6. Basic Circuitry

Question 1

cerebellum: how does it connect the CS and US inputs with CR outputs?

Answer 1

3 techniques to study this:

1. neuroanatomy
2. electrophysiology
3. manipulation

Question 2

1. neuroanatomy

Answer 2

• what connects what with what
• for any connection of neurons, where to they project and what regions project to them?
• tract-tracing (anterograde and retrograde)

Question 3

tract-tracing (anterograde)

Answer 3

anterograde - around cell body, travels down axon to terminal buttons
- can use different colours to see where the cell bodies project substances to

Question 4

tract-tracing (retrograde)

Answer 4

retrograde - inject substance around cell body, substance is transported up through axons of a neuron
- usually transported back into the cell body of another neuron

Question 5

2. electrophysiology

Answer 5

physiology - what signals are carried by these connections?

• recording from the individual neuron while animal is presented with sensory stimuli, and/or performing a motor response (to establish their properties)
  e. g. orientation specificity for neurons in visual cortex

Question 6

3. manipulation

Answer 6

• what happens when a particular region is removed, activated or stimulated?
• try to predict effects from anatomy and physiology
• standard systems neuroscience

Question 7

circuitry

Answer 7

can be broken down into 5 parts:

1. unconditional reflex pathway
2. conditional reflex circuit
3. conditioned reflex pathway
4. conditioned reflex circuit: CS pathway
5. conditioned reflex circuit: US pathway

Question 8

unconditional reflex pathway

Answer 8

• periorbital shock (skin around eye) is input > unconditioned blink = output
• corneal stimulation (from air puff - register from sensory neurons) - cell bodies outside of the brain itself in the Gasserian gangloin
• after stimulation, information travels down trigeminal nerve V, synapsing onto the spinal trigeminal nucleus (STN) in the brainstem (first synapse)
• neurons in STN synapse onto neurons in the accessory abducens nucleus (second synapse)
• axons leave the nucleus and travel down abducens nerve VI (6) - retractor bulbi muscles
• UR delay = 2-3 ms (from corneal stimulation to accessory abducens nucleus) - 3 synapse reflex

Question 9

conditioned reflex circuit

Answer 9

conditioned response (shutting NM)

• Yeo et al., (1985) - lesions of deep cerebellar nucleus block CR
• how does signal get from anterior interpositus nucleus to abducens nucleus?
• goes via the CONTRALATERAL RED NUCLEUS (inactivation blocks CR)
• thought to coordinate the general face flinch

Question 10

conditioned reflex pathway: cerebellar cortex

Answer 10

where does input from the interpositus nucleus come from?
- LOBULE VI (HVI) of cerebellar cortex - inhibits interpositus nucleus whilst others are excitatory

• Yeo et al., (1987) - lesions here affect CR
• Heiney et al., (2014) - cortical output is tonically active - for NMR you need to silence cortical output
• he inhibited Purkinje cell firing through optogenetics altering eye lids response (duration, intensity, rise time)

Question 11

conditioned reflex circuit: CS pathway

Answer 11

where might the HVI get its input?

• Yeo et al (1985) - anatomy: relevant region of cerebellum receives from pons
• this connection is carried by MOSSY FIBRES .
• lesions of the pontine nucleus abolish conditioning
  can pair stimulation of pontine nucleus with US and get conditioning
• sound > pontine nucleus > mossy fibres > cerebellum: cortex and interpositus

Question 12

conditioned reflex circuit: US pathway

Answer 12

cerebellum also requires input about US - Yeo et al (1987)

• neurons in the spinal trigmental nucleus send a branch of axons to the inferior olive (base of brain below cerebellum)
• this sends climbing fibres to cerebellum
• Yeo et al., (1986) - lesions in relevant regions of inferior olive abolish conditioning and prevent learning
• Thompson and Krupa (1994) - can use electrical stimulation of climbing fibres as the US = conditioning

> oral subdivision of STN send branch of axons > inferior olive > climbing fibres > cerebellum: cortex and interpositus