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Neurons, Neurotransmitters, and
Systems
Structure of a Neuron
The Withdrawal Reflex
The Neuron in Action
 Resting Potential: an electrical charge of –70 mV across
the cell membrane (-inside, + outside) caused by the
sodium-potassium pump (pumps 3 Na+ out for every 2 K+
in) and the impermeability of the cell membrane to Na+
 Graded Potential: small changes in resting potential
caused by other neurons; hyperpolerization (inhibitory) vs.
depolarization (excitatory)
 Threshold: the point at which a neuron has been
depolarized enough to trigger an action potential
 Action Potential: an electrical impulse that surges along
an axon; caused by an influx of Na+ ions into the neuron;
causes “communication” with any neuron it contacts
Some interesting facts and ideas
 100 to 200 billion neurons in the brain alone
 A neuron with a moderate amount of dendrites
receives between 1000 and 10000 contacts
 Some neurons in the cerebellum receive 150,000
contacts!
 A mental code (i.e., mental representation) is a
pattern of neurons firing in (sometimes) several
different locations in the brain simultaneously
 Our brain has trillions of connections which can be
used to code trillions of mental representations
Laws and implications of action potentials
 All or none law: neurons either “fire” an action
potential or they do not; there are no halfway responses
 Action potentials do not vary in intensity, either within
the same neuron at different times or across different
neurons
 Information is conveyed by the number and frequency
of action potentials
 The information conveyed by an action potential
depends on the pathway it is a part of. The image of a
bee and the sound of bee are both conveyed by a chain
of action potentials, but in different parts of the brain
Terminating synaptic transmission
 If the neurotransmitters were allowed to stay in the
synaptic gap, they would continue to bind with
receptors and thus prevent new signals from being
communicated.
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So, the influence of the neurotransmitters must be
temporarily terminated; that is, the synapse needs to
“reset” itself.
 Three termination processes
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Reuptake
Enzyme deactivation
Autoreceptors: a homeostatic device
Neurotransmitters
 The inhibitory or excitatory effects of neurotransmitters
are a function of the receptor with which they bind and
not a function of the neurotransmitter itself.
 Different areas of the nervous system rely on different
neurotransmitters for interneuronal communication
 Acetylcholine
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First neurotransmitter discovered
Links motor neurons & muscles
• Curare blocks the release of ACh
• Botulism also blocks the release of ACh
• Black widow bite floods the synapse with Ach
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Important for learning
• People with Alzheimer’s have low levels of ACh
Neurotransmitters II: Monoamines
 Dopamine
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Drugs ranging from marijuana to heroin increase the amount
of dopamine in neural pathways responsible for experiencing
pleasure
High levels of dopamine in some parts of the brain have been
linked to schizophrenia
Degeneration of dopamine-producing neurons in the
substantia nigra produces Parkinson’s disease
Encephalitis lethargica and L-dopa
 Serotonin
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Low serotonin levels in severe depression; may be responsible
for sleep disturbances in depression
Low serotonin levels associated with increased aggression
 Norepinephrine
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Increases emotional arousal (fear and anxiety) and alertness
Neurotransmitters III
 Amino acids
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Gamma amino butyric acid (GABA)
• Main inhibitory neurotransmitter in the brain
• Lowers arousal and regulates anxiety
• Alcohol does the same thing
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Glutamate
• Main excitatory neurotransmitter in the brain
 Peptides: modify effects of neurotransmitters
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Endorphins
• Endogenous [produced within the body] morphine; opiates mimic the
actions of endorphins
• Elevates mood and reduces pain
Agonistic and antagonist drug effects