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The basic organization of the nervous
system
The divisions of the nervous system
The nervous system is composed of
• central nervous system
• the brain
• spinal cord
• peripheral nervous system
• somatic nervous system
- afferent nerves
- efferent nerves
• autonomic nervous system
- sympathicus
- parasympathicus
Dorsal and ventral root
The mixed nerve splits into the dorsal and the ventral root. The dorsal root
contains the sensory axons. The ventral root contains the motor axons.
The spinal cord
• White matter forms the
external part of the cord. It is
made up of myelinated
axons. These carry signals
up and down the axis of the
spinal cord, and make it
white.
• Gray matter forms the
butterfly shaped inner core
of the cord. It is made up of
neuron cell bodies, dendrites
and synapses. Here
neurotransmission takes
place.
White and gray matter
The spinal cord contains inputs and outputs (the spinal nerves) it has
ascending and descending axon tracts passing through it (some of which
form synaptic terminals) and it has collections of neurons (or nerve cell
bodies) that process information and perform functions
The reflex pathway
Fiber tracts of the spinal cord
Some properties of the spinal cord
•
Signals enter and leave the spinal cord via spinal nerves. Of
these pair exists for each spinal vertebra. They are mixed nerves
carrying sensory information to the spinal cord and motor information
from the spinal cord to the muscles.
•
The lower and upper limbs require many more sensory and
motor axons than the chest and abdomen. For this reason the spinal
cord is larger in the lumbar and cervical regions than in the thoracic
region.
•
The spinal cord ends at about L1 and the lumbar nerves
descend within a CSF filled sack down to their point of exit. This fluid
space caudal to the end of the spinal cord is called the lumbar cistern
and is the point that CSF can be sampled using lumbar puncture.
The central nervous system
The brain is composed of
• the brainstem
(rombencephalon &
mesencephalon)
• the cerebellum
• the diencephalon
• the telencephalon
(cerebrum)
Crossing fibers
The brainstem
•
consists of the medulla oblongata (caudal) the pons (middle) and the
midbrain (rostral).
•
connects to the spinal cord at its caudal end and to the diencephalon
at its rostral end.
•
like the spinal cord, the brainstem spinal cord contains inputs and
outputs, the cranial nerves. It has ascending and descending axon tracts
passing through it. Some of these form synaptic terminals. Furthermore, it has
collections of neurons that process information and perform functions.
Medulla oblongata
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•
consists of the medulla oblongata
(caudal) the pons (middle) and the
midbrain (rostral).
•
connects to the spinal cord at its
caudal end and to the diencephalon at its
rostral end.
•
like the spinal cord, the
brainstem spinal cord contains inputs and
outputs, the cranial nerves. It has
ascending and descending axon tracts
passing through it. Some of these form
synaptic terminals. Furthermore, it has
collections of neurons that process
information and perform functions.
The cranial nerves
•
spinal nerves carry sensory and motor information from the
level of the sacrum and coccyc (sacral and coccygeal nerves) to
the level of the neck (upper cervical nerves)
•
above this level, in the region of the head and face, 12
nerves enter the cranium rather than the spinal cord and are
termed cranial nerves
Pyramids the corticospinal tract fibers within the medulla form a distinct feature called
the pyramids, they decussate, which means that the axons within this fiber bundle cross
from the left side of the brain to the right side of the brain. The cortex on the right side of
the brain send and receives information from the left side of the body. I do not know why
this is the case but it means that ascending sensory information and descending motor
information has to deccusate (cross the midline) between the cerebral cortex and the
spinal cord.
Dorsal column nuclei – sensory axons that do not synapse in the spinal cord dorsal horn
ascend the spinal cord in the dorsal column nuclei and synapse in dorsal column nuclei
in the medulla
Cranial nerve nuclei – these are nuclei (collections of cell bodies) within the medulla that
receive sensory information from cranial nerves (like the dorsal horn of the spinal cord)
or are collections of motor neurons that send out information to muscles of the head and
face (like the ventral horn of the spinal cord).
The pons
•
is connected to the medulla caudally and to the midbrain rostrally.
•
Pons means “bridge” in Latin and it referes to the fact the ventral
surface of the pons looks like a bridge because of the massive numbers of fibers
(myelinated axons) that decussate (cross the midline) in this part of the
brainstem. These crossing fibers are axons which project from the pons to the
cerebellum.
•
The pons contains inputs and outputs (some cranial nerves) it has
ascending and descending axon tracts passing through it (some of which form
synaptic terminals) and it has collections of neurons (or nerve cell bodies) that
process information and perform functions.
The cerebellum
The cerebellum ("little brain") has convolutions similar to those of cerebral
cortex, only the folds are much smaller. Like the cerebrum, the cerebellum has
an outer cortex, an inner white matter, and deep nuclei below the white matter.
If we enlarge a single fold of cerebellum, or a folium, we can begin to see
the organization of cell types. The outermost layer of the cortex is called
the molecular layer, and is nearly cell-free. Instead it is occupied mostly
by axons and dendrites. The layer below that is a monolayer of large cells
called Purkinje cells, central players in the circuitry of the cerebellum.
Below the Purkinje cells is a dense layer of tiny neurons called granule
cells. Finally, in the center of each folium is the white matter, all of the
axons traveling into and out of the folia. These cell types are hooked
together in stereotypical ways throughout the cerebellum.
The mesencephalon (midbrain)
The upper part of the brainstem is called the midbrain
It also contains inputs and outputs (some cranial nerves) it has
ascending and descending axon tracts passing through it (some of which form
synaptic terminals) and it has collections of neurons (or nerve cell bodies) that
process information and perform functions.
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The most notable anatomical features of the midbrain are:
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Cerebral peduncles – contain the descending corticospinal fibers
Medial lemniscus – contain the ascending sensory fibers
Superior (and inferior) colliculus – contain visual orientation centers
Substantia nigra – very important nuclei for motor control
The diencephalon
The rostral part of the brainstem merges with the diencephalon.
thalamus – the part of the brain that recieves synapses from all
ascending sensory innervation and then sends a
projection to the appropriate part of the cerebral cortex.
hypothalamus - the command center for homeostasis within the
brain. controlling blood pressure, body temperature, blood
glucose and all
the
‘housekeeping’
functions of the
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body.
Only one cranial nerve enters at the level of the diencephalon and that is the
optic nerve carrying visual sensory information.
Ascending and descending information within the diencephalon travels in
axon tracts within the internal capsule.
The telencephalon
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Cerebrum
The cerebrum is the most
rostral part of the
central
nervous
system. It is compose
of the
Cerebral cortex
Basal ganglia
The cerebral cortex
Cortex - the outer coating of what we think of as the brain. It is in many
animals convoluted into ridges (gyri) and folds (sulci).
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The cerebral lobes
The cerebral cortex is organized
into territories with different
functions called lobes.
• The frontal lobe sends motor
commands to the brainstem and
spinal cord.
• The parietal lobe receives
somato-sensory information (pain
and temperature, touch and
pressure)
• The occipital lobe is the site of
receipt of visual information.
• The temporal lobe is site of
receipt of auditory information.
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The basal ganglia
Generally a ganglion is a collection of cell
bodies outside the central nervous system.
Not here:he basal ganglia are a collection of
nuclei deep to the white matter of cerebral
cortex.
• The name includes: caudate, putamen,
nucleus accumbens, globus pallidus,
substantia nigra, subthalamic nucleus.
• In principle the claustrum and the
amygdala are part of this collection.
However they do not really deal with
movement, nor are they interconnected with
the rest of the basal ganglia, so they have
been dropped from this section.
• Obsolete, but are still encountered: the striatum (caudate + putamen + nucleus
accumbens), the corpus striatum (striatum + globus pallidus), or the lenticular nucleus
(putamen + globus pallidus).
A first look at systems interaction
The basal ganglia and cerebellum are large collections of nuclei that
modify movement on a minute-to-minute basis. Motor cortex sends
information to both, and both structures send information right back
to cortex via the thalamus. (Remember, to get to cortex you must go
through thalamus.) The output of the cerebellum is excitatory, while
the basal ganglia are inhibitory. The balance between these two
systems allows for smooth, coordinated movement, and a
disturbance in either system will show up as movement disorders.