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 cell3. 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

1. Action potential reaches terminal2. Calcium ions enter, which causes

vesicles to fuse with membrane3. Transmitter molecules bind with

receptors to open channels in the synapse

4. The IPSPs and EPSPs in postsynaptic cell spread to the axon hillock• EPSP - excite

– Excitatory synapse• IPSP - inhibit

– Inhibitory synapse5. Synaptic transmitter inactivated by

enzymes6. Synaptic neurotransmitter is removed

rapidly from the cleft by transporters

The Synapse• Label

– Presynaptic 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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