Cold
     Physiological   zero
  – Pain: Nociceptive stimulation
 Proprioception      and interoception
       Receptors of the skin
 Glabrous skin, hairy skin, and
  mucocutaneous zones
 Free nerve endings: Close to surface
  – Unencapsulated
  – Respond to thermal changes and threat of
    damage (nociceptive stimuli)
  – Exist as dermal plexus, coiled around
    follicles, and adjacent to hair emerging
    from the skin
           More receptors...
 Pacinian  corpuscles: Encapsulated,
  deep in skin, respond to sudden
  changes in pressure, adapt rapidly
 Merkel’s disks: Single epithelial cell with
  one axon connected, near surface of
  skin, respond to gradual pressure
  increases (indentation)
 Ruffini endings: Dermal plexus, deeper,
  respond to stretch
 The hand and tactile sensitivity
 Palmar   skin sensitivity:
  – Glabrous for localization
  – Microscopic papillae detect deformation
  – Little lateral movement: precise location
  – Densely packed receptors
 Manipulation   sensitivity:
  – Muscle and joint feedback
  – Connections between pre- and post-central
    gyri
Dermatomes
   Dermatomes are the areas of skin
   served by nerve pairs for
   somatosensation. The three branches
   of the trigeminal nerve (V) serve the
   skin of the head. The rest of the body
   is served by the spinal nerves:
   C1 - C7: Neck, shoulders, outer arms
            and hands
   T1 - T12: Inner arms and trunk
   L1 - L5: Hips, tush, and legs
   S1 - S5: Genitals, anal area, heel to
            four small toes, back of leg
     Somatosensory pathways
 Dorsal-columnmedial lemniscus system
 Conveys pressure and proprioception
  – 1. Unipolar neuron (soma in dorsal root
    ganglion) travels from skin surface to
    spinal cord and up to the base of the
    medulla, where they synapse with neurons
    in the cuneate nucleus/dorsal column
    nuclei.
Dorsal-column medial lemniscus
            system
 2. Dorsal column/cuneate nucleus cells
  send axons across the medulla and up
  through the medial lemniscus to the
  ventral posterior nucleus of the
  thalamus (contralateral projection)
Dorsal-column medial lemniscus
            system
 3. VPN cells send axons to the
  somatosensory cortex, mostly to
  primary SI, some to secondary SII, and
  some to posterior parietal cortex
 Input from the face enters the brain
  contralaterally via the trigeminal nerve,
  and does not decussate within the
  brain.
Dorsal-column medial-lemniscus
            system
                    1. Unipolar neuron travels
                    from skin to spinal cord and
                    up to the medulla, where
                    they synapse with neurons
                    in the cuneate nucleus.
                    2. Cuneate nucleus cells send
                    axons across the medulla
                    and up through the medial
                    lemniscus to the ventral
                    posterior nucleus of the
                    thalamus
                  3. 3. VPN cells send axons to
                  the somatosensory cortex
     Somatosensory pathways
 Anterolateral System
 Conveys temperature and pain
  information: Less localized
 1. Most unipolar neurons of this system
  synapse at the level they enter the cord.
 2. Second neurons in the system
  decussate at the entry level and ascend
  through one of three pathways.
     Somatosensory agnosias
 Astereognosia
 Asomatognosia
 Anosognosia
 Contralateral   neglect/Lateral neglect
                     Pain
 Paradoxical    features
  – Adaptive value though painful
  – Lack of cortical mapping of pain
     Ablation of SI and SII do not affect threshold
     Hemispherectomy does not affect area sensed
     Anterior cingulate cortex active during pain,
      may be related to emotional reaction to pain.
  – Descending pain control
    Gate control theory of pain
 Melzack and Wall’s theory (1965)
 Supporting research:
  – ESB of PAG is analgesic (Reynolds, 1969)
  – PAG contains endogenous opiate
    receptors
  – Endorphins are released
  A descending analgesia circuit
   (Basbaum and Fields, 1978)
 PAG   stimulates raphe nuclei in medulla
 Raphe cells secrete serotonin onto
  interneurons in spinal cord
 Interneurons act to block dorsal horn
  pain signals.
        Supporting research
 Naloxone    and naltrexone injected into
  PAG block morphine analgesia
 ESB of raphe inhibits response of pain
  neurons in spinal cord
 Inhibiting raphe cells (lesion, section
  axons, or deplete serotonin) blocks
  analgesia by morphine or ESB of PAG
 But not all analgesia effects are blocked

				
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posted:9/2/2011
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