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.