Ion Channels, ion transporters and electrical signalling as well as their Clinical correlates

1. APPLIED
NEUROSCIENCES
Lecture 2: Ion channels, transporters,
and electrical signaling
MRH 331

2. Content
• OVERVIEW
• RESTING MEMBRANE POTENTIALS
• ELECTRICAL EXCITABILITY OF THE CELL MEMBRANE
• VOLTAGE-GATED CHANNELS
• EXTRACELLULAR LIGAND-GATED CHANNELS
• INTRACELLULAR AND INTERCELLULAR CHANNELS
• ACTIVE TRANSPORTERS
• A CROSS-COMMUNICATION BETWEEN ELECTRICAL
AND RECEPTOR-MEDIATED SIGNALING PATHWAYS
• CLINICAL CORRELATION: DEMYELINATING DISORDERS

3. OVERVIEW
• Membranes in all cell types regulate the extracellular and
intracellular ionic environment.
• In neurons and other excitable cells, regulation of the ionic
environment is also crucial for the development and maintenance of
the specific signaling pathway for these cells, known as the electrical
signaling system.
• This system is composed of two basic elements:
(1) a lipid bimolecular diffusion barrier, termed lipid bilayer, which
separates cells from their environment, and
(2) two classes of macromolecule proteins, known as ion channels
and active transporters, which regulate the movement and
distribution of ions across the lipid barrier in the plasma membrane.
• Resting neurons generate a negative potential, termed the resting
membrane potential

4. • The primary electrical signal generated by neurons is called the action
potential, which abolishes the negative resting potentials and makes
the transmembrane potential transiently positive.
• A negative membrane resting potential results predominately from a
net efflux of K+ across membrane. Generation of action potential is
achieved by a rapid raise in Na+ and/or Ca2+ permeability, leading to
depolarization and inverse polarization of cells, whereas a slower and
prolonged rise in K+ permeability accounts for repolarization of cells
to their usual negative resting level.
• The same mechanism also is responsible for propagation of action
potentials along the length of axons. Changes in permeability for
Ca2+, on the other hand, provide an effective mechanism for transfer
of electrical signals from cell to cell by initiating the release of
neurotransmitters at synapses.
• Ion channels are the membrane proteins that give rise to selective
permeability, whereas active transporters create and maintain ion
gradients. Thus, from the perspective of electrical signaling, ion
channels and active transporters are complementary.

5. • Some of the ion channels are able to sense the electrical potential
across the plasma membrane and are called voltage-gated channels.
These channels have a pore that opens and/or closes in response to
changes in the membrane potential.
• Other channels are gated by extracellular chemical signals and are
called extracellular ligand-gated channels.
• Among transporters, the most important is the Na+ pump, which
regulates the intracellular concentrations of both Na+ and K+ and
requires ATP for its action.
• The concentration gradients of Ca2+, Cl–, and H+ are also achieved
by co-transporters specific for these ions and usually sodium ion.
• Neurons, like other cell types, also signal through nonelectrical
plasma membrane–dependent mechanisms, independently of ions
and proteins responsible for ion transport.
• This receptor-mediated signaling frequently interacts with the
electrical signaling system.

6. RESTING MEMBRANE POTENTIALS

7. Lipid Bilayer Separates Cells from
Their Environment
• PLs and several other types of lipids contribute to the membrane structure.
• The major cell-membrane lipids consist of a hydrophilic head, commonly a
glycerophosphoryl ester, and a hydrocarbon tail, usually containing two
hydrophobic fatty acids.
• In the cell membrane, tails orient back-to-back
• Such a double-layered structure is known as a lipid bilayer
• Detergents transform lipid bilayers into water-soluble micelles, whereas
cholesterol stabilizes bilayers
• The bilayer structure serves several important functions in intracellular and
intercellular signaling.
(1) The hydrophobic tails of the lipid molecules tend to be chemically incompatible with water-soluble substances
containing inorganic ions (selective permeability)
(2) Separation of intracellular and extracellular conducting solutions is provided by an extremely thin hydrophobic,
insulating layer (1.5 to 3 nm). This forms a significant electrical capacitor, or storage of electrochemical energy that
drives the signaling process.
(3) The protein components of the membranes, including carriers and ion channels, are embedded in the lipid
bilayer and are oriented and grouped in a manner that serves their respective functions.

8. Ion Pumps and Ion Channels Transport Ions in
Opposite Directions
• Ion carriers including the Na+-K+ pump, the Ca2+ pump, Na+-Ca2+ exchanger, Cl–-
HCO3 exchanger, and glucose transporters
• Ion pumps require ATP to move ions from a side of low conc. to a side of high conc
eg Na+-K+ pump
• ATP-dependent Ca2+ transporters are expressed in both plasma membrane and ER
membrane and pump Ca2+ from cytosol to the extracellular fluid and the ER fluid,
respectively.
• Together with Na+-Ca2+ antiporters, these pumps maintain low concentrations of
intracellular free Ca2+ in resting cells (about 100 nM)
• ion gradients built by these pumps are required for the most basic neuronal
functions.
• However, excitation and electrical signaling are carrier-independent and are
dependent on movements of ions through ion channels determined by ion
gradients
• Most channels are ion-selective and are so named
• Some channels are continuously open to the flow of ions (leakage channels) while
others are transiently open to ion fluxes (gated channels)
• There are five major groups of channels:
1. Voltage-gated channels - open or close in response to changes in electrical potential across the cell membrane.
2. Ligand-gated channels - require a binding of a particular signaling molecule to open or close.
3. The stretch-sensitive channels - open or close in response to a mechanical force.
4. Intracellular channels - expressed in the ER and nuclear membranes.
5. Connexins and pannexins - form gap junctions, which operate as intercellular channels.

9. Signaling Proteins Other than
Channels and Transporters
• Signaling proteins of the cell membrane include
three groups of proteins.
1. Adhesion and anchor proteins - help bind one
cell to another and fasten the membrane to an
internal network of proteins respectively.
2. Receptors are proteins that bind the extracellular messengers and respond to
this by generating intracellular messengers
3. Membrane-bound enzymes - Receptors transduce signaling by activating a
chain reaction or a cascade of events, usually by activating membrane-bound
enzymes.
• The first protein in the cascade is G protein, which in turn activates other
membrane-bound proteins, including adenylyl cyclase, phospholipases C and D,
and phosphodiesterases
• This also leads to the generation of intracellular messengers, such as cyclic
adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP),
inositol 1,4,5- trisphosphate (IP3), diacylglycerol (DAG), and phosphatidic acid,
as well as activation of kinases, including protein kinase A (PKA), protein kinase
C (PKC), and protein kinase G(PKG)

10. Ion Concentrations Differ Across the
Cell Membrane
• The concentrations of various ions in the two conducting
solutions, the intracellular and extracellular are different.
All living cells maintain differential ion
• Na+, Ca2+, and Cl– are predominantly found extracellularly,
whereas K+ and organic ions are concentrated
intracellularly.
• The concentration gradient across the membrane has
two major functional consequences for excitable cells:
1. It provides the ECE to drive signaling.
2. Ca2+ influx through plasma-membrane channels (& ER) also serves as an
intracellular messenger to activate many intracellular biochemical
processes, including synaptic transmission.

11. Distribution of Free Ion Concentrations and
Equilibrium Potentials for a Mammalian Cell Model
Extracellular
ion
Intracellular
concentration
(mM)
Equilibrium
Concentration
(mM)
Potential
(mV)
Sodium 145 12 +67
Potassium 5 150 –91
Calcium 1.5 0.0001 +128
Chloride 125 5 –86

12. Every Neuron Has a
Membrane Potential
• There is a balance of electrical charge within the
cytosol and extracellular fluid compartment,
termed macroscopic electroneutrality.
• On the other hand, there is a difference in electrical potential across
the membrane of each cell.
• Experimentally, this difference can be measured by placing a glass
microelectrode into the cytoplasm and the other electrode in
external fluid
• In excitable cells, a steady electrical potential difference of about –50
to –70 mV is detected. In nonexcitable cells, the potential difference
is smaller.
• The membrane potential is always given as that of intracellular
compartment relative to the extracellular compartment.

13. The Membrane Potential as
Cellular Force
• Separation of charges of opposite sign, like those by plasma
membrane, provides the basis for electrical phenomena
• When positive and negative electrodes are placed in an ion
solution, cations will flow toward the negative pole and
anions toward the positive pole, and both carry electrical
current toward the negative pole.
• The size of current is determined by the potential difference
between the electrodes and the electrical conductance of the
solutions between electrodes.
• Because electrical capacitance is inversely related to the
thickness of the insulating region separating two conductors,
the 1.5- to 3-nm lipid barrier makes a very efficient
capacitator, of the order 1 mF/cm2of membrane surface.

14. Membrane Potential and Ion
Movements
• Three factors contribute to the movement of ions:
1. concentration gradient. A difference in concentration of the ion on the two
sides of the membrane should result in diffusion. Thus, if the membrane is
permeable for ions, they will move down its concentration. The net transfer is
proportional to the concentration difference across the cell membrane, does
not require energy, and is termed a passive transport.
2. Electrical gradient. Electrical potential differences across the cell membrane also
influence ion movement. Cations in the pore of channels will be moved toward
the negative interior of cells, whereas anions will be moved toward the positive
exterior of cells, and this transport does not require energy.
3. Transporter activity. Pumps also participate in the transport of ions across the
membrane, but the energy stored in ATP is usually required and ions are moved
against their concentration gradients (active transport).

15. Single-Ion Electrochemical
Equilibrium
• Lets adopt a hypothetical cell model that contains sodium chloride extracellularly,
potassium aspartate intracellularly, and a plasma membrane expressing K+
channels only
• Initially, both intracellular and extracellular sides will be electrically neutral,
because of the equal numbers of cations and anions
• the diffusion of each K+ makes the inside slightly more negative relative to
outside, leading to the generation of a PD, as well as the generation of an
electrical field across the plasma membrane
• Initially, the efflux of K+ will dominate, followed by greater influx driven by
growing potential difference.
• At equilibrium, a steady state in which the tendency for further changes
disappears, a membrane potential will be established based on a single ion and
the selective permeability of the channels in membrane.
• The PD at which the equal movements occur is called the equilibrium potential,
also known as Nernst potential

16. •EK = RT/zF ln[K]o/[K]i
•ENa = RT/zF ln[Na]o/[Na]i
•ECa = RT/zF ln[Ca]o/[Ca]i
•ECl = RT/zF ln[Cl]i/[Cl]o
where R is the gas constant, T is temperature (in degrees Kelvin),
z is valence of the ion, and F is the Faraday constant (the amount
of electrical charge contained in one mol of a univalent ion).

17. Real Cells Are Not at Equilibrium
• the resting potential of cells (about –60 mV) is different from the
equilibrium potential for all four major ions, indicating that in the
physiologic-like situation, none of them is in equilibrium and that
the steady state of the resting potential is reached by other forces
• There is more force moving Na+ into the cells than there is moving
K+ out (31 mV for K+ and 127mVfor Na+).
• But, at resting potential, only a few Na+ conducting channels are
open (very low permeability).
• There is also a small but constant K+ efflux.
• Finally, the passive influx of Na+ and efflux of K+ is balanced by
the action of the Na+-K+ pump, which brings two K+ for three
Na+ that are expelled.

18. Multi-ion Electrochemical Equilibrium
• An ion electrochemical gradient illustrates the forces on ion movement through
a single channel type
• Real cells express more than one ion channel type
• Also, the dissociation between the resting potentials and equilibrium potentials
introduces a need to integrate the permeability of the membrane for ions (in
addition to the concentration of the ions in two conductors), which is not a
factor in the Nernst equation
• Thus, the equilibrium potential for each permeant ion and the permeability for
each ion are needed to calculate membrane potential in a multi-ion system, we
need to know
• This equation was derived by Goldman (1943) and Hodgkin and Katz (1949):
Em = RT / F ln
P K [ K + ] i + P N a [ N a + ] i + P C l [ C l - ] o
P K [ K + ] o + P N a [ N a + ] o + P C l [ C l - ] i
R, T, F, and z are as defined for the Nernst equation, and PNa, PK, and PCl are the permeabilities of the membranes
to Na+, K+, and Cl–, respectively.

19. • Because absolute permeabilities are difficult to measure, the
equation can be also expressed in terms of relative permeabilities
• The frequently used permeability ratios for PK:PNa:PCl are
1:0.04:0.45.
• Two important conclusions are derived from ion concentrations and
permeabilities.
1. The resting potential most closely approximates the calculated Nernst
equilibrium potential for the most permeable ion.
2. The membrane potential is most influenced by changes in concentrations of
the permeable ion e.g. if a membrane is equally permeable to Na+ and K+, the
membrane potential would be about –15 mV.
• However, because the measured resting potentials in neurons
closely approximate the calculated equilibrium potential for K+, it is
likely that the membrane is more permeable to K+ than to Na+ at
rest.
Em = RT / F ln
[ K + ] i + ( P N a / P K ) [ N a + ] i + ( P C l / P K ) [ C l - ] o
[ K + ] o + ( P N a / P K ) [ N a + ] o + ( P C l / P K ) [ C l - ] I

20. ELECTRICAL EXCITABILITY OF THE CELL
MEMBRANE

21. Excitable Cells Fire Action Potentials
• Action potential is a brief transient reversal of membrane potential that speeds
along the axons of nerve cells but also over the membranes of many muscle
and endocrine cells.
• Cells that can make action potentials spontaneously or in response to an
electrical shock are known as electrically excitable
• When both postsynaptic and receptor potentials reach the threshold level,
they generate action potential
• The ECE of membrane potential and the sequence of the permeability change
to ions account for the generation of action potential
• The initiation of action potential in axons depends on Na+ influx and in some
cells on Ca2+ influx.
• Both the concentration gradient for Na+ and the negative intracellular
potential facilitate Na+ influx, which further depolarizes the cell and activates
more Na+ channels to open and more Na+ entry, further depolarization, and
an action potential.
• At the peak membrane potential, the membrane is about 20 to 50 times more
permeable to Na+ than to K+—the reverse situation of the relative
permeabilities for these two channels at resting membrane potential.

22. • The repolarization phase of action potential that returns membrane to the
resting level results from changes in the permeability of both K+ and Na+
channels.
• The initial depolarization of cells mediated by Na+ also increases the opening
probability of K+ channels, but slightly later than the gates of Na+ channels.
• This results in a delay of the peak flow of K+ current, which is known as
delayed rectifier K+ current
• Although the membrane is depolarized, Na+ channels became non-conducting
because of a phenomenon called channel inactivation
• This leaves the activated K+ channels to dominate the membrane permeability
and to repolarize membrane to resting level.
• The continuous opening of K+ channels at that time-point accounts for
development of afterhyperpolarization
• Eventually, K+ channels close at more negative potentials, and the resting
potential is restored
• changes in the permeabilities of channels and passive movement of ions are
sufficient for the generation of action potential

23. Changes in conductance of Na+
andK+ during action potentials.
(A) Simplified diagram illustrating
the voltage dependent opening
and closing of Na+ and K+
channels.
(B) Time-course of changes in
membrane potential and
conductance for Na+ and K+
channels

24. Action Potential Propagation
• Long-distance communication in the
nervous system is possible because action
potentials propagate smoothly down an axon maintaining the
same amplitude
• The action potential–induced depolarization of cellmembrane
spreads a small distance in either direction inside the axon.
• An action potential–affected area smoothly depolarizes the region
ahead of the action potential.
• Once this depolarization reaches the threshold, opening of Na+
channels further advances the wave of excitation
• The refractory period is an important feature that forces action
potential to travel in only one direction
• The refractory period also limits the number of action potentials
that neurons can produce per unit time.

25. Myelin Enhances the
Speed of Action Potential
• The speed of action-potential propagation
depends on the density of Na+ channels
and the diameter of axon.
• Neither of these two features is physiologically attractive,
because of the energy cost and space
• majority of mammalian nerves are packed with hundreds of
axons, with diameters of only 10 to 20 mm, but their conductance
velocity is approximately 50 m/s
• In myelinated neurons, the majority of the axon is covered by
myelin (1-1.5mm, 20um)
• In such axons, depolarization also spreads from an excitable to a
non-excitable patch by localized currents, but the action potential
develops only at the nodes of Ranvier
• a phenomenon known as salutatory conduction

26. VOLTAGE-GATED CHANNELS
• Voltage-gated channels are the superfamily of ion channels that include
sodium, calcium, and potassium channels.
• Several subtypes of voltage-gated chloride channels have also been identified,
as well as numerous transient receptor potential (TRP) ion channels.
• Voltage-gated channels are macromolecular complexes in the lipid membrane
containing the aqueous pores and voltage-sensors
• A part of the pore known as the ionic selectivity filter is narrow enough to
distinguish among Na+, K+, Ca2+, and Cl–
• The voltage sensor is the charged component that senses the electrical field in
the membrane and drives conformation changes, leading to opening and
closing of the gates near the mouth of the pore
• The voltage-dependent opening of ionic channels is known as activation.
• After only a few milliseconds, or in as much as several hundred milliseconds,
the channel inactivates and the flow of ion is again blocked.
• After inactivation, the channel returns to its resting state until the next
membrane depolarization triggers the whole process again

27. Voltage-Gated Na+ Channels
Depolarize Cells
• Voltage-gated Na+ (Nav) channels consist
of the pore-forming α-subunit associated
with auxiliary β-subunits
• The α-subunit is sufficient for functional expression, but the
channel gating is modulated by the β-subunits
• In some cells, these channels are solely responsible for the rapid
and regenerative upstroke of an action potential.
• In others, they act in conjunction with voltage-gated Ca2+
channels to depolarize cells.
• The channels from this family can also control pace making
• Nav channels are also expressed in non-excitable cells at lower
level, where their physiologic role is unclear

28. Voltage-Gated Calcium Channels Have Dual Functions
• The Ca2+-selectivity and voltage-sensitivity of these channels are common
features between two major groups of voltage-gated Ca2+ (Cav) channels, which
are separated by their sensitivity to changes in membrane potential
• The first group of channels requires only weak membrane depolarization to
open. Consequently, they are activated at relatively hyperpolarizedmembrane
potentials and are known as low-voltage activated (LVA) Cav channels
• Because of its strong inactivation properties, these channels are often referred
to as transient or T-type Cav channels.
• The second group of Cav channels requires moderate to strong membrane
depolarization to open and are known as high-voltage activated (HVA) Cav
channels
• Among this group, biophysical and pharmacologic studies have identified
multiple subtypes that can be distinguished by their single-channel
conductance, pharmacology, and metabolic regulation: L-,N- , P/Q-, and R-type
Ca2+ channels
• Purification of the channel has identified five subunits, a large α1 (200 to 260
kDa) subunit and four smaller ancillary subunits: α2, β, γ, δ,.

29. • Cav channels serve two major functions in cells:
electrogenic and regulatory
• The most important process regulated by these channels is the release of
neurotransmitters at synapses
• Cav channels differ in their activation and inactivation properties, which makes
them preferential for electrogenic or regulatory processes
• LVA Cav channels exhibit rapid and complete voltage-dependent inactivation
and are unlikely candidates to promote sufficient Ca2+ influx. The major
function of these channels is electrogenic; at the resting potential
• HVA channels inactivate incompletely and are solely responsible to keeping the
cells depolarized for a prolonged period. Such action potentials drive Ca2+ to
generate intracellular Ca2+ signals of sufficient amplitude to trigger Ca2+-
dependent processes such as neurotransmission
• In accordance with this, HVA channels are found at synaptic endings and Na+
channels in axons.
Voltage-Gated Calcium
Channels Cont’d

30. Other Voltage Gated Channels
• Potassium channels tend to reduce excitation
• Cyclic Nucleotide–Modulated channels are
nonselective cation channels
• TRP ion channels in the nervous system
• Chloride channels also tend to reduce
Excitation

31. EXTRACELLULAR LIGAND-GATED CHANNELS
• The activation of ligand-gated receptor channels depends on the delivery and
binding of a ligand to the extracellular domain of these receptor channels
• Because ligand-gated channels are generally activated by neurotransmitters, they
are also known as neurotransmitter-controlled channels
• Termination of activities of these channels requires removal of the ligand, which is
usually mediated by a specific pathway for ligand degradation and/or uptake
• During prolonged occupancy of receptors, the conductance through ligand-gated
receptor channels decreases in a process called desensitization which is
analogous to inactivation of voltage-gated channels
• There are two classes of ligand-gated channels, the excitatory cation-selective
channels, operated by acetylcholine, glutamate, 5-hydroxytryptamine (5-HT), and
adenosine 5’-triphosphate (ATP), and the inhibitory anion-selective channels,
activated by GABA and glycine
• The 5-HT3, GABA, and glycine receptor channels possess structural features
similar to the nicotinic acetylcholine–activated receptor channel, and they are
grouped as one family, known as ligand-gated ion channels of the Cys-loop
family

32. Nicotinic Acetylcholine and 5- Hydroxytryptamine
Receptors Are Cation-Permeable Channels
• The nAChRs are critical for neuromuscular coupling; Upon binding of
acetylcholine, nAChR channels open to allow Na+ to flow through the channel
• The resulting plasma membrane depolarization opens Cav channels and
initiates Ca2+ release from sarcoplasmic reticulum through ryanodine receptor
channels
• In skeletal muscle, activation of voltage-gated Ca2+ influx in the T-tubule
plasma membrane is the primary signal that activates intracellular Ca2+ release
channels and ultimately stimulates muscular contraction
• Although the nAChRs are found in most parts of the brain, their functional
significance is not well-characterized
• Both Ca2+ influxes through activated nAChRs and Ca2+ potentiation probably
account for the physiologic actions of these channels
• Calcium potentiation is a process in which Ca2+ influx through one channel
regulates the efficacy of other ligand-gated channels, leading to the modulation
of membrane excitability in neurons, as well as their ability to integrate synaptic
and paracrine signals.

33. • Furthermore, nAChR-dependent Ca2+ signals enhance
protein-kinase activity in myotubes, leading to
phosphorylation of the nAChR γ-subunit.
• Because this process is dependent on Ca2+ influx,
it can be considered as autoregulation of
phosphorylation by nAChRs
• Recent results indicate that point mutation in this receptor may abolish
desensitization, increase the affinity for agonists, and convert the effects of
competitive antagonists into the agonist responses
• Such mutations also occur spontaneously in humans and may be involved in
diseases such as congenital myasthenia or frontal-lobe epilepsy.
• The 5-HT3 receptor is expressed throughout the central and peripheral nervous
systems and mediates a variety of physiologic functions.
• This includes fast excitatory synaptic transmission in neocortical interneurons,
amygdala, and visual cortex
• These receptor channels are also present on presynaptic nerve terminals,
where they contribute to the control of neurotransmission

34. GABA and Glycine Receptors Are Anion-
Permeable Channels
• γ-Amino butyric acid (GABA) is a major inhibitory transmitter in the vertebrate
CNS that acts through two different receptor channels, GABAA and GABAC, in
addition to G protein–coupled GABAB receptors
• In immature neurons, however, GABA-mediated responses are often depolarizing,
caused by Cl– efflux due to the high intracellular Cl– concentration
• These receptors are targets for many drugs in wide clinical use, including
benzodiazepines, barbiturates, neurosteroids, ethanol, and general anesthetics,
which increase the conductance through GABA channels
• Bicuculline inhibits GABAA but not GABAC channels.
• At the single-channel level, barbiturates increase the opening time of channels,
whereas benzodiazepines increase the number of channel openings
• Glycine is the other main inhibitory neurotransmitter in the CNS, particularly in
the spinal cord and in the brain stem, whereas GABA is more abundant in rostral
parts of the CNS
• At synapses, GlyRs are clustered at the postsynaptic membrane directly opposite
the presynaptic release sites and are linked to the subsynaptic cytoskeleton by a
membrane protein named gephyrin

35. Some Glutamate Channels Are Voltage and Ligand-Gated
• L-Glutamate is the major excitatory neurotransmitter in the CNS, and it acts
through a variety of ionotropic and metabotropic receptors
• iGluRs are encoded by 18 genes that assemble to form four major subtypes: the α-
amino-3-hydroxy-5-methyl-4-isoxazole propionicacid (AMPA), kainate, N-methyl-D-
aspartate (NMDA), and delta receptor channels
• NMDA channels exhibit a different excitation behavior than those not activated by
NMDA
• These channels are both ligand- and voltage-gated; full activation of the NMDA
receptor requires application of two ligands, L-glutamate and glycine
• The NMDA receptors only become fully activated by glutamate after their Mg2+
block has been relieved by membrane depolarization
• The NMDA receptor exhibits low-binding-affinity sites for Ca2+, which results in a
low selectivity among cations (because of the lower-affinity binding, Ca2+ moves through the pore rapidly)
• Their kinetics is much slower, resulting in a large Ca2+ influx and long term
metabolic or structural changes
• Some of the most important functions of the nervous system, such as synaptic
plasticity, are dependent on the behavior of NMDA receptor channels and Ca2+
influx through these channels

36. ATP-Gated Channels Are Expressed in Excitable and Non-excitable Cells
• With the use of molecular cloning techniques, seven purinergic receptor channel
(P2XR) subtypes have been identified
• In general, Ca2+ is a charge carrier through these channels, although the
permeability of Ca2+ versus Na+ varies widely among different cell types.
• Thus, these channels can serve as Ca2+ influx channels.
• These channels also facilitate Ca2+ influx indirectly, by depolarizing cells and
activating Cav
• In addition to stimulating intracellular Ca2+ signals, the paracrine actions of ATP on
purinergic receptors can generate the cell-to-cell spread of Ca2+ signals in glial cells
in the absence of gap-junctional communication
• The best-characterized agonist role of ATP is in synaptic transmission from
sympathetic nerves, where ATP acts as a co-transmitter with noradrenaline
• ATP has also been implicated in parasympathetic-, sensory-, and somatic-
neuromuscular transmission
• About 40% of hypothalamic neurons in culture respond to ATP by a rapid increase
in intracellular Ca2+ because of Ca2+ entry through P2XRs.
• Purinergic receptor channels are expressed in neurons, as well as in other excitable
cells, including pituitary cells, and nonexcitable cells, including gonadal cells and
lymphocytes

37. ENaC/DEGENERIN FAMILY OF CHANNELS
• The recently discovered epithelial sodium channel (ENaC)/degenerin (DEG) gene
superfamily encodes Na+ channels involved in various cellular functions
• This superfamily of channels include ENaC, acid-sensing ion channels (ASICs),
FMRF-amide activated channels of Helix aspersa, DEG of Caenorhabditis elegans,
and orphan channels
• In contrast with Nav channels, ENaC is localized in apical membrane of polarized
epithelial cells, where it mediates Na+ transport across tight epithelia
• The ENaC-mediated reabsorption of Na+ occurs in the epithelial lining of the distal
part of the kidney tubule, the alveolar epithelium, and the distal colon epithelium.
• Similar to other ENaC/DEG superfamily members, ENaC is very sensitive to
amiloride
• ASICs are directly activated by a drop in extracellular pH
• The functional role of ASICs in the peripheral nervous system was linked with
nociception, perception of sour taste, modulation of synaptic transmission, and
mechanosensory transduction
• These channels are also expressed in the brain, but their function in the central
neurons is still not clarified

38. INTRACELLULAR AND INTERCELLULAR CHANNELS
• The expression of ion channels is not limited to the plasmamembrane.
• Two types of channels, ryanodine receptor channels (RyRs) and inositol 1,4,5-
trisphosphate receptor channels (IP3Rs), are expressed in the ER/sarcoplasmic
reticulum membrane and nuclear membrane
• RyRs provide an effective mechanism for transduction and translation of
electrical signals inside of cells
• whereas IP3Rs are activated by two classes of plasma membrane receptors
known as Ca2+-mobilizing receptors, their activation is independent of the
electrical status of cells and represents the major pathway for Ca2+ signaling in
nonexcitable cells
• However, this signaling pathway is also operative in excitable cells, including
neurons.
• Activation of IP3Rs leads to stimulation of voltage-insensitive Ca2+ channels
expressed on the plasma membrane, and this process is known as capacitative
Ca2+ entry.
• Channels that accommodate capacitative Ca2+ entry are also expressed in both
excitable and nonexcitable cells.

39. Voltage-Gated Ca2+ Influx Activates Ryanodine Receptors
• RyRs were originally identified as Ca2+ release channels expressed in the
sarcoplasmic reticulum in skeletal muscle fibers and cardiac myocytes, where they
play a central role in excitation-contraction coupling
• These channels are also expressed in neurons, chromaffin cells, sea urchin eggs,
and several non-excitable cell types
• Mammalian tissues express three isoforms: RyR1 is expressed predominately in skeletal
muscle, RyR2 is expressed in cardiac muscle, and RyR3 has a wide tissue distribution,
including the non-excitable cells
• RyRs are the largest known ion channels and are susceptible to many different
modulators, including cytosolic calcium, membrane potential, and several
intracellular messengers
• As in the regulation of IP3Rs, intracellular Ca2+ is a major regulator of RyRs; at low
concentrations, Ca2+ promotes release, whereas higher concentrations are
inhibitory
• The ability of Ca2+ to stimulate its release from the endoplasmic/sarcoplasmic
reticulum via RyRs is known as Ca2+-induced Ca2+ release
• Others are ryanodine, cytosolic pH, Mg2+ and other cations, Cl– and other anions,
nucleotides, cyclic adenosine 5’- diphosphate ribose, several protein kinases,
calmodulin, and other Ca2+-binding proteins, Caffeine, dantrolene etc

40. ACTIVE TRANSPORTERS
• The primary purpose of active transporters is to generate and maintain the
trans-membrane ionic concentration gradients, which in turns provide the
driving forces for ion channels to generate electrical signals
• In contrast with channels, which can conduct thousands of ions through the
pore each millisecond, ion translocation by active transporters requires ion
binding and unbinding for transporter, a process that requires several
milliseconds
• Also, transporters need energy for ion movement against their concentration
gradients
• The energy for such uphill transport is provided by two sources
• One group of transporters requires ATP for its action and they are called ATPase
pumps also called primary active transporters e.g Na-K pump, Ca2+ pump
• Other transporters are responsible for the electrochemical gradients, such as Cl–
, Ca2+, and H+ classified as secondary active transporters as the driving force
for catalysis is not coupled directly to the hydrolysis of ATP but is derived from
the electrochemical gradients established for one of the solutes (usually Na+)
that drives counter-transport of the (also called ion exchangers: antiporter and
symporter)

41. Na-K-ATPase Has Multiple Functions
• The Na-K ATPase, also knows as Na pump, is a membrane-bound protein that
establishes and maintains the high internal K+ and low internal Na+ concentrations
typical of most animal cells
• The activity of this transporter is estimated to account for 20% to 40% of the brain
energy consumption
• By using the energy from the hydrolysis of one molecule of ATP, this pump
transports Na+ out of the cell in exchange for K+ transported into the cell
• The pump with bound ATP binds three intracellular Na+ ions, then ATP is hydrolyzed,
leading to phosphorylation of the pump at a highly conserved aspartate residue, the subsequent release
of ADP, and a conformational change in the pump exposing the Na+ ions to the outside.
• The phosphorylated form of the pump has a low affinity for Na+ ions, so they are
released.
• The pump then binds two extracellular K+ ions, causing the dephosphorylation of
the pump and reverting it to its previous conformational state, and exposing the K+
ions to the inside of the cell.
• As the unphosphorylated form of the pump has a higher affinity for Na+ ions than
for K+ ions, the two bound K+ ions are released.
• The pump generates an electrical current that can hyperpolarize the membrane
potential. Such transport is termed electrogenic

42. • The electrochemical gradient the Na pump generates
is critical in maintaining RMP/excitability of cells &
their osmotic balance
• In the kidney, the Na pump plays a major role in
driving the reabsorption of Na+ and water;
finally, the Na+ gradient provides the energy for
the Na-coupled transporters.
• Na pump is an oligomer composed of two major polypeptides, the α- and β-
subunits.
• The α- subunit spans the membrane 10 times, whereas the β- subunits
spans the membrane once.
• The α- subunit contains the binding sites for cations, ATP, and ouabain, an
inhibitor of this pump, and the β- subunit is essential for the normal activity
of the enzyme
• A third protein, termed the γ subunit, has also been identified in purified
preparation of the enzyme
• There are four subtypes of a subunits, α1, α2, α3, and α4, and three b
isoforms, β1, β2, and β3

43. Na-Ca Exchanger and Ca-ATPase Control
Intracellular Calcium
• The intracellular Ca2+ concentration is controlled by two major pathway, sodium-
calcium exchanger (often denoted Na+/Ca2+ exchanger [NCX] or exchange protein)
and Ca2+-ATPases or Ca2+ pumps
• NCX uses the energy that is stored in the electrochemical gradient of Na+ by
allowing this ion to flow down its gradient across the plasma membrane in
exchange for the counter transport of Ca2+
• The stoichiometry of NCX is generally accepted to be removal of a single calcium
ion in exchange for the import of three sodium ions
• it has been shown that ion flux ratio can vary from 1:1 to a maximum of 4:1,
depending on the intracellular Na+ and Ca2+ concentrations
• The NCX may operate in both forward and reverse directions simultaneously in
different areas of the cell, depending on the combined effects of Na+ and Ca2+
gradients
• The NCX does not bind very tightly to Ca2+ (has a low affinity), but it can transport
the ions rapidly (has a high capacity), transporting up to 5000 Ca2+ ions per second
• It is therefore needed in a neuron after an action potential

44. • Another, more ubiquitous transmembrane pump that
exports Ca2+ from the cell is the plasma membrane Ca2+-
ATPase
• It has a much higher affinity for Ca2+ but a much lower
capacity
• Because this pump is capable of effectively binding to Ca2+
even when its concentrations are quite low, it is more
appropriate for maintaining the very low concentrations of
intracellular Ca2+ that are normally within a cell
• Therefore, the activities of the NCX and the plasma
membrane Ca2+-ATPase complement each other
• There is another subtype of Ca2+-ATPase, located in the ER
and sarcoplasmic reticulum membranes, which pumps Ca2+
from cytosol to the lumen of these organelles

45. Several Transporters Maintain Chloride Gradient
• There are three major secondarily active transport proteins involved in
establishment and maintenance of neuronal chloride gradient: the cation-chloride
co-transporters (CCCs), the Na+-dependent anion exchangers (NDBE), and the Na+-
independent anion exchangers (AEs)
• As HCO3– is a substrate for AEs and NDBSs, these two transporters are also directly
involved in intracellular pH regulation
• Transport of Cl– mediated by CCCs is electroneutral and the energy for net
transports derived from the cation gradients generated by Na pump
• The CCC gene family consists of two major groups: K+-Cl– co-transporters (KCCs)
and Na+-K+-Cl– co-transporter (NKCCs); the third gene encodes the atypical Na+-
Cl– co-transporter (NCC), exclusively expressed in kidney
• NCCs and NKCCs mediate active chloride uptake, promoting intracellular
accumulation of this ion, and this transport is driven predominately by the energy
taken from the Na+ concentration gradient.
• In contrast, KCCs mediate active chloride extrusion, and K+ gradient provides the
energy for the operation of this exchanger
• The opposite effects of NKCC1 and KCC2 on intracellular chloride concentrations
play a major role in developmental transition of GABA channels from excitatory in
prenatal life to inhibitory in postnatal life

46. Acid-Base Transporters
• Intracellular pH in mammalian tissues is regulated through coordinated action
of several acid-base transporters, including the above-mentioned NDBEs and
AEs, Na+-H+ exchangers (NHEs), and Na+:HCO3- co-transporters (NBCs)
• NHE are ubiquitously expressed integral membrane proteins that regulate
intracellular pH by removing a proton in exchange for an extracellular Na+
• NHE activity also facilitates the progression of other cellular events such as
adhesion, migration, and proliferation
• NBC1 is essential for bicarbonate transport across plasma membranes in both
epithelial and nonepithelial cells
• The direction of the NaHCO3 movement in secretory epithelia is opposite to
that in reabsorptive epithelia
• In secretory epithelia (such as pancreatic duct cells), NBC is responsible for the
transport of bicarbonate from blood to the cell for eventual secretion at the
apical membrane.
• In reabsorptive epithelia (such as kidney proximal tubule cells), NBC is
responsible for the reabsorption of bicarbonate from cell to the blood
• In nonepithelial cells, this transporter is mainly involved with cell pH regulation

47. Other Na+ Gradient–Driven Transporters
• At least three, and up to six, Na+-dependent glucose transporters
(SGLT1 to SGLT6) may exist
• Similarly, 13 members of the family of facilitative sugar
transporters (GLUT1 to GLUT12 and HMIT) are now recognized
• These various transporters exhibit different substrate specificities,
kinetic properties, and tissue expression profiles
• Sodium glucose transport proteins are a family of glucose
transporters found in the intestinal mucosa of the small intestine
(SGLT1) and the proximal tubule of the nephron (SGLT2 and SGLT1)
• These proteins use the energy from a downhill sodium gradient to
transport glucose across the apical membrane against an uphill
glucose gradient
• Therefore, these co-transporters are an example of secondary
active transport

48. A CROSS-COMMUNICATION BETWEEN ELECTRICAL
AND RECEPTOR-MEDIATED SIGNALING PATHWAYS
• Hormones and neurotransmitters acting through their respective receptors can
modulate the gating properties of voltage-gated and ligand-gated channels.
• This modulation is dependent on signal-transduction pathways of receptors
• These includes direct action of heteromeric G-proteins on channels (termed
membrane-delimited pathway) and indirect action, through intracellular
messengers, including Ca2+, cyclic nucleotides, nitric oxide, ATP, and PKA, PKC,
and PKG (termed intracellular messenger–dependent pathway)
• The activity of Cav channels can also be modulated in a second messenger–
dependent manner e.g. in cardiac myocytes, activation of PKA augments Ca2+
influx throughCav channels
• Receptors can also modulate the activity of another K+ channel, the M-type
• Electrical activity and the associated Ca2+ influx also influence the receptor-
mediated intracellular signaling
• Thus, two pathways for signaling in neurons, electrical and receptor-controlled,
are not independent of each other but interact to provide the synchronized
control of cellular functions.

49. CLINICAL CORRELATION
(Demyelinating Disorders)
• Disorders of myelin can be divided into conditions in which
there is destruction of myelin—the demyelinating disorders—
and those in which there is an abnormality in the makeup of
myelin, known as the dysmyelinating disorders
• Demyelination can result from a great variety of causes,
including autoimmune, toxic, metabolic, infectious, or
traumatic conditions
• Dysmyelinating disorders are heritable conditions resulting
from inborn errors of metabolism that affect myelin
• Most clinical syndromes related to disorders of myelin
primarily affect the brain and spinal cord

50. Impaired or Blocked Impulse
Conduction
• The cause of most clinical symptoms related to myelin disorders
• Demyelination prevents saltatory conduction instead, impulses are
propagated along the axon by continuous conduction, or in the extreme,
there is total conduction block
• The block may result from the markedly prolonged refractory period in
demyelinated segments, explaining why rapidly arriving, repetitive
impulses are especially likely to be blocked
• Demyelination may also affect conduction by rendering axon function
more susceptible to changes of the internal milieu, such as alterations of
pH or temperature
• Uhthoff’s phenomenon, is well-known and can be the cause of prominent
and precipitous neurologic symptoms after temperature elevation in
patients with multiple sclerosis (monocular blindness, extremity
weakness, or incoordination)

51. Diseases that affect myelin
• Adrenoleukodystrophy is an example of a metabolic disorder of myelin
• It is an X-linked peroxisomal disorder in which there is impaired oxidation
of very-long-chain fatty acids, with resultant abnormalities of myelin
• Progressive multifocal leukoencephalopathy is an infectious condition
resulting in demyelination.
• It is caused by an opportunistic viral infection of oligodendroglial cells in
immunosuppressed patients
• Radiation is an example of a form of trauma that can lead to
demyelination
• Toxic agents such as cancer chemotherapy drugs can result in
demyelination, especially when instilled directly into the CSF
• Autoimmune mechanisms can also result in demyelination. This process is
believed to be important in the pathogenesis of one of the most common
demyelinating disorders, multiple sclerosis

52. MS has a distinct age, gender, race, and geographic
profile
• The prevalence of MS in the United States and Northern Europe is
approximately 100 per 100,000
• Women are affected 1.5 times more often than men
• The age distribution of MS is distinct in that onset before the age of 15 years or
after the age of 45 years is unusual
• multiple plaques of demyelination are found in the CNS. Within these areas,
there is evidence of inflammation, proliferation of glial tissue, and severe
destruction of myelin
• Minimal remyelination of fibers may occur in some plaques, resulting in areas
with scant myelin known as ‘‘shadow plaques.’’
• The pathogenesis of demyelination within these plaques is not fully
understood, but it is generally agreed that it involves an immune response
mediated by T-lymphocytes that recognize myelin components of the CNS
• Plaques may occur anywhere in the CNS, but they have a predilection for
certain parts of the brain and spinal cord (the optic nerves, the cerebellum, the
periventricular white matter of the cerebral hemispheres, the white matter of the spinal cord, the
root-entry zones of spinal or cranial nerves, and the brain stem, especially the pons)

53. The symptoms of MS remit and reappear in
characteristic fashion
• MS is marked by a wide range of disparate neurologic symptoms with a
tendency for symptoms to appear, spontaneously improve, and then reappear
or be replaced by new symptoms over time
• Typically, serious neurologic symptoms appear and disappear over a period
ranging from weeks to several months, independent of medical therapy
• Certain neurologic symptoms are much more likely to occur in MS, reflecting the
predilection for certain anatomic sites of involvement alluded to previously.
• These include monocular visual loss (reflecting involvement of the optic nerve),
limb incoordination (cerebellum), double vision (brain stem, or medial
longitudinal fasciculus), and paraparesis (spinal cord)
• Other common symptoms include vertigo, extremity numbness, slurred speech,
and bladder dysfunction
• Lhermitte’s sign is a frequent finding in MS: It consists of a shock-like sensation
whenever the neck is flexed forward.
• This phenomenon is caused by the heightened sensitivity of demyelinated
dorsal column axons to the mechanical stimulation of stretch when the spinal
cord is flexed by the offending neck motion

54. The diagnosis of MS can be aided by Imaging studies
and Laboratory tests
• Magnetic resonance imaging (MRI) of the brain and spinal cord is a sensitive
means of demonstrating multiple areas of demyelination, many of which may
prove to be clinically silent.
• Active plaques tend to enhance after the administration of gadolinium contrast,
indicating a disruption of the blood-brain barrier
• Examination of the cerebrospinal fluid (CSF) is used to screen for CNS
immunoglobulin (IgG) abnormalities, which are common in MS
• When CSF is subjected to immunoelectrophoresis, it demonstrates one or more
discrete bands of IgG known as oligoclonal bands not seen in a paired serum
sample
• Another useful CSF study involves myelin basic protein, a breakdown product of
myelin
• Electrophysiologic studies such as visual and somatosensory evoked potentials
are used to detect subtle physiologic abnormalities in MS that are not yet
sufficiently advanced to produce clinical symptoms
• These tests measure the speed and completeness of conduction in sensory
pathways

55. Immunosuppression is the most common form
of therapy used in MS
• The propensity for MS symptoms to remit spontaneously makes the scientific
evaluation of potential therapies difficult
• A number of agents now exist for the treatment of MS, which presumably act
through an immuno-modulatory mechanism
• interferon-beta-1b (Betaseron) and interferon-beta-1a (Avonex): The Putative
mechanisms for these medications include inhibition of autoreactive T cells,
inhibition of MHC class II expression, metalloproteinase inhibition, altered
expression of cell-associated adhesion molecules, and induction of
immunosuppressive cytokines and inhibition of pro-inflammatory cytokines.
• A third agent, glatiramer acetate (Copaxone), is a polypeptide containing
random arrangements of four basic amino acids, believed to mimic myelin basic
protein
• This agent is believed to act by inducing myelin-specific suppressor T cells and
inhibiting myelin-specific effector T cells
• Natalizumab (Tysabri), a humanized monoclonal antibody that binds to the
a4b1 integrins of leukocytes, blocks attachment to cerebral endothelial cells,
thus reducing inflammation at the bloodbrain barrier

56. Immunosuppression Cont’d
• For the treatment of acute relapses, the most commonly used
treatments are the corticosteroid drugs, prednisone or
methylprednisolone
• These agents are usually administered at high doses and are
believed to help reduce inflammation and restore the blood-
brain barrier, thereby shortening the course of relapse and
speeding recovery

57. SYNAPTIC TRANSMISSION

58. PRESYNAPTIC AND POSTSYNAPTIC
RECEPTORS