Vertebrates and invertebrates (e.g., arthropods: insects, crustaceans, etc.) have two different neuro-muscular systems given their differential construction: the former have an endoskeleton and the latter have an exoskeleton. In the case of vertebrates, each motor neuron is connected to a single muscle, whereas in the case of invertebrates the fewer motor neurons are connected to shared muscles (1 to 13 neurons per muscle), such that there is the need for coordination between the motor neurons to bring about a smooth muscle contraction (Belanger 2005). Furthermore, some motor neurons in invertebrates are excitatory and some are inhibitory, and the contraction of the muscles can be modified by hormones. In mammals the number of muscles can surpass 40,000 as found in the elephant, mostly for the control of the trunk (see Footnote 1; Longren, Brecht et al. 2023). Mammalian motor neurons are always excitatory. Humans have over 600 muscles (Smoliga and Zavorsky 2016), with 30 muscles dedicated to each hand and 35 muscles controlling the vocal apparatus (Gesslbauer et al. 2017; Hiatt and Gartner 2010; Saran et al. 2024). It is our hands and vocal apparatus, as connected to the central nervous system, that distinguishes us from other animals (Kimura 1993; Ojemann 1991; Penfield and Roberts 1966).
In both vertebrates and invertebrates, the contraction of muscles from body position 1 to body position 2 requires a differential innervation of signals to maintain equilibrium at a particular collection of muscle lengths or tensions and joint angles (Feldman 1974; Giszter, Mussa-Ivaldi, Bizzi 1991). Once a new position is assumed, a re-afference signal is sent to the brain so that any future shift in body posture can be made in an intelligent manner based on sensory contingencies; this is especially important if position 2 is suboptimal, namely, registering an error according to an animal’s efference-copy plan (a plan that is stored in the cerebellum and is continuously updated in vertebrates, Tehovnik, Hasanbegović, Chen 2024). All animals are equipped with sensors, muscles, and a memory apparatus (which is found in the telencephalon and cerebellum of all vertebrates) that can store behavioral routines; once an animal is behaviorally automated, a particular behavioral context evokes a specific set of muscle contractions that differs from anything previously stored by way of learning or genetic programing (Hebb 1949, 1968; Kandel 2006). The process of re-afference continues for the duration of an animal’s life. Note that most genetic programs can be altered by experience no matter how simple an animal, including single-celled animals (Nakagaki et al. 2000; Saigusa et al. 2008). In short, an animal is not ‘a rock’ (see Koch 2013); it has an internal constitution that can be modified over the duration of its life (Hebb 1949, 1968).
Footnote 1: The large number of muscles of the elephant is related to the high number of neurons in its cerebellum, estimated to be 261 billion (Herculano-Houzel, Manger et al. 2014). By comparison, humans with fewer muscles have some 69 billion neurons in the cerebellum (Herculano-Houzel 2009). Both elephants and humans have complex communication systems; how this fact affects the cerebellum is not well understood.
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