Download The Brain in a Nutshell 2010

The brain in a nutshell - page 1
The Brain in a Nutshell - a compendium of structures and functions
Version HS 2010
H.-P. Lipp, Institute of Anatomy
Stuff relevant for general knowledge (and exams) is marked in red
The brain in a nutshell - page 2
Definitions
Gray matter
Composed of neurons and their dendrites, glial cells (astroglia and oligodendrocytes, see below), capillaries, and traversing axons.
Nuclei are well-delineated aggregations of neurons. Projection means a bundle of axons terminating in a defined location.
Massive bundles of myelinated axons, sparse capillaries, oligodendrocytes producing the whitish myelin, and some astroglia cells
White matter nurturing the axons. Afferent fibers refer to axons reaching a structure, efferent fibers to axons leaving a structure. Fascicle denotes a
visible fiber bundle, alternatively also denoted as tract.
Dorsal = upper part; ventral = lower part; medial = inner side; lateral = outer side; rostral = forwards (rostrum = beak); caudal =
Orientational
backwards (cauda = tail); rostral and caudal are often synonymous with anterior and posterior, respectively, but not at the level of the
terminology
spinal cord. Contralateral = opposite side; ipsilateral = same side, bilateral = both sides.
A thick layer of tough connective tissue encapsulating the entire central nervous system. Is pain-sensitive, this pain being perceived as
Meninges
Dura mater
headaches. Duplications of the dura mater known as sinus draw the venous blood from the brain.
Loose connective tissue bridging the liquor-filled space between dura mater and pia mater. Large bridgings are refrred to as cysternae.
Arachnoidea All larger blood vessels run in this so-called subarachnoideal space. Smaller vessels then contact the pia mater and dive into the brain.
Ruptured large vessels cause so-called subarachnoideal hemorrhages.
A fine layer of connective tissue covering the entire surface of the central nervous system. Follows blood vesssels for some distance
Pia mater
into the brain. Later on, astroglia covers the vessels with plates, creating a blood-brain barrier.
Large cavities in the brain corresponding to dilations of the lumen of the embryological neural tube. Filled with cerebrospinal liquor. The
walls of the ventricles are covered by ependymal cells with microcilia moving the cerebral liquor. Liquor is produced in organs protruding
into the ventricles, called choroid plexuses. There are 4 ventricles. Two lateral ones within each hemisphere, interconnected by an
Ventricles
opening, the intraventricular foramen located at the rostral end of the third ventricle, which is confined to the diencephalon (see below).
and cerebral
The third ventricle connects to the fourth one by means of the cerebral aquaeduct (see below). The fourth ventricle has two lateral and
liquor
one dorsal openings to the subarachnoideal space, permitting liquor to flow around the brain and spinal cord. Eventually, liquor is
resorbed be veins located in in protrusions of the arachnoidea across the dura mater into the skull called Pacchioni granulations.
Further resorbtion occurs in pouches of the dura mater where spinal nerves emerge. There, liquor is resorbed by lymph vessels. Liquor
can be sampled for diagnostic purposes by puncturing a space below the ending of the lumbar spinal cord.
Originates from the interior carotid arteries crossing the base of the skull, and from the vertebral arteries climbing through holes in the
vertebral processes and merging into a common basilar artery ascending along the ventral side of the brain stem. The carotid system
branches into the ophtalmic, anterior and middle cerebral artery supplying the eye, medial and lateral side of the hemispheres,
respectively. The basilar system gives off arteries to the brain stem, cerebellum, and the posterior cerebral artery supplying the dorsal
Blood supply
thalamus and basal and occipital parts of the hemispheres. Carotid and basilar system communicate through small arteries forming the
circle of Willis. Venous blood flows in veins in the subarachnoideal space that abutt into the sinus system of the dura mater.
Interruptions of arterial blood supply (by obstruction or rupturing) cause strokes entailing destruction of the target regions. Some
especially long and thin arteries in the human brain form typical predilection sites for strokes, often in white matter such as the internal
capsule (see below).
Segmentally arranged gray matter and ascending and descending fiber tracts controlling striate & smooth musculature and sensory
information from skin, viscera, muscles and tendons. Motor fibers leave through ventral roots. Sensory neurons are located in the dorsal
root ganglion, one branch of the axon entering the spinal cord as dorsal roots, the other connecting to peripheral sensory cells or ending
Spinal cord
freely. Neuronal networks coordinate movements, while reflex loops connect sensory input directly to motoneurons. The dorsal parts of
the spinal cord are linked to sensory processing and ascending fiber tracts, the ventral portions to motor processing and descending
fiber tracts.
The brain in a nutshell - page 3
Rostral continuation of the spinal cord within the skull. Contains ascending and descending fiber tracts, a huge computational network
(reticular formation, RF). Embedded in it are recognizable clusters of neurons (nuclei) sending and receiving fibers from head organs
and viscera. Some ill-defined neuron clusters ("centers") regulate vital automatisms (breathing, blood pressure) by integrating input and
output nuclei.
Rostral continuation of the medulla oblongata containing similar structures. In addition, contains the pontine gray nuclei formed by
neurons receiving fibers from all the cortex and relaying the impulses to the opposite (contralateral) cerebellum via the middle cerebellar
tract. Harbors, in the floor of the 4th ventricle, the locus coeruleus, a group of noradrenergic neurons innervating large portions of the
brain.
Medulla
oblongata
Pons
Straddles the brain stem, shows two lateral lobes (divided into anterior and posterior lobe), a merging zone between lobes (vermis) and
ventrally small appendices (one nodulus, two lateral flocculi) . Through the lower cerebellar tract, it receives fibers from the spinal cord
carrying sensory and positional information (proprioception) from trunk, neck and extremities, plus information from the vestibular and
oculomotor systems. The middle cerebellar tract (see above) provides information received from descending fibers of the neocortex,
plus copies of movement impulses sent to the spinal cord. The bulk of arriving (afferent) fibers are cerebellar mossy fibers; a special
connection from the olivary nuclei contains climbing fibers. Integration of the multiple information takes place in the cerebellar cortex
formed by many small granule cells and relatively few large Purkinye cells. The latter project (inhibitively) to the so-called deep
cerebellar nuclei that send axons to both motor structures in the contralateral brain stem and to the contralateral motor thalamus, which,
in turn, innervates the motor cortex generating movements.
Cerebellum
Midbrain
Tectum
Cerebral
aquaeduct
Tegmentum
The phylogenetically old control center of the vertebrate brain controlling body physiology, species-specific behavior and sensori-motor
integration.
Contains the colliculus superior (a relay structure for retinal fibers from the eye to brain stem nuclei) and the colliculus inferior (a relay
station for ascending auditory information). Receives also fibers from the neocortex informing the midbrain about cortical activation
zones.
Small channel connecting the 3d and 4th ventricle. Causes hydrocephalus if flow of cerebrtospinal liquor is blocked. Is surrounded by
the central gray, a region mediating nociception (pain).
Formed largely by the reticular formation. Contains the red nucleus (a motor relay structure), the substantia nigra (sending
dopaminergic fibers to the striatum), other dopaminergic cell groups projecting to the forebrain, and serotoninergic neurons at the
midline (raphe nuclei) also innervating the forebrain.By means of these ascending monoamine systems, the midbrain can control
activity states (sleep/wake) and generate differentially activated zones in the forebrain (= diencephalon & telencephalon).
The brain in a nutshell - page 4
Diencephalon
Extends between optic chiasm and colliculus superior, lining the third ventricle.
Hypothalamus
The ventral (lower) part of the diencephalon containing many neurons with receptors for numerous hormones circulating in the
bloodstream. Controls the pituitary gland by sending axons to the neurohypophysis, and by neurohormones delivered into capillaries
(adenohypophysis), and orchestrates concomitantly body physiology including feeding and drinking by sending fibers to the visceral
control centers in the brain stem. Connects reciprocally to the midbrain through the medial forebrain bundle that also extends to the
basal forebrain. The medial hypothalamic zone deals with aversive events and feelings, its lateral zone mediates consummatory
behavior, sex and rewarding emotions. Receives modulating fiber bundles from hippocampus, prefrontal cortex and amygdala.
Contains nuclei controlling activity of the hippocampal formation by means of ascending fibers, and sends fibers to the intralaminar and
limbic thalamus.
Thalamus
A large mass of relay nuclei that are reciprocally connected with defined areas of the neocortex and limbic cortex. The posterior ventral
nuclei relay sensory impulses to auditory, visual and sensory neocortex, the anterior ventral nuclei convey to the frontal cortex motor
impulses originating in the dentate cerebellar nuclei. The most anterior portion relays impulses sent by the reticular formation of the
midbrain and the hypothalamus to prefrontal and limbic cortex. The dorsal nuclei partially relay impulses from the midbrain reticular
formation to various associative areas of the neocortex, thus tuning the cognitive functions of the neocortex according to information
from the brain stem. The intralaminar system is formed by a smaller group of nuclei found in the center of the thalamus or and
scattered along the midline. The fibers of intralaminar nuclei modulate larger portions of overlying neocortex and have a controlling
(enabling) function used by subcortical systems to tune activation of the neocortex. They also regulate sleep/wake cycles.
Subthalamus
Essentially a rostral extension of the midbrain reticular formation forming the so-called zona incerta and the fields of Forel. Contains
also the subthalamic nucleus belonging to the basal ganglia,
Hemispheres
Originate embryologically from two vesicles emanating laterally from the rostral end of the neural tube, and contain the lateral ventricles.
The tissue of the vesicles overgrows rostrally and caudally the rostral part of the neural tube. In primates, this process results in
cerebral hemispheres with various lobes representing the bulk of the telencephalon, leaving visible only the brain stem and cerebellum.
Basal
forebrain
A group of nuclei extending along the medial base of the forebrain. Boundaries of cell groups are often ill-defined. Includes the septal
nuclei (in lower mammals), the preoptic area, the diagonal band of Broca (DB), the substantia innominata and the basal nucleus of
Meynert (BM). Medial septum, DB and BM send cholinergic fibers to hippocampus, limbic and associative cortex.These projections are
important for maintaining cognitive functions. Degeneration of the BM is a hallmark of Alzheimer's disease. The basal forebrain
connects reciprocally to the hypothalamus through the medial forebrain bundle.
The brain in a nutshell - page 5
A mass of gray substance to the left and right of the lateral ventricles. Largely developed beneath the frontal cortex, tapering off along
the posterior course of the ventricles. The basal ganglia form an interconnected system by which topographically ordered activity from
various cortical regions is communicated to subcortical structures, being progressively condensed and ending, still topographically
ordered, in the motor thalamus and motor control regions of the midbrain. In this way, ongoing overall activity of the neocortex is
blended into the ongoing motor activity handled by motor cortex, midbrain, spinal cord and cerebellum. This connective design
underlies the so-called "motor planning function" of the basal ganglia.
Basal ganglia
Caudoputamen
A large mass of relatively uniform GABA-ergic cells receiving descending excitatory fibers from corresponding overlying cortex regions.
In humans, the mass is divided by the fibers of the capsula interna into an inner portion, the putamen, and an outer portion forming a
nucleus lining the lateral ventricle, the caudate nucleus with a bulky rostral head and a tapering tail along the ventricle, almost reaching
the amygdala. In rodents, descending cortical fibers penetrate the the cell mass everywhere, therefore the term striatum. Although
uniform in appearance, the caudoputamen possesses a complicated inner chemoarchitecture caused by differential distribution of
neuromediators. Hereditary degeneration of striatal neurons is the hallmark of Huntington's disease.
Nucleus
accumbens
septi
A portion of the striatum beneath the anterior commissure and receiving fibers from limbic structures such as hippocampus and
amygdala. Sends GABA-ergic fibers to the ventral pallidum.
Also simply known as pallidum. A wedge-like nucleus containing GABA-ergic cells targeted by converging GABA-ergic axons from the
caudoputamen. Is divided into an external (outer) pallidum (GPE) and an inner part (GPI). GABA-ergic fibers from the GPI inhibit the
Globus
motor thalamus and with this, the flow of impulses from the cerebellar dentate nuclei. They also inhibit species-specific motor
pallidum
automatisms orchestrated by the midbrain. Note that impulses from the caudoputamen reaching directly the GPI cause actually a
disinhibition of the tonically active target structures of the GPI!
Ventral
Receives fibers from the nucleus accumbens septi, its inhibitory GABAergic axons also reach the anterior (limbic) thalamus and
pallidum
intralaminar thalamic nuclei. This is the most important non-motor output from the basal ganglia.
Receives inhibitory GABA-ergic fibers from the external pallidum (GPE). The STN neurons are excitatory and connect to the inner
Subthalamic pallidum (GPI) which thus receives a double innervation, excitatory from the STN, and an inhibitory one directly from the
nucleus
caudoputamen. Therefore, the final inhibition level at the the motor thalamus can be carefully tuned. Lesions of the STN result in
overshooting movements (ballism or hemiballism).
Substantia
nigra
Claustrum
Visible to the naked eye in the unstained brain because of a high content of melatonin in neurons. Contains two parts. One portion
corresponds functionally to the inner pallidum (GPI) and has similar connections, and is named the substantia nigra pars reticularis. The
other, the substantia nigra pars compacta, is formed by dopaminergic neurons that receive direct inhibitory fibers from the
caudoputamen, and send back dopaminergic axons to the striatal neurons. Striatal neurons have dopamine receptors (D1, D2, D3 etc)
that either reinforce or inhibit the synaptic transmission of cortical descending fibers. Degeneration of dopaminergic cells causes
Parkinson's disease by ultimately increasing the pallidal inhibition of motor impulse transmisson in the thalamus .
A mass of gray substance between the basal ganglia and neocortex, more voluminous in the frontal forebrain. Connects reciprocally
with the overlying cortex. Functions unknown or speculative.
The brain in a nutshell - page 6
Neocortex
general
Frontal lobe
Parietal lobe
Occipital lobe
Temporal
lobe
Contains pyramidal neurons (excitatory) and granule cells. The latter include excitatory interneurons (stellate cells) or inhibitory ones
(basket cells). Other neuron types are less frequent. Glia cells include astroglia intertwining with neurons and oligodendrocytes
wrapping axons into myelin sheets. Dendrites of neurons and glia cells, traversing axons and many capillaries form the the so-called
neuropil where most synaptic actions take place. Cortical neurons are arranged in layers containing differential proportions of pyramidal
and granule cells. The neocortex is characterized by 5-6 layers, archi- and paleocortex by 1-3 layers. Local variations of these
proportions have been used to create a cytoarchitectonic map characterizing different cortical regions, named and numbered after
Brodmann. The functional division of the neocortex is imposed by subcortical input conveyed and distributed by the thalamus. Most
axons in the neocortex are short and connect reciprocally to neighbored regions. Long axons originate from larger pyramidal cells and
travel either downwards (subcortical), over long distances in the same hemisphere, or to the opposite (contralateral) hemisphere.
Denotes cortical areas receiving specific input from sensory organs (mostly through the thalamus except for olfaction), or from the
Primary
contralateral cerebellar dentate nucleus through the motor thalamus. The fibers arriving from the thalamus generally preserve the
sensory and topographical organization of the peripheral sensory organs (skin, eye). Thus, information from fibers sensing touch is reflected in
motor
activation of a cortical zone resembling a little (distorted) human, a "sensory homunculus". The motor cortex contains a so-called
neocortex
"motor homunculus" since the pyramidal cells send axons to motoneurons and spinal cord clustered topographically according to the
body scheme. Note that these homunculi stand upside down in the brain.
Associative cortex denotes regions that transfer and analyze impulses originating and spreading from the primary input and output
Associative zones. Neighboured regions are defined as unimodal association cortex. Cortical regions that receive input from different modalities are
neocortex
said to be multimodal (or higher-order) association cortex. The latter characterize cortical regions subserving merely cognitive
processes. Lesions of associative cortex areas do often not result in overt neurological symptoms.
All cortex rostral to the central sulcus dividing motor and sensory neocortex.
The motor output region (named area 4) lies in the precentral gyrus, bordered rostrally by motor association cortex orchestrating
Motor cortex smooth output activity. Area 43 (Broca's area) is a motor association cortex orchestrating neurons in area 4 that activate motoneurons
for tongue and layrnx. Its destruction destruction causes aphasia.
Rostral and medial portions of the frontal lobe. Can be considered as a higher-order motor association cortex orchestrating so-called
Pefrontal
executive functions. Is activated from limbic and dorsomedial thalamus and thought to set directionality and intentions. Also important
cortex
for organisation and recall of memory, and control of impulsivity.
Includes the postcentral sulcus containing a sensory body map (area 1, 2, 3), and extends caudally to the occipital lobe. Contains large
areas with multimodal association cortex blending somatosensory, auditory and visual information with information from the brain stem
conveyed through the nuclei of the posterior dorsal thalamus. Important for many cognitive functions. Depending on lesion sites,
neuropsychological symptoms may include agraphia, acalculia and the inability to interprete internal (egocentric) and external
(allocentric) spatial relations. Important gyri serving as landmarks: gyrus angularis, gyrus supramarginalis.
Includes the caudal pole of the hemispheres including the medial surface. Receives, via the lateral geniculate body of the thalamus,
topographically ordered fibers representing the activated spots of the retina in the eye. The termination zone denoted as area 17 is
surrounded by unimodal associative areas 18 and 19. Landmark: calcarine sulcus.
Curves downwards and forwards (rostrally). Ist upper and inner portions contain the primary auditory cortex (gyri of Heschl), and
unimodal association cortex for auditory information (planum temporale). Blends caudally into multimodal association areas shared with
the parietal and occipital lobe. One of them is Wernicke's area whose destruction causes sensory aphasia. Large fiber bundles connect
these regions with the motor associative speech area of Broca. The ventral and rostral portions of the temporal lobe are considered as
part of the limbic system (see below)
The brain in a nutshell - page 7
Hidden part of the hemisphere overgrown by frontal, parietal and temporal lobe. The portions covering the insula are called opercula
(little covers). Contains in its posterior portion areas receiving thalamic fibers carrying gustatory information. The anterior portion of the
insula iconsists of polymodal sensory association cortex including a representation of pain.
A loosely defined collection of cortical regions connected with subcortical nuclei in the basal forebrain, hypothalamus, ventral midbrain
and anterior thalamus.
Insula
Limbic
system
Cingulate
gyrus
In humans, a large gyrus running in the medial mantle of the hemispheres just above the corpus callosum. Its foremost portion blends
with the medial prefrontal cortex. Posteriorly, it blends with the gyrus parahippocampalis (entorhinal cortex). Is a higher-order
association cortex collecting pre-processed information from frontal, parietal and temporal lobes. Contains a region activated by pain.
All these impulses are sent via a large internal fiber bundle (cingulum) to the entorhinal cortex. The cingulate gyrus receives also fibers
from the anterior part of the thalamus (limbic thalamus) conveying information from the midbrain reticular formation and hypothalamus.
Entorhinal
cortex
General term for all mammalian species, in humans referred to as parahippocampal gyrus. Located at the inner rostral side of the
temporal lobe. Collects pre-processed multimodal information from all neocortical motor and sensory areas. This topographically
ordered information is sent to the hippocampal formation by fibers crossing and penetrating a small sulcus. This projection is know as
the perforant path and angular bundle. The entorhinal cortex receives feedback from the hippocampal formation through the so-called trisynaptic loop (see below). This feedback travels back along the cingulum to multimodal cortical association areas. The entorhinal cortex
is, as the cingulate gyrus, reciprocically connected with the limbic thalamus sending information from the reticular formation of the midbrain and from the hypothalamus. It also receives axons from the olfactory tract, being thus considered by some as primary olfactory
cortex.
Hippocampus
The last rim of the hemispheric mantle, rolled inwards into the space of the lateral ventricle and encapsulated by a thick fiber bundle, the
alveus. The first portion of the in-rolling formation is the subiculum, a 2-3 layered continuation of the entorhinal cortex. This is followed
by hippocampal subregions consisting of one-layered archicortex, CA1, CA2, CA3 and CA4 (CA denoting cornu ammonis, Ammon's
horn). CA4 is also referred to as the hilus of the dentate gyrus. The end of the hippocampal formation is the dentate gyrus (DG) forming
kind of cap over CA4 and CA3. The dentate gyrus is one of the few zones in the mammalian brain known for adult neurogenesis.
Tri-synaptic
loop and
memory
The intrinsic fiber connections of the hippocampal formation form the so-called tri-synapic loop. Topographically orderered fibers from
the entorhinal cortex terminate on the dendrites of granule cells in the dentate gyrus. Their axons (hippocampal mossy fibers) carry
impulses to the pyramidal cells in CA3 whose axons (Schaffer collaterals) then carry the impulse pattern to CA2 and CA1, from there to
the subiculum, which projects back to the entorhinal cortex. CA3-Schaffer collaterals also extend between the parallel tri-synaptic loops,
while the subiculum sends a fiber bundle, the fornix, to the hypothalamus and basal forebrain. Thus, the hippocampus forms the topassociative system of the entire neocortex, where highly processed information from many associative areas reverberates in parallel
along the tri-synaptic loops. Associative interactions between these parallel ("lamellar") loops occurs by means of the CA3 Schaffer
collaterals. Thus, the hippocampus is crucial for short-term memory, plus encoding and retrieval of different memories stored elsewhere
in the brain. Its destruction causes anterograde amnesia and loss of .episodic memory
The brain in a nutshell - page 8
Amygdala
Olfactory
system
Major fiber
tracts
Forebrain
associativ
Consist of heavily myelinated and compact bundles of axons visible to the naked eye
Fasciculus
frontooccipitalis
Fasciculus
arcuatus
Fasciculus
uncinatus
Cingulum
Fasciculus
longitudinals
dorsalis
Medial
forebrain
bundle
Ansa
lenticularis
Striae
medullares
Forebrain
commissural
An almond-shaped collection of subcortical nuclei located rostral to the hippocampus in the tip of the temporal lobe. They are of
different embryological origin and have different connections. Corticomedial and central portions are said to be derived from the cortical
mantle, while the origin of the basolateral nuclei is unclear. Receives direct sensory input from olfactory cortex, and, bypassing the
thalamus, unfiltered auditory information from the midbrain. Other input derives from the entorhinal cortex and cingulum, and from the
limbic thalamus. The latter structures connect reciprocically. By means of subcortically descending connections to the medial and lateral
hypothalamus, the amygdala coordinates the activity of tonically acting systems subserving positive and negative emotions. The central
nucleus also projects directly to visceral control centers in the brain stem. The amygdala tags memories with positive or negative
connotations, thus playing an important role in selecting memory for events with particular significance for the organism. It interacts with
the hippocampal formation by means of short connections, and, indirectly, by feedback through the hypothalamus.
Primary olfactory receptor cells in the olfactory epithelium send short axons into the olfactory bulb, conveying a topographical pattern of
excitation caused by segregated receptors for odorants. The olfactory bulb shows a complex layered structure and synaptic
specializations. Mitral cells in the glomerulal layer receive the impulses and send them further along the olfactory tract to basal forebrain
where a number of fibers end in the anterior olfactory nuclei and the septal nuclei of the basal forebrain. Others continue to the primary
olfactory cortex (orbitofrontal), to the entorhinal cortex and to the corticomedial amygdala. Thus, primary olfactory information is
blended into the ongoing activity of the limbic system and acts powerfully on memory. Orbitofrontal olfactory association cortex
analyzes olfactory information in more detail, blending this information into prefrontal motor association areas thought to induce
following of odors.
Connects reciprocally regions of the frontal lobe involved in eyeball control with occipital visual association cortex ("visuomotor
pathway")
Connects reciprocally the sensory-associative Wernicke language region with the motor-associative Broca region ("Language pathway")
Connects reciprocally lateral prefrontal cortex with the inner side oif the temporal lobe ("Cognitive processing")
Connects reciprocally the multimodal associative regions opf the gyrus cinguli with the entorhinal cortex, funneling highly pre-processed
multimodal information towards the entorhinal-hippocampal system.
Also called the bundle of Schütz. Connects the nuclei of the medial hypothalamus with the central gray of the midbrain. Important for the
modulation of negative emotions.
Tractus telencephalicus medialis. A lage fiber system visible in the lateral hypothalamus. Numerous short and long fibers that connect
nuclei in the basal forebrain, lateral hypothalamus and the ventral tegmental region. Contains also ascending dopaminergic fibers to
basal forebrain and prefrontal cortex (mesolimbic system). Electrical stimulation is rewarding.
Fibers of the Globus pallidum penetrating/undercrossing the capsula interna to reach the motor thalamus (Tr. pallido-thalamicus).
Fine myelinated fibers running from the septal nuclei (are subcallosa in man) on the dorsal surface of the thalamus to the habenular
nuclei. They carry limbic and olfactory information, are considered part of the limbic system.
Large bundles of myelinated fibers that primarily connect homotopic (mirror-like) regions between left and right structures. Contribute to
functional lateralization, that is, specialization of left and right hemisphere for different tasks.
The brain in a nutshell - page 9
Corpus
callosum
About 100 million fibers connecting corresponding regions of the frontal, parietal, occipital and (partially) temporal lobe. Primary sensory
cortical regions and motor cortex have few callosal fibers, the majority originating from association cortex. Only present in mammals.
Anterior
cimmissure
A smaller bundle of fibers at the rostral end of the diencephalon, interconnecting olfactory association cortex and left and right
amygdala, and parts of entorhinal cortex. Present in all vertebrates.
Hippocampal Barely present in humans where it is denoted as psalterium dorsale. Strongly developed in rodents where it is located beneath the
commissure corpus callosum. Interconnects regions CA3 and CA4 of the hippocampus in a sophisticated pattern of terminations.
Posterior
Interconnect regions of the so-called pretectal area at the junction of midbrain and diencephalon.
commissure
Habenular
Interconnects the habenular nuclei located at the dorso-posterior end of the thalamus. These are are part of the limbic system.
commissure
Forebrain
subcortical
Brain stem
Capsula
interna
Located between thalamus and basal ganglia. Contains ascending thalamo-cortical fibers, and many descending fibers from all cortical
areas. These fibers merge into the cerebral peduncles and terminate mostly in the pontine gray nuclei, which relay the impulses to the
contralateral cerebellum. All these fibers form the first part of the cortico-ponto-cerebello-thalamo-cortical loop.
Pyramidal
tract
Large pyramidal cells in area 4 (and 3), the only direct motor output structure of the cortex, send axons that run within the fibers of the
capsula interna, maintaining the topography of the body muscles. They pass through the genu of the internal capsule where they are
prone to be interrupted by strokes. In the pons they continue, some terminating in the brainstem (bulbar) motor nuclei, the others
forming later a (ventrally protruding) name-giving structure, the pyramids. The pyramidal fibers then bifurcate at the spino-medullary
junction, the majority crossing (decussating) to the contralateral side and running in the latero-ventral white matter of the spinal cord. A
smaller ipsilateral bundle runs in the white matter near the midline. All pyramidal fibers terminate on large neurons (alpha-motoneurons)
that send axons to muculature in trunk and limbs. The pyramidal tract thus permits movements directly controlled by the cortex.
Lemniscus
medialis
Lemniscus
lateralis
Fasciculus
longitudinal
medialis
Originates in somatosensory relay nuclei in the lower medulla oblongata that receive ascending fibers from the dorsal spinal cord,
crosses to the other side and ends in the posterior ventral nuclei of the thalamus. Conveys a body map of touch sensations to the
somatosensory cortex. The pathways for pain run an indirect and more complicated course along the so-called anterolateral system of
the spinal cord and brain stem and are not visible in cross-sections.
A fiber bundle visible in the pontine region conveying auditory information from auditory relay nuclei in the brain stem to the inferior
colliculus, which projects then to the medial geniculate body of the thalamus that, in turn, sends fibers to the primary auditory cortex.
Note: the tract carries auditory information from both the left and the right ear.
A bundle of thickly myelinated axons near the midline of the brain stem. Interconnects, on both sides, the nuclei of the eye muscles,
parts of the vestibular nuclei, and extends to the motoneurons controlling neck muscles. Forms a major substrate for the oculomotor
control. Also known as MLF (medial longitudinal fascicle).
The brain in a nutshell - page 10
Cranial
Nerves
I.
II.
III.
IV.
V.
VI.
The cranial nerves consist of twelve pairs of nerves emanating from the brain through openings in the skull. The efferent fibers originate
from neurons located in the brain stem. They control striated muscles, glandular tissue, smooth or specialized muscles (e.g., heart).
The sensory components of cranial nerves originate from groups of cells that are located outside the brain called sensory ganglia
(nodes). They correspond to the dorsal root ganglia of the spinal cord. They send out a branch that bifurcates: a branch that enters the
brain and one that is connected to a sensory organ. Examples of sensory organs are pressure or pain sensors in the skin, muscle
spindles, and more specialized ones such as taste receptors of the tongue.
The olfactory tract is not really a cranial nerve, but formed by mitral cells in the olfactory bulb (see above), Therefore, it is considered as
a part of the CNS.
Likewise, the optic nerve formed by specialized neurons in the retina is a tract of the CNS. It conveys impulses to the thalamus (lateral
geniculate body), 50% of fibers (in humans) crossing at the optic chiasm to the opposite side. From this point on it is called the optic
tract.
The oculomotor nerve controls several muscles moving the eye-ball and the eye lid. Some fibers innervate the smooth muscles
controlling the diameter of the pupilla (M. ciliaris, M. sphincter pupillae).
The trochlear nerve originates from the trochlear nucleus in the midbrain and innervates specifically one muscle rotating the eyeball.
The trigeminal nerve is composed of three large branches emanating from the trigeminal ganglion: ophthalmic (V1, sensory), maxillary
(V2, sensory) and mandibular (V3, motor and sensory). Motoneurons innervate muscles of the jaw and oral cavity. Sensory information
includes touch from the facial skin, proprioception of joints, tendons and muscle spindles, and pain from face (trigeminal pain) and dura
mater.
The abducens nerve innervates specifically one eyeball muscle, the M. rectus lateralis.
VII.
The facial nerve is a mixed cranial nerve. Motoneurons of the facial nucleus control the mimic musculature of the face. Fibers from
other nuclei control salivation in the submandibular and sublingual gland. Finally, it contains gustatory afferents from the tongue.
VIII.
The vestibulocochlear nerve carries primarily afferent fibers from the inner ear (Ganglion spirale) and the vestibular organs (Ganglion
spirale). They convey auditory information and positional information to groups of auditory and vestibular nuclei in the brainstem.
IX.
The glossopharyngeal nerve is a mixed nerve. Motor fibers innervate a muscle lifting the pharinx (M. stylopharyngeus). Visceromotor
parasympathetic fibers control salivation in the parotid gland. Afferents include touch and pain sensations from the external ear, middleear, posterior one-third of the tongue and the upper pharynx. The fibers from the tongue carry also gustatory information. Viscerosensory nerve fibers transmit impulses from the carotid body (oxygen tension) and carotid sinus (blood pressure).
X.
The vagus nerve is also mixed nerve. Motor fibers from the Ncl. ambiguus innervate muscles of the pharynx and larynx.
Parasympathetic viscero-efferent fibers control glandular secretion in these structures, and descend to lungs, heart and digestive tract.
Viscerosensory information originates from a variety of receptors in thoracic and abdominal viscera. Other fibers convey tactile
information and pain from the larynx, pharynx, auditory canal, and the posterior meninges.
XI.
The spinal accessory nerve is formed by fibers of neurons mostly located in the cervical spinal cord and partly in the caudal medulla
oblongata. Branchiomotor fibers from the spinal cord ascend through the foramen magnum and exit the cranium through the jugular
foramen, innervating muscles in the neck and back. The neurons in the caudal medulla are considered as part of the vagus nerve.
XII.
The hypoglossal nerve innervates all intrinsic and most extrinsic muscles of the tongue. The fibers originate in the hypoglossal
nucleus located along the midline below the floor of the caudal part of the fourth ventricle.