CPG circuit modulation + flexibility

rhythms are not fixed programs

• flexibility as animals don’t always behave the same way

• circuit output can change without changing wiring

• stable connectivity can support multiple activity patterns

what can vary in a rhythmic circuit

pattern expression - which activity the circuit controls

phase relationships - coordination between circuit elements

firing patterns - tonic vs bursting

frequency - the speech at which the rhythm runs

neuromodulation

• operates on slow timescales (s to hrs)

• alters neuronal excitability and intrinsic properties

• shifts the operating point of the circuit

intrinsic neuromodulation (IN)

from neurons within the CPG

adjusts network activity

extrinsic neuromodulation (EN)

from neurons outside the CPG e.g. higher order neurons, sensory neurons, or other CPGs

controls CPG activity and mode

IN example - tritonia swimming

modulation (serotonin) reinforces and stabilises the existing oscillatory network

• stabilises pattern timing

• extends swimming bouts

EN example - lobster digestion (STG)

STG neurons do not provide significant neuromodulatory input, these are instead primarily descending from higher ganglia:

• commissural ganglia

• oesophageal ganglion

lobster STG (pyloric rhythm)

network oscillator CPG with intrinsic bursting generating a 3-phase rhythm

AB → drives CPG network, intrinsically bursting

PD → dilated pylorus, open to admit food, conditionally bursting

LP → begins pylorus constriction, follower neuron

PY → finishes pylorus constriction, follower neuron

lobster STG (gastric mill rhythm)

half centre CPG circuit generating a 2-phase rhythm emerging from:

• reciprocal inhibition

• intrinsic properties of certain neurons

• descending neuromodulatory drive

STG - rhythm generation, interaction + flexibility

• rhythms can occur independently or together (interact through shared neurons)

• pattern expression is flexible (same circuit produced different rhythms and phase relationships)

• modulatory state determines (which neurons participate, their firing mode, how rhythms interact)

• degeneracy ensures robust circuit (similar motor outputs can arise from different combinations of intrinsic properties and synaptic interactions)

hierarchy of behaviour

many behaviours are mutually incompatible (e.g. crawl vs swim) and therefore organised into a behavioural hierarchy but controlled by a single circuit

likelihood that a behaviour is expressed depends on context and internal state

mechanisms of behaviour switching

• some circuits can generate multiple motor pattern

• in others, distinct autonomous CPGs exist

  • behaviour switches by selecting between circuits