Neurons, Neurotransmitters, and Systems

1. Neurons, Neurotransmitters, and Systems

2. Structure of a Neuron

3. The Withdrawal Reflex

4. 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

7. 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

8. 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

9. 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

10. 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 • 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

11. Neurotransmitters II: Monoamines • Dopamine • 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 • Low serotonin levels in severe depression; may be responsible for sleep disturbances in depression • Low serotonin levels associated with increased aggression • Norepinephrine • Increases emotional arousal (fear and anxiety) and alertness

12. Neurotransmitters III • Amino 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

13. Agonistic and antagonist drug effects