Microneurography

Microneurography was first developed in the mid-1960s by Karl-Erik Hagbarth and Åke Vallbo, in Sweden. The technique was introduced in Marseille in the early 1980s by Jean-Pierre Roll and Jean-Pierre Vedel, when soon after, Edith Ribot-Ciscar joined the team and still continues to conduct microneurography in Marseille today. In the early 2000s, Jean-Marc Aimonetti joined the team and trained in microneurography and in 2015, Rochelle Ackerley became the latest permanent microenurography researcher. The microneurography group has grown in recent years and is one of the largest in the world, currently with 3 permanent members of staff, two post-docs and 1 PhD student practising the technique.

“Microneurography is, in principle, a very simple technique, but in practice is quite demanding.” (Vallbo, 2018)

Microneurography is used to study activity of nerve responses in humans. In our experiments, we insert a fine electrode (thinner than ones used in acupuncture) through the skin and into a peripheral nerve, typically in the arm or leg, although studies have been carried out on other body areas, such as the face and mouth. Once the nerve has been reached, it is possible to ‘hear’ and ‘see’ the neuronal responses. There are many different types of afferent and efferent nerve fiber that can be found, in single unit (i.e. from one individual fiber) and in multi-unit recordings.

Single-unit microneurography
In Marseille, there has been a long tradition to examine single unit muscle spindle activity, as well as recordings from cutaneous afferents. Similarly, Åke Vallbo the co-founder of the technqiue practised single unit recordings for many years in Umeå, Sweden (where he trained Roland Johansson, who later had a long carrier in microneurography there) and then after in Gothenburg, Sweden. The lab in Gothenburg is now run by Johan Wessberg and is where Rochelle Ackerley trained, with others, such as Helena Backlund Wasling, Line Löken, Roger Watkins, and Mariama Dione. There is an ongoing technical and scientific collaboration between the microneurographers in Marseille and Gothenburg.

Single unit microneurography involves recording from an individual fiber. This can be challening even with large-diameter, myelinated fibers, but it is also possible to record from the small, unmyelinated C-fibers. Many studies have been condcuted on the A-beta mechanoreceptive afferents of the glabrous (non-hairy) skin of the hands, namely (Johansson & Vallbo, 1979; Vallbo & Johansson, 1984):

• Fast-adapting type I (FAI; Meissner) afferents
• Slowly-adapting type I (SAI; Merkel) afferents
• Fast-adapting type II (FAII; Pacini) afferents
• Slowly-adapting type II (SAII; Ruffini) afferents

Other studies have investigated hairy skin responses, such as on the arm, leg, or face. Here, the FAI-Meissner mechanoreceptors are not thought to be present, but the following A-beta mechanoreceptive afferents can be found (Vallbo et al., 1995; Nagi et al., 2019):

• Slowly-adapting type I (SAI; Merkel) afferents
• Fast-adapting type II (FAII; Pacini) afferents
• Slowly-adapting type II (SAII; Ruffini) afferents
• Fast-adatping hair afferents (innverating hairs)
• Fast-adapting field afferents

Further, C-fibers can be found all over the skin, including C-tactile (CT) afferents that are numerous in the face (Nordin, 1990) and arm (Vallbo et al., 1993, 1999; Wessberg et al., 2003; Löken et al., 2009; Ackerley et al., 2014, 2018; Watkins et al., 2017). They are found at lower density in the leg (Edin, 2001), but recently a sparse projection from the glabrous hand skin has also been found (Watkins et al., 2021).

As well as CTs, many other C-fibers can be recorded from, such as sympathetic C-fiber efferents, C-nociceptors, and C-thermoreceptors (Bostock et al., 2003; Campero et al., 2009; Macefield, 2013; Ackerley & Watkins, 2018).

The muscles and the joints of the body are well-innervated by myelinated and unmyelinated afferents (Macefield, 2005), with many microneurography studies focusing on muscle spindle afferents, which are divided into type Ia (most common), type II, and Golgi tendon organs. The muscles receive descending, efferent input from motoneurons (alpha, beta, and gamma) and the gamma fusimotor neurons can exert adjust the sensitivity of muscle spindles to adapt to the behavioral situation (Hospod et al., 2007; Ribot-Ciscar et al., 2009; Ackerley et al., 2017).