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2
Box
Demyelinating diseases
2.2
Multiple sclerosis is a progressive autoimmune disease, first
named by the neurologist Jean Martin Charcot in 1838.
It primarily affects young adults, occurring more often in
females than in men. Areas of the myelin sheath, mainly
in the brain, brainstem, cerebellum, spinal cord and optic
nerve, are damaged, impairing nerve conduction. This is a
chronic inflammatory process and there is evidence of both
genetic susceptibility and environmental causes, which trigger an immune response. There are two main theories as
to how the disease arises, termed the ‘outside- in’ theory
and the ‘inside- out’ theory. In the former, dysregulation
of the peripheral immune system leads to an autoimmune
response against CNS myelin, while the latter proposes that
damage to oligodendrocytes triggers an innate immune
response and subsequently an autoimmune reaction. There
are four major types of multiple sclerosis: primary progressive, secondary progressive, relapsing- remitting and clinically
isolated syndrome. The relapsing- remitting form accounts
for ∼75% of multiple sclerosis cases. Areas of demyelination, called plaques, evolve into areas of scar tissue that may
be partially remyelinated, restoring much of the function,
so the disease shows periods of remission. However, other
plaques form and eventually there is an accumulation of
damage that causes permanent deficits. The loss of myelin
in multiple sclerosis causes the failure of saltatory conduction, as there are no ion channels on the denuded axon.
Major symptoms of multiple sclerosis include muscle weakness, lack of coordination, visual disturbances, spasticity
and pain. These symptoms are due to a slowing or lack of
nerve conduction. In many patients, the disease has phases
of remission, which vary significantly in their duration. These
are thought to be caused by temporary remyelination or
by the insertion of new voltage- dependent Na+ channels in
the area lacking myelin. There is no cure for multiple sclerosis and treatment is symptomatic. Drugs aim to speed up
the remission phases or slow the demyelination phases. This
involves the use of disease- modifying immunomodulatory
drugs such as beta interferon, glatiramer acetate, cladribine,
natalizumab, alemtuzumab, ocrelizumab and siponimod.
Another type of demyelinating disease is the Guillain–
Barré syndrome, in which infection by a variety of bacteria or viruses triggers an inflammatory demyelination of
neurons in the peripheral nervous system. It classically presents with distal lower limb paraesthesia—proximal muscle
weakness that rapidly affects the rest of the body. Facial
weakness occurs in 50% of cases. Treatment consists of
measures to shorten the course of the disease (plasmapheresis or immunoglobulin therapy to inhibit the autoantibodies) and most patients make a good recovery.
only myelinate a single axon and the cell body is closely
apposed to the myelin sheath in the outermost layer.
Schwann cells play a role in the regeneration of periph-
eral axons following injury. In order for a peripheral nerve
to regrow, the tip of the axon must make contact with a
Schwann cell. This stimulates mitosis in the Schwann cell,
which then extends processes towards the growth cone of
the axon. The axon regrows at a rate of 2–5 mm per day
along the Schwann cells, which remyelinate the new axon.
Damaged neurons in the CNS do not regenerate successfully; one of the reasons is because CNS glial cells release
a host of factors that specifically inhibit axon growth.
Another major reason is that Schwann cells have a basement membrane along which the regenerating axon can
grow, whereas oligodendrocytes do not.
Not all axons are myelinated. These are seen in the
brain and spinal cord as grey matter, which consists of
cell bodies and unmyelinated axons. In contrast, white
matter consists of axons, most of which are myelinated. Dendrites are never myelinated. However, all
axons have an oligodendrocyte or Schwann cell covering, although unmyelinated axons have only a single
layer (Fig. 2.5), meaning that there are both myelinating and non-myelinating subtypes of Schwann cells and
oligodendrocytes.
Neuron excitability
The basic function of a neuron is to receive signals,
either directly from the environment or from other
cells. The signals, which may be chemical or electrical, all produce graded electrical changes in the neuron. The input signals mainly occur in the dendrites
and cell body of the neuron and may be excitatory or
inhibitory. The graded electrical potentials produced
are called excitatory postsynaptic potentials (EPSPs)
or inhibitory postsynaptic potentials (IPSPs). These
graded electrical potentials summate over time and
space (temporal and spatial summation) and, if their
total exceeds a threshold value, the trigger zone of the
neuron initiates or ‘fires’ an electrical impulse called
an action potential. This impulse is of a fixed size and
is propagated unchanged along the nerve axon to the
synapse, where the neuron makes contact with either
another neuron or an effector. At the synapse endogenous compounds called neurotransmitters are released,
which affect the postsynaptic target cell, making it
more or less likely to produce a response. In specific
cases, such as at the neuromuscular junction, a single
action potential in the nerve is sufficient to produce
muscle contraction.
Resting membrane potentials
All cells in the body have a voltage difference across the
plasma membrane, called the resting membrane potential (RMP). In neurons, this is approximately –75 mV (the
inside is negative relative to the outside). This is a consequence of the unequal distribution of ions across cell
membranes and the different permeabilities of the neurons to these ions.
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
39THE NERVOUS SYSTEM
2
Ax
SCSC
N
N
SC
A
Fig. 2.5 Myelinated and unmyelinated axons. (A) Electron micrograph of myelinated axon in the peripheral nervous system (PNS) shown in cross
section. The axon (Ax) is seen at the centre within a sheath consisting of multiple wrappings of the Schwann cell’s cytoplasmic membrane. The
Schwann cell soma (SC) is at the upper right. (B) Electron micrograph of unmyelinated axons in the PNS. Nine axons (asterisks) cut in cross section
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
are seen embedded in a Schwann cell whose nucleus is at the centre (N). In the lower right area a portion of a myelinated axon is visible (from
Lowe J. (2020). Stevens & Lowe’s Human Histology. Oxford: Elsevier Ltd.).
Table 2.1 Ionic concentrations of intracellular fluid and
extracellular fluid
Intracellular
concentration
Ions
+
Na
+
K
−
Cl
Large anions (A–) 140 –
2+
Ca
The two most common cations found in total body
water are sodium (Na+) and potassium (K+). Their distribution in intracellular and extracellular fluids are very
different. Intracellular fluid (ICF) contains predominantly
K+, while extracellular fluid (ECF) contains predominantly
Na+ (Table 2.1). It is this difference in ionic concentration
between ICF and ECF that allows the generation of the
RMP and provides the energy that drives the action potential. The predominant extracellular anion is chloride (Cl–),
whereas the intracellular cations are balanced by the presence of large intracellular anions (A–), mainly phosphates
and the negatively charged side chains of proteins.
(mM)
12 145
139 4
4 120
<0.0002 1.8
Extracellular
concentration
(mM)
B
The concentration gradients for Na+ and K+ are established by the active transport of Na+ and K+ by an Na+/
K+- ATPase known as the Na+/K+ pump (Fig. 2.6). This
uses the energy derived from breaking down the highenergy phosphoanhydride bond of ATP to pump both
Na+ and K+ against their concentration gradients. Because
the ATPase pumps three Na+ ions out for every two K+
ions, this produces a small potential of approximately 5
mV. On the extracellular face of the Na+/K+ pump is a
binding site for the glycoside ouabain, which inhibits the
activity of the pump. The Na+/K+ pump is essential to all
cells, not just excitable cells, as the ion gradients it establishes are used to power many transport processes.
For a voltage to be generated current must flow and
this occurs due to the presence in the plasma membrane
of protein channels that selectively allow ions to diffuse
passively down their concentration gradients. At rest, the
plasma membrane is slightly permeable to K+ and almost
impermeable to Na+. This means that K+ will tend to diffuse out of the cell, but this cannot be electrically balanced
by the inward diffusion of Na+. Therefore as K+ diffuses
out, down its concentration gradient, an electrical potential develops, with an excess of negative charge inside
the neuron. The neuron also contains negatively charged
ions, but these cannot leave the cell as they are too large
to cross the plasma membrane. The negative charge
inside the neuron creates an electrical gradient, which
will tend to pull positively charged ions into the cell. As
40 SYSTEMS OF THE BODY
E
61
.5
[C ]
]
1
Neuronal
membrane
bilayer
=145
[K
V
+
+
–
i
+
i
+
[Na+]
[K+]
+
[K
[Na+]
2
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
Box
+
Ouabain
Na+/K
ATPase
ADP
ATP
+
+P
2K
+
Extracellular
fluid
Lipid
Cytoplasm
i
]
3Na
Goldman–Hodgkin–Katz equation
2.3
e
PKK
PKK
= RT log
m
where PK, PNa and PCl are the relative permeabilities of the
plasma membrane to K+, Na+ and Cl–. R, gas constant; T,
absolute temperature (in Kelvins); Vm, voltage across membrane; Loge, Log10.
o
+
+ PNaNa
PNaNa
o
+
+ PClCl
PClCl
i
–
o
Fig. 2.6 Diagram of an Na+/K+ pump. The square brackets represent
ionic concentrations, a large symbol indicates a high concentration,
and a small symbol indicates a low concentration.
Intracellular
+
K
+
]
=139
[Na+]
=12
Fig. 2.7 Passive movements of ions across the plasma membrane at
rest. Intracellular and extracellular concentration are shown in mmoles
per litre.
Electrical
gradient
−
A
Extracellular
Concentration
gradient
+
K
+
Na
[Na+]
[K
+
]
= 4
the membrane at rest is virtually impermeable to Na+, the
only ion that can be attracted into the cell is K+ (Fig. 2.7).
This means that there is an equilibrium position,
whereby the amount of K+ leaving the cell down its concentration gradient is balanced by the amount being
drawn into the cell by the electrical gradient, and there
is no net movement of K+ across the cell membrane.
However, at this equilibrium position, there is a small
excess of negative charge inside the cell and this produces
the RMP. Because this potential difference is generated
across a very small distance—the width of the plasma
membrane—the actual difference in the number of ions
needed on both sides of the membrane to produce the voltage difference is very small and represents a concentration
difference of approximately one part in 108. If the membrane is assumed to be permeable only to K+, the predicted
RMP can be calculated using the Nernst equation.
At 37°C, the Nernst equation reduces to:
log
0
10
C
[
=
m
Z
where Z= valency of the ion; Co= external concentration;
Ci= internal concentration; and Em= membrane potential.
If the values for K+ (Co = 4 mM, Ci = 139 mM, Z = + 1)
are inserted in the equation, the value calculated is
approximately –95 mV (negative inside). This is called
the K+ equilibrium potential (E
+
) and is slightly more
K
negative than the measured RMP of approximately
–75 mV. It is also called the reversal potential, as at values above and below this potential the current flowing
through the channel goes in opposite directions. If the
values for Na+ are used instead, the Na+ equilibrium
potential (E
) is approximately +66 mV. Thus it is obvi-
+
Na
ous that the movement of K+ is largely responsible for
determining the RMP.
However, if the very slight permeability of the membrane to Na+ (~1/75th of that of K+) is taken into account,
a more complicated equation, called the Goldman–
Hodgkin–Katz equation (Box 2.3), is used to calculate the
RMP. This allows the relative permeabilities of Na+, K+
and also Cl– to be taken into account and is a better reflection of the roles of all these ions. At rest, the permeability
(P) ratios are PK/PNa/PCl = 1.0:0.04:0.45, which gives an
RMP of –75 mV for the concentrations in Table 2.1.
The membrane permeability to K+ (and Na+) is due to
the presence of integral transmembrane proteins that form
ion channels. The channels are passive, leak channels,
which do not seem to be regulated and are always open.
These channels are ion- selective and the greater permeability of the plasma membrane to K+ is simply due to the fact
that there are more K+ leak channels than Na+ channels.
However, present in the membranes of excitable
cells, such as neurons and muscles that can fire action
potentials, are also other types of ion channel that are
voltage- activated and do not normally open in the resting state.
Action potentials
All cells have an RMP, but cells that can generate and
conduct action potentials are said to be excitable—a
principal characteristic of neurons. Action potentials
are rapid changes in the potential difference across the
axonal plasma membrane. During an action potential
the membrane potential first depolarizes very rapidly
from –75 mV to approximately +40 mV (positive inside).
It then repolarizes to approximately –95 mV (a slight
hyperpolarization) before returning relatively slowly to
its RMP (Fig. 2.8, which shows a typical action poten-
41THE NERVOUS SYSTEM
RMP
+100
(mV)
Time (ms)
2
+50
Vm
0
−50
A
−100
0 123
Fig. 2.8 A typical action potential. For events occurring at positions
A–H, see text. RMP, Resting membrane potential.
tial as recorded from a neuron). The action potential is
an all- or- nothing phenomenon, in that once initiated it
has a fixed size. The ability of neurons to generate action
potentials is due to the presence in their plasma membrane of ion channels that respond to changes in the
membrane potential; they are voltage- sensitive. The different phases of the action potential are due to the open-
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
ing of two types of voltage- sensitive channel, which are
Na+- and K+- selective. The different properties of these
two types of channel determine the characteristics of the
action potential.
A neuron may be initially at rest (see Fig. 2.8, position
A). The initial stimulus for the generation of an action
potential is depolarization of the neuron at the trigger
zone (how this is produced is explained later).
Voltage- sensitive Na+ channels, which have a particularly high density in this part of the axon, will open
almost instantaneously, within microseconds, in response
to depolarization—a process called activation—and
Na+ ions will flow down their concentration gradient
into the neuron. However, these channels will also close
again within approximately 0.5–1 ms and be in a state in
which they cannot be reopened; they are inactivated. If
the depolarization of the trigger zone is small then only
a few Na+ channels will open. Unless the total Na+ permeability (gNa+) is greater than the resting K+ permeability (gK+), after they close nothing else will happen. This
occurs when the graded input signals to the neuron are
weak and the depolarization is subthreshold.
However, when the depolarization of the trigger zone
is larger (see Fig. 2.8, position B), usually to approximately –55 mV, a greater number of Na+ channels are
activated. Once the Na+ permeability exceeds the resting K+ permeability, positive feedback occurs, by which
depolarization of the membrane will open more Na+
channels, depolarizing the membrane further until all
local Na+ channels are open, allowing Na+ influx.
The voltage then changes rapidly (see Fig. 2.8, position C) to a value approaching + 40 mV. The local area
D
C
B
E
F
G
H
of membrane is now effectively permeable only to Na+—
the resting K+ permeability being very small compared
Na
happen the cell would develop a new equilibrium posi-
to the Na+ permeability—and if nothing else were to
+
E
tion, as predicted by the Nernst equation at the Na+ equilibrium potential, of E
= +66 mV.
+
Na
However, the depolarization of the neuron stops
before E
is reached (see Fig. 2.8, position D), due
+
Na
to three main factors. First, the accumulation of positive ions inside the neuron starts to slow the Na+ influx,
through repulsion of positive charges. Second, the inactivation of the Na+ channels stops the continued influx
+
E
K
of Na+. Third, the voltage- sensitive K+ channels open.
These channels are activated by the same depolarization that triggered the opening of the Na+ channels, but
they open more slowly. The axon now becomes highly
permeable to K+ ions, which can now leave the neuron
down their concentration gradient (see Fig. 2.8, position
E). This process is initially encouraged by the positive
charge that repels K+. The K+ permeability is now higher
than at rest and the membrane potential falls to close to
the K+ equilibrium potential (–95 mV) (see Fig. 2.8, position G). This is called hyperpolarization. When the axon
is repolarized below the normal RMP, the K+ channels
close and the resting membrane permeability allows the
axon to regain its normal RMP (see Fig. 2.8, position H).
The small number of Na+ and K+ ions that have moved
through the voltage- dependent channels are eventually
redistributed by the Na+/K+- ATPase in order to maintain
the concentration gradients. However, the pump has no
active role in action potential generation.
Immediately after an action potential, the Na+ channels remain completely inactivated for a short period and
cannot be opened by depolarization, however large. The
Na+ channels only regain their normal resting state once
the membrane has repolarized (see Fig. 2.8, position F).
This is called the absolute refractory period (see Fig. 2.8,
positions B–F) and ensures that action potentials occur
separately and do not propagate in the reverse direction
along an excited axon. However, for a short time after
this, during the period when some K+ channels are still
open and when the membrane is hyperpolarized, it is
possible to initiate another action potential if the stimulus is larger than normal—a suprathreshold stimulus—
but the action potential remains a fixed size. This second
period is called the relative refractory period (see Fig. 2.8,
positions F–H). Table 2.2 lists a summary of the events
during an action potential.
The all- or- nothing nature of the action potential, and
the enforced time gap between action potentials, means
that the action potential is a binary signal. Action
potentials are clearly separated one from another,
which ensures that the number of action potentials in
a given period is clear. This is very important in a system where stimulus strength is coded by frequency.
Binary- coded information can be transmitted with less
degradation due to noise, an important consideration
when the action potentials are transmitted across large
distances.
42 SYSTEMS OF THE BODY
2
Table 2.2 Sequence of events during an action potential
(see Fig. 2.8)
Ion
Event Channels
Initial depolarization
to threshold
(approximately
–55 mV) (A)
Depolarization to
approximately
+40 mV
Repolarization, then
hyperpolarization
(–95 mV)
Return to resting
membrane
potential (H)
Na+ channels
open (B)
Na+ channels
close and
K+ channels
open (D)
K+ channels
close (G)
movements Notes
Na+ influx
(C)
K+ efflux (E)
Movement
of K+
through
leak
channels
(G–H)
Absolute
refractory
period
(B–F)
Relative
refractory
period
(F–H)
Nerve conduction
So far we have considered only the activity at a given
position on the axon, but it is the inactivation of Na+
channels that also enables the action potential to travel
along the axon from the trigger zone to the synapse.
The depolarization caused by the opening of Na+
channels at any given point will spread passively along
the axon equally in both directions. However, any Na+
channels in the direction from which the action potential has come will still be inactivated and only those Na+
channels further down the axon, which have not yet
been opened, will be activated by the depolarization.
In this way the wave of depolarization (like a Mexican
wave) will propagate from the trigger zone to the synapse, with the depolarization being continually regenerated by newly opening Na+ channels.
In unmyelinated axons voltage- gated Na+ and K+
channels are located all along the axon and the action
potential travels continuously along the axon. However,
in myelinated axons there is a different mode of propagation. Owing to the tight insulation of the axon by oligodendrocytes (in the CNS) or Schwann cells (in the PNS)
the axon is effectively separated from the ions that would
flow in, except at tiny gaps where the myelin sheath does
not cover the surface of the axon. Occurring at regular
intervals, these gaps in the myelin are called nodes of
Ranvier (see Fig. 2.1) and are zones where the Na+ and
K+ channels are concentrated. Depolarization at the preceding node spreads passively along the interior of the
axon but, as the myelinated axon cannot depolarize,
this spread is more rapid than in bare axons. The action
potential does not decay as rapidly as in unmyelinated
axons, because of the insulation, and at the next node the
depolarization is sufficiently large to trigger the opening of large numbers of Na+ channels. This will regenerate the depolarization to its full voltage. In this way the
action potential jumps rapidly from node to node, a process called saltatory conduction. Failure of saltatory conduction may underlie certain pathologies (see Box 2.2).
The advantage of myelination is an increase in the
speed of conduction without a large increase in metabolic cost. Another way of increasing the speed of conduction is by increasing the size of the axon, which
reduces the internal electrical resistance and increases
the passive depolarization. This strategy is seen in its
most exaggerated form in the giant axons of some invertebrates, particularly the giant squid. The large size of
these axons means that they are easier to examine experimentally than smaller mammalian axons. Much of the
early work on the action potential was done on these
axons by Hodgkin and Huxley in the 1950s.
In humans the fastest axons are both large and myelinated. As detailed in Table 1.7, nerve fibres can be catego-
rized based on conduction velocity according to their
axon diameter. A difference in conduction velocity can
be seen when a mechanical trauma is experienced. The
sensory input from mechanical nociceptors travels along
myelinated axons (Aδ fibres) and arrives before the signal
carried by unmyelinated axons (C fibres). This gives rise
to fast and slow pain. Although the C fibres are smaller,
most of the difference in velocity is due to the myelin.
Because myelination allows rapid conduction with a
smaller diameter, this allows more axons to be packed
into a smaller volume, a significant advantage when wiring something as complex as a brain.
Although the electrical polarity of the membrane
changes during an action potential the numbers of Na+
and K+ ions which flow into and out of the neuron in
order to bring about this change are relatively small and
do not significantly change the internal concentration of
either ion. This is because the RMP is only a local charge
separation, involving only a very small proportion of the
positive charges present, and the main bulk of the intracellular ions is electrically balanced. This can be shown
clearly when an axon is poisoned with ouabain (a plant
derived toxin). Despite the inhibition of the Na+/K+ATPase the axon is able to fire many thousands of action
potentials, only stopping eventually when the ion gradients are dissipated.
Synaptic transmission
The specialised site where the axon terminal meets the
target cell is the synapse. This is where the signal is
transmitted either to another neuron or to an effector,
such as a muscle or a gland. There are two types of synapses: electrical and chemical.
Electrical synapses are present throughout the CNS,
for example, in the cortex, hippocampus, thalamus, locus
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
43THE NERVOUS SYSTEM
2
Mitochondrion
Neurotransmitter
Synaptic cleft
channels
Myelin sheath
A
Presynaptic
membrane
xon
MM
vesicle
V
B
Postsynaptic
receptors
Fig. 2.9 Simplified diagram of a synapse. For clarity, only a few
vesicles are shown.
Ca
2+
membrane
Postsynaptic
coeruleus and olfactory bulb. In an electrical synapse the
two cellular elements are connected by gap junctions.
At these junctions, cells interact through a cluster of
integral membrane proteins around a pore called a con-
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
nexon. Each connexon is formed from six connexin proteins and is aligned with a connexon in the opposing cell.
This forms a channel between the cells that allows the
free passage between them of water and small molecules
(<1.2 nm in diameter) including some signalling mol-
Fig. 2.10 Electron micrograph of a synapse. The synaptic bouton (B)
is making contact with the dendrite (D). The bouton contains many
small vesicles (V) and mitochondria (M). The synaptic cleft (C) contains
faint granular material above the postsynaptic membrane (P) which
presents an area of thickening (from Lowe J. (2020). Stevens & Lowe’s
Human Histology. Oxford: Elsevier Ltd.).
ecules such as inositol trisphosphate (IP3) and Ca2+. The
intercellular space is approximately 2–4 nm. Gap junctions play important roles in the electrical and metabolic
coupling of cells. They are particularly important in coordinating electrical activity in smooth and cardiac muscle.
They are also found between astrocytes and between
neurons in the retina, where horizontal cells are electrically coupled. Electrical synapses are bidirectional—electrical signalling can flow both ways across the synapses.
In humans, most synapses are chemical synapses and
are unidirectional—the presynaptic element releases
chemicals called neurotransmitters into the small gap
between the neuron and the subsequent cell (postsynaptic element). The gap measures approximately 15–25
nm and is called the synaptic cleft. A simplified image of
the chemical synapse in shown in Fig. 2.9. There is evidence that the synaptic cleft is not devoid of molecules
as it contains some proteinaceous material. There is also
evidence of specific protein interactions that establish
discrete molecular bridges across the cleft (e.g. between
the cell- adhesion proteins, neurexin on the presynaptic
side, and neuroligin on the postsynaptic side). These complexes stabilize synapses and maintain the perfect alignment of presynaptic and postsynaptic membranes.
At chemical synapses, when the propagated action
potential reaches the axon terminal, release of neurotransmitters (one or several different transmitters)
occurs by a process called exocytosis. The neurotransmitter diffuses across the gap and interacts with receptor
proteins in the postsynaptic membrane of the target cell.
The presynaptic terminal contains numerous membranebound organelles such as mitochondria and smooth endoplasmic reticulum. However, the most distinguishing
feature is the presence of large numbers of storage vesicles,
which contain neurotransmitters. These can be easily visualized on electron micrographs (Fig. 2.10). Vesicles have a
variety of morphological characteristics corresponding to
the type of neurotransmitter that they contain.
While many of the vesicles are distributed throughout the presynaptic terminal, there are some that are
anchored close to the synaptic cleft at the presynaptic
membrane, ready to be released in areas known as active
zones. When an action potential invades the presynaptic
terminal the depolarization opens voltage- sensitive Ca2+
channels, which allow Ca2+ to flow into the synapse (the
concentration of Ca2+ is much higher in the ECF than in
the ICF (see Table 2.1)). Ca2+ acts at a number of sites to
facilitate exocytosis. During exocytosis, SNARE (from
SNAP Receptor) proteins play an essential role in vesicle
docking, priming, fusion with the plasma membrane and
neurotransmitter release into the synaptic cleft. SNARE
proteins can be divided into vesicle, or v- SNAREs (part
C
P
D
44 SYSTEMS OF THE BODY
Dynamin
Endocytosis
Endosome
Clathrin
Exocytosis
Fig. 2.11 Neurotransmitter is released by exocytosis. The vesicle
membrane is coated with clathrin and the vesicle is retrieved by
endocytosis. The vesicle then fuses with an endosome. New vesicles
are budded from the endosome and filled with neurotransmitter,
ready for release.
2
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
Table 2.3 Examples of neurotransmitters
Type Neurotransmitter
Amino acids Glutamate
Aspartate
γ- Aminobutyric acid (GABA)
Glycine
Amines Acetylcholine
Dopamine
Noradrenaline
Adrenaline
Serotonin (5- hydroxytryptamine, 5- HT)
Peptides Endorphins
Enkephalins
Substance P
nerves and skeletal muscles, the skeletal neuromuscular
junction, a single action potential releases approximately
30 vesicles containing neurotransmitter, which is sufficient to produce a twitch of the target muscle.
of the membranes of vesicles), and target, or t- SNAREs
(associated with the presynaptic membrane), and these
two classes interact during exocytosis. Synaptobrevin,
syntaxin and synaptotagmin are examples of SNARE
proteins involved in the process of exocytosis. First, the
vesicle is released from the cytoskeleton. Second, it partially fuses with the presynaptic membrane with the
help of SNARE proteins in the vesicle and presynaptic
membranes. The active zone of the presynaptic membrane contains the primed vesicles and a high density of
SNARE proteins. The influx of Ca2+ following depolarisation activates synaptotagmin, which allows the complete fusion and development of a pore through which
the neurotransmitter can diffuse into the synaptic cleft.
Exocytosis incorporates vesicle membrane into the
presynaptic membrane. If the vesicle membrane were
to remain there the surface area of the neuron would
increase and the plasma membrane would also contain vesicle membrane proteins. Therefore, in order to
retrieve the vesicle for reuse, the vesicle membrane is
recycled back into the synapse by a process called endocytosis (Fig. 2.11). The vesicle membrane is first coated
with a protein called clathrin, which forms a ‘cage’
around the vesicle; this is then pulled back into the cell
by the cytoskeleton, with the help of a ‘collar’ of dynamin. The retrieved vesicle is fused with an endosome,
from which new vesicles can bud off. The empty vesicles
can then be refilled with neurotransmitter and reused.
Patterns of vesicle release vary enormously across
synapses. At many synapses in the CNS, not every action
potential will release vesicles; the average probability of
release is usually less than one. However, at the other
extreme, in the PNS, at the synapses between motor
Neurotransmitters
The first endogenous compound to be identified as a
neurotransmitter was acetylcholine (ACh), when its
effects on the frog heart were described by Otto Loewi
in 1926. Since then, a wide range of endogenous compounds have been identified which act as neurotransmitters, which are responsible for signalling at chemical
synapses (Table 2.3). There are distinct patterns of distri-
bution for various neurotransmitter systems and in some
cases particular neurotransmitters are associated with
specific functions in the brain.
Peripheral neurotransmitters
In the ANS the major neurotransmitters are noradrenaline and adrenaline, which are released onto target
organs and tissues by the sympathetic nervous system,
and ACh, which is released by the parasympathetic nervous system. ACh is also released at all skeletal neuromuscular junctions, although the receptor subtypes on
the muscle are different from those in the ANS. ACh is
also the neurotransmitter that acts at all autonomic ganglia, both sympathetic and parasympathetic.
Central neurotransmitters
In the CNS the major excitatory and inhibitory neurotransmitters are the amino acids glutamate and γ-
aminobutyric acid (GABA), respectively. These are the
main mediators of fast signalling in the brain and spinal
cord. Other neurotransmitters include a number of monoamines, such as noradrenaline, 5- hydroxytryptamine
45THE NERVOUS SYSTEM
2
(5- HT, serotonin) and dopamine. These monoamine neurotransmitters mainly act in a slower manner than amino
acids and are often described as ‘neuromodulators’, as
their (excitatory or inhibitory) effects depend on the receptor type stimulated. Many neurons release more than
one neurotransmitter, although the patterns of excitation required to stimulate their release may differ. One
of the neurotransmitters may be, for example, glutamate,
whereas the second neurotransmitter may be a peptide.
Peptides are released at a higher firing frequency. The
slow- acting neurotransmitters frequently have longlasting modulatory effects on their target neurons.
Histamine, as well as acting as an important neurotransmitter in the brain, is also an inflammatory mediator, released from mast cells and basophils, and promotes
vasodilatation and increased capillary permeability.
Following its discovery as a key molecule in the control of blood pressure, nitric oxide (NO), a short- lived
gas, is a more recent addition to the list of neurotransmitters. It is produced from L- arginine, through the action
of NO synthases. One of these synthases is expressed in
neurons, and NO can be produced postsynaptically and
act after diffusion on the presynaptic element as a retrograde signalling agent. While it fulfils the criteria for a
neurotransmitter, it does not act on cell surface receptors
but binds to the intracellular enzyme guanylate cyclase,
causing changes in intracellular biochemistry. It has been
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
suggested that one role for NO in the CNS may be in the
hippocampus, in the formation of new memories. There
are also lipid- derived neurotransmitters such as the
endogenous cannabinoids (e.g. anandamide), which can
be produced on demand by the postsynaptic element,
and diffuse across the synapse to exert their effects on
receptors located on the presynaptic terminal—another
example of retrograde signalling.
Postsynaptic events and postsynaptic
receptors
The neurotransmitter that is released into the synaptic cleft diffuses to the postsynaptic membrane, where
it binds to specific receptors. Typically, this takes less
than 5 μs, as the cleft is very narrow, measuring approximately 15–25 nm. Not all the released neurotransmitter
reaches the receptors, as diffusion out of the cleft and
specific inactivation mechanisms may reduce the amount
available for receptor binding.
The action of a given neurotransmitter on a target
neuron (or peripherally on a target organ) depends on
the identity of the receptors present on the postsynaptic
target. Almost all neurotransmitters identified and characterised so far have multiple receptor subtypes that
they can activate. Receptors for most neurotransmitters
(unlike those for some hormones and NO) are integral
membrane proteins, which have their neurotransmitter binding sites in receptor structural domains located
near the extracellular surface. There are two major types
of receptors. The first type comprises ionotropic receptors, which contain an integral ion channel; hence they
are alternatively named ligand- gated ion channels. The
second type comprises metabotropic receptors, which are
linked to intracellular signalling pathways via a group
of intermediate proteins called G- proteins (so called
because they bind the guanosine phosphates, GTP and
GDP, as described later in the text). These metabotropic
receptors are also known as G- protein- coupled receptors
(GPCRs). GPCRs, while sharing a common structural
motif, are extremely diverse, with hundreds of different
types having been identified. Within each of these two
major types, there are structural and functional similarities between receptors for different neurotransmitters,
suggesting molecular evolutionary divergence from
common precursors.
There is a third group of cell surface receptors consisting of an extracellular binding site and an intracellular catalytic site that, when the receptor is activated by
the ligand, phosphorylates tyrosine residues. These are
receptor tyrosine kinases (RTKs). There are more than 20
subclasses of RTK (e.g. the epidermal growth factor family, the fibroblast growth factor family and the vascular
endothelial growth factor family) and they are the targets for peptide growth factors (including neurotrophic
factors such as nerve growth factor (NGF) and brain
derived neurotrophic factor (BDNF)), cytokines and various hormones. They can exist as single subunit receptors
and also as multiple subunit complexes. The catalytic
domain responsible for the kinase activity of these receptors is located in the C- terminal domain of the subunits,
which is intracellular. After activation, RTKs can undergo
internalisation in a manner similar to metabotropic
receptors (see later).
Some ligands, such as the steroid hormones, are lipophilic and can diffuse across the cell membrane. Others,
such as thyroid hormone, are internalized by specialized
transport mechanisms. These ligands bind to nuclear
receptors which act as transcription factors, thus affecting the level of gene transcription.
Ionotropic receptors
Ionotropic receptors are integral membrane- spanning
proteins with multiple subunits (hence the name ‘multimeric’) that group together to form an ion channel
complex that has different ion selectivities and multiple
associated ligand- binding sites. Receptors of this type
(Table 2.4) can be divided into many subtypes. In some
cases the distribution of the subtypes clearly varies over
developmental time and by location (see later chapters for further details on specific receptors). Ionotropic
receptors are anchored to the membrane through a complex scaffolding which consists of proteins with specific
structural domains that enable them to couple both to
receptor subunits and to the cytoskeleton or to intracellular signalling complexes. Ionotropic receptors are responsible for fast signalling such as on the millisecond scale.
46 SYSTEMS OF THE BODY
2
hA
α3β
α4β
A
Table 2.4 Ionotropic receptors
Neurotransmitter Receptor type
Acetylcholine Nicotinic acetylcholine
Glutamate NMDA
AMPA
Kainate
GABA GABA
Glycine Glycine
Serotonin 5- HT
5-HT, 5-Hydroxytryptamine; AMPA, amino-3-hydroxy-5-methyl4-isoxazole propionic acid; GABA, γ-aminobutyric acid; NMDA,
N-methyl-D-aspartate.
A
3
An example of an ionotropic receptor is the nico-
tinic ACh receptor (nAChR) (Fig. 2.12; see also Chapter
17). It is formed of five subunits—α, β, γ, δ and ɛ—as a
homomer (the same subunit) or a heteromer (different
subunits). The open nAChR allows the flow of Na+, Ca2+
and K+ across the membrane. The subunit composition
dictates the biophysical and pharmacological properties
of this type of receptor. This is a general principle applicable across all ionotropic receptors.
Neurotransmitter binding to ionotropic receptors triggers a conformational change that allows ions to flow
selectively through the channel. The subsequent current
flow sets up a potential difference called a postsynaptic
potential, which may be excitatory (EPSP) or inhibitory
(IPSP), depending on the ion selectivity and the membrane potential, and makes the neuron more (EPSP) or
less (IPSP) likely to fire an action potential. If the current flowing through each type of channel is measured
at different membrane potentials, there is a voltage at
which the net current is zero. For example, if the channel is only permeable to K+, this will occur at the Nernst
potential for K+. This is because at this voltage—called
the reversal potential—the numbers of ions moving
in and out of the neuron are exactly balanced (see earlier). For any given channel, if the membrane potential is
higher than the reversal potential, activation of the channel will produce an IPSP, and if it is lower, it will produce an EPSP.
If we consider the case of ACh receptors at the neuromuscular junction (also called the motor endplate),
the RMP will be approximately –85 mV. On activation,
current will flow through the ACh receptor, making the
membrane potential more positive, and in skeletal muscle this depolarization is sufficient to activate voltagesensitive Na+ channels, producing an action potential,
which, via the subsequent release of Ca2+ from the sarcoplasmic reticulum, causes the muscle to contract.
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
Homomeric nAChRs
α7
α7 α7
α7 α7
α7
Heteromeric nAChRs
α3
(+)
β4
(-)
α3
β4
(+)
(-)
4
Fig. 2.12 The multimeric structure of the nicotinic neuronal and muscle acetylcholine (ACh) receptors. (A) Homomeric or heteromeric
combinations of various subunits define the ion channel and form receptors with different properties. The ACh binding sites are illustrated
as triangles. The different subunits are labelled α, β, δ⁄ε and γ. (B) Location of the muscle- type nACh receptor in the membrane showing its
quaternary structure—the straight arrows indicate the binding sites for the neurotransmitter and the curved arrow indicates the flow of sodium
though the channel. (A, From Ho Thao N.T., Abraham N., Lewis R.J. (2020). ‘Structure-Function of Neuronal Nicotinic Acetylcholine Receptor
Inhibitors Derived From Natural Toxins’, Frontiers in Neuroscience, 14. http://doi:10.3389/fnins.2020.609005; B, From Karlin A. (2002). ‘Emerging
structure of the Nicotinic Acetylcholine receptors’, Nature Reviews Neuroscience, 3:102-114.
ε/γ
(-)
α1
(+)
δ
β1
(-)
αβε(γ)δ
β4
α9 α9
α9 α9
α1
(+)
β2
(-)
α4
(+)
α9
α
α9
α4
(+)
β2
β2
(-)
2
AC
β
αγ
B
δ
Na
+
Ch
Extracellular
Intracellular
47THE NERVOUS SYSTEM
2
In studies on the motor endplate, it was observed
that small EPSPs called miniature endplate potentials
(MEPPs) occurred spontaneously at a rate of approximately 1 per second. They were of similar size (~ 0.5 mV)
and duration (rising rapidly and decaying more slowly
in a total of ~5 ms), and all EPSPs were multiples of
MEPPs. This is because neurotransmitter release occurs
in distinct packets called quanta. Each MEPP represents
the release of a single vesicle containing ACh, which is
the minimum possible amount that can be released. The
amount of ACh in each vesicle is approximately the same
and the stimulated neurotransmitter release therefore
consists of release from several vesicles, so the response
will be multiples times the effect of a single vesicle.
This phenomenon has been more difficult to establish
in the CNS, possibly due to the variable size and content of the vesicles and the smaller number of vesicles
released by each action potential. The amount of neurotransmitter reaching the postsynaptic receptors may
be affected by the high- affinity uptake of neurotransmitter into glia and surrounding neurons, through specialised transporters (see below). It is also important to note
that the neurotransmitter released from the presynaptic
terminal can also activate presynaptic receptors. These
are called ‘autoreceptors’ and they often act through a
negative feedback mechanism: if neurotransmitter in
the synaptic cleft reaches too high a level, the activation
ELEMENTS OF CELLULAR AND MOLECULAR NEUROSCIENCE
of autoreceptors may limit further release or even ultimately lead to reduced neurotransmitter synthesis.
Metabotropic receptors
Metabotropic receptors, or GPCRs, while having a
common mode of action, are extremely varied in their
ligands. They are the largest family of membrane- bound
receptors and are responsible not just for mediating the
effects of neurotransmitters as diverse as small amino
acids and large peptides, but also for the sensory reception of light (where the ligand is a photon) and for
detecting some tastes and all odours (Table 2.5). GPCRs
are the target of approximately 50% of all drugs used at
present.
Table 2.5 A selection of endogenous ligands that have
metabotropic receptors
Acetylcholine Adenosine
Adrenaline Serum calcium
Noradrenaline Anandamide
Serotonin Bradykinin
Dopamine Oxytocin
Histamine Somatostatin
Glutamate Vasopressin
GABA β- endorphin
The common structure of these receptors is that of a
single protein with seven transmembrane segments, with
an extracellular domain that often contains the ligandbinding site and an intracellular domain that is involved
in the interaction with G- proteins (see Fig. 2.13). Some
metabotropic receptors are anchored to specific postsynaptic proteins, in a manner similar to ionotropic receptors. G- proteins are trimers of membrane- associated
proteins and consist of alpha (α), beta (β) and gamma (γ)
subunits. The inactive α subunit has guanosine diphosphate (GDP) bound to it. This is exchanged for guanosine
triphosphate (GTP; present in the cytoplasm) upon activation of the G- protein trimer by a receptor, resulting in
the α subunit separating from the β and γ subunits. The
activated α subunit then alters the activity of an enzyme
involved in the synthesis of a second messenger. The α
subunits come in different forms, which interact with different intracellular enzymes, and can either stimulate or
inhibit these enzymes. A few seconds after the GDP/GTP
interchange, the molecule of GTP is converted to GDP
by the intrinsic GTPase activity of the α subunit, which
renders the α subunit inactive, whereupon it recombines
with the β and γ subunits. The β and γ subunits, which
are also released from the receptor, remain bound to each
other throughout, and have roles distinct from that of the
α subunit. At its simplest, the sequence of events triggered by the binding of a ligand to a metabotropic receptor is as follows (see Fig. 2.13):
1. The ligand binds to the receptor.
2. The receptor binds the G- protein trimer.
3. The α subunit exchanges GDP for GTP and separates
4. The activated α subunit binds to its target enzyme.
5. Production of second messengers is either increased
6. The α subunit dissociates from the enzyme and
The target enzymes are commonly either adenylate
cyclase or phospholipase C (PLC) (Fig. 2.14). At least
17 genes code for α subunits and they can be grouped
according to the way in which they modulate intracellular signalling, for example, whether they stimulate
or inhibit adenylate cyclase (αs or αi) or they activate
PLC (αq).
Adenylate cyclase catalyses the conversion of ATP
to cAMP, which itself activates other intracellular
enzymes such as cAMP- dependent protein kinase A
(PKA). PLC catalyses the breakdown of the membrane
lipid phosphatidylinositol 4,5- bisphosphate into two
second messengers, diacylglycerol (DAG) and inositol
1,4,5- trisphosphate (IP3). DAG is an activator of protein
kinases, in this case protein kinase C (PKC). IP3 diffuses
into the cytoplasm and binds to specific IP3 receptors on
intracellular organelles containing Ca2+ stores, causing
from the β and γ subunits.
or reduced until the GTPase activity of the α subunit
dephosphorylates GTP to GDP.
recombines with the βγ subunit.
48 SYSTEMS OF THE BODY