Neurons and Glia
o The brain is an organ no need to spate mind from brain; focus on different types of
cell in the nervous system: neurons and glia
Although there are many neurons in the human brain (about 100 billion) the
glia outnumber neurons by tenfold; however neurons are most important cell
sfor the unique functions of the brain
It is the neurons that sense changes in the environment and communicate
these changes to other neurons and command the bodys responses to these
sensations.
Glia or glial cells, are thought to contribute to brain function mainly by
insulating, supporting, and nourishing neighboring neurons; keep the brain
from running out of our ears
o Neurons size is at or beyond the limit of what can be seen by the naked eye. Thus
microscopes were the scientists way of observing neurons, it was necessary to make
very thin slices. But brain tissue has a consistency like a bowl of jelly: not firm enough
to make slices. Thus scientists needed a method to harden the tissue without
disturbing the structure fixing tissues by immersing them in formaldehyde and
developed special device called a microtome to make very thin slices
Technical advances spawned the field of histology, the microscopic study of
the structure of tissues l
Faced another problem since freshly prepared brain had a uniform,
cream-colored appearance under the microscope; the tissue has no
differences in pigmentation to enable histologists to resolve individual
cells.
Final breakthrough in neurohistology was the introduction of stains that
could selectively color some, but not all, parts of the cells in brain
tissue
o Franz Nissl, German neurologist, introduce one stain and showed that a class of basic
dyes would stain the nuclei of all cells and also stain clumps of material surrounding
the nuclei of neurons
Clumps are called the Nissl bodies, and the stain is known as the Nissl stain
extremely useful for two reasons:
a. It distinguishes neurons and glia from one another
b. It enables histologists to study the arrangement, or cytoarchitecture is
an arrangement of neuronal cell bodies in various parts of the brain,
of neurons in different parts of the brain
o Camillo Golgi, Italian histologies, discovered that by soaking brain tissue in a silver
chromate solution, now called the Golgi stain, a small percentage of neurons became
darkly colored in their entirety
This revealed that the neuronal cell body, the region of the neuron around the
nucleus
The gain in the brain is mainly in the stain
Golgi stain shows that the neurons have at least two distinguishable parts: 1. A central region that contains the central region
2. The swollen region containing the cell nucleus; can be called cell body,
soma, and perikaryon
The thin tubes that radiate away from the soma are called
neuritis and are of two types:
i. Axons is of uniform diameter throughout its length, and
if it branches, the branches generally extend at right
angles; can travel over great distances in body, act like
wires that carry the output of the neurons
ii. Dendrites rarely extend more than 2mm in length,
extend from the cell body and generally taper to a fine
point. Dendrites come in contact with many axons, they
must act as the antennae of the neuron to receive
incoming signals, and input
o Santiago Ramon y Cajal, Spanish skilled histologist and artist, who learned about
Golgis method in 1888 and make a remarkable series of publications over the next 25
years, Cajal used the Golgi stain to work out the circuitry of many regions of the
brain.
Ironically concluded different conclusions from Golgi
Golgi championed the view that the neuritis of different cells are fused
together to form a continuous reticulum, or network, similar to the arteries
and veins of the circulatory system
Cajal, on the other hand, argued forcefully that the neurites of different
neurons are not continuous with one another and must communicate by
contact, not continuity
This idea that the neuron adhered to the cell theory came to be known
as the neuron doctrine.
Invention of electron microscope in 1950s with increased resolving
power, it was finally possible to show that the neuritis of different
neurons are not continuous with one another
o Then neuron also called the nerve cell consists of several parts: the soma, the
dendrites, and the axon. The inside of the neuron is separated from the outside by the
limiting skin, the neuronal membrane, which lies like a circus tent on an intricate
internal scaffolding, giving each part of the cell its special 3D appearance.
o The soma, the roughly spherical central part of the neuron. The cell body of the
typical neuron is about 20 m in diameter.
The watery fluid inside the cell, called the cytosol, is a salty, potassium-rich
solution that is separated from the outside by the neuronal membrane
Within the soma are a number of membrane-enclosed structures called the
organelles, the cell body of the neuron contains the same organelles that are
found in all animal cells
The important organelles include the nucleus, the rough endoplasmic
reticulum, the smooth endoplasmic reticulum, the Golgi apparatus, and
the mitochondria Everything contained within the confines of the cell membrane, including the
organelles but excluding the nucleus is referred to collectively as the
cytoplasm
o The nucleus of the cell is spherical, centrally located, and about 5-10 m across. It
is contained within a double membrane called the nuclear envelope which contains
perforated pores.
Within the nucleus are the chromosomes, which contain genetic material,
DNA (deoxyribonucleic acid). The DNA in each of your neurons is the same,
and it is the same as the DNA in the cells of your liver and kidney.
Difference distinguished by the specific parts of the DNA that are used
to assemble the cell. These segments are called genes
The reading of the DNA is known as gene expression. The final
product of gene expression is the synthesis of molecules called
proteins, which exist in a wide variety of shapes and size, and
perform many different functions, and bestow upon neurons virtually
all of their unique characteristics
Protein synthesis, the assembly of protein molecules, occurs in the cytoplasm.
Messenger ribonucleic acid or mRNA carries the genetic message to the
sites of protein synthesis; it consists of four different nucleic acids
The process of assembling a piece of mRNA that contains the information of a
gene is called transcription, and the resulting mRNA is called the transcript
More
Less
