The+Structure+of+Neurons+and+Glia

=**The Structure of Neurons and Glia**=


 * 1. Know the major morphological components of neurons (soma, axon, dendrites) and be able to recognize and classify neurons based on their morphology (e.g. pseudounipolar, bipolar, multipolar).**

Generally, neurons are composed of a cell body (soma), dendrites, axon, and terminals. Depending on the cell type, there may be one or many dendrites. There is typically only one axon but it usually highly branched and end in multiple terminals at their distal ends.

Unipolar neurons have a single long primary process and are rare in mammals. Bipolar neurons have two primary processes, one for signal reception and one for signal transmission, each arising at opposite poles of the soma. Pseudounipolar neurons have a single process that bifurcates near the soma with one process directed peripherally (axon) and one directed centrally (dendrite). Pseudounipolar neurons are found in spinal and cranial nerve ganglia. Multipolar neurons have many primary processes, each of which typically branch. Typically multipolar neurons like spinal motor neurons and cortical pyramidal cells and have one process for impulse conduction and many for impulse reception.


 * 2. Be able to describe the function of the soma, axons, dendrites, and terminal boutons of the neuron.**

The soma is the metabolic and signal integration portion of the neuron. It has a high degree of protein synthesis which may be cytosolic, mitochondrial/nuclear, or membranous. Its rough ER is well-developed and visible as nissl bodies.

Axons are the signal transmitting portion of the neuron. It is usually a long cytoplasmic process responsible for conducting action potentials over long distances. Axons may be branched to give off several collateral targets. The larger the diameter, the faster the conductance.

Dendrites are the signal receiving portion of the neuron and are typically highly branched and possess numerous cytoplasmic synaptic contacts with axon terminals of other neurons. Dendritic spines are outpockets of dendritic membrane specialized to receive synaptic contacts.

Terminal boutons are bulb-like swellings at the terminal ends of axons where they form synaptic connections with other neurons. Synaptic densities are aggregations of electron dense material on the cytplasmic surface of synaptic membranes called “active zones” and are formed by docking proteins. These densities are on both sides of the synapse and may be asymmetric with postsynaptic density thicker or symmetric with both sides relatively equal.


 * 3. Be able to describe the intracellular ultrastructure and organelles in the soma of the neuron.**

Neuron nuclear mRNA produces three classes of proteins: cystolic, mitochondrial/nuclear, and membranous. Cystolic proteins form the fibrillar elements of the cytoskeleton and produce enzymes necessary for metabolic reactions. Mitochondrial/nuclear proteins are synthesized in the cytosole and incorporated into the nucleus, mitochondria, or peroxisomes. Membranous proteins are synthesized in association with the cell membrane and secretory products and include the unit membranes of cell and cytoplasmic organelles, secretory products of the neuron, and transmembrane proteins and receptors.

The rough ER of neurons are highly-developed and have numerous polysomes arranged in rosettes and free ribosomes interspersed between rough ER cisternae.

Microtubules play a role in transport of membrane proteins and organelles throughout the neuron. Neurofilaments are intermediate filaments aligned in closely packed parallel bundles. Neurofibrillary tangles are associated with brain aging and Alzheimer’s disease. Microfilaments are thin actin filaments and function in structural support.


 * 4. Understand the mechanisms and function of axoplasmic transport.**

Because most neuronal protein is produced at the soma, they must often be transported to the axon terminals by axoplasmic transportation using microtubules. Organelles are ATP-propelled along the microtubule scaffold by dynein and kinesin. Antrograde transport uses kinesin as a motor and has a slow and a fast speed. Slow antrograde transport moves at 1-4 mm/day and transport cytoskeletal proteins (actin, myosin, clarthrin, alpha- and beta-tubulin) and cytosolic enzymes. Fast antrograde transport moves at 50-400 mm/day and transports membrane associated proteins, organelles, and neurotransmitters. Retrograde transport occurs at 200 mm/day and uses dynein as a motor. Retrograde transport moves “worn out” membrane components for recycling in the soma and plays a role in feedback control of trophic interactions between axon terminals and the cell body.


 * 5. Understand the function of myelin and the process of myelination.**

All axons are ensheathed by the cytoplasmic processes of surrounding glial cells. Myelinated axons are large diameter axons wrapped by a lipid-based, segmented, multilayered covering called Myelin derived from glial cell cytoplasm. This allows for insulation necessary for saltatory conduction. Unmyelinated axons are smaller diameter axons wraped by only a single layer of glia cytoplasm.

Oligodendroctyes myelinates several axons in the CNS. Schwann cells only myelinates one axon in the PNS in a 1:1 relationship.

Myelin is not a continuous sheath over the entire length of the axon but interrupted by short unmyelinated gaps called the nodes of ranvier which allow for an action potential to travel faster by leaping form node to node during saltatory conduction.


 * 6. Describe the morphology of a synapse.**

When axons near their termination or point of contact with a target neuron, they form a synapse. The ends of the axon enlarge to form a synaptic bouton and becomes closely apposed with the membrane of the target neuron (usually the dendrite). The membrane of the synaptic bouton at this junction is called the presynaptic membrane; the membrane of the target is the postsynaptic membrane. These two membranes are separated by the synaptic cleft.

The three major types of synaptic contacts are axodendritic (including axospinous), axosomatic, and axoaxonic contacts. Chemical synapse are the most common type, using vesicle bound neurotransmitters and peptides to initiate neural excitation. Alternatively, synapses can be electrical, with synaptic membranes connected by gap junctions, utilizing voltage gated iontophores to induce excitiation.

There are three major types of synaptic vesicles: round clear, flattened clear, and dense core. Round clear vesicles are usually excitory and found in asymmetric synapses. Flattened clear vesicles are usually inhibitory gamma-amino butyric acid (GABA) and found at symmetric synapses. Dense core vesicles are thought to be associated with catecholamines.


 * 7. Be able to identify the support cells of the nervous system and describe their function.**

The glial cells are the support cells of the nervous system. They function in providing scaffolding of neurons, myelin formation, K+ buffering following depolarization, marcophages, blood/brain barrier function, and guidance of developing axons. All neuronal surfaces except synaptic sites are enclosed by glial cells. Glial cells are capable of self-renewal by mitotic division or from precursor stem cells but are also a primary source of brain tumors.

In the CNS, there are three major types of glial cells: oligodendrocytes, astrocytes, and microglia. Oligodendrocytes form myelin around neurons of the CNS. There are two types of astrocytes: fibrous astrocytes are found in areas of high myelin (white matter; axon tracts) while protoplasmic astrocytes are found associated with neuroncal cell bodies (gray matter; ganglia) or in close association with blood vessels.

Astrocyte processes are opposed to capillaries, axons, and neuronal cell bodies, providing a mechanism for nutrient/waste exchange. Furthermore, astrocyte processes may also play a role in uptake and deactivation of neurotransmitter agents at the synapse. Lastly, astrocytes proliferate at the site of neuronal damage.

Microglia are phagocytic cells of the CNS and part of the mononuclear phagocytes system. They are the smalless glial cells with the must numerous and highly branched processes.

In the PNS, there are Schwann cells and satellite cells. Schwann cells are myelin forming cells, myelinating axons at a 1:1 ratio. Satellite cells surround neuron cell bodies and appear to have some nutritive function.


 * 8. Know the general processes of neuronal degeneration.**

Neurons do not have the capability for mitotic division or replenishment from a stem cell population. That is, damaged neurons in the brain cannot be replaced. Neuronal damage can result from a variety of causes:

Ischemic insult – loss of blood supply or nutritional support Excitotoxicity – over stimulation leading to the release of neuronal toxins Direct traumatic damage – head injury Neurodegenerative disease – Alzheimers, Huntingtons, Parkinsons, etc. Demyelinating disease – Multiple Sclerosis


 * 9. Describe the general components of the blood/brain barrier.**

The blood brain barrier is a specialized barrier between the brain cells and the systemic circulation. It provides for the exchange of nutrients/waste between blood and neurons and also protects neurons from bloodborne molecules that act as neurotransmitters, and from dangerous substances like toxic drugs or bacterial toxins.

To reach through the barrier, a molecule must pass through the capillary endothelial cells (which have tight junctions), pericytes (mesenchymal-like cells surrounding endothelial cells), the basement membrane, and the foot-like processes of astroglial cells which abut the basement membrane.

Transport through the blood brain barrier is usually through active transport using specialized receptors on the endothelium. The foot-like processes of astrocyte glial cells pick up substances and transport them to neurons via their processes.