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
III. Afferent Pathways to Fear  			back to  Fear Behavior 

	A. Multisensory input possible
	
		1. Visual via optic nerve
		
		2. Auditory via auditory nerve
		
			a. vestibular
		
		3. Smell via the olfactory and accessory olfactory nerves
		
		4. Tactile - spinal sensory input
		
		5. Nociceptive pathways - foot shock
		
		
	B. All roads lead to the thalamus in the diencephalon
	
		1. thalamus is a relay station for the cortex
			
			a. processes/distributes almost all sensory and motor information
	
				i. may regulate levels of awareness and emotional
				     aspects of sensory experiences via the cortex 
					 
				
	C. Fear conditioning paradigms often use visual CS signals
	
		1. Visual signals begin in the retina
		
			a. Glutamatergic neurons
			
			b. terminate in the lateral geniculate nucleus of the thalamus
			
				i. LGN thalamocortical cells
				
				ii. NMDA and AMPA receptors
				
				iii. modified by presynaptic 5-HT activity 
				      on 5-HT1 receptors

	
		2. LGN neurons project to the occipital lobe of the cortex
		
			a. 1o visual sensory cortex
			
				i. Glu neurons
				
				ii. other LGN neurons contain GABA/NPY 
				
					(1) interneurons
					
					(2) intergeniculate leaflet neurons that project
					    to the SCN (biological clock)
		
		3. LGN neurons also project to the limbic cortex
		
			a. perirhinal cortex
		
		4. LGN also directly projects to BLA (basolateral amygdala) and LA (lateral amygdala)
	
			
	D. Acoustic startle and auditory CS signals
	
		1. Acoustic startle begins with a sharp sound
		
			a. ear via auditory (nVIII) nerve 
			
				i. auditory nerve contains embedded cochlear root neurons (CRN)
				
					(1) specialized auditory neurons
					     associated with startle
					
						(a) short-latency auditory output
						
				ii. CRN are Glu neurons
				
					(1) NMDA and AMPA receptors
			
			b. CRN to PnC (nucleus reticularis pontis caudalis = caudal pontine reticular nucleus)

				i. giant neurons
				
					(1) high firing thresholds
					
					(2) broad frequency tuning
						
					(3) sensitive to changes in stimulus rise time
						
					(4) sensitive to paired-pulse stimulation
						
					(5) repetitive acoustic stimulation: habituation
					
				ii. also receive input from the Central Nucleus
				    of the Amygdala (CeA)
						
					(1) via the deep superior colliculus
					
					(2) amygdaloid activity enhances PnC response
					    to acoustic stimuli
					
				iii. the connecting point for CS and US signals
				
					(1) sensorimotor interface
					
						(a) via reticulospinal tract
					
				iv. Balance provoked startle responses are mediated
				     mainly via the vestibulospinal tract
				
					(1) not the reticulospinal tract. 

			c. PnC reticulospinal tract to spinal cord
			
				i. Summation between vestibulospinal and reticulospinal
				    pathways mediating startle is proposed to occur in
				    the ventral spinal cord