8.5: Nerve-Muscle Interaction
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Motor Units
Recall from Chapter 6 that skeletal muscle cells are stimulated to contract by nerve cells called somatic motor neurons. The axon from each motor neuron branches as it passes through its muscle. Some motor neurons have only a few branches, while others have several hundred.
Each branch from a motor neuron axon ends on a muscle cell, and each muscle cell receives a branch from only one motor neuron (Figure 8.6a). Thus, the muscle cells in a muscle are organized into groups, with all the cells in each group being controlled by one motor neuron. The combination of one motor neuron and all the muscle cells it controls is the functional unit of the muscle and is called a motor unit.
When an impulse travels down a motor neuron, it passes along every branch of its axon. Therefore, every muscle cell in the motor unit is stimulated to contract; it is not possible to cause only some of these cells to contract.
The strength of each contraction is determined by which motor units and how many motor units are activated at a given time. (Figure 8.6b). Since more varied combinations of numbers of muscle cells can be selected in muscles with small motor units, a person has more control over the amount of strength provided by each contraction in such muscles (e.g., finger muscles). It is more difficult to select precise levels of strength from muscles with large motor units because the muscle cells contract in larger groups (e.g., thigh muscles). Thus, a person has more control in controlling and modulating the strength of finger muscle contractions than large thigh muscle contractions. The difference in the degree of control is similar to the difference between the ability to pay an exact amount when one has many one-dollar bills and small change and the ability to pay any exact amount when one has only bills of large denominations.
Changes in Motor Units
Since motor units change in many ways as people get older, the functioning of a muscle also changes. Some of these changes and their consequences were described in Chapter 6.
One change is an exponential decrease in the number of motor neurons. The loss may reach 50 percent by age 60. This is the main reason for the decrease in the number of muscle cells because a muscle cell degenerates and dies if it does not receive stimulation from a motor neuron. As more motor neurons and their muscle cells are lost, the maximum strength of contraction the muscle can produce diminishes.
Fortunately, many surviving motor neurons produce additional axon branches that connect to the orphaned muscle cells. These adopted muscle cells survive and function. This compensatory process helps slow the decline in the strength of the muscle. Note, however, that the size of the remaining motor units increases. This means that there is a decrease in control of the strength of each contraction. This may be one reason people have a reduced ability for fine movements as age increases. Also, Type II fibers are often "adopted" by motor neurons from Type I fibers. This alteration speeds up the conversion of Type II fibers to Type I fibers.
A second age change is a slowing in the passage of impulses to muscle cells. There is a variable amount of slowing among the motor neurons controlling a muscle. As mentioned in Chapter 6, three alterations in the overall contraction of the muscle result. First, it takes longer for the muscle to reach its peak strength of contraction. Second, the peak amount of strength is lower. Third, the entire contraction takes more time. These alterations further reduce the maximum amount of strength a muscle can produce and make it more difficult to perform very quick movements.
Another change in motor neurons is a decrease in the frequency with which impulses are sent to the muscle. Normally, a motor neuron sends a volley of impulses in rapid succession so that the muscle cells contract rapidly. A rapid series of contractions—incomplete tetany—provides a fairly smooth and strong contraction that can be maintained for a long time. Since age changes in muscle cell action potentials decrease the frequency at which muscle cells can contract, reducing the frequency of neuron impulses may be compensatory. Sending impulses faster than the muscle cells can respond would be wasteful of neuron energy and neurotransmitter materials.
Other Nerve-Muscle Interactions
Several other changes in the nervous system alter the operation of muscles as people age. Recall from Chapter 6 that age changes in sensory neurons, synapses used by reflex pathways, and other areas of the central nervous system involved in controlling voluntary movements all affect adversely the ability of the muscle system to maintain homeostasis and the quality of life.