In one breath
The thalamus is the relay station that every sensation except smell passes through on its way to the cortex. A Y-shaped internal medullary lamina divides it into anterior, medial and lateral groups; by function, its nuclei are specific relay, association, nonspecific and motor nuclei. Besides relaying, it perceives crude sensation, drives arousal and EEG rhythm, and links the basal ganglia and cerebellum to the motor cortex. A VPL lesion causes thalamic syndrome: discriminative touch is lost on the opposite side, while pain, temperature and crude touch suffer less.
Builds on: Ascending tracts: dorsal column and anterolateral system · Pain pathways and perception · Trigeminal pathway and trigeminal neuralgia · Leads to: Somatosensory cortex · Sensory lesions and sensory function tests · Basal ganglia: circuits and functions
Where it sits
- An egg-shaped block of nuclei, set at a slant on top of the brainstem. There is one on each side.
- At the front, the third ventricle lies between the two thalami. At the back, the corpora quadrigemina lie between them.
- Inputs: the ascending sensory pathways, fibres of the ascending reticular formation, and corticothalamic fibres, mostly from layer VI of the cortex.
- Output: to the sensory areas of the cortex. Hence its name, the sensory relay station.
Thalamic nuclei
The two laminae
- External medullary lamina: a sheet of thalamocortical and corticothalamic fibres over the lateral surface. It separates the reticular nucleus from the main mass of the thalamus.
- Internal medullary lamina: a Y-shaped sheet inside the thalamus. It splits the thalamus into anterior, medial and lateral groups.
Anatomical groups
| Group | Nuclei |
|---|---|
| Anterior | Anterior nuclei, in the fork of the Y |
| Medial | Medial, midline, centromedian, intralaminar |
| Lateral, dorsal tier | Lateral dorsal, lateral posterior, pulvinar |
| Lateral, ventral tier | VA, VL, VP, MGB, LGB |
- VA is the ventral anterior nucleus and VL the ventral lateral.
- The ventral posterior (VP) nucleus is also called the ventrobasal complex. It has a lateral part, VPL (ventral posterolateral), and a medial part, VPM (ventral posteromedial).
Physiological groups
By what they do, the nuclei fall into four groups.
1. Specific relay nuclei. An ascending pathway synapses here, and the next neuron carries the signal to its own area of cortex.
- Ventrobasal complex: takes the dorsal column pathway, the anterolateral system and corticothalamic fibres. It projects to the somatosensory cortex (areas 3, 1 and 2) and relays all body sensation, face included.
- VP, face part: the trigeminal lemniscus brings general sensation from the face, together with the taste fibres. VP relays them to the postcentral gyrus.
- LGB: optic tract → visual cortex. Vision.
- MGB: auditory pathway, via the inferior colliculus → auditory cortex. Hearing.
- Anterior nucleus: the mammillothalamic tract brings it input from the hypothalamus (mammillary body). It projects to the cingulate gyrus and serves memory and emotion.
2. Association nuclei: the pulvinar, the lateral dorsal nucleus and the lateral posterior nucleus.
- Input comes from the sensory cortex and the limbic system. Output spreads widely, to the association cortex or to subcortical structures.
- Pulvinar: sends fibres to the inferior parietal lobe. More broadly, it trades fibres with the parietal, temporal and occipital lobes, and brings somatic, auditory and visual information together.
- Lateral dorsal nucleus: linked both ways with the parietal lobe (the precuneate gyrus). It serves language and speech.
3. Nonspecific nuclei: the intralaminar (IM), midline, centromedian (CM) and reticular (RET) nuclei.
- Input comes mainly from the reticular formation and the paleospinothalamic tract, with more from the striatum, the hypothalamus and other thalamic nuclei.
- Output is diffuse: to wide areas of cortex, to the thalamus itself and to the limbic system.
- Midline nuclei: take input from all the ascending fibres, the hypothalamus and the reticular formation. They project to the neocortex, basal ganglia and hypothalamus, and blend somatic with visceral sensation. They also serve arousal.
- Intralaminar nuclei: take input from the reticular activating system and basal ganglia. They project to the neocortex and prefrontal cortex, and serve arousal and motor functions.
- Dorsomedial nucleus: Table 122.1 files it here too. It takes input from the hypothalamus and prefrontal cortex, sends output to the prefrontal cortex, and pulls crude somatic sensation together.
4. Motor nuclei: the lateral nuclei, VA and VL.
- VL: gets cerebellar input through the dentato-rubro-thalamic tract, and pallidal input through the pallidothalamic tract. It projects to motor areas 4 and 6, feeding proprioceptive information into the control of voluntary movement.
- VA: gets basal ganglia input and projects to the premotor cortex.
Extrinsic and intrinsic nuclei
- The specific relay nuclei are the extrinsic nuclei. All the others are intrinsic.
- Somatosensory information reaches every one of these types of nucleus.
Specific or nonspecific: the anterior nucleus
The chapter text and its summary count the anterior nucleus as specific, but Table 122.1 lists the anterior group under nonspecific. Go with the text: like the other specific nuclei, it passes one input to one cortical area (the cingulate gyrus). For the viva, the specific nuclei are the ventral, lateral, anterior and posterior groups. The nonspecific ones are the intralaminar and midline groups.
Where the pathways end
Most of the signals start in receptors on the opposite side of the body.
| Pathway | Crosses | Ends in |
|---|---|---|
| Dorsal column–medial lemniscus | Medulla | Mainly VPL |
| Anterolateral (spinothalamic) | Spinal cord | VPL, PO, IM |
| Spinoreticulothalamic | Mostly uncrossed | IM, midline or CM, RET |
- VPL and PO project to cortical areas.
- IM projects diffusely to the cortex.
- RET trades fibres only with the other thalamic nuclei.
VPL nucleus
- Many ascending pathways end here, each in its own typical pattern.
- The core takes mostly cutaneous input. The dorsal part takes mostly deep input, and the anterior surface takes muscle input. So VPL is a core of skin input inside a shell of deep input.
- Somatotopy: face, arm and leg are laid out from medial to lateral.
Posterior nuclei (PO)
- PO receives nociceptive input, and its cells answer many somatic and non-somatic modalities.
- Its main job is passing on pain signals: a large PO lesion causes analgesia, and stimulating PO causes pain.
- Unlike VPL, PO has no somatotopic map.
Intralaminar and reticular nuclei
- IM cells have large somatic receptive fields, and painful and other stimuli excite them. How strongly they respond depends on arousal, attention and affect.
- IM projects strongly but diffusely to the cortex, the striatum and the limbic system.
- RET cells answer many somatosensory modalities. RET sends no fibres to the cortex. It is linked both ways with the other thalamic nuclei, so it modulates thalamic activity.
Functions of the thalamus
Every afferent signal bound for the sensory cortex stops in the thalamus first.
- Relay of all somatic sensation, through thalamocortical fibres. In humans and higher animals the thalamus is mainly a relay; in lower animals it is where sensation is integrated.
- Relay of the special senses, except smell. Vision relays in the LGB and hearing in the MGB. Your book gives VPL for taste (see the warning below).
- Arousal: thalamocortical fibres from the nonspecific nuclei form part of the reticular activating system, which keeps us awake.
- Subcortical perception: pain, temperature and pressure (crude touch) are perceived to some degree in the thalamus, which integrates them as well as relaying them. That is why they largely survive a cortical lesion.
- Motor functions:
- Motor loop: globus pallidus → thalamus (pallidothalamic tract) → motor cortex (thalamocortical fibres) → back to the basal ganglia. Through this loop the thalamus shapes postural movements.
- Cerebellar link: the dentato-rubro-thalamo-cortical tract joins the cerebellum to the motor cortex. Through it the thalamus helps plan and programme movements.
- Memory and emotion: the anterior nucleus is part of the Papez circuit. It receives the mammillothalamic tract from the mammillary body, and serves recent memory and emotion.
- EEG synchronization: stimulating the intralaminar nuclei at a low frequency synchronizes the EEG over the cortex of the same side. This is the recruiting response. A high frequency does the reverse: arousal and desynchronization.
- Sleep: the thalamocortical loop sets the pattern of brain activity across the sleep–wake cycle. Inhibitory neurons of the reticular nucleus are thought to help bring on sleep.
- Sensorimotor coordination: all sensory input arrives here, and the thalamus works closely with the basal ganglia, cerebellum and motor cortex. It supplies the sensory feedback that corrects and refines movement.
- Language and speech: the lateral dorsal nucleus is linked both ways with the parietal lobe. It serves language, speech and complex integrated functions.
A slip in your book: what reaches VPL
The book sends three inputs to VPL that, by its own account elsewhere, end in other nuclei.
- Taste: function 2 says VPL, but the organization section sends taste with the trigeminal lemniscus to VP. The standard answer is VPM.
- Pallidal fibres: function 5 says the globus pallidus projects mainly to VPL. The motor-nuclei section and Table 122.1 give basal ganglia input to VA and VL, so write VA/VL.
- Cerebellar fibres: the thalamic syndrome section says VPL relays cerebellar input. The organization section gives that job to VL.
Thalamic syndrome
- Cause: a lesion of VPL, from thrombosis of the posterolateral branch of the posterior cerebral artery.
- Sensory loss, opposite side: discriminative touch and pressure are badly impaired. Diffuse touch, temperature and pain are often less affected.
- Motor signs: reduced muscle tone, marked weakness and ataxia. Your book puts these down to the loss of cerebellar input on its way to areas 4 and 6 (see the slip above).
- Emotion may be disturbed.
- Face spared: vascular lesions usually leave the medial thalamus, with its VPM, intact. So sensation in the face and head is often preserved.
Applied: thalamic pain
This chapter leaves out the feature patients dread most. Your book’s pain chapter describes it (see Specific pain syndromes): long-lasting, intense and deeply unpleasant pain that arises on its own or from a stimulus that should not hurt, with nothing wrong in the periphery. There the lesion is in the posterior thalamic nuclei, and the vessel is the thalamogeniculate branch of the posterior cerebral artery.
- Worth knowing, though not in your pages: the syndrome is also called the Dejerine–Roussy syndrome. On the affected side the threshold for feeling is raised, but a stimulus that gets through feels exaggerated and unpleasant (hyperpathia).
Other deficits
- The cortex is so closely tied to the thalamus that thalamic damage also takes away many cortical functions.
- An electrical lesion of the intralaminar nuclei relieves chronic, suffering-type pain, yet acute pain is still felt.
Epithalamus
- The structures that make up the roof of the third ventricle: the pineal gland, the habenular nuclei and the striae medullaris.
- From the outside, it can be seen above the superior colliculi.
- Worth knowing, though not in your pages: the pineal gland secretes melatonin, which helps set the sleep–wake rhythm.
Subthalamus
- It lies between the upper end of the substantia nigra and the thalamus.
- Contents:
- sensory fasciculi;
- forward (rostral) extensions of midbrain nuclei;
- fibre bundles from the cerebellum and the globus pallidus;
- the subthalamic nuclei.
- Fibres of the supramammillary commissure link the subthalamic structures with one another.
- The subthalamic nucleus (body of Luys) is connected both ways with the globus pallidus. A lesion of it causes hemiballismus.
- Your book’s basal ganglia chapter adds that the flinging movements come from a bleed into the subthalamic nucleus of the opposite side (see Parkinson’s and Huntington’s disease).
Draw it: the nuclei of the thalamus
Draw the thalamus as an egg lying on its side, narrow end forward. Inside, draw a Y with its fork opening forward: the internal medullary lamina. Write anterior nucleus in the fork. Medial to the stem, write medial (dorsomedial) nucleus, with the midline nuclei along the medial edge. The intralaminar nuclei and centromedian go inside the lamina itself. Lateral to the stem, draw two rows from front to back. The dorsal row is lateral dorsal, lateral posterior, then the large pulvinar. The ventral row is VA, VL, then VPL with VPM just medial to it. Hang the MGB (medial) and LGB (lateral) below the pulvinar. Finish with the external medullary lamina along the lateral surface and the thin reticular nucleus outside it.
Exam-answer skeleton: "Describe the connections and functions of the thalamus, with a labelled diagram" (long essay)
- Position of the thalamus; the external and internal medullary laminae.
- The anatomical groups (anterior, medial, and lateral with dorsal and ventral tiers), with the Y-lamina diagram.
- The physiological groups (specific relay, association, nonspecific, motor), with the input and output of each.
- Where the pathways end: medial lemniscus → VPL; spinothalamic → VPL, PO, IM; spinoreticulothalamic → IM, CM, RET.
- VPL (core and shell, medial-to-lateral map), PO (pain, no map), IM and RET.
- The ten functions, a line each: relay, special senses, arousal, crude perception, motor loops, Papez circuit, recruiting response, sleep, sensorimotor coordination, language.
- Applied: thalamic syndrome, and intralaminar lesions for chronic pain.
Asked in exams
- Thalamus: nuclei and functions: Nov 2012, 4 marks · Jul 2018, 10 marks · Jan 2021, 3 marks · Jan/Feb 2022, 5 marks · Jan/Feb 2022, 4 marks · May 2022, MCQ · May 2022, MCQ · Jan 2023, MCQ · Aug 2024, MCQ · Oct 2024, 5 marks
- Thalamic syndrome: May 2022, 4 marks