Lecture 22: Classical Conditioning using the Air Puff Eye Model

outputs of the cerebellum

Outputs: Purkinje cells are the sole (INHIBITORY) output neurons of the cerebellum, projecting to cerebellar nuclei, including the interpositus nucleus, and from there to dorsal motor systems and spinal motor tracts

Purkinje cells are the key site of

Plasticity

Eye blink conditioning circuit: unconditioned stimulus-> unconditioned response

US

Learning requires that the CS is paired with the _____.

Unconditioned stimulus (air puff) Meet within a very specific time window (around 200ms) Meet at purkinje cell dendritic syrface

How exactly does CS-US convergence (mossy + climbing fiber convergence) result in LTD?

1. the CS, via mossy fibers and via granule cells and their parallel fiber system, depolarize spines on Purkinje dendritic tree: glutamate released upon ionotropic and metabotropic glutamate receptors that sit in the dendritic tree of purkinje cells 2. Calcium release from intracellular stores (caused through a complex 2nd messenger mechanisms) slowly and partly inactivates glutamate receptors. The "slowness" is key for understanding CS-US delays: CS: glutamate (AMPA) receptors are "tagged" and vulnerable for a while (couple of 100 ms). 3. If a US is administered in time, climbing fibers depolarize and the postsynaptic effects "meet" tagged & vulnerable parallel fiber AMPA receptors, knocking them out for while (internalization), causing Parallel fiber long-term depression (LTD) due to loss of synaptic strength from parallel fiber system to purkinje cells. If a US is not administered in time (within 200-300ms), glutamate receptors lose their tag and thus a late US will not result in LTD. Lots of reflexes can be conditioned to a tone....(1 parallel fiber contacts 150K Purkinje cells...).

How exactly does CS-US convergence (mossy + climbing fiber convergence) change Purkinje cells and result in LTD? The simplified version:

AMPA receptors in the dendritic membrane of Purkinje cells: "marked for death" by the CS - and for a little while (100-300 ms). If UCS follows with another glutamate pulse onto these receptors - they are actually "killed", so to speak. Thus, Purkinje cells rendered to be less excitable, less GABA release... No inhibition of interpositus-> CS can drive CR

To get to the cerebellum

Access to cerebellum through conditioned stimulus pathway occurs as mossy fibers make contact with granule cells, which give rise to parrallel fiber system and allows many purkinje cells to listen to tone— cerebellum Access to cerebellum for reflex pathway (unconditioned stimulus) is availble, but it can only be used after learning

Unconditioned stimulus

Air puff

Air puff to the cornea: unconditioned stimulus causes

Air puff to the cornea: unconditioned stimulus; causes eye blink in humans nictitating membrane response in rabbits (unconditioned response). Sensory fibers from the cornea run along the trigeminal nerve (Vth cranial) to cranial motor nucleus and then activate motor projections to cause the eyelids to close. Reflex pathway is unchanged, access is granted to conditioned stimulus so that it can illicit eyeblink response

Mossy fibers could drive cerebellar output before conditioning if it were not for

Before conditioning, CS (tone) tries to depolarize interpositus neurons, and can depolarize purkinje cells, but bc of GABAergic inhibition activated by by the depolarized purkinje cells counteracts CS effects on interpositus neurons Air puff makes eye blink response After conditioning, purkinje cells stop responding to CS during learning, resulting in CS to be able to depolarize interpositus neurons

Eye Blink Conditioning circuit: controlled stimulus

Begin with tone -> auditory machinery-> mossy fiber system-> synapsing onto granule cells-> axons of granule cells make contact with purkinje cells-> will fire purkinje cells resulting in glutamate release and will try to fire interpositus neurons but it wont happen bc GABA from purkinje cells inhibits their firing-> NO tone (conditioned stimulus) induced eye blink (controlled reposone)

Kakegawa et al. 2018: inhibited Purkinje cell AMPA receptor internalization during learning...

Blocked learning entirely because AMPA recptors dont get killed and CS still depolarizes purkinje cells

Following successful conditioning A. a corneal airpuff no longer elicits an eyeblick response B. the demonstration of a UR requires an intact cerebellum C. the demonstration of a CR requires an intact cerebellum D. a and b E. all of the above

C. the demonstration of a CR requires an intact cerebellum

After learning

CS has access to reflex pathway

Inputs to cerebellum

Climbing fibers: from inferior olive to Purkinje cells; one climbing fiber contacts ~one Purkinje cell; but: one fiber makes 300 contacts which allow for massive depolarization of Purkinje cells. Mossy fibers: rom brain stem regions (e.g., pontine nucleus) and spinal cord to granule cells which, via their extensive parallel fiber network, excite Purkinje cells (1 parallel fiber contacts 75 Purkinje cells; one Purkinje cell is contacted by 150,000 parallel fibers). Both cell types converge onto purkinje cells

Following conditioning A. The CS can depolarize interpositus neurons B. The CS alone can recruit, via several synapses, the motor neurons that cause an eye blink C. The UCS no longer has access to the eyeblink reflex circuit D. a and b E. all of the above

D. a and b

After conditioning, which of the following manipulations can block the CR? A. Stimulation of GABA receptors expressed by interpositus neurons by, e.g., applying a GABA agonist B. Administering glutamate receptor antagonists onto the dendritic tree of Purkinje cells C. Lesioning the climbing fiber system D. Lesioning of the granule cells and thus the parallel fiber system E. a and d

E. a and d

Purkinje cells... A. are the output neurons of the cerebellum B. release GABA onto neurons of the nucleus interpositus C. are postsynaptic to climbing fibers and granule neurons D. a and c E. all of the above

E. all of the above

Following conditioning A. Purkinje cells are more profoundly depolarized by a CS then before conditioning B. Purkinje cells tend to remain silent in response to a CS C. More GABA is released from Purkinje cells in response to a CS than before conditioning D. Interpositus neurons are more readily depolarized than before E. b and d

E. b and d

During conditioning A.climbing fibers are now inactive B.mossy fiber action potentials translate into depolarization of Purkinje dendritic membrane C. climbing fiber action potentials "import information" about the corneal airpuff to Purkinje cells D. the convergence of mossy and climbing fiber depolarization changes Purkinje cells so that they become less excitable E. b, c and d

E. b, c and d

metabotropic receptors

Illicit secondary messanger machinery that in turns activates0 many different pathways

Mutant mice showing Purkinje cell degeneration exhibit

Impaired eye blink conditioning Shows that purkinje cells are a key site of plasticity

During conditioning: interpositus activity

Increases CS increasingly deporlaizes interpositus due to removal of brakes from purkinje cells

ionotropic receptors

Like AMPA receptors that depolarize membrane via Na+ channels

Electrical stimulation of mossy fibers replaces________ and Electrical stimulation of climbing fibers replaces _____

Presentation of CS; presentation of US

During condition, CS and US pathways are nearly

Simulatneously active CS advances unconditones stimulus by a couple 100 ms allowing for activty at the same time as US This double polarization of purkinje cells by both CS and US allows for plasticity and the ability to lose inhibitory effects on interpositus neurons after learning

conditioned stimulus (CS)

Tone

How can changes in the contacts between Purkinje cells and the interpositus allow the tone to serve as a CS, triggering eye blinks?

Tone drives interpositus bc glutamate from purkinje cells is ineffective due to LTD of purkinje cells CS now inhibits purkinje cell output, and removes inhibition from interpositus neurons Interpositus neurons can now fire and in response to CS and stimulute the red nucleus (RN) which has access to cranial motor neurons of reflex pathway Conditioned stimulus now has access to conditioned response

Stimulation of mossy and climbing fibers simultaneously results in

long-term depression of Purkinje cell activity.

Anatomy we need to know

• CS: Pontine nucleus (PN) --> mossy fibers (MF); granule cells (GR)--> parallel fibers (PF)—> depolarizes purkinje cells—> inhibit interpositus (before learning) • US: Inferior olive (IO) --> climbing fibers (CF)—-> eye blink response • Purkinje cells (PC) release GABA • Nucleus Interpositus (Int): inhibited by PC and stimulated by MF

Eye Blink Conditioning circuit: tone followed by air puff 200ms later

• US information also reaches interpositus (Int) and Purkinje cells, via interior olive (IO) and climbing fibers (CL); • Which depolarizes purkinje cells which inhibits Int. • But cerebellar output not needed as US makes UR every time via the reflex circuit. Climbing fibers (bring in air puff) and parralel fibers (bring in tone) synase on adjacent places on dendritic tree of purkinje cells