Physiology
Dr. Basim Mohamad Alwan Lecture (7)THE THALAMUS
The two thalami are large ovoid masses of gray matter situated at the lateral walls of the third ventricle, one on each side. They are interconnected by a short communicating bar of white matter (Massa intermedia) which traverses the third ventricle. Thalamus operates in close association with cerebral cortex, so both are sometimes called thalamo-cortical system.
All the nervous signals which go to the cerebral cortex pass first through and relay in the thalamus. That is why the thalamus is sometimes called the "secretary of the cerebral cortex ".
ANATOMICAL DIVISIONS OF THE THALAMUS
In each thalamus there are 5 groups of nuclei:
FUNCTIONAL DIVISIONS OF THE THALAMUS
Functionally, the thalamic nuclei could be classified into four categories:[I] Specific projection nuclei (cortical relay nuclei):
a. Ventral posterolateral nucleus (VPL), which is the site ofrelay of the somatic sensory pathways from the trunk and limbs
b. Ventral posteromedial nucleus (VPM), which is the siteof relay of the trigeminal sensory pathway from the head.
c. Medial geniculate body (MGB); site of relay of the auditorypathway.
d. Lateral geniculate body (LGB): site of relay of the visual pathway.
AFFERENT CONNECTIONS: From the medial, spinal and trigeminal lemnisci, optic tract, hearing and olfactory pathways.
EFFERENT CONNECTIONS: Specific thalamic projection system to specific points in the somatic audio and visual sensory areas of the cerebral cortex.
[II] Nonspecific projection nuclei:
a. Intralaminar and midline nuclei.
b. Ventroanterior nucleus.
c. Anterior nuclei.
AFFERENT CONNECTIONS: From the ascending reticular activating system (ARAS), and the paleospinothalamic tracts.
EFFERENT CONNECTIONS: Nonspecific thalamic projection fibers to all parts of the cerebral cortex. The anterior nuclei are connected to the hypothalamus and limbic system.
[III] Association nuclei:
a. Dorsolateral nucleus.
b. Posterolateral nucleus.
AFFERENT CONNECTIONS: From other thalamic nuclei
EFFERENT CONNECTIONS: Dorsomedial nucleus projects to prefrontal cortical area, while the dorsolateral nucleus projects to the cortical association areas.
[IV] Motor nuclei:
The most important of these is the ventrolateral nucleus VL
AFFERENT CONNECTIONS: From the basal ganglia and cerebellum.
EFFERENT CONNECTIONS: The motor cortex.
FUNCTIONS OF THE THALAMUS
It is a relay station for all the sensory pathways in their way to the cerebral cortex except olfaction. Sensory signals are processed in the thalamus before reaching the cortex (thalamus is the secretary of the cortex).It acts as a final sensory center for conscious perception of some sensations: i.e. slow pain, high or low grades of temperature and crude touch.
Facilitation of the cerebral cortex, raising its excitability up to the level necessary to do all cerebral functions. Without the thalamus, the cerebral cortical functions are markedly depressed. The cerebral cortex and thalamus are sometimes considered as one unit called "the thalamocortical system".
Identification of the stimulus affect (pleasant or unpleasant), and controlling the emotional and motor reactions to it. This is done in collaboration with the cerebral cortex, the limbic system and hypothalamus
It is part of the limbic circuit which is concerned with recent memory and memory search.
It is part of the caudate and putamen circuits which control the motor activity
It is involved in control of high intellectual cortical functions and the behavior and personality through its connections with the cortical association areas and with the prefrontal cortical areas respectively.
THE THALAMIC SYNDROME (The thalamic hypersthetic anesthesia)
The most common cause of this syndrome is thrombosis of the thalamogeniculate artery which is a branch of the posterior cerebral artery. This leads to degeneration of the posterior and ventral parts of the thalamus. The manifestations which appear on the contralateral side of the lesion include:SENSORY EFFECTS
First, there is complete hemi anesthesia at the onset of the disease except the upper part of face sensation is retained. Few weeks later protopathic sensation (primitive sensations which include crude touch, pain and high or low temperatures) are recovered. The threshold of pain is elevated but gives very unpleasant severe agonizing pain when it is reached, exaggerated central effect. Epicritic sensations (fine sensations, which include fine touch, proprioceptive sensations and intermediate grades of temperature) are permanently and irreversibly lost.
MOTOR EFFECTS
The damage of the ventrolateral nucleus (main motor nucleus) lead to loss of cereberllar and basal ganglion control on the motor cortical areas. There is hemiparesis (weakness of the muscles), hemiataxia and choreoathetoid movements.SENSORY FUNCTIONS OF THE CEREBRAL CORTEX
The cerebral cortex is the highest center for conscious perception of fine sensations. Crude sensations such as pain, temperature and crude touch can be perceived at a lower level in the thalamus. Somatic sensations are perceived in the parietal lobe, visual sensations are perceived in the occipital lobe and auditory sensations in the temporal lobe.There are three somatic sensory areas in the cerebral cortex; the primary somatic sensory area (SI), the secondary somatic sensory area (SII) and the somatic sensory association area.
THE PRIMARY SOMATIC SENSORY AREA (SI):
This area is located in the postcentral gyrus of the parietal lobe (Brodmann areas 3, 1 and 2) (fig. 7-1). It receives projection fibers from the ventrobasal complex of the thalamus (VPLN and VPMN).In this area, there are the final sensory neurons of the somatic sensory pathways of fine sensations.
BODY REPRESENTATION: It is crossed and inverted representation (fig. 7-2). The upper half of the face is bilaterally represented.
Crossed representation means that each half of the body is represented on the contralateral cerebral hemisphere.
Inverted representation means that upper parts of the body are represented in the lower part of the cortex and lower parts of the body in upper part of the cortex.
Figure 7-1: Map of Brodmann areas of the cerebral cortex. It is based on histological typing of cortical cells (first published in 1909).
The area of representation of each part is proportional to the number of receptors in this part, not to its size, e.g. the lips and thumb are represented by relatively large areas, whilst the trunk is represented by relatively small area.
Figure 7 - 2: Body representation in the postcentral sensory gyrus
Cells in the sensory cortex are arranged in columns projecting inwards from the surface. Points in the cortex that represent a certain area of the body contain several columns each column is specialized in perception of a specific sensory modality. So, modality representation is included within the topographic representation.Topographic areas of representation are not permanent or unchangeable, but can be changed and modified. E.g., if a finger is amputated, the cortical representation of the neighboring fingers creep into the area of the amputated finger.
FUNCTIONS: The primary somatic sensory area is essential for the perception of:
1. Fine touch sensation, i.e. tactile localization and discrimination.
Localization of pain and temperature sensations, SI is not essential for the perception of pain and temperature sensations, it is only needed for their accurate localization.
Intensity discrimination of different stimuli.
Texture of material.
Proprioception; static and dynamic.
SI area projects connection fibers to the secondary somatic sensory area (SII) and the somatic sensory association area for interpreting the meaning and significance of the sensory information; E.g. stereognosis.
THE SECONDARY SOMATIC SENSORY AREA (SII)
LOCATION: In the supramarginal gyrus (Brodmann area 40), behind the lower part of SI.BODY REPRESENTATION: Bilateral representation with poor topographic representation. The head area is generally in the anterior part and the leg area in the posterior part.
SII receives connection fibers which convey input signals from SI, the visual and auditory cortical sensory areas, and the thalamic nuclei on both sides of the body.
FUNCTIONS: SII is a potentiator of SI. It cannot work independent of SI. SI, however, can carry out its functions without SII. So, SI can work without SII, but the opposite is not true.
SOMATIC SENSORY ASSOCIATION AREA
(SOMATIC INTERPRETATIVE AREA)
LOCATION: In the posterior parietal cortex (Brodmann areas 5 and 7), behind SI and above SII.
CONNECTIONS: It receives sensory signals from SI, SII, and the thalamus.
FUNCTIONS:
1. It combines all sensory signals to give meaning to the complex sensory input.
2. Stereognosis. This area is the center of stereognosis.
3. Spatial orientation of the body with its surroundings. The personrecognizes the position of each part of the body relative to otherparts and to the surrounding objects.
4. Memory. This area is the memory store of previous sensory experience. Stimulation of this area produces sensory hallucinations.
effect of LESION: A lesion in the somatic sensory association area results in:
1. Astereognosis, i.e. inability to identify objects by their touch, shape, weight and texture.
2. Autotopagnosia, loss of recognition of part or whole contralateral side of the body. The patient does not acknowledge the existence of the affected part and fails to include it in planning of voluntary movement. E.g. he does not swing the arm of the affected side during walking, he puts clothes on one side of the body and ignores the other or he shaves only one side of his beard.
3. Impaired memory and decreased intelligence.
