CHAPTER 1
Introduction to the Nervous System and the Structure of Nerve Cells
The necessity for a nervous system
The cells comprising multicellular organisms are highly interdependent. Individual cells and
tissues within multicellular organisms have become so specialized in their functions that no cell
or tissue is capable of an independent existence. An absolute prerequisite for multicellular life is
therefore some means by which cells can communicate with one another; that is, a means of
intercellular communication. Such communication is necessary so that the specialized functions
of individual cells and tissues can be coordinated to insure successful interactions with the
external environment (e.g., attainment of food or avoidance of danger). Intercellular
communication is also necessary for the maintenance within an animal of an internal
environment supportive of life.
Evolution has provided two solutions to the problem of intercellular communication: the
endocrine system and the nervous system. In the case of the endocrine system, communication
is accomplished by means of a chemical agent, or hormone, which is secreted by one cell and
then diffuses or is carried by circulation to other cells. An example of a hormone is adrenalin.
Adrenalin is released into the blood system by cells in the adrenal gland in response to certain
stimuli, such as fright. When adrenalin is carried by the circulation and reaches its target cells
elsewhere in the body, it induces specific responses in those cells. For example, the adrenalin
interacting with heart tissue causes an acceleration of the heartbeat.
However, a characteristic of intercellular communication via hormones is its slowness, since
the response time is limited by the rate of the blood flow. For example, a hormone released in
the head of a mouse could reach the tail in approximately 10 seconds. However, if a mouse were
dependent on such blood-borne communication to avoid being eaten by a cat, there would be no
mice.
Rapid Communication Over Long Distances is the Principle Rationale for a Nervous
System
The system designed for rapid intercellular communication over long distances with high
fidelity is the nervous system. The cells that comprise the nervous system are called neurons.
In the nervous system, communication occurs by means of pulses of electrical signals called
action potentials. Neurons have long cellular processes, called axons (see below) over which
action potentials can be conducted at speeds of up to 120 meters/sec (a football field is about 100
meters long). Thus, by nervous conduction the speed of communication between the head and
tail of our mouse can be reduced from 10 seconds to 20-40 milliseconds (1 millisecond is 1/1000
of a second).
Structure and Function of a typical Neuron
Understanding the brain and nervous system requires that you know something about the
specialized cells called neurons that comprise the brain, what they look like, what their various
parts are called, and what function is served by each part. Therefore, a brief description of a
typical neuron, together with a brief explanation of how neurons work, is given next.
Each neuron has four major parts, which are shown in Fig. 1.
1) The first part is its cell body. The cell body is also known as the soma. The soma, or cell
body, contains the nucleus, nucleolus, endoplasmic reticulum, Golgi apparatus and all the
1
organelles and features that characterize all cells. The cell body is where the majority of protein
synthesis occurs, and thus is the neuron’s industrial center.
2) The second part, or really parts, are called dendrites. Dendrites are a series of processes
that extend from the cell body, and are the parts of the neuron that receives information from
other neurons, as described below. Dendrites of some neurons are simple, others somewhat more
complex and in many cells they are exceptionally elaborate.
3) The third part is the axon. The axon is a process that extends out from the cell body and is
the part of the neuron that conducts information from the cell body to distant sites. The axon is
basically like a wire in an electrical circuit. Each neuron only has one axon, in contrast to the
numerous dendrites that neurons often possess.
4) The fourth part of the neuron is the axon terminal. The end of the axon, the axon
terminal, is where the axon comes into close proximity with the dendrite or cell body of another
cell. This close arrangement of the axon’s terminal and the dendrite or cell body of another cell
occurs at a special site called the synapse, and is the place where one neuron, in essence, talks to
another neuron. The axon terminal at the synapse has numerous specializations, and the most
obvious are the numerous synaptic vesicles. These appear as small “balloons” in the terminal.
The “balloons” are actually membranous vesicles that contain a chemical called a
neurotransmitter.
Fig. 1. A drawing of a “typical neuron” showing its dendrites, its cell body or soma, its single axon and
the axon terminals. The insert shows a synapse made by one of the small axon terminals and the dendrite
or soma of another cell.
The operation of a neuron, in its most basic form, is illustrated in Fig. 2. The major idea
2
is that when a signal is received at one of the dendrites, the signal triggers an electrical pulse, an
action potential, at the junction where the cell body meets the axon. This particular portion of
the axon is called the axon hillock (Fig. 1). The action potential, which is initiated at the axon
hillock, then propagates (travels) down the axon until it reaches the end of the axon (Fig. 2).
When the action potential reaches the axon terminal, it causes the vesicles in the terminal to
release their chemical neurotransmitter into the very narrow cleft that separates the axon terminal
from the dendrite of another cell; that is, the chemical is released into the synaptic cleft. The
portion of the dendrite within the synaptic cleft has special receptors in its membrane that bind
the chemical released from the axon terminal. When the receptors bind the chemical, they open
pores in the receptors and allow electricity, in the form of positive ions, to flow into the cell. The
influx of positive charges into the cell then triggers an action potential at the axon hillock of the
receiving cell. The action potential then propagates down its axon of the second cell until it
reaches the axon terminal. The axon terminal releases a neurotransmitter that then binds onto the
receptors in the dendrites of the third cell, and the entire cycle is repeated.
Fig. 2. The way neurons conduct action potentials and how they communicate with other neurons by
releasing a neurotransmitter at the axon terminal is illustrated. The basic operation of the neuron is
broken into six major steps.
The entire rationale of the nervous system distilled
The nervous system is composed of billions of cells that make trillions of synaptic
contacts with each other. In short, it is a system designed for communication on a massive scale
3
among billions of cells. The entire rationale of the nervous system can be condensed into a
single idea; communication simply means that one cell influences the electrical state of
another cell by the release of chemicals at points of synaptic contact. The cell bodies of cells
that are in communication with each other are almost always separated by relatively long
distances. Thus the way that one cell is able to influence another cell’s electrical state is via
action potentials that are conducted along an axon. The communication between two cells,
where one releases neurotransmitter and the other has receptors that bind the neurotransmitter,
occurs at the synapse.
Action potentials are the universal conveyors of information in all multicellular animals
Axons and action potentials have three basic and essential attributes for conducting
information.
1) The first is that information can be conducted over long distances. Distances can be as great
as many meters, as would occur in the long axons of giraffes or whales.
2) The second is that action potentials convey information with high fidelity. What high fidelity
means is that the signal along the axon does not die out nor is it changed in any way. The same
action potential that is generated at the axon hillock will appear in exactly the same form at the
axon terminal, even if the axon hillock is many meters away from the axon terminal.
3) The third attribute is that action potential are propagated rapidly down axons, which
conduction velocities that can be as fast as 120m/sec in the fastest axons in our bodies.
Remember that a football field is roughly 100 meters long, and thus the fastest axons can
conduct action potentials over a distance of about 1 football field plus an additional 20 yards in
1.0 millisecond (1/1000 of a second). The rapidity with which the nervous system works allows
the time scales for communication among cells, even in large animals, to be on the order of tens
or hundreds of milliseconds, rather than seconds or tens of seconds, as would occur with
chemical messages carried by the circulation.
The communication between cells is intimate and private
I would also like to make an additional comment about the nature of the communication
between two cells at a synapse. For convenience, I will refer to the first cell, the cell in which an
action potential has reached its axon terminal, as the presynaptic cell, and the other cell as the
postsynaptic cell. In essence, the presynaptic cell talks to the postsynaptic cell at the synapse.
In this case, “talking” is obviously not accomplished with sound, as it would be if I spoke to you,
but rather via chemicals; specifically the release of neurotransmitter by the presynaptic cell. It is
also obvious that the postsynaptic cell does not have “ears” that detect sound, but rather the
“ears” of the postsynaptic cell are its receptors that “listen” to what the presynaptic cell “said” by
binding the chemical released by the presynaptic cell. The postsynaptic cell then responds to
what it “heard” by opening pores in the receptors when the receptors bind neurotransmitter. The
open pores allow electrical currents to enter the cell thereby changing the electrical state of the
postsynaptic cell. If the electrical change is great enough, the postsynaptic cell will then itself
generate an action potential, which in turn will influence the electrical state of another
postsynaptic cell with which it has a synaptic connection.
The communication that one cell has with another cell is intimate and private. An
analogy can best illustrate what I mean by “intimate and private”. Imagine you are in a room
with lots of people, including your girlfriend (or boyfriend). Now you want to say something to
your girlfriend (or boyfriend) but it is something that you do not want anyone else to hear. So
what you do is whisper whatever you wish to say into his or her ear. She, and only she hears
what you said; no one else heard a thing.
4
Fig. 3. Connections between a pre-synaptic neuron and a post-synaptic neuron. One of the synapses
made on the postsynaptic cell is shown in the insert. Notice that synapses are made mostly on the
dendrites but some are also made on the cell body.
There is a similar situation at the synapse; the pre-synaptic cell conveys information only
to the postsynaptic cell at, and only at, the synapse. Additionally, the neurotransmitter is
released only in the synaptic cleft, so that it can bind only to the receptors of the postsynaptic cell
in the synaptic cleft, as shown in Fig. 3. Thus, the axon terminal of the pre-synaptic cell almost
touches the postsynaptic cell at the synapse, similar to you coming close to your girlfriend’s ear.
And you do not announce your message to the entire room but rather whisper a personal message
directly into her ear. Similarly, the neurotransmitter released by the pre-synaptic cell is not
sprayed over a large region, but rather the neurotransmitter is released into the synaptic cleft, and
thus can only affect the receptors of the postsynaptic cell that are in the cleft at that synapse.
Neural communication between cells is thereby rendered private and intimate.
The Knee Jerk Reflex
Before we consider how electricity is used in the nervous system, let us first pause to get an
overview of how the nervous system operates. For this purpose, I will consider a simple reflex
I'm sure you're all familiar with: the so-called knee jerk reflex. The term reflex refers to a
stereotyped response (in this case extension of the leg) that occurs in response to a sensory
5
stimulus (a tap to the knee). The reflex response is involuntary--it occurs in response to the
stimulus independent of my own volition.
Here is how the reflex is produced. There are two types of neurons involved: sensory
neurons and motor neurons (Fig. 4). Let us begin with the tap of the hammer and follow the
events through to the movement of the leg as shown in Fig. 4. First, the tap of the hammer does
something to a muscle that is located in the thigh. This muscle, the quadriceps femoris, is an
extensor muscle because when this muscle contracts it causes the leg to extend (the opposite of
extension is flexion and there are also flexor muscles that we need not now consider). This
extensor muscle is attached to the bone of the lower leg by a fibrous cord called the tendon.
When a physician strikes your knee with a small hammer, the tap stretches the tendon, which in
turn, stretches the muscle, thereby evoking the knee jerk reflex.
Fig. 4. The basic circuit underlying the knee jerk reflex. See text for further explanation.
There are special structures in the muscle that sense whenever the muscle is stretched. These
"stretch receptors" are called muscle spindles (Fig. 5). Associated with each muscle spindle is
the end of a process of a sensory neuron, also shown in Fig. 5. This sensory neuron has a long
axon that runs in a peripheral nerve to the spinal cord. Note that there is a single process that
emerges from the cell body of this sensory neuron and then bifurcates - one process goes into the
spinal cord and one runs in the opposite direction to the muscle spindle. Both of these processes
are called axons and this sensory neuron processes no dendrites. Thus, it is an exception to the
generalized structure of the neuron we discussed earlier. The axon directed toward the spinal
cord enters the spinal cord where it branches and makes synapses with the dendrites of a neuron
located in the spinal cord. This neuron sends an axon out of the spinal cord to make a synaptic
connection with the same extensor muscle that was stretched by tapping the knee with a hammer.
But, we've departed a bit from the sequence of events we started out to describe. The hammer
tap stretched the muscle and the muscle spindle sensed this stretch. It does this because the
ending of the sensory axon applied to the muscle spindle is also stretched, and this stretch causes
6
the generation of an electrical signal in the sensory axon ending. This electrical change in the
sensory axon ending has a name; it is called a receptor potential because it is generated in a
sensory receptor (Fig. 5). When we consider sensory transduction later this semester, we will
examine the receptor potential in more detail.
If the receptor potential is sufficiently large, it will initiate a second type of electrical signal
near the afferent axon endings on the muscle spindle. This second type of potential change is the
action potential that will propagate along axon from the muscle spindle into the spinal cord.
Thus the same features of the action potential generated in the sensory ending of the axon will
appear a short time later at the axon terminal in the spinal cord (action potentials convey
information with high fidelity).
Fig. 5. The sequence of events that occurs when a muscle is stretched. The stretched muscle also
stretches the sensory receptors on the muscle spindle. The stretched receptor changes the electrical state
of the receptor that is called a receptor potential. The receptor potential then triggers an action potential
that propagates down the axon and into the spinal cord. In the spinal cord, the axon innervates a motor
neuron, shown as the second neuron in the circuit. The release of neurotransmitter from the axon terminal
of the sensory axon opens pores in the receptors of the motor neurons. The open pores allow positive
charges to enter the soma of the motor neuron thereby changing its electrical state. The change in
electrical state of the motor neuron is called a synaptic potential, because it is generated by the release of
neurotransmitter at the synapse. The synaptic potential then triggers an action potential that travels down
the motor neuron’s axon to innervate muscle fibers in the extensor muscle. Those fibers then generate
action potentials, which lead to the fibers shortening that causes the leg to extend upward.
The action potential propagates along the sensory axon at a finite speed - somewhere in the
neighborhood of 100-120 m/sec. The action potential continues to be conducted until it reaches
the synapses made by the axon terminals. There the action potential causes the axon terminal to
release a chemical transmitter, also known as a neurotransmitter. The neurotransmitter diffuses
across the small space, the synaptic cleft, separating the membranes of the sensory terminal and the
motor neuron dendrite. When the transmitter substance reaches the membrane of the motor
neuron, the transmitter binds to the receptors located in the membrane of the motor neuron. The
binding causes pores to open in the receptors thereby allowing the influx of positive ions. The
influx of positive ions, in turn, changes the electrical state of the motor neuron; bringing in positive
ions makes the inside of the motor neuron more positive than it was before. The increase in
positivity is confined to the region of the motor neuron near the synapse and it is graded in
proportion to the amount of transmitter released at the synapse. If the increased positivity is large
7
enough, then the positivity will exceed a threshold in the motor neuron and an action potential will
be initiated at the axon hillock.
The motor neuron action potential then propagates (again at a speed of about 100-120 m/sec)
away from the spinal cord and back to the quadriceps, the extensor muscle in the thigh. Here the
motor neuron innervates (i.e., makes synapses with) muscle cells. At the synapse that the motor
neuron makes with a muscle fiber, the story is repeated once again. The action potential causes the
motor axon terminals to release a chemical neurotransmitter. This neurotransmitter diffuses across
the synaptic cleft to the membrane of a muscle fiber, combines with receptors, opens pores in the
receptors and causes an influx of positive ions into the muscle fiber. If the increase in positivity
exceeds a threshold value (and at synapses in mammalian skeletal muscle fibers it always does),
then the muscle fiber generates an action potential, which in turn causes the fiber to contract and
shorten.
Glial Cells, the Satellite Cells of the Nervous System
Neurons are not the only cells in the nervous system and are not even the majority of cells.
Indeed most cells in the nervous system are called glial cells or just glia. Glial cells do not
participate directly in electrical signaling; that is they do not make synapses with other glia or
neurons. They do not have axons and they do not generate action potentials. Although these cells
can have elaborate processes that emanate from their cell body, the processes do not act like
dendrites in that they do not have synaptic connections on their processes and thus do not receive
signals from neurons or other glial cells via the release of chemical transmitter. There are four
major types of glial cells in the nervous system (Figs. 6-7); 1) astrocytes; 2) oligodendorcyes; 3)
Schwann cells; and 4) microglia. Each type of glial cell has a different function.
Astrocytes have elaborate processes and are only found in the brain and spinal cord. They are
not present on peripheral nerves, such as on the axons of motoneurons or on sensory nerves. Their
major function is to maintain a normal chemical environment for neuronal signaling.
Oligodendrocytes are also restricted to the brain and spinal cord, and form the myelin sheaths
around some, but not all neurons (Fig. 6). The processes of these glial cells wrap around a portion
of and axon thereby creating the myelin sheath. We will consider the role that myelin plays for the
conduction of action potentials in a later chapter.
Schwann cells are related to oligodendrocytes, but are only found in the peripheral nervous
system (the nerves outside of the brain and spinal cord) where they form the myelin sheaths around
peripheral neurons (not shown). For example, motoneurons, the neurons that leave the spinal cord
to innervate muscle fibers, have myelinated axons where the myelin is formed by Schwann cells.
Microglia, in contrast to the other glial types, act like the macrophages in other tissues. They
are primarily scavenger cells that remove cellular debris from sites of neuronal injury.
Fig. 6. Three types of glial
cells.
The process of the
oligodendrocyte in C wraps
around an axon to form its myelin
sheath. Schwann cells are not
shown but form the myelin
sheaths on axons in peripheral
nerves in a manner similar to the
way
oligodendrocytes
form
myelin on axons in the brain and
spinal cord.
8
Take home messages
There are a few simple take home lessons that were made in this chapter. They are important
since each will be used again in subsequent chapters to build a picture of how neurons generate
and conduct action potentials.
1) The nervous system evolved for rapid communication among cells, where cells that are in
communication with each other are almost always separated by relatively long distances.
2) One cell communicates with other cells by influencing the electrical state of the other cells
through the release of chemicals at points of contact called synapses.
3) The way that one cell is able to influence another cell’s electrical state is via action potentials
that are conducted along an axon.
4) Action potentials are the universal conveyors of information in all multicellular animals. They
are ideally suited for conveying information over long distances, rapidly and with high fidelity.
Thus, the same action potential that is generated at the axon hillock will appear in exactly the
same form at the axon terminal a short time later, even if the axon hillock is many meters away
from the axon terminal.
9