Thalamus

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.

 

Sensory

Virtually all sensory information passes through the thalamus (except olfaction) en route to the sensory cortex. The thalamus and acortex 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.

 

 

Structure

The thalamus is a big, oval collection of nuclei separated into two. The halves are separated by the narrow third ventricle.

 

Thalamic nuclei can be grouped into three regions - anterior, medial, and lateral nuclear groups. The internal medullary lamina subdivides the three regions.

 

 

• anterior nucleus is the principle relay nucleus for the limbic system, receiving information from the mammilothalamic tract of the hypothalamus and projecting to the cingulate gyrus.
• dorsomedial nucleus makes up the medial group and is interconnected with the prefrontal cortex, influencing drive and executive functions.
  • inputs from both PFC and limbic structures such as the amygdala, via basal ganglia

The lateral group composes most of the thalamus and can be further divided into the dorsal and ventral tiers.

 

dorsal tier:

• lateral dorsal (LD) nucleus
  • projects to the cingulate gyrus in the limbic system
• lateral posterior (LP) and pulvinar nuclei
  • considered together due to their continuity ande similiarity in connections
  • largest nucleus, though function largely unknown
  • receives inputs from association and visual cortex

 

ventral tier:

• ventral anterior (VA) nucleus
  • inputs from substantia nigra and globus pallidus, output to premotor cortex to control basic movements
• ventral lateral (VL)
  • cerebellar (dentate nucleus) and basal ganglia inputs, primary motor cortex outputs
• ventral posterolateral (VPL) nucleus
  • receives ascending pain/temperature information from spinothalamic tract, directly and through reticular formation
  • receives ascending touch/vibrational/proprioreceptive information from the dorsal column/medial lemniscus
  • output to primary sensory cortex
• ventral posteromedial (VPM) nucleus
• receives touch and propriorecptive information from the principle nucleus of the trigeminal
• receives pain and temperature information from the spinal nucleus of the trigeminal, via the spinothalamic tract
• output to primary sensory cortex
• lateral geniculate nucleus
  • receives information related to vision from the optic tract and superior colliculus
  • output to primary visual cortex
• medial geniculate nucleus
  • receives auditory information from the inferior colliculus, sending it to the auditory cortex

 

intralaminar thalamic nucleus

• from reticular formation, with widespread output

 

thalamic reticular nucelus

• projects to other thalamic nuclei with inhibitory GABA-ergic connections

Thalamic Neurons

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 aciton potentials.

Burst mode occurs when neurons are hyperpolarized. SLight depolarization opens voltage-gated Calcium channels, resulting in a few action potentials.

 

 

Pineal body

tends to calcify with age; normally midline; deviation is useful during imaging

 

Problems with 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