Microanatomy, also called histology, is the microscopic study of tissue
structure. It's amazing that things that are so small can have such a
significant impact on how a person lives and functions. An attack by the immune
system on the myelin covering nerve fibers might be easier to understand if it
weren't so small. Many people find that it's easier to understand a problem if
they can see or hold it, but in this case the human body and especially the
nervous system doesn't allow this luxury. We are at the mercy of science and
doctors to tell us what is happening to our bodies and how to best deal with it.
The central nervous system (CNS) and peripheral nervous system (PNS) work
together constantly. The integrative activity of the nervous system, which
underlies motor, sensory, cognitive, and psychological behavior, all depends on
electrical signaling between neurons. Each neuron encodes its message in the
form of action potentials, or small electrical impulses that are carried to
other neurons via axons, the wire-like fibers that extend from neuron cell
bodies. Many axons within the brain and spinal cord are myelinated for proper
functioning to be achieved.
In this case what we have to look at is at the microscopic level. These small
things, however, have such a significant role in the human body and how well it
functions or not. Many people try to "think big", but in this case, thinking
small is where it counts.
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A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated
extension (axon) for sending impulses and usually many branches (dendrites) for
receiving impulses. Each large axon is surrounded by oligodendrocytes in the brain
and spinal cord and by Schwann cells in the PNS.
The membranes of neurons consist of a fat (lipoprotein) called myelin. The
membranes are wrapped tightly around the axon, forming a multilayered sheath. This
myelin sheath resembles insulation, such as that around an electrical wire. Nerve
impulses travel much faster in nerves with a myelin sheath than in those without
one. If the myelin sheath of a nerve is damaged, nerve transmission slows or stops.
Neurons are highly specialized for the processing and transmission of cellular
signals. Given the diversity of functions performed by neurons in the different
parts of the nervous system, there is a wide variety in the shape, size, and
electrochemical properties of neurons. One example of size variation is the soma
of a neuron; it can vary from 4 to 100 micrometers (microns) in diameter. As a
reference, a strand of spider web has an average diameter of 4 to 5 microns and
a human hair is 40 to 50 microns in diameter.
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The soma is the central part of the neuron. It contains the
nucleus of the cell and is where most protein synthesis occurs.
The nucleus ranges from 3 to 18 micrometers in diameter. |
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The dendrites of a neuron are cellular extensions with many
branches, and metaphorically this overall shape and structure is
referred to as a dendritic tree. This is where the majority of
input to the neuron occurs. |
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The axon is a finer, cable-like projection which can extend
tens, hundreds, or even tens of thousands of times the diameter
of the soma in length. The axon carries nerve signals away from
the soma and also carries some types of information back to it.
Many neurons have only one axon, but this axon may - and usually
does - undergo extensive branching, enabling communication with
many target cells. The part of the axon where it emerges from
the soma is called the axon hillock. |
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The axon terminal contains synapses, specialized structures
where neurotransmitter chemicals are released in order to
communicate with target neurons. |
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Transmission Direction
Just like a telephone line, neurons send and receive information down the line.
Neurons are basically cells that are electrically excitable in the nervous
system that process and transmit information. Neurons are the main components of
the brain, the vertebrate spinal cord, and the peripheral nerves. Afferent and
efferent can refer generally to neurons which, respectively, bring information
to or send information from the brain region.
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Interneurons connect neurons to other neurons within specific
regions of the CNS. |
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Afferent neurons convey information from tissues and organs into
the CNS and are sometimes also called sensory neurons. These
neurons respond to touch, sound, light and many other stimuli
effecting sensory organs by sending signals to the spinal cord
and brain. |
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Efferent neurons transmit signals from the CNS to the effector
cells and are sometimes called motor neurons. These neurons
receive signals from the brain and spinal cord and cause muscle
contractions and affect glands. |
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Action on Other Neurons
A neuron affects other neurons by releasing a neurotransmitter that binds to
chemical receptors. The effect upon the target neuron is determined not by the
source or the neurotransmitter, but by the type of receptor that is activated. A
neurotransmitter can be thought of as a key, and a receptor as a lock: the same
type of key can, in principle, be used to open many different types of locks.
Receptors can be classified broadly as excitatory (causing an increase in firing
rate), inhibitory (causing a decrease in firing rate), or modulatory (causing
long-lasting effects not directly related to firing rate).
Neurotransmitters are the brain chemicals that communicate information
throughout our brain and body. They relay signals between neurons until they
reach their destination. The brain uses neurotransmitters to tell your heart to
beat, lungs to breathe, and stomach to digest. They can also affect mood, sleep,
concentration, weight, and can cause adverse symptoms when they are out of
balance.
Neurotransmitter levels can be depleted many ways. It's estimated that around
86% of Americans have suboptimal neurotransmitter levels. Items such as stress,
poor diet, neurotoxins, genetic predisposition, drugs (prescription and
recreational), alcohol and caffeine usage can cause these levels to be out of
optimal range.
In principle, a single neuron, releasing a single neurotransmitter, can have
excitatory effects on some targets, inhibitory effects on others, and modulatory
effects on others - just like a conference call.
Connectivity
Neurons communicate with one another via synapses, where the axon terminal of
one cell impinges upon another neuron's dendrite, soma, or less commonly,
axon. Neurons such as Purkinje cells in the cerebellum can have over 1000
dendritic branches, making connections with tens of thousands of other cells.
Other neurons, such as the magnocellular neurons of the supraoptic nucleus,
have only one or two dendrites, each of which receives thousands of synapses.
Synapses can be excitatory or inhibitory and will either increase or decrease
activity in the target neuron. Some neurons also communicate via electrical
synapses, which are direct, electrically-conductive junctions between cells.
In a chemical synapse, the process of synaptic transmission is as follows: when
an action potential reaches the axon terminal, it opens voltage-gated calcium
channels, allowing calcium ions to enter the terminal. Calcium causes synaptic
vesicles filled with neurotransmitter molecules to fuse with the membrane,
releasing their contents into the synaptic cleft. The neurotransmitters diffuse
across the synaptic cleft and activate receptors on the postsynaptic neuron.
The human brain has an astonishing number of synapses. Each of the brains
estimated one hundred billion neurons has on average 7,000 synaptic connections
to other neurons. It's been estimated that the brain of a three-year-old child
has about 1 quadrillion synapses (1,000,000,000,000,000). This amount does
decline with age and stabilize by adulthood. For an average adult, it's
estimated the range is from 100 to 500 trillion synapses (100,000,000,000,000 to
500,000,000,000,000). This could be the reason why it becomes difficult to teach
an old dog new tricks.
Mechanisms for Propagating Action Potentials
The cell membrane of the axon and soma contain voltage-gated ion channels which
allow the neuron to generate and propagate an electrical signal (an action
potential). These signals are generated and propagated by charge-carrying ions
including sodium, potassium, chloride, and calcium.
There are several stimuli that can activate a neuron leading to electrical
activity, including pressure, stretch, chemical transmitters, and changes of the
electric potential across the cell membrane. Stimuli cause specific ion-channels
within the cell membrane to open, leading to a flow of ions through the cell
membrane, changing the membrane potential.
Thin neurons and axons require less metabolic expense to produce and carry
action potentials, but thicker axons convey impulses more rapidly. This can be
compared somewhat to a garden hose and fire hose - it doesn't take much pressure
or effort to get water out of a garden hose, but when water is sent through a
fire hose, it takes more effort and it comes out more quickly.
To minimize metabolic expense while maintaining rapid conduction, many neurons
have insulating sheaths of myelin around their axons. The sheaths are formed by
glial cells: oligodendrocytes in the CNS and Schwann cells in the PNS. The
sheath enables action potentials to travel faster than in unmyelinated axons of
the same diameter, whilst using less energy. The myelin sheath in peripheral
nerves normally runs along the axon in sections about 1 mm long, punctuated by
unsheathed nodes of Ranvier which contain a high density of voltage-gated ion
channels.
MS is a neurological disorder in that it results from demyelination of axons in
the CNS. However, the cause of all this isn't neurological, but rather
autoimmune. So in this case, an autoimmune cause gives a neurological result.
Some neurons don't generate action potentials, but instead generate a graded
electrical signal, which in turn causes graded neurotransmitter release. Such
nonspiking neurons tend to be sensory neurons or interneurons, because they
can't carry signals long distances.
Axonal Conduction Is Impaired in Demyelinated Axons
Following damage to the myelin, conduction velocity (speed) is reduced, and
conduction slows along the demyelinated axon. Just as with electrical wire, the
severity of damage to the wires insulation and what the exposed wire is in
contact with, may determine the loss of electricity running through it.
A complete conduction failure can occur in demyelinated axons. When conduction
failure occurs, the axon potential isn't propagated from one end of the fiber to
the other and information is lost. This type of action where information is lost
produces a clinical deficit. Conduction failure in demyelinated axons is now
known to result not only from loss of the insulating myelin, but also from the
molecular organization of the axon membrane. Following damage to the myelin,
internodal parts of the axon membrane (previously been covered by myelin) are
uncovered.
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