C ells of the Nervous System
• TTX blocks the formation and transmission of electrical signals, or action
potentials, within nerve cells. Action Potentials: The nerve impulse arising in an axon.
Without signals from the nervous system, lungs and other essential organs, don’t work.
• The nervous system is made of two cells: neuron: A cell of the nervous system that
is specialized for information processing and communication. Glia: Cells in the nervous
system that support the activities of the neurons. There are atleast 10-50 glia for
The Structure of Neuron
• All animal cells including neurons, have membranes, nuclei, and small internal
structures as organelles. Many of these structures are found within the main
mass of the neuron known as the cell body or soma.
• Neurons differ structurally from other cells in that they have specialized
branches extending from the cell body, known as the axons and dendrites. Axon:
the branch of the nervous system usually responsible for carrying signals to
other neurons. Dendrite: The branch of a neuron that generally receives
information from other neurons.
• The primary task of any cell membrane is to form a boundary between the cell
and its external environment.
• The neural membrane must separate the intracellular fluid of the cells interior
from the extracellular fluid surrounding the neuron. Intracellular fluid: the fluid
inside a cell. Extracellular fluid: the fluid surrounding a cell.
• The chemical composition of extracellular fluid and intracellular fluid are quite
different. Maintaining this difference is essential to the process of generating
and sending neural messages.
• The neural membrane is made up of a double layers of pholipids do not dissolve in
water. Water and oil do not mix. As a result of lipid structure, the neural
membrane is able to restrain the water-based fluids on either side, maintaining
the structural integrity of the cell. Membranes are able to do this in spite of
being only two phospholipid molecules wide, with resulting thickness of only five
nanometers. (1 nanometer= 1 billion of a meter, or 10^-9 meters)
• Permeability: a property of a substance that determines the extent to which
other substances may pass through it. There are two primary types of protein
structures of neural function, ion channels and ion pumps. Ion channels: a protein
structure embedded in a cell membrane that allows ions to pass without the use
of additional energy. Ion pump: a protein structure embedded in a cell membrane
that uses energy to move ions across the membrane. These structures provide
pores, or channels, through which specific ions, or electrically charged particles,
can move into or out of neuron. Ion: an electrically charged particle in solution. Both channels and pumps show ion selectivity, or the ability to let a particular
type of ion pass and no others. The amino acids, or protein building blocks, that
make up the ion channel or pump determine which ions will be allowed to pass
through the membrane.
• Ion channels have the ability to open and close in response to stimuli in their
immediate vicinity. Some ion channels known as voltage-dependent channels, open
and close in response to the electrical status of adjacent areas of membrane.
These changes form an important part of our discussion of electrical signalling
within the neuron. Voltage-dependent channels: An ion channel that opens or
closes in response to the local electrical environment.
• Ligand-gated channels: open when they come in contact with specific chemicals.
These chemicals are typically our naturally occurring chemical messengers but
can be drugs from artificial sources as well. Ligand-gated channels become very
important as we discuss events taking place at the synapse, or the junction
between two neurons. Synapse: the junction between two neurons at which
information is transferred from one to another.
• The two most important pumps in neurons are the sodium-potassium pumps and
the calcium pumps. Sodium-potassium pumps: an ion pumps that helps maintain
the difference in chemical composition between the intracellular and
extracellular fluids. Sodium-Potassium pumps do a “prisoner exchange” across
the neural membrane by sending three sodium ions out of the cell while
collecting two potassium ions from the extracellular environment. This process
comes at a high cost to the neuron. 20-40 percent of the energy required by the
brain is used to run the sodium-potassium pumps. Calcium pumps perform a
similar function, although they do not collect another type of ion in exchange for
the calcium they pump out of the cell. The release of chemical messenger or
neurotransmitters by a neuron, it is essential to maintain low levels of calcium
within the cell. Neurotransmitters: a chemical messenger that transfers
information across a synapse.
The Neural Cytoskeleton
• Structural support that maintains the shape of a neuron is provided by the
cytoskeleton. Cytoskeleton: a network of filaments that provides the internal
structure of a neuron.
• Three types of filament, or fiber, make up the neural cytoskeleton. These
structural fibers also move elements within the cell and anchor the various
channel and receptor proteins in their approximate places on the neural
• Microtubules: largest of the three types of cytoskeleton, formed in the shape of
hollow tubes with a diameter of about 25nm, responsible for the movement of
materials within the cell, movement along the microtubules from the cell body to
the axon terminal is known as anterograde transport, and movement back to the cell body from the periphery of the neuron is known as retrograde transport,
implicated (connected) in the development of Alzheimer’s disease. This condition
is characterized initially by memory loss, followed by progressive decline in
cognitive and physical functions, eventually leading to death. One symptom of
Alzheimer’s disease is the presence of neurofibrillary tangles. These tangles are
made from a protein known as tau, which normally plays a big role in holding
together the microtubules that form an important part of the neural
cytoskeleton, In Alzheimers the tau protein detach and the microtubules
• Neurofilament: a neural fiber found in a cell cytoskeleton that is responsible for
structural support. (middle sized fibers)
• Microfilaments: the smallest fiber found in the cell cytoskeleton that may
participate in the changing of the length and shape of axons and dendrites.
• The neural cell body: specialized participate in the communication function of the
• Nucleus: a substructure within a cell body that contains the cells DNA. Contains
DNA information, contains substructure called nucleolus. Nucleolus builds
organelles known as ribosomes, which engage in protein synthesis. The ribosomes
produce proteins either on their own or in association with the endoplasmic
reticulum, another small structure, or organelle located in the cell body.
Mitochondria: organelles that provide energy to the cell by transforming pyruvic
acid and oxygen into the molecules of adenosine triphosphate.
• Dendrites: branches, tree like
• Dendrite spines: de