Main Difference – Axon vs Dendrite
Axon and dendrite are two components of nerve cells. Nerve cells are the structural and functional units of the nervous system of animals. They transmit nerve impulses to the brain, spinal cord, and to the body to coordinate the functions of the body. An axon is a long conical prolongation of the cell body of the nerve cell. Every nerve cell has an axon. The short structures that extend from the cell body are called dendrites. A single nerve cell has many dendrites. The main difference between axon and dendrite is that axon carries nerve impulses away from the cell body whereas dendrites carry nerve impulses from synapses to the cell body.
Key Areas Covered
1. What is an Axon
– Definition, Characteristics, Function
2. What is a Dendrite
– Definition, Characteristics, Function
3. What are the Similarities Between Axon and Dendrite
– Outline of Common Features
4. What is the Difference Between Axon and Dendrite
– Comparison of Key Differences
Key Terms: Axon, Axon Hillock, Cell Body, Dendrites, Myelin, Myelinated Nerve Fibers, Nerve Cells, Non-Myelinated Nerve Fibers
What is an Axon
An axon is single, long projection of a nerve cell. Axons carry nerve impulses away from the cell body. The membrane that covers the axon is called the axolemma. Axoplasm is the cytoplasm of the axon. Axons are branched at their terminal ends. The tips of the branched ends are formed by telodendria. The axon terminals are the swollen ends of the telodendria. The axon terminals form the synaptic connection with a dendron of another neuron or with an effector organ. The membrane of the axon terminal is linked to the membrane of the target cell. Vesicles that contain neurotransmitters are present in the axon terminals to transmit the nerve impulses by means of chemical signals through the synaptic gap. The axon hillock is the initial segment of an axon. It initiates the action potential. A cross-section of an axon is shown in figure 1.
Figure 1: Axon cross section
1 – Axon, 2 – Nucleus of the Schwann cell, 3 – Schwann cell, 4 – Myelin sheath
The two types of axons are myelinated axons and non-myelinated axons. The myelin sheath forms an insulation on the axon to increase the speed of transmission of nerve impulses through the axon. This type of transmission of nerve impulses is called saltatory conduction. Schwann cells secrete myelin on the axons of the peripheral nervous system. Oligodendrocytes secrete myelin on the axons of the central nervous system. Myelinated axons are white in colour. The gaps in the myelin sheath are called the nodes of Ranvier. The white matter of the brain and the spinal cord is composed of myelinated axons.
What is a Dendrite
A dendrite is a short-branched extension, which carries nerve impulses to the cell body from the synapses. Many dendrites are extended from a single cell body of a nerve cell. Dendrites are highly branched structures. This highly-branched nature increases the surface area that can receive signals from the synapses. Dendrites and axons of nerve cells are shown in figure 2.
Figure 2: Dendrites and Axons
Dendrites possess tapering ends. Since dendrites are short projections, they are not myelinated.
Similarities Between Axon and Dendrite
- Both axon and dendrite are projections of the cell body of a nerve cell.
- Both axon and dendrite transmit nerve impulses.
- Both axon and dendrite are branched structures.
- Both axon and dendrite contain neurofibrils.
Difference Between Axon and Dendrite
Definition
Axon: Axon is the long thread-like part of a nerve cell which conducts nerve impulses away from the cell body.
Dendrite: Dendrite is the short branched extension of a nerve cell, which transmits nerve impulses to the cell body from synapses.
Number
Axon: A nerve cell has only one axon.
Dendrite: A nerve cell has many dendrites.
Origin
Axon: An axon arises from a conical projection called axon hillock.
Dendrite: Dendrites arise directly from the nerve cell.
Length
Axon: Axons are very long (several meters).
Dendrite: Dendrites are very short (around 1.5 mm).
Diameter
Axon: Axons have a uniform diameter.
Dendrite: Dendrites have tapering ends; therefore diameter continuously decreases.
Branching
Axon: Axons are branched at their ends.
Dendrite: Dendrites are branched all along.
Synaptic Knobs
Axon: The tips of the terminal branches of the axon are enlarged to form synaptic knobs.
Dendrite: No synaptic knobs occur at the tips of the branches of the dendrites.
Vesicles
Axon: The synaptic knobs of the axons contain vesicles with neurotransmitters.
Dendrite: Dendrites do not have vesicles that contain neurotransmitters.
Nissl’s Granules
Axon: Axons do not contain Nissl’s granules.
Dendrite: Dendrites contain Nissl’s granules.
Myelinated/Non-myelinated
Axon: Axons can be either myelinated or non-myelinated.
Dendrite: Dendrites are non-myelinated.
Direction of the Transmission
Axon: Axons carry nerve impulses away from the cell body.
Dendrite: Dendrites carry nerve impulses towards the cell body.
Afferent/Efferent
Axon: Axons form the efferent component of the nerve impulse.
Dendrite: Dendrite form the afferent component of the nerve impulse.
Conclusion
Axon and dendrite are the two types of projections of a nerve cell. Both axons and dendrites transmit nerve impulses. Axon is longer than a dendrite. The diameter of an axon is uniform while dendrites consist of tapering ends. Some axons are myelinated to speed up the transmission of nerve impulses. Axons transmit nerve impulses away from the cell body, and dendrites transmit nerve impulses towards the cell body. Therefore, the main difference between axon and dendrite is the direction of the transmission of nerve impulses.
Reference:
1.“ Axon.” Wikipedia, Wikimedia Foundation, 1 Sept. 2017, Available here. Accessed 12 Sept. 2017.
2.“Dendrite.” Wikipedia, Wikimedia Foundation, 1 Sept. 2017, Available here. Accessed 12 Sept. 2017.
Image Courtesy:
1. “Myelin sheath (1)” By Ralph Walterberg – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Neuron Part 1” By BruceBlaus – Own work (CC BY-SA 4.0) via Commons Wikimedia
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