The auditory system
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Last updated 13 March 1999
The diencephalon are forebrain structures derived from
the embryological diencephalic vesicles.
It is surrounded by cerebral cortex.
- This is conventionally divided into 4 parts:
- The epithalamus, which included the pineal
gland and nearby structures;
- The thalamus (G. inner chamber);
- The subthalamus;
- The hypothalamus.
Borders of the Diencephalon
: plane through the optic chiasm and anterior commissure.
Caudal: plane through the posterior commissure
and the caudal edge of the mammillary
- The rostral and caudal boundaries are approximate and semiarbitrary.
- Functionally continuous tissues extend through both boundaries.
- Dorsal: the fornix and floor
of the lateral ventricles.
- Because of the cephalic flexure, the axis
of the diencephalon is inclined about 100 degrees
with respect to the axis of the midbrain.
This includes the pineal gland and the habenular
nuclei and their connections.
This is a midline, unpaired
It is situated just rostral to the superior
- It resembles a pinecone in shape and hence its name.
- The pineal gland consists of pinealocytes (collection
of secretory cells), some glial cells and a rich vascular network.
- It receives light-regulated input via a circuitous
Retina --> one or more relays in hypothalamus --> intermediolateral
column of spinal cord --> preganglionic
sympathetic fibres --> postganglionic
neurons of the superior cervical ganglion --> pineal gland
- In fish, amphibians, and many reptiles, it is involved in circadian
and circannual rhythms.
- The mammalian pineal is an endocrine
organ involved with reproductive cycles.
- It has, however, no known neural output.
- It secretes an antigonadotropic hormone called melatonin at relatively high rates during darkness.
- Therefore, an increase in day lengths leads to increased gonadal function.
- By the age of 17, calcareous concretions accrue in it
- This makes it opaque to x-rays, hence making it a
useful radiological landmark.
This is a small swelling on each side, rostral to the stalk of the pineal gland.
Underlying each habenula are the habenular nuclei.
The 2 habenula are interconnected by the small habenular commissure.
This extends from the habenula to the interpeduncular nucleus.
This is located between the cerebral
peduncles in the reticular formation of the rostral midbrain.
Stria Medullaris Thalami
This underlies a horizontal ridge on the dorsomedial surface of the thalamus.
To this surface, the roof of the third
ventricle is attached.
- The fibres of this originate in various limbic structures.
- The pathway through the habenula is one route the limbic system can influence
the brainstem reticular
The midbrain tegmentum
continues into the diencephalon as the subthalamus.
This area is completely surrounded by neural tissue.
- Inferior to the thalamus;
- Lateral to the hypothalamus;
- And medial to the basis pedunculi
and internal capsule.
- The subthalamus contains rostral potions of the red nucleus and substantia nigra.
- It is traversed by somatosensory
pathways on their way to the thalamus, as well as
several pathways involving the cerebellum and basal ganglia.
This is a lens-shaped, bi-convex
It is located medial and superior
to portions of the basis pedunculi
and internal capsule.
- This nucleus is interconnected with the basal ganglia.
This is a small mass of grey
matter intervening between the subthalamic nuclei
and the thalamus.
It appears to be a continuation of the midbrain reticular formation.
It has widespread connections, including direct
projections to the cerebral cortex.
- Its function is largely unknown.
This is a large, egg-shaped
It makes up about 80% of the mass of the diencephalon.
- Anteriorly to the interventricular foramen;
- Superiorly to the transverse cerebral fissure;
- Inferiorly to the hypothalamic sulcus;
- And posteriorly it overlaps
- The thalamus is part of a remarkably large number of pathways.
- All sensory pathways relay in the thalamus.
- Many anatomical loops comprising cerebellar, basal ganglionic, and limbic pathways
also involve thalamic relays.
- The various systems utilise separate portions of the
- The thalamus is therefore subdivided into a series of nuclei.
Overall Pattern of Thalamic
Each thalamic nucleus receives inputs
from one or more subcortical sources.
They also receive feedback inputs from the cortical areas to which the nucleus projects.
- These feedback inputs appear to provide a means by
which the cerebral cortex can control
its own inputs.
- The thalamic nuclei can be distinguished from each
- Their topographical locations in
- And the pattern of their connections
with the cerebral cortex.
Topographical Subdivisions of the
Click here for a schematic diagram of the
Internal Medullary Lamina
This is a thin, curved sheet
of myelinated fibres.
It divides most of the thalamus into medial and lateral groups of nuclei.
- Anteriorly, the internal medullary lamina bifurcates
and encloses the anterior nucleus (AN).
Medial Group of Thalamic Nuclei
This group contains only one large nucleus, the dorsomedial nucleus (DM).
Lateral Group of Thalamic Nuclei
This group comprises the bulk of the thalamus.
It is further subdivided into a ventral
and dorsal tier.
- The dorsal tier consists of:
- The large pulvinar;
- The lateral posterior nucleus (LP);
- And the lateral dorsal nucleus (LD).
- The lateral posterior nucleus is continuous
with the pulvinar.
- Both nuclei have somewhat similar connections so the two are sometimes referred together
as the pulvinar/LP complex.
- The ventral tier consists of 3
nuclei arranged along an anterior-posterior
- The ventral anterior nucleus (VA);
- The ventral lateral nucleus (VL);
- And the ventral posterior nucleus (VP).
- The ventral posterior nucleus is further subdivided
into the ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei.
- The VPL is the somatosensory
relay of the body.
- The VPM is the somatosensory
relay of the head.
Other Thalamic Nuclei
Some nuclei do not fit into the above framework:
- The lateral geniculate nucleus (visual system) and medial geniculate system (auditory
system) are located posterior and a bit ventral to the VPL/VPM.
The internal medullary lamina, a certain locations, splits and encloses groups of cells.
- They protrude posteriorly alongside the midbrain.
- These 2 nuclei are conveniently considered as a posterior extension of the ventral tier.
The external medullary lamina is a 2nd
curved sheet of myelinated fibres that cover the lateral
surface of the thalamus.
- These nuclei are collectively called the intralaminar nuclei.
- The 2 largest are the centromedian
(CM) and parafascicular (PF) nuclei.
- The centromedian nucleus is a large,
round nucleus located medial
to the VPL/VPM.
- The VPM conforms to the rounded
shape of the CM.
- The thin sheet of cells
that intervenes between the external medullary lamina and
the internal capsule is the reticular
The midline nuclei of the thalamus are a thin layer of cells that covers part
of the medial surface of the thalamus.
- The reticular nucleus looks continuous
with the zona incerta.
- This, however, appears to be of no functional significance.
- They are essentially rostral continuations of parts of
the periaqueductal grey.
- The midline nuclei of the 2 sides
fuse in the interthalamic adhesion (when it is
Functional Subdivisions of the
Specific Relay Nuclei
These receive well-defined bundles of fibres and projects specifically to particular
functional areas of the cerebral cortex.
- The sensory relay nuclei include:
- The VPL/VPM;
- And the geniculate nuclei.
- The motor relay nuclei receive superior cerebellar peduncle and various outputs from the basal ganglia
and project to the motor and premotor
- They include:
- The VA;
- And the VL.
- The AN is the principle relay
nucleus of the limbic system.
- It receives the mammillothalamic tract and projects to
the cingulate gyrus.
These nuclei receive their inputs from a variety of
places and project to fairly broad areas of association cortex.
The 2 great areas of association
- The prefrontal cortex anterior to the motor areas of the frontal lobe;
- And the parietal-occipital-temporal association cortex,
occupying the area surrounded by the primary somatosensory,
visual, and auditory cortices.
- Corresponding to these 2 large areas are 2 large association nuclei.
- The dorsomedial nucleus is reciprocally
interconnected with the prefrontal cortex.
- It is involved with prefrontal functions such as affect
- Bilateral damage to the DM or its
connections has effects similar to those of a prefrontal lobotomy.
- Major inputs to the DM, in addition to those of the prefrontal cortex, come from various elements of the limbic system, e.g., the amygdala.
- The pulvinar/LP complex is reciprocally interconnected
with the parietal-occipital-temporal association cortex.
- The major inputs to this complex, apart from those arising in the association cortex,
come from parts of the visual system.
- The role of the pulvinar and the LP
is, however, almost entirely unknown.
These nuclei project to extremely widespread areas of
- The inputs to these nuclei are derived from:
- The basal ganglia;
- The cerebellum;
- The brainstem reticular
- And the spinothalamic and spinoreticulothalamic fibres (carrying information about dull,
- The major output projections are to noncortical
targets such as the basal ganglia.
- These efferents, however, also send collaterals
to the cerebral cortex.
- The broad extent of the cortical
connections of the non-specific nuclei make them suitable for general roles such as regulation the level of cortical excitability.
- They are considered to be one route through which the ascending reticular activating system affects the cortex.
- Although the centromedian nucleus is one of the intralaminar nuclei, it has a special
relationship with the basal ganglia.
- It probably has a different role
to that of the other non-specific nuclei.
This nucleus has no projections to the cerebral cortex at all.
- The output of each portion of the reticular
nucleus goes to that thalamic nucleus from
which it receives its input.
- The reticular nucleus controls thalamic
- There are some indicators that it may have a major role
in modulating transmissions from the thalamus
to the cortex during different
phases of the sleep-wake cycle.
Table derived from Nolte (1993) The Human Brain 3rd Ed. Table 7, p. 259 and
Dr. Robinson UNSW Neuroanatomy (ANAT 2007) Lecture Handout, 31 August 1998.
||Name of Nucleus
||Primary visual cortex (area 17)
||Inferior colliculus and lateral lemniscus
||Primary auditory cortex (areas 41, 42)
||Medial lemniscus, spinothalamic tract
||Primary somatosensory cortex (areas 1-3)
||Pain, temperature, touch, proprioception (body)
||Trigeminothalamic tracts and solitary nucleus
||Primary somatosensory cortex (areas 1-3)
||Pain, temperature, touch, proprioception (head)
||Dentate nucleus, substantia nigra, globus pallidus
||Primary motor cortex (area 4), premotor cortex (area 6)
||Initiation of movement
||Globus pallidus, substantia nigra
||Premotor cortex (area 6) and prefrontal cortex
||Planning of movement
||Hippocampus (fornix), mammillothalamic tract
||Emotion and memory (limbic)
||Retina, superior colliculus, secondary visual cortex (areas
||Parietal-occipital-temporal association cortex
||Superior colliculus (**unknown**)
||Parietal association cortex
||Spinothalamic tract, amygdala & Entohinal cortex
||Emotional response to pain and memory
||Part of ventral anterior
||Other thalamic nuclei(?)
||Reticular formation, spinothalamic, trigeminothalamic tracts
||Frontal and parietal lobes, caudate nucleus and putamen
||Alertness and response to pain
||All thalamic nuclei and cortical regions
||All thalamic nuclei
Blood Supply of the Thalamus
This is mostly from branches (ganglionic or
perforating) of the posterior cerebral artery.
Specific branches of the posterior choroidal artery supply some dorsomedial
The Internal Capsule
This is a compact bundle of fibres
through which the large collections of fibres pass, including:
- The thalamocortical and corticothalamic
- The corticopontine, corticobulbar
and corticospinal fibres pass through
the internal capsule and then the cerebral peduncles;
- And the fibres projection from the cerebral cortex to
the various nuclei of the extrapyramidal
system (e.g., the putamen and caudate nucleus).
Shape of the Internal Capsule
It is a continuous sheet of fibres
that forms the medial boundary of the lenticular
It continues around posteriorly and inferiorly
to partially envelop this nucleus.
- Inferiorly, many of the fibres of the internal capsule funnel
into the cerebral peduncles.
- Superiorly, the fibres fan out into the corona radiata.
- Here, they travel in the cerebral white matter to reach
their cortical origins or destinations.
- The entire fibre system is shaped like a trumpet with a large notch cut
out of its bell.
- The notch corresponds to the location of where this
sheet of fibres is interrupted by the lateral
- The lenticular nucleus sits where a mute
would sit in a trumpet.
- The internal capsule is divided into 5
- The anterior limb is the portion between the lenticular nucleus and the head
of the caudate nucleus;
- The posterior limb is the portion between the lenticular nucleus and the thalamus;
- The genu the portion at the junction
of the above 2 parts and is adjacent
to the interventricular foramen;
- The retrolenticular part is the portion posterior to the lenticular nucleus;
- The sublenticular part is the portion inferior to the lenticular nucleus.
This table was contributed by David Boshell.
||Anterior thalamic radiation
- Cingulate gyrus
- Prefrontal cortex
||Relays to motor areas
- Premotor cortex
- Primary motor cortex
- Motor pathways:
- Corticospinal tract
- Corticobulbar tract
- Somatosensory relays
- Motor cortex
- Motor pathways:
- Somatosensory relays:
- Primary somatosentory cortex
- Association relay
- Optic radiation
- Association cortex
- Visual cortex
- Optic radiation
- Auditory radiation
- Visual cortex
- Auditory cortex
- Lesions to the internal capsule are called subcortical
Click here to go to "The Hypothalamus".
- This is a small portion of the diencephalon
- It is, however, an important in the pathways concerned
with autonomic, endocrine,
emotional, and somatic functions.
- Interconnections with various components of the limbic
- Outputs which influence the pituitary gland;
- Interconnections with various visceral and somatic nuclei.
Extent of the Hypothalamus
The inferior surface is directly exposed to the subarachnoid space.
It is bounded by the optic chiasm, the optic tracts, and the posterior edge
of the mammillary bodies.
- The medial surface extends anteriorly
to the lamina terminalis.
- It extends superiorly to the hypothalamic
- It extends posteriorly to the caudal
edge of the diencephalon.
Subdivisions of the Hypothalamus
It is subdivided longitudinally into 3 regions:
- The anterior
region, is above the optic chiasm;
- The tuberal
region, is above the tuber cinereum;
- And the posterior
region is above and including the mammillary bodies.
- Furthermore, the entire hypothalamus on each side is divided into medial
and lateral regions by a parasagittal
plane through the fornix.
Nolte (1993) The Human Brain, An Introduction to its Functional Anatomy 3rd Ed.
Mosby-Year Book, Inc., St. Louis, Missouri, USA. Table 8, p. 265.
||Medial preoptic nucleus
|Lateral preoptic nucleus
Part of supraoptic nucleus
Arcuate (infundibular) nucleus
Lateral tuberal nuclei
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