The Muscular System

or “Everything you ever wanted to know about Muscles, but were afraid to ask” !!!

Did you know that ? -more than 50% of body weight is muscle ! -And muscle is made up of proteins and water

The Muscular System Muscles are responsible for all movement of the body There are three basic types of muscle – Skeletal – Cardiac – Smooth

Info About Muscles  Only body tissue able to contract  create movement by flexing and extending joints  Body energy converters (many muscle cells contain many mitochondria)

3 Types of Muscles

Three types of muscle SkeletalCardiacSmooth

Classification of Muscle Skeletal- found in limbs Cardiac- found in heart Smooth- Found in viscera Striated, multi- nucleated Striated, 1 nucleus Not striated, 1 nucleus voluntaryinvoluntary

Characteristics of Muscle Skeletal and smooth muscle are elongated Muscle cell = muscle fiber Contraction of a muscle is due to movement of microfilaments (protein fibers) All muscles share some terminology – Prefixes myo and mys refer to muscle – Prefix sarco refers to flesh

Shapes of Muscles Triangular- shoulder, neck Spindle- arms, legs Flat- diaphragm, forehead Circular- mouth, anus

Skeletal Muscle Most are attached by tendons to bones Cells have more than one nucleus (multinucleated) Striated- have stripes, banding Voluntary- subject to conscious control Tendons are mostly made of collagen fibers Found in the limbs Produce movement, maintain posture, generate heat, stabilize joints

Structure of skeletal muscle Each cell (fibre) is long and cylindrical Muscle fibres are multi-nucleated Typically 50-60mm in diameter, and up to 10cm long The contractile elements of skeletal muscle cells are myofibrils

Microscopic Anatomy of Skeletal Muscle Myofibrils are aligned to give distinct bands – I band = light band Contains only thin filaments – A band = dark band Contains the entire length of the thick filaments

Microscopic Anatomy of Skeletal Muscle Figure 6.3b

Microscopic Anatomy of Skeletal Muscle Sarcomere—contractile unit of a muscle fiber Organization of the sarcomere – Myofilaments Thick filaments = myosin filaments Thin filaments = actin filaments

Microscopic Anatomy of Skeletal Muscle Thick filaments = myosin filaments – Composed of the protein myosin – Has ATPase enzymes – Myosin filaments have heads (extensions, or cross bridges) – Myosin and actin overlap somewhat Thin filaments = actin filaments – Composed of the protein actin – Anchored to the Z disc

Microscopic Anatomy of Skeletal Muscle Figure 6.3c

Microscopic Anatomy of Skeletal Muscle At rest, there is a bare zone that lacks actin filaments called the H zone Sarcoplasmic reticulum (SR) – Stores and releases calcium – Surrounds the myofibril

Microscopic Anatomy of Skeletal Muscle Figure 6.3d

Skeletal Muscle Attachments Epimysium blends into a connective tissue attachment – Tendons—cord-like structures Mostly collagen fibers Often cross a joint due to toughness and small size – Aponeuroses—sheet-like structures Attach muscles indirectly to bones, cartilages, or connective tissue coverings

Stimulation and Contraction of Single Skeletal Muscle Cells Excitability (also called responsiveness or irritability)—ability to receive and respond to a stimulus Contractility—ability to shorten when an adequate stimulus is received Extensibility—ability of muscle cells to be stretched Elasticity—ability to recoil and resume resting length after stretching

The Nerve Stimulus and Action Potential Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron

Figure 6.4a The Nerve Stimulus and Action Potential

Figure 6.4b

The Nerve Stimulus and Action Potential Neuromuscular junction – Association site of axon terminal of the motor neuron and muscle

The Nerve Stimulus and Action Potential Figure 6.5a

The Nerve Stimulus and Action Potential Synaptic cleft – Gap between nerve and muscle – Nerve and muscle do not make contact – Area between nerve and muscle is filled with interstitial fluid

The Nerve Stimulus and Action Potential Figure 6.5b

Transmission of Nerve Impulse to Muscle Neurotransmitter—chemical released by nerve upon arrival of nerve impulse – The neurotransmitter for skeletal muscle is acetylcholine (ACh) Acetylcholine attaches to receptors on the sarcolemma Sarcolemma becomes permeable to sodium (Na+)

Transmission of Nerve Impulse to Muscle Figure 6.5c

Transmission of Nerve Impulse to Muscle Sodium rushes into the cell generating an action potential Once started, muscle contraction cannot be stopped

The Sliding Filament Theory of Muscle Contraction Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere This continued action causes a sliding of the myosin along the actin The result is that the muscle is shortened (contracted)

The Sliding Filament Theory of Muscle Contraction Figure 6.7a–b

Skeletal muscle - Summary Voluntary movement of skeletal parts Spans joints and attached to skeleton Multi-nucleated, striated, cylindrical fibres

Smooth Muscle No striations Spindle shaped Single nucleus Involuntary- no conscious control Found mainly in the walls of hollow organs

Smooth muscle Lines walls of viscera Found in longitudinal or circular arrangement Alternate contraction of circular & longitudinal muscle in the intestine leads to peristalsis

Structure of smooth muscle Spindle shaped uni-nucleated cells Striations not observed Actin and myosin filaments are present( protein fibers)

Smooth muscle - Summary Found in walls of hollow internal organs Involuntary movement of internal organs Elongated, spindle shaped fibre with single nucleus

Cardiac Muscle Striations Branching cells Involuntary Found only in the heart Usually has a single nucleus, but can have more than one

Cardiac muscle Main muscle of heart Pumping mass of heart Critical in humans Heart muscle cells behave as one unit Heart always contracts to it’s full extent

Structure of cardiac muscle Cardiac muscle cells (fibres) are short, branched and interconnected Cells are striated & usually have 1 nucleus Adjacent cardiac cells are joined via electrical synapses (gap junctions) These gap junctions appear as dark lines and are called intercalated discs

Cardiac muscle - Summary Found in the heart Involuntary rhythmic contraction Branched, striated fibre with single nucleus and intercalated discs

Muscle Control Type of muscle Nervous control Type of control Example Skeletal Controlled by CNS Voluntary Lifting a glass Cardiac Regulated by ANS Involuntary Heart beating Smooth Controlled by ANS Involuntary Peristalsis

Types of Responses Twitch- – A single brief contraction – Not a normal muscle function Tetanus – One contraction immediately followed by another – Muscle never completely returns to a relaxed state – Effects are compounded

Contraction of Skeletal Muscle Muscle fiber contraction is “all or none” Within a skeletal muscle, not all fibers may be stimulated during the same interval Different combinations of muscle fiber contractions may give differing responses Graded responses—different degrees of skeletal muscle shortening

Contraction of Skeletal Muscle Graded responses can be produced by changing – The frequency of muscle stimulation – The number of muscle cells being stimulated at one time

Types of Graded Responses Figure 6.9a

Types of Graded Responses Figure 6.9b

Types of Graded Responses Unfused (incomplete) tetanus – Some relaxation occurs between contractions – The results are summed

Types of Graded Responses Figure 6.9c

Types of Graded Responses Fused (complete) tetanus – No evidence of relaxation before the following contractions – The result is a sustained muscle contraction

Types of Graded Responses Figure 6.9d

Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction Initially, muscles use stored ATP for energy – ATP bonds are broken to release energy – Only 4–6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

Energy for Muscle Contraction Figure 6.10a

Energy for Muscle Contraction Aerobic respiration – Glucose is broken down to carbon dioxide and water, releasing energy (ATP) – This is a slower reaction that requires continuous oxygen – A series of metabolic pathways occur in the mitochondria

Energy for Muscle Contraction Figure 6.10b

Energy for Muscle Contraction Anaerobic glycolysis and lactic acid formation – Reaction that breaks down glucose without oxygen – Glucose is broken down to pyruvic acid to produce some ATP – Pyruvic acid is converted to lactic acid This reaction is not as efficient, but is fast Huge amounts of glucose are needed Lactic acid produces muscle fatigue

Energy for Muscle Contraction Figure 6.10c

Muscle Fatigue and Oxygen Deficit When a muscle is fatigued, it is unable to contract even with a stimulus Common cause for muscle fatigue is oxygen debt – Oxygen must be “repaid” to tissue to remove oxygen deficit – Oxygen is required to get rid of accumulated lactic acid Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Exercise and Muscles Isotonic- muscles shorten and movement occurs ( most normal exercise) Isometric- tension in muscles increases, no movement occurs (pushing one hand against the other)

Muscle Tone Some fibers are contracted even in a relaxed muscle Different fibers contract at different times to provide muscle tone The process of stimulating various fibers is under involuntary control

Effect of Exercise on Muscles Exercise increases muscle size, strength, and endurance – Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue Makes body metabolism more efficient Improves digestion, coordination – Resistance (isometric) exercise (weight lifting) increases muscle size and strength

How are Muscles Attached to Bone? Origin-attachment to a movable bone Insertion- attachment to an immovable bone Muscles are always attached to at least 2 points Movement is attained due to a muscle moving an attached bone

Flexion Types of Musculo-Skeletal Movement

Extension

Hyperextension

Abduction, Adduction & Circumduction

Rotation

Special Movements Figure 6.13e

More Types of Movement…… Inversion- turn sole of foot medially Eversion- turn sole of foot laterally Pronation- palm facing down Supination- palm facing up Opposition- thumb touches tips of fingers on the same hand

The Skeletal Muscles There are about 650 muscles in the human body. They enable us to move, maintain posture and generate heat. In this section we will only study a sample of the major muscles.

Naming Skeletal Muscles By direction of muscle fibers – Example : Rectus (straight) By relative size of the muscle – Example : Maximus (largest)

Naming Skeletal Muscles By location of the muscle – Example : Temporalis (temporal bone) By number of origins – Example : Triceps (three heads)

Naming Skeletal Muscles By location of the muscle’s origin and insertion – Example : Sterno (on the sternum) By shape of the muscle – Example : Deltoid (triangular) By action of the muscle – Example : Flexor and extensor (flexes or extends a bone)

Arrangement of Fascicles Figure 6.14

Head and Neck Muscles Facial muscles – Frontalis—raises eyebrows – Orbicularis oculi—closes eyes, squints, blinks, winks – Orbicularis oris—closes mouth and protrudes the lips – Buccinator—flattens the cheek, chews – Zygomaticus—raises corners of the mouth Chewing muscles – Masseter—closes the jaw and elevates mandible – Temporalis—synergist of the masseter, closes jaw

Head and Neck Muscles Neck muscles – Platysma—pulls the corners of the mouth inferiorly – Sternocleidomastoid—flexes the neck, rotates the head

Head and Neck Muscles Figure 6.15

Muscles of Trunk, Shoulder, Arm Anterior muscles – Pectoralis major—adducts and flexes the humerus – Intercostal muscles External intercostals—raise rib cage during inhalation Internal intercostals—depress the rib cage to move air out of the lungs when you exhale forcibly

Anterior Muscles of Trunk, Shoulder, Arm Figure 6.16a

Muscles of Trunk, Shoulder, Arm Muscles of the abdominal girdle – Rectus abdominis—flexes vertebral column and compresses abdominal contents (defecation, childbirth, forced breathing) – External and internal obliques—flex vertebral column; rotate trunk and bend it laterally – Transversus abdominis—compresses abdominal contents

Anterior Muscles of Trunk, Shoulder, Arm Figure 6.16b

Muscles of Trunk, Shoulder, Arm Posterior muscles – Trapezius—elevates, depresses, adducts, and stabilizes the scapula – Latissimus dorsi—extends and adducts the humerus – Erector spinae—back extension – Quadratus lumborum—flexes the spine laterally – Deltoid—arm abduction

Muscles of Posterior Neck, Trunk, Arm Figure 6.17a

Muscles of Posterior Neck, Trunk, Arm Figure 6.17b

Muscles of the Upper Limb Biceps brachii—supinates forearm, flexes elbow Brachialis—elbow flexion Brachioradialis—weak muscle Triceps brachii—elbow extension (antagonist to biceps brachii)

Anterior Muscles of Trunk, Shoulder, Arm Figure 6.16a

Muscles of Posterior Neck, Trunk, Arm Figure 6.17a

Muscles of the Lower Limb Gluteus maximus—hip extension Gluteus medius—hip abduction, steadies pelvis when walking Iliopsoas—hip flexion, keeps the upper body from falling backward when standing erect Adductor muscles—adduct the thighs

Muscles of the Pelvis, Hip, Thigh Figure 6.19a

Muscles of the Pelvis, Hip, Thigh Figure 6.19c

Muscles of the Lower Limb Muscles causing movement at the knee joint – Hamstring group—thigh extension and knee flexion Biceps femoris Semimembranosus Semitendinosus

Muscles of the Pelvis, Hip, Thigh Figure 6.19a

Muscles of the Lower Limb Muscles causing movement at the knee joint – Sartorius—flexes the thigh – Quadriceps group—extends the knee Rectus femoris Vastus muscles (three)

Muscles of the Pelvis, Hip, Thigh Figure 6.19c

Muscles of the Lower Limb Muscles causing movement at ankle and foot – Tibialis anterior—dorsiflexion and foot inversion – Extensor digitorum longus—toe extension and dorsiflexion of the foot – Fibularis muscles—plantar flexion, everts the foot – Soleus—plantar flexion

Muscles of the Lower Leg Figure 6.20a

Muscles of the Lower Leg Figure 6.20b

Sternocleidomastoideus Flexes and Rotates Head

Masseter Elevate Mandible

Temporalis Elevate & Retract Mandible

Trapezius Extend Head, Adduct, Elevate or Depress Scapula

Latissimus Dorsi Extend, Adduct & Rotate Arm Medially

Deltoid Abduct, Flex & Extend Arm

Pectoralis Major Flexes, adducts & rotates arm medially

Biceps Brachii Flexes Elbow Joint

Triceps Brachii Extend Elbow Joint

Rectus Abdominus Flexes Abdomen

External Oblique Compress Abdomen

External Intercostals Elevate ribs

Internal Intercostals Depress ribs

Diaphragm Inspiration

Forearm Muscles Flexor carpi—Flexes wrist Extensor carpi—Extends wrist Flexor digitorum—Flexes fingers Extensor digitorum—Extends fingers Pronator—Pronates Supinator—Supinates

Gluteus Maximus Extends & Rotates Thigh Laterally

Rectus Femoris Flexes Thigh, Extends Lower Leg

Gracilis Adducts and Flexes Thigh

Sartorius Flexes Thigh, & Rotates Thigh Laterally

Biceps Femoris Extends Thigh & Flexes Lower Leg

Gastrocnemius Plantar Flexes Foot & Flex Lower Leg

Tibialis Anterior Dorsiflexes and Inverts Foot

Superficial Muscles: Anterior Figure 6.21

Superficial Muscles: Posterior Figure 6.22