Excitability
1. Excitability Definitions:
Table of Contents
- Excitability is defined as the reaction or response of a tissue to irritation or stimulation. It is a physicochemical change. The muscle can be excited by both direct stimulation and indirect (through its nerve) stimulation.
- Stimulus is the change in environment. It is defined as an agent or influence or act, which brings about the response in an excitable tissue.
2. Types Of Stimulus:
There are four types of stimuli, which can excite a living tissue:
- Mechanical stimulus (Pinching)
- Electrical stimulus (Electric shock)
- Thermal stimulus (By applying a heated glass rod or Intensity of Stimulus ice piece)
- Chemical stimulus (By applying chemical substances like acids).
Read And Learn More: Medical Physiology Notes
An electrical stimulus is commonly used for experimental purposes because of the following reasons:
- Electrical Stimulus can be handled easily
- The intensity, i.e. strength of stimulus can be adjusted easily
- The duration of stimulus can be adjusted easily
- The stimulus can be applied to the limited (small) area on the tissues
- Damage caused to tissues is nil or at least.
3. Qualities Of Stimulus:
To excite a tissue, the stimulus must possess two characteristics:
- Intensity or strength
- Duration.
- Intensity Of Stimulus
- The intensity or strength of a stimulus is of five types:
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- Subminimal stimulus
- Minimal stimulus
- Submaximal stimulus
- Maximal stimulus
- Supramaximal stimulus.
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The stimulus whose strength (or voltage) is sufficient to excite the tissue is called threshold or liminal or minimal stimulus. Other details are given under the heading ‘Factors affecting the force of contraction’ in this chapter.
- Duration of Stimulus:
- Whatever may be the strength of the stimulus, it must be applied for a minimum duration to excite the tissue. However, the duration of a stimulus depends upon the strength of the stimulus. For a weak stimulus, the duration is longer and for a stronger stimulus, the duration is shorter.
- The relationship between the strength and duration of the stimulus is demonstrated by means of an excitability curve or strength-duration curve.
4. Excitability Curve Or Strength – Duration Curve: The excitability curve is the graph that demonstrates the exact relationship between the strength and the duration of a stimulus. So, it is also called the strength-duration curve.
- Method to Obtain the Curve: In this curve, the strength of the stimulus is plotted (in volts) vertically, and the duration (in milliseconds) horizontally.
- To start with, a stimulus with higher strength or voltage (4 or 5 volts) is applied. The minimum duration, taken by the stimulus with particular strength to excite the tissue is noted. The strength and duration are plotted in the graph.
- Then, the strength of the stimulus is decreased and the duration is determined. Like this, the voltage is decreased gradually and the duration is determined every time. All the results are plotted and the curve is obtained.
- Characteristic Features of the Curve:
- The shape of the curve is similar in almost all the excitable tissues. The following are the important points to be observed in the excitability curve:
- Rheobase
- Utilization time
- Chronaxie
- The shape of the curve is similar in almost all the excitable tissues. The following are the important points to be observed in the excitability curve:
1. Rheobase: Rheobase is the minimum strength (voltage) of stimulus which can excite the tissue. The voltage below this cannot excite the tissue, whatever may be the duration of the stimulus.
2. Utilization Time: It is the minimum time required for the basic strength of stimulus (threshold strength) to excite the tissue.
3. Chronaxie: Chronaxio is the minimum time required for a stimulus with double the rheobase strength (voltage) to excite the tissue.
Importance of Chronaxie:
- Measurement of chronaxie determines the excitability of the tissues. It is used to compare the excitability in different tissues. The longer the chronaxie, the lesser is the excitability. Chronaxie in human skeletal muscles varies from 0.08-0.32 milliseconds. In a frog’s skeletal muscle, it is about 3 milliseconds.
- Chronaxie is 10 times more in the skeletal muscles of infants than in the skeletal muscles of adults. It is longer in paralyzed muscles than the normal muscle. And, during progressive neural diseases, chronaxie is prolonged gradually.
- Chronaxie is shortened by increased temperature and prolonged in cold temperatures. It is shorter in warm-blooded (homeothermic) animals than in cold-blooded (poikilothermic) animals. Chronaxie is shorter in red muscles than in pale muscles.
Contractility
The skeletal muscle gives a response to a stimulus in the form of contraction. The contraction is defined as the internal events of the muscle, which are manifested by changes in either the length or tension of the muscle fibers.
1. Types Of Contraction:
Muscular contraction is classified into two types based on changes in the length of muscle fibers or tension of the muscle:
- Isotonic contraction
- Isometric contraction.
1. Isotonic Contraction: Isotonic contraction is the type of muscular contraction in which the tension remains the same and the length of the muscle fiber is altered (Iso = same: Tonic = tension). An example is the simple flexion of the arm, where the shortening of muscle fibers occurs but the tension does not change.
2. Isometric Contraction: Isometric contraction is the type of muscular contraction in which the length of muscle fibers remains the same and the tension is increased. An example is pulling any heavy object when muscles become stiff and strained with increased tension but the length does not change.
2. Simple Muscle Contraction Or Twitch Or Curve:
- The contractile property of the muscle is studied by using a frog’s gastrocnemiussciatic preparation. It is also called muscle-nerve preparation.
- When the stimulus with threshold strength is applied, the muscle contracts and then relaxes. These activities are recorded graphically by using suitable instruments. The contraction is recorded as an upward deflection from the baseline. And, relaxation is recorded as a downward deflection back to the baseline.
- The simple contraction is called a simple muscle twitch and the graphical recording of this is called a simple muscle curve.
Important Points in Simple Muscle Curve:
- Four points are to be observed in a simple muscle curve:
- Point of stimulus (PS) – Denotes the time when the stimulus is applied
- Point of contraction (PC) – Indicates the time when muscle begins to contract
- Point of maximum contraction (PMC) – The point up to which the muscle contracts. It also indicates the beginning of relaxation of the muscle
- Point of maximum relaxation (PMR) – Indicates the complete relaxation of the muscle.
Periods of Simple Muscle Curve:
All the four points mentioned above divide the entire simple muscle curve into three periods:
- Latent period (LP)
- Contraction period (CP)
- Relaxation period (RP).
1. Latent period: Latent period is the time interval between the point of stimulus and the point of contraction. The muscle does not show any mechanical activity during this period.
2. Contraction period: The contraction period is the interval between the point of contraction and the point of maximum contraction. Muscle contracts during this period.
3. Relaxation period: It is the interval between the point of maximum contraction and the point of maximum relaxation. The muscle relaxes during this period.
Duration of different periods in a typical simple muscle curve:
Latent period : 0.01 second
Contraction period : 0.04 second
Relaxation period : 0.05 second
Total twitch period : 0.10 second
The contraction period is always shorter than the relaxation period. It is because contraction is an active process and relaxation is a passive process.
Causes of Latent Period:
- It is the time taken for the impulse to travel along the nerve from the place of stimulation to the muscle.
- It is the time taken for the onset of initial chemical changes in the muscle.
- It is due to the delay in the conduction of impulses at the neuromuscular junction
- It is due to the resistance offered by the viscosity of The muscles which have a large number of type II fibers muscle
- It is due to the inertia of the recording instrument also.
The latent period is not constant. It varies even in physiological conditions. It decreases in high temperatures. It increases in low temperatures, during fatigue, and in the presence of more weight.
3. Contraction Time:
- The contraction time or total twitch period in the simple. muscle twitch varies from species to species. It is less in homeothermic animals than in poikilothermic animals In the same animal, it varies in different groups of muscles.
- Based on the contraction time, the skeletal muscles are classified into two types, the red muscles, and the pale muscles. Similarly, depending upon contraction time and myosin ATPase activity the muscle fibers are also divided into two types, type 1 and type 2 fibers.
- Type 1 fibers (slow fibers or slow twitch fibers) have small diameters. Type 2 fibers (fast fibers or fast twitch fibers) have large diameters. Most of the skeletal muscles in human beings contain both types of fibers.
Red Muscles: The muscles which contain a large number of type I fibers are called red muscles. These muscles are also called slow muscles or slow twitch muscles. The red muscles have a longer contraction time. The back muscles and gastrocnemius muscles are red muscles.
Pale Muscles:
- The muscles which have a large number of type 2 fibers are called pale muscles. These muscles are also called white muscles, fast muscles or fast twitch muscles. The white muscles have a shorter contraction time. Hand muscles and ocular muscles are pale muscles.
- The characteristic features of red and pale muscles are given in
4. Factors Affecting Force Of Contraction:
The force of contraction of the skeletal muscle is affected by the following factors:
- Strength of stimulus
- Number of stimuli
- Temperature
- Load.
1. Effect of Strength of Stimulus:
When the muscle is stimulated by stimuli with different strengths (voltage of current), the force of contraction also differs. The strength of stimuli is of five types:
- Subminimal or subliminal stimulus: It is less than minimal strength and does not produce any response in the muscle if applied once
- Minimal stimulus, threshold stimulus or liminal stimulus: It is the least strength of stimulus at which minimum force of contraction is produced
- Submaximal stimulus: It is more than minimal and less than maximal strength of the stimulus. It produces more force of contraction than minimal stimulus
- Maximal stimulus: It produces almost the maximum force of contraction
- Supramaximal stimulus: produces the maximum force of contraction. Beyond this, the force of contraction cannot be increased.
2. Effect of Number of Stimulus: The force of contraction of the muscle is affected by changing the number of stimuli. One stimulus produces a simple muscle twitch. However, two or more than two (multiple) stimuli produce different effects.
Effects of two successive stimuli: When two stimuli are applied successively to a muscle, three different effects are noticed depending upon the interval between the two stimuli:
- Beneficial effect
- Superposition or wave summation
- Summation effect.
1. Beneficial Effect: When two successive stimuli are applied to the muscle in such a way that the second stimulus falls after the relaxation period of the first curve, two separate curves are obtained and the force of the second contraction is greater than that of the first one. This is called a beneficial effect.
- Cause for beneficial effect: During the first contraction, the temperature increases. It decreases the viscosity of muscle. So, the force of the second contraction is more.
2. Superposition:
- While applying two successive stimuli, if the second stimulus falls during the relaxation period of the first twitch, two curves are obtained. However, the first curve is superimposed by the second curve.
- This is called superposition or superimposition or incomplete summation. Here also, the second curve is bigger than the first curve because of the beneficial effect.
3. Summation:
- If the second stimulus is applied during the contraction period, or during the second half of the latent period, the two contractions are summed up and, a single curve is obtained. This is called a summation curve or complete summation curve.
- The summation curve is different from the simple muscle curve because the amplitude of the summation curve is greater than that of the simple muscle curve. This is because the two contractions are summed up to give rise to one single curve. The base of the summation curve is also broader than that of the simple muscle curve.
- Effect of multiple stimuli: In a muscle nerve preparation, the multiple stimuli cause two types of effects depending upon the frequency of stimuli:
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- Fatigue
- Tetanus.
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Fatigue
Fatigue Definition: Fatigue is defined as the decrease in muscular activity due to repeated stimuli. When stimuli are applied repeatedly, after some time, the muscle does not show any response to the stimulus. This condition is called fatigue.
Fatigue curve:
- When the effect of repeated stimuli is recorded continuously, the amplitude of the first two or three contractions increases. It is due to the beneficial effect. Afterward, the force of contraction decreases gradually. It is shown by a gradual decrease in the amplitude of the curves.
- All the periods are gradually prolonged. Just before fatigue occurs, the muscle does not relax completely. It remains in a partially contracted state. This state is called contracture or contraction remainder.
Causes for fatigue:
- Exhaustion of acetylcholine in motor endplate
- Accumulation of metabolites like lactic acid and phosphoric acid
- Lack of nutrients like glycogen
- Lack of oxygen.
Site (seat) of fatigue:
- in the muscle nerve preparation of frogs, the neuromuscular junction is the first seat of fatigue. It is proved by direct stimulation of the fatigued muscle. The fatigued muscle gives a response if stimulated directly.
- However, the force of contraction is less and the contraction is very slow. The second seat of fatigue is the muscle. And the nerve cannot be fatigued.
In the intact body, the sites of fatigue are in the following order:
- Betz (pyramidal) cells in the cerebral cortex
- Anterior gray horn cells (motor neurons) of the spinal cord
- Neuromuscular junction
- Muscle.
Recovery of the muscle after fatigue: Fatigue is a reversible phenomenon. The fatigued muscle recovers if given rest and nutrition. For this, the muscle is washed with saline.
Causes of recovery:
- Removal of metabolites
- Formation of acetylcholine at the neuromuscular junction
- Re-establishment of the normal polarized state of the muscle
- Availability of nutrients
- Availability of oxygen.
- The recovered muscle differs from the fresh resting muscle by having an acid reaction. The fresh resting muscle is alkaline. But the muscle, recovered from fatigue is acidic. So it relaxes slowly.
- In the intact body, all the processes involved in recovery are achieved by circulation itself. In human beings, fatigue is recorded by using Mosso’s ergograph.
Tetanus
Tetanus Definition: Tetanus is defined as the apparent sustained contraction of muscle due to repeated stimuli of high frequency. When multiple stimuli are applied at a higher frequency in such a way that the successive stimuli fall during the contraction period of the previous twitch, the muscle remains in a state of tetanus. It relaxes only after the stoppage of stimulus or when the muscle is fatigued.
Tetanus and genesis of tetanus curves:
- The genesis of tetanus and tetanus in frog muscles is recorded by using an instrument called a vibrating interrupter. It is used to adjust the frequency of stimuli as 5, 10, 15, 20, 25, 30, and 35/second. While increasing the frequency, the fusion of contractions increases every time and finally complete tetanus occurs.
- Nowadays, the electronic stimulator is used. By using this instrument stimuli with different strengths and frequencies are obtained. When the frequency of stimuli is not sufficient to cause tetanus, the fusion of contractions is not complete. It is called incomplete tetanus or clonus.
Frequency of stimuli necessary to cause tetanus and clonus:
- In frog’s gastrocnemius-sciatic preparation: Here, the frequency of stimuli required to cause tetanus is 40/second and for clonus, it is 35/second.
- In the gastrocnemius muscle of a human being: Here, the frequency required to cause tetanus is 60/seconds. And for clonus, the frequency of stimuli necessary is 55/ second.
Pathological Tetanus:
- The sustained contraction of muscle due to repeated stimuli of high frequency is usually called physiological tetanus. It is distinct from pathological tetanus which refers to the spastic contraction of the different muscle groups in pathological conditions.
- This disease is caused by bacillus Clostridium tetani found in the soil, dust, and manure. The bacillus enters the body through a cut, wound or puncture caused by objects like metal pieces, metal nails, pins, wood splinters, etc.
- This disease affects the nervous system and its common features are muscle spasm and paralysis. The first appearing symptom is the spasm of the jaw muscles resulting in the locking of the jaw. Therefore, tetanus is also called lockjaw disease.
- The manifestations of tetanus are due to a toxin secreted by the bacteria. If timely treatment is not provided, the condition becomes serious and it may even lead to death.
Treppe or Staircase Phenomenon: Treppe or staircase phenomenon is the gradual increase in the force of contraction of a muscle when it is stimulated repeatedly with maximal strength at a low frequency. It is due to beneficial effects. Treppe is distinct from the summation of contractions and tetanus.
Effect of Variations in Temperature
If the temperature of the muscle is altered, the force of contraction is also affected (Fig. 30-6). If warm Ringer solution with a temperature of about 40°C is applied over the muscle-nerve preparation, the force of contraction increases, and all the periods are shortened because of the following reasons:
- The excitability of muscles increases
- The Chemical Processes involved in muscular contraction are accelerated.
- The viscosity of muscle decreases.
Cooling the muscle-nerve preparation (with a Ringer solution of 10°C) produces the reverse effects. The force of contraction decreases and all the periods are prolonged because of the following reasons:
- The excitability of muscle decreases
- Chemical processes are slowed or delayed
- The viscosity of the muscle increases
Heat rigor:
- Rigor refers to the shortening and stiffening of muscle fibers. Heat rigor is the rigor that occurs due to increased temperature. If a hot Ringer solution with a temperature above 60°C is applied, the muscle develops heat rigor.
- The cause of heat rigor is the coagulation of muscle proteins actin and myosin. Heat rigor is an irreversible phenomenon.
Other types of rigors:
- Cold rigor – Due to the exposure to severe cold. It is a reversible phenomenon
- Calcium rigor – Due to increased calcium content. It is also reversible
- Rigor mortis – Develops after death.
Rigor mortis:
- Rigor mortis refers to after death condition of the body which is characterized by the stiffness of muscles and joints (the Latin word rigor means stiff). It occurs due to the stoppage of aerobic respiration which causes changes in the muscles.
- Soon after death, the cell membrane becomes highly permeable to calcium. So a large number of calcium ions enters the muscle fibers and promotes the formation of an actomyosin complex resulting in the contraction of the muscles.
- A few hours after death, all the muscles of the body undergo severe contraction and become rigid. The joints also become stiff and locked.
- Normally for relaxation, the muscle needs to drive out the calcium which requires ATP. But during continuous muscular contraction and other cellular processes after death, the ATP molecules are completely exhausted. New ATP molecules can not be produced because of a lack of oxygen. So in the absence of ATP, the muscles remain in a contracted state until the onset of decomposition.
Medicolegal importance of rigor mortis:
- Rigor mortis is useful in determining the time of death. The onset of stiffness starts between 10 minutes and 3 hours after death depending upon the condition of the body and environmental temperature at the time of death. If the body is active or the environmental temperature is high at the time of death, the stiffness sets in quickly.
- The stiffness develops first in facial muscles and then spreads to other muscles. The maximum stiffness occurs around 12-24 hours after death. The stiffness of muscles and joints continues for 1-3 days.
- Afterward, the decomposition of the general tissues starts. Now the lysosomal intracellular hydrolytic enzymes like cathepsins and calpains are released. These enzymes hydrolyze the muscle proteins, actin, and myosin resulting in the breakdown of the actomyosin complex. It relieves the stiffness of the muscles. This process is known as the resolution of rigor.
Effect of Load
The load acting on muscle is of two types:
- After load
- Free load.
1. After load:
- After load is the load, that acts on the muscle after the beginning of muscular contraction. An example of afterload is lifting any object from the ground.
- The load acts on the muscles of the arm only after lifting the object off the ground, i.e. only after the beginning of the muscular contraction.
2. Free load: Free load is the load, which acts on the muscle freely, even before the onset of contraction of the muscle. It is otherwise called for the load. An example of the free load is filling water from a tap by holding the bucket in hand.
Frank-Starling law:
- Frank-Starling law states that the force of contraction is directly proportional to the initial length of muscle fibers within physiological limits.
- Among these two types, the free load is more beneficial (advantageous) because, in this condition, the force of contraction as well as the work done by the muscles are greater than in after loaded condition. It is because, in free-loaded conditions, the muscle fibers are stretched and the initial length of muscle fibers is increased.
Experiment to prove Frank-Starling law:
- Frank-Starling law can be proved by using the muscle- nerve preparation of frogs. First, one simple muscle curve is recorded with 10 gm weight in after the loaded condition of the muscle.
- Then, many contractions are recorded by increasing the weight every time, until the muscle fails to lift the weight or till the curve becomes almost flat near the baseline. The work done by the muscle is calculated for every weight.
- The effects of increasing the weight in after loaded condition are:
- The force of contraction decreases gradually
- Latent period prolongs
- Contraction and relaxation periods shorten.
- The effects of increasing the weight in after loaded condition are:
- Afterward, the muscle (with weight added for the last contraction) in after loaded condition, is brought to the free loaded condition and stimulated. Now, the muscle contracts, and a curve is recorded. The work done by the muscle is calculated.
- The work done in the free-loaded condition is more than in after loaded condition. This proves the Frank-Starling law, i.e. the force of contraction is directly proportional to the initial length of muscle fiber.
Work done by the muscle:
- It is calculated by the formula:
- Work done= W x h
- Where W = Weight lifted by the muscle
- h = Height up to which the weight is lifted
- ‘h’ is determined by the formula h = IxH/L
This formula is derived as follows: Δ ABC =Δ A DEC
- L = Length between the fulcrum and writing point
- I = Length between fulcrum and point where weight is added
- H = Height of the curve
- h = Height up to which the weight is lifted
- So work done by the muscle =W x IxH/L gm cm.
- Work done is expressed as ergs or gm cm.
Optimum load: It is the load at which the work done by the muscle is maximum.
5. Length-Tension Relationship:
- The tension or force developed in the muscle during resting conditions and during contraction varies with the length of the muscle.
- The tension developed in the muscle during resting condition is known as passive tension. The tension developed in the muscle during isometric contraction is called total tension.
Active Tension: The difference between passive tension and total tension at a particular length of the muscle is called active tension. The active tension is considered as the real tension that is generated in the muscle during the contractile process. It can be determined by the length-tension curve.
Length-Tension Curve:
- The length-tension curve is the curve that determines the relationship between the length of muscle fibers and the tension developed by the muscle. It is also called length – force curve.
- The curve is obtained by using the frog’s gastrocnemiussciatic preparation. The muscle is attached to a micrometer on one end and to a force transducer on the other end. The muscle is not allowed to shorten because of its attachment on both ends.
- The micrometer is used to set the length of the muscle fibers. The force transducer is connected to a polygraph. A polygraph is used to measure the tension developed by the muscle during isometric contraction.
- To begin with, the minimum length of the muscle is set by using the micrometer. The passive tension is determined by using a force transducer. Then the muscle is stimulated and total tension is determined. From these two values, the active tension is calculated. Then the length of the muscle is increased gradually.
- At every length, both passive tension and total tension are determined followed by the calculation of active tension. All the values of active tension at different lengths are plotted to obtain the length-tension curve. From the curve, the resting length is determined.
Resting Length:
Resting length is the length of the muscle at which the active tension is maximum. The active tension is proportional to the length of the muscle up to the resting length. Beyond resting length, the active tension decreases.
Tension vs Overlap of Myofilaments:
- The length-tension relationship is explained on the basis of the sliding of acting filaments over the myosin filaments during muscular contraction. The active tension is proportional to the overlap between actin and myosin filaments in the sarcomere and the number of cross-bridges formed between actin and myosin filaments.
- When the length of the muscle is less than the resting length there is an increase in the overlap between the actin and myosin filaments and the number of cross-bridges. The active tension gradually increases up to the resting length.
- During stretching of the muscle beyond resting length, there is a reduction in the overlap between the actin and myosin filaments and the number of cross-bridges. And the active tension starts declining beyond resting length.
6. Refractory Period:
The refractory period is the period at which the muscle does not show any response to a stimulus. It is because already one action potential is in progress in the muscle during this period. The muscle is unexcitable to further stimulation until it is repolarized. The refractory period is of two types.
- Absolute refractory period
- Relative refractory period
1. Absolute Refractory Period: Absolute refractory period is the period during which the muscle does not show any response at all, whatever may be the strength of the stimulus
2. Relative Refractory Period: Relative refractory period is the period, during which the muscle shows some response if the strength of stimulus is increased to maximum.
- Refractory Period in Skeletal Muscle: In skeletal muscle, the whole of the latent period is the refractory period. The absolute refractory period falls during the first half of the latent period (0.005 sec). And, relative refractory period extends during the second half of the latent period (0.005 sec). Totally, it is 0.01 sec.
- Refractory Period in Cardiac Muscle: In cardiac muscle, the absolute refractory period extends throughout the contraction period (0.27 sec). And, the relative refractory period extends during the first half of the relaxation period (about 0.26 sec). Totally it is about 0.53 sec. Thus, the refractory period in cardiac muscle is very long compared to that of skeletal muscle.
Significance of long refractory period in cardiac muscle: Because of the long refractory period, cardiac muscle by higher centers in the brain. does not show:
- Complete summation of contractions
- Fatigue
- Tetanus.
Muscle Tone
- Muscle Tone Definition: Muscle tone is defined as continuous and partial contraction of the muscles with a certain degree of vigor and tension. More details on muscle tone are given in.
- Maintenance Of Muscle Tone:
- In Skeletal Muscle: Maintenance of tone in skeletal muscle is neurogenic. It is due to the continuous discharge of impulses from gamma motor neurons in the anterior gray horn of the spinal cord. The gamma motor neurons in the spinal cord are controlled by higher centers in the brain.
- In Cardiac Muscle: In cardiac muscle, maintenance of tone is purely myogenic, i.e. the muscles themselves control the tone. The tone is not under nervous control in cardiac muscle.
- In Smooth Muscle: In smooth muscle, tone is myogenic. It depends upon calcium level and the number of cross-bridges.
Applied Physiology – Abnormalities Of Muscle Tone
The abnormalities of muscle tone are:
- Hypertonia
- Hypotonia
- Myotonia.
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