**3. Results**

#### **3.1. Number of myelinated axons**

We estimated the total number or number per unit area of myelinated axons (NM) in the peripheral nerves (Table 1). The myelinated nerve fibers appeared as a blue-green myelin sheath surrounding a dark purple or black axon (Fig. 2). According to the data, there was no statistically significant difference in the total number of myelinated axons between the female and male specimens of the peripheral nerves, except for the vestibular nerve (*P* < 0.05; Table 1).

#### **3.2. Average transverse area of myelinated axons**

The average transverse area of myelinated axons (ATA) in the peripheral nerves was calculated (Table 1). According to the data, there was no statistically significant difference in the average transverse area of myelinated axons between the female and male specimens of all calculated peripheral nerves (*P* < 0.05; Table 1).

#### **3.3. CR of myelinated axons**

The average CR of myelinated axons (ACR) in the peripheral nerves was calculated (Table 1) and there was no statistically significant difference in the average CR of myelinated axons between the female and male specimens of the peripheral nerves, except for the vagus nerve (*P* < 0.05; Table 1).

#### **4. Discussion**

Researchers have reported that a decrease in the number and size of myelinated axons influences the occurrence of peripheral nerve palsy or neuropathy [3-6], and a lower CR of myelinated axons has been partly implicated in the degeneration of nerve fibers [7]. A smaller NM, ATA, and lower ACR of myelinated axons would help explain the sex difference in the incidence of peripheral nerve palsy and neuropathy.

The Relationship Between Sexually Dimorphic Peripheral Nerves and Diseases 131

Average Transverse Area (μm2 )

5.54 ± 1.26 0.87 ± 0.06

1.55 ± 0.62 0.86 ± 0.11

Average Circularity Ratio

Number /Unit area

p value P=0.93 P=0.43 P=0.35 P=0.71 95% CI −10.8 to 10.0 −1 to 2 −0.62 to 1.33 −0.06 to 0.04

p value P=0.66 P=0.43 P=0.88 P=0.96 95% CI −12.3 to 9.1 −3 to 8 −0.67 to 0.52 −0.08 to 0.10

Specimens Age Total Number or

R M 15 72.9 ± 12.1 9 ± 2 / 16×16 μm2

R M 15 72.1 ± 12.5 14 ± 7 / 16×16 μm2

significant sex difference in the incidence of oculomotor nerve palsy.

Splanchnic R F 15 73.3 ± 15.6 8 ± 2 / 16×16 μm2 5.18 ± 1.36 0.88 ± 0.07

Splanchnic R F 15 73.7 ± 15.5 12 ± 6 / 16×16 μm2 1.63 ± 0.96 0.85 ± 0.10

Cabrejas et al. reported epidemiological data on oculomotor nerve palsy that there were 59.1% males, with no statistically significant difference between females and males (p = 0.574) [8]. Moreover, Ohguro et al. reported finding differences in oculomotor nerve palsy with causative disease incidence according to sex, but they reported finding no significant difference in oculomotor nerve palsy with unknown cause incidence according to sex [9]. According to the data in this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the oculomotor nerve (*P* < 0.05; Table 1). My findings may partly explain why there is no

Researchers have reported that the incidence rate of trigeminal neuralgia (TN) was slightly higher for females than for males. For example, the female-to-male ratio was 1.74:1 in the Katusic et al. study [10] and 3:2 in another study by Ashkenazi and Levin [11]. It has been proposed that the symptoms of TN are caused by demyelination of the nerve leading to ephaptic transmission of impulses. Surgical specimens have demonstrated this demyelination and close apposition of demyelinated axons in the trigeminal root of patients with TN [12]. Results from experimental studies suggest that demyelinated axons are prone to ectopic impulses, which may transfer from light touch to pain fibers in close proximity (ephaptic conduction) [12]. Current theories regarding the cause of this demyelination center on vascular compression of the nerve root by aberrant or tortuous vessels. Accepting current theories, neurovascular (or microvascular) decompression, when a pad is placed between a vessel and the nerve, has been found empirically to be an effective treatment for TN in cases resistant to medical therapy, and as many as 90% of cases have been relieved [13-15]. Barker et al. reported that 706 patients (around 60%) were female of 1185 patients

Nerve Side Sex Number of

**\*** Indicates a significant difference (p < 0.05).

**Table 1.** Nerve fiber analysis of peripheral nerves in humans.

Each value is the mean ± SD CI confidence interval

**4.1. Oculomotor nerve** 

**4.2. Ophthalmic nerve** 

Greater

Lesser


The Relationship Between Sexually Dimorphic Peripheral Nerves and Diseases 131


**\*** Indicates a significant difference (p < 0.05).

Each value is the mean ± SD

CI confidence interval

130 Sexual Dimorphism

Inferior

Recurrent

Nerve Side Sex Number of

smaller NM, ATA, and lower ACR of myelinated axons would help explain the sex

Number /Unit area

R M 10 72.5 ± 8.9 18,523 ± 5,700 6.28 ± 1.95 0.83 ± 0.06 p value P=0.65 P=0.79 P=0.62 P=0.82 95% CI −6.5 to 9.9 −5,359 to 4,595 −0.97 to 2.34 −0.06 to 0.06

R M 22 73.3 ± 15.7 36 ± 7 / 16×16 μm2 4.88 ± 1.38 0.84 ± 0.06 p value P=0.82 P=0.47 P=0.94 P=0.65 95% CI −15.8 to 12.0 −7 to 11 −1.75 to 1.79 −0.04 to 0.07

R M 11 75.6 ± 7.3 23,623 ± 6,684 30.95 ± 8.76 0.86 ± 0.05 p value P=0.32 P=0.55 P=0.62 P=0.77 95% CI −4.0 to 10.2 −4,680 to 8,157 −11.84 to 3.93 −0.05 to 0.03

R M 10 75.6 ± 9.3 2,139 ± 502 3.46 ± 0.50 0.76 ± 0.06 p value P=0.68 P=0.40 P=0.09 P=0.75 95% CI −10.5 to 14.1 −341 to 911 −1.81 to 0.05 −0.14 to 0.07

R M 10 65.1 ± 15.2 6,466 ± 735 6.27 ± 0.74 0.79 ± 0.03 p value P=0.91 P=0.26 P=0.86 P=0.85 95% CI −13.8 to 15.4 −357 to 1,244 −0.71 to 0.85 −0.13 to 0.10

R M 12 74.2 ± 10.2 21,002 ± 4,636 **\*** 3.56 ± 0.93 0.86 ± 0.08 p value P=0.69 P=0.04 P=0.56 P=0.93 95% CI −9.7 to 7.2 −256 to 6,217 −0.88 to 0.48 −0.07 to 0.03

R M 12 72.0 ± 17.9 26,598 ± 6,177 1.78 ± 0.30 0.86 ± 0.02 p value P=0.75 P=0.30 P=0.69 P=0.62 95% CI −19.7 to 26.4 −4,449 to 10,450 −0.48 to 0.43 −0.02 to 0.01

R M 15 77.8 ± 11.2 1,331 ± 192 1.21 ± 0.30 0.83 ± 0.06 **\*** p value P=0.79 P=0.72 P=0.57 P=0.03 95% CI −8.1 to 11.9 −257 to 94 −0.15 to 0.32 −0.00 to 0.09

R M 10 73.0 ± 8.8 14 ± 3 / 33×33 μm2 10.38 ± 2.57 0.89 ± 0.06 p value P=0.28 P=0.96 P=0.96 P=0.51 95% CI −15.5 to 5.3 −3 to 3 −2.77 to 3.41 −0.05 to 0.08

R M 10 76.4 ± 11.8 30.4 ± 10.9 6.09 ± 1.84 0.92 ± 0.07 p value P=0.31 P=0.52 P=0.31 P=0.70 95% CI −20.0 to 5.4 −9.5 to 16.9 −0.94 to 3.01 −0.09 to 0.07

R M 10 82.5 ± 9.9 1,491 ± 1,061 3.20 ± 0.75 0.90 ± 0.09 p value P=0.61 P=0.72 P=0.07 P=0.57 95% CI −16.2 to 11.5 −602 to 1,538 −0.11 to 1.31 −0.12 to 0.05

Average Transverse Area (μm2 )

Average Circularity Ratio

Specimens Age Total Number or

Oculomotor R F 10 70.8 ± 8.6 18,905 ± 4,861 5.59 ± 1.54 0.83 ± 0.07

Ophthalmic R F 22 75.2 ± 13.8 34 ± 11 / 16×16 μm2 4.86 ± 2.27 0.83 ± 0.05

Alveolar R F 11 72.5 ± 8.6 21,885 ± 7,711 34.90 ± 8.97 0.87 ± 0.03

Abducent R F 10 73.8 ± 7.4 1,854 ± 343 4.34 ± 0.75 0.80 ± 0.08

Facial R F 10 65.9 ± 15.9 6,023 ± 955 6.34 ± 0.92 0.80 ± 0.04

Vestibular R F 12 75.4 ± 9.6 18,022 ± 2,780 **\*** 3.76 ± 0.65 0.88 ± 0.03

Cochlear R F 12 68.7 ± 18.0 23,597 ± 5,377 1.81 ± 0.40 0.86 ± 0.01

Vagus R F 15 75.9 ± 15.4 1,413 ± 274 1.12 ± 0.33 0.79 ± 0.06 **\***

Laryngeal R F 10 78.7 ± 10.2 14 ± 2 / 33×33 μm2 10.06 ± 3.15 0.88 ± 0.04

Femoral R F 10 83.7 ± 9.9 26.7 ± 11.7 5.06 ± 1.54 0.93 ± 0.08

Tibial R F 10 84.9 ± 12.3 1,023 ± 680 2.60 ± 0.39 0.94 ± 0.05

difference in the incidence of peripheral nerve palsy and neuropathy.

**Table 1.** Nerve fiber analysis of peripheral nerves in humans.

#### **4.1. Oculomotor nerve**

Cabrejas et al. reported epidemiological data on oculomotor nerve palsy that there were 59.1% males, with no statistically significant difference between females and males (p = 0.574) [8]. Moreover, Ohguro et al. reported finding differences in oculomotor nerve palsy with causative disease incidence according to sex, but they reported finding no significant difference in oculomotor nerve palsy with unknown cause incidence according to sex [9]. According to the data in this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the oculomotor nerve (*P* < 0.05; Table 1). My findings may partly explain why there is no significant sex difference in the incidence of oculomotor nerve palsy.

#### **4.2. Ophthalmic nerve**

Researchers have reported that the incidence rate of trigeminal neuralgia (TN) was slightly higher for females than for males. For example, the female-to-male ratio was 1.74:1 in the Katusic et al. study [10] and 3:2 in another study by Ashkenazi and Levin [11]. It has been proposed that the symptoms of TN are caused by demyelination of the nerve leading to ephaptic transmission of impulses. Surgical specimens have demonstrated this demyelination and close apposition of demyelinated axons in the trigeminal root of patients with TN [12]. Results from experimental studies suggest that demyelinated axons are prone to ectopic impulses, which may transfer from light touch to pain fibers in close proximity (ephaptic conduction) [12]. Current theories regarding the cause of this demyelination center on vascular compression of the nerve root by aberrant or tortuous vessels. Accepting current theories, neurovascular (or microvascular) decompression, when a pad is placed between a vessel and the nerve, has been found empirically to be an effective treatment for TN in cases resistant to medical therapy, and as many as 90% of cases have been relieved [13-15]. Barker et al. reported that 706 patients (around 60%) were female of 1185 patients

who underwent microvascular decompression, and female sex was a risk factor for recurrence after microvascular decompression (hazard ratio 1.3; *P* = 0.06) [16]. According to the data here, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the ophthalmic nerve (*P* < 0.05; Table 1). Therefore, a vascular abnormality in the female TN may be one reason why TN has a female preponderance, but morphology in the characteristic nerve does not appear to explain the sex difference in the incidence of TN.

The Relationship Between Sexually Dimorphic Peripheral Nerves and Diseases 133

Campbell and Brundage [34] reported that the incidence rate of Bell's palsy (BP) was slightly higher for females than for males (rate ratio = 1.16). Meanwhile, Monini et al. [35] reported that males were slightly more affected (53.7%) than females. However, many researchers have reported finding no significant difference in BP incidence according to sex, as Tiemstra and Khatkhate [36] recently reported. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male facial nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is no

The incidence of vestibular dysfunction has a female preponderance in a textbook description [37]. There was a marked female preponderance among individuals with vestibular vertigo (one year prevalence ratio female to male of 2.7:1.0) [38]. Neuhauser et al. also reported that prevalence and incidence rates of vestibular vertigo were consistently higher in females than in males, for example, the lifetime prevalence ratio of female to male was 10.3:4.3, and the population incidence ratio (one year) female to male was 1.9:0.8 [39]. This female preponderance tended to increase with age [40]. Yin et al. reported that adults (18-65y) had a ratio of affected females to males of 59.1:40.9, but elderly adults (>65y) had a ratio of 60.6:39.4 [40]. With regard to Menière's disease, a female preponderance can be assumed based on the data from Rochester (61% women) [41] and is confirmed by the latest data from Finland [42]. The results here showed that NM was 2,980 (mean value) higher in the male vestibular nerve than the female vestibular nerve. My data also indicated a significant sex difference (*P* = 0.04; Table 1), but there was no statistically significant difference in the ATA and ACR of myelinated axons between the female and male vestibular nerve specimens (*P* < 0.05; Table 1). The lower NM of myelinated axons in the female vestibular nerve may be one of the reasons why vestibular disorders have a female preponderance, but the findings here on ATA and ACR of myelinated axons did not appear

There are some reports regarding the incidence of cochlear dysfunction with sex difference. The incidence of tinnitus has a female preponderance in a textbook description [43]. Meanwhile, Graham [44] and the National Study of Hearing [45] reported that the incidence rate of tinnitus was higher for females than for males until the mid-fifties, but after the midfifties, that of tinnitus was higher for males than for females. Møller et al. reported that of the 72 patients who underwent microvascular decompression of the intracranial portion of the auditory nerve, 54.8% experienced total relief from tinnitus or marked improvement [46]. This report indicated that vascular compression of the auditory nerve was a factor in tinnitus. Therefore, vascular abnormalities in tinnitus patients may be one reason why tinnitus shows a sex difference, but morphology in the characteristic nerve does not appear

**4.5. Facial nerve** 

**4.6. Vestibular nerve** 

**4.7. Cochlear nerve** 

significant sex difference in the incidence of BP.

to explain the sex difference in the incidence of these diseases.

#### **4.3. Inferior alveolar nerve**

Inferior alveolar nerve (IAN) damage can occur after an IAN block [17, 18] or following oral and maxillofacial surgical procedures [19-24]. With regard to the incidence of IAN damage after these procedures, Haas and Lennon reported no significant sex difference (ratio of affected females to males, 72:68) [25]. Kipp et al. also reported that the incidence was 7% in both sexes, indicating no significant sex difference [26], while Harn and Durham reported that there was little sex difference in postinjection lingual nerve injuries (ratio of affected females to males, 24:17) [27]. Meanwhile, sexual dimorphism that results in the incidence being almost twice as high in females than in males has been reported [17, 28, 29]. Pogrel and Thamby [17] found the difference in referral rates for male and female patients difficult to explain. They mentioned that there have been studies suggesting that nerves respond differently to injury in female animals than in male animals [30]. Coyle et al. [31] reported that female rats were more prone to developing tactile allodynia than male rats after partial sciatic nerve ligation. These reports [30, 31] may partially explain the indicated sex difference in the incidence of IAN damage. According to this study's data, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male IAN specimens (*P* < 0.05; Table 1). Heasman and Beynon [32] reported a difference between the total number of myelinated axons in the human IAN of dentate and edentulous groups as significant (P < 0.001) and suggested axonal atrophy in the main nerve trunk following tooth loss. As each cadaver in this study had 7 teeth (central incisor, lateral incisor, canine, first premolar, second premolar, first molar, and second molar) on the side of the mandible that I used, I considered that this result was not affected by the dentulous condition. Therefore, the results of this study suggest a nonsignificant sex difference in the incidence of IAN damage, supported by the morphometric analysis. These findings may partly explain why there is no significant sex difference in the incidence of IAN damage.

#### **4.4. Abducent nerve**

Patel et al. reported 69 male (50%) and 68 female (50%) cases of abducent nerve palsy or paresis [33]. According to this study, ATA in the female abducent nerve was larger than that in the male abducent nerve, but there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the oculomotor nerve (*P* < 0.05; Table 1). My findings may partly explain why there is no significant sex difference in the incidence of abducent nerve palsy or paresis.

#### **4.5. Facial nerve**

132 Sexual Dimorphism

explain the sex difference in the incidence of TN.

**4.3. Inferior alveolar nerve** 

**4.4. Abducent nerve** 

who underwent microvascular decompression, and female sex was a risk factor for recurrence after microvascular decompression (hazard ratio 1.3; *P* = 0.06) [16]. According to the data here, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the ophthalmic nerve (*P* < 0.05; Table 1). Therefore, a vascular abnormality in the female TN may be one reason why TN has a female preponderance, but morphology in the characteristic nerve does not appear to

Inferior alveolar nerve (IAN) damage can occur after an IAN block [17, 18] or following oral and maxillofacial surgical procedures [19-24]. With regard to the incidence of IAN damage after these procedures, Haas and Lennon reported no significant sex difference (ratio of affected females to males, 72:68) [25]. Kipp et al. also reported that the incidence was 7% in both sexes, indicating no significant sex difference [26], while Harn and Durham reported that there was little sex difference in postinjection lingual nerve injuries (ratio of affected females to males, 24:17) [27]. Meanwhile, sexual dimorphism that results in the incidence being almost twice as high in females than in males has been reported [17, 28, 29]. Pogrel and Thamby [17] found the difference in referral rates for male and female patients difficult to explain. They mentioned that there have been studies suggesting that nerves respond differently to injury in female animals than in male animals [30]. Coyle et al. [31] reported that female rats were more prone to developing tactile allodynia than male rats after partial sciatic nerve ligation. These reports [30, 31] may partially explain the indicated sex difference in the incidence of IAN damage. According to this study's data, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male IAN specimens (*P* < 0.05; Table 1). Heasman and Beynon [32] reported a difference between the total number of myelinated axons in the human IAN of dentate and edentulous groups as significant (P < 0.001) and suggested axonal atrophy in the main nerve trunk following tooth loss. As each cadaver in this study had 7 teeth (central incisor, lateral incisor, canine, first premolar, second premolar, first molar, and second molar) on the side of the mandible that I used, I considered that this result was not affected by the dentulous condition. Therefore, the results of this study suggest a nonsignificant sex difference in the incidence of IAN damage, supported by the morphometric analysis. These findings may partly explain why there is no significant sex difference in the incidence of IAN damage.

Patel et al. reported 69 male (50%) and 68 female (50%) cases of abducent nerve palsy or paresis [33]. According to this study, ATA in the female abducent nerve was larger than that in the male abducent nerve, but there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male specimens of the oculomotor nerve (*P* < 0.05; Table 1). My findings may partly explain why there is no

significant sex difference in the incidence of abducent nerve palsy or paresis.

Campbell and Brundage [34] reported that the incidence rate of Bell's palsy (BP) was slightly higher for females than for males (rate ratio = 1.16). Meanwhile, Monini et al. [35] reported that males were slightly more affected (53.7%) than females. However, many researchers have reported finding no significant difference in BP incidence according to sex, as Tiemstra and Khatkhate [36] recently reported. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male facial nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is no significant sex difference in the incidence of BP.

#### **4.6. Vestibular nerve**

The incidence of vestibular dysfunction has a female preponderance in a textbook description [37]. There was a marked female preponderance among individuals with vestibular vertigo (one year prevalence ratio female to male of 2.7:1.0) [38]. Neuhauser et al. also reported that prevalence and incidence rates of vestibular vertigo were consistently higher in females than in males, for example, the lifetime prevalence ratio of female to male was 10.3:4.3, and the population incidence ratio (one year) female to male was 1.9:0.8 [39]. This female preponderance tended to increase with age [40]. Yin et al. reported that adults (18-65y) had a ratio of affected females to males of 59.1:40.9, but elderly adults (>65y) had a ratio of 60.6:39.4 [40]. With regard to Menière's disease, a female preponderance can be assumed based on the data from Rochester (61% women) [41] and is confirmed by the latest data from Finland [42]. The results here showed that NM was 2,980 (mean value) higher in the male vestibular nerve than the female vestibular nerve. My data also indicated a significant sex difference (*P* = 0.04; Table 1), but there was no statistically significant difference in the ATA and ACR of myelinated axons between the female and male vestibular nerve specimens (*P* < 0.05; Table 1). The lower NM of myelinated axons in the female vestibular nerve may be one of the reasons why vestibular disorders have a female preponderance, but the findings here on ATA and ACR of myelinated axons did not appear to explain the sex difference in the incidence of these diseases.

#### **4.7. Cochlear nerve**

There are some reports regarding the incidence of cochlear dysfunction with sex difference. The incidence of tinnitus has a female preponderance in a textbook description [43]. Meanwhile, Graham [44] and the National Study of Hearing [45] reported that the incidence rate of tinnitus was higher for females than for males until the mid-fifties, but after the midfifties, that of tinnitus was higher for males than for females. Møller et al. reported that of the 72 patients who underwent microvascular decompression of the intracranial portion of the auditory nerve, 54.8% experienced total relief from tinnitus or marked improvement [46]. This report indicated that vascular compression of the auditory nerve was a factor in tinnitus. Therefore, vascular abnormalities in tinnitus patients may be one reason why tinnitus shows a sex difference, but morphology in the characteristic nerve does not appear to explain the sex difference in the incidence of tinnitus. Next, with regard to the incidence of hearing acuity, Kacker reported that there was no significant sex difference [47]. Meanwhile, Hinchcliffe and Jones reported that the hearing acuity in males was better than that in females [48]. Researchers also reported that the differences in hearing levels between females and males depended on the frequencies or race [49, 50]. Moreover, Star et al. reported that there were four females and six males among 10 patients with auditory nerve disease [51]. The main lesion in auditory nerve disease is thought to be demyelination or degeneration of cochlear nerve fibers. Finally, Nakashima examined the nationwide epidemiological study of sudden deafness in 1993, and reported that there was no significant sex difference [52]. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male cochlear nerve specimens (*P* < 0.05; Table 1). The findings here may partly explain why there is no significant sex difference in the incidence of cochlear dysfunction.

The Relationship Between Sexually Dimorphic Peripheral Nerves and Diseases 135

between the two sexes. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male recurrent laryngeal nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is no

Shinoda analyzed data on adult motor neuropathy in past reports, and mentioned that the vulnerability of male motor neurons was higher than that of female motor neurons [69]. With regard to amyotrophic lateral sclerosis (ALS), a progressive disorder of motor neurons, the incidence of ALS was slightly higher for males than for females (male/female rate was 2.0% or less) in reports including 100 cases or more. For example, Collins [70], Bonduelle et al. [71], Boman and Meurman [72], Erbslöh et al. [73], Kondo [74], and Haberlandt [75] reported that the male/female rate ratio was 1.1, 1.2, 1.3, 1.5, 1.5, 2.0, respectively. According to my data, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male femoral and tibial nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is little significant sex difference in the incidence of motor neuropathy. Therefore, sex difference in the incidence of motor neuropathy is considered to be caused not only due to the morphology of the motor

Shinoda analyzed data on adult autonomic neuropathy in past reports and mentioned that the vulnerability of male autonomic neurons was higher than that of females [69]. Hogarth and coworkers' study demonstrated that females have a lower central sympathetic nerve activity to the periphery, the mechanism of which involves a greater baroreceptor reflex inhibitory effect on this activity in females than in males [76]. These findings could have implications regarding the lower number of cardiovascular events observed in females than in males. Muneta et al. reported that the activation of the sex center regulating gonadotropin secretion may be a causative factor in the baroreflex impairment in females [77]. They also mentioned that changes in blood pressure in females are more sensitive to mental stress, but less so to isometric stress than those of males. These findings suggest that ovarian dysfunction is another important factor influencing the baroreflex function in addition to aging and blood pressure, and that the baroreflex impairment in females characterizes the sex difference in the pathophysiology of essential hypertension. Hinojosa-Laborde et al. reported that clear evidence exists for differences in the regulation of the sympatho-adrenal nervous system between males and females [78]. At each level of neural control examined in their review, females were able to limit the activation or enhance the inhibition of the sympathetic nervous system (SNS) more effectively than males during at least part of the oestral/menstrual cycle. These observations suggest that the ability of females to more tightly control the SNS and, subsequently, arterial pressure may serve as a mechanism whereby sex hormones protect females against hypertension. Here, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and

significant sex difference in the incidence of recurrent laryngeal nerve paralysis.

neurons, but also because of sex hormones and other factors.

**4.11. Greater splanchnic and lesser splanchnic nerve** 

**4.10. Femoral and tibial nerve** 

#### **4.8. Vagus nerve**

Araújo et al. assessed vagal activity using heart rate response to a short (4s) bicycle exercise test during maximal inspiratory apnea. This study aimed to evaluate the role of sex and physical activity patterns on vagal activity. As a result, no sex effect could be identified [53]. With regard to vagoglossopharyngeal neuralgia, researchers reported that there was no preponderance regarding sex [54, 55]. I gave a supplementary explanation for the term "vagoglossopharyngeal neuralgia". As researchers took the central or peripheral overlap between the glossopharyngeal nerve and vagus nerve into consideration, they grouped glossopharyngeal and vagal neuralgia together, and used the more useful vagoglossopharyngeal neuralgia in clinical practice [56, 57]. Meanwhile, Khasar et al. reported that under normal conditions, responses to noxious stimuli were modulated by vagus nerve activity in males, but not in females [58]. My results showed that ACR was 0.04 (mean value) higher in the male vagus nerve than the female vagus nerve. My data also indicated a significant sex difference (*P* = 0.03; Table 1). However, there was no statistically significant difference in the NM and ATA of myelinated axons between the female and male vagus nerve specimens (*P* < 0.05; Table 1). The higher ACR of myelinated axons in the male vagus nerve may be one reason why vagus nerve activity to modulate pain has a male preponderance. My findings regarding the NM and ATA of myelinated axons may partly explain why there is no significant sex difference in the incidence of vagoglossopharyngeal neuralgia.

#### **4.9. Recurrent laryngeal nerve**

With respect to sex, males with recurrent laryngeal nerve paralysis were more frequent than females in some reports [59-65] whereas in some other reports [66-68], there were more females than males with that condition. However, overall the above data indicated that there were 1,526 females with recurrent laryngeal nerve paralysis (48.5%) and 1,618 males (51.5%) [64]. Therefore, there was no significant difference in the number of patients between the two sexes. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male recurrent laryngeal nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is no significant sex difference in the incidence of recurrent laryngeal nerve paralysis.

#### **4.10. Femoral and tibial nerve**

134 Sexual Dimorphism

**4.8. Vagus nerve** 

neuralgia.

**4.9. Recurrent laryngeal nerve** 

to explain the sex difference in the incidence of tinnitus. Next, with regard to the incidence of hearing acuity, Kacker reported that there was no significant sex difference [47]. Meanwhile, Hinchcliffe and Jones reported that the hearing acuity in males was better than that in females [48]. Researchers also reported that the differences in hearing levels between females and males depended on the frequencies or race [49, 50]. Moreover, Star et al. reported that there were four females and six males among 10 patients with auditory nerve disease [51]. The main lesion in auditory nerve disease is thought to be demyelination or degeneration of cochlear nerve fibers. Finally, Nakashima examined the nationwide epidemiological study of sudden deafness in 1993, and reported that there was no significant sex difference [52]. In this study, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male cochlear nerve specimens (*P* < 0.05; Table 1). The findings here may partly explain why there is no

Araújo et al. assessed vagal activity using heart rate response to a short (4s) bicycle exercise test during maximal inspiratory apnea. This study aimed to evaluate the role of sex and physical activity patterns on vagal activity. As a result, no sex effect could be identified [53]. With regard to vagoglossopharyngeal neuralgia, researchers reported that there was no preponderance regarding sex [54, 55]. I gave a supplementary explanation for the term "vagoglossopharyngeal neuralgia". As researchers took the central or peripheral overlap between the glossopharyngeal nerve and vagus nerve into consideration, they grouped glossopharyngeal and vagal neuralgia together, and used the more useful vagoglossopharyngeal neuralgia in clinical practice [56, 57]. Meanwhile, Khasar et al. reported that under normal conditions, responses to noxious stimuli were modulated by vagus nerve activity in males, but not in females [58]. My results showed that ACR was 0.04 (mean value) higher in the male vagus nerve than the female vagus nerve. My data also indicated a significant sex difference (*P* = 0.03; Table 1). However, there was no statistically significant difference in the NM and ATA of myelinated axons between the female and male vagus nerve specimens (*P* < 0.05; Table 1). The higher ACR of myelinated axons in the male vagus nerve may be one reason why vagus nerve activity to modulate pain has a male preponderance. My findings regarding the NM and ATA of myelinated axons may partly explain why there is no significant sex difference in the incidence of vagoglossopharyngeal

With respect to sex, males with recurrent laryngeal nerve paralysis were more frequent than females in some reports [59-65] whereas in some other reports [66-68], there were more females than males with that condition. However, overall the above data indicated that there were 1,526 females with recurrent laryngeal nerve paralysis (48.5%) and 1,618 males (51.5%) [64]. Therefore, there was no significant difference in the number of patients

significant sex difference in the incidence of cochlear dysfunction.

Shinoda analyzed data on adult motor neuropathy in past reports, and mentioned that the vulnerability of male motor neurons was higher than that of female motor neurons [69]. With regard to amyotrophic lateral sclerosis (ALS), a progressive disorder of motor neurons, the incidence of ALS was slightly higher for males than for females (male/female rate was 2.0% or less) in reports including 100 cases or more. For example, Collins [70], Bonduelle et al. [71], Boman and Meurman [72], Erbslöh et al. [73], Kondo [74], and Haberlandt [75] reported that the male/female rate ratio was 1.1, 1.2, 1.3, 1.5, 1.5, 2.0, respectively. According to my data, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and male femoral and tibial nerve specimens (*P* < 0.05; Table 1). These findings may partly explain why there is little significant sex difference in the incidence of motor neuropathy. Therefore, sex difference in the incidence of motor neuropathy is considered to be caused not only due to the morphology of the motor neurons, but also because of sex hormones and other factors.

#### **4.11. Greater splanchnic and lesser splanchnic nerve**

Shinoda analyzed data on adult autonomic neuropathy in past reports and mentioned that the vulnerability of male autonomic neurons was higher than that of females [69]. Hogarth and coworkers' study demonstrated that females have a lower central sympathetic nerve activity to the periphery, the mechanism of which involves a greater baroreceptor reflex inhibitory effect on this activity in females than in males [76]. These findings could have implications regarding the lower number of cardiovascular events observed in females than in males. Muneta et al. reported that the activation of the sex center regulating gonadotropin secretion may be a causative factor in the baroreflex impairment in females [77]. They also mentioned that changes in blood pressure in females are more sensitive to mental stress, but less so to isometric stress than those of males. These findings suggest that ovarian dysfunction is another important factor influencing the baroreflex function in addition to aging and blood pressure, and that the baroreflex impairment in females characterizes the sex difference in the pathophysiology of essential hypertension. Hinojosa-Laborde et al. reported that clear evidence exists for differences in the regulation of the sympatho-adrenal nervous system between males and females [78]. At each level of neural control examined in their review, females were able to limit the activation or enhance the inhibition of the sympathetic nervous system (SNS) more effectively than males during at least part of the oestral/menstrual cycle. These observations suggest that the ability of females to more tightly control the SNS and, subsequently, arterial pressure may serve as a mechanism whereby sex hormones protect females against hypertension. Here, there was no statistically significant difference in the NM, ATA, or ACR of myelinated axons between the female and

male specimens of the greater splanchnic and lesser splanchnic nerve (*P* < 0.05; Table 1). These findings do not explain why there is a significant sex difference in the incidence of autonomic dysfunction. Therefore, the morphology of autonomic neurons may not be the cause of sex differences in the incidence of autonomic dysfunction and other factors such as sex hormones may be the cause.

The Relationship Between Sexually Dimorphic Peripheral Nerves and Diseases 137

[11] Ashkenazi A, Levin M (2004) Three common neuralgias. How to manage trigeminal,

[12] Love S, Coakham HB (2001) Trigeminal neuralgia: pathology and pathogenesis

[13] Apfelbaum R (1988) Surgical management of disorders of the lower cranial nerves. In: Schmideck H, Sweet W, (eds) Operative neurosurgical techniques. 2nd edn, Grune &

[14] Wilkins RH (1988) Surgical therapy of neuralgia: Vascular decompression procedures.

[15] Jannetta PJ (1990) Cranial rhizopathies. In: Youmans JR (ed) Neurological surgery. 3rd

[16] Barker FG II, Jannetta PJ, Bissonette DJ, Larkins MV, Jho HD (1996) The long-term outcome of microvascular decompression for trigeminal neuralgia. N Engl J Med

[17] Pogrel MA, Thamby S (2000) Permanent nerve involvement resulting from inferior

[18] Lambrianidis T, Molyvdas J (1987) Paresthesia of the inferior alveolar nerve caused by

[19] Pogrel MA, Bryan J, Regezi J (1995) Nerve damage associated with inferior alveolar

[20] Giuliani M, Lajolo C, Deli G, Silveri C (2001) Inferior alveolar nerve paresthesia caused by endodontic pathosis: A case report and review of the literature. Oral Surg Oral Med

[21] Panula K, Finne K, Oikarinen K (2001) Incidence of complications and problems related to orthognathic surgery: A review of 655 patients. J Oral Maxillofac Surg 59:1128-1136 [22] Blanas N, Kienle F, Sàndor GKB (2002) Injury to the Inferior alveolar nerve due to

[23] Kraut RA, Chahal O (2002) Management of patients with trigeminal nerve injuries after

[24] Teerijoki-Oksa T, Jääskeläinen S, Forssell K, Virtanen A, Forssell H (2003) An evaluation of clinical and electrophysiology tests in nerve injury diagnosis after mandibular

[25] Haas DA, Lennon D (1995) A 21 year retrospective study of reports of paresthesia following local anesthetic administration. J Can Dent Assoc 61:319- 320, 323-326, 329-

[26] Kipp DP, Goldstein BH, Weiss WW Jr (1980) Dysesthesia after mandibular third molar surgery: A retrospective study and analysis of 1,377 surgical procedures. J Am Dent

[27] Harn SD, Durham TM (1990) Incidence of lingual nerve trauma and postinjection complications in conventional mandibular block anesthesia. J Am Dent Assoc 121:519-

[28] Howe GL, Poyton HG (1960) Prevention of damage to the inferior dental nerve during

periodontal-endodontic pathosis. Oral Surg Oral Med Oral Pathol 63:90-92

[published correction appears in Brain. 2002;125:687]. Brain 124:2347-2360

occipital, and postherpetic pain. Postgrad Med 116:16–32

Stratton, New York, pp. 1097-1109

edn, WB Saunders, Philadelphia, pp. 4169–4182

alveolar nerve blocks. J Am Dent Assoc 131:901-907

thermoplastic gutta percha. J Oral Maxillofac Surg 60:574-576

mandibular implant placement. J Am Dent Assoc 133:1351-1354

the extraction of mandibular third molars. Br Dent J 109:355-363

sagittal split osteotomy. Int J Oral Maxillofac Surg 32:15-23

nerve blocks. J Am Dent Assoc 126:1150-1155

Oral Pathol Oral Radiol Endod 92:670-674

Semin Neurol 8:280–285

334:1077-83

330

523

Assoc 100:185-192
