Physiological organisation and control of behaviour Flashcards
Describe the NS structure of Aglantha digitale jellyfish.
Simple radial NS consisting of net around velum, rings and giant axons.
Condensed not diffuse nerve net.
Giant inner nerve ring innervated by pacemaker neurones - coordinats subumbrellar epithelum contraction via 8 radial symmetrical giant motor neurons (MG).
Hair cell synapses with ring giant axon –> Synapsed onto MG rootlets (inner ring) –> electrical coupling to MG axons (8 in radial fashion)
Describe the cycle of slow feeding behaviour in jellyfish.
Sinks passively with tentacles extended to fish.
Exhaust food supply so swimming resumes but realises it’s upside down - righting behaviour at bottom of feeding cycle to tilt, swim upwards pulsing every 2 seconds until it encounters a new food source which inhibits swimming again.
Cycle restarts - tilts to drift downwards upside down
Describe the neuronal and muscular process of slow feeding behaviour.
Subumbrellar contracts in uniform manner, expelling water to propel medusa.
Relaxation before cycle restarts (hydrostatic, reforms original shape).
Gap junctional coupling within rootlet allows synchronisation of MGF via ring which couples pacemakers by reciprocal synapses to “entrain” them.
Muscle contracts in coordinated manner.
Pacemaker must be synchronised to drive MG oscillation.
MG unmyelinated nerves with conduction velocity of 0.4m/s compared to 50-100m/s in vertebrates.
How do jellyfish know when to change their orientation?
8 Statocysts consisting of statoliths (mineralised cell mass) surrounded by sphere of mechanosensitive cells with setae.
Statolith drops down due to gravity and deflects setae on specific cells which send information to sensory neurone.
Not fully understood but it somehow knows to tilt towards the food.
Describe the ionic basis of the slow feeding behaviour in jellyfish.
Slow rising and decaying EPSP from pacemaker on motor giant rootlets via MGR ring
Activate low threshold ~-50mV, Ca2+ AP
Small amplitude 30mv: Depolarisation (T-type Ca2+ channels, low threshold transient), Repolarisation (fast V-sensitive K+ channels curtails AP height.)
Small amplitude leads to slow propagation of Ca2+ dependent AP along MG slow swimming.
Not full depth of myoepithelium contracts so contraction is weak
Too small to evoke a Na+ spike
If we were to increase this potential and actually cross the threshold for action of sodium channels, you’d get a standard sodium type AP
Describe the neuronal and muscular process of escape behaviour in jellyfish.
Pulse every 83ms (12Hz), travel 1m in 5 seconds
Uses same axon for two different purposes
Hair cell input to RG. Due to small size of RG, the furthest MG is reached by an AP within 6msec of hair cell EPSP, bypass pacemaker. Sufficiently fast that the firing of MGs are almost synchronised.
Contact with object/predator/water vibration.
Detection of hair cells-mechanoreceptors on tentacle base which interact with giant ring axon (fast).
Stimulated outer giant ring axon which feeds MGR
Ring coordinates subumbrellar myoepithelium contraction via giant motor neurones.
Short chemical synaptic delay (amongst fastest in invertebrates).
AP goes around outer ring, synapse onto giant ring, different type of AP to pacemaker causing different nerve conduction velocity than slow feeding (now 3m/s in fast escape due to different ion channels).
Contraction 10x faster but why beat faster 0.5 to 12 Hz
Speed up pacemaker unlikely, dominated by other processes.
Timing possibly set by refractory period of escape APs in RGA. If firing at maximum frequency as permitted by refractory, such that Contraction-expulsion and relaxation-refilling of umbrella limited by physical constraints of muscle contraction/relaxation and mesoglea morphing.
Pulse 10 or 15 times before going back to slow swimming (when it essentially runs out of sodium channels)
Volley of APs by stimulating hair cells.
Describe the ionic basis of escape swimming behaviour in jellyfish.
Very Fast rising and decaying EPSP from hair cells on MG
Activate high threshold ~-33mV Sodium AP
Large amplitude ~100mV AP: greater electronic speed and conduction; repolarisation high threshold V-sensitive K+ channels
Fast propagation along motor giants fast swimming
Full depth of myoepithelium contracts so contraction is strong
Describe the feeding behaviour of the long veined squid.
Lateral fins have rhythmic undulations to propel squid forward.
Prey caught by extended hydrostatic tentacles.
Mantle volume filled and primed with sea water in readiness for escape response.
Mantle made up of hydrostatic skeleton.
Describe the escape behaviour of the long veined squid.
Circular mantle muscles undergo spontaneous contraction forcing water through siphon orifice, pushing squid forwards, mantle relaxes, and water is drawn in through inhalant openings, contracts again and repeats 3-4 times
Describe the brain of long veined squid.
Sensory information relayed to 1st order giant located in magnocellular region of brain.
Fused ring of neurones.
Coordinates input, rapid synchronised output to 2nd order giants.
Largest goes to synapse with 3rd order giants (located in stellate ganglia, bilateral), others control siphon etc.
Each ganglia has multiple 3rd order giants: motor neurones of differing diameters: Fattest/fastest/innervates furthest muscle (thinnest is opposite) - all reach and contract at same time
Describe the flying squid escape response.
Leave water and have less resistance so move faster and further.
Reaches heights of up to 25 at 11m/s - once airborne glides using fins.
Due to difference in refractive index between air and water, predators location of prey misplaced (inverse of apparent depth)
Describe the response of earthworms to mechanical stimuli.
Medial nerve 0.07mm D - 23m/s apical escape response.
Lateral nerve 0.05mm D - 2m/s posterior escape response.
Mechanical stimulus to sensory cell in skin –> sensory interneuron –> median giant fibre.
Interneuron can enhance response by acting as a central pattern generator to lead to repetitive stimulation and firing of giant fibre.
Weak stimulus, lateral fibre fires 1 AP – head might move a bit
Medium stimulus, lateral fibres fire low frequency APs – may contract a bit more
Strong stimulus, CPG activated lateral fibre fires high frequency APs, muscle contracts – full escape response
Describe the response to mechanical stimuli in crayfish.
Receptors in tail
Sensory neurone
Lateral giant interneuron (LG)
MoG
Flexor muscle
Describe feeding behaviour of acorn barnacles.
Open at high tide, closed at low tide.
Cirri controlled by adductor muscle are pushed out at high tide to attempt to grab something and bring to mouth.
Median photoreceptor –> 1st order photoreceptor axon –> 2nd order I-cell in ganglion –> COnnective –> Ventral ganglia site of CPG that drives rhythmical activity for feeding.
Describe what is meant by the shadow reflex.
Normal response to light: MPR stimulated, release NT, inhibiting excitatory synapse, no response.
When a fish swims over and casts shadow, MPR no longer stimulated, hyperpolarisation, decreased Ca2+dependent histamine release from synapse, I-cell no longer hyperpolarised so depolarises, increases excitatory output, increases glutamate secretion, increased output to motor neurones, adductor muscle contracts, withdrawal of cirri and closure of the plates, overriding feeding behaviour.
