Touch Receptors and Axons Lecture 13 PSY391S John Yeomans.
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Transcript of Touch Receptors and Axons Lecture 13 PSY391S John Yeomans.
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.
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.
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.
Lesions of SS Cortex
• Loss of 2-point discrimination.
• Loss of skin temperature discrimination.
• Loss of finest sensitivity and motor control.
Association Areas ofPosterior Parietal Cortex
• 3D Object Recognition
• Body Form (Amorphosynthesis)
• Hand-Eye Coordination
• Movement and Spatial Perception
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.
Opiates
• Opium, heroin and morphine.
• Enkephalins
• Endorphins
• Dynorphins
• Receptors: mu, delta, kappa.
• Analgesia, reward, drug abuse.
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.
Muscle Fibers and Inputs
• Extrafusal fibers with alpha motor neurons.
• Intrafusal fibers with gamma motor neurons.
• Neuromuscular junction.
• Ach release by Ca++.
• Nicotinic receptorsEPPsAPs
• APsCa++Actin and myosin sliding together.
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.
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
• Monosynaptic stretch reflex.
• Disynaptic tendon reflex (clasp-knife).
• Flexion reflex.
• Scratching and walking.