Chapter 24- Neural Control and the Senses (25)
Neurons are the basic units of communication in the nervous system. Neurons detect and
integrate information about external and internal conditions. The direction of flow of a
signal in a neuron is as follows, a signal is received by dendrites, then transmitted to and
integrated in the cell body, where a response is initiated that results in the transmission of
an action potential along the axon onwards to a synapse, which then chemically transmits
the response to an effector such as another neuron or to a muscle cell etc. Thus neurons
act on muscles and glands in ways that produce appropriate responses.
Action potentials are the means by which messages travel along neurons through the
nervous system, but chemical signals are required to bridge the gaps between neurons.
The nervous system includes the brain, spinal cord, and many nerves. The brain and the
spinal cord make up the central nervous system. Paired nerves that thread through the rest
of the body make up the peripheral nervous system. The brain is complex in both
structure and function. In its three main regions - the forebrain, the midbrain, and the
hindbrain - are the centers that receive, integrate, store, and respond to information
Be sure to understand the organization of the nervous system and be able to distinguish
between the different types of neurons in the central nervous system. Be sure to
understand the operation of a neuron and how it communicates with other neurons. A
common mis-perception is that an action potential can vary in intensity depending on the
strength of the stimulus that induces it, this is FALSE! an action potential is "all or
nothing" once initiated, it cannot be stopped and remains at the same intensity while it is
being propagated along the length of the axon and to the synapse.
None of the material on the brain and the senses covered on pages 480 to 488 will be
tested in this coursNeurons, Nerves, and the Nervous System
Nerves respond to a stimulus by generating an electrical signal - an action potential.
The functional units of the nervous system are cells called neurons.
All neurons have a cell body; filament-like and often branching dendrites conduct
signals to the cell body, which then integrates the stimuli and initiates the response that is
transmitted onwards by a long cell extension called the axon.
The terminal end of an axon is also called the synaptic terminal, and a single axon can
have many branches that form more than one synaptic terminal, in some cases there are
many synaptic ends to an axon, each of which can communicate with a different neuron. There are three types of neuron. Sensory - which relay their response to a stimulus to the
brain or spinal cord, where interneurons integrate information with other signals and
send their response to effector neurons - such as motor neurons that cause glands and
muscles to act.
Neurons in the brain are physically supported and nourished by support cells called
neuroglia (glia for short), some of which also form the insulating membrane sheath that
is wrapped around many long axons in nerves.
All cells in our body (i.e., not just neurons) need to control the numbers of sodium and
potassium ions they contain, in order to perform their specific functions. For that reason,
all cells, be they muscle, nerve, skin - whatever - have channel proteins in their
membranes that allow sodium, potassium, chloride and other ions to move back and forth
by non ATP requiring passive transport, (diffusion) in response to their concentration
All cells also have sodium and potassium “pumps” - these are transport proteins that
move sodium and potassium ions by active transport, across a cell membrane against
their concentration gradient, and require ATP to do so. These pumps use ATP derived
energy to transport ions from where they are in low numbers to where they are in high
A given cell type, nerve, muscle etc, will have a characteristic distribution of sodium and
potassium ions across the cell membrane that has been produced by the ATP fueled
action of the Na-K pumps acting in concert with the movement of Na and K ions allowed
by the passive diffuser channel proteins.
Just as with any other type of cell, this process of active adjustment of Na and K levels
occurs in the neuron, across its axonal membrane, to achieve the correct balance of ions
inside and out.
In neurons this process of sodium and potassium transport through diffuser channel
proteins and through sodium-potassium pumps, acts in conjunction with large negatively
charged proteins in the cytoplasm (that are too big to cross the cell membrane) to cause
the interior of the neuron - the cytoplasm - to have a negative electrical charge with
respect to the exterior (the cytoplasm happens to be at about -70 mv).
The particular distribution of sodium and potassium ions maintained across the
membrane of the axon by the two types of transport proteins is such that there is a high
potassium concentration inside the axon, and a high sodium concentration outside the
axon (in relative terms).
In this condition, in the absence of a signal (the action potential from the cell body of the
neuron), the electrical charge across the axonal membrane is said to be at resting
potential (-70 mv) and it will stay at this potential in the absence of a suitable stimulus to change. In this resting condition, with a marked difference of electrical charge across the
membrane, the axon is said to be in a polarized state.
How is it that a neuron can generate an electrical signal that passes down an axon and
acts to transmit a message on to another neuron or to a muscle? What is special about the
neuron is a unique kind of transport protein spanning its cell membrane - what is called a
voltage gated channel protein.
This voltage gated channel protein does not require ATP for its operation, it is a passive
transport protein, but it has the unique property of only acting (opening the gate) when it
receives a suitable electrical signal.
When it has received such a signal, the gate will open and allow ions to stream in or out.
Once the signal is ended the gate "swings shut". One such voltage gated channel protein
works with sodium, another with potassium. Which way the ions diffuse when the gate
opens, is solely determined by the concentration gradient, because the gated channel
protein does not undertake active transport.
Imagine that a neuron has received electrical stimuli from other neurons into its dendrites
region, and that the cell body region of the neuron has integrated these stimuli and
“decided”, in response, to generate an action potential.
The action potential is a sudden “all or nothing” electrical signal that causes a reversal of
the ion distribution across the cell membrane, at the beginning region of the axon, which
will then propagate down the length of the axon - this is what is called a wave of
The signal is called a wave of depolarization because the “negative inside - positive
outside” state of the resting potential across the axonal membrane is suddenly reversed,
and this state propagates down the axon. This reversal of charge across the membrane is
the electrical signal.
During this wave of depolarization a given (very tiny) region of the axon receives the
voltage signal of the action potential - this voltage causes the voltage gated protein
channels to open in that tiny region.
When the gates open in that region, they allow sodium ions to flood in and potassium
ions to flood out in a manner that suddenly causes the axon cytoplasm to become positive
with respect to outside the axon, about +50 mv instead of -70 mv when the neuron is in
its resting state.
Now this tiny region of the axon has reversed its electrical voltage state (i.e., it
depolarized) and this reversal acts as the voltage signal to cause the voltage gated
channel proteins in the next small region down the axon to open and for that region to
depolarize, and so on - this wave of depolarization propagates down the length of the
axon to the synaptic terminal and so delivers its electrical signal. To be more precise, when a patch of an axon receives the action potential, it reacts first
by the voltage gated sodium channels opening, so that the sodium ions flood in and
reverse the potential across the membrane, then, as the signal has passed on to the next
patch of the axon membrane, the voltage gated potassium channels open to release
potassium ions to the exterior, which starts the re-establishment of the resting membrane
potential ready for the next impulse.
Very soon after the electrical signal has passed on, the voltage gated channel proteins
close, and the sodium-potassium pumps go into action to restore the ion distribution to
what it was during the resting state, re-establishing the resting electrical potential and
readying the neuron for its next response.
In that short (thousandths of a second) time when the resting potential is being re-
established, the axonal membrane is said to in a refractory state - it cannot respond to
another action potential, and it will not allow the wave of depolarization to travel
Neurons transmit their electrical signal down axons away from the cell body, and that
signal will be passed on to other neurons, or to special contact points on muscles or
The signal does NOT (as a rule) pass from one neuron to another or to a muscle or gland
electrically - by direct electrical contact with the receiving structure, instead, it passes on
as a chemical message. There is a definite but tiny gap between neurons, at the end of
axons and the start of dendrites, or between the ends of axons and muscle or gland cells.
This tiny gap is called a synapse.
When the wave of depolarization hits the end of the axon at the synapse (the presynaptic
membrane) a set of processes come into play which cause the pre-synaptic m