Download COGNITIVE SCIENCE 107A Sensory Physiology and the Thalamus

COGNITIVE
SCIENCE
107A
Sensory
Physiology and
the Thalamus
Jaime A. Pineda, Ph.D.
Sensory Physiology
Energies (light, sound, sensation, smell, taste)
Pre neural apparatus (collects, filters, amplifies)
Sensory receptors (transduce energies to neural signals)
Subcortical [thalamus]
Cortical
Sensory Physiology
•  Transduction
–  a change in the membrane permeability of the receptor
cell produced by the effect of stimulus energy, which
then changes the membrane potential of that receptor
and triggers and electric (ionic) signal
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Chemoreceptors – chemicals
Mechanoreceptors – movement and pressure
Photoreceptors – light
Auditory receptors – sound
Chemoreception
•  Receptor
Cells
•  Gustatory
Salt & Sour
-cation influx
Sweet & Bitter
-g-protein
response
•  Olfactory
Similar to
sweet/bitter
g-protein
Mechanoreception
•  Broad category includes:
–  Temperature
–  Pain
–  Stretch
–  Pressure
–  Propioception
–  Orientation in space
•  Most are mechanically gated
Auditory Receptors
•  Stereocillia
–  Respond to bidirectional
input
•  Move one way, K+
channels close
•  Move the other way, K+
channels open
Photoreception
•  Rods
–  Sensitive to low light
–  Located around periphery
of retina
–  Large receptive field
•  Cones
–  Less sensitive to light
–  Detect 3 color types
–  High density in middle of
retina (fovea)
Photoreceptors Cont
Default (rest) is
releasing NTs
When hit by light, gprotein closes up Na
+ channels, causing
hyperpolarization
All sensory pathways lead to
thalamus (except odor)
Thalamus:
(Gr. Inner Chamber)
Sensorimotor input/output
State-dependent
gating function
Biogenic amines
(Amino Acids)
(Biogenic
Amines)
Principles of Thalamic Organization
•  Thalamus is the gateway to cortex
–  All externally generated sensory information relays there
•  except olfaction
–  All internally generated sensory information relays there
•  corticocortical pathways have an indirect connection through
thalamus
•  Information flow is controlled/modulated by:
–  Behavioral state
•  modulatory systems instantiate “behavioral state” control of
thalamic gate
–  Cortex
•  larger number of feedback than feedforward signals
Principles of Thalamic Organization
•  Maintains the separation of inputs
–  subnuclei segregate information flow
•  Lateral inhibitory network
–  filters/sharpens/gates information within/between
subnuclei
•  Output to cortex synapses in layer IV
•  Feedback from cortex arises in layer VI
•  Motor efferents (from cortex to spinal cord)
bypass thalamus
Thalamic Function
As the gateway to cortex, it’s believed to control
how much and what type of information can get
through – thus it performs a “filtering” or “gating”
function and may provide a substrate for
important attentional mechanisms (within and
between sensory modalities).
Consciousness arises from a continuous
‘dialogue’ between cortex and thalamus
R. LLINAS
Orderly slowing down of system
Increased inhibition (hyperpolarization) of
thalamic cells
Higher amplitude
Lower frequency
REM
M
L
A
Lateral part of thalamus has
expanded considerably in
humans relative to other
Primates.
Thalamic Subnuclei
•  Specific relay
–  Receive input from specific areas and relay output to
specific areas (point-to-point; one-to-one)
•  Association (diffuse relay)
–  Receive input from specific areas but relay output to
three major association areas (one-to-many; divergent)
•  Non-specific
–  Receive input from many areas and relay output to
many areas (global systems)
THALAMIC SUBNUCLEI
SPECIFIC RELAY NUCLEI
Inputs from
Thalamic nuclei
Projects to
Cochlea
MGN
Primary auditory cortex
Retina
LGN
Primary visual cortex
Limbic areas
A/LD
Cingulate cortex;
hippocampus
Spinothalamic (body)
VPL
Somatosensory cortex
Trigeminothalamic (head)
VPM
Somatosensory cortex
Basal ganglia
VA
Prefrontal; M1, other motor
areas
Cerebellum
VL
Prefrontal, M1, other motor
areas
ASSOCIATION NUCLEI (DIFFUSE RELAY)
Inputs from
Thalamic nuclei
Projects to
Superior colliculus
LP
Parietal association cortex
Amygdala, hypothalamus
DM
Prefrontal association cortex
Retina, superior colliculus,
striate cortex, pretectum
Pulvinar
Parietal-temporal-occipital
association cortex
NONSPECIFIC NUCLEI
Inputs from
Thalamic nuclei
Projects to
Many areas, e.g.,
hypothalamus, ARAS
Midline and intralaminar
Noncortical areas, sends
collaterals to cortex
RETICULAR NUCLEUS (nRT):
A special thalamic subnuclei that surrounds the lateral part of the
thalamus. Receives input from thalamus and projects back to
thalamus (negative feedback loop – the basis for filtering/gating).
Reticular Nucleus circuitry
THALAMIC CELLS
Relay cells
(maximize transmission of distal postsynaptic potentials to the soma)
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Comprise 75% of thalamic neurons
Receive ~4000 synapses (axodendritic)
Sensory input/nRT feedback to proximal dendrite
Project to layer IV of cortex
Cortical feedback to distal dendrite
Dendritic arbor equals 1 length constant
Time constant = 8-11 ms
Follow Rall’s 3/2 branching rule
Use Glutamate
Rall’s 3/2 rule
•  The diameter of the daughter dendrites
raised to the 3/2 power and summed equals
the diameter of the parent dendrite raised to
the 3/2 power
X3
X1
P
X2
P3/2 = X13/2 + X23/2 + X33/2….
Impedances are matched at branching points allowing
signals to flow efficiently in both directions.
Interneurons
•  Comprise 25% of thalamic neurons
•  Do not follow the 3/2 rule
–  This leads to poor current flow across the branch points
which results in the activity at various clusters being
essentially isolated and thus independent from other
clusters and soma (local computations)
•  Use GABA
•  May be connected in a lateral inhibitory network
Basic thalamic
circuit
Inputs contact both relay and
interneurons using excitatory
connections (Glu and NMDA
receptors). They go to proximal
zone of relay cell dendrites.
Relay cells project to layer IV
of cortex and contact nRT cells.
Feedback from layer VI goes
to distal zone of relay cell
dendrites and contacts nRT cells
and interneurons. Use Glu.
Thalamus Circuitry
Thalamic circuit
(cont.)
nRT cells contact relay cells
using GABA
Interneurons contact relay
cells using GABA
Non-sensory extrathalamic
systems contact relay cells,
interneurons, and nRT cells.
RETINOTHALAMOCORTICAL SYSTEM
Thalamic relay neurons can fire in
one of two modes
Tonic:
single spike
or relay
mode
Phasic/Burst:
Multiple spike
mode
To switch from tonic
to burst mode the
cell is slightly
hyperpolarized
(Vm goes from
-55 to -70 mV)
Functional Implications
•  Tonic mode
–  Info is channeled rapidly to
cortex
–  No loss of fidelity
–  Linear
–  Awake/alert individual
–  20-80 Hz oscillations (beta
activity)
–  NE/ACh depolarize relay
cells (promote tonic mode)
•  Burst (phasic) mode
–  Info is not transferred, only its
presence or absence
•  Signals change in the
environment (wake-up call)
–  Non-linear
–  Less alert/drowsy/quiet or nonREM sleep
–  10 Hz oscillations (alpha
activity)
–  NE/5-HT depolarize nRT cells
(promote burst mode)
Functional Implications:
Role of Feedback
•  Massive positive feedback from cortex to
thalamus increases the “gain” of the input – this
feedback loop may serve to lock or focus the
appropriate circuitry onto the stimulus feature.
•  nRT negative feedback hyperpolarizes relay cells
and they enter burst mode. It also entrains its
oscillations (normally at 10 Hz) onto them. nRT
cell activity a function of extrathalamic inputs.