Download Netter`s Atlas of Neuroscience - 9780323265119 | US Elsevier

4
Overview of the Nervous System
Dendrites
Dendritic spines
(gemmules)
Rough endoplasmic
reticulum (Nissl substance)
Ribosomes
Mitochondrion
Nucleus
Axon
Nucleolus
Axon hillock
Initial segment of axon
Neurotubules
PR
SA O
M PE
PL R
E TY
C O
O F
N E
TE L
N SE
T V
- N IE
O R
T
FI
N
AL
Golgi apparatus
Lysosome
Cell body (soma)
Axosomatic synapse
Glial (astrocyte) process
Axodendritic synapse
1.1 Brain Imaging: MRI (Magnetic
Resonance Imaging)­—Coronal and
Horizontal T1—Weighted Images
Neuronal structure reflects the functional characteristics of the
individual neuron. Incoming information projects to a neuron
mainly through axonal terminations on the cell body and dendrites. These synapses are isolated and protected by astrocytic
processes. The dendrites usually provide the greatest surface area
of the neuron. Some protrusions from dendritic branches (dendritic spines) are sites of specific axo-dendritic synapses. Each
specific neuronal type has a characteristic dendritic branching
pattern, called the dendritic tree, or dendritic arborizations. The
neuronal cell body varies from a few micrometers (m) in diamater to more than 100 m. The neuronal cytoplasm contains
extensive rough endoplasmic reticulum (rough ER), reflecting
the massive amount of protein synthesis necessary to maintain
the neuron and its processes. The Golgi apparatus is involved in
packaging potential signal molecules for transport and release.
Large numbers of mitochondria are necessary to meet the huge
energy demands of neurons, particularly related to maintenance
of ion pumps and membrane potentials. Each neuron has a single
(or occasionally no) axon. The cell body tapers to the axon at
the axon hillock, followed by the initial segment of the axon,
containing the Na+ channels, the first site where action potentials are initiated,. The axon extends for a variable (up to one
meter or more) distance from the cell body, and if greater than
1-2 m in diameter are insulated by sheaths of myelin provided by
oligodendroglia in the CNS or Schwann cells in the PNS. An
axon may branch into more than 500,000 axon terminals, and
may terminate in either a highly localized and circumscribed
zone (e.g. primary somatosensory axon projections for fine discriminative touch), or may branch to many disparate regions
of the brain (e.g. noradrenergic axonal projections of the locus
coeruleus). Neurons whose axons terminate at a distance from
its cell body and dendritic tree are called macroneurons or Golgi
type I neurons, and neurons whose axons terminate locally, close
to its cell body and dendritic tree are called microneurons, Golgi
type II neurons, local circuit neurons, or interneurons. There is
no “typical” neuron, as each type of neuron has its own specialization. However, pyramidal cells or lower motor neurons often
are used to portray the “typical” neuron.
CLINICAL POINT
Neurons require extraordinary metabolic resources to sustain their
functional integrity, particularly related to the maintenance of membrane potentials for the initiation and propagation of action potentials. Neurons require aerobic metabolism for the generation of ATP,
and have virtually no ATP reserve. This requires continuous delivery
of glucose and oxygen to the brain, generally in the range of 15–20%
of the body’s resources, a disproportional consumption of resources.
During starvation, when glucose availability is limited, the brain can
shift gradually to use of beta-hydroxybutyrate and acetoacetate as an
energy source for neuronal metabolism; however, this is not an acute
process and is not available to buffer acute hypoglycemic episodes. An
ischemic episode of even 5 minutes, from a heart attack or an ischemic
stroke, can lead to permanent damage in some neuronal populations,
such as pyramidal cells in the CA1 region of the hippocampus, and
with longer ischemia, widespread neuronal death. Because neurons
are postmitotic cells, except for a small subset of interneurons, dead
neurons are not replaced. One additional consequence of the post-
Advance Sample Chapter -- NOT FINAL PRODUCT
Neurons and Their Properties
13
Schematic of synaptic endings
Dendrite
Axon il
Axon
Initia hillock
segme
Initial segment
Node
Dendrites
Axon
Myelin sheath
of
presynaptic
neurons
ermi knobs)
ating
Numerous
boutons
(synaptic
a m to n neurons
ron an terminating
ts dendri e
ofnpresynaptic
on a motor neuron and its dendrites
Neurofilaments
PR
SA O
M PE
PL R
E TY
C O
O F
N E
TE L
N SE
T V
- N IE
O R
T
FI
N
AL
Enlarged section of bouton
Neurotubules
Axon (axoplasm)
Axolemma
Mitochondria
Glial process
Synaptic vesicles
Synaptic cleft
Presynaptic membrane
(densely staining)
Postsynaptic membrane
(densely staining)
Postsynaptic cell
1.9 Synaptic Morphology
Synapses are specialized sites where neurons communicate with
each other and with effector or target cells. The upper figure shows
a typical neuron that receives numerous synaptic contacts on its
cell body and associated dendrites, derived from both myelinated and unmyelinated axons. Incoming myelinated axons lose
their myelin sheaths, exhibit extensive branching, and terminate
as synaptic boutons (terminals) on the target (in this example,
motor) neuron. The lower figure shows an enlargement of an
axo-somatic terminal. Chemical neurotransmitters are packaged
in synaptic vesicles. When an action potential invades the terminal region, depolarization triggers Ca2+ influx into the terminal,
causing numerous synaptic vesicles to fuse with the presynaptic
membrane, releasing their packets of neurotransmitter into the
synaptic cleft. The neurotransmitter can bind to receptors on
the postsynaptic membrane, resulting in graded excitatory or
inhibitory postsynaptic potentials, or in neuromodulatory effects
on intracellular signaling systems in the target cell. There is sometimes a mismatch between the site of release of a neurotransmitter and the location of target neurons possessing receptors for the
neurotransmitter (can be immediately adjacent, or at a distance).
Many nerve terminals can release multiple neurotransmitters,
regulated by gene activation and by the frequency and duration
of axonal activity. Some nerve terminals possess presynaptic
receptors for their released neurotransmitter(s). Activation of
these presynaptic receptors regulates neurotransmitter release.
Some nerve terminals also possess high-affinity uptake carriers
for transport of the neurotransmitter (e.g. dopamine, norepinephrine, serotonin) back into the nerve terminal for repackaging and reuse.
Advance Sample Chapter -- NOT FINAL PRODUCT
72
Overview of the Nervous System
Origin and Spread of Seizures
Normal firing pattern of cortical neurons
Thalamus
E
–
–
+
P
P
+
Recurrent
inhibitory
circuit
Recurr
Single stimulus
I
+
P
+
E+
+
P
+
+
+
–
–
–
Recurrent
excitatory
circuit
Recurr
PR
SA O
M PE
PL R
E TY
C O
O F
N E
TE L
N SE
T V
- N IE
O R
T
FI
N
AL
–
+
+
i
–
Ceretral
cortex
–
–
P
+
+
Action potentials
(nonsynchronous)
ction p (E)
en
Normal activation of cortical neurons (P) modulated by excitatory
E
and inhibitory (l) feedback circuits
E
Substantia nigra
Corpus striatum
Excitatory pathways between cerebral cortex and thalamus
u r at
modulated by tonic midbrain inhibitory stimuli
Epileptic firing pattern of cortical neurons
–
+
P
Depolarization ↑ field potential
–
E
+
+
+
Depressed
inhibition
P
+
+
Cortex
+
+
+
High frequency
I
+
Repetitive stimuli
–
–
Depolarization ↑ extracellular K+
–
P +
P
+
+
+
E
+
Thalamus
Substantia nigra
Corpus striatum
+
E
+ –
–
Increased
excitation
P +
–
Burst firing action potentials
(hypersynchronous)
Repetitive cortical activation potentiates excitatory transmission and depresses
inhibitory transmission, creating self-perpetuating excitatory circuit (burst) and
facilitating excitation (recruitment) of neighboring neurons.
2.2 Normal Electrical Firing Patterns
of Cortical Neurons and the Origin
and Spread of Seizures
The collective electrical activity of the cerebral cortex can be
monitored by electroencephalography. Normal cortical electrical activity reflects the summation of excitatory and inhibitory
actions, modulated through feedback circuits. Thalamic inputs
Cortical bursts to corpus striatum and thalamus block inhibitory
projections and create selfperpetuating feedback circuit.
th r r i w
ISBN
Autho
F g. # can driveDoelectrical
um #nt nam excitability; the
to the cortex
can
(if ne midbrain
ded)
F lt n
0OK
Cor
Initials
provide inhibitory
control
over
this
process.
Repetitive
cortical
Fi
D
D
5
activation can
inhibition,
enhance excitatory
feedback
-2/ dampen 022-NB86 eps
Date
I ti l
OK
ev o
Artist/ recruit
R
Dat excitatory
circuits, and
repetitive
in adjacent
CE's revi w
07/25/07 circuitry
4/C
2/C
xx
Bpu/df
Da
e
OK
Corr
Initials
cortical neurons.
These
self-perpetuating
excitatory
feedback
Che k f rev sion
G aphic
Wor
d
Illust
a
ion
Stud
o
• St L
circuits can
initiate
and
spread
seizure
activity.
4/C X
2/C
xxx
B W
Advance Sample Chapter -- NOT FINAL PRODUCT
74
Overview of the Nervous System
Cingulate gyrus
Precentral sulcus
Central (rolandic) sulcus
Cingulate sulcus
Paracentral lobule
Medial frontal gyrus
Corpus callosum
Sulcus of corpus callosum
Precuneus
Fornix
Superior sagittal sinus
Choroid plexus of
3rd ventricle
Septum pellucidum
Interventricular foramen
(of Monro)
Parietoccipital sulcus
Interthalamic adhesion
Anterior commissure
Hypothalamic sulcus
Subcallosal
(parolfactory) area
Paraterminal gyrus
Gyrus rectus
Lamina
terminalis
Optic recess
Optic chiasm
Tuber
cinereum
Mammillary body
PR
SA O
M PE
PL R
E TY
C O
O F
N E
TE L
N SE
T V
- N IE
O R
T
FI
N
AL
Thalamus
Habenular
commissure
Calcarine sulcus
Lingual gyrus
Calcarine cortex
(lower bank)
Pineal gland
Straight sinus
(in tentorium
cerebelli)
Great cerebral
vein (of Galen)
Posterior (epithalamic)
commissure
Superior and inferior
colliculi
Cerebellum
A P
Pituitary gland (anterior and posterior)
Midbrain
Pons
Medulla
oblongata
3.1 Anatomy of the Medial (Midsagittal)
Surface of the Brain In Situ
The entire extent of the neuraxis, from the spino-medullary
junction through the brain stem, diencephalon, and telencephalon, is visible in mid-sagittal section. The corpus callosum, a
major commissural fiber bundle interconnecting the two hemispheres, is a landmark separating the cerebral cortex above
from the thalamus, fornix, and subcortical forebrain below. The
ventricular system, including the interventricular foramen (of
Munro), the third ventricle (diencephalon), the cerebral aqueduct (midbrain), and the fourth ventricle (pons and medulla),
is visible in midsagittal view. This subarachnoid fluid system
provides internal and external protection to the brain and also
may serve as a fluid transport system for important regulatory molecules. The thalamus serves as a gateway to the cortex.
Stria medullaris
of thalamus
Cuneus
Calcarine cortex
(upper bank)
Cerebral aqueduct
(of Sylvius)
Superior medullary velum
4th ventricle and
choroid plexus
Inferior medullary velum
The hypothalamic proximity to the median eminence (tuber
cinereum) and the pituitary gland reflects the important role
of the hypothalamus in regulating neuroendocrine function. A
midsagittal view also reveals the midbrain colliculi, sometimes
called the visual (superior) and auditory (inferior), tecta.
CLINICAL POINT
The foramina of the skull are tightly confined openings for the
passage of nerves and blood vessels. Under normal circumstances,
there is enough room for comfortable passage of these structures
without traction or pressure. However, with the presence of a tumor
at a foramen, the passing structures can be compressed or damaged.
A tumor at the internal acoustic meatus can result in ipsilateral facial
and vestibuloacoustic nerve damage, and a tumor at the jugular foramen can result in damage to the glossopharyngeal, vagus, and spinal
accessory nerve.
Advance Sample Chapter -- NOT FINAL PRODUCT