**2. Distribution of typical proprioceptors in cephalic muscles**

Typical proprioceptors of human cephalic muscles are represented by neuromuscular spindles as most of them lack Golgi tendon organs since they lack true tendons.

Muscle spindles have been found in muscles innervated by the trigeminal nerve while in the territory of the other cranial nerves, with very rare exceptions, are absent [6]. Recently, Junquera [45] determined the relative density of muscle

**69**

mechanoprotein [34].

*Proprioceptors in Cephalic Muscles*

muscles (**Table 1**; see below).

**Figure 1.**

intrafusal fibers and the capsule was less developed [45].

cervical segment of the muscle than in the suprahyoid one.

**3. Atypical putative proprioceptors cephalic muscles**

**3.1 Criteria to characterize atypical proprioceptors**

*DOI: http://dx.doi.org/10.5772/intechopen.96794*

spindles in human jaw muscles (**Figure 1**; **Table 1**). The *M. temporalis*, *m. masseter*, *m. perygoideus medialis* and m. *pterygoideus lateralis* contained numerous muscle spindles whereas they were less abundant in the *digastricus* and *mylohyoideus* muscles [45, 46]. The absence [45, 47] or presence [48] of muscle spindles in the *tensor veli palatini* muscle, also innervated by the trigeminal nerve, has been reported. It should be noted that atypical proprioceptors were also found in these

*Longitudinal and transversal sections of muscle spindles from different cephalic muscles. MS: muscle spinde.*

In muscles where the density of muscle spindles is higher, they consist of thick capsule, a shallow intracapsular space filled with variable number of intrafusal muscle fibers (ranging from 4 to 12). In muscles where the density of neuromuscular spindles was low, in general, the size of the spindles was smaller, had fewer

In the territory of the facial nerve one muscle spindle was found in the muscle *orbicularis oculi* in one pediatric specimen [49] whereas abundant muscle spindles have been found in the *platysma colli* [40]. Junquera [45] in her doctoral dissertation also observed typical muscle spindles in the *plastysma colly* more numerous in the

The identification of putative sensory receptors in the cephalic muscles that may serve as proprioceptors was based on the following criteria: independence of the nerve trajectory, be placed in close relation to muscle fibers, show a morphologically differentiated aspect, and display immunoreactivity for any putative *Proprioception*

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

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

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

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

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 mechanoreceptors-

**2. Distribution of typical proprioceptors in cephalic muscles**

Typical proprioceptors of human cephalic muscles are represented by neuromuscular spindles as most of them lack Golgi tendon organs since they lack

Muscle spindles have been found in muscles innervated by the trigeminal nerve while in the territory of the other cranial nerves, with very rare exceptions, are absent [6]. Recently, Junquera [45] determined the relative density of muscle

to the Banks [8] and Macefield and Knellwolf [16].

depends on the trigeminal nerve [6, 18].

connections with the facial nerve [28, 31, 32].

mesencephalic nucleus [19].

proprioceptors (see [44]).

**68**

true tendons.

**Figure 1.** *Longitudinal and transversal sections of muscle spindles from different cephalic muscles. MS: muscle spinde.*

spindles in human jaw muscles (**Figure 1**; **Table 1**). The *M. temporalis*, *m. masseter*, *m. perygoideus medialis* and m. *pterygoideus lateralis* contained numerous muscle spindles whereas they were less abundant in the *digastricus* and *mylohyoideus* muscles [45, 46]. The absence [45, 47] or presence [48] of muscle spindles in the *tensor veli palatini* muscle, also innervated by the trigeminal nerve, has been reported. It should be noted that atypical proprioceptors were also found in these muscles (**Table 1**; see below).

In muscles where the density of muscle spindles is higher, they consist of thick capsule, a shallow intracapsular space filled with variable number of intrafusal muscle fibers (ranging from 4 to 12). In muscles where the density of neuromuscular spindles was low, in general, the size of the spindles was smaller, had fewer intrafusal fibers and the capsule was less developed [45].

In the territory of the facial nerve one muscle spindle was found in the muscle *orbicularis oculi* in one pediatric specimen [49] whereas abundant muscle spindles have been found in the *platysma colli* [40]. Junquera [45] in her doctoral dissertation also observed typical muscle spindles in the *plastysma colly* more numerous in the cervical segment of the muscle than in the suprahyoid one.
