Physiology Of Muscular System

PHYSIOLOGY OF MUSCULAR SYSTEM

What is Anatomy and Physiology?

Anatomy and Physiology is the study of the human body.

Anatomy is concerned with the structure of a part. For example, the stomach is a J-shaped, pouch like organ. The stomach wall has thick folds, which disappear as the stomach expands to increase its capacity.

What is physiology ?

Physiology is concerned with the function of a part. For example, the stomach temporarily stores food, secretes digestive juices, and passes on partially digested food to the small intestine.

It is a branch of biology that deals with the functions and activities of living things and physical and chemical phenomena involved. Physiology is simply the science of life.

What is Pathophysiology?

Refers to the study of abnormal changes in body functions that are the causes, consequences, or concomitants of disease processes.

• Components of pathophysiology- Aspects of disease process.
• Cause(aetiology)

Pathogenesis (mechanism of its development) Morphological changes (the structural alterations induced in the cells and organs of the body) Clinical significances (functional consequences of the morphologic changes)

A: What is muscular system ?

Is an organ system consisting of skeletal, smooth and cardiac muscles which permits movement of the body, maintains posture, breathing and circulates blood throughout the body.

The muscular system is controlled by the nervous system by voluntary and involuntary mechanism.

Together with the skeletal system it forms the musculoskeletal system, which is responsible for movement of the body. They are highly integrated

Anatomy and physiology of the MSS

• Musculoskeletal system includes:
• Bones
• Joints
• Muscles
• Tendons
• Ligaments
• Bursae

Musculoskeletal system

Characteristics of muscles

• Excitability – Capability to respond to the stimulus carried from the motor neurons.
• Contractility – ability to contract or shorten its size.
• Extensibility – Ability to stretch

Elasticity – Ability to return to its original length after stretching

Functions of the muscular system

Locomotion

Vasoconstriction and vasodilatation- constriction and dilation of blood vessel Walls are the results of smooth muscle contraction.

Peristalsis – wavelike motion along the digestive tract is produced by the Smooth muscle.

Cardiac motion

Posture maintenance- contraction of skeletal muscles maintains body posture and joint stability Heat generation – about 75% of ATP energy used in muscle contraction is released as heat.

B: Types of muscles

• There are three types of muscle depending on their situation
• Skeletal muscle
• Smooth (non-striated) muscle

Cardiac muscle

Smooth muscles

Also called involuntary muscles, they are found in within the walls of organs and tubular structures such as esophagus, stomach, intestines, bronchi, uterus, ureters, bladder and blood vessels.

• Smooth muscle cells contains on one nucleus and no striations.

Cardiac muscles

Also is an involuntary muscle but it is striated in structure and appearance. Also contains one nucleus per cell. Found in the walls of heart only.

3. Skeletal muscles

• Also known as the voluntary muscles, the stripped or striated muscles.

Skeletal muscle cells are multinucleated with the nuclei located peripherally.

Used to move the skeleton, support of the body, maintain body temperature, protecting internal organs, provide joint stability etc.

Other classifications.

• Depending on Striation
• Striated muscles i.e. cardiac muscles and skeletal muscles
• Non striated muscles i.e. smooth muscles
• Depending on control
• Voluntary muscles i.e. skeletal muscles

Involuntary muscles i.e. smooth and cardiac muscles NB: Voluntary muscles are innervated by somatic nerves while involuntary muscles are innervated by Autonomic nerves.

Skeletal muscle is divided into…

Type I, ”slow twitch” or Red skeletal muscle fibers , is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color, it can carry more oxygen and sustain aerobic activities, and can contract for long periods of time (eg. Solues).

Type II, “fast twitch” have few mitochondria and myoglobin, reduced ability to carry on aerobic respiration and tend to fatigue rapidly. Designed for speed further divided into type IIa, Type IIx and Type IIb.

Muscle Fatigue : is the decrease in muscular activity due to repeated stimuli.

A fatigued muscle loses its ability to contract. Muscle fatigue is due to accumulation of lactic acid and ATP exhaustion due to repeated stimulation.

Defining of terms

Striation: only present in skeletal and cardiac muscles. Absent in smooth muscle. In that they contain sarcomere and are packed in highly-regular arrangement of bundles. Striated muscles is often used in short, intense bursts, where as smooth muscle sustains longer or even near-permanent contraction.

Nucleus: smooth and cardiac muscles are uni-nucleated(one nucleus per cell), skeletal muscle is multinucleated (several nuclei per cell ).

Transverse tubule ( T tubule ): well developed in skeletal and cardiac muscles to transport calcium. Absent in smooth muscle.

Intercalated disk: specialized intercellular junction that only occurs in cardiac muscle.

Control: skeletal muscle is always under voluntary control‚ with some exceptions ( the tongue and pili arrector muscles in the dermis). smooth and cardiac muscles are under involuntary control.

Skeletal muscle fiber

Each skeletal muscle fiber is a single muscle cell , which is the unit of contraction .Muscle fibers are cylindrical cells with many nuclei .

The cell membrane is called sarcolemma, the cytoplasm is called sarcoplasm .

The sarcoplasm contains abundant , parallel thread like myofibrils , that run in parallel fashion .

The myofibrils contain 2 kinds of protein filaments .

• Thick filaments – composed of myosin .
• Thin filaments – composed of Actin , troponin and tropomyosin .

Striations are produced by alternating light and dark filaments .

Skeletal muscle

Striation pattern of skeletal muscles

The I bands (The light bands) – extends from the edge of one stack of thick filaments to the edge of next stack of thick filaments . The I band is composed of thin actin filaments .

The A bands (The dark bands) – entire length of the thick filament composed of thick myosin filaments Myosin filaments are held together by Z lines (not attached) .

A band consist of a region Where the thick and thin filaments overlap , and a region called central region (H zone) , consisting of only thick filaments . In the center of A band is a dark band called the M line .

Sarcomere : The segment of myofibrils that extends from one Z line to the next Z line.

M line contains three proteins

• -Myomesin
• -C proteins

Creatine kinase (metabolic enzyme)

The M line accessory protein that stabilizing the thick filament by anchoring to the titin.

Titin stabilizing the thick filament by anchoring the thick filament to the Z disc and connecting to the M line.

Muscle response

• All – or – none response
• if a muscle fiber contracts at all , it will contract completely .
• motor units respond in an all – or – none manner .

Threshold stimulus is the minimal stimulus needed to elicit a muscular contraction .

Twitch : single , short contraction reflecting stimulation of some motor units in a muscle.

Latent period is the time between stimulus and responding muscle contraction .

Refractory period : During his period immediately following contraction , a muscle can not respond .

C: Types of muscle contraction.

• There are three general types of muscle contractions
• Skeletal muscle contraction
• Smooth muscle contraction

Cardiac muscle contraction.

For most muscles, contraction occurs as a result of conscious effort originating in the brain.

The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber.

However, cardiac muscles do not contract as a result of conscious effort, are said to be Autonomic.

i) Skeletal muscle contraction

Motor Response.

= Motor nerve and a myofibril from a neuromuscular junction where gap (called synapse) occurs between the two structures. at the end of motor nerve‚ neurotransmitter (i.e. acetylcholine) is stored in synaptic vesicles which will release the neurotransmitters using exocytosis upon the stimulation of a nerve impulse.

Neuromuscular Junction

Major Events(steps) of skeletal-muscle contraction

• An action potential reaches the axon of the motor neuron.
• The distal end of a motor neuron releases Acetylcholine .

Acetylcholine diffuse across the gap at the neuromuscular junction.

The sarcolemma is stimulated , and a muscle impulse travels over the surface of the muscle fiber and deep into the fiber through the transverse tubules and reaches the sarcoplasmic reticulum.

The Ca2+ ions diffuse from the sarcoplasmic reticulum(SR) into the sarcoplasm bind to troponin molecules.

• Tropomyosin molecules move and expose specific sites on actin filament.
• Actin and myosin filaments form linkages.
• Actin filaments are pulled inward by myosin cross – bridges.

muscle fiber shortens as a contraction occurs.

Physiology Of Muscular System

In skeletal muscles depolarization occurs due to opening of sodium channels and entry of sodium ions from extracellular fluids to muscle fiber.

Sodium channel open and closes faster

Types of skeletal muscle contractions

Isotonic : when a muscle contracts and its ends are pulled closer together. More energy is used by the muscle and contraction lasts longer than isometric contraction Isometric : muscle does not shorten during contraction and does not require the sliding of myofibrils but muscles are stiff.

when a muscle contracts but attachments do not move.

Isokinetic : when the force a muscle generates is less than that required to move or lift an object , the contraction is called isokinetic.

Oxygen Debt

During rest or moderate exercise, O2 is sufficient to support aerobic respiration (using many ATP molecules) .

During strenuous exercise, O2 deficiency may develop and lactic acid may accumulate as a result of anaerobic respiration .

The amount of O2 needed to convert accumulated lactic acid to glucose and restore supplies of ATP and creatine phosphate is called oxygen dept.

Role of Calcium in muscle contraction

• Promotes neurotransmitter release .
• Triggers Ca+ release from sarcoplasmic Reticulum (SR).
• Triggers sliding of myofilaments and ATPase activity .

Promotes glycogen breakdown & ATP synthesis .

Involuntary muscle movement.

Muscle Spasm – A sudden , involuntary smooth or skeletal muscle twitch , can range from mild to very painful irritation.

Multiple spasms are referred as either Seizure or Convulsions Muscle Cramp – A localized and prolonged muscle spasm that happens after strenuous activity and it is painful.

ii) Smooth Muscle Contraction

• Smooth muscles contain filaments of actin and myosin .

Lack transverse tubules and sarcoplasmic Reticulum (S.R) is not well developed .

Display rhythmicity (spontaneous repeated contractions) , responsible for peristalsis (alternate contraction and relaxation) .

Lack troponin (protein that binds to Ca2+) , instead calmodulin binds to Ca2+ .

Both neurotransmitters i.e Acetylcholine & norepinephrine depolarize the membrane of smooth muscles fibers leading to slight contraction.

• Hormones and stretching affect smooth muscle contractions .

Can contract for a long period of time .

Physiology Of Muscular System

In smooth muscles depolarization occurs due to opening of calcium channels and entry of calcium ions from extracellular fluids to muscle fiber.

Unlike the fast sodium channels, calcium channels close and open slowly responsible for prolonged action potential with plateau in smooth muscles

iii) Cardiac muscle contraction

Unique arrangement of actin and myosin filaments produces the cross- striations (an optical illusion the microscope)‚ and rapid contraction with powerful forces involved.

Muscle cells are joined by intercalated disks‚ and allow muscle groups to form branching networks – both features are necessary for cardiac muscle to function as a unit (″ sancytium″ ).

SR and T tubules are well developed‚ so a large amount of calcium can be released rapidly through the T tubules.

Contains more mitochondria in each muscle cell than skeletal and smooth muscles‚ providing more ATP energy for continuous contraction.

B. MUSCULAR DISORDERS

(MYOPATHY)

INTRO…

Myopathy is a muscular disorder in which the dysfunction of muscle fiber leads to muscular weakness. Myopathies may be acquired or genetically derived.

These diseases may or may not involve the nervous system.

1.Muscle Atrophy

Dfn: is the progressive muscle wasting.

Majority is in the population is due to disuse especially to people with sedentary jobs, bed-ridden and senior citizens can loose muscle tone and develop significant muscle atrophy. This type of Atrophy is reversible with vigorously exercises Muscle atrophy resulting from diseases can be due to Resulting from damage to the nerves that supply the muscle.

Disease of the muscle itself (this includes muscular dystrophy, myotonia congenita and myotonia dystrophy)

Causes:

• Age
• Spinal injury
• Cerebrovascular accident (stroke)
• Neuropathies (Peripheral neuropathy, diabetic neuropathy)
• Poliomyelitis
• Prolonged immobilization.
• myotonia congenita

Guillain-Barre syndrome

2. Muscular Dystrophy

Muscular dystrophy(MD) is a group of rare inherited diseases characterized by progressive degeneration of muscle fibers without the involvement of nervous system.

The muscles fail to regenerate, resulting in progressive weakness and confinement to a wheelchair.

In the late stages of muscular dystrophy, muscle fibers are often replaced by fat and connective tissue.

In some types of muscular dystrophy, heart muscles, other involuntary muscles and other organs are affected.

The most common types of muscular dystrophy appear to be due to a genetic deficiency of the muscle protein dystrophin.

There's no cure for muscular dystrophy, but medications and therapy can slow the course of the disease.

Common types of muscular dystrophy are Duchenne muscular dystrophy – is a sex­-linked recessive disorder due to absence of gene dystrophin in the X chromosome.

• Dystrophin is necessary for the stability of sarcolemma

-This disease is characterized by degeneration and necrosis of muscle fibers

Becker Muscular Dystrophy – is also a sex­linked disorder

• It occurs due to the reduction in quantity or alteration of dystrophin
• Common features of this disorder are slow progressive weakness of legs and pelvis, pseudohypertrophy of calf muscles, difficulty in walking, fatigue and mental retardation.

3. DISEASES OF MUSCLE TONE

• Hypertonia – is a muscular disease characterized by increased
• muscle tone and inability of the muscle to stretch.

CAUSES:

• + Upper motor neuron lesion (Lesion from the brain and
• spinal cord above the anterior horn cells)

* Cerebral palsy, stroke, TBI,

ii. Hypotonia – is the muscular disease characterized by decrease in muscle tone.

• Muscle becomes flaccid(lack of firmness) and the condition is
• called flaccidity.

CAUSES:

• + Lower motor neuron lesion (lesions from the anterior horn cell to
• the muscles e.g. peripheral nerve trauma and poliomyelitis
• + central nervous system dysfunction, genetic disorders or

muscular disorders.

Clinical conditions associated with hypotonia are:

• Down syndrome
• Myasthenia gravis
• Kernicterus
• Congenital cerebellar ataxia (incoordination).
• Muscular dystrophy
• Congenital hypothyroidism

Hypervitaminosis D

Infant botulism (paralysis due to botulinum toxin)

Physiology Of Muscular System

iii. Myotonia – is a congenital disease characterized by continuous contraction of muscle and slow relaxation even after the cessation of voluntary act.

• The main feature of this disease is the muscle stiffness, which is sometimes referred as cramps. Muscle relaxation is delayed
• Myotonia sets in during early to late childhood and it is not progressive.

4. MYASTHENIA GRAVIS

Dfn: Is an autoimmune disease of neuromuscular junction caused by antibodies to cholinergic receptors.

It is characterized by grave weakness of the muscle due to the inability of neuromuscular junction to transmit impulses from nerve to the muscle.

It is a serious and sometimes a fatal disease.

Causes:

MG is caused due to the development of autoantibodies (IgG autoantibodies)against the receptors of acetylcholine That is, the body develops antibodies against its own acetylcholine receptors.

These antibodies prevent binding of acetylcholine with it receptors or destroy the receptors.

So, though the acetylcholine release is normal, it cannot execute its action.

symptoms

Muscles which are more susceptible for myasthenia gravis are muscles of neck, limbs, eyeballs and the muscle responsible for eyelid movements, chewing, swallowing, speech and respiration.

• Slow & weak muscular contraction because of the defective neuromuscular activity
• Inability to maintain the prolonged contraction of skeletal muscle
• Easy fatigability when the patient attempts repeated muscular contractions
• Weakness and fatigability of arms and legs.
• Double vision and droopy eyelids due to the weakness of ocular muscles
• Difficulty in swallowing due to weakness of throat muscles

Difficulty in speech due to weakness of muscles of speech.

In severe conditions, there is paralysis of muscles. Patient dies mostly due to the paralysis of respiratory muscles.

Clinical Terms:

Convulsion : series of involuntary contractions of various voluntary muscles .

Fibrosis : Degenerative disease in which connective tissue replaces skeletal muscle tissue .

Myalgia : pain resulting from any muscular disorder .

Myasthenia gravis : an autoimmune , chronic disease characterized by muscles that are weak and easily fatigue . it results from the immunes systems attack on neuromuscular junctions .

Paresis : partial or slight paralysis of the muscle .

Muscular dystrophy : progressive muscle weakness and atrophy caused by deficient dystrophin protein .

Clinical Terms

• Myopathy : Any muscular disease .
• Paralysis : loss of ability to move a body part .
• Myotonia : prolonged muscular spasm .

Myositis : inflammation of skeletal muscle tissue .

Spasm : A sudden , involuntary smooth or skeletal muscle twitch , can range from mild to very painful irritation .

• Tics : spasm of eye–lid or facial muscles .

Cramp : a prolonged spasm that cause a muscle to become taut and painful .

CRITICAL THINKING QUESTIONS

• Why is elasticity an important quality of muscle tissue?

What are the five primary functions of skeletal muscle?

How would muscle contractions be affected if skeletal muscle fibers did not have T-tubules?

How would muscle contractions be affected if ATP was completely depleted in a muscle fiber?

How are cardiac muscle cells similar to and different from skeletal muscle cells?