The neuron is a very specialized type of cell that serves to process and transmit information within the nervous system. Much like other cell types, neurons are surrounded by a phospholipid bilayer that acts as the cellular membrane. Within the membrane, one can find various proteins such as receptors, ion channels, transporters and signalling molecules. These proteins serve as an interface between processes occurring within the cell and external changes which illicit specific responses from the cell. For instance, different types of neurons have different sets of receptors that allow specific ligands to bind in order to signal the cell to respond appropriately, which may involve the opening of an ion channel which allows the natural ionic flow based on either concentration gradients or electric potential gradients between the inside of the cell and the outside of the cell.
Structurally, neurons can be very different from other cells. They are composed of a cell body, dendrites, axons and presynaptic terminals. The cell body is where the bulk of the cellular metabolism occurs and it is where the majority of the organelles are located. It contains the nucleus, which stores genetic material as well as the endoplasmic reticulum that is involved in protein synthesis. The golgi apparatus is located outside of the nucleus, and it is where proteins are processed, packaged and labelled. This is an important organelle, which directs proteins coming from the endoplasmic reticulum to their respective destinations within the cell or outside of the cell (secretion). Much like other cells, neurons also rely on the mitochondria for cellular respiration where ATP is produced. Other organelles found in neurons are lysosomes, responsible for the processing and degradation of cellular waste; and peroxisomes used for the breakdown of lipids to eliminate potentially harmful toxins.
The cytoskeleton is what determines the shape of a neuron. It contains three important filamentous structures, namely microtubules, neurofilaments and actin microfilaments. Microtubules provide a structural framework for the cell as well as having involvement in the transportation of molecules along axons. This is often done with the help of kinesins, kinectins and dyneins which serve as motor proteins along the microtubules. Neurofilaments are the “bones of the cytoskeleton”, which unlike microtubules, are very stable. Actin microfilaments are the smallest of the three types of fibers. They are usually found in short polymer species and they form the cellular matrix. Microfilaments are involved in the growth and development of axons; much like microtubules, they exist in a dynamic state.
From the cell body, usually two kinds of processes can be found, dendrites and axons. Dendrites are relatively small branching structures filled with knobby projections called dendritic spines. Dendrites are the main apparatus for receiving incoming signals from outside the neuron. The axon on the other hand are long extending processes that conducts signals for the purpose of communicating with other cells. The axons are capable of moving electrical currents (action potentials) of up to 100mV in amplitude from the cell body to the appropriate destination. These action potentials are initiated in a specialized region of the axon called the axon hillock, and is carried down the axon much like electricity in a copper wire. Myelinated axons are wrapped in myelin sheaths that serve as electrical insulators, increasing the speed of AP transmission. Nodes can be found between myelin segments where membrane proteins can be found. Lastly, axons terminate in synaptic knobs called buttons, where granules or vesicles are stored and secreted upon the arrival of action potentials.
Protein synthesis in neurons occurs in organelles called ribosomes, where the translation of mRNA into polypeptide sequences occurs. Following the translation of proteins, protein folding occurs where the amino acid sequence forming the protein will determine its native conformation. This can be done spontaneously or with the assistance of molecular chaperones. In many cases, post-translational modifications are often needed before the protein takes its active form. The movement of proteins inside neurons can be more challenging than in other cells, and often requires the use of the previously mentioned molecular motors that allow proteins to be carried along long axons.
Ganong WF (2005). Review of Medical Physiology, 22nd edition. McGraw-Hill
Kandel ER, Schwartz JH, Jessell TM (2000). Principles of Neural Science, 4th edition. New York: McGraw-Hill
Kalisch B (2013). Lecture notes for NEUR6000: Lecture #1 on 2013-01-10. Retrieved from CourseLink at Guelph.
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