Behavioral Neuroscience, lecture on Leech and its behavior
USD Department of Biology
Behavioral Neuroscience
Leech Shortening
Sensory Stimulation of Shortening
S&R Sensitization
Motor Efferents
Neuromuscular Function
Leech figures
Shortening Neurocircuitry
end     Acronyms/Abbreviations
Sensitization - Leech
X. Neuromuscular Function 			back to  Motor Stimulation of Shortening

	A. ACh secreted from L, VE, & DE motor neurons projecting from each ganglion
	   to the longitudinal, oblique and circumferential 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) 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

	C. Useful Movement: contraction of longitudinal trunk muscles

		1. Contractile force summates for each muscle cell

			a. 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
			d. motor units are recruited in a fixed order

				i. weakest to strongest

					(1) allows fine motor actions

					(2) integrated changes during swimming
			e.  firing rate   force

				i. more effective summation       (see C 1)

XI. Integrated Story of Leech Sensitized Shortening