Behavioral Neuroscience Summers

Sexual Behavior Reflexes Sensory Reception - Tactile Receptors Spinal Synapses Afferent Spinal Pathways

Hypothalamic Gating Efferent Motor Cascade Neuromuscular Production of Lordosis Integrated Story of Lordosis VIP Substance P Estrogens

5-HT GnRH ACh Lordosis figures   Sensory Receptors Spinal Cord   Lordosis Circuitry end     Acronyms/Abbreviations

LORDOSIS
XIV. Neuromuscular Function  	

	A. ACh secreted from motor neuron projecting from the ventral horn 
	
		1. Lumbosacral spinal cord 
		
			a. to lateral longissimus and transversospinalis muscles 

		2. may be inhibited by spinal GABA interneurons
		
			a. acting on GABAA receptors

	B. ACh binds to Nicotinic receptors at a neuromuscular synapse

		1. stumulates action potential (= end-plate potential or EPP) 
		
		2. Opening transmitter-gated Na+, Ca++ and K+ channel 
		   (a large diameter channel)


			a. EPP passes 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: bilateral dorsiflexion of entire vertebral 
	    column by lateral longissimus and transversospinalis 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

			d. motor units are recruited in a fixed order

				i. weakest to strongest

					(1) allows fine motor actions

				ii. not necessary for Lordosis
				
					(1) deep back muscles contract with maximum
					    force to support male
						
					(2) termination of descending axons across
					    a wide range of lumbosacral spinal cord levels
						produce a ballistic muscular ressponse

			e. ñ firing rate ñ force


				i. more effective summation       (see C 1)