Structural Biochemistry/Neurons

The neuron is a cell that transmits information via electrical and chemical signaling throughout the nervous system. There are many different neurons of various sizes and function.

Components

Neuron Structure: 1-Dendrites, 2-Axon, 3-Node of Ranvier, 4-Axon Terminal, 5-Schwann's Cell, 6-Cell Body, 7-Nucleus

All the brain and nervous system are based on communication among nerve cells, known as neurons. Each neuron is like any other cell in the body. Each neuron is surrounded by a membrane and filled with liquid and has a nucleus containing its genetic material. Neurons are specialized to receive and transmit information. All of the neurons gather information either from other cells of the body or from the environment. They transmit information to other neurons and/or other kinds of cells. A typical neuron has an enlarged area, which is the cell body. The cell body contains the nucleus. Neurons have branches or nerve fibers. The branches on which information is received are known as dendrites. The dendrites are a branched structure that receives signals from other cells. Each neuron has a longer tail-like structure, or axon. The axon transmits information to other cells. Axons can be branched at the tips. The axons of many kinds of neurons are surrounded by a fatty, segmented covering called the myelin sheath. The covering acts as a kind of insulation and improves the ability of axons to carry nervous system signals rapidly. A single neuron may be capable of receiving messages simultaneously on its dendrites and cell body from several thousand different cells. Most neurons usually have a soma, dendrites, axons, and a terminal button. The soma is the cell body of the neuron, and it contains the nucleus and is responsible for many of the processes of the cell.

Classification

Neurons are classified based on (1) the number of extensions from the cell body (soma) and (2) the neuron's function:

Sensory neurons receive sensory signals from sensory organs. These signals are then sent to the central nervous system via short axons. These neurons are also called Pseudo-unipolar neurons due to the short extension that divides them into two branches. One of these two branches functions as an axon, while the other functions as a dendrite.

Motor neurons are neurons that control motor movements of the body. They take commands from the cortex and send the signal to the spinal cord, or to the spinal cord to the muscles. Motor neurons and interneurons make up the family of multipolar neurons that possess a single axon and many dendrites.

Interneurons also called, associated neurons, are neurons that interconnect various neurons within the brain or spinal chord. The majority of these neurons are seen in the brain, connected densely. These neurons relay information and conduct signals between neurons. Interneurons may also be called bipolar neurons due to their two main extensions, a single dendrite and an axon.

Interneurons can be either efferent neurons, which carry signals away from the brain, or afferent neurons, which carry signals to the brain.

Neuron Classification

Neurons can be organized in two ways: 1) based on anatomy and 2) based on function. Neurons can be Pseudounipolar, bipolar, anaxonic or multipolar. Pseuounipolar neurons have a single axon and their soma are off to the side. Bipolar neurons have axons extending off both sides of the soma. Anaxonic neurons have no obvious axon but have a soma and dendrites. Multipolar neurons do not have long axons, but have extremely branched dendrites. Pseudounipolar and bipolar neurons are sensory (or afferent) neurons. Anaxonic and multipolar neurons are interneurons within the Central Nervous System. Multipolar neurons also function as efferent neurons.

Neuron Degradation

Neuron replacement is limited in the brain, though neuron phagoptosis does occur. Viable neurons are phagocytosized by lipopolysaccharide (LPS). Microalgia is responsible for the eating of viable neurons, but it is also responsible for eating apoptopic neurons which may be beneficial because it may reduce debris and inflammation. Inflammation in the brain can cause microalgia to eat viable neurons, however this can be blocked by blocking phagooptic signaling. Microalgia also kills developing neurons in the protein in the cerebellum and hippocampus.

Synapse

The synapse is a junction between the terminal button of an axon of one neuron and the dendrite of another neuron. In this junction, one neuron sends information to another neuron via electrical or chemical signaling. The process for sending information is called action potential where electrical impulses are sent down an axon of a neuron.

Also, the synapse is a small gap, or commonly referred to as a connection, between two cells that allows for the first cell (the presynaptic cell) to communicate with the second cell (the postsynaptic cell) through a chemical signal. These chemical signals are called neurotransmitters, and once they are released by the presynaptic cell, they act on the postsynaptic cell through specialized protein molecules called neurotransmitter receptors.

A synapse is a connection which allows for the transmission of nerve impulses. Synapses can be found at the points where nerve cells meet other nerve cells, and where nerve cells interface with glandular and muscular cells. In all cases, this connection allows for the one-way movement of data. The human body contains trillions of synapses, and at any given time, huge numbers of these connections are active.

Axonal Transport

Axons lack ribosomes and an endoplasmic reticulum and because of this, the cell body must synthesize proteins and send them through the axon via axonal transport. There are two main types of axonal transport: slow and fast. Slow axonal transport is used for moving proteins through the axon that are not used up quickly by the cell, such as enzymes and cytoskeletal proteins. Fast axonal transport is used for moving proteins down the axon that are needed much more quickly in the cell, such as organelles.

Microtubules and Their Role in Axonal Transport

Microtubules provide a crucial role in fast axonal transport systems that supply synaptic vesicles with vital chemical messengers by providing the long cells with highways for material to be transported through. Two families of proteins, Dynein and Kinesin, are in charge of vesicle transportation through microtubules. With Dynein being charge of retrograde transport and Kinesin being in charge of ante-retrograde transportation, their combined proportions provides the axon with variability in transport velocity as well as the potential for intentional halts in vesicle transport.

Information Processing

Information processing is the brain's process of interpreting the receiving information and knowledge. There are three stages in information processing: sensory input, integration, and motor input. There are also different types of neurons in the information processing. They are sensory neurons, interneurons, motor neurons, and neurons coming out of the brain. Sensory neurons transmit information from sensors like the ears that detect stimuli like sound. Interneurons are neurons that make up most of the neurons in the brain. Motor neurons are the ones transmit signals to muscle cells so that they can contract. Lastly, neurons that come out of the brain are nerves that instigate the reaction or motor output. In addition, there are two main nervous systems that help to interpret the information. They are the central and peripheral nervous systems. Central nervous system consist of the brain and the nerve cord where the neurons that are in charge of integration are here. And the peripheral nervous system consists of neurons that receive sensory input and result in the motor output. [edited:"There are three stages in information processing: sensory input, integration, and motor input." - correct is "and motor -output-"]

Information processing

Chemical Signaling

Chemical signaling is the physical chemical interchange that takes place in the synaptic cleft. Vesicles containing neurotransmitters are released by an incoming axon and received by receptors on opposing ends to induce a response on the recipient neuron. Chemical signaling via molecules secreted from the cells and moving through the extracellular space. Signaling molecules may also remain on cell surfaces, influencing other cells. Chemical signaling can involve small molecules (ligands) or large molecules (cell-surface signaling proteins). This signaling can be received either on the surface of cells by receptor proteins or within the interior of cells but also by receptor proteins. An example can be within-cell reception of signals is of steroid hormones. Signals also can be intentionally provided, such as is the case of hormones, or instead can be present for reasons that are not specifically for the purpose of providing a signal. The example can be carbon dioxide levels in blood.

The movement of neurotransmitters in a synaptic cleft

Glial Cells

Glial cells are not neurons. They significantly outnumber neurons and are therefore vital to the role of the nervous system. It was previously thought that Glial cells merely aided with physical support within the nervous system. However, Glial cells actually electrically communicate with neurons and provide important biochemical support to them. Common types of Glial cells include 1. oligodendrocytes 2. astrocytes 3. microglia and 4. ependymal cells, all of which are found in the Central Nervous System. Glial cells found within the Peripheral Nervous System include 5. Schwann Cells and 6. satellite glial cells.

1. Oligodendrocytes

Oligodendrocytes are one of the types of the neuroglial cells that is mainly responsible for myelinating central axons in central nervous system. Myelination refers to the act of oligodendrocytes wrapping around the axon with myelin sheath that is made of lipid and protein. Myelination of the oligodendrocytes have crucial effects on the transmission of neural signals by increasing the speed at which action potentials are conducted along axons. This action allows the neural signal to travel long distance with short energy and time.

Astrocytes stained for GFAP, with end-feet ensheathing blood vessels

2. Astrocytes

Astrocytes constitute 20-50% of the volume in most brain areas, especially in the central nervous system. It is mainly responsible for the physical and metabolic support of the brain. It has many other functions including generating numerous proteins such as N-CAM, laminin, fibronectin, growth factors as well as cytokines, which is responsible for signaling proteins involved in the immune system.

3. Microglia

Microglia is one type of the neuroglial cells that is mainly responsible for acting as macrophages. Microglia takes up about 5-20% of the mammal brains that act as mediators of immune response. Microglia cells constantly move around within the central nervous systems analyzing for damaged neurons, plaques, and infectious agents.

4. Ependymal cells

Epedymal cells aid in separating the fluid components of the Central Nervous system by creating an epithial layer and are also a source of neural stem cells. Epedymal cells in the ventricular system of the brain form capillaries that form chroid plexus in each ventricle of the each hemisphere of the brain. Chroid Plexus then produces cerebrospinal fluid (CSF). 60-80% of CSF comes from chroid plexus and rest from extrachrodial sources.

5.Schwann cells

Schwann cells' functions are very similar to oligodendrocytes. They myelinate neurons within the peripheral nervous systems instead of the neurons in the central nervous system. The main difference is that schwann cells are about 100 micrometres long that only covers the portion of the axons individually whereas one oligodendrocytes can mylinate multiple axons by stretching out their dendrites.

6. Satellite glial cells

Satellite glial cells are a type of glial cells that cover the exterior side of neurons in the peripheral nervous system. Satellite glial cells' functions are similar to astrocytes in the central nervous system. Although there is still ongoing research to discover the specific functions and mechanisms, it is so far discovered that the satellite glial cells supply nutrients to the peripheral neurons as well as regulating neurotransmitter by uptaking and inactivating the neurotransmitters.

Cerebrospinal fluid (CSF)

Cerebrospinal fluid(CSF) is bodily fluid that circulates around the nervous system and throughout body that is produced in choroid plexus of each ventricle system of the brain. It is commonly used for diagnostic information about the normal and pathological states of the nervous system.

Functions of CSF:

1. It provides buoyancy and support to brain and chord that protects against rapid movements and trauma.

2. The fluid delivers nutrition for both neurons in both CNS and PNS and for other glial cells.

3. It functions like lymphatic system and removes wastes out from the nervous system.

4. It controls homeostasis of the ionic composition of the local microenviroment of the cells of the nervous system.

5. It acts as a transport system for releasing factors, hormones, neurotransmitters, metabolites.

6. The fluid controls H+ and CO2 concentrations (pH levels) in the CSF that may affect both pulmonary ventilation and cerebral blood flow.

7. The fluid is essential in medical fields in which it provides diagnostic information about the nervous system.

References

Gorazd B. Stokin and Lawrence S.B. Goldstein, "Axonal Transport and Alzheimer's Disease". Annual Review of Biochemistry Vol. 75: 607-627 (Volume publication date July 2006) Print

Silverthorn, D. (2012) Human Physiology: An Integrated Approach, 6th edition. Prentice Hall.

Purves, Dale, Principles of Cognitive Neuroscience, Sinauer Associates, Inc., 2008.

http://commons.wikimedia.org/wiki/File:Astrocytes.jpg

Dubuc, Bruno. <http://thebrain.mcgill.ca/flash/a/a_01/a_01_cl/a_01_cl_ana/a_01_cl_ana.html>