Thalamus Flashcards
The Diencephalon
Majority of sensory, motor and limbic pathways relay in the diencephalon
Includes 4 major parts:
- Epithalamus
- Subthalamus
- Hypothalamus
- Thalamus
The epithalamus
Includes the Habenula and the pineal gland
Habenula
Pathway for the limbic system to influence brainstem reticular formation
Pineal gland Endocrine organ Releases Melatonin Codes photoperiodism Inhibited by light Useful marker in CT scans Tumors associated with early puberty
Subthalamus
Subthalamic nucleus
Major role in indirect pathway of basal ganglia-cortex modulation
Zona incerta
Rostral continuation of the midbrain reticular formation
Thalamus
ALL SENSORY PATHWAYS relay in thalamus
– Exception of olfaction
A portal to the cortex, not just a passive relay
Decisions about what should reach the cortex
10X as many fibers from cortex as from subcortical afferents
Also cerebellar, basal ganglia and limbic pathways involve thalamus
31 nuclei comprise thalamus
Most separated by internal medullary lamina
Thin, curved sheet of myelinated fibers
Separated into three divisions
Anterior
Medial (or “dorsomedial”)
Lateral (Dorsal, Ventral)
2 directions information flow (thalamus)
Sending ahead to cortex
Eg. Spinothalamic
THALAMOCORTICAL RELAYS
Regulates if information leaves thalamus
Attention, consciousness
THALAMIC RETICULAR NUCLEI
Anterior nucleus
Group of nuclei in the “anterior” division
Afferent input from mammillothalamic tract
Originates in mammillary body
Efferent info to cingulate gyrus
Limbic system info
Dorsomedial nucleus = DM
Association nucleus
Gate info between cortical areas
Afferents = Prefrontal cortex, olfactory and limbic structures
Efferents = Prefrontral cortex
Laterodorsal nucleus = LD
Association
Similar in function and connection to anterior nucleus
Afferent = hippocampus
Efferent = cingulate gyrus
Lateral posterior (LP)/pulvinar
Association nuclei Pulvinar-LP complex Largest nucleus Connected reciprocally with parieto-occipital-temporal association cortex *Vision
Ventral anterior (VA), ventral lateral (VL)
Relay nuclei of the striato-cortical circuit (basal ganglia)
Grouped together, but
VA mostly basal ganglia
VL mostly cerebellum
Ventral Posterior Lateral and Medial
(VPL and VPM)
The main processor and relay nuclei for somatosensory input
Somatotopic
VPL
Afferent = medial lemniscus, spinothalamic (trunk and extremities)
Efferent = Somatosensory cortex
VPM
Afferent = trigeminothalamic (head region)
Efferent = Somatosensory cortex
clinical point with VPL and VPM
Modality segregation
Proprioceptive (anterior), Tactile (middle) and Nociceptive (posterior)
Vascular accidents in the thalamogeniculate arteries
Major damage to VPL/VPM, thalamic pain
Central pain from otherwise painless tactile stimulus
Extensive damage also cause ataxia and tactile insensitivity. Together with thalamic pain = “thalamic syndrome”
Occurs contralateral to lesion
Medial geniculate nucleus (MGN)
Major auditory relay
Tonotopic
Lateral geniculate nucleus
Major visual relay
Retinotopic maps
Optic nerve—LGN—1° Visual cortex
Optic nerve fibers post-chiasm = “optic tract”
Intralaminar nuclei
Nonspecific nuclei (also paleospinothalamic pathway) Afferents = brainstem, cortex Efferents = cortex, basal ganglia Landmark nuclei: -- Centromedian Efferent to putamen and motor cortex
– Parafascicular
Efferent to caudate and prefrontal cortex
Thalamic reticular nuclei = TRN
External medullary lamina is thin sheet of fibers covering lateral surface of thalamus
Between this and internal capsule = TRN
NO CEREBRAL PROJECTION
GABA-ergic projections back to the thalamus
Thalamocortical projections pass through TRN
TRN inhibit specific thalamic neurons, modulating ascending flow
*Mechanism for thalamic gating
Tonic state of neurons
state = normal depolarization of thalamic neuron = Action potential chain
burst state of neurons
Burst state = thalamic neuron is hyperpolarized, sensory info blocked
due to control by TRN neurons
tonic vs burst state
TRN neurons control tonic/burst state of thalamic neurons
Attention = no TRN inhibition, high thalamocortical activity
Sleep = high TRN inhibition, low thalamocortical activity
EEG
is a summary of action potentials in an area.
Each large amplitude deflection in “slow wave sleep” is a “slow wave.” Main EEG aspect of slow wave, restorative sleep. Body is devoting its resources to metabolic regeneration, esp. synapses (requires a lot of energy). When thalamocortical neurons are quiet we are good at regenerating.
In wake/ rem sleep, low amplitude EEG (TRNs not as active). REM sleep interesting– behaviorally still, but the brain looks like it’s awake. It’s when there are vivid dreams.
types of waves on EEG
Alpha waves = quiet wakefulness
Beta waves = awake, high activity
Theta waves = light sleep
Delta waves = deep sleep
Thalamocortical relays and TRN functions regulate behavior state
NE, HA, 5-HT, Ach— all from ascending reticular activating system neurons. Project to layer 6 in cortex which joins it in telling the nRt to shut up. When we go to sleep, this circuit gets shut off so that the thalamic reticular neurons act and tell thalamocortical neurons to stop getting through to the cortex anymore.
