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 100A alpha

Somatosensory Pathways and Cortex

Lecture 14

PSY391S

John Yeomans

Dorsal Column Pathway

Trigeminal Nucleus V

A, Aβ fibers

Dermatomes

Double innervation of each skin area.

Trigeminal V

Spinothalamic Pathway

Trigeminal V

Parietal postcentral gyrus

Skin Temperature Sensation

Cool-Menthol R1Vanilloid R1

Vanilloid-like AδCMR1, VR1 C fibers

Somatosensory Cortex

Cortex Plasticity in Human

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.

Peripheral Pain Mechanisms

Skin Temperature Sensation

Cool-Menthol R1Vanilloid R1

Vanilloid-like Aδ

Analgesia Pathways

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.

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 receptorsEPPsAPs

• APsCa++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

• Monosynaptic stretch reflex.

• Disynaptic tendon reflex (clasp-knife).

• Flexion reflex.

• Scratching and walking.

Stretch Reflex