Muscular System

Skeletal

• attached to bones

• striated

• voluntarily controlled

Cardiac

• located in the heart

• striated

• involuntarily controlled

Smooth

• Located in blood vessels, hollow organs

• Non-striated

• involuntarily controlled

THE MUSCULAR SYSTEM

Functions

1. Movement

2. Maintain posture

3. Respiration

4. Production of body heat

5. Communication

6. Heart beat

7. Contraction of organs and vessels

Properties of Muscles

Contractility - the ability of muscle to shorten forcefully, or contract

Excitability - the capacity of muscle to respond to a stimulus

Extensibility - the ability to be stretched beyond it normal resting length and still be able to contract

Elasticity - the ability of the muscle to recoil to its original resting length after it has been stretched

SKELETAL MUSCLE STRUCTURE

⁃ Skeletal muscle, or striated muscle, with its associated connective tissue, constitutes approximately 40% of body weight.

⁃ Skeletal muscle is so named because many of the muscles are attached to the skeletal system.

⁃ Some skeletal muscles attach to the skin or connective tissue sheets.

⁃ Skeletal muscle is also called striated muscle because transverse bands, or striations, can be seen in the muscle under the microscope.

⁃ Individual skeletal muscles, such as the biceps brachii, are complete organs, as a result of being comprised of several tissues: muscle, nerve, and connective tissue.

Connective Tissue Coverings

⁃ Each skeletal muscle is surrounded by a connective tissue sheath called the epimysium.

⁃ A skeletal muscle is subdivided into groups of muscle cells, termed fascicles.

⁃ Each fascicle is surrounded by a connective tissue covering, termed the perimysium.

⁃ Each skeletal muscle cell (fiber) is surrounded by a connective tissue covering, termed the endomysium.

Muscle Fiber Structure

⁃ A muscle fiber is a single cylindrical cell, with several nuclei located at its periphery.

⁃ Muscle fibers range in length 1 cm to 30 cm and are generally 0.15 mm in diameter.

⁃ Skeletal muscle fibers contain several nuclei that are located at the periphery of the fiber.

⁃ The sarcolemma (cell membrane) has many tubelike inward folds, called transverse tubules, or T tubules.

⁃ T tubules occur at regular intervals along the muscle fiber and extend into the center of the muscle fiber.

⁃ The T tubules are associated with enlarged portions of the smooth endoplasmic reticulum called the sarcoplasmic reticulum.

⁃ The enlarged portions are called terminal cisternae.

⁃ T tubules connect the sarcolemma to the terminal cisternae to form a muscle triad.

⁃ The sarcoplasmic reticulum has a relatively high concentration of Ca2+, which plays a major role in muscle contraction.

⁃ The cytoplasm of a muscle fiber is called the sarcoplasm, which contains many bundles of protein filaments.

⁃ Bundles of protein filaments are called myofibrils.

⁃ Myofibrils consist of the myofilaments, actin and myosin.

THE SARCOMERE

⁃ A sarcomere extends from one Z disk to the next Z disk.

⁃ The sarcomere is the basic structural and functional unit of a skeletal muscle because it is the smallest portion of a skeletal muscle capable of contracting.

⁃ Z disks form a network of protein fibers that both serve as an anchor for actin myofilaments and separate one sarcomere from the next.

⁃ A sarcomere extends from one Z disk to the next Z disk.

⁃ The organization of actin and myosin myofilaments gives skeletal muscle its striated appearance and gives it the ability to contract.

⁃ The myofilaments slide past each other, causing the sarcomeres to shorten.

⁃ Each sarcomere consists of two light-staining bands separated by a dark-staining band.

⁃ Light bands, consist only of actin, and are called I bands that extends toward the center of the sarcomere to the ends of the myosin myofilaments.

⁃ Dark staining bands are called A bands, that extend the length of the myosin myofilaments.

⁃ Actin and myosin myofilaments overlap for some distance on both ends of the A band; this overlap causes the contraction.

⁃ Actin myofilaments are made up of three components: actin, troponin, and tropomyosin.

⁃ Troponin molecules have binding sites for Ca2+and tropomyosin filaments block the myosin myofilament binding sites on the actin myofilaments.

⁃ Myosin myofilaments, or thick myofilaments, resemble bundles of tiny golf clubs.

⁃ Myosin heads have ATP binding sites, ATPase and attachment spots for actin.

Excitability of Muscle Fibers

⁃ The electrical charge difference across the cell membrane of an unstimulated cell is called the resting membrane potential.

⁃ Muscle cells (fibers) have a resting membrane potential, but can also perform action potentials.

⁃ The resting membrane potential is due to the inside of the membrane being negatively charged in comparison to the outside of the membrane being positively charged.

⁃ Action potentials are due to the membrane having gated channels.

Resting Membrane Potential

The resting membrane potential exists because of:

• The concentration of K+ being higher on the inside of the cell membrane and the concentration of Na+ being higher on the outside

• The presence of many negatively charged molecules, such as proteins, inside the cell that are too large to exit the cell

• The presence of leak protein channels in the membrane that are more permeable to K+ than it is to Nat

• Na+ tends to diffuse into the cell and K+ tends to diffuse out.

• In order to maintain the resting membrane potential, the sodium-potassium pump recreates the Nat and K+ ion gradient by pumping Nat out of the cell and K+ into the cell.

Action Potential

⁃ To initiate a muscle contraction, the resting membrane potential must be changed to an action potential.

⁃ Changes in the resting membrane potential occur when gated cell membrane channels open.

⁃ In a skeletal muscle fiber, a nerve impulse triggers gated Na+ channels to open and Na+ diffuses into the cell down its concentration gradient and toward the negative charges inside the cell.

⁃ The entry of Na+ causes the inside of the cell membrane to become more positive than when the cell is at resting membrane potential.

⁃ This increase in positive charge inside the cell membrane is called depolarization.

⁃ If the depolarization changes the membrane potential to a value called threshold, an action potential is triggered.

⁃ An action potential is a rapid change in charge across the cell membrane.

⁃ Depolarization during the action potential is when the inside of the cell membrane becomes more positively charged than the outside of the cell membrane.

⁃ Near the end of depolarization, the positive charge causes gated Nat channels to close and gated K+ channels to open.

⁃ Opening of gated K+ channels starts repolarization of the cell membrane.

⁃ Repolarization is due to the exit of K+ from the cell.

⁃ The outward diffusion of K+ returns the cell to its resting membrane conditions and the action potential ends.

⁃ In a muscle fiber, an action potential results in muscle contraction.

Depolarization

⁃ change in charges inside becomes more + and outside more - Na+ channels open

Repolarization

⁃ Nat channels close change back to resting potential

Nerve Supply

⁃ A motor neuron is a nerve cell stimulates muscle cells.

⁃ A neuromuscular junction is a synapse where a the fiber of a nerve connects with a muscle fiber.

⁃ A synapse refers to the cell-to-cell junction between a nerve cell and either another nerve cell or an effector cell, such as in a muscle or a gland.

⁃ A motor unit is a group of muscle fibers that a motor neuron stimulates.

⁃ A presynaptic terminal is the end of a neuron cell axon fiber.

⁃ A synaptic cleft is the space between the presynaptic terminal and postsynaptic membrane.

⁃ The postsynaptic membrane is the muscle fiber membrane (sarcolemma).

⁃ A synaptic vesicle is a vesicle in the presynaptic terminal that stores and releases neurotransmitter chemicals.

⁃ Neurotransmitters are chemicals that stimulate or inhibit postsynaptic cells.

⁃ Acetylcholine is the neurotransmitter that stimulates skeletal muscles.

Muscle Contraction

1. An action potential travels down motor neuron to presynaptic terminal causing Ca2+ channels to open.

2. Ca+ causes synaptic vesicles to release acetylcholine into synaptic cleft.

3. Acetylcholine binds to receptor sites on Na+ channels, Na* channels open, and Nat rushes into postsynaptic terminal (depolarization).

4. Na+ causes sarcolemma and t-tubules to increase the permeability of sarcoplasmic reticulum which releases stored calcium.

5. Ca2+binds to troponin which is attached to actin.

6. Ca2+binding to troponin causes tropomyosin to move exposing attachment sites for myosin.

7. Myosin heads bind to actin.

8. ATP is released from myosin heads and heads bend toward center of sarcomere.

9. Bending forces actin to slide over myosin.

10. Acetylcholinesterase (enzyme breaks down acetylcholine) is released, Nat channels close, and muscle contraction stops.

ATP and Muscle Contractions

⁃ Energy for muscle contractions is supplied by ATP

⁃ Energy is released as ATP → ADP + P

⁃ ATP is stored in myosin heads

⁃ ATP help form cross-bridge formation between myosin and actin

⁃ New ATP must bind to myosin before cross-bridge is released

⁃ Rigor mortis will occur when a person dies and no ATP is available to release cross-bridges

Muscle Twitch

⁃ A muscle twitch is a single contraction of a muscle fiber in response to a stimulus.

⁃ A muscle twitch has three phases: latent phase, contraction phase, and relaxation phase.

⁃ The latent phase is the time between the application of a stimulus and the beginning of contraction.

⁃ The contraction phase is the time during which the muscle contracts and the relaxation phase is the time during which the muscle relaxes.

Summation and Recruitment

⁃ In summation, individual muscles contract more forcefully.

⁃ Tetanus is a sustained contraction that occurs when the frequency of stimulation is so rapid that no relaxation occurs.

⁃ Recruitment is the stimulation of several motor units.

Skeletal Muscle Fiber Types

Slow twitch fibers

• contract slowly

• fatigue slowly

• have a considerable amount of myoglobin

• use aerobic respiration

• are dark in color

• used by long distance runners

Fast twitch fibers

• contract quickly

• fatigue quickly

• use anaerobic respiration

• energy from glycogen

• light color

• used by sprinters

Skeletal Muscle Fiber Types:

A muscle has a blend of types, with one type dominating. Humans have both types of fibers

The distribution of fibers is genetically determined.

Energy for Muscle Contractions, Muscle fibers

⁃ Muscle fibers are very energy-demanding cells whether at rest or during any form of exercise.

⁃ This energy comes from either aerobic (with O2) or anaerobic (without O2) ATP production

⁃ ATP is derived from four processes in skeletal muscle.

1. Aerobic production of ATP during most exercise and normal conditions.

2. Anaerobic production of ATP during intensive short-term work

3. Conversion of a molecule called creatine phosphate to ATP

4. Conversion of two ADP to one ATP and one AMP (adenosine monophosphate) during heavy exercise

Muscle Fatigue

⁃ Fatigue is a temporary state of reduced work capacity.

⁃ Without fatigue, muscle fibers would be worked to the point of structural damage to them and their supportive tissues.

Mechanisms of fatigue include:

• Acidosis and ATP depletion due to either an increased ATP consumption or a decreased ATP production

• Oxidative stress, which is characterized by the buildup of excess reactive oxygen species (ROS; free radicals)

• Local inflammatory reactions

Types of Contractions

There are two types of muscle contractions:

⁃ isometric and isotonic.

⁃ The isometric contraction has an increase in muscle tension, but no change in length.

⁃ The isotonic contraction has a change in muscle length with no change in tension.

⁃ Concentric contractions are isotonic contractions in which muscle tension increases as the muscle shortens;

⁃ Eccentric contractions are isotonic contractions in which tension is maintained in a muscle, but the opposing resistance causes the muscle to lengthen.

Muscle Tone

⁃ Muscle tone is the constant tension produced by body muscles over long periods of time.

⁃ Muscle tone is responsible for keeping the back and legs straight, the head in an upright position, and the abdomen from bulging.

⁃ Muscle tone depends on a small percentage of all the motor units in a muscle being stimulated at any point in time, causing their muscle fibers to contract tetanically and out of phase with one another.

Smooth Muscle

⁃ Smooth muscle cells are non-striated small, I spindle-shaped muscle cells, usually with one nucleus per cell.

⁃ The myofilaments are not organized into sarcomeres.

⁃ The cells comprise organs controlled involuntarily, except the heart.

⁃ Neurotransmitter substances, hormones, and other substances can stimulate smooth muscle.

Cardiac Muscle

⁃ Cardiac muscle cells are long, striated, and branching, with usually only one nucleus per cell.

⁃ Cardiac muscle is striated as a result of the sarcomere arrangement.

⁃ Cardiac muscle contraction is autorhythmic.

⁃ Cardiac muscle cells are connected to one another by specialized structures that include desmosomes and gap junctions called intercalated disks.

⁃ Cardiac muscle cells function as a single unit in that action potential in one cardiac muscle cell can stimulate action potentials in adjacent cells.

Skeletal Muscle Anatomy

⁃ A tendon connects skeletal muscle to bone.

⁃ Aponeuroses are broad, sheetlike tendons.

⁃ A retinaculum is a band of connective tissue that holds down the tendons at each wrist and ankle.

⁃ Skeletal muscle attachments have an origin and an insertion, with the origin being the attachment at the least mobile location.

⁃ The insertion is the end of the muscle attached to the bone undergoing the greatest movement.

⁃ The part of the muscle between the origin and the insertion is the belly.

⁃ A group of muscles working together are called agonists.

⁃ A muscle or group of muscles that oppose muscle actions are termed antagonists.

Nomenclature

Muscles are named according to:

1. Location - a pectoralis muscle is located in the chest.

2. Size - the size could be large or small, short or long.

3. Shape - the shape could be triangular, quadrate, rectangular, or round.

4. Orientation of fascicles - fascicles could run straight (rectus) or at an angle (oblique).

5. Origin and insertion. The sternocleidomastoid has its origin on the sternum and clavicle and its insertion on the mastoid process of the temporal bone.

6. Number of heads. A biceps muscle has two heads (origins), and a triceps muscle has three heads (origins).

7. Function. Abductors and adductors are the muscles that cause abduction and adduction movements.

Muscles of Mastication

• Temporalis

• Masseter

• Pterygoids (two pairs)

Thoracic Muscles

External intercostals:

• elevate ribs for inspiration

Internal intercostals:

• depress ribs during forced expiration

Diaphragm:

• moves during quiet breathing

Abdominal Wall muscles

Rectus abdominis:

• center of abdomen

• compresses abdomen

External abdominal oblique:

• sides of abdomen

• compresses abdomen

Internal abdominal oblique:

• compresses abdomen

Transverse abdominis:

• compresses abdomen

Upper Scapular and Limb Muscles

Trapezius:

• shoulders and upper back

• extends neck and head

Pectoralis major:

• chest

• elevates ribs

Serratus anterior:

• between ribs

• elevates ribs

Deltoid:

• shoulder

• abductor or upper limbs

Upper Limb Muscles

Triceps brachii:

• 3 heads

• extends elbow

Biceps brachii:

• "flexing muscle"

• flexes elbow and shoulder

Brachialis:

• flexes elbow

Latissimus dorsi:

• lower back

• extends shoulder

Forearm Muscles

• Flexor longus

• Flexor carpi radialis

• Flexor carpi ulnaris

• Flexor digitorum profundus

• Flexor digitorum superficialis

• Pronator

• Brachioradialis

• Extensor carpi radialis brevis

Pelvic Floor Muscles

• Levator ani

• Ischiocavernosus

• Bulbospongiosus

• Deep transverse perineal

• Superficial transverse perineal

Muscles of Hips and Thighs

Iliopsoas:

• flexes hip

Gluteus maximus:

• buttocks

• extends hip and abducts thigh

Gluteus medius:

• Hip

• abducts and rotates thigh

Muscles of the Upper Leg

The quadriceps femoris is comprised of 4 thigh muscles:

The rectus femoris:

• front of thigh

• extends knee and flexes hip

The vastus lateralis:

• extends knee

The vastus medialis:

• extends knee

The vastus intermedius:

• extends knee

Gracilis:

• adducts thigh and flexes knee

Biceps femoris, semimembranosus,

semitendinosus:

• Hamstring

• back of thigh

• flexes knee, rotates leg, extends hip

The rectus femoris:

• front of thigh

• extends knee and flexes hip

Muscles of Lower Leg

Tibialis anterior:

• front of lower leg

• inverts foot

Gastrocnemius:

• calf

• flexes foot and leg

Soleus:

• attaches to ankle

• flexes foot