Neural Circuitry and Conditioning in Rabbit Eyeblink Response

Nictitating membrane response (NMR)

Sliding of a third eyelid over the eyeball in rabbits caused by retraction of the eyeball; used as a measure of conditioning.

Delay conditioning

A form of classical conditioning where the CS precedes and overlaps with the US, producing an anticipatory CR.

Why is NMR conditioning useful?

It is simple, has a measurable behavioural output, and its neural circuitry can be mapped in detail.

Core finding of NMR conditioning research

The cerebellum is essential for delay conditioning, whereas the forebrain is not required.

Three techniques used to establish neural circuitry

Neuroanatomy (tract tracing), electrophysiology, and manipulation (lesions, inactivation, stimulation).

Retrograde transport

Tracer method that reveals inputs to a brain region by travelling backward from axon terminals to cell bodies.

Anterograde transport

Tracer method that reveals outputs from a brain region by travelling from cell bodies to axon terminals.

Electrophysiology

Recording neuronal activity to determine the functional properties of neural connections.

Manipulation technique

Altering brain regions through lesions, inactivation, or stimulation to test their function.

Muscimol

A GABA agonist used to reversibly inactivate brain regions.

Unconditioned response (UR)

An innate reflexive response to the US that is present before learning.

Conditioned response (CR)

A learned anticipatory response that occurs before the US after conditioning.

UR pathway

US → trigeminal nerve (CN V) → Gasserian ganglion → Sp5O → accessory abducens nucleus → retractor bulbi muscle.

UR reflex arc

A three-neuron reflex arc consisting of sensory neuron, interneuron, and motor neuron.

UR latency

Approximately 20 ms from stimulus to response.

Trigeminal nerve (CN V)

Sensory cranial nerve carrying information from the face and eye.

Abducens nerve (CN VI)

Motor cranial nerve controlling the retractor bulbi muscle.

Retractor bulbi muscle

Muscle that retracts the eyeball, causing the nictitating membrane to slide over the eye.

Gasserian ganglion

Sensory ganglion of the trigeminal nerve containing cell bodies of sensory neurons.

Sp5O (spinal trigeminal nucleus, oral subdivision)

Interneuron nucleus in the UR pathway that also sends US information to the inferior olive.

Anterior interpositus nucleus

Deep cerebellar nucleus critical for NMR conditioning; lesions abolish the CR while sparing the UR.

Evidence for anterior interpositus involvement

Lesions abolish retention and acquisition of conditioned responses while leaving unconditioned responses intact.

CR output pathway

Anterior interpositus nucleus → contralateral red nucleus → accessory abducens nucleus → retractor bulbi muscle.

Red nucleus

Midbrain structure that relays CR signals from the interpositus to motor output pathways.

Evidence for red nucleus involvement

Muscimol inactivation blocks CR performance, which returns when the drug wears off.

HVI (hemisphere of lobule VI)

Region of cerebellar cortex involved in eyeblink conditioning that projects to the anterior interpositus.

Primary input to the interpositus nucleus

The cerebellar cortex, particularly HVI.

Purkinje cells

Inhibitory GABAergic neurons that provide the sole output of the cerebellar cortex to deep cerebellar nuclei.

Purkinje cell activity during rest

Tonically active, continuously inhibiting the anterior interpositus nucleus.

Disinhibition

Removal of inhibition; silencing Purkinje cells allows the interpositus to become active and generate a CR.

Key inhibitory logic of eyeblink conditioning

Silencing cerebellar cortical output disinhibits the interpositus, allowing the CR to occur.

Evidence for cerebellar cortex control of CRs

Optogenetic inhibition of Purkinje cells produces eye blinks.

CS pathway

CS → sensory processing areas → pontine nuclei → mossy fibres → HVI and anterior interpositus.

Pontine nuclei

Relay nuclei that transmit conditioned stimulus information to the cerebellum.

Mossy fibres

Axons carrying CS information from the pons to the cerebellar cortex and deep nuclei.

Mossy fibre targets

Both cerebellar cortex and anterior interpositus nucleus.

Evidence that the pons carries CS information

Pontine lesions abolish conditioning and pontine stimulation can serve as a CS.

US pathway

US → Sp5O → inferior olive → climbing fibres → HVI and anterior interpositus.

Inferior olive

Brainstem nucleus that relays US information to the cerebellum.

Climbing fibres

Axons of inferior olivary neurons that carry US teaching signals to the cerebellum.

Climbing fibre targets

Purkinje cell dendrites and deep cerebellar nuclei.

Evidence that climbing fibres carry US information

Inferior olive lesions abolish conditioning and climbing fibre stimulation can serve as a US.

Teaching signal in cerebellar learning

The climbing fibre/inferior olive pathway is thought to provide the error or teaching signal.

Difference between mossy and climbing fibres

Mossy fibres carry CS information from the pons, whereas climbing fibres carry US information from the inferior olive.

Effect of decerebration on delay NMR conditioning

Delay conditioning remains possible despite forebrain removal.

Effect of cerebellar damage on delay NMR conditioning

Conditioning is blocked when critical cerebellar regions are damaged.

Difference between UR and CR motor output

Both use the same motor pathway, but the UR is driven by the US reflex circuit while the CR is driven by cerebellar output.

Exam focus: five circuit components

UR pathway, CR output pathway, cerebellar cortex-interpositus interaction, CS pathway via mossy fibres, and US pathway via climbing fibres.

Tract-tracing

Neuroanatomical method for mapping connections between brain regions using transported tracers.

Optogenetics

Technique using light to activate or inhibit specific neurons, such as Purkinje cells.

Lesion evidence

Demonstrates the necessity of a brain region by examining behavioural deficits after damage.

Nictitating membrane

A third eyelid that passively slides over the eye when the eyeball retracts.

Difference between delay and trace conditioning

Delay conditioning does not require the hippocampus, whereas trace conditioning does because there is a temporal gap between CS and US.