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
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) 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 X 1 X 2 X 3 P 3/2 = X 3/2 1 + X 3/2 2 + X 3/2 3. 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 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.