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Somatic Sensory System

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Somatic Sensory System Sensation arising from skin muscle joints Allow you to survive in your environment and make appropriate motor responses Somatic Sensation The ... – PowerPoint PPT presentation

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Title: Somatic Sensory System


1
Somatic Sensory System
  • Sensation arising from skin muscle joints
  • Allow you to survive in your environment and make
    appropriate motor responses

2
Somatic Sensation
  • The ability to feel your physical environment, to
    ache, feel temperature, pain, to know where your
    body position is
  • Pressure, position of joints/muscles,
    temperature, distension of bladder, stomach
  • Over-stimulation of body can be damaging

3
Sensory Receptors
  • Spread throughout body
  • Senses 4 types of information aka sensory
    modality
  • Touch, pressure, vibration
  • Body position, propioceptive receptors
  • Temperature
  • Pain-Nociception

4
Encoded Information
  • Intensity
  • Duration
  • Position
  • Direction

5
Propioception
  • Sensory Information regarding joint and limb
    position
  • Allow you to know your body position without
    looking in the mirror
  • Muscle spindles- intrafusal fibers and
  • Golgi Tendon organs are specialized structures
    that are innervated by DRGNs and send information
    to the spinal cord

6
Touch
  • Skin largest sensory organ
  • Epidermis (outer layer) dermis (inner layer)
  • Sensitive to raised dot 0.006mmH x 0.04mmW
    braille dot is 167 times bigger
  • Skin receptors
  • Hairy glabrous (hairless)

7
12.1
8
Mechanoreceptors
  • Found in skin for sensing contact with physical
    world
  • Found in bladder, blood vessels, heart digestive
    organs and teeth to sense pressure
  • Mechanoreceptors are innervated by myelinated
    axons
  • Axons have mechanosensitive ion channels gated by
    stretch tension changes

9
Types of Mechanoreceptors Found in Skin
  • Pacinian corpuscle
  • Found in dermis
  • Meissner corpuscle
  • Found in ridges of glabrous skin
  • Merkels disc in epidermis
  • Rufini endings
  • Found in hairy glabrous skin

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11
Primary Afferent Axon
  • Axons of varying diameters with soma in DRG and
    enter Spinal cord through dorsal roots into
    dorsal horn
  • Different diameter axons carry different types of
    somatosensory information
  • Project locally in spinal cord and have long
    ascending branches to contact secondary
    somatosensory axons

12
Receptive Field
  • Area of skin that is monitored by a single
    mechanoreceptor
  • Meissner merkel have small RF (2-3mm)
  • Pacinian ruffini have large RF (entire finger/
    ½ palm)

13
F 12.2
14
Dermatomes
  • The area of skin innervated by the right and left
    dorsal roots (1st order neuron) of a single
    spinal segment
  • When mapped, dermatomes form sets of bands
    representing surface of body innervated by axons
    in one level of sc.

15
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18
2-point discrimination
  • Ability to discern 2 closely position points as 2
    rather than 1.
  • Varies 20 fold throughout body
  • Fingertips have highest resolution
  • Due to high density of mechanoreceptors
  • Receptor subtypes with small receptive fields
  • More cortical neurons dedicated to deciphering
    sensory information

19
F 12.6
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21
First and Second Order Neurons
  • Primary and secondary somatosensory or afferent
    neurons
  • The primary neuron has the sensory receptor
  • The secondary neurons gets information from the
    primary and can project information or modify
    locally activity of primary neurons.

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23
Second order Neurons aka Interneurons
  • Neurons that receive synaptic input form DRG
    neurons
  • Reside in dorsal horn and trigger reflex
    responses
  • Also ascend to brainstem and thalamus
  • Also reside in brain stem and are involved in
    perception

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27
Dorsal Column-Medial Leminiscus Pathway
  • Touch/vibration position (proprioception) info
    travels to brain separate from pain/temperature
  • Afferent/central axon of large sensory (AB)
    fibers ascend ipsilaterally in dorsal columns
    with tactile info and limb position info
  • DC also have 2nd order ascending axons from
    dorsal horn neurons

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Dorsal column nuclei
  • Axons terminate in DCN in medulla
  • Then decussate (cross) and ascend as medial
    lemniscus tract through pons midbrain to
    synapse in ventral posterior nucleus (VPN) of
    thalamus
  • VPN axons then project to primary somatosensory
    neurons in parietal cortex (S1)

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31
Somotopic Organization
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Somotopy
  • Mapping of body areas sensation onto the cortex
  • Somotopic map called homunculus that shows that
    the largest number of neurons in S1 receive
    sensory information from hand and mouth

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38
Somatosensory Cortex
  • In Parietal lobe, posterior to central sulcus
  • Carries on higher order processing of sensory
    information. Called S1
  • Receives synaptic input from VP nucleus of
    thalamus
  • Respond to somatosensory info
  • Lesions in S1 impair somatic sensation
  • Electrically stimulate S1 and you feel a
    sensation on the appropriate body part

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40
S1 Cortex
  • Reciprocal (bidirectional) connections between
    cortical areas
  • Association pathways
  • Restriction of information some cortex areas
    specialize in decoding texture, size shape
  • Thalamic input is to cortical layer IV which send
    axons to other layers in same area

41
S2 and Parietal Posterior Cortex
  • S2 is lateral to S1 and is association area
  • PPC is posterior to S1 and is involved in
    perception/recognition of sensation
  • Neurons in S2 and PPC have complex receptive
    fields which can include sensory information as
    well as attention and visual and movement
    planning.

42
Posterior Parietal Cortex
  • Injury causes neglect syndrome Do not recognize
    body part as your own so you do not dress it,
    wash it.
  • Agnosia Inability to recognize objects including
    your own body parts.
  • Astereoagnosia inability to recognize something
    by touch, but recognized by sight

43
Trigeminal Touch Pathway
  • Two trigeminal nerves CN5
  • Each divided into 3 PNs that innervate face,
    mouth and anterior 2/3 tongue dura mater

44
Sensory CNs
  • CN 7 facial, 9 glossopharyngeal, 10 vagus
  • Large diameter axons carry tactile info from skin
    mechanoreceptors
  • Synapse on ipsilateral trigeminal nucleus in pons
  • Decussate and project to medial VP nucleus of
    thalamus
  • Project to somatosensory cortex

45
Add 12.15-19
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