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

Glu DA ACh Transmitter Figures

Figures of C-Start Behavior C-Start Circuitry Acronyms/Abbreviations end

Fish Escape Behavior
X. Neuromuscular Function  		

	A. ACh secreted from motor neuron projecting from the spinal cord
	   to the trunk, fin, eye, jaw and operculum muscles 

	   
	B. 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) Ca++ binds to troponin


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


							(i) myosin head binds 
							    to actin	  	   


							(ii) conformational change 
							     (muscle contracts)

							(iii) ATP allows unbinding
							      of actin


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


	C. Useful Movement: contraction of contralateral trunk muscles

		1. Contractile force summates for each muscle cell

			a. requires repeated EPP's (before fiber relaxes)

				i. for C-start escape this is occurs in Stage 2

					1) Mauthner AP is a single pulse
                    				
				ii. \requires Ca++ restoration to cisternae

				iii. \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. variable angles in C-start stage 2

				
			d. motor units are recruited in a fixed order

				i. weakest to strongest

					(1) allows fine motor actions

					(2) unlike C-start, more line fine fin movements

					
				ii. this is why the Mauthner cell does not directly stimulate 
				    stage 2
				
					(1) everything fires with one Mauthner AP
					
					
			e. ñ firing rate  ñ force

				i. more effective summation       (see C 1)