Innervation of the wing membrane in the bent-winged bat Miniopterus fuliginosus

1 INTRODUCTION

The bent-winged bat Miniopterus fuliginosus of the family Vespertilionidae is widely distributed in Afghanistan, India, China and Japan (Maeda, 1996). It performs seasonal migration to its roosting sites between summer and winter and travels up to 200 km (Iida et al., 2017; Xu et al., 2005). The distance of flight differs depending on its wing morphology. There are two parameters indicating the wing morphology: wing loading and the aspect ratio. Wing loading is the body weight divided by the total area of the wing membrane. Higher the wing loading value, smaller is the wing membrane compared to the body. The aspect ratio is the ratio of the wingspan to the mean chord length or the square of the wingspan divided by the wing area. Higher aspect ratios correspond to long and narrow wings, while lower aspect ratios correspond to short and broad wings. The wing type of bats with a high wing loading value and aspect ratio, such as that of Miniopterus species, is suitable for a wide range and long distance of flight (Norberg & Rayner, 2016). In contrast, the wing type of bats with a low wing loading value and aspect ratio is suitable for a narrow range of flight and for skilful flight in narrow spaces but not for a long distance of flight (Norberg & Rayner, 2016). The Rhinolophus species wing appears to have a similar shape, but it is not known if it actually has a low wing loading value and aspect ratio. Ecology studies attaching metal tags to the hindlimbs of bats reported a wide area of flight for M. fuliginosus (Xu et al., 2005). In contrast, a narrow area of flight is seen in the Rhinolophidae family (Kuramoto, 1977).

Bats have sensory hairs on the surface of the wing membrane, which can sense airflow during flight. The Merkel nerve endings at the root of the sensory hair conduct information received by the wing to the central nervous system (Sterbing-D’Angelo et al., 2011). The skeletal muscles in the wing membrane help control the wing shape during flight (Cheney, Allen, & Swartz, 2017). As small-sized insectivorous bats show both acute turning and straight flights, their neural circuits are considered to be complex. However, there is little neuroanatomical knowledge on the innervation that controls the wings of small-sized insectivorous bats. To clarify the neural circuits involved in flight, we conducted this anatomical survey on the innervation of the upper arm to the fingertips and wing membrane of M. fuliginosus.

2 MATERIALS AND METHODS

After obtaining approval from the Wakayama Prefecture (approval number: 04110001), we captured M. fuliginosus from a cave in Tanabe, Wakayama Prefecture (33.708N 135.413E). The animals were euthanized and subsequently subjected to immersion fixation in 10% formaldehyde. This study was performed after obtaining approval from the Nagoya University Animal Experiment Committee (approval number: 2019031245). The nerves distributed from the upper arm to the fingertips and wing membranes were studied under the stereomicroscope. In this study, three male M. fuliginosus were used.

3 RESULTS

Figure 1 shows the bat wing anatomy. No special differences were observed among the three bats used in this study.

3.1 Spinal nerves comprising the brachial plexus and the radial, median, and ulnar nerves

The brachial plexus of M. fuliginosus comprised of nerves arising from C6–8 and T1 (Figure 2). The nerves arising from C7–T1 mainly innervated the wings and branched out as the radial, ulnar and median nerves. The radial, median and ulnar nerves were formed by the spinal nerves C7–T1, C8–T1 and C8–T1, respectively. The musculocutaneous nerve was formed by the nerves C7–T1.

3.1.1 Radial nerve (C7–T1)

The radial nerve ran along the extensor surface of the humerus, and subsequently, reached the flexor side of the forearm, near the flexor surface of the elbow joint (Figure 3). Finally, it ran along the caudal edge of the propatagium up to the end of the first digit (Figure 4a).

3.1.2 Median nerve (C8–T1)

The median nerve extended along the extensor surface of the humerus, and subsequently, reached the extensor surface of the forearm via the dorsal side of the elbow joint (Figure 3). It continuously ran along the extensor side of the forearm and branched out at the extensor side of the carpal joint to supply the digits (Figure 4b), extending from the second to fifth digits up to the ends of the phalanges (Figure 5). The digital nerves running along the third to fifth digits also branched out radially from around the metacarpophalangeal and distal interphalangeal joints, innervating the dactylopatagium (DP) area (Figure 6). However, the final branch of the nerve that ran to the DP area could not be traced under stereomicroscopy.

3.1.3 Ulnar nerve (C8–T1)

The ulnar nerve ran from the fossa axillaris to the extensor surface of the humerus and plagiopatagium (PLP), near the extensor surface of the elbow joint (Figures 3 and 6). A few millimetres after the elbow joint, a few nerves branched-off at intervals and extended caudally towards the trailing edge of PLP (Figure 7).

3.1.4 Musculocutaneous nerve (C6–T1)

The musculocutaneous nerve ran along the ventral side of the humerus. After covering two-thirds of the humerus, it branched out radially to the PLP area (Figure 3). Several fine branches were confirmed in the musculocutaneous nerve that ran to the PLP area, but could not be traced more under stereomicroscopy.

3.1.5 Other nerves

Two nerves running from the dorsal side of the body to the wing membrane were found (Figures 3 and 7). One nerve ran along the extensor surface of the humerus, and subsequently, curved to extend caudally to the trailing edge of the wing membrane. The origin of the nerve could not be traced under the stereomicroscope, as it ran inside the deltoid muscle via the fossa axillaris and was rendered invisible. The other nerve travelled along the extensor surface of the humerus and innervated PLP. It ran parallel to the ulnar nerve at the elbow joint. Tracing the upstream pathway of this nerve revealed that it branched out from the ulnar nerve. These two nerves may be axillary nerves．Both of these two nerves and the ulnar nerve ran towards the trailing edge of PLP, but their final or fine branches could not be confirmed under stereomicroscopy.

4 DISCUSSION

In this study, we examined the innervation of the wings of M. fuliginosus, focusing on the radial, ulnar and median nerves and their branches. The brachial plexus of M. fuliginosus comprised of the nerves arising from C6 to T1. The radial, median and ulnar nerves were formed by the spinal nerves C7–T1, C8–T1 and C8–T1, respectively. These results indicated that the spinal nerves C6–T1 innervate the bat wings. Previous studies have revealed a pattern of innervation of the brachial arm in the bat embryo, with the radial, median, ulnar, musculocutaneous and axillary nerve mainly composed of C6 –T1 and a slight C5 component (Tokita, Abe, & Suzuki, 2012). These findings are almost consistent with the results obtained in this study on adult bats. However, more spinal nerves innervate the wings of the big brown bat (Eptesicus fuscus) (Marshall et al., 2015). Neural tracing using injection of the fluorescent cholera toxin revealed that the sensory neurons innervating the wing membrane, digits and upper arms arise from C4 to T9 (Marshall et al., 2015). Although not confirmed in this study, more spinal nerves innervate the wing membrane of M. fuliginosus. Consistent with the stereoscopic findings of this study, a previous study stated that over 80% of the sensory neurons in the bat wings arise from C6 to T1 (Marshall et al., 2015). Nearly 80% of the sensory nerves were observed under the stereomicroscope in this study. In the little Japanese horseshoe bats (Rhinolophus cornutus), the brachial plexus comprises C5–T2, and the radial, median and ulnar nerves are formed by C7–T1, C8–T1 and C8–T2, respectively (Aoyama, Kurihara, & Sugita, 2019). Compared to M. fuliginosus, the brachial plexus of R. cornutus, which flies short distances, comprises of more pairs of spinal nerves. The brachial plexus comprises of four pairs of spinal nerves (C13–T1) in non-flying chicken breeds (Kato, 1995), same as M. fuliginosus, while of seven pairs of spinal nerves (C11–T2) in Falco columbarius (Çevik-Demirkan, 2014). Thus, the composition of the brachial plexus may not be associated with the flight distance.

The nerves ran along the third to fifth digits branched out radially from the metacarpophalangeal and distal interphalangeal joints, innervating the dactylopatagium area. Mechanoreceptors, including Merkel nerve endings, are distributed in the phalanges and phalangeal joints (Marshall et al., 2015). These findings suggest that the nerves in the dactylopatagium area innervate the mechanoreceptors.

The musculocutaneous nerve and suspected axillary nerves were found in M. fuliginosus but not in R. cornutus (Aoyama et al., 2019). It is possible that in R. cornutus, these nerves have too few or too thin branches to be visible under a stereomicroscope.

The wing membrane of bats, in particular PLP, has muscles that control the shape of the wing during flight (Cheney et al., 2017). In Eptesicus fuscus, over 98% of the motor neurons arise from T1 to T3 innervate the muscles of PLP (Marshall et al., 2015). In M. fuliginosus, the ulnar nerve (C8–T1) and suspected axillary nerves probably branch-out to innervate the muscles of PLP. Actually, in M. fuliginosus, several muscles continuous from the upper arm and entering the PLP are known to be innervated by C5 – T1 (Tokita et al., 2012).

Bats with a high wing loading value and aspect ratio, such as M. fuliginosus, can fly in open spaces at a high speed (Altringham, 1998). In contrast, bats with a low wing loading value and aspect ratio, such as R. cornutus, can fly through intricate forests without slowing down and can turn around more rapidly (Altringham, 1998). Thus, it seems characteristic that there was a great morphological difference in the nerves distributed in the wing membrane between the two species having different flight patterns and wing morphologies. However, its functional significance has not been clarified in this study.

Our study revealed the anatomical features of bat wing membrane innervation. However, functions of each nerve fibre are not to understand. Recent studies have reported that the motor nerve bundle also coexists with the sensory nerve, which is thought to contribute to fine system control (Aszmann, Gesslbauer, Tereshenko, Mayer, & Wiedemann, 2019). The information on functions of nerve fibre in bats is even more important for flight patterns and distribution.