Neuronal communication
Neuronal communication
While the basic structures of the neuron play a role in neuronal communication, the signal itself moves through the neuron and and then jumps to the next neuron, where the process is repeated.
The neuronal membrane
In neuronal membrane, the neuron exists in a fluid environment that is surrounded by extracellular fluid and contains intracellular fluid. The neuronal membrane keeps these two fluids separate, which is a critical role because the electrical signal that passes through the neuron depends on the intra and extracellular fluids being electrically different.
This difference in charge across the membrane, called the membrane potential, provides energy for the signal.
The electrical charge of the fluids is caused by charged molecules (ions) dissolved in the fluid. The semipermeable nature of the neuronal membrane somewhat restricts the movement of these charged molecules, and, as a result, some of the charged particles tend to become more concentrated either inside or outside the cell.
A series of states
On receipt of a signal (an event), the neuron's potential goes through a series of states:
resting potential
depolarisation
action potential
repolarisation
hyperpolarisation
resting potential
In the absence of any stimulation, the resting potential is generally a constant state.
Lights, camera, action (potential)
Between signals, the neuron membrane's potential is held in a state of readiness, called the resting potential. Like a rubber band stretched out and waiting to spring into action, ions line up on either side of the cell membrane, ready to rush across the membrane when the neuron goes active and the membrane opens its gates.
Ions in high-concentration areas are ready to move to low-concentration areas, and positive ions are ready to move to areas with a negative charge.
The following diagram shows the movement of ions across the membrane. At resting potential, sodium (Na+), shown as blue pentagons, is more highly concentrated outside the cell in the extracellular fluid (shown in blue), whereas potassium (K+), shown as purple squares, is more highly concentrated near the membrane in the cytoplasm or intracellular fluid, and will tend to move out of the cell.
Other molecules, such as chloride (Cl-), shown as yellow circles, and negatively charged proteins (A) shown as brown squares, help contribute to a positive net charge in the extracellular fluid and a negative net charge in the intracellular fluid.
In addition, the inside of the cell is slightly negatively charged compared to the outside. This provides an additional force on sodium, causing it to move into the cell. From this resting potential state, the neuron receives a signal and its state changes abruptly.
When a neuron receives signals at the dendrites due to neurotransmitters from an adjacent neuron binding to its receptors small pores, or gates, open on the neuronal membrane, allowing Na+ ions, propelled by both charge and concentration differences, to move into the cell. With this influx of positive ions, the internal charge of the cell becomes more positive.
If that charge reaches a certain level, called the threshold of excitation, the neuron becomes active and the action potential begins.
Many additional pores open, causing a massive influx of Na+ ions and a huge positive spike in the membrane potential, the peak action potential. At the peak of the spike, the sodium gates close and the potassium gates open. As positively charged potassium ions leave, the cell quickly begins repolarisation.
At first, it hyperpolarises, becoming slightly more negative than the resting potential, and then it levels off, returning to the resting potential.
This positive spike constitutes the action potential: the electrical signal that typically moves from the cell body down the axon to the axon terminals. The electrical signal moves down the axon like a wave, at each point, some of the sodium ions that enter the cell diffuse to the next section of the axon, raising the charge past the threshold of excitation and triggering a new influx of sodium ions.
The action potential moves all the way down the axon to the terminal buttons.
The all-or-none phenomenon
The action potential is an all-or-none phenomenon. This means that an incoming signal from another neuron is either sufficient or insufficient to reach the threshold of excitation.
There is no in-between, and there is no turning off an action potential once it starts.
Think of it like sending an email or a text message. You can think about sending it all you want, but the message is not sent until you hit the send button. Furthermore, once you send the message, there is no stopping it.
Because it is all or none, the action potential is recreated, or propagated, at its full strength at every point along the axon. Much like the lit fuse of a firecracker, it does not fade away as it travels down the axon. It is this all-or-none property that explains the fact that your brain perceives an injury to a distant body part like your toe as equally painful as one to your nose.
The release of neurotransmitters into the synapse
When the action potential arrives at the terminal button, the synaptic vesicles release their neurotransmitters into the synapse. The neurotransmitters travel across the synapse and bind to receptors on the dendrites of the adjacent neuron, and the process repeats itself in the new neuron (assuming the signal is sufficiently strong to trigger an action potential).
Once the signal is delivered, excess neurotransmitters in the synapse drift away, are broken down into inactive fragments, or are reabsorbed in a process known as reuptake.
Reuptake involves the neurotransmitter being pumped back into the neuron that released it, in order to clear the synapse.
Clearing the synapse serves both to provide a clear 'On' and 'Off' state between signals and to regulate the production of neurotransmitter (full synaptic vesicles provide signals that no additional neurotransmitters need to be produced).
Neuronal communication is often referred to as an electrochemical event. The movement of the action potential down the length of the axon is an electrical event, and movement of the neurotransmitter across the synaptic space represents the chemical portion of the process.
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