Behavioral Neuroscience Summers

Crayfish Escape Behavior Crayfish Neuroanatomy Fundamentals of Neurocircuitry Sensory Afferent input for Tail Flip

Gating the Full Flip Gating the Tail Flip for Upward Thrust Parallel Gating of Crayfish Escape Motor Neuron output for Crayfish Escape Integration: Crayfish Escape

Acetylcholine ACh GABA 5-HT

Crayfish figures Crayfish Escape Neurocircuitry end     Acronyms/Abbreviations

Escape - Crayfish
XI. Motor Neurons and the Crayfish tail-flip Escape 	

	A. Two sets of motor neurons

		1. Motor Giants (MoG 1-6)

			a. bilateral neurons for each abdominal segment

			b. required for rapid sterotypical single flip escape
            
				i. Two types of Escape

					1) Full-flip Retreat

					2) Upward Thrust

		2. Fast Flexor motor neurons (FF 1-6)

			a. multiple segmental neurons

			b. recruited for stereotypical and flexible tail-flips

				i. required for adjustments to direction, speed or duration

					1) required for swimming


	B. MoG

		1. Stimulated directly by MGs or LGs

		2. MGs stimulates all MoGs

			a. LGs stimulates only rostral MoGs (MoG 1-3)

		3. MGs and LGs commensurately stimulate FFs

			a. MGs stimulates all FFs (FF 1-6)

			b. LGs stimulate only* rostral FFs (FF 1-3)

		4. MoG output produces only one tail flip


	C. FF motor neurons

		1. Stimulated by a parallel network of premotor interneurons
        
 			a. premotor interneurons stimulated by Non-Giant (NG) neurons

		2. Also stimulated by MG or LG à SG connections

			a. SGs à premotor

				i. premotor à FF

			b. SGs à FF

			c. LG-SIs inhibit FF 4-6
                           
		3. Fast flexors alone produce slower tail flips
 
 			a. FF enhance the action of MoG fast tail flips
        
	D. ACh secreted from MoG and FF motor neurons projecting from each ganglion
	   to the longitudinal, oblique and circumferential muscles 

	   
	E. ACh binds to Nicotinic receptors at a neuromuscular synapse

		1. stumulates action potential (= end-plate potential or EPP) by opening
		   transmitter-gated Na+ and K+ channel (even Ca++ passes; a large
		   diameter channel)


			a. along  sarcolemma and into transverse tubules


				i. opens voltage gated Ca++ channels in
				   sarcoplasmic reticulum  						       


					(1) binds to troponin


						(a) conformational change in  
						    tropomyosin uncovers binding  
							site for myosin on actin						        


							(i) myosin binds 
							    to actin	  	   


							(ii) conformational change 
							     (muscle contracts)

							(iii) ATP allows unbinding
							      of actin


				ii. Ca++ATPase returns Ca++ to cisternae
				    of sarcoplasmic reticulum


	F. Useful Movement: contraction of longitudinal abdominal hemi-segmental muscles

		1. Contractile force summates for each muscle cell

			a. MoG + FF produce maximal summation and contraction
            
				i. in just one flip
            
			b. for FFs summation requires repeated EPP's (before fiber relaxes)

				i. \ requires Ca++ restoration to cisternae


				ii. \ contractile force depends on initial length	     
				

		2. the Nervous System Grades the force of muscle contraction

			a. single motor axon innervates a single muscle fiber (cell)

			b. one motor neuron (many axons) innervates a number 
		 	   of muscle fibers = Motor Unit

				i. smallest functional unit

					1) magnitude of contractile function depends 
					    on innervation ratio (fibers/neuron)


			c. graded force is dependent upon motor unit recruitment
			
				i. degree of shortening

				ii. local direct motor innervation
				
					1) segmental gradation in FF tail flips
				
			d. motor units are recruited in a fixed order

				i. weakest to strongest in FF

					1) allows fine motor actions

					2) integrated changes during tail flip swimming
	
						a) changes in speed or direction

					3) all abdominal motor units are recruited under MG drive
                    
					4) all rostral abdominal motor units are recruited under LG drive
                                            					
			e. ñ firing rate  ñ force

				i. more effective summation       (see C 1)

					1) for FF modulated tail flip swimming
                    
				ii. not pertinent for MG or LG stimulation
                
	G. Sudden rostral sensory stimulation produces MG mediated full tail flip
    
		1. sudden caudal stimulation produces LG mediated upward thrust tail flip                   

		2. MG full tail flip or LG upward thrust are often followed by NG swimming
        
			a. making the escape response more variable and \ more adaptive