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.
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 Vanilloid R1 Vanilloid-like Aδ CMR1, VR1 C fibers
Cortical Layers and Columns Mountcastle
Column Plasticity in Monkey Cut ulnar nerveLose 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 ofPosterior Parietal Cortex • 3D Object Recognition • Body Form (Amorphosynthesis) • Hand-Eye Coordination • Movement and Spatial Perception
Pain and Analgesia Lecture 15 PSY391S John Yeomans
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.
Skin Temperature Sensation Cool-Menthol R1 Vanilloid R1 Vanilloid-like Aδ
Opiates • 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.
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.