Showing posts with label Muscles. Show all posts
Showing posts with label Muscles. Show all posts

Friday, May 4, 2018

Clinical anatomy of muscles

Basic Anatomy , Muscles
Clinical anatomy of muscles

Clinical anatomy of muscles



The following clinical points related to the muscles of body are of interest for medical students.

Paralysis


Loss of motor power (power of movements) in muscles is called paralysis. This causes inability of the muscles to contract. The root cause of paralysis can be of two types;

  • Damage to motor neural pathways
  • Inherent disease of muscles
In the type of paralysis caused by damage to motor neural pathways, either the upper or lower motor neuron might be the exact point of damage. In the case of upper motor neuron, spastic paralysis is caused which is accompanied by exaggerated tendon jerks. In the case of lower motor neurons, flaccid paralysis is caused in which there are no tendon jerks.

Disuse atrophy and Overuse hypertrophy


The muscles which arc not used for long times become thin and weak. This is called disuse atrophy. Conversely, adequate or excessive use of particular muscles causes their better development, or even hypertrophy (Fig. 1.2). Muscular ‘wasting’ (reduction in size) is a feature of lower motor neuron paralysis and generalized debility.



Muscle atrophy



Muscle hypertrophy 
(John Cena)

Regeneration of skeletal muscle


Skeletal muscle is capable of limited regeneration. If large regions are damaged, regeneration does not occur and the missing muscle is replaced by connective tissue.

Hyperplasia


Hyperplasia means increase in muscle mass. The most common cause is the increase in number of smooth muscle fibers. Usually occurs in uterus during pregnancy.

Myasthenia Gravis


Myasthenia gravis is an autoimmune disease of muscle of unknown origin. Antibodies are produced that bind to acetylcholine receptor and block it. The nerve impulse transmission to muscle fibers is therefore blocked. This leads to extensive and progressive muscle weakness although the muscles arc normal. Extraocular and eyelid muscles are affected first, followed by those of the neck and limbs. It affects more women that men and usually those between age of 20 and 40 years.

Polymyositis


Polymyositis is a disease of muscle characterized by inflammation of the muscle fibers. It starts when white blood cells (immune cells of inflammation) spontaneously invade the muscle. Muscles close to trunk or torso are mostly affected by polymyositis that results in severe weakness. Polymyositis associated with skin rash is referred to as “dermamyositis”.

Fibrillation


Fibrillation is the abnormal contraction of cardiac muscle. The cardiac chambers do not contract as a whole resulting in the disruption of pumping action. In atrial fibrillation, there is rapid and uncoordinated, contraction of atria, ineffective pumping and abnormal contraction of the AV node. Ventricular fibrillation is characterized by very rapid and disorganized contraction of ventricle. This leads to disruption of ventricular function.

Angina Pectoris


Angina pectoris is episode of chest pain due to temporary ischemia of cardiac muscle. It is usually relieved by rest and nitrites.

Myocardial ischemia


Persistent ischemia due to blockage of more than one artery results in necrosis (death) of the cardiac muscle. Pain is not relieved by rest and gets referred to the left arm, chest and neighboring areas.





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Actions of skeletal muscles

Basic Anatomy , Muscles
Actions of skeletal muscles

Actions of skeletal muscles


Broadly, when a muscle contracts, it shortens by one-third (30%) of its belly-length, and brings about a movement. The range of movement depends on the length of fleshy fibers, and the power or force of movement on the number of fibers.
However, the actual behavior of muscle contraction is more complex. During contraction the length of the muscle may decrease (isotonic contraction), may remain unchanged (isometric contraction) or may increase, according to the functional demands of the body. In each circumstance the tension generated at the ends may either increase, persist, or decrease, depending upon the number and state of its active motor units and the external conditions like loading.

Types of Skeletal muscles on the basis of action


Each movement at a joint is brought about by a coordinated activity of different groups of muscles. These muscle groups are classified and named according to their function.

Prime Movers


Prime movers (agonists) bring about the desired movement. When a prime mover helps opposite action by active, controlled lengthening against gravity, it is known as action of paradox. For example, putting a glass back on the table is assisted by gravity but controlled by a gradual active lengthening of biceps (paradoxical or eccentric action).


Antagonists


Antagonists (opponents) oppose the prime movers. They help the prime movers by active controlled relaxation, so that the desired movement is smooth and precise. Thus, the antagonists cooperate rather than oppose the prime movers. This is due to reciprocal innervation of the opposite groups of muscles, regulated by the spinal cord through stretch reflex.



Bicpeps and Tricepts: 
The best example of antagonist muscles

Fixators


Fixators are the groups of muscles which stabilize the proximal joints of a limb, so that the desired movement at the distal joint may occur on a fixed base. Muscles acting on shoulder joint fix it for better movement of fingers.

Synergists


When the prime movers cross more than one joint, the undesired actions at the proximal joints are prevented by certain muscles known as synergists. For example, during making a tight fist by long digital flexors the wrist is kept fixed in extension by the synergists (extensors of wrist). Thus, the synergists are special fixators and partial antagonists to the prime movers. Two or more muscles causing one movement are synergist.



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Nerve Supply of Cardiac Muscles

Basic Anatomy , Muscles
Nerve Supply of Cardiac Muscles

Nerve Supply of Cardiac Muscles


Heart is supplied by sympathetic and parasympathetic nerve fibers. Sympathetic nerves stimulate both the heart rate and blood pressure and dilate the coronary arteries. The sensory fibers convey painful impulses from heart.

Parasympathetic fibers decrease the heart rate. Their sensory fibers are involved with visceral reflexes.

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Nerve Supply of Smooth Muscles

Basic Anatomy , Muscles
Nerve Supply of Smooth Muscles

Nerve Supply of Smooth Muscles


The pattern of nerve supply differentiates smooth muscles into two different categories. It is better to explain these classes to understand the nerve supply of smooth muscles.

Single-Unit type smooth muscles


These muscles are seen in the intestine. The nerve impulses reach one muscle cell and are transmitted to neighboring cells by mechanical pull through the fused cell membrane. Thus the nerve supply is sparse. This type of smooth muscle is know as single-unit type because when an impulse arrives, the whole mass contracts simultaneously as one single unit.

Multi-Unit type smooth muscles


They are seen in the muscles of ductus deferens. Each muscle cell receives a separate nerve fiber. Because of separate innervation of muscle fibers, these muscles allow fine control over their contraction, much similar to the case of skeletal muscles. Their nerve supply is very rich.



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Nomenclature of Muscles

Basic Anatomy , Muscles
Nomenclature of Muscles

Nomenclature of Muscles


The muscles have been named in a number of ways

  1. According to their shape, e.g. trapezius, rhomboideus, scrratus anterior, latissimus dorsi, etc.
  2. According to the number of heads of origin, e.g. biceps, triceps, quadriceps, digastric, etc.
  3. According to their gross structure, e.g. scmilcndinosus, semi-‘membranosus, etc.
  4. According to their location, e.g. temporalis, supra-spinatus, intercostales.
  5. According to their attachments, e.g. stylohyoid, cricothyroid, etc.
  6. According to their action, e.g. adductor longus, flexor carpi ulnaris, abductor pollicis longus, etc.
  7. According to direction of their fibers, e.g. rectus abdominis, transversus abdominis, orbicularis oculi.
  8. A muscle with two bellies with an intervening tendon is called digastric muscle. Muscle with number of intervening tendons or intersections is the rectus abdominis.
  9. The muscles that extend over two or more joints are called diarthric or polyarthric muscles, e.g. flexor carpi radialis and flexor digitorum profundus.



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Fascicular Architecture of Muscles

Basic Anatomy , Muscles
Fascicular Architecture of Muscles

Fascicular Architecture of Muscles


The arrangement of muscle fibers varies according to the direction, force and range of habitual movement at a particular joint. The force of movement is directly proportional to the number and size of muscle fibers, and the range of movement is proportional to the length of fibers. The muscles can be classified according to the arrangement of their fasciculi into the following groups.



Morphological Types of Muscles

Muscles with Parallel Fasciculi


These are muscles in which the fasciculi are parallel to the line of pull. These muscles may be:

  1. Quadrilateral, for example thyrohyoid,
  2. Strap-like, for example sternohyoid and sartorius.
  3. Strap-like with tendinous intersections, for example rectus abdominis.
  4. Fusiform, for example biceps brachii, digastric.
The range of movement in such muscles is maximum.

Muscles with Oblique Fasciculi


When the fasciculi arc oblique to the line of pull, the muscle may be triangular, or pennate (feathcr-like) in the construction. This arrangement makes the muscle more powerful, although the range of movement is reduced. Oblique arrangements are of the following types:

  1. Triangular, e.g. temporalis, adductor longus.
  2. Unipennate, e.g. flexor pollicis longus, extensor digitorum longus, pcroncus tcrtius, palmar intcrossci.
  3. Bipennate, e.g. rectus femoris, dorsal interossei, pcroncus longus, flexor hallucis longus.
  4. Multipennate, e.g. subscapularis, deltoid (acromial fibers].
  5. Circumpennate, e.g. tibialis anterior.

Muscles with Spiral or Twisted Fasciculi


Spiral or twisted fibers are found in trapezius, pectoralis major, latissimus dorsi, supinator, etc. In certain muscles the fasciculi are crossed. These are called cruciate muscles, e.g. sternocleidomastoid, masseter and adductor magnus.



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Slow and Fast muscle fibers

Basic Anatomy , Muscles
Slow and Fast muscle fibers

Slow and Fast muscle fibers



The fibers of the skeletal muscles of the entire body are not same. There are two primary variants, which differ from one another significantly. These variants are:

Slow (Type I) muscle fibers


These type of fibers show a slow ‘tonic’ contraction characteristic of postural muscles. These are red in color because of large amounts of myoglobin. The fibers arc rich in mitochondria and oxidative enzymes, but poor in phosphorylases.  Because of a well-developed aerobic metabolism, slow fibers are highly resistant to fatigue. There features can be summarized as:

  • Large amounts of myoglobin.
  • Many mitochondria.
  • Many blood capillaries.
  • Generate ATP by the aerobic system, hence the term oxidative fibers.
  • Split ATP at a slow rate.
  • Slow contraction velocity.
  • Resistant to fatigue.
  • Found in large numbers in postural muscles.
  • Needed for aerobic activities like long distance running.

Fast (Type II) muscle fibers


Type II muscle fibers are further divided into two categories: Type IIa and Type IIb.

Type IIa fibers:

They represent a variant of type II (fast) fibers which are relatively resistant to fatigue, although less than type I fibers. They are red in color because of presence of significant amount of myoglobin in them. Oxidative phosphorylation in this type of fibers is more developed that Type I fibers. Their features can be summarized as:

  • Large amounts of myoglobin.
  • Many mitochondria.
  • Many blood capillaries.
  • High capacity for generating ATP by oxidation. Split ATP at a very rapid rate and, hence, high contraction velocity
  • Resistant to fatigue but not as much as slow oxidative fibers.
  • Needed for sports such as middle distance running and swimming.

Type IIb fibers:

They show a fast ‘phasic’ contraction, required for large-scale movements of body segments. These are paler (white) in color because of small amounts of myoglobin. The fibers arc rich in glycogen and phosphorylases, but poor in mitochondria and oxidative enzymes. Because of a glycolytic respiration, the fast fibers are quite easily fatigued. There features can be summarized as:

  • Low myoglobin content.
  • Few mitochondria.
  • Few blood capillaries.
  • Large amount of glycogen.
  • Split ATP very quickly.
  • Fatigue easily.
  • Needed for sports like sprinting.

Muscles of the body are mixture of fiber types


In man, most of the skeletal muscles show a mixture of above 3 fiber types, however, a specific muscle may show a dominance of one fiber type. This depends on the function of the muscle in body. Muscle required for fast phasic contractions have a dominance of type II fibers. On the other hand muscles required for tonic long term contractions have a dominance of type I fibers. It should however be kept in mind that a single motor unit of a skeletal muscle contains only one type of fibers.

Muscles of the body can be modified to contain specific types of fibers in relative abundance. It depends on the type of work performed by the muscles. For instance, muscle performing tonic work with long term contractions will eventually develop a lot of type I fibers.



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Nerve supply of skeletal muscles

Basic Anatomy , Muscles
Nerve supply of skeletal muscles

Nerve supply of skeletal muscles



The nerve supplying a muscle is called motor nerve. In fact it is a mixed nerve and consists of the following types of fibers.

Motor fibers


These fibers make up to 60% of a nerve to skeletal muscle. They comprise of:

  • Large myelinated alpha efferents which supply extrafusal muscle fibers.
  • Smaller myelinated gamma efferents which supply intrafusal fibers of the muscle spindles which refine and control muscle contraction.
  • The fine non-myelinated autonomic efferents which supply smooth muscle fibers of the blood vessels.

Sensory fibers


They form up to 40% of a nerve to the muscle and comprise of:

  • Myelinated fibers distributed to muscle spindles for proprioception, also to tendons.
The axon of the motor nerve branches many times after entering a target muscle. Each of these branches makes its way to different set of muscle fibers (motor unit, see below). Near the midpoint of the muscle fiber, the each branch splits again, forming a small cluster of terminal branches, which form the neuromuscular junction.

Neuromuscular Junction


Neuromuscular Junction is a synapse between the motor neuron and the muscle fiber. It is an important structure through which the impulse is transferred from the motor nerve to the muscle fiber. This phenomenon eventually results in muscle contraction. Visit the page: Neuromuscular Junction for detailed study.


Muscle Spindles


Muscle spindles are spindle-shaped sensory end organs of the skeletal muscle. Each spindle contains 6 to 13 intrafusal muscle fibers which are of two types, the larger nuclear bag fibers, and the smaller nuclear chain fibers. The spindle is innervated by both the sensory and motor nerves. The sensory endings are of two types, the primary sensory endings (annulospiral endings) around the central nuclear region of the intrafusal fibers, and the secondary sensory endings {flower spray endings) beyond the nuclear region on either side of these fibers.
The motor nerve supply of the spindle is derived from gamma motor neurons of the spinal cord. Muscles spindles act as stretch receptors. They record and help regulate the degree and rate of contraction of the extrafusal fibers by influencing the alpha neurons.

Motor Point


It is the site where the motor nerve enters the muscle. It may be one or more than one. Electrical stimulation at the motor point is more effective.

Motor unit (myone)


It is defined as a single alpha motor neuron together with the muscle fibers supplied by it. The size of motor unit depends upon the precision of muscle control. Small motor units (5-10 muscle fibers) are found in muscles of fine movements (extra¬ocular muscles). Large motor units (100-2000 muscle fibers) are found in muscles of gross movements (proximal limb muscles).

Composite/hybrid muscle


Muscle supplied by two different motor nerves with different root values is called a composite or hybrid muscle. Examples of composite muscles are adductor magnus, flexor digitorum profundus and pectoralis major.



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Neuromuscular Junction

Basic Anatomy , Muscles
Neuromuscular Junction

What is Neuromuscular Junction ?



Neuromuscular junction can be defined as a synapse the motor neuron and the muscle fiber. Actually the synapse occurs between the “synaptic end bulbs” of the motor neuron and “motor end plate” of muscle fiber. The motor end plate is in fact the highly excitable region of muscle fiber plasma membrane and it is responsible for initiating action potentials across the muscle surface. This effect ultimately results in muscle contraction.






Neuromuscular Junction



Anatomy of Neuromuscular Junction 



The neuromuscular junction is composed of three parts:

  • Synaptic end bulbs:

As the axon of the motor neuron enters the skeletal muscle, it forms many branches called axon terminals. At the end of each axon terminal, there is a bulbous swelling called “synaptic end bulb”. Each synaptic end bulb contains many synaptic vesicles. These vesicles contain the all-important neurotransmitter substances such as acetylcholine. These neurotransmitter substances are responsible for transmission of impulse from axon to muscle fiber through the synapse.

  • Motor end plate:

It is the part of the sarcolemma of muscle cell, which is in closest proximity to the synaptic end bulb. It shows certain specific features different than those of other regions of muscle cell sarcolemma, including:

Synaptic Gutter: It is the invaginated membrane, which forms space for the synaptic end bulbs to reach close to the muscle fiber sarcolemma.
Subneural Clefts: These are small folds of the muscle membrane present at the bottom of the synaptic gutter. They greatly increase the surface area at which the neurotransmitter can act.
Increased number of mitochondria: The area of the muscle fiber surrounding the motor end plate shows a considerable increase in the number of mitochondria. The obvious reason for this is the energy demand of the neuromuscular junction.

  • The Synaptic cleft:

Synaptic cleft is the space between the motor end plate (muscle fiber part) and synaptic end bulb (motor neuron part) of the neuromuscular junction. It is 20 to 30 nanometers wide. Because of this cleft, the connection between the motor neuron and the muscle fiber is not continuous and there is a break. This break is traversed by the neurotransmitters. This shows the importance of these substances in the activity of muscles (and all other nervous control mechanisms).

Difference between synapse and Neuromuscular Junction


Synapses and neuromuscular junctions are physiologically the same, however, the neuromuscular junction is a specific type of synapse that occurs between motor neuron and muscle fiber. At other places in the body, such as the Central Nervous System, the structure with similar role is known as synapse.



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Basic structure of a skeletal muscle

Basic Anatomy , Muscles
Basic structure of a skeletal muscle

Basic structure of a skeletal muscle



A typical skeletal muscle consists of two types of tissues: Contractile tissue and Supporting tissue.

Contractile tissue


The contractile tissue of each muscle is composed of numerous muscle fibers. Each muscle fiber is a multinucleated, cross-striated cylindrical cell. The length of each muscle fiber is between 1 and 300 mm. It consists of a cell membrane (sarcolemma), which encloses the cytoplasm (sarcoplasm).
There are two types of substances embedded in the sarcoplasm.


  1. Several nuclei arranged at the periphery beneath the sarcolemma.
  2. A number of evenly distributed longitudinal threads called myofibrils.
Each myofibril shows alternate light and dark bands. Dark bands are Anisotropic and thus are known as A-bands. The light bands are Isotropic and thus are known as I-bands. The bands of adjacent fibrils are aligned transversely so that the muscle fiber appears cross striated. In the middles of the A band (dark band) there is a light H band. In the middle of the H band there is a dark M line. In the middle of the I band (light band) there is a dark Z disk also known as Krause’s membrane. The segment of myofibril between two Z discs is called sarcomere.



Structure of Skeletal Muscle 
(Image source: Deglr6328/Wikipedia)

Supporting tissue


It helps in organization of the muscle. Endomysium surrounds each muscle fiber separately. Perimysium surrounds bundles (fasciculi or myonemes) of muscle fibers of various sizes. Epimysium surrounds the entire muscle. The connective tissue of the muscle becomes continuous with the tendon.




Structure of Muscle



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Parts of a typical skeletal muscle

Basic Anatomy , Muscles
Parts of a typical skeletal muscle

Parts of a typical skeletal muscle


A typical skeletal muscle consists of two ends and two parts.



Parts of Skeletal Muscle

Ends


A typical skeletal muscle consists of two ends. These are;

  1. Origin is one end of the muscle which remains fixed during its contraction.
  2. Insertion is the other end which moves during its contraction. In the limb muscles, the origin is usually proximal to insertion.
However, it should be kept in mind that the terms, origin and insertion, are at times interchangeable (e.g. climbing action of latissimus dorsi), and at other times difficult to define, as in the intercostal muscles. They are commonly used for muscles which have clear distinction between the two ends.

Parts


A typical skeletal muscle consists of two parts. These are;

  1. Fleshy part is contractile, and is called the ‘belly’.
  2. Fibrous part is non-contractile and inelastic. When cord-like or rope-like, it is called tendon; when flattened, it is called aponeurosis.
The fleshy part of the muscle, the belly, is red in color while the fibrous part is white in color. The difference in color makes it easy to differentiate the active contractile area of the muscle from the non-contractile part.

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Wednesday, May 2, 2018

Muscles and their types

Basic Anatomy , Muscles
Muscles and their types

Muscles and their types


The term “muscle” is derived from Latin word “Musculus” diminutive of “mus” meaning mouse. They were named so because their belly resembles body of the mouse and their tendons resemble mouse’s tail. Muscles are contractile tissues that bring about movements of different body parts. They can be regarded as motors of human body because they provide all the force necessary to perform different types of movements. Whether it is the locomotion of body as a whole or just motion of its parts, its all because of muscles.

Types of Muscles


Muscles are of three types; skeletal, smooth and cardiac. Characteristic features of each of them are described below.



Types Of Muscles
Link to Image Source : Earth Physiology

Skeletal Muscles


  • They are also known as striped, striated, somatic and voluntary muscles
  • They are the most abundant type and are found attached to the skeleton. For this reason they are called skeletal muscles.
  • They are innervated by somatic nervous system and are therefore under voluntary control. They obey the will of human beings.
  • They respond quickly to stimuli and are capable of rapid contractions. They get fatigued easily because of their rapidity
  • Each muscle fiber is multinucleated cylindrical cell containing groups of myofibrils. The myofibrils are in turn made up of myofilaments of three types namely actin, myosin, and tropomyosin. Thus the skeletal muscles have three structural levels namely muscle fibers, myofibrils and myofilaments.
  • Examples of skeletal muscles include all muscles of body wall.

Smooth Muscles


  • They are also known as plain, unstriped, visceral and involuntary muscles.
  • Unlike skeletal muscles, they do not exhibit cross striations under the microscope and thus they got the name “smooth”.
  • They are supplied by autonomic nervous system and therefore they are involuntary in their action. They do not obey the will of human being.
  • They respond slowly to stimuli but are capable of long time sustained contractions. They do not get fatigued easily because of their slowness of response.
  • They provide motor power for regulating internal environment related to digestion, circulation, secretion and excretion.
  • Each smooth muscle fiber is an elongated spindle shaped cell with a single nucleus placed at the center. They also possess actin and myosin filaments but the structural arrangement of these filaments is very different as compared to the skeletal muscles.
  • Examples of smooth muscles include muscles of blood vessels, and muscles of the gut etc.

Cardiac Muscles


  • They form the myocardium of human heart.
  • Cardiac muscle is intermediate in structure lying between the skeletal and smooth muscles. They are striated like skeletal muscles but at the same time they are involuntary and have uninuclear cells like smooth muscles.
  • They are meant for automatic rhythmic contractions for long period of time.
  • Each muscle fiber has a single centrally placed nucleus. The fibers branch and anastomoses with each other to form a syncitium. Neighboring cells are joined by intercalated discs which provide conductive pathways from one cell to another.

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