cognitive science 107a sensory physiology and the thalamuspineda/cogs107a/lectures/thalamus.pdf ·...
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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
• 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, g-protein 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
REM
Higher amplitude Lower frequency
Increased inhibition (hyperpolarization) of thalamic cells
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)
• 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
P X1
X2
X3
P3/2 = X13/2 + X2
3/2 + 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
Tonic:
single spike or relay mode
Phasic/Burst:
Multiple spike mode
Thalamic relay neurons can fire in one of two modes
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 non-
REM 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.