Portage Learning BIO 152 The Brain
The Brain: Introduction and Protective structures
The two main divisions of the central nervous system are the brain and spinal cord. The
human brain consists of the following divisions: the cerebrum, brainstem, diencephalon,
and the cerebellum (Figure 2.1).
Figure 2.1 Cross section of the human brain. The cerebrum, diencephalon (blue), brainstem, and
cerebellum.
The protective structures of the brain are important because neurons are not
replaceable. Nervous tissue is damaged by even the slightest pressure. The brain is
protected by the skull of the skeletal system, meninges, a cushion of cerebrospinal fluid,
and the blood-brain barrier. There are four ventricles inside of the brain. Ventricles are
cavities within the brain that are filled with cerebrospinal fluid (CSF). The ventricles
include the two lateral ventricles, third ventricle, and fourth ventricle (Figure 2.2). The
fourth ventricle drains into the central canal of the spinal cord, which also contains CSF.
Figure 2.2 Cross section of the brain with ventricles highlighted in blue. There are two lateral
ventricles, one third ventricle and one fourth ventricle, which drains CSF into the central canal of the
spinal cord.
The meninges are three layers of connective tissue membranes that cover and protect
central nervous system organs and enclose cerebrospinal fluid. The leathery dura
,Portage Learning BIO 152 The Brain
mater is the double-layered outer meninx. The middle arachnoid meninx is a loose
layer separated from the dura mater by the subdural space. Beneath the arachnoid
meninx is the subarachnoid space, which contains blood vessels and is filled with
cerebrospinal fluid. The inner pia mater meninx is a thin connective tissue tightly
attached to the brain.
Cerebrospinal fluid (CSF) is a special fluid formed in the walls of the ventricles from
blood plasma by permeating through the choroid plexus. The choroid plexus is formed
from a network of blood vessels within the ventricles. CSF circulates through the
ventricles and into the meninges. It cushions the brain and spinal cord by providing
buoyancy, allowing the brain to float. Once formed, the CSF circulates through the
ventricles and into the subarachnoid space, bathing the brain in fluid.
The blood-brain barrier is a diffusion barrier that prevents most particles from entering
the central nervous system tissue, keeping the brain and spinal cord separate from
general blood circulation (Figure 2.3). The blood-brain barrier is formed by the relatively
impermeable brain capillaries, due to the glial cells astrocytes. Maintenance of the
blood-brain-barrier is important to provide a stable chemical environment for the
nervous system. A stable internal environment is important to protect neurons from
chemical variations that could cause uncontrollable firing of neurons.
Figure 2.3 Astrocytes help to form the blood-brain barrier by wrapping tightly around capillaries of the
central nervous system, preventing harmful substances from harming neurons.
The Cerebrum
The cerebrum, the foremost part of the brain, is the largest part of the brain in humans
comprising about 83% of total brain mass. It consists of two large masses called the left
and right cerebral hemispheres (Figure 2.4). The median longitudinal
fissure (Figure 2.4) separates the left and right cerebral hemispheres from one
another. The brain is highly specialized in its control over body function. The left
cerebral hemisphere controls the right side of the body, and the right cerebral
hemisphere controls the left side of the body.
,Portage Learning BIO 152 The Brain
Figure 2.4 Superior view of the left and right cerebral hemispheres, divided by the median
longitudinal fissure. Deep grooves are called sulci, and ridges are called gyri in the cerebral cortex.
The cerebral cortex is marked by raised ridges of tissue called gyri separated by
shallow grooves called sulci. The deeper grooves, called fissures, separate large
regions of the brain. The transverse fissure separates the cerebrum from the
cerebellum (Figure 2.5).
, Portage Learning BIO 152 The Brain
Figure 2.5 Posterior view of the brain: The cerebrum (blue) is separated from the cerebellum by the
transverse fissure.
The cerebral hemispheres are connected by a bridge of white nerve fibers called
the corpus callosum (Figure 2.6). The outer portion of the cerebral hemispheres,
the cerebral cortex, (Figure 2.6) is highly convoluted and gray in color.
Figure 2.6 The cerebral cortex is highly ridged with deep grooves to increase the surface area. The
corpus callosum (blue) connects the left and right cerebral hemispheres.
Some of the deeper sulci divide each hemisphere into four surface lobes: frontal,
parietal, temporal, and occipital (Figure 2.7). Different functions are associated with
each lobe. The frontal lobe controls higher level executive functions, such as reasoning
and decision making. The frontal lobe also controls motor functions and permits control
over voluntary muscle actions. The parietal lobe receives sensory information from
receptors in the mouth for taste and located in the skin, such as those for touch,
pressure, and pain. The occipital lobe interprets visual input. The temporal lobe has
sensory areas for hearing and smelling.
Certain areas of the cerebral cortex have been mapped in detail. Primary
areas (Figure 2.7) in each lobe receive or send information for one type of sensory or
motor information. Association areas (Figure 2.7) act mainly to integrate more than
one type of sensory information for purposeful action. For example, a portion of the
frontal lobe is called the primary motor cortex. The primary motor cortex (also called
the precentral gyrus or the somatomotor cortex) controls voluntary motor
movements for all parts of the body (Figure 2.7). The parietal lobe receives sensory
information from the same parts of the body in the primary sensory cortex (also called
the postcentral gyrus or the somatosensory cortex). Each of the four lobes of the
cerebral cortex contains an association area (Figure 2.7), which receives information
from the other lobes and integrates it into higher, more complex levels of
consciousness. Some examples of higher levels of consciousness are intellect,
artistic/creative ability, learning, and memory.
The Brain: Introduction and Protective structures
The two main divisions of the central nervous system are the brain and spinal cord. The
human brain consists of the following divisions: the cerebrum, brainstem, diencephalon,
and the cerebellum (Figure 2.1).
Figure 2.1 Cross section of the human brain. The cerebrum, diencephalon (blue), brainstem, and
cerebellum.
The protective structures of the brain are important because neurons are not
replaceable. Nervous tissue is damaged by even the slightest pressure. The brain is
protected by the skull of the skeletal system, meninges, a cushion of cerebrospinal fluid,
and the blood-brain barrier. There are four ventricles inside of the brain. Ventricles are
cavities within the brain that are filled with cerebrospinal fluid (CSF). The ventricles
include the two lateral ventricles, third ventricle, and fourth ventricle (Figure 2.2). The
fourth ventricle drains into the central canal of the spinal cord, which also contains CSF.
Figure 2.2 Cross section of the brain with ventricles highlighted in blue. There are two lateral
ventricles, one third ventricle and one fourth ventricle, which drains CSF into the central canal of the
spinal cord.
The meninges are three layers of connective tissue membranes that cover and protect
central nervous system organs and enclose cerebrospinal fluid. The leathery dura
,Portage Learning BIO 152 The Brain
mater is the double-layered outer meninx. The middle arachnoid meninx is a loose
layer separated from the dura mater by the subdural space. Beneath the arachnoid
meninx is the subarachnoid space, which contains blood vessels and is filled with
cerebrospinal fluid. The inner pia mater meninx is a thin connective tissue tightly
attached to the brain.
Cerebrospinal fluid (CSF) is a special fluid formed in the walls of the ventricles from
blood plasma by permeating through the choroid plexus. The choroid plexus is formed
from a network of blood vessels within the ventricles. CSF circulates through the
ventricles and into the meninges. It cushions the brain and spinal cord by providing
buoyancy, allowing the brain to float. Once formed, the CSF circulates through the
ventricles and into the subarachnoid space, bathing the brain in fluid.
The blood-brain barrier is a diffusion barrier that prevents most particles from entering
the central nervous system tissue, keeping the brain and spinal cord separate from
general blood circulation (Figure 2.3). The blood-brain barrier is formed by the relatively
impermeable brain capillaries, due to the glial cells astrocytes. Maintenance of the
blood-brain-barrier is important to provide a stable chemical environment for the
nervous system. A stable internal environment is important to protect neurons from
chemical variations that could cause uncontrollable firing of neurons.
Figure 2.3 Astrocytes help to form the blood-brain barrier by wrapping tightly around capillaries of the
central nervous system, preventing harmful substances from harming neurons.
The Cerebrum
The cerebrum, the foremost part of the brain, is the largest part of the brain in humans
comprising about 83% of total brain mass. It consists of two large masses called the left
and right cerebral hemispheres (Figure 2.4). The median longitudinal
fissure (Figure 2.4) separates the left and right cerebral hemispheres from one
another. The brain is highly specialized in its control over body function. The left
cerebral hemisphere controls the right side of the body, and the right cerebral
hemisphere controls the left side of the body.
,Portage Learning BIO 152 The Brain
Figure 2.4 Superior view of the left and right cerebral hemispheres, divided by the median
longitudinal fissure. Deep grooves are called sulci, and ridges are called gyri in the cerebral cortex.
The cerebral cortex is marked by raised ridges of tissue called gyri separated by
shallow grooves called sulci. The deeper grooves, called fissures, separate large
regions of the brain. The transverse fissure separates the cerebrum from the
cerebellum (Figure 2.5).
, Portage Learning BIO 152 The Brain
Figure 2.5 Posterior view of the brain: The cerebrum (blue) is separated from the cerebellum by the
transverse fissure.
The cerebral hemispheres are connected by a bridge of white nerve fibers called
the corpus callosum (Figure 2.6). The outer portion of the cerebral hemispheres,
the cerebral cortex, (Figure 2.6) is highly convoluted and gray in color.
Figure 2.6 The cerebral cortex is highly ridged with deep grooves to increase the surface area. The
corpus callosum (blue) connects the left and right cerebral hemispheres.
Some of the deeper sulci divide each hemisphere into four surface lobes: frontal,
parietal, temporal, and occipital (Figure 2.7). Different functions are associated with
each lobe. The frontal lobe controls higher level executive functions, such as reasoning
and decision making. The frontal lobe also controls motor functions and permits control
over voluntary muscle actions. The parietal lobe receives sensory information from
receptors in the mouth for taste and located in the skin, such as those for touch,
pressure, and pain. The occipital lobe interprets visual input. The temporal lobe has
sensory areas for hearing and smelling.
Certain areas of the cerebral cortex have been mapped in detail. Primary
areas (Figure 2.7) in each lobe receive or send information for one type of sensory or
motor information. Association areas (Figure 2.7) act mainly to integrate more than
one type of sensory information for purposeful action. For example, a portion of the
frontal lobe is called the primary motor cortex. The primary motor cortex (also called
the precentral gyrus or the somatomotor cortex) controls voluntary motor
movements for all parts of the body (Figure 2.7). The parietal lobe receives sensory
information from the same parts of the body in the primary sensory cortex (also called
the postcentral gyrus or the somatosensory cortex). Each of the four lobes of the
cerebral cortex contains an association area (Figure 2.7), which receives information
from the other lobes and integrates it into higher, more complex levels of
consciousness. Some examples of higher levels of consciousness are intellect,
artistic/creative ability, learning, and memory.
