Download Gross Organization I

Gross Organization I
The Brain
Reading:
BCP Chapter 7
Layout of the Nervous System
Central Nervous System (CNS)
• Located inside of bone
• Includes the brain (in the skull) and
the spinal cord (in the backbone)
• Interprets sensory input, initiates
movement, and mediates complex
cognitive processes
Peripheral Nervous System (PNS)
• Located outside of bone
• Includes nerves
• Serves to bring sensory information
into the CNS (called afferents) and
carry motor signals out from the
CNS (efferents)
Major Parts of the Nervous System
External
environment
Sensory
input
A “system of twos”
Motor
output
Internal
environment
Anatomical References
All vertebrates (including humans) have
the same basic body plan — they are
strictly bilaterally symmetrical in early
embryonic stages and largely bilaterally
symmetrical in adulthood. Thus, there are
standard anatomical terms of location for
all vertebrates (and many invertebrates).
Positional descriptive terms are with
respect to the organism in its standard
anatomical position:
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anterior/rostral – towards the front/nose
posterior/caudal – towards the back/tail
dorsal – towards the top
ventral – towards the bottom
medial – towards the middle
lateral – towards the side
Structures on the same side of the head
are said to be ipsilateral; if on opposite
sides of the head, then contralateral.
Anatomical Directions in Humans
In humans, the directions in the cerebral hemispheres are rotated by 90°in
comparison to those in the spinal cord (and brain stem) because of the unusual
upright posture of humans. Thus, for example, the top of the head and the back of
the body are both dorsal even though the directions are different.
Planes of Section
To view the internal structures of
the brain, it is usually necessary
to slice it up. A slice is called a
section, and there are three
standard perpendicular planes of
section:
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sagittal – left vs right (a midsagittal
cut separates the left and right
halves of the brain)
horizontal – top vs. bottom (parallel
to the ground)
frontal (coronal) – front vs back
A cross-section is cut at a right
angle to a long narrow structure
(e.g., the spinal cord)
The Brain
The brain weighs about 3 lbs.
Visual inspection (lateral view)
reveals three parts that are
common to all mammals:
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the cerebrum – top-most, split
into two cerebral hemispheres
that each receive sensory input
from and control motor output to
the contralateral side of the body.
the cerebellum – behind/below
the cerebrum, primarily a motor
control center, two hemispheres
each concerned with movement
of the ipsilateral side of body.
the brain stem – forms the stalk
from which the cerebrum and
cerebellum sprout, fibers of
passage, cranial nerves, basic
functions (e.g., breathe rate).
cerebrum
brain stem
cerebellum
The Cerebrum 1
The cerebrum is noteworthy for its
convoluted surface (to allow more
cortical surface area to exist in the
confines of a smaller cranium):
• fissures: large grooves
• sulci: small grooves
• gyri: bumps
Three major fissures (longitudinal,
central and lateral) partially divide each
hemisphere into four lobes: frontal;
parietal; temporal and occipital
Parietal
Frontal
Three large gyri: precentral; postcentral
and superior temporal
Lobes are not functional units
Occipital
Temporal
The Cerebrum 2
The cerebral hemispheres are
connected by several fiber
tracts called commissures.
The largest commissure is
called the corpus callosum. It is
visible if the dorsal surfaces of
the two hemispheres are gently
pulled apart at the longitudinal
fissure. The medial view (midsagittal cut) of the brain shows
the callosum in cross section.
Dorsal view
Medial view
The Cerebrum 3
A frontal section of the cerebrum
reveals two additional features of
the gross organization of the
brain.
First, the cerebrum comprises
three different areas:
• outer area of neuronal cell
bodies (called gray matter or
cerebral cortex)
• inner area of myelinated
axons (called white matter)
• subcortical areas of gray
matter
Second, the brain contains fluidfilled caverns and canals, called
the ventricular system. The
cerebrum surrounds the paired,
lateral ventricles.
The Cerebellum
The cerebellum (Latin for “little brain”), like the cerebrum, is a highly folded
structure consisting of two hemispheres, each of which is divided into lobes.
Each ridge or gyrus is called a folium, with gray matter at the edge and white
matter inside.
Medial view
Fourth ventricle
The Brain Stem 1
The brain stem consists of four
divisions:
• diencephalon (either side of
third ventricle)
-- thalamus
-- hypothalamus
• midbrain (cerebral aqueduct)
-- tectum
-- tegmentum
• pons (below 4th ventricle)
• medulla (below 4th ventricle)
The medulla is continuous with
the spinal cord.
Cerebral aqueduct
Fourth ventricle
The Brain Stem 2
The underside of the brain
shows the ventral aspects of
the brain stem including the
hypothalamus (diencephalon),
tegmentum (midbrain), pons
and medulla (hindbrain).
In addition, 12 pairs of cranial
nerves can be observed, most
of which emerge from the brain
stem. The cranial nerves
provide sensory and motor
innervation mainly to structures
in the head and neck. Note that
the axons from the eyes (optic
nerve) cross (or decussate) as
necessary at the optic chiasm
prior to entering the brain.
Imaging the Brain 1
The gross structural organization
of the brain, can now be obtained
in the living brain.
MRI series (top to bottom of brain)
The are two main methodologies
to detect the structure (and
changes to it) in the living brain:
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computer tomography (CT)
-- measures opacity to X-rays
magnetic resonance imaging (MRI)
-- measures hydrogen atom
response to magnetic fields
diffusion tensor imaging (DTI)
used to visualize large bundles of axons
Imaging the Brain 2
Imaging methods can also be used to reveal “functional” organization in the living
brain. Such techniques are based on the premise that cerebral blood flood and
neuronal activation are tightly coupled. Two techniques are in widespread use.
Positron emission tomography (PET)
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inject a radioactive substance
have subject perform behavior (active cells take
up substance)
scan a horizontal slice of brain
Functional magnetic resonance
imaging (fMRI)
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Paired
image
subtraction
no substance injected
have subject perform behavior (active
cells need more oxygen/glucose)
scan brain for de/oxygenated hemoglobin