Motor Unit Neural Adaptations to Strength Training


Introduction:
On a basic, general level all movements involving the skeletal muscle involve motor units. They are essential for performing a movement. Specifically, these units are front and center in the realm of strength training. It has even been said that as a result of training neural adaptations within the motor unit occur- and they are the cause of some of the enhanced physical capabilities of a trained individual. The purpose of this project is to first explore the motor unit's basic anatomy and function, followed by the main focus: what neural adaptations actually occur.

Anatomy:
The motor unit is composed of the alpha motor neuron, or somatic motor neuron, and all of the muscle fibers that it innervates, as seen in Figure 1. The alpha motor neuron's cell body originates in the ventral horn of the spinal cord, and from there it's axon travels toward the muscle, which it then synapses on. Figure 1. also serves to show that a single motor neuron innervates multiple muscle fibers within a particular muscle. For a given motor unit, the alpha motor neuron innervates on average roughly 200 muscle fibers- although this number can fluctuate greatly toward higher and lower values. Smaller motor units function to control fine movements, such as movement of eye muscles, whereas large motor units may innervate up to 1,000 muscle fibers. The function of this extensive innervation is to provide muscle strength, such as that which is needed in strength training (1).
Figure 1. Motor Unit
Figure 1. Motor Unit


A closer look:
Currently there are two known types of skeletal muscle fibers in the body. They are type I and type II. The first type is also called slow-twitch, for the reason that they take longer to reach their peak tension. The second type are fast-twitch fibers due to their quicker ability to achieve peak tension. Within these two categories of muscle fibers, only type II has multiple forms. They are: type IIa, IIc, and IIx. Type I fibers excel at providing aerobic endurance when exercising, whereas type II fibers provide poor aerobic endurance. However, type II fibers are superior at producing anaerobic movements. It is the alpha motor neuron controlling these different fiber types that determines which type the fiber will be. There is much to know regarding these fibers and their different characteristics, but for the sake of focusing on the neural adaptations of the motor unit to training, much of the detail will be left for further reading and a brief representation in Table 1 (2).
Table 1. Muscle Fiber Type Differences
Table 1. Muscle Fiber Type Differences



Input to Motor Unit
Neural input to the motor units comes from higher brain centers. Specifically the motor cortex. The primary motor pathway is the corticospinal pathway (shown in Figure 2.). It starts in the precentral gyrus of the cerebral cortex. Other areas of input include the primary somatosensory cortex. The corticospinal pathway officially starts mainly as the axons of the Betz cells in layer V of the precentral gyrus. From there the axons enter the white matter of the brain below layer VI of the primary motor cortex. They descend as the internal capsule and then as the cerebral peduncles at the level of the midbrain. Afterwards they enter the pons, and from there they travel to the medulla. Right before the spinal cord, still in the medulla, the fibers decussate. After this point they enter the spinal cord in the grey matter and travel to their appropriate area. It may be the cervicalspinal cord to innervate upper limbs, or somewhere lower in the spinal cord for a lower limb or specific muscle. Once the fibers are in the ventral horn of the spinal cord they synapse with the alpha motor neurons, or interneurons (3).
Figure 2. Corticospinal pathway (ventral)
Figure 2. Corticospinal pathway (ventral)


Output from Motor Unit
Once the nerve impulse reaches the alpha motor neuron it travels down the neuron's axon until it reaches the neuromuscular junction. This is where the neurotransmitter acetylcholine (ACh) travels across the neuromuscular junction to the muscle fiber. ACh only does so after the impulse reaches the junction, which then stimulates a process that releases ACh from the junction. ACh is then received by receptors on the other side of the junction in the plasma membrane of the muscle. Once ACh is received a series of processes occur, including the propagation of the action potential to the muscle fibers, and ending with the contraction of the muscle (1).

Motor units not only send signals to the muscle fibers to cause a contraction- they also control how much force is created by the fibers. The first way that force is controlled is via rate code. Rate code is the amount of action potential being sent to the muscle. If the motor neuron needs to increase the muscle force it can do so by increasing the rate code. This is accomplished by sending one action potential very soon after another before the muscle is able to relax after the previous impulse. This process of increasing muscle force is called summation. The second process the motor unit uses is the size principle: small motor neurons are recruited before larger ones, and larger ones are recruited afterwards. This process occurs as the strength of the input onto the the motor neurons increases (4).

Neural Adaptations
One of the adaptations that results from strength training over a period of time is strength gain. Although strength gain can certainly be attributed to increases in muscle fiber size, which allows for greater force production- motor unit recruitment, stimulation frequency, and other neural factors play an essential role as well (5). One example of support for this is a study of strength trained athletes which took place over a period of 6 months. Eleven male subjects saw a 26.8% increase in maximal isometric strength, and most of their gains during the intense part of their training were not a result of major hypertrophy in the muscle (6).


Normally motor units are recruited at different times. As a result of strength training however, changes may occur in the connections between motor neurons located in the spinal cord. The change may result in the recruitment of motor units at the same time, which facilitates muscular contraction and can lead to a greater production of muscle force. (X phys book to this point) A study done with sedentary individuals, musicians who were skilled yet not strength trained, and strength trained individuals showed that motor unit synchronization increased in the order from sedentary to musician to strength trained subjects (7).

Other studies have shown that the human body is incapable of maximally activating all of its motor units- the term used for this activation of motor units is recruitment. Techniques have been used to stimulate motor units as a person is exercising to see if there are additional units that can be activated. Right after stimulation 2-5% more force is generated by the muscle, indicating that it has not been activated to its full force-producing capabilities by the person on their own (7). A study comparing the muscle activation of 4 strength athletes to 4 nonathletes showed that after forced repetitions with knee extensions (8RM load and 4 additional reps) the strength athletes were more capable because they showed less neural fatigue then their counterpart (8).


Yet another adaptation seen in studies is an increase in the firing rate (same as rate coding mentioned before) of the motor units. A study of young and older adults who were randomly placed in a control or training group showed that after 6 training sessions focused on the dorsiflexor muscles of the foot, there was a 6.8% increase in firing rate for the young adults, and a 24.3% increase for the older adults. These same, trained subjects also increased their maximum voluntary force after training (17.4% for young adults, 19.8% for older adults) (9).


But Why?
A particular review article in Sports Medicine offers research based-suggestions for the reasons that these motor unit adaptations occur (7). According to the article the motor neurons themselves may change and cause the increased motor unit activation as a result of training. However, the complete determination of the motor neuron as actually having greater excitability as a direct result of training is not possible yet. This increased excitability that is recorded however may be the result of spinal reflexes- which are affected by training. Renshaw cells are inhibitory interneurons that synapse on the motor neurons and inhibit their ability to fire. However, through changes in spinal reflexes, Renshaw cells can be inhibited, and thus allow greater excitability of the motor neuron. Renshaw cells may also be regulated by supraspinal centres (7).

Increased neural drive in the descending cortical tracts that bring impulses to the motor neurons is another possibility (7).

The synapses that the motor neurons make may become enhanced. The histochemical and morphological characteristics of the these synapses are able to change as a result of physical activity. The axon of the motor neuron may become hypertrophic and as a result conduct impulses to the muscle fibers at a greater speed (7).

Golgi tendon organs may be involved in the increased excitability of the motor unit as well. Normally when the Golgi tendon organ is activated it sends information to the spinal cord which inhibits the agonist muscle via the Ib inhibitory interneuron and excites the antagonist muscle via the Ia inhibitory interneuron. Muscle force is kept under control through these mechanisms, which are shown below in Figure 4. Yet through training one study showed that the Ib feedback to the agonist muscle through the inhibitory interneurons may be decreased. The suggested reason is modulation of this mechanism via supraspinal pathways (7).
Figure 3. Golgi tendon reflex
Figure 3. Golgi tendon reflex


Summary
Composed of an alpha motor neuron and many or little muscle fibers that it innervates, the motor unit is an essential structure in the execution of muscular movements. It also plays a role in regulating what the movement will be like- through rate coding and the size principle. Additionally, it is an active presence in the explanation for strength adaptations and other adaptations as a result of strength training. Although explicit evidence for inherent adaptations of the motor unit itself are not conclusive, much evidence exists which leaves the idea a possibility. It has the potential ability to become more efficient at recruitment, synchronization, amount of activation, and firing rate. These adaptations also may not be the explicit work of the motor unit, but instead changes in spinal reflexes, increased neural drive of descending pathways, and less inhibition of the agonist muscle via modulation of Golgi tendon reflexes. Or it could be a change in the motor unit and its own neurological characteristics. More research is crucial to the exact identification of the neural adaptations of the motor unit and the structures that are responsible.

Glossary of Terms:
precentral gyrus- another term for the area of the primary motor cortex
decussate- cross to the other side
interneurons- neuron that connects other neurons with each other
neuromuscular junction- the small gap in space between the end of the motor neuron and the plasma membrane of the muscle fiber
isometric contraction- a muscle contraction that generates force without causing a change in the muscle's length
neural drive- frequency of action potential propagation
agonist muscle- the muscle creating the contraction that is in focus (ie. the biceps in a bicep curl)
antagonist muscle- the muscle opposing the movement of the agonist (ie. the triceps in a bicep curl)
supraspinal- above the spinal cord


Suggested readings and relevant links:
http://sportsmedicine.about.com/od/anatomyandphysiology/a/MuscleFiberType.htm
- Further detail on muscle fiber types

http://jap.physiology.org/content/101/4/1228.full
- More information on motor unit adaptations; specifically related to endurance training

http://www.unmc.edu/physiology/Mann/mann14.html
- Detailed information about how the muscle contracts

Quiz Questions:
True/False
1. The Motor Unit neuron is the beta motor neuron.
2. Smaller motor units are involved with fine muscle movements.
3. There are three categories of muscle fiber types.
4. The descending input to the motor neurons in the spinal cord decussates at the medulla.
5. This input terminates in the dorsal horn of the spinal cord.

Multiple Choice
6. Which of the following is not a neural adaptation talked about in this project?
a) enhanced recruitment
b) better synchronization
c) new alpha motor neurons
d) increase in firing rate

7. Which of the following muscle fiber types is optimal for producing aerobic movements?
a) type IIc
b) type I
c) type Ib
d) type IIx

Short Answer
1. Explain the mechanism by which the Golgi tendon organ can allow greater muscle force to be exerted
2. What are the two ways that the motor unit can influence the amount of muscle force being exerted?

Essay
Explain the possible neural mechanisms that may account for the neural adaptations seen in the studies that were mentioned.

Answers:
F
T
F
T
F
c
b

References
1. Saladin KS. Anatomy & Physiology. New York: McGraw-Hill, 2010, p. 410-411.
2. Wilmore JH, Costill DL, Kennedy WL. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics, 2008, p. 35-37.
3. The Washington University School of Medicine. 1997. Basic Motor Pathway.
http://thalamus.wustl.edu/course/ [16 December 2011].
4. Knierim J. 1997-present. Chapter 1: Motor Units and Muscle Receptors [Online]. Department of Neuroscience, The John Hopkins University. http://neuroscience.uth.tmc.edu/s3/chapter01.html [16 December 2011].
5. Wilmore JH, Costill DL, Kennedy WL. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics, 2008, p. 206-210.
6. Hakkinen K, Alen M, Komi, PV. Changes in isometric force and relaxation-time, electromyographic and muscle fibre characteristics of human skeletal muscle during strength training and detraining. Acta Physiologica Scandinavica 125: C573-C585.
7. Gabriel, D. A., Kamen, G., & Frost, G. (2006). Neural Adaptations to Resistive Exercise: Mechanisms and Recommendations for Training Practices. Sports Medicine, 36(2), 133-149.
8. Ahtiainen JP, Hakkinen K. (2009). Strength athletes are capable to produce greater muscle activation and neural fatigue during high-intensity resistance exercise than nonathletes. Journal of Strength and Conditioning Research 23(4), 1129-34.
9. Christie A, Kamen G. (2010). Short-term training adaptations in maximal motor unit firing rates and afterhyperpolarization duration. Muscle Nerve 41(5), 651-60.