Afferent pathways of the somesthesia

The information collected by the receptor must pass along three neurons to reach the brain. These neurons are simply called first, second and third order neurons. The receptor receives stimulation to which it is sensitive, transforms this energy (mechanical, chemical or thermal) into an action potential and this nerve impulse passes from neuron to neuron thanks to the synapses that bind them.

The pathways for mechanoreceptors (Aß), proprioceptors (A-alpha), and thinly myelinated or non-myelinated fibres (A∂ and C) are different, but they are always (or almost always) a three-neuron organisation. So we will have three main paths (but here too, it is a real puzzle and whoever searches well can find a multitude of them).

1- The dorsal column medial lemniscus pathways.

This is the way of mechanoreceptors and of conscious proprioception. It is subdivided because, let us not forget, the central nervous system has several somatotopic maps and the spinal cord is also organised into specialised regions. Fibres from the lower limbs do not follow exactly the same path as fibres from the upper limbs or trunk and in each subdivision there are others that are specific to each region of the lower limb, trunk or lower limb.

 

 

The dorsal column pathway comprises the fasciculus gracilis (lower limbs and sacral region of the trunk) and fasciculus cuneatus (upper limbs and thoracic trunk) which carry discriminative touch and conscious proprioception information (Meisness, Merkel, Pacini and Ruffini).

The neurons of both fasciculus travel through this ipsilateral pathway to the respective gracilis and cuneatus nuclei located in the medulla. They synapse with second-order neurons that decuss at this level and travel to the ventral posterolateral area of the thalamus. Then, with a second synapse, third order neurons are recruited and information can reach the primary somatosensory cortex also known as the postcentral gyrus.

2- The spinothalamic tract.

This is the pathway of C fibres, those that carry information about temperature, pleasant touch, tickling, itching and pain. The afferent C fibre neurons penetrate the Lissauer tract where they can ascend or descend from one to two levels. Then they reach the Lamina 1 of the posterior horn where they will synapse with a second order neuron.

Second order neurons decuss at this level through the anterior white commissure, then move to the contralateral spinothalamic tract where they begin their ascension to the brainstem and to the thalamus.

Finally, all current physiology textbooks tell us that the third order neuron of this ascending pathway projects to the somatosensory cortex, like those of the dorsal column pathway. This is an error that the neuroanatomist Arthur D. Craig brought to light. Instead, these neurons project into the anterior cingulate cortex, area 3a of the somatosensory cortex and, above all, into the insular cortex.

Arthur D. Craig, "How do you feel ?
An Interoceptive Moment with Your Neurobiological Self, Princeton University Press, 2015, p. 14

3- The spinocerebellar pathways

The spinocerebellar pathways are complex. They consist of the dorsal spinocerebellar fasciculus, the ventral spinocerebellar fasciculus, the cuneocerebellar fasciculus, the rostral spinocerebellar fasciculus, the spino-olivary fasciculus and the spinovestibular fasciculus. These fasciculi, like those of the posterior columns, have somatotopic organisations and all the afferences which travel from the periphery to the cerebellum will reach, by a system of two neurons, the ipsilateral cerebellum although some fasciculi must decuss twice.

One of the main roles of the cerebellum is to coordinate movements. To achieve this, the cerebellum receives information from the limbs, the trunk and the brain. It also receives visual and vestibular signals to ensure the body's balance. He must make sure that the movement we make will be exactly the movement we intend to make. He can "compare" the intentional movement of the brain with the actual movement that can be performed and "correct" the intentional movement according to the actual state of the body and send the information back to the brain via the thalamus before the brain passes the command to the efferent neurons towards the brainstem and spinal cord, all this so that the movement is well coordinated.

Proprioceptive information is transmitted to the cerebellum by proprioceptors (neuromuscular spindles and Golgi tendon organs) and some pressure-sensitive skin receptors. The cerebellum is constantly aware of the position of the limbs and the body, even if we are not always aware of it. This is called unconscious proprioception.

Why is it important to know these pathways? Because these pathways will project to different areas of the brain and will activate specific areas.

Depending on the type of stimulation I do, I may have a stronger impact on one area of the brain or another.

That's awesome!

© 2018 Louise Tremblay

 

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