It’s all about maintaining homeostasis in Episode 36!
It’s all about maintaining homeostasis in Episode 36! Neurons are specialized cells for conducting electrical signals and releasing chemical neurotransmitters (2:02). Axons conduct an action potential through the movement of sodium and potassium ions (3:00). The message changes from electrical to chemical at a synapse (4:30). Neurotransmitters act as ligands, binding to the postsynaptic membrane (5:20).
The Question of the Day asks (6:44) “Which autoimmune disorder degrades the myelin sheath of an axon? ”
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap Signal Transduction and the Nervous System
Lets Zoom out:
Unit 4 - Cell Communication and Cell Cycle
Topic - 4.1-4.4
Big idea - Information storage and transmission
Your left hand needs to know what your right hand is doing. Coordinated movement, processing, and responding requires an incredible amount of cell communication. Sending and receiving chemical signals allows organisms to maintain homeostasis.
Lets Zoom in:
This episode will talk about the nervous system. I know, I know - human body systems aren’t explicitly on the AP exam. Quite a few AP teachers were outraged when they were removed from the curriculum - This stuff is important! And they are right. There will be some vocabulary I’ll mention in this episode that is unfamiliar to you. But this is good practice - because there will be vocabulary on the AP exam you’ve never seen as well. Maximize the understanding, minimize the need for memorization. The key takeaway is to understand how cells communicate and respond. Then, take that understanding and apply it to an unknown scenario. Just like figuring out a math equation. You’ve never seen it before, but you are familiar with the process of solving for X. So here we go - solving for communication in the nervous system.
All membranes are made of phospholipids with embedded proteins. These proteins are often ligand specific contributing to selective permeability. Meaning, some molecules can cross the membrane, while others cannot. Instead they may bind, cause a conformational change and trigger a signaling cascade. For a full recap on cell communication - check out Episode 13.
Neurons, cells of the nervous system, have a very cool design. Some may stretch the entire distance from your spine to finger tip. Just like the electrical wires in your house, they are insulated - to conduct the signal more efficiently. But when the message gets to the end of a neuron - it has to transform temporarily from electrical to chemical to cross a gap and get to the next cell (could be a gland, muscle, or another neuron). The entire process is just like a relay race with batons and hurdles. Neurons, and their chemical messengers, the neurotransmitters, are an example of paracrine signaling because their cells aren’t touching, but are fairly close by.
Electrical to chemical to electrical and response. That’s the goal of the signaling pathway. Let’s start with an electrical message traveling along a neuron’s axon (the long insulated part of the cell). Perhaps this signal is coming from the spinal cord with the message “that stove is hot! we need to move the hand immediately!” This signal transduction within a neuron's axon is called an action potential. You might have seen this graphed as millivolts on the y axis and time on the x axis - showing how the electricity of the neuron changes through signaling. If you’ve seen ANY medical drama on TV - it's always CLEAR! and an electrical shock is sent through paddles to trigger a heartbeat, to trigger an action potential. All related to neurons and membrane potential.
Without any signal, the resting potential of an axon is around -70 millivolts. When a stimulus is significant enough to meet the threshold (about -55 millivolts) a series of voltage gated Na+ channels open, allowing Na+ to rush into the axon. This causes a depolarization or the membrane and a spike in charge to +40 millivolts. Remember that sodium ions are positively charged. Immediately following, potassium ions (also positively charge) leave the axon through channels. This causes a repolarization of the membrane and a drop in charge. The entire process of moving ions in and out of the axon cascades down the axon in a domino effect. Since the axon is insulated (myelin sheath covers up some of the ion channels) - the impulse jumps from node to node, where the next exposed channels are located.
When the impulse reaches the end of the neuron, called the axon terminal, it's time to cross the synapse. Depend ing on the type of neuron, the strategy of transferring the signal across a synapse can be different. This episode is going to discuss chemical signals through neurotransmitters. The action potential with the message “its hot! move your hand!” has gone down the axon and reached an axon terminal. Waiting within that axon terminal are a series of vesicles that contain chemical messages known as neurotransmitters. Also in the presynaptic neuron are voltage gated calcium channels. The action potential triggers the opening of the calcium channels, allowing calcium ions to enter the presynaptic terminal. The calcium ions bind to the waiting vesicles and allow their docking and release of neurotransmitters into the synapse through exocytosis. The signal has now changed from electrical to chemical.
The postsynaptic membrane has docking proteins known as chemically-gated channels. The neurotransmitters do not enter the postsynaptic cell, but bind to the surface proteins. Neurotransmitters can be excitatory or inhibitory. Their presence will either allow the flow of sodium ions - causing action potentials or the flow of potassium or chloride - moving away from action potentials. Some examples of neurotransmitters include dopamine and epinephrine (which you probably recognize by its other name - adrenaline). The neurotransmitters are often taken back up by the presynaptic membrane, stored again in vesicles - so that the process can happen again.
All that work - But did you move your hand from the hot stove? The postsynaptic membrane may signal another action potential or connect to an effector. In this example, muscles in your hand cause it to pull back. This particular example is a reflex arc. Maximize the understanding and minimize the memorization. But if you DO remember something, that means you’ve created more post synaptic membrane ion channels to receive neurotransmitter signals. Good job!
To recap….
Communication through neurotransmitters in a synapse is an example of paracrine signaling. The movement of sodium and potassium ions create an action potential through voltage gated channels. The coordination between electrical and chemical communication allows you to remember information for the AP Exam! Thank you neurons.
Today’s Question of the day is about disorders
Question “Which autoimmune disorder degrades the myelin sheath?”
Coming up next on the Apsolute RecAP Biology Edition: Chromosomal Inheritance.