1. WHAT IS THE GROSS ANATOMY OF THE BRAIN?
2. WHAT IS THE MICROANATOMY OF THE BRAIN?
3. WHAT ARE THE FUNCTIONS OF THE DIFFERENT PARTS OF THE BRAIN?
RESEARCH
LEARNING GOAL 1: GROSS ANATOMY OF THE BRAIN
DEVELOPMENT OF THE BRAIN
The brain and spinal cord begin as an
embryonic structure called the neural tube.
As soon as the neural tube forms, its
posterior (caudal) part becomes the spinal
cord and its anterior (rostral) end begins to
expand, and constrictions appear that mark
off the three primary brain vesicles
The primary vesicles give rise to the
secondary brain vesicles:
- Prosencephalon (forebrain) → telencephalon and diencephalon
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain) → metencephalon and myelencephalon
Each of the 5 secondary vesicles then develop to produce the major structures of the adult
brain.
The central cavity of the neural tube remains continuous and enlarges in 4 areas to form the
fluid-filled ventricles.
Because the brain grows more rapidly than the membranous skull, it folds up. Also, during
development the midbrain and cervical flexures move the forebrain toward the brain stem.
This causes the cerebral hemispheres to grow posteriorly and laterally. Ultimately, the
cerebral hemispheres fold into convolutions, increasing the surface area.
GRAY AND WHITE MATTER
The basic pattern of the CNS is a central cavity surrounded by gray matter (mostly neuron
cell bodies), external to which is white matter (myelinated fiber tracts). The spinal cord
exhibits this basic pattern, but the brain has additional regions of gray matter not present in the spinal cord.
Both the cerebral hemispheres and the cerebellum have an outer layer of gray matter called a cortex. This
pattern changes with descent through the brain stem—the cortex disappears but scattered gray matter nuclei
are seen within the white matter. At the caudal end of the brain stem, the basic pattern is evident.
,VENTRICLES
The brain ventricles are continuous
with one another and with the central
canal of the spinal cord. The hollow
ventricular chambers are filled with
cerebrospinal fluid and lined by
ependymal cells, which forms a
permeable barrier between the CSF
and the cells of the CNS. The CSF
buffers shocks, mediates buoyancy,
rids waste products, and plays a part in
homeostasis in the CNS. The CSF is
produced by the choroid plexuses.
The paired lateral ventricles are anteriorly close together, separated only by a thin median membrane called the
septum pellucidum.
Each lateral ventricle communicates with the narrow third ventricle in the diencephalon via the interventricular
foramen.
The third ventricle is continuous with the fourth ventricle via the cerebral aqueduct that runs through the
midbrain. The fourth ventricle lies in the hindbrain dorsal to the pons and superior medulla. It is continuous with
the central canal of the spinal cord inferiorly. Three openings mark the walls of the fourth ventricle: the paired
lateral apertures in its side walls and the median aperture in its roof. These apertures connect the ventricles to
the subarachnoid space: a fluid-filled space surrounding the brain.
TELENCEPHALON: CEREBRAL HEMISPHERES
The cerebral hemispheres are the superior part of the
brain (83% total brain mass). It has elevated ridges of
tissue called gyri, sing. gyrus. It is separated by
shallow grooves called sulci, sing. sulcus. Deeper
grooves separate large regions of the brain and are
called fissures.
The more prominent gyri and sulci are important
anatomical landmarks. The median longitudinal
fissure separates the cerebral hemispheres. Another
large fissure, the transverse cerebral fissure,
separates the cerebral hemispheres from the
cerebellum below.
There are 5 lobes: frontal, parietal, temporal, occipital
lobes, and the insula. These are separated by sulci.
The insula is buried deep within the lateral sulcus.
Each cerebral hemisphere has 3 basic regions:
- Superficial cerebral cortex of gray matter
- Internal white matter
- Basal nuclei, islands of gray matter deep within the white matter
, Cerebral cortex
The cortex enables us to be aware of ourselves and our
sensations, to communicate, remember, understand, and
initiate involuntary movements. Some functions may center
in a specific cortical area, domain, but higher mental
functions as language and memory are spread out.
The cerebral cortex is composed of gray matter: neuron cell
bodies, dendrites, associated glia, and blood vessels, but no
fiber tracts. Its many convolutions triple its surface area.
4 generalizations of the cerebral cortex
- It contains 3 kinds of functional areas: motor areas,
sensory areas, and association areas. All neurons in the cortex are interneurons
- Each hemisphere is concerned with the sensory and motor functions of the contralateral side of the body
- The two hemispheres are not equal in functions
- No functional area of the cortex acts alone
Cerebral white matter
The white matter is
responsible for
communication between
cerebral areas and between
the cortex and lower CNS
centers. It consists largely of
myelinated fibers bundled
into large tracts. These fibers
and tracts are classified
according to the direction in
which they run as association,
commissural, or projection.
▪ Association fibers connect different parts of the same hemisphere. Short association fibers connect
adjacent gyri. Long association fibers are bundled into tracts and connect different cortical lobes.
▪ Commissural fibers connect corresponding gray areas of the two hemispheres. These commissures allow
the two hemispheres to function as a coordinated whole. The largest commissure is the corpus
callosum, which lies superior to the lateral ventricles, deep within the longitudinal fissure. Less
prominent examples are the anterior and posterior commissures.
▪ Projection fibers either enter the cerebral cortex from lower brain or cord centers or descend from the
cortex to lower areas. Sensory information reaches the cerebral cortex and motor output leaves it
through these projection fibers. They tie the cortex to the rest of the nervous system and to the body’s
receptors and effectors.
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