Nolte Chapter 16 – The Thalamus and Internal Capsule: Getting to and from the Cerebral Cortex



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Nolte Chapter 16 – The Thalamus and Internal Capsule: Getting to and from the Cerebral Cortex

The diencephalon is made up of the epithalamus(pineal), subthalamus, hypothalamus(which includes the mammillary bodies and the infundibular stalk) and the thalamus.



Epithalamus

  • includes the pineal gland and habenular nuclei.

  • the pineal is just rostral to the superior colliculi and resembles a pinecone.

  • the pineal arises in fish as photoreceptors and is suspected of monitoring the day.

  • in mammals there are a collection of secretory cells known as pinealocytes that receive a light-regulated input and sense seasonal cycles of reproduction.

  • the pineal secretes a hormone derived from serotonin known as melatonin at high rates during darknes.

    • this has an antigonadotropic effect

  • the pineal gland is attached to the dorsal surface of the diencephalon by a stalk. Caudally at the base of the stalk is the posterior commissure. Rostrally is a small swelling on each side called a habenula

  • Input to habenula is the stria medullarias

    • these fibers originate in the globus pallidus and limbic structures (amygdala).

  • output of habenula is the habenulointerpeduncular tract (fasiculus retroflexus) which goes to the interpeduncular nucleus and the reticular formation.

Subthalamus

  • the substantia nigra and red nucleus.

  • traversed by somatosensory pathways and pathways between cerebellum and basal ganglia

  • the subthalamic nucleus is medial and superior to parts of the cerebral peduncle and internal capsule. It’s very involved in the basal ganglia circuitry.

Thalmus

  • The intermedullary lamina divide up most of the thalamus into medial and lateral groups of nuclei

    • this splits on the anterior end and encloses the anterior nucleus

  • the medial group of the thalamus consists of one, single large nucleus – the dorsomedial nucleus

  • the lateral group has a dorsal and ventral tier

    • dorsal: lateral dorsal, lateral posterior, and pulvinar

    • ventral: ventral anterior(motor relay), ventral lateral(motor relay), ventral posterior

      • ventral posterior: ventral posterolateral(somatosensory relay) and ventral posteromedial(somatosensory relay for the head).

    • also consists of the lateral geniculate(visual relay) and the medial geniculate(auditory relay)

  • the intralaminar nuclei consist of the centromedian and the parafasicular

    • the habenulointerpeduncular passes through the centromedian

  • the reticular nucleus partially surrounds the lateral surface thalamus

    • covers the lateral surface

  • there are midline nuclei that cover the medial surface of the thalamus and are basically rostral continuations of the periaqueductal gray.

  • The thalamus is thought to transform information and extract features and decisions are implemented about which information should reach the cerebral cortex accurately for further processing.

  • all the neurons are projections (except for reticular)

  • inputs are specific or regulatory

    • specific convery information that a given thalamic nucleus may pass on accurately to the cerebral cortex

    • regulatory are those that contribute to decisions about the form in which information leaves a thalamic nucleus. regulatory inputs come from the cerebral cortex, but some also come from the reticular nucleus and reticular formation.

  • There are distinctive types of nuclei

    • relay nuclei receive well-defined bundles of specific input fibers and project to particular functional areas of the cerebral cortex

    • association nuclei project to association areas and receiv their major specific inputs from the cerebral cortex itself and some subcortical structures

    • intralaminar and midline nuclei function with the basal ganglia and limbic system.

  • There are two states

    • projection neurons that are slightly depolarized are in a tonic mode and behave normally

      • their frequency is a function of input magnitude

      • focusing attention palces appropriate thalamic projection neurons in tonic mode.

    • projection neurons hyperpolzaried beyong the tonic range enter a burst mode where just a slight depolarization causes transient opening of CA2 channels, followed by their inactivation. This send depolarizing waves sufficient to trigger Na based action potentials.

      • this is mainly during sleep, but their function can be good for sensitivity to occurrences of events without very specific transmission for detailed information. Instead think of them as serving a “lookout” function in this state.

  • ventral anterior(motor relay) and ventral lateral(motor relay) get input from the superior cerebellar peduncle.

  • The anterior nucleus is the principal relay for the limbic system and receives the mammillothalamic tract and projects to the cingulate gyrus. Lateral dorsal also projects to the cingulate.

  • The Dorsalmedial Nucleus and Pulvinar are the principal association nuclei

    • dorsalmedial goes to prefrontal cortex and gets its inputs from the prefrontal cortex, and limbic system(amygdala)

    • Pulvinar (along with the lateral posterior) is interconnected with the parietal-occipital-temporal association cortex

      • inputs come from the association and the visual system.

  • The midline and intralaminar nuclei have inputs from cortex, basal ganglia, cerebellum, brainstem, reticular formation, spinothalamic, and spoinoreticulothalamic.

    • they project to multiple cortical areas, basal ganglia, and limbic strctures.

    • parallel circuit.

  • The thalamic reticular nucleus has no projections to the cerebral cortex

    • it receives inputs from the crtex and from thalamic projection neurons and send inhibitory signals back into the thalamus.

Internal Capsule

  • between the lenticular nucleus and the thalamus/head of the caudate

  • almost all neural traffic to and from the cerebral cortex comes through the internal capsule

    • mainly by and through the thalamus, buy some also go straight to the cerebral peduncle to reach pontine nculei(corticopontine), motor nuclei of cranial nerves (corticobulbar), and the spinal cord motor neurons (corticospinal).

  • The corona radiate are fibers that fan out just above the internal capsule to ineterconnect to different cortical areas in the centrum smeovale of each hemisphere.

  • the medial boundary of the lenticular nucleus is formed by the internal capsule and continues around posteriorly and inferiorly to partially envelop this nucleus

    • inferior many fibers funnel down into the cerebral peduncle

    • superiorly they fan out as corona radiate

  • the anterior limb is between lenticular and the caudate

    • has fibers connecting anterior nucleus with cingulate gyrus and the dorsomedial nucleus and the prefrontal and the frontal lobe to the pontine nuclei.

  • the posterior limb seperates the lenticular from the thalamus.

    • fibers interconnection the ventral anterior and ventraolateral with motor areas of cortex. it also contains corticospinal and corticobulbar fibers. It also contains fibers projecting from ventroposteriolateral/medial to the postcentral gyrus. Corticobulbar fibers that are going to motor nuclei of cranial nerves as well.

    • the most posterior part is the retrolenticular and gives rise to the optic radiation, fibers passing between parietal and occipital association with the pulvinar lateral posterior complex. it also contains corticopontine fibers from the parietal lobe.

    • inferior to this is the sublenticular part which contains more optic radiation and the auditory radiation with fibers from the medial geniculate nucleus that dip under the lenticular nucleus and then superior to the tranverse temporal gyri.

  • the genu is the portion at the junction and occurs at the anterior end of the thalamus(interventricular foramen)

    • contains some frontopontine fibers and fibers interconnecting the dorsomedial nucleus and prefrontal cortex


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