Excitable Cells: Revision Notes

EXCITABLE CELLS: YEAR 1 SEMESTER 2
LECTURE 1 – INTRODUCTION
LECTURE 2 – IN AND OUT OF THE BAG
All known cells exist in an aqueous environment, so it is necessary to know the properties of water. Water is
one of the most complex molecules known, oxygen is electronegative so draws charge from the hydrogen
atoms, giving them a δ+ charge and the bonding angle of water is 105°. Water easily forms hydrogen bonds
between molecules in a solution and is essentially a dynamic charged, hydrogen bonded polymer. The
implications of the structure of water mean that it has an anomalously high boiling point and is an excellent
solvent for polar molecules. Solvents are liquids which dissolve solutes. Polar molecules have polar groups
such as –OH, NH2, COOH and any ion; non-Polar groups includealkyl chains,benzene rings and cyclohexane.
The important ions for membrane biology can be divided into two groups: those which are physiologically
useful, such as Na+, K+ and Cl-, and those which are biochemically useful, such as Mg2+. Some ions also fall into
both categories, such as Ca2+. Ions attract water and therefore have hydration shells, the size of which depends
on the ion radius; small ions have a high charge density and therefore attract more water molecules tha n high
large ions. This means that the size of an ion depends on its hydration shell radius , the bigger the hydration
shell,the less motile itis and hydration therefore impedes ion flow.
Biological membranes are made of water insoluble lipids; phospholipids, glycoli pids or cholesterol. The main
feature of membrane lipids is that they are amphipathic, having regions which are polar (head) and non-polar
(backbone). Since the head group is polar it attracts water molecules and vice versa, allowing a lipid bilayer
structure to be established.
LECTURE 3 – THE PROTEINS: PUMPS, CARRIERS AND CHANNELS
The lipid bilayer is very impervious to polar molecules, especially ions, allowing the concentrations to differ on
either side of the membrane. Concentration gradients represent a source of energy and big concentration
gradients can do more work than littleones. Concentration gradientenergy can be used in the followingways:
Chemical work ATP synthesis,transport
Mechanical work Rotation of flagella
Cell volume regulation Generation of osmotic potentials
Cell homeostasis Efflux of toxic compounds
Chemiosmosis Generation of ATP with H+ ions in ETC
Signal transduction Action potentials,Ca2+ signalling

Cells use special protein pumps to transport ions
against their gradient, which requires energy in
the form of ATP. They work fairly slowly and
nearly always move cations, such as Ca2+ and
Mg2+. The sodium (Na+/K+ ATPase) pump is
extremely important and cells expend 25% ATP to
sustain these pumps. It extrudes 3Na+ and 2K+ for
every molecules of ATP hydrolysed and generates
a gradient as shown to the left. Inhibitors of this
pump, such as digoxigenin, are therapeutically
and toxically important. Since it blocks the pump,
Na+ concentration builds up in heart cells, causing
increased [Ca2+] to accumulate, thus increasing
the contractility of the heart; however too much
digoxigenin can causeheartand nerve failure.
Ion channels use an electrochemical gradient and are often selective. Voltage gated ion channels are tetramers
which have a central pore, a voltage sensor and a coupling mechanism, as well as an inactivation mechanism
to close the channel. Many toxins block voltage-gated channels, such as conotoxins from the venom of marine
cone snails, which plug the channel pore mouth. Ligand gated on channels are pentamers which have a central
pore, a ligand binding site, a coupling mechanism and desensitisation mechanisms, which close the channel if
the ligand remains bound for too long.
LECTURE 4 – DIFFUSION, PERMEABILITY AND ELECTRICITY
Diffusion is the spontaneous movement of molecules from regions of high to low concentration, which
dissipates the concentration gradient. Molecules that punch holes in membranes a re said to facilitate diffusion
and examples include:
Detergents Saponin
Antibiotics Gramicidin,Valinomycin,Alamethicin
Toxins Mellitin (bee stings)
Viruses Influenzavirus M2 protein
Molecules in liquids are in constant motion due to thermal agitation, for water molecules the average centre
to centre distance is about 2.8Å. Fick showed that the number of molecules (N) moving across an interface is
proportional to the area of the interface (A) and the concentration gradient.
Einstein showed that diffusion was due to a random walk of molecules and that the time it takes for a
molecule to travel away from its starting point depends on whether the molecule moves in 1/2/3 dimensions.
A molecule can diffuse much faster if it travels in three dimensions because the chances of bumping into other
molecules are lower. This is why catalysts work; by providing a surface area to allow reactions on. Signalling
molecules therefore have longer ranges if they are not bound to membranes.
𝑜𝑛𝑒 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛: 𝑡 =
𝑑2
2𝐷
𝑡𝑤𝑜 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑠 : 𝑡 =
𝑑2
4𝐷
𝑡ℎ𝑟𝑒𝑒 𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑠: 𝑡 =
𝑑2
6𝐷
Where t is time, d is the root mean square distance in cm, and D is the diffusion constant in cm2/sec.
The movements of ions under the influence of an electric field are called electrophoretic movements, which
add to, or subtractfrom diffusion.The total gradientis called the electrochemical gradient.
𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡 = 𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑓𝑟𝑜𝑚 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 − 𝐺𝑟𝑎𝑑𝑖𝑒𝑛𝑡 𝑓𝑟𝑜𝑚 𝑒𝑙𝑒𝑐𝑟𝑜𝑝ℎ𝑜𝑟𝑒𝑡𝑖𝑐 𝑚𝑜𝑣𝑒𝑚𝑒𝑛𝑡

The rate of ion movement across a membrane depends on four key factors; the size of the electrochemical
gradient, the nature of the ion, the number of open ion channels and the ion channel’s properties , such as its
selectivity and permeability for specific ions.
LECTURE 5: RADIOTRACERS, DYES AND ELECTROPHYSIOLOGY
We need to be able to detect channels, assays show that cells make channel proteins, but to determine
whether or not the channel is actually functional, we need to be able to assay its activity. Ion flux can be
measured using radioactive dyes, imaging dyes or currents using electrophysiology. Radioactive dyes show
increased radioactivity in proportion to ion flux through the channels and radiotracer measurements can also
be performed on cells:
Types of radioactive ions
Channel Ion
Calcium 45Ca2+
Potassium 86Rb+
Sodium 22Na+
Chlorine 36Cl-
45Rb2+ is used instead of a
radioisotope of K+ because
it has the same function
whilst being safer to use
than radioactivepotassium.
The disadvantages of radioactive ion flux assays is that radioisotopes are hazardous and have poor spatial
resolution, meaning specific areas in cells or tissues are hard to resolve clearly, whilst also having poor
sensitivity, meaning we can’t look at a few
channels. Another major problem is time
resolution. If the system is left long enough,
vesicles will still fill even if the protein channel is
not fully functional. “Long enough” generally
refers to a length of time in milliseconds, which is
far too fast to measure manually, so rapid kinetic
millisecond events need to be recorded, meaning
horrific mathematics and expensiveequipment.
Ion sensitive dyes bind ions and change colour (wavelength) or brightness (intensity), a change which can be
detected optically using an imaging microscope or spectophotometry. In the example below, the colour of the
solution changes according to the calcium ion concentration. Knowing the fluorescence intensity we can
determine the Ca2+ concentration through comparison beforeand after the Ca2+ was added.

The fluorescence intensity in the absence of Ca2+ is 10 units and in 50nM Ca2+ is 60 units. What is the [Ca2+]
when the fluorescence is 40 units?
𝐸𝑞𝑢𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑎 𝑠𝑡𝑟𝑎𝑖𝑔ℎ𝑡 𝑙𝑖𝑛𝑒: 𝑦 = 𝑚𝑥 + 𝑐
𝐼𝑛 𝑡ℎ𝑖𝑠 𝑐𝑎𝑠𝑒: 𝐹 = 𝑚 ∙ [𝐶𝑎2+
] + 𝐹𝑚𝑖𝑛
𝐴𝑡 𝐹 𝑚𝑖𝑛: 10 = 𝑚 ∙ 0 + 10 ∴ 𝐹 𝑚𝑖𝑛 = 10 𝑢𝑛𝑖𝑡𝑠
𝑊ℎ𝑒𝑟𝑒 [𝐶𝑎2+
] = 60: 60 = 𝑚 ∙ 50 + 10 ∴ 𝑚 = 1 𝑢𝑛𝑖𝑡/𝑛𝑀
𝐴𝑡 𝐹 = 40: 40 = 1 ∙ [𝐶𝑎2+
] + 10 ∴ [𝐶𝑎2+
] = 30𝑛𝑀
Other dyes are also being developed; the idea is to get them to be selective for specific ions. So far dyes to
measure Na+, K+ and Cl- have been created, but none are good as Ca2+ selective dyes. These dyes are so
important so calcium fluxes can be looked at in real time and so the spatial distribution of ion flux can be
determined. The problem with indicator dyes is that they are polar molecules and so cannot get into cells! The
polar molecule is therefore made into a non-polar molecule by adding an ester group, and then cleaved by
naturally occurringesterases in thecell,allowingthe polar dye to be liberated and to bind Ca2+.
Electrophysiology allows currents in physiological samples, like cells, to be measured using recording
equipment. Ion flow is equivalent to current and voltage is potential difference, there can be no ion flow
(current) if no potential difference exists. A higher resistance (R) produces smaller currents for the same
voltage. This principal isOhm’s law (V=IR), but electrophysiologists usea re-arranged equation:
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 ( 𝐼) = 𝑉𝑜𝑙𝑡𝑠 ( 𝑉) × 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒 (
1
𝑅
)
Electrophysical recordings are extremely fast events, ranging from the sub-millisecond timescale upwards,
they are extremely sensitive; as little as one ion channel can be detected, the spatial resolution is good and it
allows thedetails of individual channelsto be recorded, such as activation,inactivation and poreproperties.
Radiotracers Ion-sensitive dyes Electrophysiology
Speed Poor Fast Fast
Spatial resolution Poor Excellent Good
Sensitivity Poor Good Excellent
Major uses Biochemical assays Imagingion fluxes in cells Channel properties and distributions
LECTURE 6 – ELEMENTS OF NEURONAL BIOPHYSICS
Newton’s first law states that a body remains at rest, or moves in a straight line at a constant velocity, unless
acted upon by a net outside force. Newton’s second law states that the acceleration of an object is
proportional to the force acting upon it. F=ma. Newton’s third law states that whenever one body exerts a
force upon a second body, the second body exerts an equal and oppositeforce upon the firstbody.
The SI unit for quantity of electricity or electric charge is the coulomb (C), which represents approximately
6.24x1018 elementary charges (the charge carried by a single proton or electron). An electron can be removed
from an atom, which gives it a charge. A positive ion is called a cation (electrons taken) and a negative ion is
called an anion (electrons added).
A capacitor is an electronic device that can store energy in the electric field between a pair of conductors
("plates"), and biological membranes behave very much in the same way so the electric potential within a

membrane and an ion channel is approximately constant. Electric potential (V) is a measure of how much
kinetic energy (½ mv2) can be generated by the electric field acting on a unit charge. Given a static distribution
of electric charges, the electric potential is defined for every point in space, such as the difference in electric
potential between two points A and B (∆𝑉 = 𝑉𝐵 − 𝑉𝐴 ), which is a measure of how much kinetic energy a
charge will acquire when moving freely from point A to point B. Biological membrane voltages are usually best
dealt with in millivolts(mV).
𝑇𝑟𝑎𝑛𝑠𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 = 𝐼𝑛𝑡𝑟𝑎𝑐𝑒𝑙𝑙𝑢𝑙𝑎𝑟 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 − 𝐸𝑥𝑡𝑟𝑎𝑐𝑒𝑙𝑙𝑢𝑙𝑎𝑟 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙
Electric current is a flow of charged particles and its rate of flow is measured with an ammeter. The quantity of
charge passing through a surface is equal to the rate of charge flow x time. Q = It. When a charge “falls”
through an electric field it loses electrical potential energy, which can be maintained using a power source.
Accordingto Ohm’s lawthe resistanceof a conductor increases as its temperature increases.
LECTURE 7 – ORIGINS OF MEMBRANE POTENTIALS
The intracellular fluid in cells is neutral; charge can only
accumulate on the internal membrane surface and an equal and
opposite charge will always appear on the external side of the
membrane, which is necessary to ensure the extracellular fluid
has the same potential (equipotential). All neurons are negative at
rest and typical resting values range from -50mV to -80mV; the
electric field is therefore directed from the outside towards the
inside of the cell and a cation will follow it, while an anion will go
against it. The transmembrane potential gradient is also present
within the ion channels that are located across the lipid bilayer,
but diffusional processes arealso atwork within the cell.
All systems move towards a state of equilibrium, thermal agitation causes particles in a solution to move all
the time in a random fashion,if non-charged particles are placed in two compartments, only diffusional forces
act upon them. Concentrations aremeasured in moles/litre,and fluxes in moles/second.
A bath is set up with two compartments, separated by a membrane containing
pores only permeable to K+. A high concentration of a salt, KA, is introduced into
the left side, and a low concentration into the right. At first the voltmeter reads
0mV since both sides are neutral, then potassium ions immediately start to
diffuse down their concentration gradient into the right side and a net positive
charge builds up; an electrical potential difference appears across the
membrane because the anion cannot cross it. The positive side soon begins to
repel K+ ions until an equilibriumvalue(Ek) is established.
𝐴𝑡 𝐸𝑘 , 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 = 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛𝑎𝑙 𝑓𝑜𝑟𝑐𝑒, 𝑠𝑜 𝑠𝑦𝑠𝑡𝑒𝑚 𝑠𝑡𝑜𝑝𝑠 𝑐ℎ𝑎𝑛𝑔𝑖𝑛𝑔
Equilibrium potentials are essential in any description of biological membrane potentials. A formula can be
found for Ek, the equilibrium potential for K+ ions. This is the Nernst equation. Ek must be more positive on the
sidewhere there is less K.
𝐸 𝐾 =
𝑅𝑇
𝐹
𝑙𝑛
[𝐾] 𝑜
[𝐾] 𝑖
Where R is the thermodynamic gas constant, Tis the absolute temperature and F is the faraday constant.
At room temperature (20°C), RT/F is about 25mV
V=Ek

The physical meaning of the Nernst equation: If we have a membrane permeable to K + ions and we want to
maintain a concentration difference between the two sides, we have to apply a transmembrane electric
potential difference to counteract the diffusional force which tends to move ions down their concentration
gradients. We call this the equilibrium potential. To maintain a concentration difference, the electric field must
be directed from the less concentrated side to the more concentrated side. This implies that the electric
potential must be larger on the less concentrated side. In neurons the K+ concentration is much more
concentrated insidethe cell than outside, therefore EK is negative (more positivepotential outside).
LECTURE 8 – THE ACTION POTENTIAL I
The equilibrium potential is given by the Nernst equation and depends logarithmically on the ratio of K+
transmembrane concentrations, to maintain the concentrations at a specific ratio, at a given temperature the
same transmembrane potential must exist. This cannot be created by potassium ions alone, so other ions must
be present on the two sides of the membrane.
EK is not necessarily the actual value of transmembrane potential given by Ko and Ki, it is the value required for
a net flux of potassium through the channels of the membrane. If the membrane potential (Vm) is different
from EK, there will be a net ionic movement through the potassiumchannels.Thecurrent caused by this is:
𝐼 𝐾 = 𝑔 𝐾 (𝑉𝑚 − 𝐸 𝐾 )
Where I is the ionic driving force and g is the ionic conductance
To calculate the resting membrane potential of the cell consider a cell with a membrane permeable to K+, Na+
and Cl-. (For these purposes we can leave Ca2+ out, because it is less important in electrical signalling).
Electrical equilibrium means there must be no net current flowing through the membrane, since the
membrane potential does not change at rest. By rearranging the above equation we can obtain a “weighted
average” of ENa, EK and ECl.
𝑉𝑚 =
( 𝐸 𝑁𝑎 × 𝑔 𝑁𝑎
) + ( 𝐸 𝐾 × 𝑔 𝐾
) + ( 𝐸𝐶𝑙 × 𝑔𝐶𝑙
)
𝑔 𝑁𝑎 + 𝑔 𝐾 + 𝑔𝐶𝑙
In neurons at rest, potassium and chloride conductances are larger than sodium. Although this is a rough
model of the membrane potential, a more complete description should also take into account the presence of
the Na/K ATPase pump.
The ionic basis of the action potential was understood through
experiments on the squid giant axon while the squid was still alive
(1939-1950). The large diameter of the axon allowed the use of
several intracellular electrodes; the axon could then be excited by
an electric shock which was delivered by a stimulating electrode.
In 1939 it was shown that the action potential was due to a
selective increase in Na+ permeability rather than a generic
breakdown in membrane resistance. Membrane resistance
breakdown would cause the membrane potential to collapse to
0mV, where an increase in Na+ permeability leads the cell towards ENa, the equilibrium potential for Na+ ions. It
was found that reducingthe [Nao] decreased the action potential overshoot.
Larger
electric
potential
Electric fielddirection

So what is it that causes the falling phase of the action potential? If it were just Na+ channels closing, the
membrane potential would slowly return to the resting level. The answer is that there is a second large
increase in membrane permeability caused by the opening of voltage-activated potassium channels. The
action potential results from a quick increase in the sodium permeability of the membrane where Na+ entry
drives Vm towards ENa. Repolarisation is caused by a slower increase in potassium permeability where K+ exit
drives Vm toward EK.
A neuron’s membrane contains thousands of Na+ and K+ channels, each of which can either be open or closed,
their behaviour is probabilistic rather than deterministic and the probability of finding them open depends on
the membrane potential (more will be found open at more depolarised potentials). The effec t of a
depolarisation on sodium conductance is regenerative; a greater depolarisation leads to a greater number of
Na+ channels opening, and more Na+ entering as a result (positive feedback). The voltage-dependent activation
of potassium conductance is self-limiting; Depolarisation leads to more K+ channels opening, but a greater
potassiumefflux causes repolarisation which triggers the channels to shut(negative feedback).
LECTURE 9 & REVISION LECTURE – THE ACTION POTENTIAL II
When an ion channel opens, ions flow down their concentration gradient and a blip of current is recorded. Ion
channels work according to an on or off mechanism, but some channels have a really big conductance which
lets through may more ions and produces a greater current. Over long periods of time, channels open many
times and activation of channels increases the probability of them opening. Rarely channels may also open
spontaneously without any external stimulus being present. If a current leaves the cell, deflection is shown on
the graph as going upwards, while current entering deflects the graph downwards. When many channels open,
tiny blips build on top of each other and all sorts of shapes may be produced, such as waves or nerve action
potentials.
Neher and Sakmann developed a revolutionary method of studying individual channels. This was termed patch
clamping and gained them the Nobel prize in 1991. Patch clamping uses a tiny capillary tube, which is heated
in the middle and slowly pulled apart to make a micro-pipette that is finer than a human hair and this is used
as an electrode. The pipette (electrode) is filled with an electrical conducting solution and connected to a very
fast amplifier and recording equipment. An electric controller is used to set the desired membrane potential.
The living cell is kept in a bath solution to keep it alive throughout the experiment. With the current and
voltage as known values, conductance can be deduced from a graph. If the voltage is kept constant, the
current allows the channel conductance to be determined, which measures how well the channel works, and
the various properties of the channel. We can use this technique to see how the channels are regulated by the
cells and drugs and toxins can be added to see how the channels areaffected.

Whole cell configuration is used to record currents through active channels in the whole
cell by inserting the pipette into the cytoplasm of the cell. It is good for looking at cell
currents in response to drugs added from the outside, or the regulation of channels by
the cell itself.
Cell attached configuration allows the current through a few active channels to be
recorded at the cell surface by sucking the pipette onto the surface over the cell over a
certain type of channel. This focuses the attention on single channel currents in response
to the regulation of the whole cell.
When we don’t want any interference of the cell on the functioning of the channel, a piece of membrane can
be ripped off and analysed away from the cell. Voltage gated channels al ways have an intracellular face, which
binds cytoplasmic regulators and enzymes, and an extracellular face, which binds drugs and toxins. The same is
true for ligand binding.Solutionsin patch pipettes cannot be easily changed,but the bath solution can.
Inside-out configurations are used to record currents though a channel away from the cell by removing the
membrane and placing it in a bath, the environment of which can be changed. This is good for looking at
agents that modulate the channel at its intracellular face. Agents can be added to the bath solution to change
the cell environment. Outside-in configurations use the same technique, but using agents that modulate the
channel at its extracellular face.
LECTURE 10 – GROSS ORGANISATION OF THE NERVOUS SYSTEM
The nervous system is divided up into the central and peripheral nervous systems. The peripheral nervous
system is further divided into the somatic PNS; which innervates and collects information from the skin,
muscles and joints,and the visceral PNS; which innervates smooth musclein blood vessels and glands.
The telencephalon (cerebrum) has two hemispheres and consists of the cortex
(the outermost layer of the brain),and the olfactory bulb.
The diencephalon is located at the midline of the brain, above the
mesencephalon (midbrain) of the brain stem and contains the thalamus and the
hypothalamus.
The mesencephalon contains the tectum (dorsal midbrain) and tegmentum
(ventral midbrain). The midbrain controls movement and sensory input.
The rhombencephalon (hindbrain) contains the pons, which connects the
cerebellum to the cortex, the medulla (sensory functions), and cerebellum.
The spinal cord is protected by the spinal column and is the primary channel for messages from skin, joints and
muscles to and from the brain. Dorsal roots bring information into the spinal cord and ventral roots send
information from the spinal cord. The cerebrum is the largest part of the brain, which contains two
hemispheres, separated by the sagittal fissure running down the centre of the brain. The right hemisphere
controls the left side of the body, whilst the left hemisphere controls the right side (decussation). The cortex
controls voluntary actions, cognition and perception and the number of neurons is related to intelligence.
Because the skull is a confined area, this needs to be kept to a minimum area, and so the cortex is kept thin
and folded; peaks are called gyri and troughs are called sulci. The cerebellum is the old part of the brain that
co-ordinates movement and contains extensive connections to the cerebrum and spinal cord. Diseases of the
cerebellum include ataxias, which result in coordination problems. Babies have floppy heads because their
cerebellum has not yet fully developed. The brain stem is the oldest part of the brain that controls vital
functions such as breathing.

In acute schizophrenic disease patients are
unable to separate real from unreal
experiences. Patients with disorganised
schizophrenia may be difficult to understand
as their speech may be incomprehensible
and their behaviour is often inappropriate or
bizarre. Patients with paranoid schizophrenia
experience hallucinationsand delusions.
David Berkowitz, a paranoid schizophrenic,
killed 6 people in NY, 1977 and believed he
was possessed with demonic power.
Brain activity can be measured through the use of electroencephalograms; measuring brain waves in response
to activity. This is fast and cheap to do, but produces a poor resolution and is hard to interpret. Functional
Imaging is safe and produces high resolution results, but is expensive. An fMRI (Functional Magnetic
Resonance Imaging) scan detects differences in the way
hydrogen nuclei of water behave in different situations and PET
(positron emission tomography) scans detect the positrons
emitted after injection of a labelled drug. A positron is the
antiparticle counterpart of the electron with a charge of +1.
Both methods detect changes in blood flow and metabolism
within the brain. The image to the left shows a panic disorder
patient (right) with a significant global reduction in binding sites
(too few GABA receptors), mainly in the orbitofrontal and
temporal areas which indicateanxiety.
LECTURE 11 – CELLULAR ORGANISATION OF THE NERVOUS SYSTEM
The brain is a hierarchical structure: brain → systems → pathways → local circuits → single neurons. In 1880,
new improvements in glass making allowed the advance of microscopes. Methods were developed to preserve
(fix) nervous tissue and cut it into thin sections and to stain nervous tissue, such as the Golgi silver stain. Cajal
examined sections from throughout the nervous system and identified that neurons exist in different forms, but
most have a cell body (soma) with two types of processes: Axons, specialized for transmission of information,
and dendrites, specialized for the receipt of information. He also discovered that neurons communicate
indirectly by contact,rather than fusion, with synapses.
The more recent introduction of the electron microscope allows images to be resolved to 0.1nm, confirming the
existence of synapses. Fluorescence microscopy, with the aid of Green Fluorescent Protein, allowed a powerful
way to determine the protein distribution in cells, but the disadvantage is that this is limited due to the range of
antibodies available and confocal microscopy allowed 3D images of live cells to be rendered with the use of
lasers as a light source.
The two major cell types in the nervous systemare glia and neurons. Glia outnumber the neurons by 10:1, their
primary role is to support neurons, and they have the ability to divide. In the PNS, glia exist as Schwann cells,
which myelinate the periphery neural axons. In the CNS, the glial cells exist as astrocytes, which fill the space
between neurons and regulate the composition of the extracellular fluid, oligodendrocytes, which myelinate the
central neural axons, and microglia, which act as the main form of active immune defence in the CNS.
Neurons are highly polarised cells, simple polarised cells include those with different apical and basolateral
surfaces,and a simple non-polarised cell is just a standard cell with a regular, unchanging membrane shape.

Main features Axon Dendrite
Physiological Propagate information Receive information
Organelles Synaptic vesicles ER, ribosomes,Golgi
Structural Long (mm-m), branch at 900 Short, tapered, branched
Specialisations Synapses Dendritic spines
Myelination/ nodes Often Never
mRNA Never
How does the polar structure of neurons arise? The mechanisms are still unclear. Do the proteins have address
markers on them and if so what reads this address? The neuronal cytoskeleton gives structural support to the
shape and calibre of axons and dendrites, allows the transport of cargo from axons and dendrites and tethers
components at the membrane surface. It contains Microtubules; large tubulin polymers that run longitudinally
down axons and dendrites, providing structure and transport with the ability to undergo polymerization and
depolymerisation. Neurofilaments are 10nm wide protein threads that provide the neuron with mechanical
strength, and Microfilaments are 5nm wide actin polymers, that are tethered to the membrane and mediate
shapechange.
Diseases of the neuronal structure include mental retardation, which occurs due
to an impoverished environment during the “critical period” of brain
development, and Alzheimer’s disease, which has genetic origins and is
characterised by neurofibrils (dead and dying neurons) and neurofibrillary
plaques full of cytoskeletal junk. Some signs appear in 80% of 80 year olds,
consistingof mental impairment; irritability and forgetfulness.
LECTURE 12 – TRANSMISSION
Currents that pass down dendritic cells become smaller over time, to understand this better we can use the
analogy of the transatlantic telegraph cable; if it were the perfect conductor with perfect insulation you would
get out what you put in, but the cable core resists current flow and current also lea ks through the insulator.
The principleequation demonstrates an exponential decrease in current over a greater distance.
𝑉 = 𝑉𝑂 𝑒𝑥𝑝
−
𝑥
𝜆
λ is the length constant; the distance over which the voltage drops to 37% ofthe original value
𝑤ℎ𝑒𝑛 𝑥 = 𝜆, 𝑡ℎ𝑒𝑛 𝑉
𝑉𝑂
⁄ = 0.37 (37%)

Cables with big length constants transmit further than cables with small length constants, which depends on
Rm (leakiness), Ri (conductivity) and d (diameter). The ideal cable should be fat, with thick insulation and a
heavy core. In neurons, the core conductivity and insulation are 106 times worse than the transatlantic cable.
The typical dendrite is 1-4µm in diameter, which limits the transmission to short ranges of <1mm. Dendrites
combat this problem by havinglots of synapses,which help to maintain current flow.
In axons, the currents that pass down do not get smaller as they travel because they have active membranes
where dendrites have passive membranes; axons have a high density of Na+ channels to fire action potentials
which enable signal transmission over long distances. To increase the conductivity of an axon, either the
diameter or insulation needs to increase, an increase in diameter was employed by primitive animals such as
squids, but for more complex mammals, this proved a problem as there is simply not enough space to fit these
fatter neurons in our bodies.Therefore the insulation mustbe better and this is achieved through myeli nation.
There are two types of glial myelinating cells in the nervous system: oligodendrocytes in the CNS, and Schwann
cells in the PNS. Most axons are wrapped 30 – 50 times by a myelin sheath, and some parts of the axon, known
as Nodes of Ranvier are left unmyelinated at regular intervals along the axon. The unmyelinated regions
contain high proportions of Na+ channels, which allow salutatory conduction through repetitive firing, rather
than a constant signal.Theappearanceof white matter is due to myelination.
Multiple sclerosis is an immune mediated demyelinating disease of the CNS, where the immune system breaks
down the myelin in the axons. The myelin sheath can be revealed in microscopy by staining with fluorescent
antibodies to myelin basic protein, and bare axon can be revealed by staining with antibodies to the
cytoskeletal filament Neurofilament H. In MS, the brain starts to repair itself by the fourth month, but
eventually the processes take over, leading to an effective shrinking of the brain, or cerebral atrophy. Other
important types of de-myelinating disorders include optic neuritis; the inflammation of the optic nerve that
may cause a complete or partial loss of vision, and Guillain-Barré syndrome; an acute, autoimmune,
polyradiculoneuropathy (deranged function and structure of peripheral motor, sensory, and autonomic
neurons), affecting the peripheral nervous system, usually triggered by an acute infectious process.
LECTURE 13 – SYNAPTIC TRANSMISSION
Synapses are important for integration; convergence allows us to recognise important objects in the
environment, some specific neurons (grandmother cells) allow us to recognise particular people. Divergence
makes sure we behave in a certain way to a certain stimulus, such as when you tread on a pin. Synapses are
also importantfor plasticity;learningand memory, and as targets for drug action to reduce pain.
Because the extracellular fluid has a lower resistance than the postsynaptic axon, action poentials cannotjump
synapses as they follow the pathway with lower resistance through the ECF. Charles Sherringtion, in 1932,
coined the term “synapse”, in 1921 Otto Loewi demonstrated chemical transmission, and between 1970 and
2000, Eric Kandel proposed a cellular/ molecular model of learning and memory; that short-term memory was
linked to functional changes in existing synapses, while long-term memory was associated with a change in the
number of synaptic connections.
Electrical synaptic transmission occurs at gap junctions, where pores exist for the free travel of ions. This leads
to fast conduction, but the signal is weaker at the other side due to resistance from the pores. This system is

used in escape pathways in invertebrates, such as in the giant motor synapse of crayfish, because the relative
speed of electrical synapses also allows for many neurons to fire synchronously. Approximately 10% of
synapses work in this way. Chemical synaptic transmission is slower because the signal needs to be converted
from an electrical signal,to a chemical one and then back again.
LECTURE 14 – CHEMICAL SYNAPSES & NEUROTRANSMITTER RELEASE
Action potentials are all or nothing, but post synaptic
potentials (PSPs) can vary in strength as they can be iPSPs
(inhibitory) and ePSPs (excitatory). When a chemical such
as curare (a potent paralysing agent) is added to a synapse,
a much smaller postsynaptic potential (iPSP) is observed.
End-plate potentials (EPPs) are the postsynaptic potentials
induced at the neuromuscular junction. The EPP is not an
action potential, but it partially depolarizes the membrane
and can initiate an action potential in the postsynaptic cell.
Miniature EPPs occur spontaneously in muscle cells.
Depolarisation causes Ca2+ influx in presynaptic terminals via voltage-gated Ca2+ channels and images showing
calciumlevels can beproduced usingcalciumsensitivedyes such as aequorin and FURA.
Neurotransmitters are released in vesicles called “quanta” that each contain several thousand ACh molecules.
Each EPP is made up of about 200 quanta and as extracellular Ca2+ is lowered, EPP amplitude decreases in a
stepwise manner. This is the vesicle hypothesis. SNARE
proteins at the plasma membrane mediate fusion of
cellular transport vesicles with the cell membrane at a
target compartment.
SNARE proteins can be classed as v-SNAREs (vesicle), which
are incorporated into the membranes of transport vesicles
during budding, and t-SNARE (target), which are located in
the target membranes. Synaptotagmin is a Ca2+ sensor that
triggers synaptic vesicle fusion with the presynaptic
membrane.
LECTURE 15 – CHEMICAL SYNAPSES & PSP INDUCTION
2ACh molecules bind to the alpha subunits on nicotinic acetylcholine receptor channels at neuromuscular
junctions and cause them to open, allowing Na+ and K+ to flow down their electrochemical gradient leading to
the depolarisation of the membrane (EPP/ePSP). The nicotinic acetylcholine receptor is made up of five
subunits: 2α, β and γ, arranged to form a central pore. There are a great number of neurotransmitters, and
neurons can contain or releasemore than one type of neurotransmitter, this is called co-localisation.
Amino acids Glutamate, glycine,GABA
Amines Acetylcholine,dopamine, noradrenaline,serotonin
Peptides Enkephalin,substanceP, neuropeptide Y
Serotonin is responsible for arousal, sleep and appetite, Glycine and GABA are widespread inhibitors and
peptide neurotransmitters are neuromodulators. Enkephalin is a natural painkiller in the brain that regulates
pain and nociception,dopamine depletion leads to Parkinson’s disease.

Amino acids and amines are made in the presynaptic terminal by synthetic enzymes and transported in
vesicles, neuropeptides are made in the cell body, synthesised in the rough ER, cleaved in the Golgi apparatus
and transported in secretory granules. The action of a neurotransmitter depends on the receptor present;
nicotinic ACh receptors excitatory at skeletal muscle,but muscarinic Ach receptors are inhibitory on the heart.
Repolarisation is due to the removal of a chemical stimulus. This is done by acetylcholinesterase action at the
synapse, not all ACh actually reaches the post synaptic membrane since acetylcholinesterase is always present.
Nerve toxins that attach cholinergic synapses include
Botulinum toxin From Clostridium botulinum Destroys SNARE proteins,prevents ACh release
Black widow spider venom From black widow spiders Induces massiveACh releaseand depletion
α- Bungarotoxin From cobra venom Irreversibly bindsto ACh receptors
Organophosphates In insecticides and nervegas Irreversibly inhibits acetylcholinesterase
Myasthenia gravis, muscle weakness, is caused by a reduction in the number of ACh receptors on skeletal
muscle, an autoimmune response targets antibodies to the α subunit of the nicoti nic ACh receptor, causing a
reduction EPPs and mEPPs. Edrophonium is an acetylcholinesterase inhibitor. In people with myasthenia gravis
involvingthe eye muscles,administration of this drugwill briefly relieveweakness.
LECTURE 16 – INTEGRATION
Primary motor cortex
Sensory
association area
Visual
association
area
W
B
Motor
association
cortex
Prefrontal
cortex
Auditory
association
Primary Somatosensory area

The primary motor cortex initiates voluntary movement, while the motor
association cortex (premotor area) coordinates complex movements. The
prefrontal cortex deals with planning,emotion and judgement.
Synaptic integration involves summation of ePSPs and iPSPs on the
postsynaptic membrane. PSPs travel to the axon hillock in a passive,
detrimental form, where decision making occurs. The axon hillock has a high
density of Na+ channels, while the postsynaptic neuron filters and integrates
the signals. The average neuron forms about 1,000 synapses and receives up
to 10,000 connections, most PSPs are only a few mV and do not take the
membrane to threshold.
Summation refers to the addition of a number of impulses, the cell has to “decide” what to do depending on
the inputs; spatial summation is due to multiple pathways, and threshold is reached when the impulses from
the presynaptic neurons fire at the same time (the theory behind event association). Temporal summation
works according to the rate at which impulses are fired along one pathway, firingat a higher frequency caus es
the action potentials to reach threshold as they do not have time to fully repolarise before the next signal
arrives.In effect, the signal justbuilds up gradually to reach threshold.
In hyperekplexia (literally “excessive startle”), an affected adult will startle easily at a sudden sound or
unexpected touch or bump and may fall and be injured. This is usually inherited as an autosomal dominant
trait and due to a single amino acid mutation in the glycine receptor chlorine ion channel, which reduces
inhibition of the central nervous system.
LECTURE 17 – NEUROMODULATION
Neuromodulation refers to setting the activity level of neural pathways by changing their threshold (due to
more channels, or channels having a greater sensitivity), or altering synaptic strength (with more postsynaptic
receptors or neurotransmitters). For example noradrenaline acts on hippocampal neurons to make the cell
more responsive to a neurotransmitter (glutamate) by causing K+ channels to close. In the long term the
neuronal membrane properties or synaptic strength may permanently change.
Classical synaptic transmission Fast, shortlived - ionotropic ion channels ACh on skeletal muscle
Local neuromodulation Slower, longer lived - metabotropic receptors ACh on cardiac muscle
Extrasynaptic neuromodulation Slow, long lived - metabotropic receptors Oxytocin on brain pathway
Ionotropic receptors include nicotinic ACh receptors and GABAA receptors, metabotropic receptors are G-
protein coupled receptors, and include muscarinic ACh receptors (where the ion channel is directly activated)
and Serotonin 5HT receptors (where a secondary messenger cascadeactivates the ion channel).

GABA is a neuromodulator which inhibits calcium channels when released; the blue neuron effectively acts as
an on/off switch to vary the strength of the action potential of the main neuron. Serotonin acts in the opposite
way to GABA, and usually leads to association of events when both neurons are stimulated at the same time.
LECTURES 18 & 18B – THE BASIS OF LEARNING AND MEMORY
Behaviour changes as a result of experience; learningis the process of acquisition of knowledge, while memory
is concerned with storage and retrieval. An altered stimulus → response relationship is also known as
Pavlovian (or classical) conditioning. Short term memory is a reverberation of electrical activity caused by
changes in synaptic activity, and long term memory is due to new or enhanced synaptic connections or new
gene expression. Most animals with a nervous system can learn. Drosophila can detect certain odours from
food and by pairingcertain “bad”odours with a mild electric shock they learn to avoid them.
In sea hares, gills retract when the pressure is applied to protect
them, this reflex action is a result of classical conditioning. When the
stimulus is repeated and no form of “punishment” is administered,
the response decreases, showing a habituation response. But when
an electric shock is administered with the stimulus, the response
quickly increases again. Synaptic efficacy is reduced with repetitive
use, due to a reduction in Ca2+ influx per action potential and hence
reduced neurotransmitter release.

Sensitisation of gill withdrawal involves presynaptic facilitation. A facilitating interneuron (5HT) receives
impulses from the electrical stimulus in the tail, and causes cAMP to be released at the motoneuron, which
leads to the closing of voltage-gated potassium channels, meaning it takes longer for the action potential to
repolarise and a broader response is observed as more neurotransmitter is released. In the long term, the
signallingcascadeleads to changes in gene expression,K+ channels areinhibited and will notfunction at all.
The hippocampus is located deep in the temporal lobes of the brain and is part of the limbic system involved in
memory processing. It is important for spatial learning and London taxi drivers have large hippocampi. The
hippocampus is also severely affected in Alzheimer’s disease. Long-term potentiation (LTP) is the long-lasting
enhancement in communication between two neurons that results from stimulating them simultaneously.
Since neurons communicate via chemical synapses, and because memories are believed to be stored within
these synapses, LTP and its opposing process, long-term depression, are widely considered the major cellular
mechanisms that underlie learning and memory. LTP can be induced by strong tetanic stimulation of a single
pathway to a synapse.
LECTURES 19 AND 20 – MUSCLE
Skeletal muscle is a form of striated muscle that is attached to bones via tendons. They perform voluntary
actions such as runningand can be up to 40% of one’s body weight. Skeletal muscle contains between 100 and
10,000 muscle fibres (cells) that are made up of myofibrils and run in parallel, stretching from millimetres to
tens of centimetres long, the cells have many nuclei due to the fusion of myoblasts. The origin of striations is
due to the presence of actin and myosin filaments.
Tropomyosin stops musclecontraction as itcovers up actin bindingsites,this can be relieved through calcium
bindingto troponin and causingtropomyosin to undergo a conformational change,exposingthe bindingsites.

Genetic muscular diseases include:
Muscular dystrophy Progressiveskeletal muscledestruction and weakness
Myotonia
Sustained contraction followed by slowed relaxation due to a mutation in protein
kinase
Channelopathy Mutations in ion channels
McArdle disease Lack of phosphorylaseso glycogen cannotbe broken down
Malignant
hyperthermia
General anaesthetics can make SR Ca2+ channels open, leadingto an uncontrolled
increasein skeletal muscleoxidativemetabolismwhich overwhelms the body's
capacity to supply oxygen, remove carbon dioxide,and regulate body temperature,
eventually leadingto circulatory collapseand death if not treated quickly.
Calcium accumulates into the sarcoplasmic reticulum by the calcium ATPase pump and is released into the
cytoplasm through specialised release channels (Ryanodine receptors). The opening of these channels is
caused by a surface membrane action potential that spreads down t-tubules and makes charged particles in t-
tubule membranes move; these are connected to the SR releasechannel and causeit to open.
A nerve (motor neuron) action potential releases
ACh at the neuromuscular junction and opens the
channels at end plates, depolarising the muscle
membrane and initiatingtheaction potential.
The action potential travels down to the t-tubule,
where a charged particle opens the Ryanodine
receptors, causing calcium to be released from the
sarcoplasmic reticulum.
The calcium binds to troponin, causing the
tropomyosin to undergo a conformational change
and lever off the actin filaments allowing the
myosin heads to bind and leading to muscle
contraction by the shorteningof the sarcomeres.
On relaxation, SERCA (Sarco/Endoplasmic Reticulum
Ca2+-ATPase) takes calciumback into the SR.
A muscle action potential lasts for a similar length of time to a nerve action potential, but the muscle
contraction forcelasts much longer. Muscleimpulses can take many forms:
Singletwitch Short contractions,blink of an eye
Summation Impulseincreases gradually
Unfused tetanus Musclecontracts and relaxes quickly
Fused tetanus Muscleremains contracted, maintained movement
Smaller motor neurons have lower thresholds and activate smaller motor units, allowing for a small muscle
contraction of a certain area when greater contraction is not required, fast fibres allow for rapid shortening,
but at a high energy cost as ATP is hydrolysed quickly, slow fibres are used for posture and contain myoglobin
as an oxygen store with many mitochondria. Fatigue results in a state of decreased performance due to lactic
acid buildup in the short term, and glycogen depletion in the long term. Ageing results in the loss of motor
neurons and a decreased number of muscle fibres (which can be partially restored wi th exercise), whilst
damage to muscle cells results in them being replaced with satellite cells, leading to muscle growth. Other
factors such as diseasealso havedifferent effects on muscle (see table at top for examples)
t-tubule
muscle

Cardiac muscle is only found in the heart, it is striated and the component cells are joined by intercalated discs
which provide low resistance and allow the spread of an action potential. Cardiac muscle effectively acts as
one big cell, the mechanism of calcium release is different to that of skeletal muscle since cardiac muscle has
no need for nerve excitation, it is myogenic. Because the heart beats continuously it cannot rely on anaerobic
glycolysis and so requires its own blood supply for aerobic respiration.
Smooth muscle is not striated and consists of small spindle shaped cells with varicosities; swollen regions of
the autonomic neuron containing neurotransmitter vesicles that are released when action potentials arise.
Smooth muscle is found in hollow organs such as blood vessels and bronchi to regulate flow, and in the gut
and uterus in order to propel contents. As with striated muscle, smooth muscle is regulated by calcium, but
this is not just from the sarcoplasmic reticulum, it also comes from the extracellular fluid. Calcium is released
from the SR via both Ryanodinereceptors (RyRs) and Inosital TriphosphateReceptors (IP3Rs)
Caldesmon is a calmodulin binding protein, and Calponin is a calcium binding protein, they both inhibit smooth
muscle contraction until they are phosphorylated in the presence of agonists. Some smooth muscles have
pacemaker activity, such as those in the gut and uterus. Vascular smooth muscle controls blood flow. The
endothelium of blood vessels releases factors to control smooth muscle and noradrenaline helps to raise the
intracellular calciumconcentration.

Excitable Cells: Revision Notes

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