A °) General
Sensitivity can be divided into three types:
1. Exteroceptive sensitivity:
– it is the sensitivity of the integuments, of all that originates from the ectoderm;
– epicritic tact: fine, very differentiated tact, or epéroceptive epicritic sensitivity;
– protopathic tact: coarse, non-discriminatory tact or protopathic exteroceptive sensitivity;
– thermoalgic sensitivity: it conveys thermal or painful sensations, it is also a protopathic exteroceptive sensitivity.
2. Proprioceptive sensitivity:
It is native to the mesoderm and consists of two types:
– Conscious Proprioceptive Sensitivity which provides information on the position of a member in space;
– Unconscious Proprioceptive Sensitivity, mainly intended for information on tone.
All this information comes from receptors located in the aponeuroses, in the tendons, in the ligaments.
3. Interoceptive sensitivity:
It carries all the information from the viscera and the endoderm.
The characteristics common to the three sensitivities:
a. they are all crossed;
b. All the sensory pathways present two essential relays: the posterior horn of the spinal cord and the thalamus;
c. All pathways of conscious sensitivity will have a chain of crossed medullothlamic neurons, and there is no third thalamocortical neuron here.
B. Epicritic exteroceptive sensitivity:
She is perfectly aware, precise, discriminative and made up of three neurons. It is born by receptors located in the ectoderm, the integuments, and reaches the spinal cord. It is the dendrite of the neuron which arises from the integument and which goes towards the spinal nerve. The cell body is placed in the spinal ganglion, then the neuron enters the cord, runs along the posterior horn of the cord and goes to the posterior cords of the cord where it changes direction to become vertical.
It therefore occupies the posterior cord of the cord where there is somatotopia: the innermost part will convey information from the lower limb. Outside, the neighboring zone conveys the information of the trunk and the most lateral zone conveys the information of the upper limb.
There are two distinct beams that will colonize the whole:
– the Goll bundle, for the lower limbs and the trunk;
– the Burdach bundle, for the upper limbs.
Up to this point, the information has remained on the same side and arrives in the medulla oblongata. In the bulb, the beams of Goll and Burdach have a relay, respectively in the nuclei of Goll and Burdach in the lower part of the bulb, to reach the posterior lateral ventral part of the contralateral thalamus or second neuron.
It is a conscious path, so there is a third neuron, which is thalamocortical and which remains homolateral, it will gain the ascending or post central parietal convolution, where the somatosensitive area is located with the same somatotopic representation as ‘at the level of the precentral convolution, the ascending frontal.
C- Protopathic exteroceptive sensitivity:
It is a poorly limited, discreet but conscious sensitivity with a pathway with three neurons. It originates at the level of the integuments and from there gains the posterior root. Neurons have a cell body in the spinal ganglion. The axon will reach the posterior horn and act as a relay in the head or apex of this horn. From there, the second neuron crosses the gray substance, passes in front of the ependymal canal and unscrews and gains, contro laterally, an anterolateral beam: the croissant de Déjerine. There, it changes direction and it becomes vertical, it goes up all along the spinal cord until the bulb.
It remains homolateral to the bulb and reaches the postero-ventral nucleus of the thalamus. From there leaves the third neuron of the path which joins the ascending parietal convolution.
It is a crossed track, the crossing being made at each metameric level at the level of Déjerine’s beam.
In the crescent, we can individualize 4 sectors, from the most posterior to the most anterior:
– pressure information;
– tact information;
– information on pain;
– information on heat.
D. Proprioceptive sensitivity
It starts from receptors for tendons, muscles, bone joint capsules, and it can be conscious or unconscious.
The conscious has three neurons, and a thalamocortical relay. It can be the painful expression, or the expression of tension of bone tension, of ligament distension.
Information arrives through the posterior root at a neuron whose cell body is in the spinal ganglion. From there, the axon will go towards the posterior horn and towards the posterior tendons, where it will change direction: it will follow the posterior cords with the same somatotopy as for epicritic sensitivity. It goes up to the medullary junction, constituting the beams of Goll and Burdach.
The connection is made to the lower part of the bulb and the deutoneurone crosses the midline and reaches the thalamus in its posterolateral part where the relay with the third neuron takes place, the third neuron which will gain the parietal cortex.
The unconscious has significantly different pathways that do not reach the cortex. There is an additional relay constituted by the cerebellum.
The sensory afferents will pass through the posterior root, the cell body is in the spinal ganglion. There are then two routes depending on the source of the afferents: the trunk or the upper and lower limbs together:
– for the trunk, unconscious proprioceptive sensitivity, the fibers reach the posterior horn of the cord and there articulate with a second neuron which reaches the posterolateral part of the cord, outside the posterior cords and constitutes the direct cerebellar spino bundle of Flechsig. It will go into the cerebellum.
– for the limbs, the fibers reach the cord. Their cell body is located in the spinal ganglion. The fibers then pass through the posterior horn, behind the ependymal canal and constitute, on the opposite lateral edge of the cord, a crossed cerebellar spino bundle, the Gowers bundle.
These fibers rise vertically and have a relay in the cerebellum.
Some of the fibers can leave in the posterior root of the cord a collateral which will gain the homolateral anterior horn at the same metameric level where it will anastomose with the deuto neuron either directly or by an intermediate neuron.
This is the principle of the elementary reflex arc which, from the posterior roots, there are three possibilities:
– constitution of the posterior lemniscs and cords of the exteroceptive and conscious protopathic sensibility;
– relay and constitution of the direct and crossed cerebellar spino bundle;
– direct collateral with a motor neuron, this is the reflex arc.
E. Interoceptive sensitivity
It is formed from receptors located in the innervated organs and viscera and reaches the pre-vertebral ganglion, constitutes a lateral vertebral nerve chain which will articulate with the spinal nerve via communicating branches at each metameric level.