Special Topics in Biomedical Science
Neurons
The body has two systems that help maintain
homeostasis: the nervous system and the
endocrine system.
The nervous system is a complex network of
nervous tissue that sends electrical and chemical
signals.
The nervous system includes the central nervous
system (CNS) and the peripheral nervous system
(PNS) together.
The central nervous system is made up of the brain and spinal cord, and the
peripheral nervous system is made up of the nervous tissue that lies outside
the CNS, such as the nerves in the legs, arms, hands, feet and organs of the
body.
The electrical signals of the nervous system move very quickly along
nervous tissue.
Nerve Cells
Although the nervous system is very complex, there are only two main types
of nerve cells in nervous tissue.
All parts of the nervous system
are made of nervous tissue.
The neuron is the "conducting" cell that transmits electrical
signals, and it is the structural unit of the nervous system.
1: Unipolar neuron 2: Bipolar neuron 3: Multipolar neuron 4:
Pseudounipolar neuron
The other type of cell is a glial cell. Glial cells provide a support system for the neurons, and recent
research has discovered they are involved in synapse formation.
A type of glial cell in the brain, called astrocytes, are important for the maturation of neurons and may
be involved in repairing damaged nervous tissue.
Immunocytochemical staining
of Astrocytes in culture
using an antibody against
glial fibrillary acidic
protein.
Isolated Astrocyte shown
with confocal microscopy.
Hyman astrocyte
Neurons and glial cells make up most of the brain, the spinal cord and the nerves that branch out to
every part of the body.
Both neurons and glial cells are sometimes referred to as nerve cells.
Structure of a Neuron
Every neuron has a membrane that surrounds its cytoplasm and a nucleus that contains its genes.
Neurons also have small organelles that let them produce energy and manufacture proteins.
The neurons’ main job is to transmit information, so they also have two types of highly specialized
extensions that distinguish them from other cells.
Dendrites, with their tree-like branching structure, gather information and relay it to each neuron’s cell
body.
Axons are generally very long, and each neuron has only one. This axon carries information away from
the neuron’s cell body toward other neurons, with which it makes connections called synapses. Axons
can also directly stimulate other types of cells, such as muscle and gland cells.
The special shape of a neuron allows it to pass an electrical signal to another neuron, and to other cells.
Electrical signals move rapidly along neurons so that they can quickly pass “messages” from one part of
the body to another. These electrical signals are called nerve impulses.
Neurons are typically made up of a cell body (or soma), dendrites, and an axon.
The cell body contains the nucleus and other organelles similar to other body cells. The dendrites extend
from the cell body and receive a
nerve impulse from another cell.
The cell body collects information
from the dendrites and passes it
along to the axon.
The axon is a long, membranebound extension of the cell body
that passes the nerve impulse
onto the next cell. The end of the
axon is called the axon terminal.
The axon terminal is where the
neuron communicates with the
next cell.
You can say the dendrites of the
neuron receive the information,
the cell body gathers it, and the
axons pass the information onto
another cell.
The axons of many neurons are covered with an electrically insulating phospholipid layer called a myelin
sheath. The myelin speeds up the transmission of a nerve impulse along the axon. The myelin is an
outgrowth of glial cells.
Schwann cells which are sometimes wrapped around the neuron, are a type of glial cell. Schwann cells
are flat and thin, and like other cells, contain a nucleus and other organelles.
Schwann cells supply the myelin for neurons that are not part of the brain or spinal cord, while another
type of glial cell, called oligodendrocytes,
supply myelin to those of the brain and
spinal cord.
Myelinated neurons are white in
appearance, and they are what makes up
the "white matter" of the brain.
Myelin is not continuous along the axon.
The regularly spaced gaps between the
myelin are called Nodes of Ranvier.
The nodes are the only places where ions
can move across the axon membrane,
through ion channels. In this way the nodes
act to strengthen the nerve impulse by
concentrating the flow of ions at the nodes of Ranvier along the axon.
A cross section of a myelinated neuron.
Neurons are specialized for the passing of cell signals. Given the many functions carried out by neurons
in different parts of the nervous system, there are many different shapes and sizes of neurons.
For example, the cell body of a neuron can vary from 4 to 100 micrometers in diameter.
Some neurons can have over 1000 dendrite branches, which make connections with tens of thousands
of other cells.
Other neurons have only one or two dendrites, each of
which has thousands of synapses.
A synapse is a specialized junction where neurons
communicate with each other.
A neuron may have one or many axons. The longest axon of
a human motor neuron can be over a meter long, reaching
from the base of the spine to the toes. Sensory neurons
have axons that run from the toes to the spinal cord, over
1.5 meters in adults.
Neurons form networks through which nerve impulses
travel. From each neuron’s dendrites to the sometimes very
distant tip of its axon, these impulses propagate through
the neural membrane in the form of electricity.
But the neurons communicate with one another without
touching one another. They use special molecules
called neurotransmitters to pass nerve impulses from one
neuron to the next.
This chemical transmission of nerve impulses causes the axon and the dendrites to develop specialized
structures that facilitate it.
So, dendrites have thousands of “spines” sticking up out of their surface.
The bulb-like terminal buttons of the axons, which secrete the
neurotransmitters, are
positioned opposite these
spines.
But the form of these
component structures of
the synapse varies greatly,
as does the overall form of the neurons themselves.
NEURONS
have accentuated the basic characteristics of other cells, which include transmembrane potential, the
ability to form extensions of its cytoplasm, and so on.
The extensions of neurons have
also become specialized, so that the
ion channels and receptors in
dendrite membranes are different
from those in axon membranes.
In addition, every neuron has its
own unique shape, its own unique
position in the nervous system, and
its own unique connections to other
neurons or to receptor (sensory)
cells or effector (muscle or gland)
cells.
This great variability (there are over 200 different kinds of neurons) means that some neurons deviate
from the standard basic morphology.
For example, some axons may form synapses directly with another neuron’s cell body, or even with its
axon.
Neuronal cell bodies also vary
widely both in size (small,
medium, large, and giant) and
in shape (star-shaped, fusiform,
conical, polyhedral, spherical,
pyramidal).
The geometry of a neuron’s
dendrites and axon also vary
tremendously with its role in
the neural circuit.
Neurons can also be classified
into various categories,
depending on what criteria are
used. For example:
Functional classification:
eg. sensory neurons that receive sensory signals from sensory organs and send them via short axons to
the central nervous system
Morphological classification based on the number of extensions from the cell body:
eg. pseudo-unipolar neurons with a short extension that quickly divides into two branches, one of which
functions as a dendrite, the other as an axon
Functional classification:
Eg. motor neurons that conduct motor commands from the cortex to the spinal cord or from the spinal
cord to the muscles
Morphological classification based on the number of extensions from the cell body:
eg. multipolar neurons that have short dendrites coming from the cell body and one long axon
Functional classification:
Eg. interneurons that interconnect various neurons within the brain or the spinal cord
Morphological classification based on the number of extensions from the cell body:
Eg. bipolar neurons that have two main extensions of similar lengths
GLIAL CELLS
Astrocytes, like most glial cells, were long considered essential for their role in supporting and
maintaining nerve tissue. But more and more evidence indicates that astrocytes may actually play a far
more important role in neural communication.
Astrocytes supply glucose needed for nerve
activity. Through the astrocytes’ end feet,
which are next to the walls of the capillaries in
the brain, glucose can enter the astrocytes,
which partially metabolize it, then send it on
to the neurons.
More intense synaptic activity seem to
promote a better supply of glucose by
activating this astrocytic metabolisis.
Astrocytes are connected with each other via “gap junctions” through which they can pass various
metabolites. It is through these
junctions that astrocytes send to the
capillaries the excess extracellular
potassium generated by intense
neuronal activity.
A gap junction or nexus is a
specialized intercellular connection
between a many animal cell-types. It
directly connects the cytoplasm of
two cells, which allows
various molecules and ions to move
freely between cells.
The network of intercommunicating
astrocytes forms a syncytium, it behaves like a single thing. For example, through this network, the
regulatory effects of waves of calcium ions might be propagated to large numbers of synapses
simultaneously.
T
he astrocytic extensions surrounding the synapses might have a broader control over the concentration
of ions and the volume of water in the synaptic gaps.
The network of astrocytes could act as a non-synaptic transmission system superimposed on the
neuronal system to play a major role in modulating neuronal activities.