6.9: Age Changes in Reflexes
- Page ID
- 84016
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Since aging causes many detrimental changes in sensory and motor neurons as well as in myelin, it produces deleterious effects on the reflexes that use those structures. Some of these effects were mentioned in the sections on sensory, somatic, and autonomic neurons. The decrease in number and the decline in sensitivity of certain sensory neurons mean that more stimulation is required to start many reflexes. It takes more time for the response to begin because reception takes longer and action potentials are weaker and slower. Changes in action potentials, together with decreases in the number of motor neurons and the effectiveness of certain neurotransmitters, cause the response to be weaker and of longer duration.
Age changes in the structures that surround the sensory neurons, such as the skin and blood vessels, further alter reflexes by preventing sensory neurons from properly detecting stimuli. Reflex responses are also reduced by age changes in the glands and muscles producing the responses and in the skeletal system.
Reflexes also seem to be detrimentally affected by age changes in the CNS. It has been observed that the more complicated the pathway in the CNS, the more dramatic the effect of aging on reflexes. In addition to reflexes occurring more slowly and weakly, there is a decline in the amount of coordination provided by the CNS in complicated reflex responses. Reflex contraction of large muscles is a good example.
The simplest muscle reflexes in the body are those which help maintain posture. These stretch reflexes or deep tendon reflexes use few synapses and no interneurons. A stretch reflex is initiated when a muscle is stretched, as occurs when a person's posture begins to change because of slumping, an external force causes a joint to bend, or an object hits a tendon. When the impulses in the reflex pathway reach the muscle that has been stretched, it contracts to restore the body to its original posture. The knee‑jerk reflex is an example of a stretch reflex. Such simple reflexes become weaker but only slightly slower with age. The degree of weakening indifferent individuals is highly variable. The degree ranges from virtually no change in the strength of the response to essentially total loss of the response. However, many cases of very weak or absent stretch reflex responses result not from aging but from abnormal or disease conditions such as traumatic injury, atherosclerosis, arthritis, and diabetes mellitus.
In contrast to stretch reflexes, reflexes that maintain balance while one is standing in place require the proper timing of a sequence of many muscle contractions. Keeping one's balance while there is movement of either the body or the surface on which a person is standing requires an even more complicated series of muscle contractions. Though the same sensory and motor neurons involved in stretch reflexes may be used, many interneurons and synapses in various parts of the brain and spinal cord are involved in these pathways. Sensory inputs from the eyes, ears, and skin may assist in these reflexes.
Complex reflexes such as those which maintain balance show a substantially greater slowing with age than do simple muscle reflexes. Aging also causes disturbances in the coordination required for such reflexes. For example, there is a change in the sequence in which the muscle contractions occur during these reflexes and an increase in the number of antagonistic muscle contractions. In comparison to simple reflexes, some of the additional slowing and much of the decline in coordination seen in complex reflexes seem to be due to age changes in the synapses and interneurons in the CNS.
Interestingly, some age changes in the CNS seem to involve adjustments in reflex pathways that compensate for diminished sensory functioning, muscle strength, skeletal system functioning, and confidence in one's ability to maintain balance. This can be observed in the age change in gait. Part of walking involves voluntary activity, but many of the muscles used for walking are controlled by acquired reflexes. Older individuals walk with smaller steps, at a slower pace, and with the feet more widely spread. Such a gait minimizes the risk of losing one's balance. Gradually modifying voluntary actions and reflexes to walk in this manner seems to reduce the demands on the muscles, joints, and reflexes needed to maintain balance.
In summary, reflexes undergo several age changes. They require more stimulation to be activated, and it takes longer for a response to begin. The response is weaker, takes longer to occur, and shows less coordination. These changes are caused by alterations in both the PNS and the CNS. With more complicated reflexes, aging of the CNS makes a larger contribution to alterations in reflexes than do age changes in the PNS. As aging diminishes the functioning of reflexes, it reduces their ability to provide automatic, fast, and accurate responses to changes in internal and external conditions and therefore to maintain homeostasis.