Behavioral Neuroscience Summers

Fear Afferent Path for Fear Amygdala Fear Conditioning

Afferent Shock (US) Pathway Efferent Output Neuromuscular Production of Fear Potentiated Startle Integration of Fear Conditioning

ACh BDNF Gephyrin Enkephalin

Substance P Fear figures Fear Conditioning Circuitry end     Acronyms/Abbreviations

Fear Conditioning
VIII. Neuromuscular Production of Fear Potentiated Startle  			

	A. ACh secreted from motor neuron projecting from the ventral  
	   horn of the spinal cord to quadriceps femoris
	   and gastrocnemius muscles 

		1. 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) 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) conformation change in tropomyosin 
						    uncovers binding site for myosin on actin						        


							(i) myosin binds to actin	  	   

							(ii) conformational change:
							     myosin heads bend 
							     (muscle contracts)

							(iii) ATP allows unbinding of 
							      myosin from actin


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


	C. Useful Movement: rapid sequence of muscle contractions that begin in the 
	   neck and back and extend down to the fore and hindpaws. 

		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 Startle Reflex
				
					(1) neck, back and limb muscles contract with 
					    high to maximum force
						
					(2) termination of descending axons across
					    a wide range of spinal cord levels
						produce a ballistic muscular ressponse

			e. ñ firing rate ñ force

				i. more effective summation  (see C 1)