The Neuron
Important lessons I
•	Nature does not necessarily parse human behavior the way we do.
–	Aggression 
–	Depression
•	Causation is difficult
–	Localization is complex
–	Many systems must cooperate
•	Biology is not destiny
–	Not necessarily imply lack of free will
–	Culture and experience still important
Important lessons II
•	Levels of analysis
–	Biological representation is important 
–	But experiences are also represented biologically
–	Hence silly to argue about priority
•	Scientists can choose their level
–	Don’t need to understand neurotransmitters to do therapy
–	But levels do have important relationships
•	Lower levels set limits and constraints
•	Ad higher levels suggest problems
Important lessons III
•	Generally can’t change brain directly  
–	The fallacy of “training” or exercising the brain
–	But if experience works what good is implicating the brain
The Neuron
•	The basic unit of the nervous system
–	Receives signals from neurons or sensory organs
–	Processes information
–	Sends signals to other neurons, muscles, or organs
•	The brain contains one hundred billion neurons
•	Basic building block and similar across species
The Neuron
•	Sensory neuron
•	Motor neuron
•	Interneuron
•	Projection neurons
•	Glial cell
–	Fills the gaps between neurons
–	Supports and feeds neurons
–	10 times more glial cells than neurons
•	Brain circuit
Neurons in the brain
•	Projection neurons
–	Long axons extending from area to area
–	Usually idle without stimulation
–	Usually uses glutamate as main neurotransmitter
•	Interneurons
–	Short axons found in single area
–	Regulate information with a given level of a circuit
–	Often inhibitory and often tonically active
–	Use GABA as main transmitter
The Neuron: Structure
Neural Impulses
•	Resting potential is negative
•	Action potential
–	“Firing” – positive ions enter cell and this positive “charge” travels down from dendrite to axon
–	All-or-none law
–	Refractory period
•	Myelin
–	Acts as an insulator
–	Allows signal to travel more efficiently
–	Multiple Sclerosis (MS): deteriorated myeli
Receptors
•	Each neuron connected to many others
•	Excitatory
•	Inhibitory 
–	Why necessary
•	Continual firing leads to neuron death – strokes, epilepsy, perhaps Alzheimer’s
•	Fine regulation of system
The synapse
Neurotransmitters and Neuromodulators
•	Synapse
•	Synaptic cleft
•	Neurotransmitter
•	Neuromodulator
•	Specific 
•	Circuits
Action of neurotransmitters
•	Excitatory open “gates” for + ions to enter
•	Inhibitory 
–	allow – ions to enter 
–	making the interior more negative 
–	And thus creating a for a higher “barrier” to be overcome by + ions
•	This is one main advantage of chemical transmission
–	Allows for easy adding and subtracting of impulses
Getting rid of unused neurotransmitters
•	Can’t have stray chemicals
•	Several ways of regulating
–	Reuptake
–	Glial cells
–	Other chemical reactions
Neurotransmitters
•	Glutamate and GABA are most important
–	Fast acting
–	Effects short-lived
•	Others are generally modulators
–	Slower
–	More long-lasting effects
–	Hence tend to affect subsequent firing and affect ability of cell to be fired
Common modulators
•	Hormones
–	Cortisol
–	Testosterone
•	Peptides
–	Opiates – endorphins
•	Monoamines
–	Serotonin
–	Dopamine
–	Epinephrine
–	Norepinephrine
Receptors
•	Excitatory
•	Inhibitory
•	Reuptake
Drugs and the Brain
•	Endogenous cannabinoids
•	Cannabinoid receptors
•	Agonist
•	Antagonist
•	Selective serotonin-reuptake inhibitor (SSRI)
The brain is not a chemical soup
•	Talk about brain chemistry
•	But the same neurotransmitter can be involved in different functions
•	And in some cases the same function can be modulated by several different transmitters
•	Therefore typically drugs that act on a particular transmitter have many side effects
•	So a lack of serotonin does not cause depression directly, but only because certain circuits are inactive
Brain development
•	Hot area in psychology and neurosciences
•	Genes create proteins that create neurons and neurotransmitters
–	Some genes create enzymes
–	Some turn other genes on
–	Some create barriers that guide neural migration
–	Other create chemical adhesives
•	Chemical environment in uterus affects development
The development of neurons
•	At the peak the developing embryo creates 250,000 new neurons every minute
•	Shortly after their birth neurons begin to segregate
–	Homeotic genes produce proteins that guide migration
–	Eventually neurons in different parts of the brain and parts of different circuits 
•	Have different shapes
•	Respond to different neurotransmitters
•	This is affected by local chemical environment
Creating initial synapses
•	Axons sprout growth cones
–	Chemical scouts that attach the axon to certain other neurons
–	Then synapses form
•	Only synapses that are actually used continue to function
•	Neural selectionists argue that synapses compete to stay alive – some synapses are pruned 
–	Some genetic control
–	But also experiences
–	Especially at crucial times
–	Greatest number of synapses at 2 years of age
–	Age and experience create efficiency and complexity
Clear that there are critical periods
•	Hubel & Wiesel
–	In adult cats each eye sends signals to different parts of the brain
–	In young cats this differentiation does not exist
–	Experiment
•	Closed one eye of a young cat
•	Areas of brain that should have responded to that eye now responded only to the other
•	When opened it fired few cortical cells – effectively blind in one eye
•	Humans that are blind from birth
–	Not blind when sight is restored
–	But have trouble learning shape discriminations and the like
Other examples of critical periods
•	Bird song
•	Imprinting in fowl
•	Language learning in humans
–	Viktor – Wild Boy of Aveyron
–	Genie
•	Incest taboos
Changes in synaptic transmission
•	One of the great mysteries
•	Important because it’s the way memories are stored
•	Single cells can’t store memories
–	Memories are associations
–	And highly complex
–	How could a single cell store a memory of what you had for dinner last night
•	Smell
•	Sight
•	Other people
Memories must be patterns of activation
•	But how can patterns be established
•	Long term potentiation (LTP)
•	Need to find a way to bind two or more pre-synaptic cells to one post-synaptic
•	Simultaneous firing
•	Donald Hebb
How is this accomplished?
•	Second messenger system
Changes in post-synaptic cell
•	Ca causes chemical changes that affect genes in cell 
•	These produce proteins that create new receptors or make old ones more sensitive to glutamate
•	Hence it takes less glutamate from the pre-synaptic cell to cause a reaction 
Possible changes in the pre-synaptic cell
•	Considerable speculation that changes also occur in the pre-synaptic cell
•	This would require retrograde messengers
–	Chemicals that cross the synapse in opposite direction
–	And make the pre-synaptic cell release glutamate more readily or more of it.
The net result 
•	A post-synaptic cell is linked to many pre-synaptic cells
–	Usually 1000s
•	So LPT strengthens some connections at the expense of others
•	Creating very wide patterns involving perhaps millions of cells
•	But the same cell can participate in many different patterns
•	Connectionism and modeling