The synapse: where neurons talk and drugs join in

Transcript

RyanIt's two in the morning. You get up for a glass of water, the house is dark, and you catch your little toe on the leg of the bed.

SoheeOh, I felt that just hearing it.

RyanRight? And here's a strange question. Why does a toe, about as far from your brain as you can get, hurt just as much as bumping your nose?

SoheeWelcome to EPPP Prep, the podcast. I'm Sohee.

RyanAnd I'm Ryan. Today we start the biological bases domain at the smallest scale: the neuron and the synapse. And we'll answer the toe question along the way.

SoheeThis domain can feel intimidating, especially if your training leaned clinical. But the core is a handful of mechanisms. Once they click, a lot of questions get easier, including the medication questions.

RyanLet's meet the cast. The OpenStax psychology textbook says the nervous system is made of two basic cell types: glial cells and neurons.

SoheeGlia are the support crew. They provide scaffolding, help neurons line up closely so they can communicate, provide insulation, transport nutrients and waste, and mediate immune responses.

RyanAnd neurons are the information processors. The cell body is called the soma. The branching dendrites are the input sites, where signals from other neurons are received.

SoheeThe signal then travels down the axon, which ends at terminal buttons. Those buttons hold synaptic vesicles, and the vesicles house neurotransmitters, the chemical messengers of the nervous system.

RyanSome axons are coated in myelin, a fatty substance formed by glial cells. It acts as an insulator and increases the speed of the signal. The small gaps in the myelin are called the nodes of Ranvier.

SoheeAnd myelin matters clinically. In multiple sclerosis, an autoimmune disorder, there's a large-scale loss of myelin. The interference in the signal can lead to dizziness, fatigue, loss of motor control and sexual dysfunction.

RyanPKU, phenylketonuria, which we'll meet again in the genetics episodes, also causes a reduction in myelin. It's associated with severe cognitive deficits, exaggerated reflexes and seizures.

SoheeOkay. So how does the signal actually move? This is the part people memorize the night before and then lose.

RyanLet's make it stick by thinking about why. At rest, the neuron is like a stretched rubber band, waiting. That's the resting potential. Sodium is more concentrated outside the cell, so it tends to move in. Potassium is more concentrated inside, so it tends to move out.

SoheeAnd the inside is slightly negative compared with the outside, because of the sodium-potassium pump. It moves three sodium ions out for every two potassium ions in.

RyanNow neurotransmitters from a neighbor bind at the dendrites, small gates open, and sodium rushes in. The inside becomes more positive. If the charge reaches a level called the threshold of excitation, the action potential begins.

SoheeAt the peak, the sodium gates close and the potassium gates open. Potassium leaves, and the cell repolarizes. It briefly overshoots and becomes slightly more negative than rest, which is hyperpolarization, and then it levels off at the resting potential.

RyanSo: sodium in for the spike, potassium out for the recovery.

SoheeAnd here's our toe. The action potential is all or none. An incoming signal is either enough to reach threshold or it isn't. There's no in-between, and once it starts, there's no turning it off.

RyanThe textbook compares it to sending a text message. You can think about it all you want, but nothing goes until you hit send. And once you send it, there's no stopping it.

SoheeBecause it's all or none, the action potential is recreated at full strength at every point along the axon. Like the lit fuse of a firecracker, it doesn't fade. OpenStax says that's why the brain perceives an injury to your toe as equally painful as one to your nose.

RyanMystery solved. Now the chemical half. When the action potential reaches the terminal buttons, the vesicles release neurotransmitters into the synaptic cleft, a very small space between two neurons.

SoheeThe neurotransmitter crosses and binds to receptors on the next neuron. The textbook calls it a lock-and-key relationship. Specific neurotransmitters fit specific receptors.

RyanThen comes cleanup, and this matters a lot for medications. Leftover neurotransmitter drifts away, gets broken down into inactive fragments, or is pumped back into the neuron that released it. That last one is called reuptake.

SoheeSo neuronal communication is an electrochemical event. Electrical down the axon, chemical across the synapse.

RyanWith one exception worth knowing. Some neurons connect physically through gap junctions. Those are electrical synapses. There are far fewer of them in the brain, but they're much faster than chemical synapses.

SoheeLet's run through the major neurotransmitters from the OpenStax table. Acetylcholine: muscle action and memory.

RyanDopamine: mood, sleep and learning, with effects like increased pleasure and suppressed appetite.

SoheeGABA, which stands for gamma-aminobutyric acid: brain function and sleep, with decreased anxiety and decreased tension. The textbook says GABA has a quieting effect on the brain.

RyanGlutamate: memory and learning. Norepinephrine: heart, intestines and alertness. Serotonin: mood and sleep. And beta-endorphin: pain and pleasure.

SoheeNow here's where this connects to medications. Psychoactive drugs can act as agonists or antagonists for a neurotransmitter system.

RyanAn agonist mimics a neurotransmitter at the receptor site. An antagonist blocks or impedes the neurotransmitter's normal activity at the receptor.

SoheeTwo textbook examples. Parkinson's disease is associated with low levels of dopamine, so a common treatment uses dopamine agonists, which mimic dopamine by binding to dopamine receptors.

RyanAnd certain symptoms of schizophrenia are associated with overactive dopamine transmission. Antipsychotics are dopamine antagonists. They bind the receptors without activating them, so the dopamine signal doesn't get through.

SoheePause and think. A drug binds to a receptor, produces no effect of its own, and prevents the natural neurotransmitter from acting. Agonist or antagonist?

RyanAntagonist. The tempting wrong answer is agonist, because the drug binds. But binding alone isn't the test. An agonist binds and mimics. An antagonist binds and blocks.

SoheeThen there's a third mechanism that doesn't work at the receptor at all. Reuptake inhibitors.

RyanThey prevent unused neurotransmitter from being transported back into the neuron. So it stays active in the synaptic cleft longer, which increases its effect.

SoheeThat's how SSRIs work, selective serotonin reuptake inhibitors. OpenStax names Prozac, Paxil and Zoloft. The textbook says depression has been consistently linked with reduced serotonin, and that by preventing reuptake, SSRIs give serotonin more time to act on its receptors.

RyanOne honest footnote on that serotonin link. A 2022 umbrella review in the journal Molecular Psychiatry concluded that the main areas of serotonin research provide no consistent evidence of an association between serotonin and depression. So treat the textbook's serotonin link as a classic explanation, not a settled fact.

SoheeAnd the mechanism of the drug doesn't change either way. An SSRI blocks reuptake, so serotonin stays in the cleft longer.

RyanNotice the distinction. The textbook sets reuptake inhibitors apart from agonists and antagonists, which both work by binding receptor sites. An SSRI keeps the brain's own serotonin around longer.

SoheeStimulants give us a nice contrast. Cocaine blocks the reuptake of dopamine, which the textbook says agonizes, or boosts, the dopamine system. Amphetamines also block dopamine reuptake, and on top of that, they stimulate its release.

RyanAnd depressants show a receptor example. Alcohol, barbiturates and benzodiazepines are agonists of the GABA system. They bind to sites on a GABA-gated chloride channel, which lets negatively charged chloride ions into the neuron.

SoheeWhich ties right back to the action potential. Shift the charge in a negative direction, and the neuron is pushed away from firing. That's the quieting effect.

RyanOne more point from the textbook, and it's a kind one to pass on to clients. Psychotropic drugs aren't instant. A person often takes a drug for several weeks before seeing improvement, many have significant side effects, and people vary a lot in how they respond.

SoheeIt also notes that some research suggests combining drug therapy with other forms of therapy tends to be more effective than any one treatment alone. We'll dig into those trials later in this series.

RyanSo how do questions on this usually look? Mostly mechanism matching. A stem describes what a drug does at the synapse and asks for the mechanism, or names a drug and asks which neurotransmitter system it acts on.

SoheeAnd the traps are predictable. Saying an SSRI binds the receptor and mimics serotonin, the way an agonist does. Swapping the ions, when it's sodium in on the rise and potassium out on the recovery. Forgetting that myelin speeds the signal. Or treating the action potential as something that can be half-fired.

RyanPause and think, round two. A client with multiple sclerosis describes fatigue and trouble with motor control. Which part of the neuron is affected, and why would that slow things down?

SoheeThe myelin sheath. Myelin insulates the axon and speeds the signal. When it's lost, the electrical signal is disrupted and information can't be passed along quickly.

RyanLet's pull it together. Two cell types: glia support, neurons process. Dendrites receive, the axon carries, terminal buttons release.

SoheeAt rest, the inside is negative. At threshold, sodium rushes in. Then potassium flows out and the cell recovers. The action potential is all or none and travels at full strength, which is why your toe hurts as much as your nose.

RyanNeurotransmitters cross the synaptic cleft and fit receptors like keys in locks. They're cleared by drifting away, breakdown, or reuptake.

SoheeAgonists mimic, antagonists block, and reuptake inhibitors keep the neurotransmitter in the cleft longer. Parkinson's: dopamine agonists. Schizophrenia: dopamine antagonists. Depression: SSRIs.

RyanIf biology is the domain you'd rather skip, you're in good company. But you just traced a signal from one neuron to the next, and you can name three ways a drug changes it. That's the foundation for every medication question ahead.

SoheeTake a short break, maybe wiggle those toes, and next time we'll zoom out to the whole brain. Thanks for listening to EPPP Prep.

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