By Janani Rajan
Graphic designers Josephine Wang and Olivia Tang
From the world we perceive around us, to the everyday tasks we perform, everything we sense and act upon is controlled by the brain. But how do motor signals start in the brain, and end up in our fingers? How does sensory input travel from sensory receptors to the brain? The answer is electrical conductance in neurons. When a change in our surroundings is perceived, it is picked up by sensory neurons and transmitted from neuron to neuron via electrical conduction, until it reaches the brain. Similarly, when the brain sends an electrical signal to perform a certain action, the signal is transmitted from neuron to neuron until it reaches the motor neurons, which will signal muscle cells to perform the action.
First, electrical impulses are received by the dendrites, which have a smaller diameter than axons. They are then propagated to the soma and congregate at the axon hillock, which is located between the axon and soma. The axon hillock is highly concentrated with voltage-gated sodium channels and is the deciding factor as to whether or not the impulses can propagate down the axon as a single signal. In order for them to do so, the strength of the impulses must be sufficient to depolarize the neuron, which means that the membrane potential must become more positive than the threshold potential. If they do this, they may travel to the large diameter myelinated axons as a single signal.
Myelinated axons have intermittent regions of lipid coatings called myelin sheaths. These regions of the axon with myelin do not contain voltage-gated ion channels. These myelin sheaths end at Ranvier nodes, gaps between myelin sheaths that contain high concentrations of voltage-gated sodium and potassium channels. Evolutionary pressure, which was caused by the diameter of the axon, allowed for the lengths of the lipid coatings and Ranvier gaps to vary.
The nodes of Ranvier enable high concentrations of sodium ions to enter and potassium ions to exit the axon as a signal approaches the node, by concentrating the voltage-gated ion channels at the node. Since axons do not contain voltage-gated ion channels, except for at the nodes of Ranvier, the only way to conduct current through the next myelinated region of the axon would be the passive diffusion of sodium and potassium ions inside the axon’s cytoplasm. The ions diffuse down the axon’s length to the next node before the signal loses charge. The longitudinal diffusion of these ions is facilitated by high concentrations of sodium at the node, which results in charge-charge push, an electrostatic repulsion from ions with similar charges located in close proximity to each other. Charge-charge push among the high concentrations of sodium ions at the nodes causes faster diffusion between positively charged sodium ions. Faster diffusion via charge-charge push is much faster than the simple passive diffusion of isolated non concentrated ions, and is extremely inefficient to carry ions across long distances. Furthermore, it is critical for each node of Ranvier to provide sufficient ions to diffuse down the axon’s length to the next node, before the charge of the signal dissipates.
Therefore, at each node, the electrical signal has a momentary burst of speed. As it travels down a myelinated region of the axon, the signal dissipates and slows down, until it reaches the next node where the cycle repeats again. In neuroscience, this is referred to as saltatory conduction. After a signal propagates down an axon, the high sodium and potassium ion concentrations inside the axon at the nodes of Ranvier will control the potential difference between the membrane located inside and outside the node.
With that said, a key question is, “How come propagation is a one-way street? Why doesn’t the dendrite activate ion currents in the reverse direction of propagation?” Evolution took care of this problem. As an electrical signal arrives at a node of Ranvier, a neuron is depolarized. Depolarization of the neuron causes the sodium gated ion channels at the node to change conformation. The sodium gated ion channels open up, however, a small protein sequence moves to block sodium transport into the axon or dendrite, physically clogging the sodium ion channel residing in either the intra-dendritic or intra-axon membrane. This is referred to as the inactivation of the sodium channel. The inactivation of the sodium channel prohibits sodium ions from flooding out of the axon, allowing action potentials to maintain the sufficient level of sodium ions to continue propagating through the axon.
Understanding how signals are conducted between neurons is extremely vital, as it helps us to understand the details of how the functions of the brain and body are carried out. All functions, including sensory, motor, homeostatic, and even cognitive functions all boil down to the conductance of electrical signals between neurons. With this knowledge, we can explore the realms of the different regions of the brain, their functions, and beyond.
Approved by Dr. Charles Pidgeon.
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