*4.3.4 Cuneocerebellar tract (posterior external arcuate fibers, dorsal external arcuate fibers)*

DSCT does not convey information from the upper limb since the *nucleus dorsalis* does not extend into the cervical spinal cord. Therefore, there is another proprioceptive pathway for the upper limb: the cuneocerebellar tract. The secondary order neurons of this nucleus pass to the inferior ipsilateral cerebellar peduncle to reach the spinocerebellum.

#### *4.3.5 Other spinocerebellar tracts*

There are additional ascending direct and indirect spinocerebellar pathways. The *spinocervical tract* relays in the lateral cervical nucleus and projects to the ventral postero-lateral (VPL) nucleus of the thalamus. The *spinohypothalamic* and *spinoamygdalar* tract provides sensory input to areas of the nervous system involved in controlling autonomic, endocrine and emotional responses. Sensory information from the hindlimbs is also relayed by indirect *spinoreticulocerebellar* pathways through at least two olivocerebellar pathways: the indirect *spinoreticuloolivocerebellar* tract and the direct *spinoolivocerebellar* tract.

These pathways provide necessary information regarding the current status of reflex pathways, as well as muscle tone, length and tension that consent the cerebellum to coordinate and regulate motor activity.

#### **4.4 The brain connection – conscious proprioception**

According to Proske and Gadivia [4] there are at least two reasons for including body schemas and images in the study and discussion of proprioception. "*First, while proprioceptors provide information about position and movement of the limb, they are unable to signal the length of limb segments and therefore the absolute location of the limb in space. Second, there is the issue of body ownership*". The blind movement of a limb, while proprioceptive feedback informs us about the movement, we need to be able to identify the moving limb as our own [78, 79]. Carruthers has proposed that all representations of the body are available to consciousness [80]. "On-line," newly constructed body representations, provided by inputs from vision, touch, and

#### *Proprioception*

proprioception, generate a perception of the body as it actually is at any moment in time, an image which is able to change from moment to moment. It is distinguished from an "off-line" representation constructed, in part, from current sensory inputs, in part, from stored memories and is available to consciousness both immediately and after retrieval of memories.

In the last times neuroimaging has strongly contributed to the knowledge of the central activity patterns produced by proprioceptive stimuli, to the recognition of the integration of proprioceptive inputs with inputs from other senses and the identification of central areas involved in the integration [81]. These include regions in the parietal cortex [82–85] including the primary somatosensory cortex [86, 87]. Furthermore, parts of the frontal cortex and insula [88, 89] are involved in proprioception.

### **4.5 The mesencephalic trigeminal nucleus**

The mesencephalic trigeminal nucleus is a sensory structure located at the mesopontine junction and contains the cell bodies of primary order afferent proprioceptors that innervate muscle spindles of the muscles of mastication and other muscles of the head and neck [27, 71]. Whether these primary sensory neurons are generated directly in brain or have a of neural crest origin is stell debated. Classically it has been regarded as a representation of a peripheral sensory ganglion similar to DRG that became incorporated into de brainstem during embryonic development, although molecular studies support a central origin for these cells [90, 91].

It projects to the dorsolateral division of the trigeminal motor nucleus and to the supratrigeminal nucleus, which are involved in humans in the jaw-jerk reflex and the periodontal-masseteric reflex [92] (for a review see [93]).
