Transcript
Neurons, Neurotransmitters, and Systems
Structure of a Neuron
The Withdrawal Reflex
The Neuron in ActionResting 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 aloneA neuron with a moderate amount of dendrites
receives between 1000 and 10000 contactsSome 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. So, the influence of the neurotransmitters must be
temporarily terminated; that is, the synapse needs to “reset” itself.
Three termination processes Reuptake Enzyme deactivation Autoreceptors: a homeostatic device
NeurotransmittersThe 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 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 Important for learning
• People with Alzheimer’s have low levels of ACh
Neurotransmitters II: MonoaminesDopamine
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-dopaSerotonin
Low serotonin levels in severe depression; may be responsible for sleep disturbances in depression
Low serotonin levels associated with increased aggressionNorepinephrine
Increases emotional arousal (fear and anxiety) and alertness
Neurotransmitters IIIAmino acids
Gamma amino butyric acid (GABA)• Main inhibitory neurotransmitter in the brain
• Lowers arousal and regulates anxiety
• Alcohol does the same thing Glutamate
• Main excitatory neurotransmitter in the brain
Peptides: modify effects of neurotransmitters Endorphins
• Endogenous [produced within the body] morphine; opiates mimic the actions of endorphins
• Elevates mood and reduces pain
Agonistic and antagonist drug effects
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