Health heuristics

Health heuristics opinion

Both smooth and cardiac muscles are innervated by the ANS (described above). In contrast, somatic MNs exclusively innervate skeletal muscles that are the most abundant muscle class, with around 639 health heuristics muscles in the human body (Stone and Stone, 2009). Skeletal muscles are firmly attached to health heuristics skeleton by the tendons and are responsible for both posture and movement. Developmentally, skeletal muscles derive from the paraxial mesoderm that produces the somites, which in turn generate muscle precursor cells called myoblasts.

Those cells migrate toward the periphery and fuse to form the body of the muscle. Physiologically, skeletal muscles are composed of 2 structures: (i) extrafusal fibers, generating the force and (ii) muscle spindles health heuristics proprioceptive information on the position and extension status of the muscle.

Muscle spindles are composed of several intrafusal fibers enveloped by a collagen sheath named the outer capsule. There are three kinds of intrafusal fibers with specific characteristics: (i) dynamic nuclear bag fibers (B1), (ii) static nuclear bag fibers (B2 fibers) and (iii) nuclear chain fibers.

Analogously, extrafusal fibers are divided into 3 types according to their physiological and molecular properties: (i) slow-twitch fatigue-resistant (SFR), health heuristics fast-twitch fatigue-resistant (FFR) and (iii) fast-twitch fatigable (FF). Table 2 summarizes the principal characteristics of the three health heuristics muscle fibers. Mirroring the diversity of both intra- and extrafusal fiber types in a muscle, somatic MNs are further sub-divided into 3 types: (i) alpha, (ii) beta and health heuristics gamma that will be further described below.

Alpha MNs exclusively innervate extrafusal muscle relay indications and are the key of muscle contraction (Figure 1). Anatomically, alpha MNs are characterized by a large cell body and a well-characterized neuromuscular ending. They have an important role in the spinal reflex circuitry by receiving monosynaptic innervation directly from SNs thus minimizing the delay of the response (Eccles et al.

Alpha MNs can be further divided into 3 different subtypes depending on the extrafusal fiber type they innervate: (i) SFR, (ii) FFR, and (iii) FF (Burke et al. There is no universal criteria distinguishing alpha MNs subtypes; however, some trends are observed in term of size, excitability, and firing pattern. SFR MNs tend to have a smaller cell body diameter health heuristics thus a higher input resistance making them responsive to a lower stimulation threshold.

As a result, SFR MNs are recruited first during muscle contraction. They also have the capacity of maintaining a persistent activity even after the stimulation ceased (Lee and Heckman, 1998). On the other hand, FF MNs have often a larger cell body and are firing after the initial recruitment of SFR neurons giving extra strength to the activated muscle. Lastly, little is known about FFR MNs physiology; yet, they are considered to have health heuristics characteristics between FF and SFR MNs (Figure 2).

Characteristics of alpha and gamma MNs. Schematic showing the principal characteristics of alpha and gamma MNs (adapted from Kanning et al. Alpha MNs have a larger diameter than gamma MNs. Beta MNs are not represented for simplicity. The proportion of alpha MN subtypes varies between MN pools.

Intrafusal muscle fibers (blue) reside within a muscle spindle (gray) and are exclusively innervated by gamma MNs. A single MN innervate multiple fibers all of the same type; health heuristics, for the schematic simplicity only one fiber is represented.

Beta MNs are smaller and less abundant than other somatic MN subtypes. As a result beta MNs are poorly characterized. They innervate both intrafusal and extrafusal muscle fibers (Bessou et health heuristics. Therefore, beta MNs constitute an exception to the homogeneity observed in motor-units health heuristics control both muscle contraction and responsiveness of the sensory feedback from muscle spindles.

They are further subdivided into two health heuristics depending on the type health heuristics intrafusal fibers they innervate: (i) static, innervating nuclear chain fibers and (ii) dynamic, innervating the nuclear Intrarosa Vaginal Inserts (Prasterone)- FDA fibers of muscle spindles. Static body mass index MNs increase the firing rate of type Ia and type II sensory fibers at a given muscle length whereas dynamic beta Health heuristics increase the stretch-sensitivity of the type Ia sensory fibers by stiffening the nuclear bag fibers.

Detailed innervation health heuristics a muscle spindle. Schematic of an adult muscle spindle (MS, light gray) health heuristics the longitudinal section (adapted from Health heuristics, 1997). Alpha MN (red) exclusively innervates (incoming arrow) extrafusal fibers (EF, brown).

Beta MNs (green-brown) innervate both EF and health heuristics fibers (IF, blue). Gamma MNs are divided into two subtypes: static (blue) connecting to nuclear chain (CH, light blue) and nuclear bag 2 (B2, dark blue) fibers and dynamic (purple) connecting to nuclear bag 1 fibers (B1, intermediate health heuristics. Sensory afferent axons Ia (light green) and II (pink) convey information (outgoing arrows) to sensory neurons located in the dorsal root ganglia.

The outer capsule (OC) is a dedicated membrane isolating the muscle spindle from the extrafusal fibers. Gamma MNs control exclusively the sensitivity of muscle spindles.

Their firing increases the tension of intrafusal muscle fibers and therefore mimics the stretch of the muscle. Therefore, gamma MNs do not directly participate to spinal reflexes (Eccles et al.

Upper and lower MNs are fundamentally different and their shared nomenclature can easily be misleading. In contrast, lower MNs, health heuristics the exception of visceral Health heuristics, connect directly to their muscle targets and constitute the last step of the neuronal circuitry. In addition, a single muscle is composed of several fiber types that are innervated by specific classes of MNs.

The generation of this complex architecture must rely on precise mechanisms ensuring the establishment of the correct connections between matching MN - target pairs. The spinal cord offers health heuristics relatively simple, yet, powerful experimental model to study neuronal development.



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