Thalamus and Hypothalamus

Lecture 13

What are the four divisions of the diencephalon?

Epithalamus, thalamus, subthalamus, hypothalamus.

What structures make up the epithalamus?

Pineal gland & habenula.

What hormone does the pineal gland secrete and when?

Melatonin; in response to darkness.

What is the major nucleus of the subthalamus?

Subthalamic nucleus.

What percent of the diencephalon is the thalamus?

~80%.

What is the function of the thalamus?

Relay for all sensory pathways (except olfaction); gateway to the cortex.

What subdivides the thalamus internally?

The internal medullary lamina.

Name the major thalamic relay nuclei.

VA, VL, VPL, VPM, LGN, MGN, Pulvinar.

Where do VPL and VPM receive input from?

VPL: body sensation; VPM: face sensation.

What nucleus relays visual information to cortex?

LGN → area 17.

What nucleus relays auditory information?

MGN → area 41.

What is the intralaminar nuclear group responsible for?

Nonspecific activation; arousal; basal ganglia connections.

What does the thalamic reticular nucleus do?

Inhibitory control over other thalamic nuclei; no cortical projections.

What fiber bundle contains almost all cortical afferents/efferents?

Internal capsule.

What are the longitudinal divisions of the hypothalamus?

Anterior, tuberal, posterior.

What are the medial-lateral zones of the hypothalamus?

Periventricular, medial, lateral.

Which hypothalamic nuclei regulate water balance (osmoreception)?

SON & PVN.

Which nuclei regulate feeding/satiety?

Lateral nucleus (feeding); VMN (satiety); arcuate (integration).

What nucleus controls circadian rhythms?

Suprachiasmatic nucleus.

What nuclei produce ADH and oxytocin?

PVN & SON (magnocellular cells).

Describe the four major divisions of the diencephalon and their main components.

• Epithalamus:

  • Pineal gland: releases melatonin; regulates circadian rhythms & reproductive cycles.

  • Habenula: involved in aversion, emotional processing.

• Thalamus (80% of diencephalon): major sensory relay; part of basal ganglia, cerebellar, and limbic circuits; gateway to cortex.

• Subthalamus:

  • Contains subthalamic nucleus, zona incerta, and rostral parts of red nucleus/SN.

  • Functionally tied to basal ganglia.

• Hypothalamus: controls homeostasis, autonomic, endocrine, limbic functions.

Describe the pineal gland and its functions.

• Midline, unpaired, pinecone-shaped structure.

• Secretes melatonin in response to darkness.

• Regulates circadian rhythm and reproductive cycles.

Describe the components and function of the subthalamus.

• Includes STN, zona incerta, and parts of midbrain nuclei.

• STN is major excitatory driver of GPi in basal ganglia.

• Zona incerta continuous with reticular formation.

Why is the thalamus referred to as the “gateway to the cortex”?

All sensory pathways (EXCEPT olfaction) synapse in the thalamus before projecting to cortex.

Describe the internal medullary lamina and its importance.

• A sheet of myelinated fibers dividing the thalamus into anterior, medial, and lateral nuclear groups.

• Contains intralaminar nuclei.

List the thalamic nuclear divisions and the nuclei within each.

Anterior group:

• Anterior nucleus (limbic, memory)

Medial group:

• Dorsomedial (DM) nucleus (prefrontal connections)

Lateral group:

• VA/VL: motor relay (basal ganglia & cerebellar input)

• VPL: somatosensory (body)

• VPM: somatosensory (face)

• Pulvinar: association cortex integration

• LGN: visual input

• MGN: auditory input

What are the intralaminar nuclei, and what do they do?

• Embedded in internal medullary lamina.

• Include centromedian (CM) & parafascicular (PF) nuclei.

• Important for arousal, widespread activation, basal ganglia loops.

What is the thalamic reticular nucleus?

• Thin shell adjacent to internal capsule.

• GABAergic inhibition of thalamic nuclei.

• Does NOT project to the cortex.

• Critical in sleep/wake rhythms.

Distinguish between relay, association, and nonspecific nuclei.

• Relay nuclei: receive specific input from subcortical areas → project to cortex (e.g., LGN, MGN, VPL).

• Association nuclei: receive cortical input → project back to cortex (e.g., DM).

• Nonspecific nuclei: broad projections (e.g., intralaminar).

Describe the internal capsule and its significance.

• Compact white matter bundle between lenticular nucleus (putamen + GP) and thalamus/head of caudate.

• Contains nearly all cortical afferents/efferents, incl. CST, corticobulbar, corticopontine, thalamocortical fibers.

• Damage → widespread motor/sensory deficits.

Describe the longitudinal and mediolateral divisions of the hypothalamus.

Longitudinal: anterior, tuberal, posterior.
Mediolateral: periventricular, medial, lateral.

Describe the major inputs to the hypothalamus.

• Brainstem & spinal cord: visceral and somatic sensory.

• Limbic system: amygdala, hippocampus → emotional/autonomic responses.

Distinguish magnocellular vs. parvocellular neurosecretory cells.

Magnocellular:

• Located in PVN & SON.

• Send axons to posterior pituitary.

• Release ADH & oxytocin into bloodstream.

Parvocellular:

• Project to median eminence.

• Release releasing/inhibiting hormones into portal circulation → anterior pituitary.

Explain hypothalamic control of autonomic function.

• Anterior hypothalamus: parasympathetic responses (↓ HR, pupil constriction).

• Posterior hypothalamus: sympathetic responses (↑ HR/BP, dilation).

Explain hypothalamic control of autonomic function.

• Anterior hypothalamus: parasympathetic responses (↓ HR, pupil constriction).

• Posterior hypothalamus: sympathetic responses (↑ HR/BP, dilation).

How does the hypothalamus regulate temperature?

• Preoptic area: detects temperature; initiates heat loss mechanisms.

• Posterior nucleus: activates heat production (shivering, vasoconstriction).

Describe hypothalamic regulation of drinking.

• SON/PVN detect osmotic changes (osmoreceptors).

• Activate ADH release and stimulate thirst.

Describe hypothalamic regulation of eating.

• Arcuate nucleus: integrates signals (glucose, leptin).

• Lateral nucleus: feeding center → damage = refusal to eat.

• VMN: satiety center → damage = overeating/obesity.

Explain the hypothalamic response to stress.

• PVN releases CRH → anterior pituitary releases ACTH → adrenal glands release cortisol.

• Sex differences in HPA axis response exist.

Describe hypothalamic involvement in sleep/wake cycles.

• SCN: circadian rhythm pacemaker.

• Tuberomammillary nucleus: histamine → wakefulness.