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Thalamus

I. INTRODUCTION. The thalamus is the largest division of the diencephalon. It plays an important role in the integration of the sensory and motor systems.

II. MAJOR THALAMIC NUCLEI AND THEIR CONNECTIONS (Figure 16-1)

A. The anterior nucleus receives hypothalamic input from the mamillary nucleus through the mamillothalamic tract. It projects to the cingulate gyrus and is part of the Papez circuit of emotion of the limbic system.

B. The mediodorsal (dorsomedial) nucleus is reciprocally connected to the prefrontal cortex. It has abundant connections with intralaminar nuclei. It receives input from the amygdala, substantia nigra, and temporal neocortex. When it is destroyed, memory loss occurs (Wernicke-Korsakoff syndrome). The mediodorsal nucleus plays a role in the expression of affect, emotion, and behavior (limbic function).

C. The centromedian nucleus is the largest intralaminar nucleus. It is reciprocally connected to the motor cortex (Brodmann's area 4). The centromedian nucleus receives input from the globus pallidus. It projects to the striatum (caudate nucleus and puta-men) and projects diffusely to the entire neocortex.

D. The pulvinar is the largest thalamic nucleus. It has reciprocal connections with the association cortex of the occipital, parietal, and posterior temporal lobes. It receives input from the lateral and medial geniculate bodies and the superior colliculus. It plays a role in the integration of visual, auditory, and somesthetic input. Destruction of the dominant pulvinar may result in sensory dysphasia.

E. Ventral tier nuclei

1. The ventral anterior nucleus receives input from the globus pallidus and substantia nigra. It projects diffusely to the prefrontal cortex, orbital cortex, and pre-motor cortex (Brodmann's area 6).

2. The ventral lateral nucleus receives input from the cerebellum (dentate nucleus), globus pallidus, and substantia nigra. It projects to the motor cortex (Brodmann's area 4) and the supplementary motor cortex (Brodmann's area 6).

3. The ventral posterior nucleus (ventrobasal complex) is the nucleus of termination of general somatic afferent (touch, pain, and temperature) and special visceral afferent (taste) fibers. It has two subnuclei.

a. The ventral posterolateral nucleus receives the spinothalamic tracts and the medial lemniscus. It projects to the somesthetic (sensory) cortex (Brodmann's areas 3,1, and 2).

b. The ventral posteromedial (VPM) nucleus receives the trigeminothalamic tracts and projects to the somesthetic (sensory) cortex (Brodmann's areas 3, 1, and 2). The gustatory (taste) pathway originates in the solitary nucleus and

Anterior

Anterior

Gustatory Nucleus

Figure 16-1. Major thalamic nuclei and their connections. (A) Dorsolateral aspect and major nuclei. (B) Major afferent and efferent connections. VA = ventral anterior nucleus; VL = ventral lateral nucleus; VPL = ventral posterior lateral nucleus; VPM = ventral posterior medial nucleus.

projects via the central tegmental tract to VPM, and thence to the gustatory-cortex of the postcentral gyrus (Brodmann's area 3b), of the frontal operculum and insular cortex. The taste pathway is ipsilateral.

F. Metathalamus

1. The lateral geniculate body is a visual relay nucleus. It receives retinal input through the optic tract and projects to the primary visual cortex (Brodmanns area 17).

2. The medial geniculate body is an auditory relay nucleus. It receives auditory input through the brachium of the inferior colliculus and projects to the primary auditory cortex (Brodmanns areas 41 and 42).

G. The reticular nucleus of thalamus surrounds the thalamus as a thin layer of y-aminobu-tyric acid (GABA)-ergic neurons. It lies between the external medullary lamina and the internal capsule. It receives excitatory collateral input from corticothalamic and thalamocortical fibers. It projects inhibitory fibers to thalamic nuclei from which it receives input. It is thought to play a role in normal electroencephaolgram readings.

Cingulate gyrus

Prefrontal cortex

Mamillothalamic tract] Fornix J

Globus pallidus j Substantia nigra]

Area 6

Diffuse frontal cortex

Dentate nucleus ] Globus pallidus > Substantia nigra J

Lobules Cortex Cerebral

Areas 18 and 19 Inferior parietal lobule

Figure 16-1. Major thalamic nuclei and their connections. (A) Dorsolateral aspect and major nuclei. (B) Major afferent and efferent connections. VA = ventral anterior nucleus; VL = ventral lateral nucleus; VPL = ventral posterior lateral nucleus; VPM = ventral posterior medial nucleus.

Trigeminothalamic tracts and taste pathways

Area 4

Medial lemniscus Spinothalamic tracts

Mamillothalamic tract] Fornix J

Globus pallidus j Substantia nigra]

Areas 18 and 19 Inferior parietal lobule colliculus Lateral lemniscus

Areas 41 and 42 Optic tract Area 17

Trigeminothalamic tracts and taste pathways

Cingulate gyrus

Prefrontal cortex

Amygdaloid complex Temporal neocortex Substantia nigra

Area 6

Diffuse frontal cortex

Dentate nucleus ] Globus pallidus > Substantia nigra J

Area 4

Medial lemniscus Spinothalamic tracts

III. BLOOD SUPPLY. The thalamus is irrigated by three arteries (see Figure 3-3).

A. The posterior communicating artery

B. The posterior cerebral artery

C. The anterior choroidal artery (lateral geniculate body)

IV. THE INTERAL CAPSULE (Figure 16-2) is a layer of white matter (myelinated axons) that separates the caudate nucleus and the thalamus medially from the lentiform nucleus laterally.

A. The anterior limb is located between the caudate nucleus and the lentiform nucleus (globus pallidus and putamen).

B. The genu contains the corticobulbar fibers.

C. The posterior limb is located between the thalamus and the lentiform nucleus. It contains corticospinal (pyramid) fibers as well as sensory (pain, temperature, and touch), visual, and auditory radiations.

D. Blood supply

1. The anterior limb is irrigated by the medial striate branches of the anterior cerebral artery and the lateral striate (lenticulostriate) branches of the middle cerebral artery.

2. The genu is perfused either by direct branches from the internal carotid artery or by pallidal branches of the anterior choroidal artery.

3. The posterior limb is supplied by branches of the anterior choroidal artery and lenticulostriate branches of the middle cerebral arteries.

Figure 16-2. Horizontal section of the right internal capsule showing the major fiber projections. Clinically important tracts lie in the genu and posterior limb. Lesions of the internal capsule cause contralateral hemi-paresis and contralateral hemianopia. VP = ventral posterior nucleus.

Lateral Geniculate Body

Figure 16-2. Horizontal section of the right internal capsule showing the major fiber projections. Clinically important tracts lie in the genu and posterior limb. Lesions of the internal capsule cause contralateral hemi-paresis and contralateral hemianopia. VP = ventral posterior nucleus.

Lateral geniculate body (vision)

Visual radiation to striate cortex of occipital lobe (area 17)

Lateral geniculate body (vision)

Visual radiation to striate cortex of occipital lobe (area 17)

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