PART FIVE Forms of Life
Animal Neurons and Signal Transduction
Neuron function is based on chemicals diffusing down their concentration gradients.
A signal passes down a single neuron as a positively-charged wave.
A signal passes between adjacent neurons as a chemical called a neurotransmitter.
Our nervous system is based on specialized cells called neurons. These cells are awaiting an input from another neuron or from a specific stimulus (such as stepping on a wall tack you just dropped on the floor and couldn’t locate … until now). As soon as the pointy end of the offending office product jabbed into your heel, you jerked your leg up and pulled it out. This coordinated series of actions was made possible by neurons. The cool thing is, the number of neurons needed to move a signal around your body is staggeringly large; each individual cell is between 4 μm (micrometer) and 100 μm long. The brain alone contains something along the lines of 100 billion of these cells. The path a signal must take between the area of input (your foot), up to your brain, and then back to the area of response (get that tack outta my foot!) is huge. To make the journey, nervous signals must travel along an individual neuron, and then jump between adjacent neurons.
Just for your information, a micrometer is one-millionth of a meter. A 4 μm neuron is 0.004 millimeters; a human hair is huge in comparison … it’s 75 μm wide.
The method by which a single neuron moves a signal down its own cell body is different from how the signal is then shuttled between adjacent neurons. Either way, as our must know concept states, the signals are nothing more than chemicals diffusing down their concentration gradients. First, let’s talk about how a single neuron responds to a stimulus—action potentials!
This is the general structure of a neuron:
Neurons send signals in one direction: from the dendrites, down the axon, ending at the synaptic terminals. Within a single neuron, this signal takes the form of a ~whoosh~ of positive ions diffusing along the axon of the cell. A cell that is “resting” is not actively sending a signal; there is no positive wave occurring. Therefore, the inside of a neuron at rest has a more negative charge compared to its surroundings. This difference in charge is created by the cell itself, using a special membrane pump called the sodium-potassium pump.
Whenever there is a mention of a “pump,” it should make you think of active transport. A pump requires energy and moves something against its concentration gradient (meaning it creates a high concentration of something on one side of the cell’s membrane).
In the neuron, the sodium-potassium pump moves three sodium ions (Na+) out for every two potassium ions (K+) it brings into the cell (part A in the figure below). Because more positively charged ions are being pumped out of the cell, it results in a net positive charge on the outside of the cell. Furthermore, there are these big negatively charged proteins stuck inside the cell, further contributing to the charge differential (more positive on the outside and more negative on the inside). Because of this difference in charge, the resting neuron is now ready to fire!
A resting neuron
The ability of a neuron to send a message all depends on this charge differential; it is called a membrane potential. When a neuron fires (sending the signal along its axon), it does so by opening up doorways and allowing the sodium ions to diffuse down their concentration gradient, back into the interior of the cell (step A in the figure below). Remember, our must know states that a neuron’s function is based on diffusion of chemicals!
The special doorways that offer a passive pathway through which the sodium ions can diffuse are called sodium channels. This is the beginning of the positive wave and our neuron’s action potential. The signal is moved down the length of the neuron as a sort of chain reaction; a positively charged interior at one section of the axon (B) causes more sodium channels (C) a bit farther down to open. Sodium ions enter (E) and turn that section positive, causing more sodium channels to open, and so on. This creates the ~whoosh~ of positive charge that’s moving down the axon toward the synaptic terminals.
A neuron creating an action potential
Puffer fish is a delicacy in Japan (fugu), though it is dangerous to eat unless prepared by specially trained chefs. The fish contains a toxin (tetrodotoxin) that interferes with sodium channel function if ingested. Tetrodotoxin binds to neurons’ sodium channels and blocks the diffusion of sodium ions. If sodium ions cannot diffuse into the cell, the neuron cannot fire!
In the wake of this positive charge, a second type of doorway—the potassium channel—opens, allowing K+ ions to flow out of the cell (part D in the previous figure). The potassium ions happily diffuse out of the cell, down their concentration gradient. In so doing, positive charges are removed from the inside of the cell, returning the inside to its “normal” resting negative charge.
There’s one final thing the cell needs to do in order to fire again. Even though the charge differential is correct at this point (more negative on the inside; more positive on the outside), things aren’t quite right … the sodium and potassium ions are on the wrong sides of the membrane! This brings us back to the start of the story: the sodium-potassium pump takes over, and with the help of energy (ATP), pushes the sodium ions back out and pulls the potassium ions back in. The resting conditions are reestablished and the neuron is once again ready to send a message.
But what happens when the positive wave hits the end of the synaptic terminal? How do neurons communicate with each other? Let’s move on to neurotransmitters and synapses.
Neurotransmitters and Synapses
When a single neuron fires and sends its message, it is in the form of a positive charge moving down the axon. This electrical charge, however, cannot jump from one neuron to the next across the tiny space (synapse) separating adjacent neurons. Instead, a chemical needs to diffuse across the synapse to move the signal from one neuron to the next … and it reinforces the importance of our must know for this chapter. The chemical responsible for moving the signal from one neuron (presynaptic cell) to the next (the postsynaptic cell) is called a neurotransmitter.
Neurons and their synapse
When the action potential hits the end of the first neuron (A), it signals for calcium channels to open (B), allowing calcium ions to flood inward. The high levels of Ca2+ ions cause vesicles filled with neurotransmitters (C) to fuse with the presynaptic neuron’s membrane, dumping the neurotransmitter into the synapse (D). The chemical neurotransmitters are able to diffuse across the space between the two neurons until they bind to another sodium channel protein (E) in the postsynaptic neuron. The postsynaptic cell’s sodium channel swings open, allowing sodium to diffuse through (F), starting another action potential in the second neuron.
The bacterium Clostridium botulinum is responsible for producing one of the most potent natural toxins on the planet: botulinum toxin. The botulinum toxin inhibits presynaptic release of a specific neurotransmitter called acetylcholine. If a muscle doesn’t receive acetylcholine’s signal, the muscle will not respond properly by contracting. Botulism is a deadly food poisoning caused when someone accidentally ingests food containing botulinum toxin; BOTOX is an anti-wrinkle treatment where someone intentionally injects tiny amounts of botulinum toxin into their facial muscles.
1. Choose the correct answer from each of the following pairs: A resting neuron has a higher concentration of K+/Na+ on the inside of the cell and a higher concentration of K+/Na+ on the outside of the cell. The net charge on the inside of the cell is positive/negative.
2. A neuron transmits a signal by generating an _____________, which is a wave of _____________ charge generated when _____________ ions rush into the cell.
3. Put the following events of a signal crossing a synapse in the correct order:
a. Neurotransmitter binds to the postsynaptic cell’s membrane
b. Calcium channels open
c. Neurotransmitter is dumped into the synapse
d. Vesicles containing neurotransmitter fuse to the presynaptic neuron’s membrane
e. Action potential hits the end of the neuron
f. Sodium channels on postsynaptic cell open
4. Explain how a resting neuron maintains a net negative charge on the inside of the cell.
5. Put the following events of a neuron firing in the correct order (this is a general overview of the process; not all the steps are included in this list):
a. K+ channels open
b. Sodium ions diffuse into the cell
c. Sodium-potassium pumps move ions into their starting positions
d. The positive charge inside the neuron causes adjacent sodium channels to open
e. Potassium ions diffuse out of the cell
f. Na+ channels open
g. A neuron receives a signal to fire and send a message
6. When an action potential reaches the end of the _____________ neuron, it must rely on the chemicals called __________________________ to cross the space between adjacent neurons (the _____________). Once the chemicals bind to the __________________________ neuron, sodium channels will open and the action potential will continue.