Central Nervous System + Neural Networks The adult human brain has ~180 BILLION cells ~ 80 billion neurons neurons have ~15,000 connections (synapses) EACH!

Do Now Propose an analogy for understanding the complexity of the human mind/central nervous system Draw that analogy

Neuron doctrine Billions of cells communicating across synapses The brain is composed of separate, independent neurons Information is transmitted from cell to cell across synapses

Neurons

Neuron Morphology Label the following: Afferent connection Efferent connection Axon (send) Dendrites (receive) Myelin sheath Action potential Terminal buttons (transform impulses to neurotransmitters and releases them) Soma (cell body)

Neuron Variety The shape and size indicates function

Neuron Action Potential Action potential = change in permeability Process (less than a millisecond) Neuron “starts” at resting potential Greater negative charge inside cell than outside There are few positive charges in the cell Guiding rule: concentration gradients Ions from regions of high concentrations will flood areas of low concentration

Neuron Action Potential (II) 2. Electrical charge is generated until inside charge positive This is the depolarization phase Na+, K+ flood the cell 3. Positive charge forces potassium and sodium out This is the repolarization phase 4. Tense unbalanced state restored Absolute refractory period Voltages gates closed for period Potassium pump re-establishes equilibrium Variables Size of the neuron Bigger = faster conductance Thicker myelin sheaths = faster conductance

Action Potential (III): The Party Metaphor A person is throwing a house party. At the beginning house is relatively empty with a couple of friends inside. A large group of positive friends, sodium and potassium, are waiting outside to come in. (Resting potential) The doors open and people flood in (depolarization phase) The house becomes too crowded. People are packed together and at once decide that it is much nicer outside of the house. Everyone rushes out. Its more empty than at the beginning. (refractory period) Gradually some close friends of the owner enter through the back door and equilibrium is again establish (resting state)

Neuron Action Potential (IV)

Synaptic Transmission Process Action potential reaches terminal Calcium ions enter, which causes vesicles to fuse with membrane Transmitter molecules bind with receptors to open channels in the synapse The IPSPs and EPSPs in postsynaptic cell spread to the axon hillock EPSP - excite Excitatory synapse IPSP - inhibit Inhibitory synapse Synaptic transmitter inactivated by enzymes Synaptic neurotransmitter is removed rapidly from the cleft by transporters

The Synapse Label Presynaptic neuron Postsynaptic neuron axon Postsynaptic neuron Dendrites Receptors Neurotransmitters Cleft Vesicles

Neurotransmitters (keys) Definition: a substance released by a presynaptic neuron that elicits an electrophysiological change in a postsynaptic neuron Process Indirect: through effecting the receptors on postsynaptic neuron Regulates openness GABA bonds and releases Cl (inhibitor) Secondary: effects how neuron reacts to other stimuli Alcohol Direct: influences presynaptic neuron release Regulates release or reuptake Coffee Adenosine and norepinephrine are released at the same time by the same neuron Norepinephrine causes an excitatory reaction (increased action) Adenosine effects release of NE (slows down release) Caffeine blocks adenosine (NE continually released)

Neurotransmitters in specific Types Agonist: chemicals that bond to receptors and open channels Antagonist: binds to ionotrophic channel and blocks other neurotransmitteres from attaching Doesn’t close door but prevents opening Examples of neurotransmitters: acetylchoine (memory, muscle contraction) Serotonin (arousal, sleep, pain, mood) Dopamine (movement, attention, reward-seeking behavior) GABA (inhibitory)

Neuromodulators Neuromodulators effect the post-synaptic cell in long term ways Effect responses to other neurotransmitter Slow acting and can help explain changes in mood and learning

Neurons and the connections between them are always changing Neurogenesis New cells are created within the brain regularly The debate around stem cells concerns their role in neurogenesis Hebbian Synapse: neurons that fire together, wire together Long-term Potentiation (LTP) potentiation means strengthening Strengthening of synapses If prevent LTP, learning is severely limited If drug is injected in fear learning pathway in amygdala Prevented conditioning associations If increase LTP through selective breeding of mice Memory, spatial learning, conditioning and object recognition.

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