Summary The basics of neuroanatomy
Biomedical Sciences Master Course
Radboud University, Nijmegen
Made by: Georgia Graat
,Brain anatomy
With 100 billion neurons in the brain, each neuron is connected to about
200.000 other neurons with 15.000 synapses per neuron. Neurons receive
information on the dendrites and soma. The soma is also the cell body with all
the important structures. The effector is the signal transduction. The action
potential runs over the axon, as this is for conducting information. Synaptic
transmission flows further through conducting and sending chemical
mediated electrical signals through the axon, after which it arrives at the axon
bouton and axon terminal. The synapses can release neurotransmitters or
chemical messengers. This creates a new action potential which jumps to the
new axon. The action potentials are generated by the neurotransmitters that
cause a change in sodium/potassium uptake in the next neuron.
Glial cells are 10 times more common in
the brain than neurons. Their function is structural, metabolic
and functional support. They transport nutrients, isolate axons,
provide repair and control the composition of the extracellular
fluid. Another cell is the microglia, which is the defense cell that
eats waste as a sort of macrophage. The astrocyte gives support
and nutrition transport and sits close to neurons to wrap around
it, where it also forms part of the blood-brain barrier.
Oligodendrocytes wrap around neurons to form myelin sheaths
around the axons.
The nervous system consists of the central nervous system with
the brain and spinal cord and the peripheral nervous system
with 12 cranial nerves and 31 spinal nerves. The CNS is located in the skull and the vertebral column.
The main functions are perception, movement, learning memory, personality, emotions,
consciousness, and language. The whole CNS is surrounded by cerebrospinal fluid. The CSF has the
functions of support, protection, nutrient distribution, and waste and toxins removal. It protects the
brain by functioning as a shock absorber, and reducing the weight of the brain on the skull. The
important function of maintaining homeostasis of the fluid and external environment is needed for
normal neuronal function. The CNS is actually a triptych, meaning almost all of the neurons found
there are interneurons. This means afferent/sensory input comes in, is processed, stored and
associated with other memories, and efferent/motor output is send out. Most of these interneurons
can be found in the brain and brainstem.
The CNS has the white matter on the inside of the brain and the grey matter
in the neocortex (outside). This contains 6 layers of neurons. The grey matter
mostly has the nuclei of the neurons, while the white matter contains the
long axons. The white matter in the brain also has some groups of cell
bodies/grey matter randomly in the middle. This is actually the basal
ganglia/nuclei for initiation and organization of motor movements. It is the
other way around in the spinal cord. Here, the grey matter/neuronal bodies
is located in the center surrounding the canalis centralis in a butterfly shape.
The white matter is located more peripherally and contains the ascending
and descending fiber tracts. Ascending can be found on the dorsal side and all the lateral parts. The
descending pathways are more ventral and also in the posterior columns.
, The neural development starts with the neural-plate-
fold-tube. The ectoderm makes a neural fold that
becomes larger and folds into the mesoderm. This
makes a groove and when it is all the way into the
mesoderm, the folds connect to each other and form
the neural tube. This is thus a tube of ectoderm
placed within mesoderm. This neural tube eventually
turns into the whole CNS. It becomes thicker and
broader on the cranial side where it forms the head.
The spinal cord is made caudal by increasing and
narrowing the tube. In spina bifida, the closing of the
CNS right at the end of the spinal cord does not
happen correctly, causing an open back or bump
where neurons and CSF is placed in.
During the growth of the spinal cord, many cells start lining besides it in the
neural cavity. The mitosis starts at the neural tube cavity side and divides very
fast. This becomes ependymal cells and radial glia cells nearby, as well as
neurons, glioblast, and neuroblast cells more to the outer membranes. The
spinal cord divides itself into the basal plate with motor neurons and dorsal
plate with sensory neurons.
When the neural tube starts expanding, it creates the
first part of the brain cranial. This will start to develop
into three brain vesicles. These are the forebrain (prosencephalon),
midbrain (mesencephalon), and hindbrain (rhombencephalon). This
happens as neurons grow bigger and start to move upward. The cerebrum
starts to gain 6 layers of neurons with different functions, which are all
placed into position as they crawl along the glial cells.
The forebrain then divides into the
telencephalon and the diencephalon, which is the fastest growing part
and therefore becomes the biggest growing completely over the rest of
the structures. The hindbrain divides into the metencephalon and
myelencephalon. The forebrain also gets cavities and ventricles.
After the telencephalon starts developing two hemispheres, it will also
start to get gyri and sulci. The gyri are important to achieve more
volume of the cortex meaning more storage for neurons. Sulci are the
clefts in between that separate the brain lobes. In neurodegeneration,
these sulci also disappear and the whole brain tends to shrink. While
the fornix is attached to the frontal part of the brain, the body of the
hippocampus at its end starts to move downward. This means a curve
of this whole system starts to appear in the brain. This also changes the system of the ventricles,
creating aqueducts and different ventricles throughout the cerebrum.
The system of the CSF starts in the choroid plexus. This structure produces around 500 ml per day
and secretes it into the two lateral ventricles which are located right under the fornix and corpus
callosum. The CSF volume is however only 150 ml in total, with 125 ml in the subarachnoid spaces
and 25 ml in the ventricles. From the lateral ventricles, it flows into the third ventricle (between
