Objectives

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Figure 11-1 Major thalamic nuclei and their connections. A. Dorsolateral aspect and major nuclei. LD, lateral dorsal nucleus; LP, lateral posterior nucleus; MD, medial dorsal nucleus; VA, ventral anterior nucleus; VL, ventral lateral nucleus; VPL, ventral posterior lateral nucleus; VPM, ventral posterior medial nucleus.

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 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 premotor cortex (Brodmann 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 area 4) and the supplementary motor cortex (Brodmann area 6).

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

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

b.Ventral posteromedial (VPM) nucleus receives the trigeminothalamic tracts and projects to the somesthetic (sensory) cortex (Brodmann areas 3, 1, and 2). The gustatory (taste) pathway originates in the solitary nucleus and projects via the central tegmental tract to the VPM and thence to the gustatory cortex of the postcentral gyrus, of the frontal operculum, and of the insular cortex. The taste pathway is ipsilateral.

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

5.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 (Brodmann areas 41 and 42).

F.The reticular nucleus of the thalamus surrounds the thalamus as a thin layer of γ-amino- butyric 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.

94 Chapter 11

IIIBlood Supply. The thalamus is irrigated by the following three arteries (see Figure 4-1):

A.Posterior Communicating Artery

B.Posterior Cerebral Artery

C.Anterior Choroidal Artery (Lateral Geniculate Body)

IV The Internal Capsule (Figure 11-2) is a layer of white matter (myelinated axons) that separates the caudate nucleus and the thalamus medially from the lentiform nucleus laterally. It can be divided into five parts, the:

1.Anterior limb is located between the caudate nucleus and the lentiform nucleus (globus pallidus and putamen). It contains fibers that interconnect the anterior nucleus and the cingulate gyrus, as well as fibers connecting the dorsomedial nucleus with the prefrontal cortex. Finally, it contains frontopontine fibers.

2.Genu is located near the interventricular foramen and contains corticonuclear fibers.

3.Posterior limb is located between the thalamus and the lentiform nucleus. It contains fibers

that connect the VA and VL nuclei with motor cortex, as well as fibers connecting the VP nuclei to somatosensory cortex. Descending fibers include corticospinal (pyramid) and corticonuclear fibers.

4. Retrolenticular part is composed of fibers passing posteriorly to the lentiform nucleus. This includes the optic radiations and fibers that interconnect the pulvinar nucleus with

parietal and occipital association cortices.

Figure 11-2 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 hemiparesis and contralateral hemianopia. VP, ventral posterior.

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B.Major Fiber Systems of the Hypothalamus

1.The fornix is the largest projection to the hypothalamus. It projects from the hippocampal formation to the mammillary nucleus, anterior nucleus of the thalamus, and septal area. The fornix then projects from the septal area to the hippocampal formation.

2.The medial forebrain bundle traverses the entire lateral hypothalamic area. It interconnects the orbitofrontal cortex, septal area, hypothalamus, and midbrain.

3.The mammillothalamic tract projects from the mammillary nuclei to the anterior nucleus of the thalamus (part of the Papez circuit).

4.The stria terminalis is the major pathway from the amygdala. It interconnects the septal area, hypothalamus, and amygdala.

5.The supraopticohypophysial tract conducts fibers from the supraoptic and paraventricular nuclei to the neurohypophysis, which is the release site for ADH and oxytocin.

6.The tuberohypophysial (tuberoinfundibular) tract conducts fibers from the arcuate nucleus to the hypophyseal portal system (see Figure 11-4).

7.The hypothalamospinal tract contains direct descending autonomic fibers. These fibers influence the preganglionic sympathetic neurons of the intermediolateral cell column and preganglionic neurons of the sacral parasympathetic nucleus. Interruption above the first thoracic segment (T-1) causes Horner syndrome.

C.Hypothalamic Functional Regions

1.Autonomic function

a.The anterior hypothalamus has an excitatory effect on the parasympathetic nervous system. Lesion results in unopposed sympathetic activation.

b.The posterior hypothalamus has an excitatory effect on the sympathetic nervous system. Lesion results in unopposed parasympathetic activation.

2.Temperature regulation

a.The anterior hypothalamus regulates and maintains body temperature. Destruction causes hyperthermia.

b.The posterior hypothalamus helps to produce and conserve heat. Destruction causes hypothermia.

3.Water balance regulation. The paraventricular nucleus synthesizes ADH, which controls water excretion by the kidneys.

4.Food intake regulation. Two hypothalamic nuclei play a role in the control of appetite.

a.When stimulated, the ventromedial nucleus inhibits the urge to eat. Bilateral destruction results in hyperphagia, obesity, and savage behavior.

b.When stimulated, the lateral hypothalamic nucleus induces the urge to eat. Destruction causes starvation and emaciation.

D.Hypothalamic Clinical Correlations

1.Diabetes insipidus, characterized by polyuria and polydipsia, results from lesions of the ADH pathways to the posterior lobe of the pituitary gland.

2.The syndrome of inappropriate ADH secretion may be caused by lung tumors or drug therapy (e.g., carbamazepine, chlorpromazine) and results in hyponatremia.

3.Craniopharyngioma is a congenital tumor that originates from remnants of Rathke pouch (see Chapter 2). The tumor is usually calcified. It is the most common supratentorial tumor in children and the most common cause of hypopituitarism in children.

a.Pressure on the optic chiasm results in bitemporal hemianopia.

b.Pressure on the hypothalamus causes hypothalamic syndrome (i.e., adiposity, diabetes insipidus, disturbance of temperature regulation, and somnolence).

E.Pituitary Adenomas account for 15% of clinical symptomatic intracranial tumors. They are rarely seen in children. When pituitary adenomas are endocrine-active, they cause endocrine abnormalities (e.g., amenorrhea and galactorrhea from a prolactin-secreting adenoma, the most common type).

1.Pressure on the optic chiasm results in bitemporal hemianopia.

2.Pressure on the hypothalamus may cause hypothalamic syndrome (Figure 11-5).

Figure 11-5 Coronal section through the hypothalamus at the level of the dorsomedial, ventromedial, and lateral hypothalamic nuclei. Lesions or stimulation of these nuclei result in obesity, cachexia, and rage. The column of the fornix separates the medial from the lateral hypothalamic zones. A lesion of the optic tract results in a contralateral hemianopia. FX, fornix; DM, medial dorsal nucleus of thalamus; OT, optic tract; VL, ventral lateral nucleus of thalamus; VP, ventral posterior nucleus of thalamus. (Reprinted from Fix JD. BRS Neuroanatomy. 3rd ed. Baltimore, MD: Williams & Wilkins; 1996:313, with permission.)

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CASE 11-1

A 90-year-old woman complains of an intense burning sensation on the left side of her neck and upper limb. The patient has a history of high blood pressure and diabetes. What is the most likely diagnosis?

Differentials

● Hypoglycemia; middle cerebral artery stroke; migraine

Relevant Physical Exam Findings

●Unilateral sensory loss is observed. Though the patient may complain of weakness on the affected side, no weakness is found on examination.

Relevant Lab Findings

●Normal serum glucose levels

●Thrombocytopenia

●Ischemic infarction of posterior cerebral artery seen on computed tomography scan

Diagnosis

●Infarction of the ventral posterolateral nucleus of the thalamus results in pure hemisensory loss contralateral to the lesion.

CASE 11-2

A 65-year-old diabetic man was hospitalized after an auto accident with lethargy and progressive confusion. Laboratory tests results revealed low sodium levels. The patient was discharged after serum sodium levels were elevated to 130 mmol/L.

Differentials

● Diabetes

Relevant Physical Exam Findings

●Fatigue and depression

●Slowed mental processing time

●Slowed pulse and hypothermia

●Hyporeflexia and hypotonia

Relevant Lab Findings

●Normal hepatic, renal, and cardiac function

●Hyponatremia that worsens with fluid load

●Serum hypo-osmolality

●Urine hyperosmolarity

Diagnosis

●Inappropriate secretion of antidiuretic hormone by the hypothalamus. As many as 50% of traumatic brain injury patients experience endocrine complications that may result in intracerebral osmotic fluid shifts and brain edema, affecting hypothalamic function.