diencephalon
Embryological origin of thalamus
The diencephalon gives rise to the:
•Thalamus
•Epithalamus (pineal gland, habenula, paraventricular n.)
•Hypothalamus
•Subthalamus (Subthalamic nuclei)
The Thalamus: Gross features.
Note: In this and the following images you will be expected to name the structures
indicated by the red arrows.Medial
Youview
might also be called upon to give a brief
functional description
Name the structures indicated by the red arrows
For the most part when we refer to the ‘thalamus’ we really mean
the dorsal thalamus. Most of the following material refers to the
dorsal thalamus but you should be aware of the ventral thalamus
that consists of the:
•thalamic reticular nucleus
•ventral lateral geniculate nuc.
•zona incerta
.
The dorsal thalamus is divided into a number of nuclei.
A basic definition of a thalamic nucleus is “a circumscribed region of
cytoarchitecture receiving a particular set of afferent connections
and projecting within the borders of a particular cortical field or
fields.”
The rat thalamus
Nissl stain
Name the
structures
indicated by
the red arrows
Myelin stain
The human thalamus
Location of the thalamus in
the somatosensory pathway
Dorsal Thalamic Nuclei
•Project to the cerebral cortex (and some to basal ganglia)
•Receive projections from the cerebral cortex
•No descending projections
•All sub-cortical information (except olfaction) passes through the thalamus to get
to the cortex
•All parts of cortex receives projections from thalamus
Afferents to thalamic nuclei:
What does the morphology of these afferents tell us about their function?
Retinal
afferents to
the LGn
Cortical
afferent to
thalamic cells
Lemniscal afferents to the VPL
Topology of Afferents
Retinotopy (LGn)
Somatotopy (VPL)
Neuron types in the thalamus
LGn Y-cell
LGn X-cell
LGn
local circuit neuron
Thalamic nuclei contain two basic cell types:
Projection neurons and interneurons (local circuit neurons)
Some nuclei like the LGn have more than one class of projection neuron
Brainstem afferents to the thalamus
Afferents to thalamic projection neurons have a
characteristic distribution to dendritic trees
GABA in the rat thalamus
Human thalamus
Synaptic organization of the thalamus
Extraglomerular neuropil
Synaptic organization of the thalamus
Glomerular neuropil
The thalamic glomerulus
•D thalamocortical neuron dendrite
•T1 principal afferent (Glu)
•T2 local circuit neuron presynaptic dendrite (GABA)
•G glial cell
Electrophysiological consequesnces of neuronal morphology.
Attenuation of injected current in a relay cell (A, B) and in two local circuit neurons (C, D)
Electrophysiological properties of thamic projection neurons
Tonic and Burst Response Modes:
Determined by voltage- and time-dependent state of IT
If cell is relatively depolarized by ≥5 mV for ≥50-100 msec, IT is inactivated and response
is tonic mode: sustained firing of unitary action potentials with no role for IT
40 mV
100 msec
300
200
-47 mV
100
tonic (linear)
burst (nonlinear)
0
0
-70 mV
-83 mV
-59 mV
-77 mV
-59 mV
Response (spikes/sec)
If cell is relatively hyperpolarized by ≥5 mV ≥50-100 msec, IT is de-inactivated and
response is burst mode: IT is activated, leading to all-or-none Ca2+ spike and burst of
action potentials
800
1600
2400
Current Injection (pA)
3200
Luminance
Tonic
Burst
linear
nonlinear
detectability:
poor
detectability:
good
cortical activation:
poor
cortical activation:
good
Hypothesis: Bursts as a “Wake-up Call”
tonic firing is better for stimulus analysis and bursting is better for
detecting changes or novelty in a relatively unattended scene;
bursts act as a “wake-up call”.
indirect evidence: more bursting during inattention or drowsiness
and a tendency for novel stimuli to elicit bursts.
but there is still much about burst and tonic firing in behaving
animals to be explained.
Thalamic nuclei can be categorized by their location within the thalamus
Representative thalamic nuclei
Name
Afferents
Cortical target
Lateral geniculate (LGd)
Retina
Striate Cortex area
17
Ventroposterior lateral
(VPL)
Medial lemniscus
(Dorsal columns)
Spinothalamic tract
SI and SII
Ventroposterior medial
(VPM)
Trigeminal nuclei
SI and SII
Ventrolateral (VLp)
Deep cerebellar
nuclei
Vestibular nuclei
Area 4
(Primary motor area)
(VLa)
Central lateral (CL)
Globus pallidus
Spinothalamic tract
Striatum
Area 4, SI, parietal
The relationship between the thalamus and cortex is key to understanding
the function of the thalamus and is not yet fully understood.
Corticothalamic afferents terminate in:
•Layer 4, spill into 3 and 5
•Layer 6
•Layer 1 spill into 2
Individual nuclei have projections to combinations
of layers, partly depending on cells size.
Generalized scheme of thalamic circuitry including interneurons and the
thalamic reticular nucleus
Generalized scheme of thalamic circuitry including neurotransmitters
layer 4
layer 6
Visual
Cortex
Glu
GABA
TRN
ACh
excitatory
inhibitory
relay
cells
Input to
beRetina
Relayed
interneurons
Thalamic
LGN
Relay
PBR
midbrain
New ideas on thalamo-cortical connectivity I
Conventional
Alternate based on
Parvalbumin and calbindin (Jones)
Retinal
afferents
cortical afferent
Afferents revisited:
There is strong evidence that cortical afferents form a heterogeneous population. Some have a morphology
similar to sub-cortical afferent and arise from layer 5 cortical cells. Together with the sub-cortical afferents
these can be seen as ‘drivers’ and deliver the message to be processed by the projection neuron. The small
cortical afferents from layer 6 are ‘modulators’ and together with other non-driver afferents alter the
responsiveness of the cell.
New ideas on thalamo-cortical connectivity II
First and higher order thalamic nuclei
New ideas on thalamo-cortical connectivity II contd
Thalamic function: the searchlight hypothesis
Reciprocal connections
Segregation of input into TRN
p11
Thalamic function: the searchlight hypothesis
Thalamus: Summary I
All information that goes to cortex passes through thalamus
Thalamus can act as a gate.
•TRN opens and closes the gate.
•TRN might mediate selective attention/lateral inhibition
•Synchronize input
Local circuit neurons
•Filter inputs
•Lateral inhibition
•Synchronize input
p12
Thalamus: Summary II
Low threshold calcium
conductance underlies the
‘tonic’ and ‘burst’ modes
of thalamic projection
cells
Clinical correlates
Sleep:
Two stages of sleep.
•Slow wave sleep is characterized by low frequency, high amplitude oscillations
•Desynchronized (REM) sleep high-frequency, low amplitude oscillations.
During slow wave sleep cholinergic input is reduced and cells enter burst mode
•The bursting in cells is synchronized
•Depends on TRN relay cell interactions
•Note bursting cells are not silent
During REM sleep cholinergic input increases, relay cells enter tonic mode
Epilepsy
Reverberatory circuits between TRN, relay cells and cortex
Thalamic pain syndrome Dejerine-Roussy syndrome