**Abstract**

The proprioception from the head is mainly mediated via the trigeminal nerve and originates from special sensitive receptors located within muscles called proprioceptors. Only muscles innervated by the trigeminal nerve, and rarely some muscles supplied by the facial nerve, contain typical proprioceptors, i.e. muscle spindles. In the other cephalic muscles (at the exception of the extrinsic muscles of the eye) the muscle spindles are replaced by sensory nerve formations (of different morphologies and in different densities) and isolated nerve fibers expressing mechanproteins (especially PIEZO2) related to proprioception. This chapter examines the cephalic proprioceptors corresponding to the territories of the trigeminal, facial, glossopharyngeal and hypoglossal nerves.

**Keywords:** proprioception, muscle spindles, atypical proprioceptors, cephalic muscles, PIEZO2, mechanoproteins

#### **1. Introduction**

Proprioception is a quality of the somatosensory system that informs the central nervous system about the static and dynamics conditions of muscles and joints. This type of sensitivity has been studied in deep in the muscles depending on the spinal nerves and today the neurobiology of spinal proprioception is well known [1–4]. On the contrary, the neuroanatomy as well as the cellular and molecular bases of the proprioception in the cephalic muscles is not well known. Nevertheless, it is clear that cephalic muscles permanently develop fine adjustments of stretching and tone in facial movements, regulation of chewing force, oromotor reflex behaviors, verbal and nonverbal facial communication, swallowing, coughing, vomiting or breathing [5–7].

The skeletal muscles contain an intrinsic mechanosensory system, the proprioceptive system, which provides unconscious and conscious information to the central nervous system. The proprioceptive inputs originate in specialized sensory organs (proprioceptors) present in muscles (muscle spindles [8, 9]), tendons (Golgi's tendon organs [10]), joint capsules (Ruffini-like sensory corpuscles, Pacinian corpuscles and free nerve endings [11]), and presumably also the skin but their physiological properties suggest they are not the alternative to muscle spindles [2, 12–14]. The information encoded by the propioceptors gives rise to unconscious

and conscious sensations, necessary for most basic motor functions [15]. For those interested in a recent review and in detail on both types of proprioceptors, we refer to the Banks [8] and Macefield and Knellwolf [16].

Some decades ago, Baumel [17] suggested that proprioceptive impulses from facial muscles are conveyed to the central nervous system via different branches of trigeminal nerve throughout multiple communications with the branches of the facial nerve. Actually, it is accepted that the proprioception of all cephalic muscles depends on the trigeminal nerve [6, 18].

Therefore, the first unresolved issue in cephalic proprioception is whether all cranial nerves that innervate striated muscles also collect their proprioceptive innervation. According to Lazarov [18] the proprioceptive innervation of all cephalic muscles depends exclusively on the trigeminal nerve. In other words: the sensory ganglia of cranial nerves lack of primary sensory neurons and the proprioceptors of the cephalic muscles are supplied by neurons from the trigeminal mesencephalic nucleus [19].

The second aspect pending clarification is: if the proprioception of the cephalic muscles depends exclusively, or mainly, on the trigeminal nerve, how do the fibers of this nerve reach the muscles of the territories of other nerves? This question can be answered because to extensive communications of the trigeminal nerve with other cranial nerves. The trigeminal nerve has numerous connections to the facial nerve [20–34] and the data collected from animal models indicate that the nerve fiber interchange is always from the trigeminal to the facial nerve and not on the contrary [35]. To serve facial proprioception additional connections between the facial and cervical spinal nerves exists [36, 37]. Apart from those communications no specific reference of communications between the trigeminal nerve with the glossopharyngeal, vagal and hypoglossal nerves were found. But presumably the trigeminal proprioceptive fibers pass from the trigeminal nerve to them directly on the target organs themselves (tongue, pharynx, palate) or through their connections with the facial nerve [28, 31, 32].

And the third main question of cephalic proprioception regards the identification and characterization of proprioceptors in the cephalic muscles. The skeletal muscles innervated by spinal nerves contain neuromuscular spindles and Golgi tendon organs, in addition to other types of corpuscles with less functional entity [8–11]. However, only the cephalic muscles supplied by the mandibular branch of the trigeminal nerve, and *the platysma colli* muscle contain neuromuscular spindles [38–40]. Therefore, cephalic proprioceptors, if any, have to be represented by other sensory nerve formations other than neuromuscular spindles. Recent studies, using immunohistochemistry techniques associated with specific markers related to mechanization, have shown that facial muscles [34, 41, 42] and some pharyngeal muscles [43] have differentiated sensory structures that presumably replace proprioceptors. However, it cannot be ruled out that sensitive nerve fibers reaching the muscles (especially nociceptive ones) can function as mechanoreceptorsproprioceptors (see [44]).
