Behavioral Neuroscience Summers

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

text:Kandel pages 180 Glu DA ACh Transmitter Figures Figures of C-Start Behavior C-Start Circuitry end   Acronyms/Abbreviations

Escape Behavior
VII. Response Gating - Mauthner Cells 	

	A. Pair of large neurons in the caudal midbrain
	
	
		1. found in lamprey, sharks, teleost fishes, and amphibians
			
			a. similar to identified neurons in invertebrates
			
				i. functional analogy to indentified large axon
				   systems that mediate escape in invertebrates
				
				ii. 'identified neuron concept'
	
			b. much larger than adjacent neurons
			
				i. 1.0 mm length (goldfish)
				
				ii. 0.1 mm diameter

			
	B. always have the same characteristic morphology
		
		1. T shaped: 2 lateral dendrites
		
			a. coming together at an axon
		
		2. axons extend to the spinal cord
			
			a. axons decussate
		
				i. each Mauthner neuron controls the contralateral side	
			
			b. innervate motor neurons
			
				i. e.g. right Mauthner stimlates left side
	
				
	C. also the same neurophysiology, receptor mechanisms,
			   and synaptic connectivity
			   
		1. 1o input VIIIth cranial nerve

			a. mixed Bidirectional synapse
						
				i. gap junction/connexin electrical centrally
						
				ii. chemical-Glu around edges
	
						
			b. complex & specialized regional input: 
				
				i. Glu-AMPA everywhere 
	
					
				ii. Gly (glycine, G) and GABA throughout
					
					(1) Gly/GABA inhibitory interneurons
		
				
				iii. Glu-NMDA, DA-DA1 only on distal dendrites
					
					(1) enhanced by NO synthesis
						
						(a) NOS stimulated by NMDA activity
						
				iv. colocalization of somatostatin (Glu or GABA)
			   
			   	v. 5-HT (serotonin) inputs not on dendrites
	
				 
	D. Part of a Reticulospinal Network
			
			
	E. Axon cap/hillock

		1. surrounds the base of the axon: specialized morphological adaptation
 
			a. determines the character/quality of startle response
			
				i. Absent or less developed cap
				
					(1) withdrawal response, 
					    but less like a C start (eel like fish)
				
				ii. goldfish have a prominent cap

			b. divided into peripheral and central regions
		
		3. Fibers that enter the Central cap excitatory
		
			a. integrates inputs & triggers the common C start response

		4. Fibers that enter Peripheral cap 

			a. reciprocal inhibitory firing: 
			   limits length of action potential
			
			b. inhibition: only one M-cell fires at a time	
	

	F. Large size of neurons results in large/fast response
	
		1. Mauthner cell Action Potential (AP) lasts 1 ms
	
		2. 3-5 ms after acoustic/somatosensory stimuli
	
		
		3. 2 ms for synaptic transmission to spinal motor neuron
		   + motorneuron AP + neuormuscular junction
	
		
		4. entire signal 6-8 ms, entire behavior < 100 ms
	
		
		5. large size also facilitates speed of inhibition: more important
		
			a. mediate very fast inhibition of ipsilateral or contralateral Mauthner neuron
			
				i. reciprocal inhibitory neurons impinging on axon cap
				
					(1) produce discrete pulses of muscle stimulation
				
				iii. inhibitory signals block contralateral movement
			
			b. blocks contraditory command by non-Mauthner cell
	
		
	G. What is Mauthner functional output?