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 ideas100 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