Behavioral Neuroscience, lecture on Efferent-Motor output in startle responses in fish
USD Department of Biology
Behavioral Neuroscience
C-Start Behavior
Fundamentals of Neurocircuitry
Senstory Stimulus for Startle
Mauthner Cells - Response Gating
Mauthner Efferent output
Neuromuscular Function
Integration of C-Start Circuitry and Behavior
Transmitter Figures
Figures of C-Start Behavior
C-Start Circuitry
Fish Escape Behavior
XI. Integration of Circuitry and Behavior 			

	A. Timeline - Stage 1
		1. Predator approaches = stimulus = time 0
			a. pressure and sound waves hit hair cells
				i. ear, vestibular, lateral line
		2.  from 0 - 5 ms
			a. bend hair cells
			b. AP travels down VIIIth Cranial Nerve
			c. AP transferred via gap junctions, electrical synapse
			   to Mauthner cell
			   	i. 0.2 ms
			d. Glu portion of mixed synapse potentiates signal
				i. 2 ms
			e. commissural inhibitory interneurons depolarized
				i. AP reaches contralateral Mauthner
				ii. Gly/GABA synapse
					(1) Gly-R & GABAA receptors
			f. Mauthner depolarization, and AP begins
				i. IPSP in contralateral Mauthner bocks AP
		3. from 5 - 6 ms
			a. Mauthner AP reaches recurrent branch
				i. fires commissural inhibitory interneuron
					(1) terminates at peripheral axon cap
			b. Mauthner stimulates additional inhibitory interneurons
				i. terminating on motor neurons
			c. Mauthner AP reaches motor neurons
				i. ACh - Nicotinic R
		4. 6 - 7 ms 
			a. motor neuron AP
			b. ACh released at neuromuscular junction
				i. binds Nicotinic receptors
			c. End Plate Potential (EPP) generated
			d. GABAA/Gly-R inhibition of Mauthner
				i. limiting duration of AP to 1 ms
			e. GABA inhibits ipsilataral motor neurons via IPSP
				i.  from commissural interneuron
				ii. ipsilateral muscles (convex/outside of C) remain relaxed
		5. 7- 13 ms
			a. 6 ms delay due to muscle activation
				i. EPP travels down sarcolemma, down T-tubules
				   releases Ca++ from sarcoplasmic reticulum,
				   Ca++ binds tropinin, tropomyosin moved,
				   myosin heads bind, begin to contract muscle
			b. first shortening of the contractile apparatus
			c. shorter delays for all actions rostrally
				i. 8 ms - first movement 
					(1) deviation of the head
		6. 8 - 21 ms: muscle contraction
			a. rotation around center of mass
				i. C-bending of fish's body
					(1) from head to tail
		7. 21 - 39 ms
			b. gradation into stage 2
				i. activation of neural mechanisms for stage 2
				ii. not monosynaptically stimulated by the 
				    Mauthner Neuron
	B. Command Neurons - an early concept
		1. envisioned as a neural 'decision making' cell
			a. trigger a complete behavioral act
		2. receive convergent sensory and integrative input
			a. output to 'pattern-generating circuits'
			b. \ linked to another early concept:
			   the Central Pattern Generator (CPG)
		3. 'Stereotypic' response patterns
			a. also linked to the ethological idea of the 'Fixed Action Pattern'
				i. behavioral module with a high degree of temporal 
				   and spatial constancey
					(1) stereotypy
			b. e.g. Mauthner firing correlates with electromyographic (EMG)
			   output of trunk musculature in escape behavior
		4. implied that the CNS contained arrays of command cells and CPGs
			a. dedicated to triggering fixed action patterns
			b. behavior from hierarchical serial sequence of neural signals
			c. decision to execute = firing of command cell
			d. each command cell should be 'necessary' and 'sufficient'
			   for triggering the behavior
			   	i. necessary - blocking cell firing prevents behavior, always
				ii. sufficient - artificial activation of the command cell  
				    should result in complete expression of the behavior
					(1) proposed by Kupfermann and Weiss in 1978
		5. contrasts with distibuted circuit connected in parallel to output ciricuit
			a. pattern of activity in group of cells trigger behavior
			b. no single cell unambiguously triggers output
		6. Mauthner cells stimulate a rapid stereotypic response
			a. M cell might be the prototype of a “command neuron” 

	C. Parallel Networks and Escape
		1. M cell ablations do not abolish C-Start Escape
			a. Delay its onset by a few ms
			b. but Mauthner cell activation alone can trigger stage 1
				i. resulting behavior is less variable than normal behavior

		2. C-start almost never occurs without firing the Mauthner cell
			a. Mauthner neuron always fires first
				i. before all other brainstem neurons
		3. \ Mauthner cells are “command-like” neurons
		    participating in a parallel “brainstem escape network” 
		4. Parallel escape network finely regulates the escape trajectory

			a. made up of M cell homologs 
				i. MiD2cm and MiD3cm
				ii. other descending reticulospinal neurons
			b. Mauthner homologs can fire bursts of action potentials
				i. Mauthner cells only fire 1 AP
			c. Mauthner homologs also activate spinal circuits
				i. cause the trunk musculature to contract 
				ii. contribute to the normal behavior
				iii. activated with a longer latency by sensory inputs
				iv. higher firing threshold than the Mauthner cell
				v. \ longer delay 
				   when compensating for the absence of Mauthner cells
	D. Time line - Stage 2 - using some back estimates
		1. 0 - 6 ms 
			a. initial sensory input from VIIIth cranial nerve
			   also stimulates MiD2cm and MiD3cm homologs
			   	i. via mixed electrical/Glu synapses
				ii. contribute to stage 1
		2. 6 - 29 ms (even to 82 ms see below)
			a. additional stimulation of reticular network neurons
				i. from the Mauthner neuron?
					(1) direct stimulus of Mauthner alone reduces
					    variability of stage 2 angle  
				ii. from auditory, balance, lateral line sensory neurons
					(1) from auditory & vestibular nuclei
				iii. receive input from rostral (‘higher’) motor systems 
				     and sensory input from the visual midbrain 
			b. reticulospinal system is the major motor network 
			   connecting higher motor areas to the spinal 
			   pattern generating circuits (fish, amphibians, birds)
			c. MiD2cm and MiD3cm cells depolarize
				i. MiD2cm and MiD3cm type homologs can fire bursts of APs
					(1) no recurrent inhibition
					(2) not limited to single 1 ms AP of Mauthner
			d. AP to motor neuorns
			d. ACh/Nicotinic R synapses?
				i. homologs should have the same transmitter
		2. 9 - 32 ms motor neurons begin to depolarize
			a. AP to muscle cells
			b. neuromuscular junction - ACh/Nicotinic R
			c. EPP generated
		3. 10 - 33 ms - EPP stimulates muscle activation
			a. mechanical delay
		4. 39 ms Stage 2 movement begins
			a. variable contractile results
				i. not stereotypic
				ii. although still producing a C-bend
					(1) and forward propulsion
			b. ensures unpredictable angle of escape
		3. 82 ms full startle-escape behavior is complete
			a. fish swims away
			b. bilateral undulatory movement
				i. controlled by reticulospinal network