NMR Conditioning Flashcards
We have selected NMR conditioning to investigate and have determined which parts of the brain is involved, what is the next step?
Establish the circuitry
Give an overview of what lesions indicated about what brain areas are involved in NMR conditioning
A lesion between the forebrain and the brainstem in the rats brain, cutting off the cerebral cortex and hippocampus from the brainstem and cerebellum, (decerebration) did not affect delay NMR conditioning
However, such conditioning is not possible after cerebellar damage
Must be the cerebellum
What are the main questions we want to answer by establishing the circuitry?
How does the cerebellum connect with the CS and US inputs, and with the CR outputs
What techniques can be used to establish the circuitry?
3 basic techniques
1. Neuroanatomy
2. Electrophysiology
3. Manipulation
What is involved with the neuroanatomy technique?
Finding out what connects with what
For any collection of neurons:
- Where do they project?
- Which regions project to them?
Neuroanatomy techniques often cannot be seen directly, even with a microscope, how do we overcome this?
Use tract-tracing methods
What are the two types of tract-tracing methods?
Retrograde transport
Anterograde transport
What is retrograde transport?
Used to trace neural connections from their point of termination (the synapse) to their source (cell body)
A tracer substance that will be taken up by synaptic terminals is injected into a region of interest, such as a central nervous system nucleus
The tracer is then conveyed retrogradely by axonal transport to the cell bodies of the neurons that give rise to the projection being labeled
What is anterograde transport?
Used to trace axonal projections from their source (cell body) to their point of termination (the synapse)
Inject substance into the nucleus, substance is transported to terminals
What can we find out using electrophysiology?
What signals are carried by these connections?
Establish their properties e.g., by recording from individual neurons while animal is presented with sensory stimuli, and/or performing motor responses
Example = orientation specificity of neurons in visual cortex
What does manipulation tell us about circuitry?
What happens when a particular region is removed, inactivated or stimulated?
Try to predict effects from anatomy and physiology
Standard systems neuroscience
What is the NMR conditioning circuitry broken down into?
5 parts ==
- Unconditioned reflex pathway
- Conditioned reflex circuit: conditioned response
- Conditioned reflex pathway: cerebellar cortex
- Conditioned reflex circuit: CS pathway
- Conditioned reflex circuit : US pathway
What does the unconditioned reflex pathway consist of and what is an advantage of this?
Basic three neuron reflex arc
1. Sensory neuron (in Gasserian ganglion)
2. Interneuron (in trigeminal nucleus)
3. Motor neuron (in abducens nucleus)
Only 3 synapses - 2 in the CNS and the nerve-muscle synapse
Therefore, is very fast - overall about 20msec of which 16msec is mechanical, 2.5ms time for trigeminal neurons to respond to air puff, only 2.5ms in the CNS - makes sense given the importance of protecting the eye
Explain the unconditioned reflex pathway
Synapse 1 = Cornea projects to the spinal trigeminal nucleus via the trigeminal nerve 5 with cell bodies in the Gasserian ganglion
Synapse 2 = spinal trigeminal nucleus projects to the accessory abducens nucleus
Synapse 3 = accessory abducens nucleus project to the retractor bulbi muscle via the abducens nerve 6
Retractor bulbi muscle pulls eye back into the orbit, the nictitating membrane then slides over the eyeball
What do we know about the conditioned response as part of the Conditioned Reflex Circuit: CR?
Lesions of the deep cerebellar nuclei block the CR and any pre-existing conditioning is abolished and cannot be re-learned
In fact, lesions confined to the anterior interpositus nucleus are effective
What do we want to find out about the conditioned reflex circuit: CR?
We can effectively remove the CR with little effect on the UR
Generally agreed that a properly placed lesion blocks retention of previously learnt conditioning and prevents new learning (Steinmetz et al., 1992)
Therefore, how does the CR command signal get from the anterior interpositus nucleus to the accessory abducens nucleus?
Explain the conditioned reflex pathway: CR
The anterior interpositus nucleus doesn’t project directly to the accessory abducens nucleus, but via a midbrain structure called the red nucleus
Cerebellar interpositus nucleus –> Red nucleus –> Accessory abducens nucleus
What evidence is there that the red nucleus is involved in the conditioned reflex pathway: CR?
Causes inactivation by muscimol (selective agonist of GABA A receptors) which blocks performance of the conditioned response
What is the significance of the cerebellar cortex in the conditioned reflex pathway?
Almost all output from the cerebellar cortex goes through the deep cerebellar nuclei
So the main input to the interpositus nucleus is from the cerebellar cortex
Different regions of cortex project to different parts of deep cerebellar nuclei
What region of the cerebellar cortex projects to the anterior interpositus nucleus and is concerned with eyeblink conditioning?
In the hemisphere of lobule 6 (HVI)
Explain the conditioned reflex pathway: cerebellar cortex
Cerebellar cortex lobule 6 (HVI) –> (inhibits) –> interpositus nucleus –> red nucleus –> accessory abducens nucleus
What evidence is there for the role of the cerebellar cortex in the CR pathway?
Lesions in the CR pathway affect the CR (Yeo, 1987)
Not entirely straighforward - cortical lesions probably abolish the CR if large enough (and bilateral)
What is more clear cut than lesion work?
Cerebellar cortex stimulation data
How does cerebellar cortex stimulation indicate its role in conditioning?
Output from cerebellar cortex inhibits deep cerebellar nuclei (all other synapses are excitatory)
Cortical output is tonically active, so to get an eyeblink, need to silence cortical output (disinhibition)
The output cells are called purkinje cells which are inhibitory- stopping them firing causes the cells in the deep cerebellar nuclei to fire more
What did Heiney et al (2014) find in the ‘stimulation’ of the cerebellar cortex?
Used optogenetics to selectively suppress PC activity
Found that stimulation resulted in sustained decreases in firing of PCs
Output from cerebellar cortex inhibits deep cerebellar nuclei
Cortical output is tonically active so to get an eyeblink response need to silence cortical output which was acheived by Heiney et al
What evidence did Heiney et al. (2014) find for eyeblink control?
Used photostimulation of the cerebellar cortex to reduce PC activity which resulted in closure of the ipsilateral eyelid
Demonstrated that transient decreases in PC activity are sufficient to drive movements that are specific to the cerebellar cortical area where the inhibited PCs are located
Varied intensity duration and rise time
Changing the duration of stimulation changed the duration of the eyeblink
Changing intensity of stimulation affected velocity of eyelid closure
Rise time had an affect on blink speed and the time for max speed to be reached
Why was Heiney et al.’s (2014) study important?
Nice demonstration that the parameters of the stimulation you’re applying are related to eye position & velocity = good evidence this is the mechanism that we’re looking at
What can we use to see where the HVI region of the cerebellum get its input from?
Anatomy: relevant region of cerebellum receives from pons (Yeo et al. 1985)
Describe the conditioned reflex pathway: conditioned stimulus
Pontine nuclei receive input about light and sound via the forebrain and midbrain
Mossy fibres from the pons project to the cerebellum (cortex and interpositus)
Project to the red nucleus and so on
What evidence is there for the pons carrying information about the CS?
Lesions of the pontine nuclei abolish conditioning
Can use electrical stimulation of pontine nuclei as a CS = get good learning, eyeblink response
Explain the conditioned reflex pathway: unconditioned stimulus
The cerebellum also receives information about the unconditioned stimulus
Arrives at the cerebellar cortex from the trigeminal nucleus via a relay, the inferior olive
The axons of the inferior olivary neurons are termed climbing fibres
This pathway carries the error signal in the system
What evidence is there that the climbing fibres carry information about the US?
Lesions in relevant region of inferior olive abolish conditioning and prevent relearning (Yeo et al, 86)
Can use electrical stimulation of climbing fibres as a US, and get conditioning (Thompson and Krupa, 1994)
