Vse živali - sesalci, ptiči, plazilci, ribe, dvoživke - imajo možgane. Človeški možgani pa so nekaj posebnega. Dajo nam moč misliti, načrtovati, govoriti, predstavljati si... Oglejmo si ta neverjetni organ.
Možgani izvajajo zelo veliko število nalog:
Nadzorujejo telesno temperaturo, krvni tlak, srčni utrip in
dihanje.
Od različnih čutil (oči, ušesa, nos, itd.) sprejemajo množico
podatkov o svetu okoli nas.
Uravnavajo premikanje, ko hodimo, govorimo, stojimo ali sedimo.
Omogočajo nam misliti, sanjati, sklepati in izražati čustva.
Vsa ta opravila usklajuje, nadzoruje in uravnava organ,
približno tako velik kot glava cvetače: človeški možgani.
Možgani, hrbtenjača in obrobno živčevje tvorijo povezan in zapleten sistem za
obdelavo podatkov in nadzorovanje. Veda, ki znanstveno preučuje možgane se imenuje
nevroznanost ali nevrobiologija. Ker je področje nevroznanosti tako
široko in so možgani z živčnim sistemom tako zapleteni, bomo začeli pri osnovah in
vam predstavili le pregled delovanja tega neverjetnega organa.
V tem sestavku bomo obdelali strukturo možganov in naloge njihovih delov.
Na ta način bomo sposobni dojeti koncepte kot so: motorična kontrola, vizualno
procesiranje, slušno procesiranje, zaznavanje, učenje, pomnjenje, čustvovanje...,
to bomo namreč podrobneje obravnavali v nadaljevanju.
Struktura nevrona
Možgane sestavlja približno 100 milijard živčnih celic, ki jih imenujemo nevroni. Nevroni
imajo neverjetno sposobnost zbirati in prenašati elektrokemične signale - so nekaj takega kot
vrata in vezja v računalniku.
Nevroni imajo enake lastnosti in enako sestavo kakor druge celice,
toda njihove elektrokemične lastnosti jim omogočajo kar nekaj metrov daleč prenašati dražljaje in si
medsebojno izmenjavati sporočila.
Nevroni imajo tri osnovne dele:
Celično telo - Ta, pomembni del, ima vse nujno, kar potrebuje celica,
celično jedro (ki vsebuje DNK), endoplazmatski retikulum in ribosome (za izdelavo
proteinov) in mitohondrije (za tvorbo energije). Če odmre celično telo, odmre
celoten nevron.
Axon - Ta dolgi, kablu podoben izrastek celice, prenaša elektrokemično
sporočilo (živčni impulz ali akcijski potencial) vzdolž celotne
celice.
Pri nekaterih vrstah nevronov so aksoni lahko prekriti s tanko plastjo
mielina - kot izolacija pri električni žici. Mielin je iz maščobe
in pomaga pospeševati prenos živčnega impulza po aksonu. Mielinirane nevrone
največkrat najdemo med perifernimi živci (senzorni in motorični nevroni), medtem
ko najdemo nemielinizirane nevrone v možganih in hrbtenjači.
Dendriti ali živčni končiči - Majhni, vejicam podobni izrastki
živčne celice se povezujejo z drugimi celicami in omogočajo nevronu komunikacijo z
drugimi celicami ali neposredno z okoljem. Dendrite lahko najdemo na obeh koncih celice.
Osnovni tipi nevronov
Nevroni so različnih velikosti. Senzorni nevron iz konca prsta na roki poteka, na primer, po celotni
roki, nevroni v možganih pa so lahko dolgi le nekaj milimetrov. Nevroni so tudi različnih oblik,
odvisno, čemu služijo. Motorični nevroni, ki nadzorujejo
mišične kontrakcije, imajo celično telo na enem koncu, dolg akson v sredini in dendrite na
drugem koncu; senzorni nevroni pa imajo dendrite na obeh straneh, povezane z dolgim aksonom,
s celičnim telesom v sredini.
Nekaj vrst nevronov: motorični nevron (a),
senzorni nevron (b), piramidalna celica v korteksu - možganski skorji
(c)
Nevrone delimo tudi glede na njihovo funkcijo:
Senzorni nevroni prenašajo signale z obrobja telesa (periferija) v centralni živčni
sistem.
Motorični nevroni prenašajo signale iz centralnega živčnega sistema proti obrobnim
delom telesa (mišice, koža, žleze).
Receptorji zaznavajo okolje (kemične dražljaje, svetlobo, zvok, dotik) in pretvarjajo
te podatke v elektrokemične signale, ki se nato prenašajo s senzornimi nevroni.
Internevroni povezujejo različne nevrone v možganih in hrbtenjači.
Najpreprostejši tip živčne zveze je monosinaptična (enojna zveza) refleksna povezava,
kakršno srečamo pri kolenskem refleksu. Ko zdravnik z gumijastim kladivcem udari na pravo
mestu na kolenu, receptorji prenesejo ta signal po senzornem nevronu v hrbtenjačo. Tam prenese
senzorni nevron sporočilo motoričnemu nevronu, ki nadzoruje mišice naših nog. Živčni impulzi se
nato razširijo po motoričnem nevronu in vzdražijo ustrezne mišice v nogi, da se skrčijo.
Odgovor je mišični krč, ki se zgodi takoj, brez vključevanja možganov. Ljudje imamo veliko refleksov
s takim načinom povezave, toda ko postajajo naloge bolj zapletene, postanejo take
tudi povezave - vključijo se še možgani.
Deli možganov
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.
Fish
Brain? 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
Lower
Brain 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):
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.
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 Vestibular
System
The vestibular
system is responsible for maintaining posture, balance,
and spatial orientation. Part of the system is located
in the inner ear. It also includes the vestibulocochlear
nerve (the eighth cranial nerve) and certain parts of
the brain that interpret the information the
vestibulocochlear nerve receives.
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.
Hard-wired 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.)
Homunkulus, senzorni zemljevid telesa. Nekoliko
nenavadno izgleda, ker je velikost posameznih delov telesa
povezana s številom nevronskih povezav v možganih in ne
z njihovo fizično velikostjo.
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 work 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.
Click the play button to review the different
components of the brain. If the
animation above isn't working, click
here to get the Shockwave player.
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