**3.1 Criteria to characterize atypical proprioceptors**

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 mechanoprotein [34].

#### *Proprioception*


*\*Isolated nerve fibers displaying immunoreactivity for any of the mechanoproteins investigated. \*\*facial/cervical segments.*

#### **Table 1.**

*Distribution and density of muscle spindles (MS), atypical proprioceptors (types I to III) and isolated nerve fibers (INF) in muscles supplied by the trigeminal nerve (green), facial nerve (blue), hypoglossal nerve (white) and glossopharyngeal nerve (brown).*

In agreement with the above premises, capsulated and non-capsulated corpuscle-like structures of variable size and shape containing numerous axon profiles complexly arranged, have been identified. Given the morphologic heterogeneity of the corpuscle-like structures that fulfill the preestablished criteria we attempt to classify them into three types: type I, capsulated by a thin capsule, the glial cells variably arranged and showing different morphologies; type II, partially capsulated (the capsule being continuous with the perimysium), with variable morphology and in most of the cases the direction of the long axis was parallel to the one of muscular fibers; type III, non-capsulated and both the axon and Schwann-like cells are variably arranged (**Figure 2**).

On the other hand, it is now well established that at the basis of mechanosensitivity are mechanically-gated ion channels [50]. At present acid-sensing ion

**71**

muscle [48].

**Figure 2.**

*Proprioceptors in Cephalic Muscles*

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

channel 2 (ASIC2) and Piezo2 have been detected in muscle spindles and are strong candidates to initiate the mechanotransduction in proprioceptors [50–56]. Also, the putative mechanoprotein transient-receptor potential vanilloid 4 (TRPV4) was

No typical muscle spindles have been found in the human facial muscles [42, 57–61] with the exception of on the facial part of the muscle *platysma colli* [40, 45]*.* Conversely, they contain numerous atypical proprioceptors (**Table 1**) the type II of being the predominating and the greater density being observed in

Most research have not found typical muscle spindles in the muscles innervated by the glossopharyngeal nerve although they are present in the human *palatoglossus*

Regarding the pharyngeal muscles, typical muscle spindles were never found with the exception of the *constrictor pharyngis inferior* of the crab-eating monkey (*Macaca irus*) [62]. Nevertheless, human pharyngeal muscles are richly innervated. In particular, the *constrictor pharyngis superior* and muscle *constrictor pharyngis inferior* (innervated by branches of the pharyngeal plexus, derived from the glossopharyngeal and vagal nerves, and a small contribution of facial nerve; [63]) contain type II and III putative proprioceptors and isolated nerve fibers that display

As far as we know no muscles spindles have been reported in togue muscles. Junquera [45] observed one muscle spindle in the genioglossus muscle as well as

detected in proprioceptors of the facial and pharyngeal muscles [42, 43].

*Types of putative proprioceptors in human cephalic muscles. INF: Isolated nerve fibers.*

**3.3 Distribution in the territory of the glossopharyngeal nerve**

immunoreactivity for mechanoproteins (**Table 1**) [43].

numerous putative proprioceptors (**Table 1**).

**3.4 Distribution in the territory of the hypoglossal nerve**

**3.2 Distribution in the territory of the facial nerve**

the *buccinator* and *orbicularis oris* muscles.

*Proprioceptors in Cephalic Muscles DOI: http://dx.doi.org/10.5772/intechopen.96794*

*Proprioception*

M. corrugator supercilii + M. depresor supercilii

*M. orbicularis* oculii pars palpebralis pars orbitalis

M. orbicularis oris pars marginalis pars labialis

mentalis

*\*\*facial/cervical segments.*

*and glossopharyngeal nerve (brown).*

**Table 1.**

M. depressor labii inferioris +

**70**

are variably arranged (**Figure 2**).

In agreement with the above premises, capsulated and non-capsulated corpuscle-like structures of variable size and shape containing numerous axon profiles complexly arranged, have been identified. Given the morphologic heterogeneity of the corpuscle-like structures that fulfill the preestablished criteria we attempt to classify them into three types: type I, capsulated by a thin capsule, the glial cells variably arranged and showing different morphologies; type II, partially capsulated (the capsule being continuous with the perimysium), with variable morphology and in most of the cases the direction of the long axis was parallel to the one of muscular fibers; type III, non-capsulated and both the axon and Schwann-like cells

*Distribution and density of muscle spindles (MS), atypical proprioceptors (types I to III) and isolated nerve fibers (INF) in muscles supplied by the trigeminal nerve (green), facial nerve (blue), hypoglossal nerve (white)* 

**Muscle MS Type I Type II Type III INS\*** *M. temporalis* 14 6 8 6 Yes M. masseter 23 3 3 6 Yes M. pterygoideus lateralis 18 3 14 7 Yes M. pterygoideus medialis 21 5 10 3 Yes Venter anterior m. digastricus 2 3 1 1 Yes M. mylohyoideus 1 3 1 1 Yes M. tensor veli palatini 0 2 2 1 Yes

> 0 0

> 0 0

*\*Isolated nerve fibers displaying immunoreactivity for any of the mechanoproteins investigated.*

3 1

5 7

M. zygomaticus maior 0 1 4 4 Yes M. zygomaticus minor 0 1 2 0 Yes M. buccinator 0 19 28 10 Yes

M. levator labii superioris 0 1 1 3 Yes Platysma colli\*\* 12/8 11/7 4/7 6/8 Yes/

M. genioglossus 1 16 28 10 Yes M. palatoglossus 0 0 5 3 Yes M. uvulae 0 0 7 3 Yes M. constrictor pharyngis superior 0 0 6 14 Yes M. constrictor pharyngis inferior 0 0 5 9 Yes

1 3 7 Yes

9 9

12 7

Yes Yes

Yes Yes

Yes

11 7

19 13

0 0 8 2

On the other hand, it is now well established that at the basis of mechanosensitivity are mechanically-gated ion channels [50]. At present acid-sensing ion

**Figure 2.** *Types of putative proprioceptors in human cephalic muscles. INF: Isolated nerve fibers.*

channel 2 (ASIC2) and Piezo2 have been detected in muscle spindles and are strong candidates to initiate the mechanotransduction in proprioceptors [50–56]. Also, the putative mechanoprotein transient-receptor potential vanilloid 4 (TRPV4) was detected in proprioceptors of the facial and pharyngeal muscles [42, 43].
