Embryological origin of thalamus

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Franklin Williams
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1 diencephalon Embryological origin of thalamus The diencephalon gives rise to the: Thalamus Epithalamus (pineal gland, habenula, paraventricular n.) Hypothalamus Subthalamus (Subthalamic nuclei)

2 The Thalamus: Gross features. Note: In this and the following images you will be expected to name the structures Medial view indicated by the red arrows. You might also be called upon to give a brief functional description

3 Name the structures indicated by the red arrows

4 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.

5 The rat thalamus Nissl stain Name the structures indicated by the red arrows Myelin stain

6 The human thalamus

7 Location of the thalamus in the somatosensory pathway

8 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

9 Afferents to thalamic nuclei: What does the morphology of these afferents tell us about their function? Retinal afferents to the LGn Cortical afferent to thalamic cells Lemniscal afferents to the VPL

10 Topology of Afferents Retinotopy (LGn) Somatotopy (VPL)

11 Neuron types in the thalamus 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

12 Brainstem afferents to the thalamus

13 Afferents to thalamic projection neurons have a characteristic distribution to dendritic trees

14 Human thalamus GABA in the rat thalamus

15 Synaptic organization of the thalamus Extraglomerular neuropil

16 Synaptic organization of the thalamus Glomerular neuropil

17 D thalamocortical neuron dendrite T1 principal afferent (Glu) T2 local circuit neuron presynaptic dendrite (GABA) G glial cell The thalamic glomerulus

18 Electrophysiological consequesnces of neuronal morphology. Attenuation of injected current in a relay cell (A, B) and in two local circuit neurons (C, D)

19 40 mv Response (spikes/sec) mv mv mv mv Electrophysiological properties of thamic projection neurons Tonic and Burst Response Modes: Determined by voltage- and time-dependent state of I T If cell is relatively depolarized by 5 mv for msec, I T is inactivated and response is tonic mode: sustained firing of unitary action potentials with no role for I T If cell is relatively hyperpolarized by 5 mv msec, I T is de-inactivated and response is burst mode: I T is activated, leading to all-or-none Ca 2+ spike and burst of action potentials mv -59 mv mv tonic (linear) burst (nonlinear) 100 msec 0-70 mv Current Injection (pa) 3200

20 Luminance

21 Tonic linear detectability: poor cortical activation: poor Burst nonlinear detectability: good cortical activation: good

22 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.

23 Thalamic nuclei can be categorized by their location within the thalamus

24 Representative thalamic nuclei Name Afferents Cortical target Lateral geniculate (LGd) Retina Striate Cortex area 17 Ventroposterior lateral (VPL) Ventroposterior medial (VPM) Ventrolateral (VLp) (VLa) Medial lemniscus (Dorsal columns) Spinothalamic tract Trigeminal nuclei Deep cerebellar nuclei Vestibular nuclei Globus pallidus SI and SII SI and SII Area 4 (Primary motor area) Central lateral (CL) Spinothalamic tract Striatum Area 4, SI, parietal

25 The relationship between the thalamus and cortex is key to understanding the function of the thalamus and is not yet fully understood.

26 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.

27 Generalized scheme of thalamic circuitry including interneurons and the thalamic reticular nucleus

28 Generalized scheme of thalamic circuitry including neurotransmitters layer 4 layer 6 Visual Cortex Glu GABA ACh TRN Input to be Retina Relayed relay cells interneurons Thalamic LGN Relay excitatory inhibitory PBR midbrain

29 New ideas on thalamo-cortical connectivity I 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.

30 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.

31 New ideas on thalamo-cortical connectivity II First and higher order thalamic nuclei

32 New ideas on thalamo-cortical connectivity II contd

33 Thalamic function: the searchlight hypothesis Reciprocal connections Segregation of input into TRN p11

34 Thalamic function: the searchlight hypothesis

35 Thalamus: Summary I 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

36 Low threshold calcium conductance underlies the tonic and burst modes of thalamic projection cells Thalamus: Summary II

37 Clinical correlates 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

COGNITIVE SCIENCE 107A. Sensory Physiology and the Thalamus. Jaime A. Pineda, Ph.D.

COGNITIVE SCIENCE 107A. Sensory Physiology and the Thalamus. Jaime A. Pineda, Ph.D. 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)