Download Thalamus 1







Oval, nuclear mass
Forms 80% 0f diencephalon
Anterior extent- interventricular foramen
Superiorly- transverse cerebral fissure, floor
of 3rd ventricle
Inferiorly- hypothalamic sulcus
Posteriorly- overlaps midbrain



All sensory pathways relay in thalamus.
Many circuits used by cerebellum, basal
nuclei and limbic system involve thalamus.
These utilize more or less separate portions
of thalamus, which has been subdivided into
a series of nuclei.


Nuclei can be distinguished from each other
by topographical locations within thalamus
and by input/output patterns.
Thalamus is divided into medial and lateral
nuclear groups by a thin curved sheet of
myelinated fibres called internal medullary
lamina..



It splits anteriorly to enclose a group of
nuclei, collectively called anterior nucleus,
which is close to interventricular foramen
Medial group contains one large nucleus
called dosomedial nucleus
Lateral group is subdivided into a dorsal and
ventral tier


All thalamic nuclei are a mixture of projection
neurons, whose axons provide the output of
thalamus, and small inhibitory interneurons
that use GABA as a neurotransmitter
Projection neurons account for 75% or more
of the neurons of the most thalamic nuclei,
though the relative proportions of projection
neurons and interneurons vary in different
nuclei


Dorsal tier consists of lateral dorsal, lateral
posterior nuclei and pulvinar.
Lateral posterior nucleus and pulvinar have
almost similar connections


Ventral anterior, ventral lateral- concerned
with motor control; are connected to basal
nuclei and cerebellum
Ventral posterior is subdivided into ventral
posterolateral[ smatosensory input from
body] and ventral posteromedial
[somatosensory input from head]
Lateral and medial geniculate nuclei / bodies
are considered as posterior extensions of
ventral tier
Intralaminar nuclei
 Embedded in internal medullary lamina
 Largest of this group are centromedian and
parafascicular nuclei

•
•
•
•
Lies between lateral thalamic surface and
external medullary lamina
Reticular nucleus is developmentally not a
part of thalamus.
It has distinct anatomical and physiological
properties.
Considered a part of thalamus because of
location and extensive involvement in
thalamic function.


Rostral continuation of periaqueductal gray
matter
Form interthalamic adhesion [when present]



Pipelines for flow of information to cerebral
cortex
Site where decisions are implemented about
which information should reach cerebral
cortex for processing
Any particular type of information affected by
any thalamic nucleus is a function of its input
and output connections



Specific - Regulatory
Specific inputs convey information that a
given nucleus may pass to cerebral cortex
[and for some nuclei to additional sites].
Examples; Medial lemniscus specifically to
VPL. Optic tract to LGB

Regulatory inputs contribute to decisions
about whether or in what form information
leaves a thalamic nucleus



cortical area to which the nucleus projects
thalamic reticular nucleus
diffuse cholinergic, noradrenergic,
serotonergic endings from brainstem reticular
formation
Relay nuclei
•
•
receive well defined specific input fibres and
project to specific functional areas of cerebral
cortex
deliver information from specific functional
systems to appropriate cortical areas
Intralaminar and midline nuclei seem to have
special role in function of basal nuclei and
limbic system



project to association areas of cerebral cortex
receive major inputs from cerebral cortex and
subcortical structures
probably important in distribution and gating
of information between cortical areas


Every nucleus of the thalamus except the
reticular nucleus sends axons to the cerebral
cortex, either to a sharply defined area or
diffusely to a large area.
Every part of the cortex receives afferent
fibers from the thalamus, probably from at
least two nuclei.
•
•
•
•
Every thalamocortical projection is faithfully
copied by a reciprocal corticothalamic
connection.
Thalamic nuclei receive other afferent fibers
from subcortical regions.
Probably only one noncortical structure, the
striatum , receives afferent fibers from the
thalamus.
.


The thalamocortical and corticothalamic
axons give collateral branches to neurons in
the reticular nucleus, whose neurons project
to and inhibit the other nuclei of the
thalamus
No connections exist between the various
nuclei of the main mass of the thalamus,
although each individual nucleus contains
interneurons


The synapses of the interneurons are
inhibitory, and most are dendrodendritic.
Other synapses in the thalamus are
excitatory, with glutamate as the transmitter,
and so are thalamocortical projections
Input
Collateral
branches of
thalamocortical
and
corticothalamic
axons
Output
To each thalamic
nucleus that
sends afferents
to reticular
nucleus
Functions
Inhibitory
modulation of
thalamocortical
transmission
Input
Output
Functions
Cholinergic and
central nuclei of
reticular
formation,locus
coeruleus,
collateral
branches from
spinothalamictra
cts, cerebellar
nuclei, pallidum
Extensive
cortical
projections,
especially to
frontal and
parietal lobes;
striatum
Stimulation of
cerebral cortex
in waking state
and arousal from
sleep;somatic
sensation,
especially pain
[from
contralateral
head and body];
control of
movement
Input
Output
Functions
Inferior
colliculus
Primary auditory
cortex
Auditory
pathway [from
both ears]
Input
Output
Ipsilateral halves Primary visual
of both retinas
cortex
Functions
Visual pathway
[from
contralateral
visual fields]
Input
Contralateral
gracile and
cuneate nuclei;
contralateral
dorsal horn of
spinal cord
Output
Primary
somatosensory
area
Functions
Somatic
sensation
[principal
pathway, from
contralateral
body below
head]
Input
Output
Functions
Contralateral
trigeminal
sensory nuclei
Primary
somatosensory
area
Somatic
sensation
[principal
pathway, from
contralateral
side of head:
face, mouth,
larynx, pharynx,
dura mater]
Input
Contralateral
cerebellar nuclei
Output
Primary motor
area
Functions
Cerebellar
modulation of
commands sent
to motor
neurons
Input
Pallidum
Output
Premotor and
supplementary
motor areas
Functions
Planning
commands to be
sent to motor
neutons
Input
Output
Functions
Pallidum
Frontal lobe,
including
premotor and
supplementary
motor areas
Motor planning
and more
complex
behavior
Input
Output
Functions
Spinothalamic
and
trigeminothalami
c tracts
Insula and
nearby temporal
and parietal
cortex, including
second
somatosensory
srea
Visceral and
other responses
to somatic
sensory stimuli
Input
Output
Functions
Hippocampal
formation;
pretectal area,
superior
colliculus
Cingulate gyrus; Memory ;
visual
interpretation of
association
visual stimuli
cortex
[occipital,posteri
or parietal and
temporal lobes]
Input
Output
Functions
Superior
colliculus
Parietal,
temporal, and
association
cortex
Interpretation of
visual and other
sensory stimuli;
formation of
complex
behavioral
responses
Input
Output
Functions
Pretectal area;
primary and all
association
cortex for
vision;retinas
Parietal lobe,
anterior frontal
cortex, cingulate
gyrus, amygdala
Interpretation of
visual and other
sensory stimuli,
formation of
complex
behavioral
responses
Input
Output
Functions
Etorhinal cortex, Prefrontal cortex Behavioral
amygdala
responses that
,collaterals from
involve decisions
spinothalamic
based on
tract, pallidum,
prediction and
substantia nigra
incentives
Input
Output
Funtions
Amygdala,
hypothalamus
Hippocampal
formation and
parahippocampa
l gyrus
Behaviorr;includi
ng visceral and
emotional
responses
Input
Output
Funtions
Mamillary body
Cingulate gyrus
Memory



Vascular accidents
Can involve adjacent structures
Small lesion can lead to large collection of
deficits



Paroxysms of intense pain triggered by
somatosensory stimuli
Pain may spread to involve entire one- half of
the body- analgesic resistant
Abnormal perception of stimuli that do not
cause pain



Intensity and modality may be distorted
May seem unusually uncomfortable or
unpleaseant
Similar syndrome can develop in some
patients after damage in almost any part of
Anterolateral pathway




This type of pain is called Thalamic
pain/central pain
Cause not understood
Lesions causing this pain always involve
VPL/VPM nuclei with sparing of spinothalamic
and spinoreticulothalamic fibres that end in
other thalamic nuclei
May result in imbalanced thalamic activity



Total/nearly total loss of somatic sensation in
contralateral head and body
Gradually – return of some appreciation of
painful, thermal and gross tactile stimuli
Functions associated with Medial lemniscus
tend to more severely and oermanently
impaired



Discriminative touch may be abolished
Position sense may be greatly impaired
Sensory ataxia [due to loss of proprioception]
may be present


Tahalamic pain+ hemianaesthesia+sensory
ataxia contralateral to a posterior thalamic
lesion= thalamic syndrome
It is often accompanied by mild and transient
paralysis [damage to corticospinal fibres in
Internal capsule] and various types of residual
involuntary movements [damage to adjacent
basal nuclei]
It is often accompanied by
 mild and transient paralysis [damage to
corticospinal fibres in Internal capsule]
 various types of residual involuntary
movements [damage to adjacent basal nuclei]