Every
animal you can think of -- mammals, birds, reptiles, fish,
amphibians -- all have brains. But the human brain is unique.
It gives us the power to think, plan, speak, imagine... It is
truly an amazing organ.
The brain performs an incredible number of tasks:
It controls body temperature, blood pressure, heart rate
and breathing.
It accepts a flood of information about the world around
you from your various senses (eyes, ears, nose, etc.).
It handles physical motion when walking, talking,
standing or sitting.
It lets you think, dream, reason and experience
emotions.
All of these tasks are coordinated,
controlled and regulated by an organ that is about the size of
a small head of cauliflower: your brain.
Your brain, spinal cord and peripheral nerves make up a
complex, integrated information-processing and control system.
The scientific study of the brain and nervous system is called
neuroscience or neurobiology. Because the field
of neuroscience is so vast, and the brain and nervous system
so complex, this article will start at the beginning and give
you an overview of this amazing organ.
In this edition of HowStuffWorks,
we will examine the structures of the brain and what each one
does. With this general overview of the brain, you will be
able to understand concepts such as motor control, visual
processing, auditory processing, sensation, learning, memory
and emotions, which we will cover in detail in future
articles.
Neurons Your brain is made of approximately
100-billion nerve cells, called neurons. Neurons have
the amazing ability to gather and transmit electrochemical
signals -- they are something like the gates and wires in a computer.
Neurons share the same characteristics and have the same parts
as other cells, but
the electrochemical aspect lets them transmit signals over
long distances (up to several feet or a few meters) and pass
messages to each other.
Neurons have three basic parts:
Cell body - This main part has all of the
necessary components of the cell, such as the nucleus
(contains DNA), endoplasmic reticulum and ribosomes (for
building proteins) and mitochondria (for making energy). If
the cell body dies, the neuron dies.
Axon - This long, cable-like projection of the
cell carries the electrochemical message (nerve
impulse or action potential) along the length of
the cell.
Depending upon the type of neuron, axons can be
covered with a thin layer of myelin, like an
insulated electrical wire. Myelin is made of fat, and
it helps to speed transmission of a nerve impulse down a
long axon. Myelinated neurons are typically found in the
peripheral nerves (sensory and motor neurons), while
non-myelinated neurons are found in the brain and spinal
cord.
Dendrites or nerve endings - These small,
branch-like projections of the cell make connections to
other cells and allow the neuron to talk with other cells or
perceive the environment. Dendrites can be located on one or
both ends of the cell.
Neurons come in many sizes. For example, a single sensory
neuron from your fingertip has an axon that extends the length
of your arm, while neurons within the brain may extend only a
few millimeters. Neurons have different shapes depending on
what they do. Motor neurons that control muscle
contractions have a cell body on one end, a long axon in the
middle and dendrites on the other end; sensory neurons
have dendrites on both ends, connected by a long axon with a
cell body in the middle.
Some types of neurons: motoneuron (a),
sensory neuron (b), cortical pyramidal cell
(c)
Neurons also vary with respect to their functions:
Sensory neurons carry signals from the outer
parts of your body (periphery) into the central nervous
system.
Motor neurons (motoneurons) carry signals from
the central nervous system to the outer parts (muscles,
skin, glands) of your body.
Receptors sense the environment (chemicals,
light, sound, touch) and encode this information into
electrochemical messages that are transmitted by sensory
neurons.
Interneurons connect various neurons within the
brain and spinal cord.
The simplest type of neural pathway is a
monosynaptic (single connection) reflex pathway,
like the knee-jerk reflex. When the doctor taps the the right
spot on your knee with a rubber hammer, receptors send a
signal into the spinal cord through a sensory neuron. The
sensory neuron passes the message to a motor neuron that
controls your leg muscles. Nerve impulses travel down the
motor neuron and stimulate the appropriate leg muscle to
contract. The response is a muscular jerk that happens quickly
and does not involve your brain. Humans have lots of
hard-wired reflexes like this, but as tasks become more
complex, the pathway "circuitry" gets more complicated and the
brain gets involved.
Brain Parts The simplest possible creatures
have incredibly simple nervous systems made up of nothing but
reflex pathways. For example, flatworms and invertebrates do
not have a centralized brain. They have loose associations of
neurons arranged in simple reflex pathways. Flatworms have
neural nets, individual neurons linked together that
form a net around the entire animal.
Most invertebrates (such as the lobster) have simple
"brains" that consist of localized collections of neuronal
cell bodies called ganglia. Each ganglion controls
sensory and motor functions in its segment through reflex
pathways, and the ganglia are linked together to form a simple
nervous system. As nervous systems evolved, chains of ganglia
evolved into more centralized simple brains.
Major Divisions of the
Brain
Spinal cord
Brainstem
Cerebellum
Forebrain
Diencephalon - thalamus, hypothalamus
Cerebral cortex
Brains
evolved from ganglia of invertebrates. Regardless of the
animal, brains have the following parts:
Brainstem - The brainstem consists of the
medulla (an enlarged portion of the upper spinal
cord), pons and midbrain (lower animals have
only a medulla). The brainstem controls the reflexes and
automatic functions (heart rate, blood pressure), limb
movements and visceral functions (digestion, urination).
Cerebellum - The cerebellum integrates
information from the vestibular system that indicates
position and movement and uses this information to
coordinate limb movements.
Hypothalamus and pituitary gland - These
control visceral functions, body temperature and behavioral
responses such as feeding, drinking, sexual response,
aggression and pleasure.
Cerebrum (also called the cerebral cortex
or just the cortex) - The cerebrum consists of the
cortex, large fiber tracts (corpus callosum) and some deeper
structures (basal ganglia, amygdala, hippocampus). It
integrates information from all of the sense organs,
initiates motor functions, controls emotions and holds
memory and thought processes (emotional expression and
thinking are more prevalent in higher mammals).
As you proceed from fish toward humans, you can see that
the cortex gets bigger, takes up a larger portion of the total
brain and becomes folded. The enlarged cortex takes on
additional higher-order functions, such as information
processing, speech, thought and memory. In addition, the part
of the brain called the thalamus evolved to help relay
information from the brainstem and spinal cord to the cerebral
cortex.
Lower Brain
Spinal
Cord
The spinal cord can be
viewed as a separate entity from the brain or merely as
a downward extension of the brainstem. It contains
sensory and motor pathways from the body, as well as
ascending and descending pathways from the brain. It has
reflex pathways that react independently of the brain,
as in the knee-jerk reflex.
Lower animals (fish,
amphibians, reptiles, birds) do not do much "thinking," but
instead concern themselves with the everyday business of
gathering food, eating, drinking, sleeping, reproducing and
defending themselves. Therefore, their brains reflect the
major centers that control these functions. We perform these
functions as well, and so have a "reptilian" brain built into
us.
Underside of the brain, showing the brainstem
and cranial
nerves
The basic lower brain consists of the spinal
cord, brainstem and diencephalon (the
cerebellum and cortex are also present, but will be discussed
in later sections). Within each of these structures are
centers of neuronal cell bodies, called nuclei, that
are specialized for particular functions (breathing,
heart-rate regulation, sleep):
Medulla - The medulla contains nuclei for
regulating blood pressure and breathing, as well as nuclei
for relaying information from the sense organs that comes in
from the cranial nerves.
Pons - The pons contains nuclei that relay
movement and position information from the cerebellum to the
cortex. It also contains nuclei that are involved in
breathing, taste and sleep.
Midbrain - The midbrain contains nuclei that link
the various sections of the brain involved in motor
functions (cerebellum, basal ganglia, cerebral cortex), eye
movements and auditory control. One portion, called the
substantia nigra, is involved in voluntary movements;
when it does not function, you have the tremored movements
of Parkinson's disease.
Thalamus - The thalamus relays incoming sensory
pathways to appropriate areas of the cortex, determines
which sensory information actually reaches consciousness and
participates in motor-information exchange between the
cerebellum, basal ganglia and cortex.
Hypothalamus - The hypothalamus contains nuclei
that control hormonal secretions from the pituitary gland.
These centers govern sexual
reproduction, eating, drinking, growth, and maternal
behavior such as lactation (milk-production in mammals). The
hypothalamus is also involved in almost all aspects of
behavior, including your biological "clock," which is linked
to the daily light-dark cycle (circadian rhythms).
Internal view of the lower
brain
Balancing Act The cerebellum is folded into
many lobes and lies above and behind the pons. It receives
sensory input from the spinal cord, motor input from the
cortex and basal ganglia and position information from the
vestibular system. The cerebellum then integrates this
information and influences outgoing motor pathways from the
brain to coordinate movements. To demonstrate this, reach out
and touch a point in front of you, such as the computer
monitor -- your hand makes one smooth motion. If your
cerebellum were damaged, that same motion would be very jerky
as your cortex initiated a series of small muscle contractions
to home in on the target point. The cerebellum may also be
involved in language (fine muscle contractions of the lips and
larynx), as well as other cognitive functions.
Higher Brains
Gray
Matter
The cerebrum
contains gray matter (neurons with no myelin) and white
matter (myelinated neurons that enter and leave the
cortex).
The cerebrum
is the largest part of the human brain. The cortex contains
all of the centers that receive and interpret sensory
information, initiate movement, analyze information, reason
and experience emotions. The centers for these tasks are
located in different parts of the cortex. Before we discuss
what each part does, let's look at the parts of the cerebrum.
Major Parts of the Cerebral
Cortex The cortex dominates the exterior surface of
the brain. The surface area of the brain is about 233 to 465
square inches (1,500 to 2,000 cm2), which is about the size of one to two
pages of a newspaper. To fit this surface area within the
skull, the cortex is folded, forming folds (gyri) and
grooves (sulci). Several large sulci divide the cortex
into various lobes: the frontal lobe, parietal
lobe, occipital lobe and temporal lobe. Each
lobe has a different function.
Mouse-over the part labels of
the brain to see where those parts are
located.
When viewed from above, a large groove (interhemispheric
fissure) separates the brain into left and right halves.
The halves talk to each other through a tract of white-matter
fibers called the corpus callosum. Also, the right and
left temporal lobes communicate through another tract of
fibers near the rear of the brain called the anterior
commissure.
If you look at a cutaway view of the brain, you see that
the cortical area above the corpus callosum is divided by a
groove. This groove is called the cingulate sulcus. The
area between that groove and the corpus callosum is called the
cingulate gyrus, also referred to as the limbic
system or limbic lobe. Deep within the cerebrum
lies the basal ganglia, amygdala and hippocampus.
This ends our tour of the major structures of the cortex.
Now, let's see what they do.
The brain is "hard-wired" with connections, much like a
building or airplane
is hard-wired with electrical wiring. In the case of the
brain, the connections are made by neurons that connect the
sensory inputs and motor outputs with centers in the various
lobes of the cortex. There are also connections between these
cortical centers and other parts of the brain.
Several areas of the cerebrum have specialized
functions:
Parietal lobe - The parietal lobe receives and
processes all somatosensory input from the body
(touch, pain).
Fibers from the spinal cord are distributed by the
thalamus to various parts of the parietal lobe.
The connections form a "map" of the body's surface on
the parietal lobe. This map is called a homunculus.
The homunculus looks rather strange because the
representation of each area is related to the number of
sensory neuronal connections, not the physical size of the
area. (See What
Does Your "Homunculus" Look Like? Mapping Your Brain for
details on how to determine your own homunculus.)
Homonculus, a sensory map of your body. The
homunculus looks rather strange because the
representation of each area is related to the number of
sensory neuronal connections, not the physical size of
the
area.
The rear of the parietal lobe (next to the temporal
lobe) has a section called Wernicke's area, which
is important for understanding the sensory (auditory and
visual) information associated with language. Damage to
this area of the brain produces what is called "sensory
aphasia," in which patients cannot understand language but
can still produce sounds.
Frontal lobe - The frontal lobe is involved in
motor skills (including speech) and cognitive functions.
The motor center of the brain (pre-central
gyrus) is located in the rear of the frontal lobe,
just in front of the parietal lobe. It receives
connections from the somatosensory portion in the parietal
lobe and processes and initiates motor functions. Like the
homunculus in the parietal lobe, the pre-central gyrus has
a motor map of the brain (for details, see A
Science Odyssey: You Try It - Probe the Brain
Activity).
An area on the left side of the frontal lobe, called
Broca's area, processes language by controlling the
muscles that make sounds (mouth, lips and larynx). Damage
to this area results in "motor aphasia," in which patients
can understand language but cannot produce meaningful or
appropriate sounds.
Remaining areas of the frontal lobe perform
associative processes (thought, learning, memory).
Diagram highlighting the
functional areas of the brain
Occipital lobe - The occipital lobe receives and
processes visual information directly from the eyes and
relates this information to the parietal lobe (Wernicke's
area) and motor cortex (frontal lobe). One of the things it
must do is interpret the upside-down images of the world
that are projected onto the retina by the lens of the eye.
Temporal lobe - The temporal lobe processes
auditory information from the ears and relates it to
Wernicke's area of the parietal lobe and the motor cortex of
the frontal lobe.
Insula - The insula influences automatic
functions of the brainstem. For example, when you hold your
breath, impulses from your insula suppress the medulla's
breathing centers. The insula also processes taste
information.
Hippocampus - The hippocampus is located within
the temporal lobe and is important for short-term memory.
Amygdala - The amygdala is located within the
temporal lobe and controls social and sexual
behavior and other emotions.
Basal ganglia - The basal ganglia works with the
cerebellum to coordinate fine motions, such as fingertip
movements.
Limbic system - The limbic system is important in
emotional behavior and controlling movements of visceral
muscles (muscles of the digestive tract and body cavities).
Water on the Brain Your brain and spinal
cord are covered by a series of tough membranes called
meninges, which protect these organs from rubbing
against the bones of the skull and spine. For further
protection, the brain and spinal cord float in a sea of
cerebrospinal fluid within the skull and spine. This
cushioning fluid is produced by the choroid plexus
tissue, which is located within the brain, and flows through a
series of cavities (ventricles) out of the brain and
down along the spinal cord. The cerebrospinal fluid is kept
separate from the blood supply by the blood-brain
barrier.
Ventricle system of the
brain
As you can see, your brain is a complex, highly organized
organ that governs everything you do. Now that you are
familiar with the anatomy of the brain, look for future
articles to address its specific functions.
