Chapter 6 Muscular System

Types of Muscles

Skeletal Cardiac Smooth

Smooth Muscle

• Found in hollow parts of the body (stomach, intestines, blood vessels, bladder)

• Single nucleus

• Many mitochondria (needs lots of energy)

• Arranged in layers, running in all directions – Muscles contract in all directions

• No striations

• Involuntary

• Does not fatigue easily

Cardiac Muscles

• Striated

• Involuntary rhythmic contractions

• Branched fiber

• Intercalated disks

• Single nuclei

• Found in Heart

• Never rests

Skeletal Muscle

• Striated (protein filaments )

• Voluntary

• Attached to bone via tendons

• Multinucleated

• Over 600 in your body

• Make up about 50% of body weight

Tissues that make up muscles

• Skeletal muscle

• Connective tissue (dense reg. CT = tendons)

• Nervous tissue – sends messages to contract

• Blood Vessels – bring nutrients removes wastes

Organs are made of TISSUES, so

MUSCLE is ORGAN

Functions of Skeletal Muscle

• Support the body

• Movement of bone

• Maintain constant body temperature

• Protect internal organs

• Stabilize joints

• Assists movement in cardiovascular veins and lymph moving in lymphatic vessels

Muscle cells • Each muscle cell is

called a fiber

• Smallest subunit of a fiber = myofibril (made up of actin and myosin units)

Muscle Fibers

• one muscle fiber runs the length of the entire muscle (longest sartorius)

• Contains organelles – Cell membrane = sarcolemma

• Controls what enters and exits cell

– Cytoplasm = sarcoplasm • Contains nuclei and mitochondria

– Endoplasmic reticulum = sarcoplasmic reticulum • Transport, stores calcium

– Mitochondria – • Cell respiration = ATP formation

Bundles of fibers

fiber

fascicle

Connective tissue coverings • ENDOMYSIUM =

C.T. Surrounding Muscle Fiber

• PERIMYSIUM =

C.T. Surrounding Fascicle

• EPIMYSIUM = C.T. Surrounding Muscle

Review of terms

Myofibril - fill with actin and myosin protein filaments

Muscle fibers – covered with endomysium (CT) Fascicle – covered with perimysium (CT) Muscle – covered with epimysium

(CT) continues as a strong fibrous tendon (attaches to bone & merges with periosteum)

Organization: Microscopic to macroscopic

• Myofibril – actin and myosin unit

• Muscle fiber – muscle cells with organelles

• Fascicle = bundle of muscle fibers

• Muscle = organ ( includes all above parts)

Myofibrils

• Functional unit of a myofibril = sarcomere

Sarcomere

• Protein filaments

– Thin filaments-Actin, Troponin, Tropomyosin

– Thick filaments-Myosin

• Contraction of a muscle occurs as the actin filaments move over the myosin filaments

Reading a Sarcomere

• Length of thick myosin filaments = A - band • Thick myosin without overlap of actin = H zone • Middle of H zone (thin proteins) = M line

• Thin actin only with no overlap = I band • Middle of I band (actin protein attached) = Z line

M line

Sarcolemma Plasma membrane

• Forms T (transverse) tubules that dip down into the cell

• Come in contact with the expanded parts of SR

Sarcoplasmic Reticulum

• Endoplasmic reticulum

• Encases myofibrils

• Terminal portions store Ca++

Sarcoplasm

Cytoplasm

Contains organelles and cell components such as:

• Glycogen-polysaccharide (complex carbohydrate) that stores energy for muscle contraction

• Myoglobin-red pigment which binds oxygen

• Myofilaments-thick (myosin), thin (actin, tropomyosin, troponin)

– Myofibril-bundle of myofilaments

Relaxed Muscle

• Tropomyosin covers the myosin binding sites on actin

Contraction = movement of actin over myosin

Steps for Muscle Contraction

• Nerve impulse travels down neuron • Reaches axon terminal • Ca+ travels down Axon • Synaptic vesicle fuses with presynaptic membrane of

synaptic bulb • Neurotransmitters are released into synaptic cleft • Ca+ released from SR binds troponin • Tropomyosin moves • Myosin heads attach to actin • Muscle contracts • Ca returns to SR and myosin heads release = muscle relaxes

Sliding Filament Theory

As a muscle fiber contracts…

• Thin filaments slide past the myosin filaments

• Length of thick and thin filaments does not change

• Sarcomeres shorten

• I bands shorten

• H zone almost disappears

Neuromuscular Junction

Neuromuscular Junction Axon terminals: endings of an

axon containing,

– Synaptic vesicles full of neurotransmitter (Acetylcholine, Ach)

Synaptic Cleft: space between the axon terminal and the muscle

Postsynaptic membrane: sarcolemma with receptors for Ach

Neuromuscular Junction

• Neurons generate impulse = action potential

• Axon leads to muscle or a gland

• Nerve impulse will cause muscle to contract

ATP- Energy for Muscle Contraction

3 ways muscles acquire new ATP:

• Creatine phosphate breakdown (anaerobic)

• Cell respiration (aerobic)

• Fermentation (anaerobic)

Aerobic vs. Anaerobic:

• Aerobic-requires oxygen

• Anaerobic-does not require oxygen

Oxygen supply for muscles

Myoglobin-oxygen carrier protein (similar to hemoglobin)

• Synthesized in muscle cells

• Gives reddish brown color to skeletal muscle

• Has a higher affinity for oxygen than hemoglobin – Pulls oxygen out of blood and makes it available to

mitochondria

• Temporarily stores oxygen

Muscle Responses

• All-or-none law- In a lab, a muscle fiber contracts completely or none at all

• A muscle twitch-contraction of an entire muscle when stimulated above a threshhold

– 3 Stages: latent period, contraction, relaxation

Muscle responses…

• Summation: when a whole muscle is given a series of stimuli

– Muscle responds to the next stimulus without completely relaxing

• Tetanic contraction: maximum sustained contraction

– Continues until the muscle fatigues

Muscle responses…

• Fatigue: gradual weakening after repetitive use

– ATP depleted (“out of energy”)

– Fermentationlactic acid buildup (low pH)muscle function inhibited

– Motor neurons can run out of Ach

– Brain signals a person to stop exercising even if not fatigued

Motor Unit • A nerve fiber with all of the

muscle fibers it innervates

• Obeys all-or-none law

• Fine motor control-

– Small motor units

– Less muscle fibers/nerve

• Gross motor control-

– Large motor units

– Large #of muscle fibers/nerve

Recruitment and Tone Recruitment:

• Increase in the intensity of nervous stimulation

• More and more motor units are activated

• Stronger and stronger contractions over all

Tone:

• Muscle tone at rest

• Some muscle fibers are always contracting – Posture maintainance

Use it or …?

Atrophy

• Unused muscles decrease in size

• Limbs in a cast

• Damage to a nerve ending

• Sometimes muscle fibers can even shorten

Hypertrophy

• Muscle increases in size with forceful activity

• Increase in the # of myofibrils

Slow vs. Fast Slow twitch fibers: aerobic, small motor units

• More endurance, high resistance to fatigue

• Helpful in long-distance running, biking, jogging, swimming

• Dark: many mitochondria, high myoglobin, large number of capillaries

Fast twitch fibers: anaerobic, large motor units

• Strength, quick fatigue

• Weight lifting, sprinting

• Light: low mitochondria, little myoglobin