6.7: Age Changes in Somatic Motor Functioning
- Page ID
- 84014
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Somatic Motor Neurons
Important age changes in somatic motor neurons involve their numbers, action potentials, and transmission sites. The first two changes are similar to those we have noted in sensory neurons.
Number
There is a decrease in the number of motor neurons, and this reduces the number of cells that can be stimulated in a muscle. Therefore, the maximum strength of contraction that muscles can produce declines. In the lumbar region of the spinal cord, which controls muscles in the lower half of the body, as many as 50 percent of the somatic motor neurons are lost by age 60. Muscle cells that lose their motor neurons degenerate completely because they are no longer stimulated.
The resulting decrease in muscle strength can be minimized by increasing the strength of contraction provided by muscle cells that retain their motor neurons. This effect can be achieved on a short‑term basis by increasing the amount of stimulation by the surviving motor neurons. However, using this strategy puts extra strain on the stimulated muscle cells. It also can produce the feeling that one must work harder to perform a strenuous activity which formerly was not difficult. Over the long term much of the strength of each muscle can be retained through programs of physical training and ordinary activities that require very strong muscle contractions.
Action Potentials
The second age change in motor neurons is a slight decrease in the speed of action potentials in their axons. The amount of slowing is different in different neurons. The changes in speeds caused by aging increase the original differences in speed found among young neurons. As a result, when an aging muscle is supposed to contract, the burst of impulses sent to it by the motor neurons arrives over an increasingly long period. Therefore, the contractions of muscle cells are spread out over a longer period.
Slower action potentials in motor neurons may result from age changes in motor neuron cell membranes, myelin, or blood vessels within the nerves. Aging causes some myelin in peripheral nerves to separate from its axons. Damaged myelin is removed by macrophages, and its replacement occurs more slowly with age. Age changes in blood vessels were described in Chapter 4. These changes reduce blood flow in the nerves and therefore decrease the supply of nutrients and the elimination of wastes.
Alterations in muscle contraction resulting from slower action potentials and the spreading of muscle cell contractions include slower contraction, lower peak strength of contraction, and slower relaxation. Age-related decreases in anticipatory impulses increase these changes. All these alterations reduce the maximum amount of strength a muscle can produce when it performs very quick movements.
Neuromuscular Transmission
The third age change is a substantial decrease in the speed of transmission from motor neurons to muscle cells. This decline may be from the formation of irregularities at the ends of aging motor axons. Slower transmission results in further delay in starting a motion.
All three age changes mean that activities that require strong and/or fast actions cannot be performed as well. This can have a significant impact on individuals whose careers or recreational activities depend on such actions. For other people, modifying or changing strategies to achieve their goals can help compensate for the slow decline in strength and speed.