Locomotor systems in lower vertebrates Flashcards
What are the advantages of studying locomotion in lower vertebrates?
- Simpler behaviour
- Simpler circuit (fewer neurons and classes of neuron)
- Higher accesibility
- Important mechanisms conserved
What are the characteristics of a 2 day old frog tadpole?
- No gut, circulatory system or respiratory system
- Only capable of detecting changes in light intensity
- Most neurons present related to locomotion
What are the characteristics of tadpole swimming?
- In response to touch
- Body alternates 10-25 Hz
- Contraction spreads from head to tail
- Activity can be sustained for many seconds
What components related to lomotion are located in the brain of the tadpole?
- Basal ganglia
- Mesenphalic locomotor region
- Lateral hypothalamus
What components related to lomotion are located in the brainstem of the tadpole?
- Reticulospinal neurons
- Cerebellum
What are the 4 types of CPG in the tadpole spinal cord?
cIN - commisural interneurons
aIN - ipsilateral ascending interneurons
dIN - ipsilaterally descending interneurons
mn - motor neurons
What is the function of cINs?
- Shown to have regular bursts synchronous with ipsi motor nerve discharges with one action potential per cycle
- Arrival of cIN spike on opposite side produces mid-cycle inhibition
- Believed to function as a left-right coordinator
What are the functions of dINs?
- Excite all CPG neurons on the same side
- Earliest firing neurons are dINs in the caudal hindbrain
What is the rebound hypothesis?
- Phasic EPSCs are too late for driving dIN firing
- Therefore they background depolarisation and phasic inhibition to fire
- dINs cause NMDAR excitation on to themselves while phasic cINs produce phasic inhibition
What evidence supports the rebound hypothesis?
- Optogenetic silencing of half the CNS using archearhodopsin (yellow light)
- Stimulation halts swimming
What are the advantages of using a lamprey model system?
- Readily available
- Easy to keep alive
- Comparable to tadpole swimming circuit
How does the lamprey motor system differ to that of a frog tadpole?
- Sensory stretch receptor neurons contribute to swimming rhythm generation
- Motor nerve bursts are longer than those of tadpoles with more occuring
What is the structure of the lamprey stretch receptors?
2 types: inhibitory and excitatory
- located within the spinal cord
- respond to bending of the spine
What mechanisms are behind the initiation of bursts in the lamprey CPG?
- v-dependent NMDA activated by stretch receptors
- LVA allows Ca2+ to enter cell
- Once the threshold is reached Na+ triggers action potential
What mechanisms sustain bursts in the lamprey CPG?
- Background excitation
- Phasic eIN excitation
