Neuroscience/Cellular Neurobiology/Cells of the Brain

The cellular basis for the nervous system lies in two classes of cells, neurons and glia. Neuronal cells function as the information processing and transfer cells, while glial cells are generally considered support cells (though this is not a rigid statement.)

Neuron Morphology

The neuron has a body and projections that reach out of the cell resembling a tree growing out of the ground.

Cell Membrane

The cell membrane is a phospholipid bilayer that denotes the boundary between the cell and the extracellular solution. It is responsible for the regulation of the flow of the substances inside and outside of the cell, and the bilayer has numerous proteins on it that perform various functions. Specific proteins allow for the passage of small soluble molecules through the membrane giving it a selective permeability.

The Body

The body (soma) of a neuron contains all the organelles necessary for cellular function: the nucleus, endoplasmic reticulum(smooth and rough), ribosomes, Golgi apparatus, and mitochondria all lie inside a neuron's soma.

Axon

The axon is a projection that grows out of the soma. It is one part of the cell designed specifically for the transfer of information between neurons. The axon begins with the axon hillock which leads to the beginning of the axon which continues until it reaches the synapse (a site of communication between two cells.) The cytosol of an axon is different from that of the cell body in that no protein synthesis is carried out, so all proteins to be used in the synapse will be carried down the axon to the synapse. Axons vary in length (<1mm to >1m) and diameter, and when the axon branches, the branch is called an axon collateral.

Axoplasmic Transport

This transport accounts for the movement of materials that sustain the axon and provide for the neurotransmitters in the synapse. There are three types of transport: fast axoplasmic transport, slow and retrograde. Because slow transport occurs at a rate of 1-10 mm a day, it is suitable not for active synapses at the axon terminal but for moving receptors and other membrane proteins. Neurotransmitters are brought to the terminal by fast transport that moves proteins down the axon at rates near 1000mm a day. These speeds are achieved by the protein kinesin, a biped, which, fueled by ATP, follows microtubule tracks down the axon. Essentially, kinesins carrying vesicles containing neurotransmitters, for example, "walk" towards the plus end of the microtubule, leading to the axon terminal.

Axon Terminal

The axon (and its collaterals) will eventually terminate as an axon terminal at a synapse. At the synapse, the axon terminal interfaces with the dendrite (or soma) of another cell. Axons that terminate on other cells are said to 'innervate' those cells. The area of the cell membrane that lies in the synapse of the axon terminal has a higher concentration of proteins.

Synapse

There will be more on the synapse later, but the synapse is the site of transfer of information between cells, in the form of neurotransmitters (proteins) that are spat out of one cell of the synapse (a synapse is the connection between two cells) and recognized by the other. The direction of information determines the label given to the sides. The side of the synapse releasing the ntr (neurotransmitter) will presynaptic and the side receiving postsynaptic.

Dendrites

Neurons project dendrites that branch out of the soma in a shape resembling a tree. Like the axon, the dendrites' function is the transfer of information but rather than transmitting it, they receive it. Dendrites' membrane typically has many (thousands) synapses, indicating the reception of information from many other cells. Whereas the axon is presynaptic in the synapse, the dendritic membrane is said to be postsynaptic. On this postsynaptic membrane, there are many proteins that act as receptors for neurotransmitters (think of the lock on a door as the receptor and the neurotransmitter as the key.)

Neuron Types

Cells are usually classified by the feature most relevant to the topic being discussed.

By Poles

Cells may be distinguished by the number of projections: unipolar and bipolar cells have one and two projections respectively, and multipolar cells have many projections. Polarity may be determined by looking at a Golgi stain (a slice of the brain stained so that individual cells stand out).

By Shape

In the cerebral cortex, most cells can be classified as stellate or pyramidal because their dendrite structure is reminiscent of a star (stellate) or a pyramid.

By Connections

Neurons may also be classified by the connections they make in the nervous system. Neurons that deliver sensory information from the rest of the body to the brain are primary sensory neurons, and neurons that synapse on muscles are motor neurons. Interneurons connect one neuron to another.

By Neurotransmitter Used In Function

Another significant classification is based on the neurotransmitters used by a particular neuron or system of neurons. For example, motor neurons use acetylcholine to transfer motor impulses, and these neurons are said to be cholinergic neurons.

Non-Neuronal Cells

Glial Cells

Glia are thought to be the cells that provide an environment in which neurons can function. There are various types of glial cells, each filling a highly specialized role in the nervous system.

Astrocytes:This star-shaped cell occupies most of the volume of the brain not taken up by blood vessels or neurons. An important function of the astrocyte is the regulation of the extracullular concentration of various ions.

Schwann cells: Schwann cells are the cells that provide myelination to the axons of neurons in the PNS. Their CNS equivalents are the oligodendrocytes. Schwann cells cover the myelin sheaths much like a marshmallow on a stick, secreting spiraling layers of myelin directly onto the axon surface. Schwann cells have a thin, protective outer covering called a neurolemma. The spaces between myelin sheaths and their covering Schwann cells are called Nodes of Ranvier. Schwann cells are only found in the PNS.

Oligodendrocytes: Oligodendrocytes are the cells that provide myelination to the axons of neurons in the CNS. Their PNS equivalents are the Schwann cells. The main difference in Schwann cells and oligodendrocytes is that one Schwann cell can only myelinate one small portion of an axon, whereas an oligodenrocyte can myelinate many sections of an axon, or even multiple axons.

Ependymal cells: Ependymal cells are small, ciliated epethelial cells that line the cavities of the CNS neural organs, and help to move CSF by the movements of their cilia.

Microglia: Small, spiky looking cells of the CNS that support neurons much like astrocytes, but can morph in phagocytic cells to help clean up the CNS, as immune system cells are not granted entry into the CNS.

Satellite cells: Very small cells that provide outer coverings to the somas of neurons. Their function(s) is/are largely unknown.

Resources

Glossary