Transcript Ch. 24
Ch. 24 Plasticity of Mature Synapses Invertebrate Behaviour Modification and synaptic plasticity: • Aplysia californica (sea slug) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. • Large neurons • Can identify specific neurons in particular locations within ganglia Behavioral Plasticity • Habituation - animal becomes less responsive to repeated stimulus. • Sensitization - animal generalizes an aversive response elicited by a noxious stimulus. Aplysia habituation • Light touch on siphon – Gill withdrawal – Gill withdrawal becomes weaker with repeated stimulation Aplysia habituation • The reflex circuit: – Mechanosensory neurons in the siphon – Motor neurons innervate gills – Interneurons receive input from various sensory neurons Habituation reflex • Mechanosensory neurons release glutamate onto (excitatory synapse) interneurons and motor neurons. • Motor neurons release acetyl choline onto the muscle after summed excitation from sensory neurons and interneurons. Habituation Reflex • During habituation: – Decrease in glutamatergic synapse between sensory neurons and interneurons. – Synaptic depression Synaptic depression • Depleted synaptic vesicles in the active zone during synaptic neurotransmission • Less glutamate released Aplysia sensitization • Siphon becomes habituated to touching – Sensitization by pairing a strong electrical stimulus to the animal’s tail with another touch of the siphon. – This pairing causes the siphon stimulus to again elicit a strong withdrawal reflex to light touch – Even after a single stimulus to the tail, the gill withdrawal reflex remains enhanced for at least half an hour. – With repeated stimulation, it stays for days or weeks. How sensitization? • Interneurons are recruited • Tail shock – Activates sensory neurons in the tail – Sensory neurons activate modulatory interneurons that stimulate the withdrawal pathway. Modulatory interneurons • Release serotonin (5HT) • Activation of 5HT receptor enhances more glutamate release from the sensory neuron for about an hour. 5HT and sensitization • Pre synaptic 5HT receptors on sensory neurons are coupled to G-proteins – Production of cAMP – The cAMP binds to PKA regulatory subunits – PKA catalytic subunits liberated and phosphorylate K+ channels – Decreased probability of K+ opening when phosphorylated. – Pre-synaptic membrane depolarizes.