Understanding hypotonia [low muscle tone] involves knowledge of biomechanics and motor control. The field of biomechanics and motor control is a complex one, especially in a living adult or child. Skeletal muscle and bone is the scaffold which supports our vital functions in a otherwise potentially poisonous planet we call home. We are more fragile and prone to injury of our skeletal muscles, including the skeletal muscles we depend on for proper air exchange, than we realize. …
……..still revising and creating this post and editing….
Low muscle tone
- Muscle tone is the amount of tension (or resistance to movement) in muscles. Our muscle tone helps us to hold our bodies upright when we are sitting and standing. Changes in muscle tone are what enable us to move. Muscle tone also contributes to the control, speed and amount of movement we can achieve. Low muscle tone is used to describe muscles that are floppy, which is also known as hypotonia. Children with low muscle tone may need to put in more effort to get their muscles moving properly when they are doing an activity. They may also have difficulty maintaining good posture when sitting or standing. Many children with low muscle tone have delays in their gross motor development (e.g. rolling, sitting, walking).Low muscle tone may be caused by problems with the nerves or muscles. Often the low muscle tone is idiopathic, which means the cause is unknown. https://www.rch.org.au/kidsinfo/fact_sheets/Low_muscle_tone/
Mark L. Latash, Vladimir M. Zatsiorsky, in Biomechanics and Motor Control, 2016
Muscle tone is arguably one of the most commonly used and least commonly defined notions in studies of movement, posture, and movement disorders. While most researchers imply under this expression something like “state of relaxed muscle under the spontaneous excitation by the central nervous system,” methods of assessment of muscle tone commonly used in clinical practice are likely to reflect a host of factors including properties of peripheral tissues that do not receive neural excitation. The lack of a clear and unambiguous definition for muscle tone has resulted in much misunderstanding in the literature. Mark L. Latash, Vladimir M. Zatsiorsky, in Biomechanics and Motor Control, 2016
Muscle tone [normal or not] is not well understood.
Rodger Laurent, in The Musculoskeletal System (Second Edition), 2010
Control of muscle function
Each fibre has a motor endplate on which the nerve fibre terminates. The functional unit of activity is a motor unit, which combines numerous fibres that are supplied by a single anterior horn cell and its axon. The individual muscle fibres that make up the motor unit are scattered throughout the muscle but contract together under the influence of the anterior horn cell. All muscles fibres supplied by a single motor neuron are of the same histochemical type, either type 1 or type 2.
All of the muscle fibres controlled by a single motor neuron form a motor unit. Small motor units where a motor neuron may control two or three muscle fibres are found in muscles where fine control is required. The converse is found in muscles that do not need fine control, for example the gastrocnemius or gluteus maximus. The amount of tension produced in a contracting muscle depends on the frequency of stimulation and the number of muscle units involved.
The nervous system controls the force of the contracting muscle by varying the number of motor neurons activated at any one time. For each movement, there is a progressive increase in the number of motor units contracting to provide an even increase in tension. Maximum tension in a muscle occurs when all the motor units are contracting.
Muscle tone is the resting tension in a skeletal muscle. It occurs because there are always a few motor units contracting in a resting muscle. These contractions do not cause enough tension to produce movement. Muscle tone is maintained by a normal reflex arc, whereby a signal is sent from the muscle spindles to a lower motor neuron in the posterior root ganglion which then sends a signal to the appropriate muscles to adjust the extent of their contraction. Changes in tension in a muscle result in activation of the muscle spindles so that the contraction of other muscles is altered to correct the tension in that muscle. This reflex arc is also under the control of the central nervous system.
Resting muscle tone is important for maintaining normal posture, and provides support for the joints to stabilize their position and help prevent sudden changes in the position. Muscle tone is increased in upper motor neuron lesions, for example in cerebral cortical damage that occurs in cerebrovascular accident. This is thought to be due to loss of cortical control of motor neurons, which increase their activity. There is no muscle wasting. A reduction in muscle tone, hypotonia, occurs in lower motor neuron disorders. These occur in spinal and/or peripheral nerve damage. This results in muscle atrophy. Examination of muscle tone provides important clues to the cause of muscle weakness.
Myotonia is delayed relaxation of the muscle after contraction. It is an important feature of dystrophia myotonica but can be due to other causes, where it is less severe, which include hypothyroidism, prolonged cold exposure, extreme physical exercise and medication, e.g. propranolol. DISORDERS OF SKELETAL MUSCLE Rodger Laurent, in The Musculoskeletal System (Second Edition), 2010
Paula’s family has a history of hypotonia in infancy and childhood. She is more aware of her mothers’s family history than she is of her father’s family history, but hypotonia is a feature of both. Her mother and all of her siblings [5 of them] all had hypotonia in infancy and childhood to various degrees. Two of her siblings died in early childhood, one from measles and the other from a heart defect. The rest had developmental delays in their physical milestones [sitting, standing, etc…due to varying degrees of muscle weakness. ] Paula’s mother, as an infant, had muscles too weak to be able to sit and was unable to stand or walk until age 4 . She was sent away at age of 18 months to a rehabilitation centre at the seashore, where minerals from the sea were believed to be curative for muscle weakness. It is possible that proper nutrition, daily exposure to the sun and the mineral spray of the sea helped strengthen her muscles. [Paula’s mother had lived in the city as a child and may have suffered from relative malnourishment due to poverty and/or from vitamin D deficiency and Rickets from lack of exposure to the sun and/or the family may have had some genetic issue affecting muscle.]. Paula’s father also had misshapen lower limbs so he also may have suffered from malnutrition and vitamin D deficiency . He came from even more extreme poverty. Both of Paula’s parents were short and may have had stunted growth; her dad was 5 foot 3 inches and her mom was 4 foot 11 inches .
Paula and I think that Paula has [hidden, unless baseline respiratory rate is measured at rest] muscle weakness somewhere in the ventilatory system causing her to have baseline depressed respiratory rate. We think that this is why she is forced to actively use her abdominal muscles to achieve an adequate enough minute ventilation. We think that it is important to evaluate any illness syndrome that she presents with [ including nonspecific behavioural syndromes, especially presenting with acute psychomotor change] by first measuring her basic physiological vital signs [RR,HR,BP and Body Temperature]. In our experience depressed ventilation is accompanied with lower than normal body temperature and if minute ventilation is inadequate, causing cognitive and memory problems, then sympathetic activation will cause high blood pressure [to maintain cerebral perfusion when intracranial pressure rises due to endogenous carbon dioxide retention and its effects as a cerebral vasodilator] and heart rate changes [fast heartbeat with occasional arrhythmia’s and even murmurs]. These are assumptions, yes, but they are educated assumptions [I hope] which can be tested.
This is what Dr Emile Kraepelin described at the height of his long career, when he examined the 4 baseline vital signs of his bipolar depressed patients. Dr Kraepelin had excellent knowledge of physiology during the golden age of physiology in Europe, where he did his clinical work, and he understood the importance of a ventilatory defect or injury to maintenance of homeostasis and acid base balance in the brain. He also witnessed the patients anguish, which was indescribable and not part of normal sensation or mood. [1926, Manic Depressive Insanity, chapters one and three, definition of manic depressive insanity syndrome, bodily signs of same syndrome].
We think that this is why Paula does not have a normal hyperventilation reflex to counter conditions causing hypoxia/hypercapnia episodes. Paula’s ability to raise her respiratory rate is sluggish with locomotor activity, forcing her to rely on increasing her tidal volume to make up the difference. Raising tidal volume in such a patient is even more difficult when breathing is impeded by upper respiratory obstruction or by environment s containing raised C02 [poorly ventilated and overcrowded conditions].
Failure of the skeletal muscles of the body is hard to detect and evaluate and , if this failure affects the ability to move air in and out of our bodies, then some level of respiratory ventilatory failure will occur and will require correction. If the muscles of the neck, throat and torso become weaker, especially if they were weak to begin with, then reflexes allowing for safely swallowing and eating and hydrating will be affected as well as moving air in and out in the correct ratio’s.
Evaluation of the airway, breathing [respiratory rate] , and circulation is key to investigation of potential respiratory failure and reflexes allowing for safe swallowing .
Diminished consciousness [acute impairment of cognition and personal memory] along with impairment of the facial muscles [Paula had difficulty moving her facial muscles, forming words was difficult because her face was stiffer than normal and this resulted in a non smiling facial expression [smiling requires normal muscle function].
Paula was hyper alert despite having diminished consciousness because she had a normal reflex to abnormal endogenous carbon dioxide levels. The normal reaction is alarm, anguish and unrecognized dyspnea and shortness of breath. [Unrecognized by the patient and invisible to the eye, in many cases of skeletal muscle dysfunction] . Measurement of the respiratory rate unmasks this skeletal muscle dysfunction.
The integrity of the skeletal muscles are all that really stand between the forces of gravity, the management of the air we breathe and the gases [endogenous carbon dioxide ] produced by the successful metabolism of our cells.
Skeletal muscles, from our birth [and during our development and linear growth [in height as we grow to maturity] and the effects of normal hormonal changes] and throughout our lives are easily injured and weakened through injury, blood loss, accidental malnutrition and illness through to our old age.
Yet no one checks our birth history [and possible initial injuries], our childhood history of physical injury or illness, or the basic signs of normal or pathological physiology- the 4 basic vital signs. Respiratory rate abnormalities and injuries to the ventilatory system , especially may have consequences for the ability to maintain homeostasis affecting pH, body temperature and circulation. Too much endogenous C02 affects the head and the interior parts consisting of brain tissue and liquid, under pressure, by its effect as a cerebral vasodilator increasing the flow of blood in a confined space. This is good , until it is not. Perhaps the production of endogenous C02 is even responsible for our superior mind and memory functions….till it is not, in a dose related manner, causing gradual sublethal effects of poisoning, sublethal because the brain and the body are familiar with its own products of metabolism, and most likely have many ways to limit C02 buildup even when respiratory ventilatory defects make it much harder than it otherwise would be.
The dose makes the poison, even with endogenous products of cellular metabolism.
Impairment of homeostatic reflexes will make it easier for the system to be overwhelmed under certain conditions and will make it harder to recover without supportive medical care to move air in and out and recover baseline respiratory muscle strength and protective reflexes and to correct nutritional imbalances resulting from illness.
All of this can easily be thought through once it is discovered that vital signs, especially respiratory rate and then minute ventilation, are abnormal. And treatments can be discovered that restore baseline consciousness and normal mood and locomotor activity [and speed].
The careful measurement of the 4 basic baseline vital signs at rest, if abnormal, will provide the map of what is causing changes to mood, behavior and the content of consciousness [cognition and memory].