What is an axon?
Every nerve cell (neuron) in the body has a long cable attached to the cell body. This cable is known to be an axon. The axon’s main function is to relay electrical impulses from the cell body down to the terminal buttons. By carrying nerve impulses away, signals can then be transmitted to other neurons. In fact, some axons can be really long, stretching from the spinal cord to the toe. The larger axons can even transmit messages at 90m/s!
So, what are the components that make up the axon?
Axon Anatomy
Axon Hillock
Located at the of the cell body (soma) is the axon hillock, or also known as the initial segment. The plasma membrane here generates the nerve impulses that get relayed down from the dendrites and cell body to the terminal.
Myelin Sheath
Axons are encased by an insulating layer of fat, called the myelin sheath. This insulation allows impulses to be transmitted faster. However, if it is damaged, nerve impulses will slow down, therefore signals will be sent slower than normal. In fact, damaged myelin is also related to multiple sclerosis.
Nodes of Ranvier
Gaps between myelin also exist to mediate ion exchange. These gaps are called nodes of ranvier and have many ion channels (e.g sodium and chloride) to allow for the creation of action potentials. Specifically, electrical impulses travel down the axon by jumping from node to node. This is known as saltatory conduction and it increases the velocity of action potentials.
Terminal Branches/Buttons
At the end of the axon are the terminal branches/buttons. These branches and buttons release neurotransmitters which allows neurons to communicate with one another.
Route of travel: Axon → axon collaterals → terminal branches → terminal buttons
Axon Function
Now that we know the parts of the axon, what exactly does it do?
Neural Transmission
The communication between the neurons is referred to as neural transmission.
Neural transmission is triggered when neurons are activated. For neurons to be activated, a certain threshold must be reached, specifically when excitatory postsynaptic potentials are greater than inhibitory postsynaptic potentials. Excitatory postsynaptic potential (EPSP) is the change in charge because a neurotransmitter increases the chances the neuron will fire an action potential. Meanwhile, a neurotransmitter decreasing the chances of firing an action potential would induce a change in charge, which is the inhibitory postsynaptic potential (IPSP).
Synapses are where the nerve impulses are exchanged between two neurons or between a neuron and other parts (ex. muscle cell or gland). They are specialized structures that form at the intersection point of the terminals and other neurons or muscle cells. They also contain neurotransmitters necessary for bodily functions.
Neural Firing
During neural transmission, the reaction within the cell is known as neural firing.
There are three main steps in neural firing:
1. Polarization of axon: resting potential
This is when the neuron is resting, where firing messages or ion exchange are not occuring.
During this time, the internal charge of the neuron is around -70 mV. Around the neuron, there are sodium ions on the outside, potassium ions on the inside and negatively charged chloride ions on the inside. The neuron is also “selectively permeable”, which means that ions cannot travel from inside to outside (and vice versa). Essentially, the gates of the neuron are closed.
2. Depolarization of axon: action potential
During action potential, a certain threshold is met in the dendrites. Consequently, the neuron stops being selectively permeable, which allows for ion exchange. Sodium ions come rushing in meanwhile potassium ions rushes out. Being attracted to the chloride ions, the sodium ions come in, while the potassium ions get repelled out of the neuron since it’s also positively charged. An action potential will then flow down the axon to the terminal, which will create a change in charge of the axon (it becomes positively charged). The myelin and nodes of the axon make the action potential travel faster during this stage.
3. Repolarization: refractory period
After the neuron releases neurotransmitters into synapse, the cell resets, leading to the refractory period. The neuron returns back to -70 mV charge and the polarization stage (the first stage).
Summary
Axons are long projections of a nerve cell, responsible for conducting electrical impulses away from the cell body. Within axons are axon hillocks, which play a role in relaying signals from the dendrites and cell body, to the terminals. It is at the terminal where signals can be released to other neurons for neural communication. The myelin sheath is also an important feature of the axon that accelerates signal transmission, which occurs in gaps called nodes of ranvier. Additionally, for neural transmission to be activated, a certain threshold must be reached, in which excitatory postsynaptic potentials are greater than inhibitory postsynaptic potentials. Neural firing occurs during neural transmission and has three main stages: resting potential, action potential, refractory period.
Sources
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