Muscular System

The Muscular System

B. Pimentel, M.D.

University of Makati – College of Nursing

Functions

Movement – skeletal muscle contractions move the body as a whole or in parts.

Heat production – muscle cells are numerous and very active, this results in catabolism producing the majority of heat.

Posture – maintaining body positions such as standing and sitting erect

MAJOR PROPERTIES OF MUSCLE

1. Excitability – ability to be stimulated

2. Contractility – ability to contract or shorten and produce body movement

3. Extensibility – ability to extend or stretch muscles to their original length

4. Elasticity – ability to recoil to its original length.

CHARACTERISTICS OF MUSCLE TISSUE

TYPES SKELETAL CARDIAC SMOOTH

PRINCIPAL LOCATION

Skeletal muscle groups

Wall of heartINVOLUNTARY

Walls of many hollow organs

PRINCIPAL FUNCTION

Movement of bone, heat production, posture

Pumping of blood

Movement in walls of hollow organs

TYPE OF CONTROL

VOLUNTARY INVOLUNTARY INVOLUNTARY

Epimysium (facia) – connective tissue sheath, which surrounds each skeletal muscle

Perimysium – loose connective tissue, which surrounds muscle fiber bundles

Endomysium – surrounds each muscle fiber

Sarcoplasm – myofibril cytoplasm.

Myofibril – protein fibers that extend from one end of the muscle fiber to another. Actin and Myosin myofilaments.

STRUCTURE

STRUCTURE

STRUCTURE

STRUCTURE

Sarcolemma – plasma membrane of the myofibril

Sarcoplasmic Reticulum – a network of tubules and sacs that is similar but not identical to E.R.

T-tubules – extend transversely across the sarcoplasm at right angles to the long axis of the cell. Separate from the sarcoplasmic reticulum.

Triad – a triplet of tubules with a T-tubule sandwiched between two sarcoplasmic reticulums. Allows electrical impulse to travel along a T-tubule to stimulate membranes of adjacent sacs of the sarcoplasmic reticulum.

STRUCTURE

Membrane PotentialsResting Membrane Potential

The intracellular surface of the sarcoplasm is negatively charged compared with that of the extracellular surface of the sarcoplasm. Intracellular [K+] is higher vs. extracellular [K+] The cell membrane is more permeable to [K+]

Positively charged potassium ions readily diffuse across the membrane from intra to extracellular spaces, resulting in a –70 to –90 millivolts.

Membrane PotentialsResting Membrane Potential

Equilibrium is achieved when the tendency of K+ to diffuse is opposed The increasing intracellular negative charge Shift of K+ concentration gradient

Action PotentialDepolarization

Occurs when the intracellular membrane becomes less negative than the extracellular membrane.

Gated sodium channels open when the cell is stimulated. This allows positive sodium ions to diffuse into the cell. Making the intracellular space less negative.

Once the intracellular membrane becomes positive the gated sodium ion channels close.




Action PotentialRepolarization

Begins as soon as the sodium ion channels close.

Thus the movement of sodium into the cell ceases and gated potassium channels open begins moving potassium out of the cell.

The potassium gated channels close when the resting membrane potential is reached.

NEUROMUSCULAR JUNCTION

Motor neurons – nerve cells along which action potentials travel to stimulate muscle fibers

Neuromuscular junction (synapse) – motor neuron axons and its branches innervating the muscle fibers

Presynaptic terminal – axon terminal that contains synaptic vesicles containing acetylcholine.

Synaptic cleft – space between pre and post synapse.

Postsynaptic terminal – sarcoplasm membrane opposite the presynaptic terminal or motor end plate.



Function of Neuromuscular Junction

1. Action potential arrives at presynaptic terminal, increasing the permeability of calcium.

2. Calcium enters the presynaptic terminal and initiates the release of acetylcholine (Ach).

3. Diffusion of Ach across the synaptic cleft and binding to receptor sites on the motor end plate, causing an increase of sodium permeability of the sarcoplasm.

4. Resulting in depolarization; once the threshold has been reached an action potential begins.

5. Ach is broken down by acetylcholinesterase

MUSCLE CONTRACTION

Occurs as actin and myosin myofilaments slide past one another

Sarcomeres shorten Extends from one Z line to another.

Z line – filamentous network of protein forming a disk like structure for the attachment of actin myofilaments anchoring them in place.

I band – from one side of the Z disk to the other. Consists of only actin fibers.

A band – extends the length of the myosin filaments within a sarcomere the actin and myosin filaments overlap at both ends of the A band.

H zone – center of each A band, smaller. Only myosin filaments present.

M line – is in the middle of the H zone, consists of delicate fibers that attach to the center of myosin filaments, anchoring them

The Sarcomere

MUSCLE CONTRACTION (The Sliding Filament Model)

Muscle contraction that results in the shortening of the sarcomere without changing the length of the actin and myosin filaments.

Muscle contracts when myosin crossbridges attach to actin and the molecule bends

MUSCLE CONTRACTION (The Sliding Filament Model)

What happens when muscle contracts?

The Z lines move closer together

The I band becomes shorter

The A band stays at the same length

Action Potential, Neuromuscular Junction and the Sarcomere Excitation and Contraction

Nerve impulse → the motor end plate → release of the neurotransmitter (acetylcholine) → binds to receptors on the motor end plate of the muscle → initiates an impulse that travels along the sarcolemma, through the T tubule, to sacs of the sarcoplasmic reticulum → calcium is released into the sarcoplasm → it binds to troponin molecules on the actin filaments → tropomyosin moves to expose myosin attachment sites on the actin myofilaments → a cross bridge is formed between when the head of the myosin bind with the actin myofilaments



During contraction ATP is broken down to ADP to produce energy to pull the thin filaments toward the center of each sarcomere

This cycle repeats several times per second, as long as ATP is present


Muscle Relaxation

After the impulse is over, the sarcoplasmic reticulum begins actively pumping calcium back into sacs.

As calcium is stripped from troponin molecules in the actin (thin) filaments, tropomyosin returns to its position, blocking the active sites of the actin filaments.

Myosin cross bridges are prevented from binding to actin and thus can no longer sustain contraction.

Since the thick and thin myofilaments are no longer connected, the muscle fiber returns to its resting length

Muscle Twitch, Summation, Tetanus and Recruitment

Muscle TwitchContraction of a muscle in response to a stimulus that causes an action potential in one or more muscle fibers Lag or Latent Phase – the time period between the stimulus of the

motor neuron and the beginning of contraction. Stimulus Action potential along the axon of the motor neuron Release of acetylcholine from the presynaptic terminal Opening of Na channels Release of Ca from the sarcoplasmic reticulum Ca binds to troponin Tropomyosin exposes myosin binding sites Cross bridge formation


Contraction Phase – contraction of the muscle Cross bridge movement and cycling Increase the tension produced by the muscle fibers

Relaxation Phase – relaxation of the muscle Ca diffuses away from the troponin molecules Ca is actively transported back into the sarcoplasmic reticulum Tropomyosin blocks the myosin binding sites Inhibition of cross bridge formation Tension decreases


Summation

Increasing the force of contraction of the muscle fibers within the muscle

Rapid stimulation

As the frequency of action potentials increases the frequency of contraction increases.


Incomplete Tetanus – muscle fibers partially relax between contractions.

Tetanus – action potentials are produced so quickly that there is no relaxation.

Build up of Ca in the myofibrils

Ca is released at a higher rate from the sarcoplasmic reticulum that its uptake


Recruitment

Increases the number of muscle fibers contacting

As the number of motor units stimulated increases, the more muscle fibers are stimulated to contract

ENERGY REQUIREMENTS

ATP is the immediate energy source of muscle. It must be synthesized continuously to sustain muscle contraction.

Creatine Phosphate – resting conditions, energy from aerobic respiration is used to synthesize creatine phosphate.

This accumulates in the cell and functions to store energy to synthesize ATP.

Creatine phosphate stores are quickly depleted during intense muscular contractions. Sustains maximum contraction for 8 to 10 seconds.

ENERGY REQUIREMENTSAerobic vs. Anaerobic

Anaerobic Respiration – absence of oxygen breakdown of glucose to produce 2 ATP molecules and lactic acid

Occurs in the cytoplasm of cells

Less efficient than aerobic respiration but quicker synthesis of ATP.

Used for short periods of intense exercise, such as sprinting provides up to 3 minutes of energy.

Aerobic Respiration – requires oxygen and breaks down glucose to produce ATP, Carbon dioxide, and water. Occurs in the mitochondria Net gain of 38 ATP molecules

per glucose molecule. Can utilize fatty acids and

amino acids to generate ATP

OXYGEN DEBT/EXCESS

After intense exercise, the rate of aerobic metabolism remains elevated for a time. The oxygen taken in the body is above that needed for resting metabolism

This reestablishes ATP and creatine phosphate levels in muscle fibers.

FATIGUE

Psychological fatigue Most common type of fatigue

Muscle Fatigue ATP is utilized faster than it is produced Lactic acid build up Force of contractions become weaker

TYPES OF MUSCLE CONTRACTIONS

1. Isometric contraction – length of the muscle does not change, but the tension increases during contraction process. Postural muscles.

2. Isotonic contraction – the amount of tension produced by the muscle remains constant but the length of the muscle changes. Voluntary movements.

3. Concentric contractions – isotonic contractions in which muscle tension increases as the muscle shortens

4. Eccentric contractions – isotonic contractions in which tension is maintained as the muscle lengthens

TYPES OF MUSCLE FIBERSFast and Slow Twitch Fibers

Fast Twitch – low oxidative muscle fibers, respond rapidly to stimuli and contain myosin molecules that break down ATP more rapidly than slow twitch. Less developed blood supply Few myoglobin Fewer and smaller mitochondria Large stores of glycogen and are well adapted to anaerobic

respiration Fatigue occurs quickly

TYPES OF MUSCLE FIBERSFast and Slow Twitch Fibers

Slow Twitch – high oxidative muscle fibers. Contract slower as compared with fast twitch. Better developed blood supply More mitochondria More fatigue resistant Aerobic respiration Contain large amounts of myoglobin

SKELETAL MUSCLE ANATOMYTerminologies

Tendon – connective tissue that attaches muscle to bone.

Aponeurosis – a very broad, sheetlike tendon.

Origin – the end of the muscle attached to a fixed or usually proximal segment.

Insertion – end of the muscle where the attachment to the bone moves.

Agonist – a muscle causing an action during contraction.

Antagonist – a muscle working in opposite direction to an agonist. (i.e. triceps muscle working against the biceps).

Synergists – muscles that work together to cause a movement.

Prime Mover – one muscle of a synergist group that plays the major role.

Fixator – muscles that stabilize the joints that the muscle

SKELETAL MUSCLE ANATOMYNomenclature

Muscles can be named according to their location, size, shape, orientation, origin and insertion, number of heads, and or function.

Location – pectoralis (chest), gluteus (buttock), and brachial (arm) are a few examples of name by location.

Size – maximus (large), minimus (small), longus (long) brevis (short).

Shape – deltoid (triangle), quadratus (quadrangle)

Orientation – fascicular orientation, rectus (straight), and oblique.

SKELETAL MUSCLE ANATOMYNomenclature

Origin and Insertion – sternocleidomastoid muscle is named for its origin and insertion.

Number of Heads – biceps (2 heads), triceps (3 heads).

Function – abductor muscle moves bone away from midline, adductor moves a bone towards the midline, flexor, extensor…

SKELETAL MUSCLE ANATOMYMuscles of the Head

Facial Expressions

Occipitofrontalis – raises the eyebrows

Orbicularis oculi – closes the eyelids; “crows feet”

Orbicularis oris – pucker the mouth

Buccinator – whistling

Zygomaticus – smiling

Levator labii superioris – sneer

Depressor anguli oris – frowns and pouting

SKELETAL MUSCLE ANATOMYMuscles of the Neck

Neck Flexion Muscles

Sternocleidomastoid

Origin - manubrium and medial clavicle

Insertion - mastoid process

Action - flexion of head and neck, rotation, and lateral flexion

Palpation - anterolateral side of neck

Rectus capitis anterior, Rectus capitis lateralis, Longus capitis, Longus colli, 8 pair of hyoid muscles


Neck Extension Muscles

Trapezius – extends the head and neck

Splenius capitis (O- vertebral processes, I - occipital bone), Semispinalis capitis, Splenius cervicis, Rectus capitis posterior major and minor, Obliquus capitis superior and inferior

Neck Lateral Flexion Muscles

Sternocleidomastoid, levator scapulae, scalenus anterior, cervical flexors, cervical extensors


SKELETAL MUSCLE ANATOMY

SKELETAL MUSCLE ANATOMY Muscles Moving the Vertebral Column

Trunk Extension Muscles

Erector spinae

1. Iliocostalis

2. Longissimus

3. Spinalis

Deep back muscles – extension, lateral flexion and rotation of the vertebral column

SKELETAL MUSCLE ANATOMY Muscles Moving the Vertebral Column

Trunk Flexion Muscles1. Rectus Abdominis (parallel to midline)

Origin - crest of pubis Insertion - cartilage 5,6,7 ribs and xiphoiud process

2. Internal Obliques (high medial to low lateral) Origin - external surface of lower 8 ribs

Insertion - linea alba 3. External Obliques (high lateral to low medial)

Origin - linea alba and lower 4 ribs

Insertion - iliac crest, lumbordorsal fascia

SKELETAL MUSCLE ANATOMY Thoracic Muscles

Thoracic Muscles

External intercostals – elevate the ribs

Internal intercostals – depresses the ribs; contract during forced expiration

Diaphragm – major muscle in normal respiration

SKELETAL MUSCLE ANATOMY Thoracic Muscles

SKELETAL MUSCLE ANATOMY Abdominal Wall Muscles

Linea alba – located midline the abdomen; it is composed of connective tissue

Rectus abdominis – laterally on each side of the linea alba

Tendinous intersection – traverses the width of the rectus abdominis

Group of abdominal muscle responsible for flexion, rotation or compression of abdominal contents

1. External oblique

2. Internal oblique

3. Transversus abdominis

SKELETAL MUSCLE ANATOMY Abdominal Wall Muscles

SKELETAL MUSCLE ANATOMY Upper Limb Muscles

Arm Movements

Pectoralis major – adducts the arm and flexes the shoulder

Origin - clavicle, ribs, sternum

Insertion - lateral humerus

Latissimus dorsi – medially rotates and adducts the arm; extends the shoulders

Origin - illium, sacrum, T6-L5

Insertion - anterior humerus

Deltoid – major abductor of the upper limb

Origin - clavicle, scapula, lateral acromian

Insertion - deltoid tuberosity

SKELETAL MUSCLE ANATOMYUpper Limb Muscles


Rotator cuff muscles

1. Infraspinatus

2. Subscapularis

3. Supraspinatus

4. Teres minor


Rotator cuff muscles

Muscle Origin Insertion Action Palpate

Supraspinatus

supraspinous fossa

greater tubercle abduction deep to deltoid

Infraspinatus infraspinous fossa

greater tuberclehorizontal

abduction deep to deltoid

Teres Minor posteriorly lateral scapula

greater tubercle horizontal abduction, external rotation

deep to deltoid

Subscapularis subscapular fossalesser tubercle internal rotation, adduction

deep to deltoid



Forearm Movements

Triceps brachii – primary extensor of the elbow

Biceps brachii and Brachialis – primary flexors of the elbow


Wrist and Finger Movements

Retinaculum – fibrous connective tissue that covers the flexors and extensors

Flexor carpi – flexes the wrist

Extensor carpi – extends the wrist

Flexor digitorum – flexes the fingers

Extensor digitorum – extends the fingers

SKELETAL MUSCLE ANATOMY

Wrist and Finger Movements

SKELETAL MUSCLE ANATOMY Lower Limb Muscles

Thigh Movements

Iliopsoas – flexes the hip

Tensor Facia Latae – a tense, thick band of facia on the lateral side of the thigh

Gluteus maximus – extends the hip, abducts and laterally rotates the thigh

Gluteus medius – abducts and medially rotates the thigh

Leg Movements

Quadriceps femoris – primary extensors of the knee

Sartorius – flexes the hip and knee; rotates the thigh laterally

Hamstrings – knee flexors

SKELETAL MUSCLE ANATOMYLower Limb Muscles



Ankle Movement

Gastrocnemius and Soleus – joins to form the Achilles tendon or Calcaneal; plantar flexion of the foot


ResourcesTextbook

Essential of Anatomy & Physiology Seeley, Stephens and Tate

Regional Atlas of Human Anatomy Clemente

Essentials of Human Anatomy Burkel

Textbook of Medical Physiology Guyton

Basic Histology Junqueira, Carneiro and Kelley

On-line ResourcesWayne State University – Academic Resources LibraryUniversity of Minnesota – Hematology CenterMedline PLUSMcGraw-Hill (On-line Resource) Getbodysmart.comRedcross.org

Thank You!

Documents

Muscular System