UNIT 6 - Thalamus
A. Purpose
The thalamus is a complex structure interposed between most major cortical and subcortical structures. It serves both a sensory and a motor function. The thalamus acts as (1) a relay station for information from sensory receptors on their way to the cerebral cortex (all sensory pathways, except the olfactory tracts, have direct projections to thalamic nuclei). It provides for (2) conscious awareness of specific sensory stimuli. The thalamus also plays (3) an important role in the motor pathways, because it has many connections with the basal ganglia, and it (4) helps regulate cortical activity through specific reticulothalamic relays (the Reticular Activating System). Lastly, the thalamus (5) plays a part in feelings and emotions that we experience via its connections with the prefrontal lobe of the cortex.
Fig. 1 –
B. Structure
The thalamus is covered on its outer lateral surface by a layer of myelinated fibers referred to as the external medullary lamina, outside of which the reticular thalamic nuclei are found. These nuclei separate the thalamus from the adjacent internal capsule. The thalamus is divided into three nuclear masses of gray matter by vertically myelinated fibers called the internal medullary lamina. These three subdivisions are called the anterior nuclear mass, the medial nuclear mass, and the lateral nuclear mass. The anterior nuclear mass of the thalamus is composed of the anterior group nuclei, and the medial nuclear mass of the thalamus is made up of the medial group nuclei. However, the lateral nuclear mass of the thalamus is divided into a ventral and a dorsal tier.
The ventral tier is comprised of the ventral group nuclei, which are broken down into the medial and lateral geniculate bodies, the ventral anterior nuclei (VA), the ventral posterior nucleus (VP), and the ventral lateral nucleus (VL). The dorsal tier of this nuclear mass is made up of the lateral group and posterior group nuclei. The lateral group nuclei consisting of the lateral thalamic nuclei, and the pulvinar nuclei. The posterior group consisting of part of the pulvinar nuclei located near the superior aspect of medial geniculate body, the superior barder of the medial geniculate body, the suprageniculate nuclei, and the nucleus limitans. Also note that in the internal medullary lamina is a group of nuclei called the intralaminar nuclei.
C. Functional Classification
Formally, the thalamic nuclei were divided into functional groupings based on the specificity of their ascending projections. This theory is no longer accepted, as current research has demonstrated that the diversity of connections each thalamic nucleus has with various cortical areas is far beyond what was previously thought. However, these groupings are still used for ease of reference and learning purposes.
1. Specific Relay Nuclei
The specific relay nuclei basically, project fibers to and receive fibers from well defined areas on the cerebral cortex that are considered to be related to specific function.
a. Medial Geniculate Body
Input - auditory information; mostly arising from the inferior colliculi.
Output - sends information via the auditory radiations to the auditory cortex (Area 41-42).
b. Lateral Geniculate Body
Input - information dealing with vision, arising from the optic tract.
Output - projects information via visual radiations to the calcarine cortex of the occipital lobe (Area 17).
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c. Ventral Posterior Nucleus (VP)
The ventral posterior nucleus is broken down into the ventral posterior lateral nucleus (VPL), the ventral posterior medial nucleus (VPM), and the ventral medial nucleus (VM).
1. Ventral Posterior Lateral Nucleus
Input - receives information regarding pain, temperature, touch, pressure, vibration, and conscious proprioception from the extremities, neck and trunk via major afferent pathways (Chp 5).
Output - projects information to the primary somesthetic area in the post central gyrus (Area 1,2,3)
2. Ventral Posterior Medial Nucleus (VPM)
Input - receives information of the same nature as the VPL, from the face and head area via the 5th cranial nerve. Output - to primary somesthetic area in the post-central gyrus (Area 1,2,3)
3. Ventral Medial Nucleus (VM)
Input - receives information dealing with the sense of taste from the parabrachial nuclei of the reticular formation.
Output - to the primary receiving area for taste in Area 43.
d. Ventral Lateral Nucleus (VL) and Ventral Anterior Nucleus (VA)
Input - receive information dealing with motor control from the contralateral cerebellar cortex (via the superior cerebellar peduncle) and the ipsilateral corpus striatum.
Output - sends information to the primary motor areas in the cerebral cortex (Area 4, 6 and 8), to complete the feedback system of the motor control mechanism.
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2. Less Specific (or Association) Nuclei
Projections from these less specific nuclei generally go to the association areas of the cerebral cortex.
a. Anterior Nucleus
Input - receives information primarily from the mammillary bodies (via the mammillothalamic tract), other hypothalamic nuclei, and the cingulate gyrus.& This nucleus is concerned with memory, learning, and emotional behavior relating to our basic drives for food, shelter, and sex. Also, this nucleus works closely with other limbic system structures in the coupling of our autonomic functions and emotional behavior.
Output - sends information to the cingulate gyrus of the cerebral cortex, hypothalamus, and hippocampus as well as collaterals to other limbic system structures.
b. Medial Nucleus
Input - receives somatic sensory input from all major afferent tracts, from the hypothalamus, the amygdala, and Areas 9,10,11,12. This nucleus deals with our emotional responses in relation to our internal and external environment, thus is an important part of our personality. The medial nucleus is also concerned with autonomic responses which accompany our moods and feelings as an expression of our emotional responses to our internal and external environment. Additionally, this nucleus plays a part in our overall memory mechanism and facial expression used to express our feelings and emotions.
Output - sends information to the pre-frontal lobe of the cerebral cortex (Areas 9,10,11,12), hypothalamus, hippocampus, corpus striatum and collaterals to other limbic system structures.
c. Lateral Nucleus
Input - receives information primarily from the hypothalamus and the cingulate gyrus.
Output - projects information to the cingulate gyrus and other limbic system structures. Through these connections this nucleus works closely with the anterior nucleus in regard to our basic emotional drives for food, shelter, and sex.
d. Posterior Group Nuclei
Input - receives information dealing with pain from the lateral spinothalamic tract and the 5th cranial nerve.
Output - sends information to the insular cortex, located at the base of the lateral fissure, this being part of a system dealing with the total conscious perception of pain at the cortical level.
e. Pulvinar Nuclei
The pulvinar nuclei have reciprocal connections with association areas of the cerebral cortex located within the parietal, occipital, and temporal lobes. It is thought to be involved with integration of all incoming sensory input to allow for total comprehension of that moment in time we are experiencing
3. Non-Specific Nuclei (or other Thalamic Nuclei)
a. Intralaminar Nuclei
Input - receives information from the basal ganglia, the reticular formation,
and collaterals from major afferent tracts.
Output - relays information to all parts of the cerebral cortex. These nuclei play an important role in the Reticular Activating System, the motor control system, and the perceptions of various modalities on the thalamic level (specific, non-specific pain, temperature, touch, pressure, vibration and auditory input).
b. Reticular Nuclei
Input - receive collaterals from all fibers leaving and entering the thalamus.
Output - sends information to all thalamic nuclei to help provide cortical regulation of the thalamic activity.
Fig. 2 –
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