Anatomy and Physiology, Seventh Edition

Rod R. SeeleyIdaho State UniversityTrent D. StephensIdaho State UniversityPhilip TatePhoenix College

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*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.

Chapter 09Chapter 09

Lecture OutlineLecture Outline**

Muscular System Functions

• Body movement (Locomotion)• Maintenance of posture• Respiration

– Diaphragm and intercostal contractions

• Communication (Verbal and Facial)• Constriction of organs and vessels

– Peristalsis of intestinal tract– Vasoconstriction of b.v. and other structures (pupils)

• Heart beat • Production of body heat (Thermogenesis

Properties of Muscle

• Excitability: capacity of muscle to respond to a stimulus

• Contractility: ability of a muscle to shorten with force

• Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree

• Elasticity: ability of muscle to recoil to original resting length after stretched

Types of Muscle

• Skeletal– Attached to bones– Responsible for locomotion, facial expressions, posture, respiratory

movements, other types of body movement– Voluntary in action; controlled by somatic motor neurons

• Smooth– In the walls of hollow organs, blood vessels, eye, glands, uterus, skin– Some functions: propel urine, mix food in digestive tract,

dilating/constricting pupils, regulating blood flow, – In some locations, autorhythmic– Controlled involuntarily by endocrine and autonomic nervous systems

• Cardiac– Heart: major source of movement of blood– Autorhythmic– Controlled involuntarily by endocrine and autonomic nervous systems

Connective Tissue

• Layers of Connective Tissue– Epimysium. Dense collagen fibers that

surround the whole muscle• Separates muscle from surrounding

tissues and organs• Connected to the deep fascia

– Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle

• Contains b.v and nerves

– Endomysium. Loose connective tissue that surrounds individual muscle fibers

• Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue

• Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Nerve and Blood Vessel Supply

• Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves

• Axons branch so that each muscle fiber is innervated

• Capillary beds surround muscle fibers

– Muscles require large amts of energy

– Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Skeletal Muscle Structure• Composed of muscle cells (fibers),

connective tissue, blood vessels, nerves

• Fibers are long, cylindrical, and multinucleated

• Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length

• Develop from myoblasts; numbers remain constant

• Striated appearance due to light and dark banding

Muscle Fiber Anatomy• Sarcolemma - cell membrane

– Surrounds the sarcoplasm (cytoplasm of fiber)• Contains many of the same organelles seen in other cells• An abundance of the oxygen-binding protein myoglobin

– Punctuated by openings called the transverse tubules (T-tubules)• Narrow tubes that extend into the sarcoplasm at right angles

to the surface• Filled with extracellular fluid

• Myofibrils -cylindrical structures within muscle fiber– Are bundles of protein filaments (=myofilaments)

• Two types of myofilaments– Actin filaments (thin filaments)– Myosin filaments (thick filaments)

– At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma

– When myofibril shortens, muscle shortens (contracts)

Sarcoplasmic Reticulum (SR)• SR is an elaborate, smooth endoplasmic

reticulum that mostly runs longitudinally and surrounds each myofibril

• Form chambers called terminal cisternae on either side of the T-tubules

• A single T-tubule and the 2 terminal cisternae form a triad

• SR has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR

Sarcoplasmic Reticulum (SR)

Figure 9.5

Parts of a Muscle

Sarcomeres: Z Disk to Z Disk

• Sarcomere - repeating functional units of a myofibril

– About 10,000 sarcomeres per myofibril, end to end

– Each is about 2 µm long

• Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.

– A bands: a dark band; length of thick filaments

• M line - protein to which myosins attach• H zone - thick but NO thin filaments

– I bands: a light band; from Z disks to ends of thick filaments

• Thin but NO thick filaments• Extends from A band of one sarcomere to

A band of the next sarcomere

– Z disk: filamentous network of protein. Serves as attachment for actin myofilaments

– Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

Structure of Actin and Myosin

Myosin (Thick)

Myofilament

• Many elongated myosin molecules shaped like golf clubs.

• Single filament contains roughly 300 myosin molecules

• Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

• Myosin heads1. Can bind to active sites on the

actin molecules to form cross-bridges. (Actin binding site)

2. Attached to the rod portion by a hinge region that can bend and straighten during contraction.

3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Actin (Thin) Myofilaments

• Thin Filament: composed of 3 major proteins1. F (fibrous) actin2. Tropomyosin3. Troponin

• Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.– Composed of G actin monomers

each of which has a myosin-binding site (see yellow dot)

– Actin site can bind myosin during muscle contraction.

• Tropomyosin: an elongated protein winds along the groove of the F actin double helix.

• Troponin is composed of three subunits: – Tn-A : binds to actin– Tn-T :binds to tropomyosin,– Tn-C :binds to calcium ions.

Sliding Filament Model of Contraction

• Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

• In the relaxed state, thin and thick filaments overlap only slightly

• Upon stimulation, myosin heads bind to actin and sliding begins

Sliding Filament Model of Contraction

• Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere

• As this event occurs throughout the sarcomeres, the muscle shortens

InterActive Physiology®: Muscular System: Sliding Filament TheoryPLAYPLAY

Neuromuscular Junction

• The neuromuscular junction is formed from:– Axonal endings, which have small membranous

sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

– The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction

• Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

Neuromuscular Junction

Figure 9.7 (a-c)

Motor Unit: The Nerve-Muscle Functional Unit

• A motor unit is a motor neuron and all the muscle fibers it supplies

• The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500)

• Muscles that control fine movements (fingers, eyes) have small motor units

• Large weight-bearing muscles (thighs, hips) have large motor units

Motor Unit: The Nerve-Muscle Functional Unit

Figure 9.12 (a)

Motor Unit: The Nerve-Muscle Functional Unit

• Muscle fibers from a motor unit are spread throughout the muscle– Not confined to one fascicle

• Therefore, contraction of a single motor unit causes weak contraction of the entire muscle

• Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

Smooth Muscle

• Not striated, fibers smaller than those in skeletal muscle

• Spindle-shaped; single, central nucleus• More actin than myosin

– Not arranged as symmetrically as in skeletal muscle, thus NO striations.

• Caveolae: indentations in sarcolemma; may act like T tubules

• Dense bodies instead of Z disks as in skeletal muscle; have noncontractile intermediate filaments

Smooth Muscle

Figure 9.24

Types of Smooth Muscle

• Visceral or unitary: cells in sheets; function as a unit– Numerous gap junctions; waves of contraction– Often autorhythmic

• Multiunit: cells or groups of cells act as independent units– Sheets (blood vessels); bundles (arrector pili and

iris); single cells (capsule of spleen)

Cardiac Muscle• Found only in heart• Striated fibers that branch• Each cell usually has one nucleus• Fibers joined by intercalated disks

– IDs are composites of desmosomes and gap junctions– Allow excitation in one fiber to spread quickly to adjoining fibers

• Under control of the ANS (involuntary) and endocrine system (hormones)

• Some cells are autorhythmic– Fibers spontaneously contract (aka Pacemaker cells)

Developmental Aspects• Muscle tissue develops from embryonic mesoderm

called myoblasts• Multinucleated skeletal muscles form by fusion of

myoblasts• The growth factor agrin stimulates the clustering of

ACh receptors at newly forming motor end plates• As muscles are brought under the control of the somatic

nervous system, the numbers of fast and slow fibers are also determined

• Cardiac and smooth muscle myoblasts do not fuse but develop gap junctions at an early embryonic stage

Developmental Aspects: Regeneration

• Cardiac and skeletal muscle become amitotic, but can lengthen and thicken

• Myoblast-like satellite cells show very limited regenerative ability

• Cardiac cells lack satellite cells• Smooth muscle has good regenerative ability• There is a biological basis for greater strength in men

than in women• Women’s skeletal muscle makes up 36% of their body

mass• Men’s skeletal muscle makes up 42% of their body

mass

Developmental Aspects: Male and Female

• These differences are due primarily to the male sex hormone testosterone

• With more muscle mass, men are generally stronger than women

• Body strength per unit muscle mass, however, is the same in both sexes

Developmental Aspects: Age Related

• With age, connective tissue increases and muscle fibers decrease

• Muscles become stringier and more sinewy• By age 80, 50% of muscle mass is lost

(sarcopenia)• Decreased density of capillaries in muscle• Reduced stamina• Increased recovery time• Regular exercise reverses sarcopenia