Touch Receptors and Axons
Lecture 13
PSY391S
John Yeomans
Receptors in Skin
Hairy and glabrous skin are different.
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.
Adaptation in Single Neurons
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.
Receptive Field
Receptive field is part of the environment to which a neuron responds.
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
Receptive Fields and Adaptation
Glabrous skin of palm and fingertips.
Recordings of single axons from median or ulnar nerves.
Valbo and Johansson
4 Different Feelings from
Stimulation of Single Axons
• Pacinian: No feeling unless >10 action
potentials, then “deep vibration”.
• Meissner’s: 1 AP leads to “tap”. >10 leads
to odd “buzzing” or “fluttering” feeling.
• Merkel’s: 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.
Axon Types
to 100
A alpha
Somatosensory Pathways
and Cortex
Lecture 14
PSY391S
John Yeomans
Dorsal Column Pathway
Trigeminal Nucleus V
A, Aβ fibers
Dermatomes
Trigeminal V
Double innervation of
each skin area.
Spinothalamic Pathway
Parietal postcentral gyrus
Trigeminal V
Skin Temperature Sensation
Cool-Menthol R1
CMR1, VR1 C fibers
Vanilloid R1
Vanilloid-like Aδ
Somatosensory Cortex
Cortex Plasticity in Human
Cortical Layers and Columns
Mountcastle
Column Plasticity in Monkey
Cut ulnar nerveLose cortical areas
Lesions of SS Cortex
• Loss of 2-point discrimination.
• Loss of skin temperature discrimination.
• Loss of finest sensitivity and motor control.
Association Areas of
Posterior Parietal Cortex
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3D Object Recognition
Body Form (Amorphosynthesis)
Hand-Eye Coordination
Movement and Spatial Perception
Pain and Analgesia
Lecture 15
PSY391S
John Yeomans
Pain
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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.
Peripheral Pain Mechanisms
Skin Temperature Sensation
Cool-Menthol R1
Vanilloid R1
Vanilloid-like Aδ
Analgesia Pathways
Opiates
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Opium, heroin and morphine.
Enkephalins
Endorphins
Dynorphins
Receptors: mu, delta, kappa.
Analgesia, reward, drug abuse.
Muscles and Reflexes
Lecture 16
PSY391S
John Yeomans
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.
Muscles
Sliding Filaments
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.
Neuromuscular Junction
Muscle Receptors
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.
Spindle Stretch Receptors
Spindle and Tendon Activation
Proprioceptive Pathways
• Spinal reflexes--Monosynaptic stretch.
Disynaptic GTO inhibition (clasp-knife).
• Dorsal columns to thalamus and motor
cortex.
• Spinocerebellar path.
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.
Reflexes
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Monosynaptic stretch reflex.
Disynaptic tendon reflex (clasp-knife).
Flexion reflex.
Scratching and walking.
Stretch Reflex