NMR Conditioning: Basic Circuitry Flashcards
Once you’ve picked a task to assess what happens in the brain when we learn something what two things do we do?
- determine which parts of the brain are involved- where we look for synaptic changes that underpin the learning
- Establish circuitry
Why do we need to establish circuitry?
To establish where the sites of plasticity are
What are the three basic techniques for establishing neural circuitry?
- Neuroanatomy
- Electrophysiology
- Manipulation
In terms of neuroanatomy, what do we want to find out?
What connects with what
For collections of neurons what two things do we what to know about them?
Where do they project?
Which regions project to them?
What is an issue with looking at neuronal projections in neuroanatomy?
Usually cannot see them directly, even with a microscope because there is a mass of cells and fibres
How can we establish neuronal circuitry by neuroanatomy?
By using “tract-tracing” methods
What is retrograde transport?
Inject Substance X into a nucleus around a set of cell bodies
The substance you choose will be one that then gets transported back up from the synapse, up the axon and into the cell body
This is the opposite direction from the way the cell usually functions so its retrograde/backwards transport
What is anterograde transport?
Inject Substance X into the cell and it shows you where the cells axons end in synapses, it is transported to terminals
This is known as forward/anterograde transport
What can you use retrograde/anterograde transport for?
To create demonstrations of connectivity in the brain
What does electrophysiology tell us about basic circuitry?
What signals are carried by neural connections
Electrophysiology can establish neuronal properties e.g. by recording from neurons while a sensory stimuli is presented
e.g. the orientation specificity of neurons in the visual cortex
What does manipulation tell us about circuitry?
What happens when a particular region is removed, inactivated temporarily or stimulated
Try to predict effects from anatomy anf physiology
Standard systems neuroscience
The circuitry is broken down into 5 parts, what are they?
- Unconditioned reflex pathway
- Output of anterior interpositus nucleus controls the conditioned response
- Output of HVI (eyeblink region of cerebellar cortex) controls the anterior interpositus nucleus
- HVI (eyeblink region) receives information about conditioned stimulus via mossy fibres
- HVI (eyeblink region) receives information about unconditioned stimulus via climbing fibres
The unconditioned reflex circuit is a basic three neuron arc reflex, what are the three neurons?
Sensory neuron (in gasserian ganglion)
Interneuron (in trigeminal nucleus)
Motoneuron (in abducens nucleus)
Describe the unconditioned reflex circuit.
Input is from the cornea or the skin around the eye (periorbital UCS)
It travels in the trigeminal nerve and it’s cell bodies are located in the gasserian ganglion
This projects to the spinal trigeminal nucleus oral subdivision (synapse 1)
Then projects to the accessory abducens nerve (synapse 2) and travels out in the abducens nerve to the retractor bulbi muscle that pulls the eye back in the orbit
What are the implications of the unconditioned reflex circuit only being a 3 neuron reflex arc?
Means there are only three synapses
This makes it very fast
Overall takes 20milliseconds, where 16milliseconds is mechanical (muscle movement)
Quick to protect the eye- makes sense
What does the retractor bulbi muscle do in the unconditioned reflex circuit?
Pulls the eye back into the orbit
The the nictitating membrane slides passively over the eyeball
What shows us that the output of the anterior interpositus nucleus controls the CR?
If you damage the relevant area then you remove the CR with little effect on the UCS
If we lesion the deep cerebellar nuclei we block conditioning and any pre-existing conditioning is abolished and cannot be re-learned
Lesions confined to the anterior interpositius nucleus are effective so they must control the CR
What did Steinmetz et al. (1992) find?
A properly placed lesion blocks retention of previously learnt conditioning, and prevents any new learning
How does the CR command signal get from the anterior interpositus nucleus to the accessory abducens nucleus (which controls the retractor bulbi muscle)?
Doesn’t project direct to accessory abducens, but via a midbrain structure, the (contralateral) red nucleus
What does the red nucleus do?
Its a motor coordination nucleus
Red nucleus transports the CR command to the accessory abduencns nucleus which controls the retractor bulbi muscle, so it’s suggested that the red nucleus is relevant to coordinating eye closure
How can we confirm the red nucleus is involved in transporting the CR command signal?
Through it’s inactivation by muscimol
If we block the red nucleus by the Gabba antagonist muscimol, it blocks performance of the CR
What is the conditioned reflex pathway: cerebellar cortex?
All output from the cerebellar cortex goes through the deep cerebellar nuclei
-so main input to interpositus nucleus is from cerebellar cortex
Different regions of cortex project to different parts of deep cerebellar nuclei
The particular region that projects to anterior interpositus and is concerned with eye-blink is in the hemisphere of lobule VI (HVI)
Lobule HVI inhibits the interpositius nucleus which then releases activity in the red nucleus and then in the accessory abducens nucleus
So it inhibits, and the accessory abducens nucleus activates the retractor bulbi muscle and that produces the response
What is the evidence for the role of the cerebellar cortex in the conditioned response pathway?
Lessions here cause selective loss of the CR (Yeo, 1987)
