Regional Anatomy of the CNS and
Brain Vasculature
3-17-09
Types of Neurons
-Afferent-Relative term. Any neuron entering an area is afferent to that area (a
neuron synapsing into the thalamus is afferent to the thalamus). From the
perspective of the CNS, Afferent Neurons generally refer to sensory (either
somatic or visceral or special sensory) PNS neurons that synapse onto a CNS
interneuron in the spinal cord or brain stem
-Afferent signals DO NOT need to effect efferent signals. I.e. sensory
signals can be processed and “stored” in the CNS without an immediate
effect to the outgoing signals.
-Efferent-Relative Term. Any neuron leaving an area is efferent to that area (a
neuron fiber leaving the thalamus is efferent from the thalamus). From the
perspective of the CNS, efferent neurons are usually motor neurons, or other
effector neurons (such as to glands, smooth muscles), leaving the CNS at the
brain stem or spinal cord and entering the PNS.
-Interneuron- Term for all other neurons that are part of the CNS. These neurons
connect afferent PNS signals to the various levels of the CNS, and then the
effectors’ signal to the efferent neuron in the PNS
-The More interneuron involved in a signal, in general, the more
complicated the process
-Types of Interneurons
-Commissural-Neurons within the CNS that have cell bodies on
one side of the cerebral cortex and send conductive axon to the
other side (they cross the midline in structures known as
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commusirals). They connect like areas of the CNS at the same
functional level. (Motor area for the right hand with motor area for
left hand).
-Association- Cell bodies in the CNS will send axons to same side
in brain on the same functional level. They connect different
functional areas of the brain (Axons from cell bodies in the speech
comprehension center of the brain synapse into the motor speech
center).
-Projection-Send their axon to a different functional level than the
location of its cell body. Many, NOT ALL, projection neurons
decussate (cross the midline-especially when
ascending/descending). They connect somewhat dissimilar areas
-An example of a non-decussating projection fiber would
be a neuron with a cell body in the cortex that synapses into
the thalamus (a different functional level-telencephalon to
the diencephalon)
-Long fibers tend to decussate
Reflex
-Special type of neuron interaction in which an afferent neuron synapses directly
onto an efferent neuron without going through an interneuron. This reflex
interaction requires muscles spindles. This results in very structured,
stereotypical, repeatable, and predictable responses.
Functional Levels of the CNS
-Prosencephalon
-Telencephalon
-Cerebrum
-Higher cognitive function, memory, voluntary movement
etc. gives things meaning
-Without Cerebrum we may be aware of light and dark, but
not much else
-Diencephalon
-Buried deep within the cerebrum, but have different origin and
function than cerebrum (different functional level)
-Composed of three parts
-Thalamus
-Relay of all sensory lower level CNS signals to the
cerebrum (except some olfactory)
-Hypothalamus
-Epithalamus
-Mesencephalon
-Midbrain (part of brain stem)
-Helps drive the thalamus. Because of the cranial nerve nuclei in
this area, a lesion to this area results in loss of consciousness
-Rhombencephalon
-Metencephalon
-Pons
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-Big relay center, especially for signals between cerebellum
and other parts of the CNS
-Cerebellum
-Outgrowth of the pons, aids the cerebrum in coordination
of muscle movement and timing
-Myelencephalon
-Medulla Oblongata
-Transition area between the brain stem and spinal cord
-Contains neurons that control respiratory and heart rate
-Automatic controls
-I.e. influence phrenic nerves to make the
diaphragm contract or relax
-Myelon
-Spinal Cord
-Controlled by higher functional areas
Two General Concepts about the CNS
1. CNS is a hollow tube- Consequence of the embryological origin of the CNS
(started as a neural tube..). The Subarachnoid space around the cortex/brain
stem/sc contains the outer cavity of the tube which contains CSF. 4 ventricles
buried within the cortex and areas of the brain stem manufacture the CSF and
serve as the inner cavity of the tube. The Central canal is the extension of the
4th ventricle which carries CSF inside the spinal cord.
-Walls of the tube are NOT the same thickness
-Diameter of the tube is NOT unifrom
2. CNS is NOT homogenous
a. There are areas of high cell body concentration (nuclei-grey matter),
areas of the high axon concentration (white matter), different thickness
of grey matter
i. On inside of cerebrum cell bodies referred to as nuclei
ii. On outside rim of cerebrum cell bodies referred to as cortex
iii. White matter (fibers)-commissural, projection, association
iv. Gray matter (cell bodies)-not much myelin, without the myelin
they appear pinkish gray, less myelin because of need of
synpase
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Cauda Equina
-Is made up of sensory and motor axons of the lumbar and sacral regions of the
spinal cord
Stabilization of the Spinal Cord in Fluid
-Two growths of the pia mater anchor the spinal cord vertically and laterally
within the spinal column
1. Filum Terminale-From the conus medularis to the coccyx
2. Denticulate Ligmanets-Lateral extensions of the pia mater which extend
to the dura, through the arachnoid, between the ventral and dorsal roots
Length of Neurons
-Neurons are conductory, not contractile
-Variety of shapes and sizes (1 mm in length of body)
-Primary sensory recepetive segment in big toe and dorsal root ganglion in
lower lumbar, sacral region of the spinal cord has axon terminating in the
skull
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Dura Mater and Brain Stablizaition
-Dura Mater (interperiosteum)
-Outer most layer of the menignes
-Superficial layer is the periosteum (skulls inner portion or
endocranium
-Deep Layer-Actual Dura Mater
-Tentorium Cerebelli
-Hard structure. If there is a volume expansion in the parenchyma above
the tentorium, the brain can get pushed down partly through the tentorium
-Herniation is at cranial nerve 3 (mydriasis) and brain stem is
compressed
-Limit the movement of the brain, but can become hazardous if brain
swells (herniates through notch of tentorium putting pressure in CN III
causing 3rd nerve palsy)
-Midbrain gets pressure put on it because the midbrain is located
within the notch
-Cerebellum could get pushed into the foramen magnum which
puts pressure on the medulla causing respiratory and CV pressure
-Dura reflections serve to limit the movement of the brain and are
subdividing the cranial cavity into smaller units. Pressure on the dura can
compromise the nerve but the CSF is used to prevent this from happening
-Prevents the cerebrum from pushing cerebellum outside cranial cavity
-Pia Mater
-Continuous with the membranes of the brain, brain stem, and spinal cord.
If the pia is disrupted, neural tissue expands out of position
-Inferior to the Tentorium Cerebelli is the brain stem: midbrain, cerebellum, pons,
and medulla
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-Superior: Cerebrum (occipital lobe and parts of parietal)
Organization of the Spinal Cord
Cervical Spinal Cord
-Large Ventral Horn relative to lateral and dorsal horns and white matter
-This region inervates the hand
-In general the larger the surface area that a spinal cord
segment innervates, the larger the ventral horn
-Dorsal white column, usually contains well localized fine touch and
proproception fiber tracts
-Lateral white column extends from the caudal portion to at least brain
stem, if not further
Thoracic Spinal Cord
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-In general, the lower the SC segment, the more grey matter relative to white matter
-This is the case as you ascend the spinal cord, there are more fiber tract inputs
that need to get to higher CNS areas, thus the white tract enlarges relative to the
grey matter.
-Exception: The Cervical, Lumbar, and Sacral enlargements which have
more grey matter due to innervation to the limbs
Bones of the Skull
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Regions of the Cerebrum
-Just as there is frontal, parietal, temporal, and orbital bones, there are frontal,
parietal, temporal, and orbital lobes of the right and left cerebrum
-Frontal
-Motor/Behavior/Decision Making
-Parietal
-Sensory
-Temporal
-Hearing (audition)/memory
-Occipital
-Predominantly Vision
-Insula
-When you pull back the Lateral sulcus (between frontal and temporal
lobes), there is a 5th “lobe” the Insular
-Has sections of three of the 4 lobes: frontal, parietal, and temporal
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The Production and Flow of CSF
-Lateral ventricles (right and left-1 and 2), within the lobes of the cerebrum
-Outside of tube is the subarachnoid space filled with CSF. Therefore CSF is
surrounding the entire CNS and fills spaces within the NS. All lumen of the
neural tube contain greater or lesser amounts of CSF (not homogenous)
-Lateral connects to the third ventricles via the intraventricular foramen
-Cerebral aqueduct connects the 3rd and 4th ventricle
-Median aperature (foramen luschka) connects the 4th ventricle to the
subarachnoid space
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-3rd ventricle is between the 2 thalami and the midline between the left and
right lateral ventricles
-4th ventricle is located in the pons or upper part of the medulla
-CSF provides buoyancy to the brain. If you lose CSF, one gets a bad
headache due to the brain beginning to settle upon the dura mater in the
floor of the skull. This part of the skull is innervated by the trigeminal and
vagal nerves which causes the unnecessary firing of action potentials
The Telencephalon and Important Gyri/Sulci
-The telencephalon folds, forming fissures and sulci, which by consequence forms
gyri
-Post-Central Gyri-Info from the body sent here for initial processing. It is
then packaged and sent to other areas to be put together. It is the primary
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somatosensory cortex: somatosensory, proprioception, subcutaneous,
visual and auditory
-Someone with a parietal lobe lesion on the left side tends not to
recognize that they have a right side of their body
-Central sulcus separates the frontal lobe from the parietal lobe (separates
primary motor from primary somatosensory
-Lateral fissure divides the frontal and patietal lobes above from the
temporal lobe below
Limbic System
-Just Superior to the Corpus Callosum
-Transitional system between the “pure” sensory and motor areas of the brain
-Adds emotional response to “pure” sensory/motor interaction
-Not much emotion before the limbic lobe communicates with other structures
such as the amygdale.
-Basic biological drives.
-Connects directly to the brain stem
Corpus Callosum
-Connect the left and right brain
-Collection of Commissural Fibers
-Does not degenerate as quickly as other fibers
Cell Bodies in the Cerebrum
-In the cerebrum, the majority of the cell bodies are the grey matter, which is
superior to the white matter and just inferior to the pia mater
-However, there are also deep nuclei within the cererbrum, basal nuclei (ganglia),
as well as in the thalamus
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Basal Ganglia
-improper name, actually basal nuclei because they are part of the CNS, however,
generally referred to as ganglia
-Groups of cell bodies imbedded deeply within the CNS
-Part of the Telencephalon
Thalamus
-collection of nerve bodies, size of chicken egg with one on each side
-Relay station for all sensory ascending info except some olfactory
-Functional Level: Diencephalon
-Not Homogeneous
-Consists of groups of neurons with specific relationships
-Certain systems have own nuclei, project to very specific parts of the cerebrum
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BG=Basal Ganglia-Telecephalic in origin, function largely motor
T=Thalamus= Diencephalon
HYPT=hypothalamus-diencephalic in origin
IC=Internal capsule-white matter, projection neurons
-Fiber tracts between thalamus and cerebrum, basal ganglia and thalamus, and the
cerebrum and the lower CNS
-Directly superior to the Thalamus in the picture are the lateral ventricles, which contain
the choroid plexus and CSF. In between the two halves of the thalamus is the third
ventricle, which also contains choroid plexus.
-Directly superior to the lateral ventricles is the corpus collosum, which consists of
commissural fibers connecting like areas of the two sides of the cerebrum
Cerebellum
-Outgrowth of the Pons in the brainstem
-Convoluted with sulci and gyri
-Does this to increase the surface area for a greater number of cell bodies
and greater processing capabilities
-Similar organization to the cerebrum
-Cortex on the outside
-White matter in the middle
-Deep ganglia on the inside
-Divided into three lobes
-Anterior
-Flocculonodular
- Smallest and Oldest lobe
-Vestibular function
-Receives innervation from the inner ear, special sense of position
in relation to gravity; helps coordinate body movements
-Visible on the inferior view of the cerebellum. It is sandwhiched
in between the anterior and posterior lobes
-Posterior
-Lobes divided into hemispheres by structure known as vermis (worm like)
-Vermal region of cerebellum (large midline notch)
-Has components of all three lobes
-Region tends to relate to proprioception and coordination of muscular
activity of the trunk
-Lesion here, or in paravermal region, person cannot carry out the
movement in a smooth, coordinated and integrated way
-Paravermal region
-Coordination of proximal limbs.
-Generally, along with the vermal region, coordinates activity as it is
happening
-Lateral Region
-Largest of the regions
-Involved in the planning of movements
-Involved in the planning of sequence
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-Lesion here will make the person not be able to plan movements (the
coordination, timing, and sequence of a movement). If you cannot plan the
movement, the movement does not tend to happen because you cannot
initiate it.
-Cerebellar Cortex receive and process sensory, proprioception, and positional
information from other areas of the CNS (such as fibers from CN VIIIvestibulocochlear), send this information through its axons (white matter middle
of the cerebellum) to deep cerebellar nuclei
-From their the fibers project out of the cerebellum and into appropriate areas of
the CNS
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Cranial Nerves
-PMJ=Ponto-medullary junction
-Pre-OS=Pre-olivary sulcus
-Post-OS= Postolivary sulcus
-Pyramids-2 sides of pyramids located on either side of midline. Also known as
ridges
-Olive is a medulla structure
CNS- Cranial “Nerves”
-CNI-Olfactory nerve-found in nasal mucosa, axons project to the olfactory bulb
which connects to the olfactory tract. Bulb and tract are CNS structures. The
axons project through the cribiform plate to get into the cranial cavity.
-CNII-Optic Nerve-Incorrectly known as a nerve. Not part of the PNS. Optic
nerve are the axons of ganglion cells whose axons project from the retina to from
the optic nerve and are intrinsically part of the CNS. From there the axons project
directly into the thalamus (through chiasm). Signal from there then projects to
appropriate parts of the cerebrum
CNIII-XII emerge from areas of the brainstem and are correctly named nerves
-CNIII-Oculomotor- Comes out of midbrain region
-CNIV-Trochlear- Distinct amongst true cranial nerves that it emerges from the
posterior (dorsal) aspect of the brain stem, specifically the midbrain. All others
emerge from ventro-lateral aspects of the brain stem
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-CNV-Trigeminal-Sensory and motor root emerges from the anterior aspect of the
pons
-CNVI(Abducens), CNVII(Facial), CNVIII(Vestibulocochlear) all emerge from
the pontomedullary junction
-CNIX(Glossopharyngeal) and CNX (Vagus) and the cranial portion of
CNXI(Spinal Accessory)emerge from the postolivary sulcus
-CNXII(Hypoglossal)-emerges from the preolivary sulcus
-CN Nuclei generally located deep within the part of the brain stem it emerges
from
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