Thalamus
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Introduction
The thalamus fuctions as the way station for information passing throughout the brain. It also is an important site of decisional information regarding what information should continue for further processing.
Specific inputs enter the various thalamic nuclei. Regulatory inputs arise from innervated cortical areas, as well as diffuse cholinergic, noradrenergic, and serotonergic signals from the reticular formation. These regulatory inputs outnumber the specific inputs in most cases.
Functions
Sensory
Virtually all sensory information passes through the thalamus (except olfaction) en route to the sensory cortex. The thalamus and cortex are connected by fibres running both ways within the internal capsule, which spans the striatum of the basal ganglia.
The ventral posterior and geniculate bodies are termed sensory nuclei.
The thalamus is thought to be especially important for perception of pain.
Motor
Motor information, including from the cerebellum, the subthalamus, and the basal ganglia, is referred to the thalamus before sending it to the motor cortex.
The thalamus conveys information from the cerebellum and globus pallidus to the motor cortex.
The ventral anterior and ventral lateral nuclei are termed motor nuclei.
Limbic
Information pertaining to mood and cognitive function also passes through the thalamus.
Along with the subthalamus, it receives projections from the basal ganglia important for motor function.
The pontocerebellum also provides thalamic input to regulate motor control.
Thalamic neurons can be in two physiologic states, detemined by regulatory state.
Tonic mode exists when neurons are slightly depolarized, with slight additional depolarization causing a trian of action potentials.
Burst mode occurs when neurons are hyperpolarized. Slight depolarization opens voltage-gated Calcium channels, resulting in a few action potentials.
Structure
The thalamus is a big, oval collection of nuclei separated into two. The halves are separated by the narrow third ventricle. The pineal body is normally midline within the thalmaus. It tends to calcify with age; normally midline; deviation is useful during imaging.
Thalamic nuclei can be grouped into three regions - anterior, medial, and lateral nuclear groups.
- anterior group
- medial group
- lateral group
The anterior nucleus is the principle relay nucleus for the limbic system, receiving information from the hypothalamus and projecting to the cingulate gyrus.
Dorsomedial nucleus
The dorsomedial nucleus makes up the medial group and is interconnected with the prefrontal cortex, influencing drive and executive functions. There are inputs from both the prefrontal cortex and limbic structures such as the amygdala, via the basal ganglia.
Lateral Group
The lateral group composes most of the thalamus and can be further divided into the dorsal and ventral tiers.
The dorsal tier contains the lateral dorsal nucleus, which projects to the cingulate gyrus in the limbic system. It also contains the lateral posterior and pulvinar nuclei. These are the largest nuclei and receive inputs from association and visual cortex, though their function is largely unknown.
The ventral tier contains many nuclei
- ventral anterior nucleus: inputs from substantia nigra and globus pallidus, output to premotor cortex to control basic movements
- ventral lateral nucleus: inputs from the cerebellar (dentate nucleus) and basal ganglia; output to primary motor cortex
- ventral posterolateral nucleus
- inputs from the spinothalamic tract regarding pain and temperature
- inputs from dorsal column/medical lemniscus regarding touch/vibrational/proprioreceptive
- output to primary sensory cortex
- ventral posteromedial nucleus
- inputs from the principle nucleus of the trigeminal regarding touch and proprioreception
- inputs from the spinothalamic tract regarding pain and temperature
- output to primary sensory cortex
- lateral geniculate nucleus
- inputs from the optic tracy and superior colliculus regarding vision
- output to primary visual cortex
- medial geniculate nucleus
- inputs from inferior colliculus regarding auditory information
- outputs to the auditory cortex
- intralaminar thalamic nucleus: input from reticular formation, with widespread output
- thalamic reticular nucelus: projects to other thalamic nuclei with inhibitory GABA-ergic connections
Diseases Affecting the Thalamus
The thalamus is supplied by small branches from the posterior cerebral artery, and infarcts can be far-reaching. Effects include pain on up to an entire half of the body, usually triggered by nonpainful stimuli (allodynia)
Resources and References
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