Principles of Sensory Systems

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Psy 111 Basic concepts in Biopsychology Lecture
9 Auditory and Somatosensory Systems
Website http//mentor.lscf.ucsb.edu/course/fall/p
syc111/
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Objectives

• Describe the nature and properties of sound.
• Identify the route of sound to the ear and where
  transduction occurs.
• Explain the organization of the cochlea and the
  organ of corti.
• Describe the transduction of sound waves and the
  maintenance of sound properties.
• Describe the output pathway from the ear to the
  cortex and how sound properties of encoded.
• Identify the transducers for the vestibular
  system and related circuitry
• Introduce language localization in the brain as a
  higher-order association area describe its
  lateralization.

• Define the somatosensory system(s) and describe
  differences in sensitivity.
• Describe the variety of transduction systems in
  somatosensation and their response profiles.
• Define the sub-systems of somatosensation.
  Explain why pain is considered to be multimodal.
• Describe the variety of afferent fibers (axons)
  used by the subsystems.
• Compare the parts of the spinal cord of ascending
  paths taken by touch versus pain information in
  somatosensation.
• Describe the central paths taken by touch (from
  the body), touch (from the face), and pain.
• Explain the mapping of somatosensory info on the
  postcentral gyrus.
• Describe function of the higher somatosensory
  cortices.

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Principles of Sensory Systems
Transduction Mechanism - envirnomental energy -gt
biological energy Receptive fields each neuron
is receptive to info from a specific subset of
sensory informationoften defined
anatomically Topographically arranged shape
of information is maintained Relay Centers
series of projection neurons Parallel Pathways
quality of information is maintained
Hierarchical organization convergence at
relays to cortex Cross Midline it just does
Note like all rules there are exceptions.
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Sensory systems
getting info encoded in the primary sensory
cortex

• Principles of sensory systems
• Chemical sensory systems
• Vision the photic sensory system
• Hearing (Touch) the mechanical sensory systems

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Sound Waves Air Compression
Sound waves compressions in air produced by
vibrations
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Properties of Sound Waves
Frequency pitch Human range is 20 20K Hz
Intensity loudness
Real sounds are complex mixtures of frequencies
intensities
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Properties of Sound Waves
Real sounds are a mixture of waves with different
amplitudes and frequencies which give complexity
to hearing.
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Structure of the Ear
Outer ear serves to focus sound waves and
involved in localization of sound
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The Middle Ear
Middle ear serves to transfer air compressions
(of the outer ear) to fluid compressions (of the
cochlea). This process also greater amplifies
force of wave.
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The Inner Ear
Cochlea comprises 3 interconnected tubes
(scalae) and the organ of Corti
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Organ of Corti
Scala tympani
The organ of Corti lies between the (flexible)
basilar membrane and the (rigid) tectorial
membrane
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Functional Anatomy of the Cochlea
Cochlea is a pressure-conducting, cone-shaped
tube from the Oval to Round windows
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Wave formation and Resonance
Air pressure on the Tympanic membrane causes
movement of the middle ear with the Stapes
causing vibration of the Oval window resulting in
fluid waves within the Cochlea. Waves resonate at
specific point on the (flexible) Basilar membrane
(i.e. specific anatomical site is associated with
maximal displacement of membrane). Waves
dissipate at Round Window.
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Frequency Encoding
Point of resonance is determined by frequency of
sound wave, providing a basis for anatomical
encoding of pitch. i.e. more displacement/mechanic
al force at a point will produce more
transduction
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The organ of Corti and cochlear waves
Waves move the basilar membrane but not the
tectorial membrane resulting in conformational
changes in the stereocilia of hair cells.
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Depolarization of hair cells
Movement of basilar membrane changes conformation
of stereocilia resulting in increased K
conductance. Endolymph has usually high
K. Thus. increased K conductance depolarizes
cell, opening vg-Ca channels, and increases
transmitter release.
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Intensity encoding-at same frequency
Sound intensity (loudness) corresponds to height
of wave. Higher waves spread out more along
basilar membrane.
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Hair Cell Potentials
Increased sound pressure (louder noise) produces
higher graded receptor potentials
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Frequency tuning of auditory neurons

• Neurons respond maximally to characteristic
  frequencies.
• Higher characteristic frequencies correspond to
  higher APs.

Population encoding is similar to in other senses
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Tonotopic mapping
Spatial location encodes (intermediate to high)
frequencies
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Central Auditory Pathways
Spiral ganglion cells synapse on complex brain
stem circuits that precede thalamic relay to the
cortex. Brain stem circuits involved in auditory
activated reflexes.
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Summary of Auditory Pathways

• Auditory nerve axons
• (Ipsilateral) cochlear nucleus
• Superior olives (bilateral connections sound
  localization)
• Inferior colliculi (involved in orienting)
• Medial geniculate nucleus
• Primary auditory cortex

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Auditory Cortex

• 2 - 3 areas of primary auditory cortex
• Functional columns (cells of a column respond to
  the same frequency)
• Tonotopic organization (encodes frequency)
• About 7 areas of secondary
• Secondary areas do not
• respond well to pure tones
• and have not been
• well-researched

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Auditory vs Visual Systems
thalamus
Brain stem neurons perform early (pre-thalamic)
processing of auditory info as the retina does
for visual info.
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The vestibular system
The labyrinth contains the receptor cells for
the vestibular system.
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Vestibular system activation
Head movement activates the hair cells of the
vestibular system. Transduction is similar to
auditory system except environmental signal is
produced by orientation of the body relative to
gravity.
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Auditory and Visual Association Areas in Language
Perception
Motor side of language covered later.
Dorsal Stream (processes attributes)
Visual cortex

• Specialized for language perception
• Identified in neuropsychology exams.
• Receives complex /high level auditory and visual
  inputs.
• Assymmetrically localized on left side of brain.

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Auditory and Visual Association Areas in Language
Perception
Paralyze right side of brain no langauge
loss Paralyze left side of brain severe
langauge loss (96 in right-handed 70 in
left-handed)
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Hemispheres of Split-Brain Patients Function
Independently

• Left hemisphere can tell what it has seen, right
  hemisphere can show it.
• Studies of split-brain patients
• Present a picture to the right visual field (left
  brain)
• Left hemisphere can tell you what it was
  (verbally)
• Right hand can show you, left hand cant
• Present a picture to the left visual field (right
  brain)
• Subject will report that they do not know what it
  was
• Left hand can show you what it was, right cant

Both hemispheres have some language abilities
Left is usually dominant for verbal ability.
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The Somatosensory system
Somatosensory system gathers info from all parts
of the skin and from the internal organs
(contains parts we are consciously aware of part
that we are not (autonomic).
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Somatosensation

• Somatsensory system is 3 separate and interacting
  systems
• Exteroreceptive external stimuli
• Proprioceptive body position
• Interoceptive body conditions (e.g.,
  temperature and blood pressure)
• Exteroreceptive 3 divisions
• Touch (mechanical stimuli)
• Temperature (thermal stimuli)
• Pain (nociceptive stimuli)
• Specialized receptors respond to the various
  stimuli

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Somatosensory system(s)

• Somatosensory information is transduced by a
  variety of receptors with diverse properties, all
  are sensitive to mechanical (pressure) force.
• Two distinct pathways mediate (1)
  touch/proprioception
• (2) pain/temperature.
• Receptor transduction is complex and by primary
  neurons.

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Mechanosensitive Ion Channels

• The ability to perceive and react to different
  stimuli from the surrounding environment is
  essential for all living organisms.
• These membrane proteins open in response to
  mechanical stimuli which cause confirmational
  changes in the proteins to open pore generating
  an ionic current that leads to depolarization.

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Cutaneous (touch) Receptors

• Free nerve ending
• temperature and pain
• Pacinian corpuscles
• touch adapt rapidly, large and deep onionlike
• touch sudden displacements of the skin
• Merkels disks touch gradual skin indentation
• Ruffini endings touch gradual skin stretch

• We respond to change when there is no change,
  no sensation
• Stereognosis identifying objects by touch

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Differences in Receptive Fields
Different transducers have different spatial
sensitivities.
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Sensitivity of touch receptors
Transducers have complex frequency sensitivities
that show broad tuning with maximal activation at
characteristic frequencies.
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Adaptation of touch receptors

• Response profiles are diverse and complex for
  individual somatosensory transducers.
• Typically, changes in pressure produces high
  firing with varying adaptation speeds.

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Regional Sensitivity Differences
point-to-point discrimination measures
sensitivity
Different regions of skin have different
sensitivities due to variation in receptor type
and amounts.
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Chemical Mediators of Pain
In addition to pressure responsiveness, pain
sensation involves chemical detection of
noxious molecules associated with tissue
damage. Chemical and mechanical transduction is
performed by free nerve endings.
Pain is multi-modal (chemical and mechanical)
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Thermoreceptors
Thermoreceptors are sensitive to changes in
temperature rather than absolute temperature
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Afferent fiber diversity
Touch fibers are fast (large and myelinated) Pain
fibers are slow (smaller and/or lack myelin)
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Fast and Slow Pain Afferents
Pain fibers have different response profiles.
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Organization of the spinal cord
All somatosensory info enters the spinal cord
through the dorsal horn
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Spinal Afferents

• Dermatome the area of the body innervated by
  the left and right dorsal roots of a given
  segment of spinal cord

• Nerve afferents enter spinal cord between each
  vertebrae.
• Each nerve innervates a specific body region.
• (Facial info does not go through spinal cord).

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Skin and Visceral Afferents
Dorsal root ganglion fibers are comprised of both
skin and visceral fibers that form in ascending
tracts.
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Touch Somatosensory Afferents
Touch info travels up the spinal cord in the
dorsal columns
To interneurons

• Primary sensory neuron bifurcates, axons go
• Through dorsal column up to brain (ipsilateral,
  cross over later).
• To s.c. interneurons (important for reflexsive
  motor actions)

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Pain Somatosensory Afferents

• Pain info travels up the spinal cord in the
  lateral column spinothalamic tract.
• Afferents bifurcates to interneurons and
  ascending pathways.
• Spinothalamic tract has ipsilateral and
  contralateral ascending fibers.

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Ascending Touch Pathway (for body)

• Ascending axons of
• primary neuron goes
• Through dorsal column up to medulla relay,
  dessucates in medial lemniscus and projects to
  Ventro-Posterior Lateral (VPL) thalamus, then to
  primary somatosensory cortex.
• Dorsal column-medial lemniscus pathway

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Ascending Pathway (for facial touch)
Somatosensory receptors in the face form the
trigeminal nerve (cranial), synapse in trigeminal
nucleus of the pons, then to VPL thalamus, and
to primary cortex.
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Pain and Temperature Paths

• Despite its unpleasantness, pain is adaptive and
  needed
• No obvious cortical representation (although the
  anterior cingulate gyrus appears involved in
  emotional component)
• Descending pain control pain can be suppressed
  by cognitive and emotional factors

Pain and Temperature Info is relayed and
decussates (or not) at spinal levels and travels
to VPL thalamus, then primary cortex.
Spinalothalamic tract
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Ascending Somatosensory Pathways

• Dorsal-column medial-lemniscus system
• Touch and proprioception
• 1st synapse in the dorsal column nuclei of the
  medulla
• (Antero)lateral system
• Pain and temperature
• Synapse upon entering the spinal cord
• 3 tracts spinothalamic, spinoreticular,
  spinorectal

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Comparison of Somatosensory Paths
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Ascending Regulation of Pain
Touch and pain paths interact at the spinal level
to gate pain info.
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Descending modulation of Pain

• 3 discoveries made this possible
• Electrical stimulation of the periaqueductal gray
  (PAG) has analgesic (pain-blocking) effects
• PAG and other brain areas have opiate receptors
• Existence of endogenous opiates (natural
  analgesics) - endorphins

Brain activity can help to decrease pain info
gating of pain
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Cortical mapping of somatosensation

• Postcentral gyrus is site of primary
  somatosensory cortex.
• Body parts are dis-proportionally mapped in the
  primary cortex.
• Area of cortex is proportional to sensitivity of
  the body region.

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Somatosensory cortex

• Primary somatosensory cortex (SI)
• postcentral gyrus
• somatotopic
• more sensitive, more cortex
• input largely contralateral
• SII mainly input from SI
• somatotopic, input from both sides of the body
• Much of the output from SI and SII goes to
  association cortex in posterior parietal lobe

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Secondary Association Somatosensory Cortex

• Secondary somatosensory cortex -bilateral info
  (both sides of the body)
• Damage complex effects
• Asterognosia inability to recognize objects by
  touch
• Asomatognosia the failure to recognize parts of
  ones own body

• Parietal lobe association cortex
• Apraxiadifficulty making specific movements when
  requested to do so
• Contralateral neglect inability to respond to
  stimuli on one side of the body opposite the
  lesion (usually to stimuli on the left???)

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Plasticity in Somatosensory Maps
Loss of input or sensory experience can alter the
somatosensory mapping in the cortex.
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Acupuncture
Stimulation of define points of the somatosensory
system(s) via mechanical, thermal, or electrical
energy produces changes brain function.
Conditions that purportedly respond well to
Acupuncture Acute and chronic pain Headaches
Fibromyalgia Smoking/Drug Addiction Insomnia/Depre
ssion/Stress/Anxiety Infertility (???) Weight
Loss/Appetitie Control Celiac Disease
(Gluten-intolerence) - Metabolic Syndrome
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Sensory Pathways

• Each sensory system is associated with a specific
  transduction mechanism localized in a sensory
  organ.
• Transduction converts environmental info into
  biological signals (eventually encoded as action
  potential in neurons) that is transmitted along
  specific parallel pathways.
• These parallel neuronal pathways are relayed at
  specific points in the brain and is kept separate
  to maintain stimulus info.

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Thalamus
Thalamus is a major relay for all sensory systems
(for most prior to info arriving at
cortex). MDN-olfaction VPM-taste VPL-somatosensati
on MGN-auditory LGN-visual
Also, contains many motor nuclei and some
involved in complex processes (memory, emotional,
etc poorly understood).
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Primary Cortex

• Area of cortex that first receives sensory
  information of a given modality (i.e. vision,
  auditory, somatic, olfactory etc) is referred to
  as primary sensory cortex.
• Primary motor cortex is the area that directly
  controls output to muscles (later)

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Primary, Secondary, Association Cortices

• Cortical areas that are only associated with
  function of primary cortex (motor or sensory) are
  referred to as secondary motor/sensory cortex.
• Areas that are indirectly associated with primary
  cortices (involve multiple sensory modalities or
  sensory-motor information processing) are
  referred to as association cortex mediate
  higher cognitive functions.

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Primary cortices