What is the Thalamus?

The thalamus, also known as the dorsal thalamus, is the largest oval-shaped gray matter nucleus in the midbrain. It is located on both sides of the third ventricle.

[qi no]

Dorsal thalamus

The thalamus is a relay station for sensory conduction. In addition to the sense of smell, the sensory pathways of various senses replace neurons in the thalamus and then project to the cerebral cortex. In the thalamus, only the rough analysis and synthesis of sensations is performed, and the fine analysis and synthesis of sensations is performed in the cerebral cortex.

According to the different projection characteristics of the thalamus to the cerebral cortex, the thalamus can be divided into two major systems, one is a specific projection system, and the other is a non-specific projection system (or diffuse projection system).

Specific projection system

After entering the central nervous system, the nerve impulses from various sensory bodies of the body enter the central nervous system through a fixed sensory pathway to reach a certain nucleus of the thalamus (except for the sense of smell), thereby emitting fibers that are projected to each sensory region of the cerebral cortex, producing specific feel. This transmission system is called a specific projection system.

The typical sensory pathway is usually completed by the replacement of tertiary neurons. The first level neurons are located in the spinal ganglia or related brain neurosensory ganglia; the second level neurons are located in the relevant nucleus of the spinal dorsal horn or brainstem; the third level neurons are in the ventrolateral nucleus of the thalamus. But the pathways for special sensations (vision, hearing, smell) are more complex. Therefore, the thalamus is an important relay station for the specific conduction system. It collects various afferent impulses (except for the sense of smell) and conducts preliminary analysis and synthesis to produce a rough feeling. However, for the nature and intensity of the stimulus, No accurate analysis can be performed.

_{ps: The visual pathway includes rods and cones, which take over for four neurons. The auditory conduction pathway from the periphery to the cerebral cortex is difficult to be sure to take over by several neurons . These sensations are projected into specific sensory regions of the cerebral cortex after neurons are replaced in the lateral nucleus of the thalamus, the lateral geniculate body, and the medial geniculate body to produce specific sensations.}

1. _{The specific projection system refers to the fibrous connection between the lateral nucleus, lateral geniculate body and medial geniculate body of the thalamus projecting into the cerebral cortex . Classical sensory pathways act on the cerebral cortex through the thalamic specific projection system . From the thalamus to the specific area of the cerebral cortex , there is a point-to-point projection relationship, and each sensory transmission has its own specific way.}

_{The first type of cell population, which is directed to a specific region of the cerebral cortex , has a point-to-point projection relationship.
Most of the second type of cell structure in the structure also has a specific projection relationship with the cerebral cortex , projecting to a specific region of the cortex, so it can also belong to a specific projection system .}

Thalamus nonspecific

Non-specific projection system. When sensory transmission is projected to the cerebral cortex, that is, when the fibers of the second-level neurons of the specific projection system pass through the brain stem, synaptic connections between the collaterals and neurons in the brain stem reticular structure occur, and then pass through the reticular structure. The short axon changes multiple times and projects into a wide area of the cerebral cortex. This projection system is a common way forward for different sensations. Because various sensory impulses enter the brainstem reticular structure, and after the interaction of many intricately intertwined neurons, the specificity of various sensations is lost, so projections onto the cerebral cortex no longer produce specific sensations. Therefore, this conduction system is called a non-specific projection system. One of the functions of this system is to excite the cerebral cortex to make the body in an awake state, so the non-specific projection system is also called the brain stem reticular ascending activating system. When the afferent impulses of this system increase, the cortical excitatory activity increases, which keeps the animal awake and even causes an agitated state; when the afferent impulses of this system decrease, the cortical excitatory activity weakens, leaving the animal relatively quiet State, and even a large area of the cortex turns into an inhibited state and causes sleep. The second is to regulate the excitability of each sensory region of the cortex, so that the sensitivity of various specific sensations is increased or decreased. If this system is damaged and the excitatory activity of the cortex is weakened, the animal will fall into a lethargy. Since this system is a multisynaptic forward system, it is susceptible to conduction disorders due to the effects of anesthetic drugs. Some anesthetics, such as hibernating spirit, act on the reticular structure of the brainstem, blocking this pathway, reducing the excitability of the cortex, and causing quietness and sleep.

To produce sensation in the cerebral cortex, it depends on the cooperation of specific and non-specific projection systems. Only through the impulse of the non-specific projection system can the sensory area of the cerebral cortex maintain a certain excitement. At the same time, only through the various sensory impulses of the specific projection system can specific sensations be generated in the cerebral cortex.

1. _{Non- specific projection system refers to the fibrous connections diffusely projected from the medial thalamus nucleus to a wide area of the cerebral cortex .}

2. _{The axons of the second-level neurons of the above-mentioned classic sensory conduction pathway pass through the brain stem on the way to upload, and the side branches emit synaptic connections with neurons in the brain stem reticular structure ; and then repeatedly change cells in the reticular structure to go up The nucleus group reaching the medial part of the thalamus and finally diffusely projected into a wide area of the cerebral cortex. Experiments have shown that stimulating the midbrain reticular structure can awaken sleeping animals; when the reticular structure is cut off at the head of the midbrain, it can make the animals sleep-like. It has been clinically observed that patients with damaged midbrain reticulum also appear to be drowsy. It can be seen that there is a functional system in the brainstem reticular structure that has an upward awakening effect on the cerebral cortex , which is called the brainstem reticular ascending activation system. The brainstem reticular ascending activation system works mainly through the thalamus non-specific projection system.}

Non-specific projection system refers to the third type of cell population, which diffusely projects over a wide area of the cerebral cortex and does not have a point-to-point projection relationship

Thalamic brainstem reticular structure

The ascending reticular activation system and the thalamus non-specific projection system are functionally inseparable to form a unified system. They are a common pathway for various sensory afferents, and their role is to maintain and improve the state of excitement of the cerebral cortex. Therefore, the thalamic non-specific projection system called the brain-stem reticular ascending activation system is sometimes referred to as the sensory afferent non-specific projection system.

The sensory afferent non-specific projection system plays an important role in maintaining the arousal state of the cerebral cortex. The more various afferent impulses, the more impulses that enter the brainstem reticular structure through the collaterals, and thus the stronger the upward awakening effect on the cerebral cortex, the better the state of excitement of the cortex and the feeling of uploading to the specific projection system The more perfect it is. Therefore, the sensory specific projection system and non-specific projection system are functionally interdependent and indivisible.

Brain stem reticular structure ascending reticular activation system-thalamic non-specific projection system, is a multisynaptic successor ascending system, so it is susceptible to drugs to cause conduction block. For example, barbiturates may have a hypnotic effect by blocking the conduction of the ascending agonistic system; some general anesthetics (such as ether) also first inhibit the activity of the ascending stimulatory system and cerebral cortex to exert anesthetic effects.