Touch Receptors and Axons

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Touch Receptors and Axons

• Lecture 13
• PSY391S
• John Yeomans

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Receptors in Skin
Hairy and glabrous skin are different.
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Sensitivity and Acuity

• SS receptors much less sensitive than acoustic or
  visual receptors.
• More receptors in glabrous skin of fingertips,
  lips and genitals.
• Fewer receptors in back, proximal limbs.
• Better 2-point discrimination when more
  receptors, esp. with small receptive fields.

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Adaptation in Single Neurons
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Pacinian Corpuscles

• Easiest receptor to study due to size and
  isolation.
• Sensitivity high despite deep location when
  vibratory stimuli used.
• Fire at onset and removal of 1 s stimulus--Fast
  adapting.
• Adaptation due to capsule absorbing energy--No
  adaptation when naked axons are directly
  stimulated.

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Receptive Field
Receptive field is part of the environment to
which a neuron responds.
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Single Neurons in Human Hand

• Microelectrodes in nerves isolate single neuron
  action potentials from large axons.
• 4 types of neurons, consistent with 4 receptor
  types in other animals.
• After studying receptive fields and adaptation,
  then microstimulate single axons to evoke
  perceptions!
• Perceptive fields match receptive fields.
• Valbo and Johansson

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Receptive Fields and Adaptation
Glabrous skin of palm and fingertips. Recordings
of single axons from median or ulnar nerves.
Valbo and Johansson
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4 Different Feelings from Stimulation of Single
Axons

• Pacinian No feeling unless gt10 action
  potentials, then deep vibration.
• Meissners 1 AP leads to tap. gt10 leads to odd
  buzzing or fluttering feeling.
• Merkels 4 APs cause light touch like leaf.
  10 APs cause stronger touch.
• Ruffini No feeling until at least 2 axons, then
  tugging sensation.
• Labelled lines for touch sensations.

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Axon Types
to 100 A alpha
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Somatosensory Pathways and Cortex

• Lecture 14
• PSY391S
• John Yeomans

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Dorsal Column Pathway
?Trigeminal Nucleus V
A?, Aß fibers
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Dermatomes
Trigeminal V
Double innervation of each skin area.
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Spinothalamic Pathway
Parietal postcentral gyrus
Trigeminal V
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Skin Temperature Sensation
Cool-Menthol R1
Vanilloid R1
Vanilloid-like Ad
CMR1, VR1 C fibers
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Somatosensory Cortex
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Cortex Plasticity in Human
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Cortical Layers and Columns
Mountcastle
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Column Plasticity in Monkey
Cut ulnar nerve?Lose cortical areas
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Lesions of SS Cortex

• Loss of 2-point discrimination.
• Loss of skin temperature discrimination.
• Loss of finest sensitivity and motor control.

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Association Areas ofPosterior Parietal Cortex

• 3D Object Recognition
• Body Form (Amorphosynthesis)
• Hand-Eye Coordination
• Movement and Spatial Perception

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Pain and Analgesia

• Lecture 15
• PSY391S
• John Yeomans

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Pain

• Acute pain signals tissue damage.
• Chronic Pain Syndromes
• Causalgia
• Neuralgia
• Phantom Limb Pain
• Usually involve peripheral nerve damage
  (neuropathy), but are sustained by CNS.
• Hard to treat.

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Peripheral Pain Mechanisms
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Skin Temperature Sensation
Cool-Menthol R1
Vanilloid R1
Vanilloid-like Ad
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Analgesia Pathways
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Opiates

• Opium, heroin and morphine.
• Enkephalins
• Endorphins
• Dynorphins
• Receptors mu, delta, kappa.
• Analgesia, reward, drug abuse.

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Muscles and Reflexes

• Lecture 16
• PSY391S
• John Yeomans

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Muscle Types

• Smooth muscles in viscera.
• Striated muscles to skeleton and connective
  tissue.
• Cardiac muscle--visceral striated muscle with
  rhythmic contractions.
• Fast-twitch and slow-twitch striated muscles.

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Muscles
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Sliding Filaments
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Muscle Fibers and Inputs

• Extrafusal fibers with alpha motor neurons.
• Intrafusal fibers with gamma motor neurons.
• Neuromuscular junction.
• Ach release by Ca.
• Nicotinic receptors?EPPs?APs
• APs?Ca?Actin and myosin sliding together.

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Neuromuscular Junction
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Muscle Receptors
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Kinesthetic Receptors

• Movements sensed by receptors in muscles, joints
  and tendons.
• Joint receptors respond to angle of joint.
• Pacinian corpuscles respond to vibration.
• Spindles respond to muscle stretch.
• Golgi tendon organs respond to stronger stretch.

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Spindle Stretch Receptors
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Spindle and Tendon Activation
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Proprioceptive Pathways

• Spinal reflexes--Monosynaptic stretch. Disynaptic
  GTO inhibition (clasp-knife).
• Dorsal columns to thalamus and motor cortex.
• Spinocerebellar path.

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Motor Units and Rotation

• Motor unit 1 axon and all the fibres
  innervated.
• Reciprocal inhibition of competing motor units in
  ventral horn (flexors vs. extensors).
• Size principle--small motor units first.
• Rotation of motor units, by recurrent inhibition
  in ventral horn.

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Reflexes

• Monosynaptic stretch reflex.
• Disynaptic tendon reflex (clasp-knife).
• Flexion reflex.
• Scratching and walking.

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Stretch Reflex