Characteristics features of Skeletal muscle .
Excitability .
Excitability is defined as the reaction or response of a tissue to irritation or stimulations. It is a physicochemical change.
Stimulus .
Stimulus is the change in environment. It is defined as an agent or influence or act, which causes the response in an excitable tissue.
Types of Stimulus .
Stimuli, which can excite the tissue are of four types :
1. Mechanical stimulus (pinching)
2. Electrical stimulus (electric shock) .
3. Thermal stimulus (applying heated glass rod or ice piece)
4. Chemical stimulus (applying chemical substances like acids).
2. Electrical stimulus (electric shock) use .
Electrical stimulus is commonly used for experimental purposes because of the following reasons:
[1]. It can be handled easily
[2]. Intensity (strength) of stimulus can be easily adjusted
[3]. Duration of stimulus can be easily adjusted
[4]. Stimulus can be applied to limited (small) area on the tissues
[5]. Damage caused to tissues is nil or least.
Qualities of Stimulus .
To excite a tissue, the stimulus must possess two characters:
1. Intensity or strength
2. Duration.
1. Intensity Intensity or strength .
Intensity or strength of a stimulus is of five types:
[1] . Subminimal stimulus
[2] . Minimal stimulus
[3] . Submaximal stimulus
[4] . Maximal stimulus
[5] . Supramaximal stimulus.
Stimulus whose strength (or voltage) is sufficient to excite the tissue is called threshold or liminal or minimal stimulus.
2. Duration .
[1]. 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.
[2]. For a weak stimulus, the duration is longer and for a stronger stimulus, the duration is shorter.
[3]. The relationship between the strength and duration of stimulus is demonstrated by means of excitability curve or strength-duration curve.
Excitability Curve or Strength-Duration Curve .
 |
| Strength–duration curve. R = Rheobase, UT = Utilization time, C = Chronaxie. |
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 gain the Curve .
[1]. In this curve, the strength of the stimulus is plotted (in volts) vertically and the duration (in milliseconds) horizontally.
[2]. To start with, a stimulus with higher strength or voltage (4 or 5 volt) is applied. The minimum duration, taken by the stimulus with particular strength to excite the tissue is noted.
[3]. The strength and duration are plotted in the graph. Then, the strength of the stimulus is decreased and the duration is determined.
[4]. 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. Following are the important points to be observed in the excitability curve:
1. Rheobase
2. Utilization time
3. Chronaxie.
1. Rheobase .
[1]. Rheobase is the minimum strength (voltage) of stimulus, which can excite the tissue.
[2]. The voltage below this cannot excite the tissue, whatever may be the duration of the stimulus.
2. Utilization Time .
Utilization time is the minimum time required for rheo-basic strength of stimulus (threshold strength) to excite the tissue.
3. Chronaxie .
Chronaxie is the minimum time required for a stimulus with double the rheo-basic strength (voltage) to excite the tissue.
Importance of chronaxie
[1]. Measurement of chronaxie determines the excitability of the tissues.
[2]. It is used to compare the excitability in different tissues.
[3]. Longer the chronaxie, lesser is the excitability.
Normal chronaxie
In human skeletal muscles : 0.08 to 0.32 milliseconds.
In frog skeletal muscle : 3 milliseconds.
Variations in chronaxie .
Chronaxie is:
[1] . Ten times more in skeletal muscles of infants than in the skeletal muscles of adults .
[2] . Shorter in red muscles than in pale muscles
[3] . Shorter in warm-blooded (homeothermic) animals than in cold-blooded (poikilothermic) animals
[4] . Shortened during increased temperature and prolonged during cold temperature
[5] . Longer in paralyzed muscles than in normal muscle
[6] . Prolonged gradually during progressive neural diseases.
Contractility .
[1]. Contractility is the response of the muscle to a stimulus.
[2]. Contraction is defined as the internal events of muscle with change in either length or tension of the muscle fibers.
Types of Contraction .
Muscular contraction is classified into two types based on change in the length of muscle fibers or tension of the muscle:
1. Isotonic contraction
2. Isometric contraction.
1. Isotonic Contraction .
[1]. 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).
[2]. Example: Simple flexion of arm, where shortening of muscle fibers occurs but the tension does not change.
2. Isometric Contraction .
[1]. Isometric contraction is the type of muscular contraction in which the length of muscle fibers remains the same and the tension is increased.
[2]. Example: Pulling any heavy object when muscles become stiff and strained with increased tension but the length does not change.
Simple Muscle Contraction or Twitch or Curve .
 |
| Isotonic simple muscle curve PS = Point of stimulus PC = Point of contraction |
[1]. The contractile property of the muscle is studied by using gastrocnemius-sciatic preparation from frog.
[2]. It is also called muscle-nerve preparation.
[3]. When the stimulus with threshold strength is applied, the muscle contracts and then relaxes. These activities are recorded graphically by using suitable instruments.
[4]. The contraction is recorded as upward deflection from the base line. And, relaxation is recorded as downward deflection back to the base line .
[5]. Simple contraction of the muscle is called simple muscle twitch and the graphical recording of this is called simple muscle curve.
Important Points in Simple Muscle Curve .
Four points are to be observed in simple muscle curve:
[1] . Point of stimulus (PS): The time when the stimulus is applied.
[2] . Point of contraction (PC): The time when muscle begins to contract.
[3] . Point of maximum contraction (PMC): The point up to which the muscle contracts. It also indicates the beginning of relaxation of the muscle.
[4] . Point of maximum relaxation (PMR): The point when muscle relaxes completely.
Periods of Simple Muscle Curve
All the four points mentioned above divide the entire simple muscle curve into three periods:
1. Latent period (LP)
2. Contraction period (CP)
3. Relaxation period (RP).
1. Latent period .
Latent period is the time interval between the point of stimulus and point of contraction. The muscle does not show any mechanical activity during this period.
2. Contraction period .
Contraction period is the interval between point of contraction and point of maximum contraction. Muscle contracts during this period.
3. Relaxation period .
[1]. Relaxation period is the interval between point of maximum contraction and point of maximum relaxation. The muscle relaxes during this period.
[2]. 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 .
Contraction period is always shorter than relaxation period. It is because, the contraction is an active process and relaxation is a passive process.
Causes of Latent Period .
[1] . Latent period is the time taken by the impulse to travel along the nerve from place of stimulation to muscle.
[2] . It is the time taken for the onset of initial chemical changes in the muscle.
[3] . It is due to the delay in the conduction of impulse at the neuromuscular junction.
[4] . It is due to the resistance offered by viscosity of the muscle.
[5] . It is also due to the inertia of the recording instrument.
Variations in Latent Period .
[1]. Latent period is not constant. It varies even in physiological conditions.
[2]. It decreases in high temperature.
[3]. It increases in low temperature, during fatigue and with increase in weight.
Contraction Time – Red Muscle & Pale Muscle .
[1]. Contraction time or total twitch period varies from species to species.
[2]. It is less in homeothermic animals than in poikilothermic animals.
[3]. In the same animal, it varies in different groups of muscles.
[4]. Based on contraction time, the skeletal muscles are classified into two types:
1. Red muscles
2. Pale muscles.
Similarly, depending upon contraction time and myosin ATPase activity
The muscle fibers are also divided into two types:
1. Type I fibers or slow fibers or slow twitch fibers, which have small diameter.
2. Type II fibers or fast fibers or fast twitch fibers, which have large diameter.
Most of the skeletal muscles in human beings contain both the types of fibers.
Red Muscles .
[1]. Muscles Which contain large quantity of myoglobin are called red muscles.
[2]. These muscles are also called slow muscles or slow twitch muscles.
[3]. Red muscles have large number of type I fibers.
[4]. The contraction time is longer in this type of muscles.
[5]. Example: Back muscles and gastrocnemius muscles.
Pale Muscles .
[1]. Muscles Which contain less quantity of myoglobin are called pale muscles or white muscles.
[2]. These muscles are also called fast muscles or fast twitch muscles.
[3]. Pale muscles have large number of type II fibers.
[4]. Contraction time is shorter in this type of muscles.
[5]. Examples: Hand muscles and ocular muscles.
Characteristic features of red and pale muscles .
 |
| Features of Pale &Red Muscle . |
Factors Affecting Force of Contraction .
Force of contraction of the skeletal muscle is affected by the following factors:
1. Strength of stimulus
2. Number of stimulus
3. Temperature
4. Load.
1. Effect of Strength of Stimulus .
When the muscle is stimulated by stimuli with different strength (voltage of current), the force of contraction also differs.
Types of strength of stimulus .
Strength of stimulus is of five types:
[1]. Subminimal or subliminal stimulus .
It is less than minimal strength and does not produce any response in the muscle if applied once.
[2]. Minimal stimulus, threshold stimulus or liminal stimulus .
It is the least strength of stimulus at which minimum force of contraction is produced.
[3]. Submaximal stimulus .
It is more than minimal and less than maximal strength of stimulus. It produces more force of contraction than minimal stimulus.
[4]. Maximal stimulus .
It produces almost the maximum force of contraction.
[5]. Supramaximal stimulus .
It produces the maximum force of contraction. Beyond this, the force of contraction cannot be increased.
2. Effect of Number of Stimulus .
[1]. Contractility of the muscle varies, depending upon the number of stimuli.
[2]. If a single stimulus is applied, muscle contracts once (simple muscle twitch).
[3]. Two or more than two (multiple) stimuli produce two 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 :
[1] . Beneficial effect
[2] . Superposition or wave summation .
[3]. 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 second contraction is greater than that of first one. This is called beneficial effect.
Cause for beneficial effect .
During first contraction, the temperature increases. It decreases the viscosity of muscle. So, the force of second contraction is more.
[2] . Superposition .
[1]. While applying two successive stimuli, if the second stimulus falls during relaxation period of first twitch, two curves are obtained. However, the first curve is superimposed by the second curve.
[2]. This is called superposition or superimposition or incomplete summation.
[3]. Here also, the second curve is bigger than the first curve because of beneficial effect.
[3] . Summation
[1]. If second stimulus is applied during contraction period, or during second half of latent period, the two contractions are summed up and a single curve is obtained. This is called summation curve or complete summation curve.
[2]. Summation curve is different from the simple muscle curve because, the amplitude of the summation curve is greater than that of simple muscle curve.
[3]. This is due to the summation of two contractions to give rise to one single curve. Base of the summation curve is also broader than that of the simple muscle curve.
Effects of multiple stimuli .
In a muscle-nerve preparation, the multiple stimuli cause two types of effects depending upon the frequency of stimuli:
[1] . Fatigue .
[2] . Tetanus.
1. Fatigue .
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 .
[1]. When the effect of repeated stimuli is recorded continuously, the amplitude of first two or three contractions increases. It is due to the beneficial effect. Afterwards, the force of contraction decreases gradually.
[2]. It is shown by gradual decrease in the amplitude of the curves.
[3]. All the periods are gradually prolonged. Just before fatigue occurs, the muscle does not relax completely.
[4]. It remains in a partially contracted state. This state is called contracture or contraction remainder .
Causes for fatigue .
a. Exhaustion of acetylcholine in motor endplate
b. Accumulation of metabolites like lactic acid and phosphoric acid
c. Lack of nutrients like glycogen
d. Lack of oxygen.
Site (seat) of fatigue .
[1]. In the muscle-nerve preparation of frog, neuromuscular junction is the first seat of fatigue. It is proved by direct stimulation of fatigued muscle.
[2]. Fatigued muscle gives response if stimulated directly. However, the force of contraction is less and the contraction is very slow.
[3]. 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:
a. Betz (pyramidal) cells in cerebral cortex
b. Anterior gray horn cells (motor neurons) of spinal cord
c. Neuromuscular junction d. Muscle.
Recovery of the muscle after fatigue .
Fatigue is a reversible phenomenon. Fatigued muscle recovers if given rest and nutrition. For this, the muscle is washed with saline.
Causes of recovery .
a. Removal of metabolites
b. Formation of acetylcholine at the neuromuscular junction
c. Re-establishment of normal polarized state of the muscle
d. Availability of nutrients
e. Availability of oxygen.
1. The recovered muscle differs from the fresh resting muscle by having acid reaction.
2. The fresh resting muscle is alkaline. But the muscle, recovered from fatigue is acidic. So it relaxes slowly.
3. In the intact body, all the processes involved in recovery are achieved by circulation itself.
4. In human beings, fatigue is recorded by using Mosso’s ergograph.
[2].Tetanus .
[1]. Tetanus is defined as the sustained contraction of muscle due to repeated stimuli with high frequency.
[2]. When the multiple stimuli are applied at a higher frequency in such a way that the successive stimuli fall during contraction period of previous twitch, the muscle remains in state of tetanus.
[3]. It relaxes only after the stoppage of stimulus or when the muscle is fatigued.
Tetanus and genesis of tetanus curves .
[1]. Genesis of tetanus and tetanus in frog’s muscle is recorded by using the instrument called vibrating interrupter .
[2]. It is used to adjust the frequency of stimuli as 5, 10, 15, 20, 25, 30 and 35/second. While increasing the frequency, fusion of contractions increases every time and finally complete tetanus occurs .
[3]. Nowadays, electronic stimulator is used. By using this instrument, the stimuli with different strength and frequency are obtained.
[4]. 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 .
[1]. In frog gastrocnemius-sciatic preparation, the frequency of stimuli required to cause tetanus is 40/second and for clonus it is 35/second.
[2]. In gastrocnemius muscle of human being, the frequency required to cause tetanus is 60/second. And for clonus, the frequency of stimuli necessary is 55/ second.
Pathological Tetanus .
[1]. Sustained contraction of muscle due to repeated stimuli of high frequency is usually called physiological tetanus.
[2]. 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.
[3]. The bacillus enters the body through a cut, wound or puncture caused by objects like metal pieces, metal nails, pins, wood splinters, etc.
[4]. This disease affects the nervous system and its common features are muscle spasm and paralysis. [5]. The first appearing symptom is the spasm of the jaw muscles resulting in locking of jaw. Therefore, tetanus is also called lockjaw disease.
[6]. 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 .
[1]. Treppe or staircase phenomenon is the gradual increase in force of contraction of muscle when it is stimulated repeatedly with a maximal strength at a low frequency.
[2]. It is due to beneficial effect. Treppe is distinct from summation of contractions and tetanus.
3. Effect of Variations in Temperature .
If the temperature of muscle is altered, the force of contraction is also affected .
Warm temperature .
At warm temperature of about 40°C, the force of muscle contraction increases and all the periods are shortened because of the following reasons:
[1]. Excitability of muscle increases
[2]. Chemical processes involved in muscular contraction are accelerated
[3]. Viscosity of muscle decreases.
Cold temperature .
At cold temperature of about 10°C, the force of contraction decreases and all the periods are prolonged because of the following reasons:
[1]. Excitability of muscle decreases
[2]. Chemical processes are slowed or delayed
[3]. Viscosity of the muscle increases.
High or hot temperature – Heat rigor .
[1]. At high temperature above 60°C, the muscle develops heat rigor.
[2]. Rigor refers to shortening and stiffening of muscle fibers.
[3]. Heat rigor is the rigor that occurs due to increased temperature. It is an irreversible phenomenon. [4]. Cause of heat rigor is the coagulation of muscle proteins, actin and myosin.
Other types of rigors
[1]. Cold rigor: Due to the exposure to severe cold. It is a reversible phenomenon.
[2]. Calcium rigor: Due to increased calcium content. It is also reversible.
[3]. Rigor mortis: Develops after death.
Rigor mortis .
Rigor mortis refers to a condition of the body after death, which is characterized by stiffness of muscles and joints (Latin word ‘rigor’ means stiff). It occurs due to stoppage of aerobic respiration, which causes changes in the muscles.
Cause of rigor mortis .
[1]. 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 actomyosin complex resulting in contraction of the muscles.
[2]. Few hours after death, all the muscles of body undergo severe contraction and become rigid.
[3]. The joints also become stiff and locked. Normally for relaxation, the muscle needs to drive out the calcium, which requires ATP.
[4]. But during continuous muscular contraction and other cellular processes after death, the ATP molecules are completely exhausted.
[5]. New ATP molecules cannot be produced because of lack of oxygen. So in the absence of ATP, the muscles remain in contracted state until the onset of decomposition.
Medicolegal importance of rigor mortis .
[1]. Rigor mortis is useful in determining the time of death.
[2]. Onset of stiffness starts between 10 minutes and 3 hours after death depending upon condition of the body and environmental temperature at the time of death.
[3]. If the body is active or the environmental temperature is high at the time of death, the stiffness sets in quickly.
[4]. The stiffness develops first in facial muscles and then spreads to other muscles. The maximum stiffness occurs around 12 to 24 hours after death.
[5]. The stiffness of muscles and joints continues for 1 to 3 days. Afterwards, the decomposition of the general tissues starts.
[6]. Now the lysosomal intracellular hydrolytic enzymes like cathepsins and calpains are released.
[7]. These enzymes hydrolyze the muscle proteins, actin and myosin resulting in breakdown of actomyosin complex. It relieves the stiffness of the muscles. This process is known as resolution of rigor.
4. Effect of Load .
Load acting on muscle is of two types:
[1]. After load
[2]. Free load.
After load .
[1]. After load is the load, that acts on the muscle after the beginning of muscular contraction. Example of after load is lifting any object from the ground.
[2]. The load acts on muscles of arm only after lifting the object off the ground, i.e. only after beginning of the muscular contraction.
Free load .
[1]. Free load is the load, which acts on the muscle freely, even before the onset of contraction of the muscle. It is otherwise called fore load.
[2]. Example of free load is filling water from a tap by holding the bucket in hand.
Free load Vs after load .
[1]. Free load is more beneficial (advantageous) since force of contraction and work done by the muscles are greater in free-loaded condition than in after-loaded condition.
[2]. It is because, in free-loaded condition, the muscle fibers are stretched and the initial length of muscle fibers is increased.
[3]. It facilitates the force of contraction. This is in accordance with Frank-Starling law.
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.
Experiment to prove Frank-Starling law .
[1]. Frank-Starling law can be proved by using the muscle-nerve preparation of frog.
[2]. First, one simple muscle curve is recorded with 10 g weight in after-loaded condition of the muscle . Then, many contractions are recorded by increasing the weight everytime, until the muscle fails to lift the weight or till the curve becomes almost flat near the base line.
[3]. The work done by the muscle is calculated for every weight . Effects of increasing the weight in after-loaded condition are:
1 . Force of contraction decreases gradually
2 . Latent period prolongs
3 . Contraction and relaxation periods shorten .
[4]. Afterwards, the muscle (with weight added for last contraction) in after-loaded condition, is brought to the free-loaded condition and stimulated.
[5]. Now, the muscle contracts and a curve is recorded. The work done by the muscle is calculated. Work done in free-loaded condition is more than in after-loaded condition.
[6]. This proves Frank-Starling law, i.e. the force of contraction is directly proportional to the initial length of muscle fiber.
Length Tension Relationship .
 |
| Length-Tension Curve . |
[1]. Tension or force developed in the muscle during resting condition and during contraction varies with the length of the muscle.
[2]. Tension developed in the muscle during resting condition is known as passive tension. Tension developed in the muscle during isometric contraction is called total tension .
Active Tension .
[1]. Difference between the passive tension and total tension at a particular length of the muscle is called active tension.
[2]. Active tension is considered as the real tension that is generated in the muscle during contractile process. It can be determined by the length-tension curve.
Length-Tension Curve .
[1]. Length-tension curve is the curve that determines the relationship between length of muscle fibers and the tension developed by the muscle. It is also called length-force curve.
[2].The curve is obtained by using frog gastrocnemius-sciatic preparation. Muscle is attached to micrometer on one end and to a force transducer on other end.
[3]. Muscle is not allowed to shorten because of its attachment on both the ends . A micrometer is used to set length of the muscle fibers.
[4]. Force transducer is connected to a polygraph. 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.
[5]. The passive tension is determined by using force transducer. Then the muscle is stimulated and total tension is determined. From these two values the active tension is calculated.
[6]. Then the length of muscle is increased gradually. At every length, both passive tension and total tension are determined followed by calculation of active tension.
[7]. 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 .
[1]. Resting length is the length of the muscle at which the active tension is maximum.
[2]. Active tension is proportional to the length of the muscle up to resting length. Beyond resting length, the active tension decreases.
Tension Vs Overlap of Myofilaments .
[1]. Length-tension relationship is explained on the basis of sliding of actin filaments over the myosin filaments during muscular contraction.
[2]. The active tension is proportional to overlap between actin and myosin filaments in the sarcomere and the number of cross bridges formed between actin and myosin filaments.
[3]. When the length of the muscle is less than the resting length, there is increase in the overlap between the actin and myosin filaments and the number of cross bridges.
[4]. The active tension gradually increases up to the resting length.
[5]. During stretching of the muscle beyond resting length, there is 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.
Refractory period .
[1]. Refractory period is the period at which the muscle does not show any response to a stimulus.
[2]. It is because already one action potential is in progress in the muscle during this period.
[3]. The muscle is unexcitable to further stimulation until it is repolarized.
[4]. Refractory period is of two types.
1. Absolute refractory period .
2. 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 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 .
[1]. In skeletal muscle, whole of the latent period is refractory period.
[2]. The absolute refractory period falls during first half of latent period (0.005 sec).
[3]. Relative refractory period extends during second half of latent period (0.005 sec).
[4]. Totally, it is 0.01 sec.
Refractory Period in Cardiac Muscle .
[1]. In cardiac muscle, absolute refractory period extends throughout contraction period (0.27 sec).
[2]. Relative refractory period extends during first half of relaxation period (about 0.26 sec).
[3]. 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 does not show:
i. Complete summation of contractions
ii. Fatigue
iii. Tetanus.
Muscle tone
Muscle tone is defined as continuous and partial contraction of the muscles with certain degree of vigor and tension.
Maintenance of Muscle Tone .
In Skeletal Muscle
[1]. Maintenance of tone in skeletal muscle is neurogenic.
[2]. It is due to continuous discharge of impulses from gamma motor neurons in anterior gray horn of spinal cord.
[3]. The gamma motor neurons in spinal cord are controlled by higher centers in brain .
In Cardiac Muscle
[1]. In cardiac muscle, maintenance of tone is purely myogenic, i.e. the muscles themselves control the tone.
[2]. 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 number of cross bridges.
Thanks for Visiting us .
