In the bottom picture, there are many plates on the bar requiring near maximal exertion of the exercise. Thus with any load of work you notice how the Type I fibers are always firing. Top of Page. Research Interests. These circuits execute the low-level commands that generate the proper forces on individual muscles and muscle groups to enable adaptive movements.
The spinal cord also contains complex circuitry for such rhythmic behaviors as walking. Because this low level of the hierarchy takes care of these basic functions, higher levels such as the motor cortex can process information related to the planning of movements, the construction of adaptive sequences of movements, and the coordination of whole-body movements, without having to encode the precise details of each muscle contraction.
Alpha motor neurons also called lower motor neurons innervate skeletal muscle and cause the muscle contractions that generate movement. Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction. When the acetylcholine binds to acetylcholine receptors on the muscle fiber, an action potential is propagated along the muscle fiber in both directions see Chapter 4 of Section I for review.
The action potential triggers the contraction of the muscle. If the ends of the muscle are fixed, keeping the muscle at the same length, then the contraction results on an increased force on the supports i sometric contraction. If the muscle shortens against no resistance, the contraction results in constant force isotonic contraction. The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, and the motor neurons that control head and facial movements are located in the motor nuclei of the brainstem.
Even though the motor system is composed of many different types of neurons scattered throughout the CNS, the motor neuron is the only way in which the motor system can communicate with the muscles. Thus, all movements ultimately depend on the activity of lower motor neurons. Motor neurons are not merely the conduits of motor commands generated from higher levels of the hierarchy. They are themselves components of complex circuits that perform sophisticated information processing. As shown in Figure 1.
Two terms are used to describe the anatomical relationship between motor neurons and muscles: the motor neuron pool and the motor unit. If a muscle is required for fine control or for delicate movements e. That is, each motor neuron will innervate a small number of muscle fibers , enabling many nuances of movement of the entire muscle. If a muscle is required only for coarse movements e. A motor neuron controls the amount of force that is exerted by muscle fibers.
There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle. The upper trace on the oscilloscope shows the action potentials generated by the alpha motor neuron. The lower trace shows the force generated by the isometrically contracting muscle. PLAY 1: Single spikes by the motor neuron produce small twitches of the muscle. PLAY 2: Multiple spikes in succession summate to produce larger contractions.
PLAY 3: Very high rates of spikes produce maximal contraction called tetanus. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle Play 1. With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle Play 2 and Play 3.
Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers ; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers ; and large motor neurons innervate fast-twitch, fatigable muscle fibers. The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods. These fibers are used for maintaining posture and making other low-force movements.
Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers. Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated.
These fibers produce large amounts of force, but they fatigue very quickly. Intensity, in this case, will be defined as a percentage of maximal speed, or force. Therefore to make something more intense would require an increase in speed, force, or both. A motor unit is a motor neuron and the muscle fibers it innervates.
It consists of both a neuron nerve cell responsible for stimulating a muscle cell and the muscle cells themselves. Each type of motor unit possesses different characteristics. Most people have heard the terms fast and slow twitch muscles. This refers to the specific characteristics of the different motor units. Muscles contain some combination of all three fiber types not just one motor unit is responsible for the whole thing. This chart will highlight some of the main differences between fast and slow twitch classifications.
The existence of task groups does not imply size independent selective recruitment of motor unit types. Wakeling has reported recruitment of different compartments of triceps muscles in goats and humans depending on the mechanics of the movement.
Selective recruitment of different compartments is akin to recruitment of different task groups; again, this observation does not imply selective recruitment if excitatory input is restricted to a subset of motoneurons. This same line of reasoning applies to the condition of fast paw shake in the cat Smith et al.
In these experiments EMG activity was observed in the fast LG muscle but not in the slow soleus muscle during the paw shake. This observation is often cited as evidence for selective recruitment of motor unit types.
However, there was no discussion as to whether the motor units within the LG muscle, which is composed of type I and II muscle fibers, were recruited according to size. A subsequent study that included EMG sampling from histochemically regionalized muscles showed high activation of all muscle regions during paw shaking, which is not consistent with preferential recruitment of muscle regions rich in type II fibers during this condition Chanaud et al.
We suggest that when discussing recruitment order, the motoneuron pool should be operationally defined as the group of motoneurons that receive excitatory synaptic input to drive the functional movement, not the pool of motoneurons defined by anatomy. The validity of the Size Principle should then be evaluated within this operationally defined motoneuron pool to determine if recruitment proceeds from small to large. What do orderly recruitment and selective recruitment really mean?
The accepted narrative is that orderly recruitment of motor units from small to large twitch force, results in a more precise control of force and movement; this precision is more important for small and mid range forces. By maintaining the same order of recruitment, the central nervous system minimizes the computational load across a wide range of desired outputs Henneman et al.
A range of quantitative theoretical studies support this qualitative description. For example, Senn et al. Selective recruitment, on the other hand, refers to the hypothesis that under certain conditions the central nervous system selects motor units to enhance the force and rate of force output irrespective of the rank order of the motor unit within a motoneuron pool.
To achieve this goal, selective recruitment may use preferential inhibition of small motor units. The most commonly proposed examples of selective recruitment include ballistic contractions, lengthening contractions and the preferential recruitment of fast motor units during cutaneous stimulation.
However, empirical evidence from a number of laboratories failed to support the hypothesis of selective recruitment in these conditions reviewed by Heckman and Enoka, Electrical stimulation of some pathways could produce inhibition of small motor units and excitation of larger motor units.
Yet none of the behavioral studies demonstrated selective recruitment of large units with inhibition of the small ones. A possible basis for this discrepancy comes from Kernell and Hultborn They proposed that the selective excitatory and inhibitory synaptic inputs change the gain of the input-output curve of the motoneuron pool. To increase the recruitment gain, the small motoneurons are biased with inhibitory currents. Alternatively, or concurrently, the large motoneurons can be biased with excitatory currents.
The opposite synaptic bias scheme can be used to decrease the gain. All motoneurons receiving excitatory input to drive the final movement, despite underlying bias inputs, should be considered part of a functionally defined pool of motoneurons. In the high gain situation, there will be a higher likelihood of random departures from strict recruitment as a result of noise—however, the general principle of rank ordered recruitment will remain.
Another factor that has contributed to a misunderstanding of the orderly recruitment is the expectation for precise rank ordered recruitment Henneman et al.
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