Summers

Neurobiology

text: Principles of Neural Science - Kandel, Schwartz and Jessell: Read pages 1114-1129 for this lecture end

 XV. Synaptic Connections 			back to XIV. Synapse Formation


	A. Synapses reflect wiring necessary to process sensory
	   information into coherent activity


	B.  Synaptic connections are refined by Sensory Experience
	    and Social Interaction


		1. dependent upon specific interactions with the environment


			a. learning


			b. experiences have more profound effects in early
			   postnatal development than adulthood


		2. deprivation or isolation result in abnormal development


			a. therefore abnormal behavior


	C. Critical Periods


		1. at specific points in development neurons become
		  irreversibly differentiated


			a. results in critical periods for acquisition of behavior


				i. e.g. sexual identity


		2. different regions of the brain have different critical
  	       periods of development


			a. do not have sharp time boundaries


		3. sensory deprivation  during critical periods result in
		   permanent deficits in perception

		   
			a. by structural/neuronal changes/loss in cerebral cortex


	D. Competition and Neuronal selection (synapse formation)


		1. neurons from developing organs may compete 
		   for synapse space


			a. intermingled axonal growth


				i. become arranged in a regular pattern


					(1) via selective elimination


						(a) synapse retraction


				ii. e.g. development of optic innervation


		2. random differences in density may confer a competitive
		   advantage


			a. cooperation between clustered fibers allows growth


				i. spreading to adjacent cells


				ii. fibers with the highest density retain connections


			b. competition segregates fibers


				i. fibers with less extensive contact retract


		3. organization of areas of the brain are derived from
		   competition and cooperation of developing afferent
		   fibers over one set of postsynaptic cells


			a. e.g. optic tectum


	E. Cooperation requires Synchronous activity


		1. spontaneous activity may have instructive function
		   in development


			a. resulting synchronous excitation of target strengthens
			   synapses when fibers are active together


			b. synapses are weakened in inactive fibers


		2. Coincident activity in pre- and postsynaptic elements
		   leads to strengthing of the synapse


			a. Neurons need activity


	F. Survival of neurons depends upon activity, synaptic connections,
	   postsynaptic activity, target cell production of neurotrophins
       and growth factors


		1. Environmental stimuli are at the root of all those actions

XVI. Neurotransmitters

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