diencephalon
The diencephalon gives rise to the:
•Thalamus
•Epithalamus (pineal gland, habenula, paraventricular n.)
•Hypothalamus
•Subthalamus (Subthalamic nuclei)
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
Myelin stain
Thalamic nuclei of the rat drawn by Le Gros Clark (1932)
Sir Wilfred Le Gros Clark
The human thalamus
Retinal
afferents to
the LGn
Cortical
afferent to
thalamic cells
p5
Lemniscal afferents to the VPL
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
Retinotopy (LGn)
Somatotopy (VPL)
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
p5
p9
Afferents to thalamic projection neurons have a
characteristic distribution to dendritic trees
p10
GABA in the rat thalamus
Human thalamus
Extraglomerular neuropil
Synaptic architecture
the glomerulus is a distinctive feature of many thalamic nuclei
•D thalamocortical neuron dendrite
•T1 principal afferent (Glu)
•T2 local circuit neuron presynaptic dendrite (GABA)
•G glial cell
The glomerulus
p7
Attenuation of injected current in a relay cell (A, B) and in
two local circuit neurons (C, D)
p7
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
p8
-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
p9
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.
p3
thalamic nuclei can be categorized on their location within the thalamus
Name
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)
p4
Afferents
Globus pallidus
Spinothalamic tract
Striatum
Area 4, SI, parietal
Thalamo-cortical projections
Specific and non-specific:
Thalamocortical relationships in the cat
based on degeneration studies by von
Monakow (1895)
Constantine von Monakow
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.
p6
Generalized scheme of thalamic circuitry including
interneurons and the thalamic reticular nucleus
p6
layer 4
layer 6
Visual
Cortex
Glu
GABA
TRN
ACh
excitatory
inhibitory
relay
cells
Input to
beRetina
Relayed
interneurons
Thalamic
LGN
Relay
p10
PBR
midbrain
Scheme of thalamocortical projection in the cat (Macchi, 1983)
1.
Project densely onto a single cortical area
2.
Project densely to one area and diffusely to another
3.
Project diffusely on several areas with concentration in one
4.
Project diffusely to many regions
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.
p11
First and higher order thalamic nuclei
The searchlight hypothesis
Reciprocal connections
Segregation of input into TRN
p11
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
Low threshold calcium
conductance underlies the
‘tonic’ and ‘burst’ modes
of thalamic projection
cells
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