Neurons and Synapses: The Art of Communication


Hierarchical Organization of the Brain, Neuron and Synapse
The brain communicates with itself through it's neurons, and neurons communicate with each other through their synapses
  • external image neuron.jpgThe cerebral cortex is segmented into 4 lobes: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. This cortex contains glial cells and neurons (together forming the "gray matter"). Glial cells serve to support and insulate neurons but do not conduct electrical impulses, while neurons are specialized cells that make up the basic working unit of the brain.
  • There are numerous parts of the neuron which allow it to perform its function of conducting signals to other neurons. Dendrites are extensions of the cell body that provide a large surface area to receive information from other neurons to then pass on to the soma. The soma is the cell body, containing the nucleus and many other organelles. Connected to the soma is a long axon, which is the communication route between the cell body and terminals. Schwann Cells, a type of glial cell, encircles the axon. The Nodes of Ranvier are the gaps between the myelin sheath segments of the axon along which the action potential travels. At the end of the axon is the axon terminal, at the ends of which are the synaptic end bulbs
(synaptic buttons).

It is at this point that this neuron meets with another neuron at the synapse. The synaptic cleft is the region between the axon terminal of one neuron and the dendrites of another. An impulse travels down the presynaptic neuron, forcing vesicles to its membrane, causing neurotransmitters once contained within the vesicle to be expelled into the synaptic cleft. These neurotransmitters travel across the synaptic cleft to bind to receptors in a "lock and key" arrangement. Once bound to the receptors, the neurotransmitters signal either an external image ann.gifinhibitory or an excitatory postsynaptic potential (PSP) in the postsynaptic neuron, causing the neuron to either fire or not fire an impulse (all-or-none law). After signaling a PSP, the neurotransmitter is released. The neurotransmitters are returned to the vesicles of the presynaptic neuron through the reuptake channels or is broken down by enzymes in the synaptic cleft. Millions upon millions of these interactive spaces exist, allowing the body to form a wide range of functions.

Video Showing How Neurons Fire

How the Structure of a Neuron Relates to its Function

Neurons are the building blocks of the nervous system. Their function is to transmit information throughout the body. A typical neuron consists of dendrites, a cell body (soma), an axon, and an axonal terminal.

· Most neurons have multiple dendrites, which extend out-ward from the cell body and are specialized to receive chemical signals from the axon terminal of other neurons. They extend outward because they need to be able to recieve signals from multiple neurons and connect neurons to each other.
· The cell body or soma contains the nucleus and is the site of synthesis of virtually all neuronal proteins and membranes. It is contained in a small area because the nerve cell needs to be long with many connecting sites in order to transmit messages. The nucleaus is also at the top of the neurons next to the dendrites because it needs to pass along the signal and tell the nerve cell what to do with it.
· Axons are specialized for the conduction of a particular type of electric impulse, called an action potential, away from the cell body toward the axonal terminal (outward.) There are so many so that the cell can transmitt its messages fast and to a variety of places.
The function of the myelin sheath is to speed up the electrical impulse and protect the nerve cell. Sp, it is made up of layers of a Schwann cell membrane for maximum protection and efficiency of resources. They are have gaps between them (nodes of Ranvier) so that the signal can jump from gap to gap and reach its destination quickly, instead of having to take lengthy waves.


The Series of Events of the Transmission of a Nerve Impulse

In the synapse, the Pre-Synaptic Neuron experiences an inhibitory or an excitatory signal, and when it crosses the threshold, undergoes an actNeurotransmission.gifion potential. Gated Sodium channels open, rapidly taking the voltage inside the cells from -70mV to 40mV. This occurs in a positive feedback cycle, which spreads the action potential down the axon. This rapid increase in voltage triggers the opening of Potassium channels, which quickly takes the voltage from 40mV to -70mV, restoring equilibrium, and preventing another action potential from occurring for a short time. This ensures that the action potential always travels in only one direction. When the Action Potential reaches the Axon Terminal, it triggers the vesicles filled with neurotransmitters to release the neurotransmitters by exocytosis. The vesicles bind with the pre-synaptic membrane, and release the neurotransmitters that fill the synapse, and some bind to receptors on the Post-Synaptic Neuron. Each neurotransmitter can bind with a different and specific receptor. These receptors trigger changes in the voltage of the Post-Synaptic Neuron, either raising the voltage or lowering it. If the voltage is raised to the threshold, another action potential will be propagated. Similarly, if the voltage is decreased, it will decrease the likelihood of the Post-Synaptic Neuron reaching the threshold for an action potential. The neurotransmitters that didn't bind with the receptors are left in the synaptic cleft. The remaining ones are then reabsorbed by transport proteins that take them through reuptake channels and allow neurotransmitters to diffuse back into the Pre-Synaptic Neuron to be re-packaged in vesicles for the next action potential, while any remaining neurotransmitters left in the synapse is generally broken down by enzymes.

Neurons and Action Potentials

Inhibitory and Excitatory Neurotransmitters: Their Influence

When an action potential reaches the end bulb of a neuron, neurotransmitters are released into the synaptic cleft. These neurotransmitters are either inhibitory, like GABA, or excitatory, like glutamate. These neurotransmitters affect the post-synaptic potential by binding to receptor sites on the post-synaptic neuron. Inhibitory neurotransmitters alter the transmission of an impulse by weakening the transmission. This is done by making the post-synaptic neuron "hyperpolarized," making the neuron less likely to transmit a charge. Excitatory neurotransmitters increase the intensity of the impulse by increasing the concentration of positive sodium ions and decreasing the concentration of negative potassium ions in the post-synaptic potential, making the neuron more likely to carry the impulse.

GABA, being part of the inhibitory system, slows the process of neural decreasing. GABA is the major inhibitory neurotransmitter, and helps to lessen anxiety and stress. It controls norepinephrine, adrenaline, dopamine, and serotonin. Glutamate, however, is the opposite of GABA- it is an excitatory neurotransmitter. Glutamate is the most common excitatory neurotransmitter, and increases neural impulses dramatically, which releases more dopamine; this gives us a rewarding, "good" feeling.

Neurotransmitters

The Role of the Blood Brain Barrier in Drug Transport

blood-brain-barrier-picture.jpgThe blood brain barrier is a semi-permeable membranic structure that works towards protecting the brain from chemicals in the blood and maintaining a state of homeostasis in the brain. Any drug that targets the brain must first pass the blood brain barrier, which is made of millions of tightly-packed endothelial cells that are extremely selective in what can get through. With very few exceptions, only small molecules soluble in fat can pass the barrier. Alcohol, caffeine, and nicotine, as well as antidepressants, all fit this criteria which allows them to get through the barrier. Unfortunately, in trying to protect the brain, the blood brain barrier also hinders the delivery of many potentially important diagnostic and therapeutic drugs to the brain. Today, some therapeutic drugs that may be effective in diagnosis and treatment of brain disorders are not able to cross the barrier in adequate amounts.

Other drigs of abuse are designed to cross the BBB. Methamphetamine is an Amphetamine molecule with an extra methyl group attached which gives it the ability to cross the blood brain barrier more efficiently and in turn makes it much more potent than regular amphetamine.
Functions of the BBB
The BBB has several important functions:
  1. Protects the brain from "foreign substances" in the blood that may injure the brain.
  2. Protects the brain from hormones and neurotransmitters in the rest of the body.
  3. Maintains a constant environment for the brain.

General Properties of the BBB

  1. Large molecules do not pass through the BBB easily.
  2. Low lipid (fat) soluble molecules do not penetrate into the brain. However, lipid soluble molecules, such as barbituate drugs, rapidly cross through into the brain.
  3. Molecules that have a high electrical charge are slowed.

The BBB can be broken down by:

  1. Hypertension (high blood pressure): high blood pressure opens the BBB.
  2. Development: the BBB is not fully formed at birth.
  3. Hyperosmolitity: a high concentration of a substance in the blood can open the BBB.
  4. Microwaves: exposure to microwaves can open the BBB.
  5. Radiation: exposure to radiation can open the BBB.
  6. Infection: exposure to infectious agents can open the BBB.
  7. Trauma, Ischemia, Inflammation, Pressure: injury to the brain can open the BBB.