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NEUROANATOMY IGeneral organization of the nervous system
The nervous system can be divided from a macro perspective and a micro perspective. The macroscopic
nervous system is divided in a central part and a peripheral part. The central part consists of the brain
and the spinal cord and the peripheral part consists of spinal and cranial nerves. The easiest role they
play is to connect the nervous system with the rest of the body to elaborate incoming info (e.g. via
senses) and produce a response (e.g. motor). The main cellular components are seen in the microscopic
division. Not only neurons, but also oligodendrocytes (ODCs, which produce myelin), astrocytes (for
support) and microglia (immune system of the brain) make together the nervous system.
The cerebrum of the brain consists of 2 hemispheres. It has comfortable folds that allow the brain to sit
(fossae). The skull hosts many holes (foramina) to allow the passage
of cranial nerves and vessels:
• The anterior fossa (green) holds part of the frontal lobe
• The medial fossa (blue) does the same for the temporal
lobes
• The occipital fossa (red) holds the occipital lobe and
cerebellum
• The sella turcica (purple) looks like a settle where the optic
chiasm, mammillary bodies and part of the diencephalon sits.
The connection between the frontal fossa and sella turcica is
called sphenoid.
4 important foramina are the foramen ovale, spinosum, lamina cribrosa and foramen magnum. The
foramen ovale and spinosum allow the passage of the trigeminal nerve, the lamina cribrosa for the
olfactory nerve and the foramen magnum is a central big hole in the occipital fossa for passage of the
spinal cord.
Orientation planes in neuroanatomy
3 planes are normally used to analyse brain. These are sagittal, coronal and horizontal. Specific language
to determine regions are rostral/caudal, superior/inferior, dorsal/ventral and medial/lateral.
,Meninges
The brains has developed a specific way to protect from the outside. This is mostly because of the
presence of meninges. There are 3 layers of meninges. These are the dura, pia and arachnoid. Dura (right
below skull) is the outer one and the pia the inner one. The dura mater in the encephalon is further
divided into the osteal layer and the meningeal layer. The osteal layer is more towards the skull and the
meningeal layer more towards the brain. As also the spinal cord has the same 3 layers of meninges, the
dura here does not have 2 separate layers, but just 1. In between the arachnoid mater and pia mater,
there is a subarachnoid space. This is an empty space is called the
subarachnoid cavum and functions to let the cerebrospinal fluid flow to be
drained. The drainage happens via granulations, which are small
mushrooms made by arachnoid mater and subarachnoid space, that drains
the CSF to the sinuses.
Blood supply and CSF
The brain needs a lot of O2, so 20% of the whole oxygenated blood goes directly to
the brain. 2 arteries serve blood to the brain:
1. The carotid artery (more ventral)
2. The vertebral artery (more dorsal)
When entering the cranial region, the vertebral arteries (2 branches) converge into
the basilar artery. The basilar artery and the carotid artery converge into the circle of
Willis, a big roundabout to be found in the basal part of the encephalon at the level
of the optic chiasm. The role of this circle is to prevent a lack of blood to the brain in
case one of the 2 arteries does not function. It doubles the structures letting the
blood circulate like in the ring of a highway to avoid damages in the brain. This
principle is called redundancy. From the circle of Willis, 3 main arteries depart: the
anterior, medial and superior cerebral artery. Their role is to serve blood to the whole
encephalon (the cerebellum has its own arteries). These 3 arteries sent compartmentalized blood to the
brain, where the anterior artery only perfuses the medial part of the encephalon, the medial artery the
lateral part and the posterior artery the occipital part.
The brain uses not only blood to survive, but also nutrients and salts from the cerebrospinal fluid (CSF).
The transparent fluid full of sodium, calcium, magnesium and glucose is produced in the choroid plexus
that makes and sends CSF to the ventricles. The ventricles are cavities filled by CSF that play a role in
distributing the CSF to the whole NS. There are 4 ventricles in the human brain:
• 2 lateral ventricles (1 per hemisphere)
• 3rd ventricle (very skinny, connects the 2 lateral together)
• 4th ventricle (at the level of the cerebellum/pons)
In between the lateral and the 3rd ventricle, there is the foramen of Monro,
there to allow the communication between lateral and 3rd ventricles. In
between 3rd and 4th ventricle, there is a cerebral aqueduct, a connecting tube
to spread the CSF all the way through the brain and spinal cord. After the 4th
ventricle, there is the central canal, a big tube that walks along the whole
spinal cord to provide CSF. The role of the CSF is to feed the brain and protect due to the buoyancy
principle. Which means that the mass is reduced of about 90%, this spares the neurons that reside in the
more ventral part of the brain. The last station in the CSF pathway is that the CSF is gathered in the
subarachnoid space and via the granulations sent to the sinuses. The latter are a group of venous-like
structures whose cellular organizations differs a bit from that of normal
veins. Their walls are made of dura mater. The sinuses are located in
specific parts of the encephalon and their role is mainly to let convey CSF
and blood (venous capillaries) to be drained outside the NS. Half a litre CSF
is produced per day and this needs to be drained after usage. Via the
sinuses, the CSF is sent to the jugular vein to leave the CNS, normally
through the sigmoidal sinus. Important sinuses are the superior and
inferior sagittal sinus, the transverse sinus and the sigmoidal sinus.
, MEMBRANE AND ACTION
POTENTIALS
Resting membrane potential
When an action potential arrives at a synapse, it changes to a chemical potential to pass on the potential.
This will be sensed by the post synaptic membrane and changes it to a synapse potential. Once arrived in
the NS, it will be converted to an action potential. Action potential has a huge amplitude and a shorter
period of time.
Receptor potential Synapse potential Action potential
Electrical activity can be stimulated and measured with an electrode. An electrode will change the
membrane potential. The membrane potential is equal to the resting potential. Nerve cells are
hyperpolarised, so they have a negative resting potential. It can be hyperpolarised even further, which
results in an even more negative membrane potential. Once stimulation is twice as strong, the
hyperpolarisation will be twice as big. This is called a passive
response. A positive stimulus will result in a depolarisation. If the
positive stimulus is large enough, an action potential will be
generated. This is called an active response. An even larger stimulus
will result in an increasing firing frequency. Now, 2 action potentials
will follow each other up in the same amount of time.
The ions and charged particles that will be injected, diffuse and will
become weaker in the case of a passive response. So, the signal will
die out. In a long axon, this will never work in terms of signalling. A
property of active signals is that it will remain its amplitude along the
entire axon. Active and passive signals are deviations from the
resting membrane potential. The resting membrane potential is the electrical potential difference
measured across the membrane (inside with respect to outside). This is based on 2 membrane
properties:
- Lipid bilayer is impermeable for ions
- Specialized ion channels can conduct ions selectively
It is also based on 2 principles in physics, which are diffusion of particles and electrical forces between
charges. Diffusion is the spread from high concentration to low concentration until the concentration is
equal at both sides. Equilibrium is reached when there is no net movement of particles anymore.
, Equal charges repel each other and opposites will attract. Ions are charged particles. When the amount
of potassium ions at each side of a membrane is equal to the amount of chloride ions, the charge is equal,
namely 0. This doesn’t mean that the amount of potassium ions is the same on both sides of the
membrane. The concentration can differ on both sides. Now, both potassium and chloride ions will want
to move to the low concentration KCl side. However, only potassium can
go through the selective permeable membrane. When one ion goes, the
charge will be 1- at the side it comes from and 1+ at the side it goes to.
The charge difference will now be 2-. When another ion flows, the
charge difference will be 4-. The potassium ions will move to equilibrium,
but this will result in a large charge difference and an electrical force will
be created. This will push the particles back to create an electrical
equilibrium.
When the diffusion force is equal to the electrical force, there will be no net movement of ions over the
membrane and there will be electrochemical equilibrium. The electrical potential will now be the
equilibrium potential. The number of ions that is needed for this equilibrium to be reached is very small
(one millionth of the total number of K+ ions in the solutions).
Nernst and Goldman equations
The Nernst equation can be used to calculate the equilibrium potential. This equation goes as follows:
𝑹·𝑻 [𝑿]𝒐𝒖𝒕
𝑬𝒙 = 𝒛·𝑭
· 𝐥𝐧 [𝑿]𝒊𝒏
. In this equation, Ex is the equilibrium potential in mV, R is the gas constant (= 8,31 J
K-1 mol-1),
T is the temperature in Kelvin, z is the valence (electrical charge) of the ion, F is the Faraday
constant (= 96485 C mol-1), [X]out is the concentration outside and [X]in is the concentration inside. At
58 [𝑋]𝑜𝑢𝑡
room temperature, the Nernst equation is equal to 𝐸𝑥 = 𝑧
· log [𝑋]𝑖𝑛
.
The equilibrium potential depends on the ratio between the concentration inside and outside. The slope
of the graph is the equilibrium potential. By putting in a stimulation electrode or a battery, the potential
can be changed. The ions can overcome the diffusion force. Now, there will be a net flux of K+ from
outside to inside, when making the inside more negative. Because it goes form a negative flux to a
positive flux, it is called a reversal potential.
The Goldman equation is for monovalent ions only (valence of +1 or -1). This is used, because there are
multiple channels selective for different ions in the membrane. The result of the equation is the property
𝑷𝒌[𝑲]𝒐𝒖𝒕+𝑷𝑵𝒂[𝑵𝒂]𝒐𝒖𝒕+𝑷𝑪𝒍[𝑪𝒍]𝒊𝒏
of the ion channels (opened or closed). The equation: 𝑽𝒎 = 𝟓𝟖 · 𝐥𝐨𝐠 𝑷𝒌[𝑲]𝒊𝒏+𝑷𝑵𝒂[𝑵𝒂]𝒊𝒏+𝑷𝑪𝒍[𝑪𝒍]𝒐𝒖𝒕
. Px is
the permeability of the membrane for ion X. Let’s assume that the
permeability for potassium (in rest) is much higher than sodium. This means
that the channels for potassium are open and sodium channels are closed.
Now, the ions of the closed channels can be removed from the equation. This
means that the channels are closed for a reversal potential. A lot of channels
are voltage sensitive (open upon change in charge). Once the sodium
channels open, the permeability excites that of potassium, now what is left, is
the Nernst equation of sodium, due to removal of potassium from the
equation.
NEUROANATOMY II
To understand how the brain is structured, it is useful to look at how it develops. From the development,
5 main vesicles form. In the beginning from the zygote, the foetus starts developing and, at gestational
day 19, there is already a rudimental NS. It is a flat structure, named neural plate, made by not very
differentiated cells. At day 22, the extremities fold up, creating the neural groove, which closes up after a
couple of days leaving a hole inside. That hole is the rudimental central canal from which the ventricles
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