BIO 201 Chapter 11

Muscular system 

• Specialized for one major purpose 

  • Converting the chemical energy in ATP into the mechanical energy of motion 

• Myo - muscle 

• Myology - the study of muscular system 

functions of muscle 

• Movement of 

  • Whole body 

  • Body parts 

  • Organ contents 

• Stability 

  • Maintain posture and prevent movement 

• Communication - speech, facial, expression and writing 

• Control of opening and passageways

  • Sphincters - internal muscular rings 

• Body heat production 

CT of muscle - Fascia

Separates neighboring muscles or muscle groups from each other and the subcutaneous tissue 

CT of muscle - Epimysium 

• Fibrous sheath surrounding the entire muscle 

• Outer surface grades into the fascia 

CT of muscle

• Surrounds fascicles 

  • Bundles of muscle fibers 

• Carry larger nerves and blood vessels and stretch receptor

CT of muscle - Endomysium

• Thin sleeve of loose connective tissue surrounding each muscle fiber (cell) 

• Allows room for capillaries and nerve fibers to reach each muscle fiber 

Deep fascia 

between adjacent muscle

Superficial fascia hypodermis) 

• Between skin and muscle 

• Contains adipose tissue 

CT of elements

• Tendons - attach between muscle and bones 

  • Dense irregular connective tissue composed of collagen fibers

  • The epimysium surrounding the entire muscle is continuous with collagen fibers of tendons 

    • Which in turn, are continuous with connective tissue (periosteum) of bone 

  • Collagen is somewhat extensible and elastic 

  • Stretches slightly under tension and recoils when released 

    • Resist excessive stretching and protects muscle from injury 

    • Returns muscle to its resting length 

    • Contribute to power output and muscle efficiency 

Strength of a muscle and the direction of it pulls are determined partly by

• shape

• orientation of its fascicles

Fusiform muscles 

• Thick in middle and tapered at ends

• Biceps brachii, gastrocnemius 

Triangular (convergent) muscles 

• Broad to origin and tapering to a narrower insertion 

  • Pectoralise major, temporalise 

Parallel muscles 

• Parallel fascicles 

• Can span longer distances than other shape s

  • Rectus abdominis, zygomatic major 

Circular muscles 

• Acts as sphincters 

• Ring around body opening 

• Orbicularis oculi, urethral and ana sphincters

Pennate muscles

• Fascicles insert obliquely on a tendon (feather shaped) 

• Unipennate, bipennate or multipennate 

indirect attachment to bone

• Tendons attach muscle to bone

  • Collagen fibers of endo, peri, epimysium continue into the tendon 

  • From there the tendon merges into the periosteum of bond 

  • Very strong structural continuity from muscle to bone 

    • Stress will tear the tendon before pulling it loose from either muscle to bone 

  • Biceps brachii, calcaneal (Achilles) tendon 

  • Aponeurosis - tendon is a board, flat sheet (palmaraponeurosis) 

direct (fleshy) attachment to bone

• Little separation between muscle to bone 

• Muscle seem to immerge directly from bone 

• Margin of brachialis, lateral head of triceps brachii 

Some skeletal muscle

do not insert on bone, but in dermis of skin 

• Muscles of facial expression 

Origin

Bony attachment at stationary end of muscle 

insertion

Bony attachment to mobile end of muscle 

belly

Thicker. Middle region of muscle between origin and insertion 

Action

• the effects produced by a muscle

  • To produce or prevent movement

Prime mover (agonist)

• muscle that produces most of force during a joint action (brachialis) 

Synergist

• muscle that aids the primer mover

  • Stabilizes the nearby joint 

  • Modifies the direction of movement 

  • Bicep brachii 

Antagonist

•  opposes the prime mover 

  • Relaxes the give prime mover control over an action 

  • Preventing excessive movement and injury 

  • Tricep brachii 

  • Antagonistic pairs

Antagonistic pairs

muscles that act on opposite sides of a joint (triceps brachii) 

Fixator

•  muscles that prevents movement of bone 

  • Muscles that holds scapula firmly in palace 

    • Rhomboideus

Responsiveness (excitability) 

• To chemical signals, stretch and electrical changes across the plasma membrane

Conductivity 

• Local electrical change triggers a wave of excitation that travels along muscle fiber 

Contractility

• Shortens when stimulated

Contractility

• Shortens when stimulated

Elasticity

Returns to its original resting length after being stretched 

Skeletal muscle

• Voluntary, striated muscle attached to one or more bones 

  • Striations - altering light and dark transverse bands 

    • Result from an overlapping of internal contractile proteins 

  • Voluntary - usually subject to conscious control 

• Is made up of: muscle cells called muscle fibers or myofibers 

Muscle cell is also

muscle fiber = myofibers

Myofibers

are composed by myofibrils

Myofibrils

long protein bundles that occupies the main portion of the interior of muscle fiber 

Myofibrils

are composed of myofilaments 

Myofilaments

• a protein microfilament responsible for muscle cell contraction 

  • Composed of myosin or actin proteins 

Sarcolemme

- plasma membrane of a muscle fiber 

Sacroplasm

- cytoplasm of a muscle fiber 

Mitochondria

packed in spaces between myofibrils 

Sarcoplasmic reticulum (SR)

Sarcoplasmic reticulum (SR) - smooth ER that forms a network around each myofibril - calcium reservoir 

• Calcium activates the muscle contraction process 

Terminal cisternae

Dilated end sacs of SR which cross muscle fiber from one side to the other 

T tubules

tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side 

Triad

- t tubule and 2 terminal cisterns 

Myofibers - Internal proteins

long protein bundles that occupy the main portion of the sarcoplasm

myofibers - Glycogen

stored in abundance to provide energy with heightened exercise  

myofibers - Myoglobin

red pigment that stores oxygen needed for muscle activity 

myofibers - Multiple nuclei

flattened nuceli passed against the inside of the sarcolemma 

Myoblasts

stem cells that fuse to form each muscle fiber 

Satellite cells

unspecialized myoblast remaining between the muscle fiber and endomysium 

• May multiple and produce new muscle fibers to some degree 

Myofibers - repair

by fibrosis rather than regeneration of functional muscle 

Myofilaments

• thick filaments

• thin filament

• elastic filament

Thick myofilament

• Made of several hundred myosin molecules 

  • The shaped like a golf club 

    • Double globular 

  • Heads directed outward in a helical array around the bundle 

    • Heads on one half of the thick filament angle to the left 

    • Heads on the other half angle to the right 

    • Bare zone with no heads in the middle

thin myofilaments - Fibrous (F) actin

• 2 intertwined strands 

  • String a globular (G) actin subunits each with an active site that can bind to head of myosin molecule

thin myofilament - Tropomyosin 

 

• Each blocking 6 or 7 active sites of G actin subunit 

thin myofilament - Troponin complex

small, calcium binding protein on each tropomyosin molecule 

Elastic myofilament

• Titan (connectin) - huge springy protein 

  • Flank each thick filament and anchor it to the Z disc 

  • Help the cell recoil to its resting length (elasticity) 

  • Keep thick and thin filaments aligned 

    • Help stabilize the thick filament 

    • Center it between thin filaments 

  • Prevents over stretching 

Contractile proteins

myosin and actin 

• Do the work of contracting the muscle 

• Regulatory proteins

• tropomyosin and troponin 

  • Like a switch that starts and stops contraction 

    • Contraction activated by release of calcium into sarcoplasm and its binding to troponin 

    • Troponin changes shape and moves tropomyosin off the active sites on actin 

Acessory proteins

• At least 7 other accessory proteins in or associated with thick or thin filaments 

  • Anchor the myofilaments 

  • Regulate length of myofilaments 

  • Alignment of myofilaments for optimal effectiveness 

Dystrophin

most clinically important 

• Links actin in outermost myofilaments to transmembrane proteins and eventually to fibrous endomysium surrounding the entire muscle cell 

• Transfers forces of muscle contraction to connective tissue around myofiber

• Genetic defects in dystrophin produce disabling disease muscular dystrophy 

Muscular dystrophy 

• Group of hereditary diseases in which skeletal muscle degenerate and are replaced with scar tissue and adipose tissue 

• Mainly a disease of males - duchenne MD 

  • Appears as child begins to walk 

  • Rarely live past 20 years of age 

• Normal allele makes dystrophin 

  • Absence of dystrophin leads to torn cell membranes 

• Fascioscapulohumeral MD - facial and shoulder muscle only 

  • Affects both sexes equally

Striations

• Myosin and actin are proteins that occur in all cells 

  • Functions in cellular motility, mitosis, transport of intracellular material 

• Organized in a precise way in skeletal and cardiac muscle 

A band

• dark - A strand for anisotropic

• Part of A band where thick and thin filaments overlaps is especially dark 

H band

• in middle of A band - just thick filaments 

m line

•  is in the middle of H band 

I band

• alternating lighter band - I stands for isotropic 

  • The way the band reflect polarized light 

Z disc

•  provides anchorage for thin filaments and elastic filaments 

  • Biscuits I band 

Sarcomeres

• Segments from Z disc to Z disc 

• Muscle cells shorten because individual sarcomeres shorten 

  • Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other 

• Neither thick nor thin filaments change length during shortening 

  • Only the amount the overlap changes

• During shortening dystrophin and linking proteins also pull on extracellular proteins 

  • Transfer pull to extracellular tissue

The nerve-muscle relationship

Skeletal muscle must be stimulated by a nerve or it will not contract 

• If nerve connections are severed or poisoned, a muscle is paralyzed 

• Denervation atrophy - shrinkage or paralyzed muscle when connection not restored

Somatic motor neurons

stimulate skeletal muscle

• Cell bodies are located in the brainstem and spinal cord 

• Somatic motor fibers

• axons of somatic motor neurons 

  • Lead to the skeletal muscle 

  • Each nerve fiber branches out to a number of muscle fibers 

    • 200 myofibres on average are controlled by a single somatic motor neuron

  • Each myofiber is supplied by only one motor neuron 

motor unit 

• One nerve fiber and all muscle fibers innervated by it

Myofibers of one motor unit 

• Dispersed through the muscle 

• Contract in unison 

• Produce weak contraction over wise area 

• Provides ability to sustain long term contraction as motor unit take turns contracting 

  • Postural control 

• Effective contraction usually requires the contraction of several motor units at once 

Average motor unit

200 muscle fibers per neuron 

Small motor units  

• fine degree of control 

  • 3-6 muscle fibers per neuron 

  • Eye and hand muscles 

Large motor unit

• more strength than control 

  • Powerful contractions supplied by large motor units 

  • Many muscle fibers per motor unit 

  • Many muscle fibers per motor unit 

  • Gastrocnemius - 1000 myofibers per neuron 

Synapse  

point where a nerve fiber meets its target cell 

• Neuromuscular junction (NMJ)

• when target cell is a muscle fiber 

  • Each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber 

  • One nerve fiber stimulates the muscle fiber at several points within the NMJ 

components of neuromuscular junction - Synaptic Knob

• swollen end of nerve fiber

  • Contains synaptic vesicles filled with acetylcholine 

  • Synaptic vesicles undergo exocytosis releasing ACh into synaptic cleft 

components of neuromuscular junction - Synaptic cleft

• tiny gap between synaptic knob and muscle sarcolemma 

components of neuromuscular junction - Schwann cell

• envelops and isolates all of the NMJ from surrounding tissue fluid 

components of neuromuscular junction - 50 million ACh receptors

• proteins incorporated into muscle cell plasma membrane 

  • Junctional fold of sarcolemma 

    • Increase surface area holding ACh receptors 

      • Lack of receptors can lead to paralysis 

components of neuromuscular junction - Basal lamina

• thin layer of collagen and glycoprotein separates Schwann cell and entire msucle cell from surrounding tissues 

  • Contains acetylcholinesterase (AChE) breaks down ACh after contraction causing relaxation 

Neuromuscular junction

• Toxin that interfere with synaptic function can paralyze muscles 

• Some pesticides contain cholinesterase inhibitors 

  • Bind to acetylcholinesterase and prevent it from degrading ACh 

  • Spastic paralysis - a state of continual contraction of muscles

    • Possible suffocation 

Neuromuscular junction - Tetanus

• (lockjaw) is form of spastic paralysis caused by toxin of clostridium tetani 

  • Glycine (inhibitory neurotransmitter) in the spinal cord normally stops motor neurons from producing unwanted muscle contractions 

  • Tetanus toxin block glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscle 

Neuromuscular toxins - Flaccid paralysis

• a state in which the muscles are limp and cannot contract 

  • Curare - plant poison used by South American natives to poison blowgun darts 

  • Compete with ACh for receptor sites, but do not stimulate the muscles  

Electrically Excitable Cells 

 

• Muscle fibers and neurons are electrically excitable cells 

  • Their plasma membrane exhibits voltage changes in response to stimulation  

• Electrophysiology - the study of electrical activity of cells 

in an unstimulated (resting) cell: 

• There are more anions (negative ions) on the inside of plasma membrane than on the outside 

• The plasma membrane is electrically polarized (charged) 

• There are excess sodium ions (Na+) in the extracellular fluid (ECF) 

• There are excess potassium ions (K⁺) in the intracellular fluid (ICF) 

• Also in the ICF, there are anions such as proteins, nucleic acids, and phosphates that cannot penetrate the plasma membrane

• The inside of the plasma membrane is negatively charged by comparison to its outer surface 

Voltage (electrical potential)

a difference in electrical charge from one point to another 

Resting membrane potential (RMP)

of a myofiber is about -90mV

• Maintained by sodium potassium pump 

Stimulated (active) muscle fiber or nerve cell 

• Ion gates open in plasma membrane 

• Na⁺ instantly diffuses down its concentration gradient into the cell 

• These actions override the negative charges in the ICF 

• Immediately, Na⁺ gates close and K⁺ opens 

• K⁺ rushes out of cell 

  • Repelled by the positive sodium charge and partly because of its concentration gradient 

Depolarization

• inside of the plasma membrane becomes briefly positive 

repolarization 

• Loss of a positive potassium ions turn the membrane negative again

Action potential

• quick up and down voltage shift from the negative RMP to a positive value, and back to the negative value again 

  • RMP is a stable voltage seen in a waiting muscle or nerve cell 

  • Action potential is a quickly fluctuating voltage seen in an active stimulated cell 

  • An action potential at one point on a plasma membrane causes another one to happen immediately in front of it, which triggers another one a little father long and so forth 

4 major phases of contraction and relaxation 

• Excitation

• Excitation - contraction coupling

• Contraction

• Relaxation

Excitation

•  process in which n nerve action potential lead to muscle action potentials 

Excitation - contraction coupling

•  events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract 

Contraction

•  step in which the muscle fiber develops tension and may shorten