Chapter 12. Neurochemistry
Image of a Nerve Cell
The nerve cells of the central and peripheral nervous systems are called neurons. Most neurons have three parts; an axon, a cell body or soma, and dendrites. All neurons have one soma and one axon, but while some neurons have many dendrites, others have none.
Neurons vary in size; the smallest have a diameter of 5 microns while the largest are approximately 100 microns in width. (A micron is one one-thousandth of a millimeter).
The soma of a neuron contains the cell's nucleus and cytoplasm, a jelly-like substance that surrounds the nucleus.
Chromosomes, which consist of molecules of DNA (deoxyribonucleic acid) are found in the nucleus of the neuron. RNA (ribonucleic acid) molecules are also located within the nucleus.
Nissl Bodies or Nissl Substances, which also contain RNA, and Golgi Apparati are found in the cytoplasm.
DNA forms the genetic code that determines the cell's function. As DNA cannot pass through the nuclear membrane, its commands are carried to the cytoplasm by messenger RNA which can travel outside of the nucleus. These RNA molecules link up with the Nissl substances, connecting to them as a key would fit into a lock. This combination causes the cell to work, or use glucose. The waste products generated by this process are removed from the cell by the Golgi apparati.
The axon allows the neuron to send messages to other nerve cells. Each neuron has only one axon, but this may have numerous branches which connect the cell to many other.
Axons vary in length; the axons of some pyramidal cells in the precentral gyrus are long enough to travel all the way down to the end of the spinal cord but other axons are very short.
Cells can be classified based on the length of their axons. Golgi Type I neurons have long axons while those of Golgi Type II cells are short.
Axons arise from an area on the cell body of the neuron called the axon hillock. Most axons form many branches as they extend away from the soma. At its end, each axonal branch divides into a number of telodendria. The boutones terminaux or boutones de passage, which contain neurotransmitters, are located on the telodendria.
Myelin
Many of the axons in the central and peripheral nervous systems are covered at regular intervals with a fatty insulating substance called myelin. The segments of the axon which lie between areas of myelin and are therefore in direct contact with extracellular fluid are called the Nodes of Ranvier.
Myelin coating increases the speed with which an axon can transmit messages. The neural impulse travels by a process known as saltatory conduction, jumping from one unmyelinated segment to the next. This means that the impulse does not have to be propagated through the entire area of the axon.
Dendrites
The dendrites of a neuron receive messages from the axons of other nerve cells. There are two types of dendrites, apical dendrites and basilar dendrites.
Apical dendrites have stalks filled with cytoplasm that appear to be part of the soma of the neuron to which they are attached. Most of these dendrites are found in the cerebral cortex.
Basilar dendrites do not have a stalk. They are more numerous than apical dendrites.
Links with
relevant information about nerve
cells:
For more information on Nerves and nerve cells visit the following web sites
Neural Definitions: http://psych.hanover.edu/Krantz/neural/neurldef.html
A Self-Quiz: http://psych.hanover.edu/Krantz/neural/struct3.html
The Transmission of Neural Messages
The Action Potential
The messages conducted along axons are electrochemical in nature.
Four different types of ions, or electrically charged atoms, are involved in the transmission of neural impulses; chloride ions (Cl-), sodium ions (Na+), potassium ions (K+), and organic anions (A-).
When a neuron is at rest, there are high concentrations of A- and K+ within the cell, while most Na+ and Cl- ions are located outside its membrane. The resting potential of the neuron is -70 millivolts, meaning that the electrical charge of the cell is slightly negative in comparison to that of the extracellular fluid surrounding it.
This arrangement is due to the selective permeability of the neural membrane. Cl- and K+ can pass through the membrane, but Cl- does not enter the cell in great quantities because both it and the interior of the neuron are negatively charged. Organic anions cannot move through the membrane due to their large size. As Na+ is positively charged and the interior of the nerve cell has a negative electrical potential, these ions should pass into the cell. However, sodium cannot readily pass through the membrane and most of the Na+ ions that do enter the cell are extruded by the sodium-potassium pump. This is the name given to a group of molecules located in the cellular membrane which push Na+ out of the cell and draw K+ ions inside.
The resting potential of a neuron changes when messages are received from other nerve cells. Inhibitory impulses cause the electrical charge of the neuron to become even more negative, decreasing its ability to fire, or send messages to other nerve cells. When excitatory messages are received, however, the permeability of the cell membrane changes, allowing Na+ ions to enter the neuron. This influx of positively charged ions causes the cell's electrical potential to temporarily become positive, peaking at +40 millivolts. This change of the electrical potential of the cell from negative to positive is the action potential, which ultimately causes the cell to fire.
When the charge of the cell reaches its positive peak, K+ ions are forced out of the neuron because they are positively charged. The exit of these ions causes the potential of the cell to become negative again, temporarily dipping below -70 millivolts before returning to resting potential.
After the cell fires, it goes through a refractory period during which it will not fire again. The refractory period may be divided into two phases, the absolute refractory period and the relative refractory period.
During the absolute refractory period, the neuron will not fire again, no matter how strong the excitatory messages that it receives.
The cell will fire during the relative refractory period, but only if it receives a very strong stimulus.
When a neuron fires, it communicates with other nerve cells through the release of chemicals called neurotransmitters.
Neurotransmitters are found in the boutons terminaux located on the teledendria of the axons. They are stored in circular or oval-shaped capsules called synaptic vesicles.
When an excitatory impulse of sufficient strength reaches the teledendria, the synaptic vesicles fuse with the axonal membrane and open, spilling the chemicals they contain into the extracellular fluid. The neurotransmitters then travel across a small space called the synaptic cleft to attach to the neuron that will receive their message.
After a message has been sent, excess quantities of the neurotransmitter that remain in the synaptic cleft must be cleared away in order to allow further communication between the cells. In some cases, the left-over chemicals are recycled; they are picked up and repackaged in new synaptic vesicles to be used in future transmissions. Other types of neurotransmitters are destroyed by enzymes when they remain in the synaptic cleft.
Acetylcholine (ACh) is one neurotransmitter that has been well-studied. It is the major neurotransmitter of the peripheral nervous system and is also present in the central nervous system. It carries messages controlling voluntary muscle movement, as the nerve fibers located in muscles and in the spinal and cranial nerves are acetylcholinergic. After messages have been transmitted, ACh is broken down in the synaptic cleft by an enzyme called acetylcholinesterase.
An insufficient supply of acetylcholine, whether due to excess quantities of acetylcholinesterase or resulting from inadequate synthesis of the chemical, causes Myasthenia Gravis. In this disorder, the strength of neural impulses is attenuated, causes the voluntary movement of muscles, including those involved in articulation, voicing and respiration, to be weakened.
Myasthenia Gravis is distinguished from other disorders like ALS by the administration of a derivative of curare. This drug will temporarily improve the strength of someone suffering from Myasthenia Gravis, but will have no effect in cases of ALS.
Myasthenia Gravis should not be confused with Myasthenia Laryngis, which is a localized weakness in the larynx and is not the result of a neurological condition.
Other known neurotransmitters include two groups of chemicals called the monoamines and the peptides. The monoamines, which include dopamine, norepinephrin, and serotonin, are all synthesized from proteins called amino acids. The peptides, including enkephalin, endorphins, and substance P, are large molecules which may be involved in blocking sensations of pain.
Consequences of Neuronal Damage
When cells in the rest of the body are injured, they can regenerate and repair themselves. Neurons, on the other hand, do not have this capability. If an axon is damaged, the soma of the cell may also degenerate. Also, when a cell body is injured or an axon is severed from the soma, Wallerian Degeneration, or death of the axon occurs.
When a neuron dies, it is ingested by the support cells in the nervous system. This process is called phagocytosis.