Biology 121 Anatomy and physiology

Biology 121Anatomy and physiology

Lecture 5Chapter 7

The Muscular System

VIII. The Muscular System A. Introduction 1. Kinds of Muscle (3 types). a. Skeletal-striated, voluntary b. Smooth-nonstriated, involuntary c. Cardiac-striated, involuntary 2. Number of skeletal muscles. a. Approximately 700 3. Purpose of skeletal muscle. a. Movement-by shortening (contraction)

usually in pairs b. Maintain posture c. Support soft tissue and organs d. Maintain body temperature

e. Guard entrances and exits

4. Properties of muscle tissue a. Excitability (irritability).-responds to

stimulation b. Contractility-shortens or contracts upon

stimulation c. Elasticity – can be stretched.

C. Structure of Skeletal muscle-Skeletal muscle is an organ composed of

a. muscle tissue (muscle fibers or cells)

b. connective- binds muscle forms tendons

c. Blood vessels and nerves-provide oxygen and nutrients-nerves transmit signals from brain/ sp. cord

C. Structure of a Skeletal Muscle continued (bundles of bundles) 1. Muscle –covered with connective tissue: epimysium. epimysium=collagen separates muscle from surrounding organs 2. perimysium-divides muscle into compartments of muscle fibers (facscicles)- this layer has blood vessels and nerves3. Each individual muscle fiber (cell) within a fascicle are bound to adjacent fibers with endomysium

Epimysium, perimysium, endomysiumCome together to form tendons

Special Structures of muscle fiber (cell)

• Sarcolemma=cell membrane, conduct impulses from a nerve

• Transverse tubules=openings in the cell membrane, conduct nervous impulses into cell interior

• Sarcoplasmic reticulum=ER, does job of ER in regular cell but in muscle stores calcium.– Releases calcium into sarcoplasm when stimulated from nervous

impulse– Pumps calcium back into SR when muscle relaxes

• Myofibrils=bundles of myofilaments (thick and thin) that are responsible for muscle contraction

Muscle cell ultrastructure

Sarcolemma = cell membrane

sarcoplasm =cytoplasm

Transverse tubules (T tubules)Transmit nervous stimulus (action

potential) from sarcolemma through cell

Allows entire muscle fiber to contract

simultaneouslySarcoplasmic reticulumCalcium store: releases calcium when stimulated causes myofilaments to interact

Muscle myofibrils• Myofibrils– Made up of bundles of protein filaments

(myofilaments):• Myofilaments are responsible for muscle contraction

– Types of myofilaments:• Thin filaments:

– made of the proteins actin +troponin/tropomyosin• Thick filaments:

– made of the protein myosin

MyofilamentsThin and Thick Filaments

– Thin filaments:• actin (filamentous actin):

– two twisted rows has a myosin binding site • Tropomyosin;

– prevents actin–myosin interaction • Troponin:

– controlled by Ca2+

– Thick filaments:• Contain twisted myosin subunits • Tail:

– binds to other myosin molecules • Head:

– made of two globular protein subunits– reaches the nearest thin filament

Myosin can bind and pull on actin

Overlapping arrangement of myofilaments is called a sarcomere=the unit of contraction • Hundreds of sarcomeres make up a single myofibril• Sarcomeres shorten upon contraction causing the muscle cell

to shorten

ActinMyosin

Pattern of light and dark is called??

Myofibril Contraction • In the absence of calcium myosin and actin cannot interact• In the presence of calcium Myosin binds to Actin and begins

contraction• How?

– Ca2+ binds to receptor on troponin-tropomyosin complex which

lies on actin

– Troponin–tropomyosin complex changes position

– Exposes a myosin binding (interaction) site

– Myosin interacting with actin causes the sarcomere to shorten

(contract)

The Contraction Cycle=myosin interacting with actin

• Five Steps of the Contraction Cycle

– Exposure of active sites

– Formation of cross-bridges

– Pivoting of myosin heads

– Detachment of cross-bridges

– Reactivation of myosin

Molecular Events of the Contraction Process

Calcium binds troponin and causing tropomyosin to move aside

This exposes the myosin binding site on actin

Molecular Events of the Contraction Process

Figure 7-5

Myosin bindsactin

Molecular Events of the Contraction Process

Figure 7-5

Myosin-actin releases ADP causes myosin head to pull on actin

Molecular Events of the Contraction Process

Figure 7-5

ATP binds to Myosin enabling myosin to release actin

NOTE: ATP IS REQUIRED TO RELEASE MYOSIN CROSSBRIDGE

Molecular Events of the Contraction Process

Figure 7-5

ATP is split causing myosin to “cock”

Sarcomere Shortening

Figure 7-3 A band stays the same length, H and I bands become smaller

Sarcomere Shortening

Figure 7-3

The Contraction Cycle• Relaxation– Ca2+ concentrations decrease (by SR calcium pump

moving calcium back into the Sarcoplasmic reticulum)

– Ca2+ detaches from troponin– Active sites are re-covered by tropomyosin

Initiation of contraction requires nervous system impulses this

occurs at neuromuscular junctions

D. Muscle Activity by nervous control 1. Motor unit – A nerve fiber and all the muscle fibers it stimulates. 2. Neuromuscular junction – location where the nerves motor end plate contacts the muscle fiber.

3. Neurotransmitter and action potential (impulses) a. Charges at membrane 1.) Resting potential -70 mV (millivolts) a.) More Na+ outside cell than within b.) More K+ inside than out b. The sodium-potassium exchange pump uses ATP to move Na+ out and K+ in to repolarize the cell

c. Depolarization-action potential-repolarization (-70mV +30) (+30mV -70)

-Action potentials are electrical currents (Ions) that flow across a membrane through ion channels

-The channels are normally closed and Na,K pump establishes resting potential (-70mV)

-An action potential causes ion channels to open allowing ions to flood into the cell . Result: the membrane to become temporarily +30mV

-The flip in potential causes nearby channels to also open = spread of the action potential down the cell membrane

After a period of time the Na K pump restores the resting potential

1 motor nerve controls 100’s of muscle cells

No fine control ut lots of force

Common in muscles of the back

1 nerve controls a few muscle cells

Fine control

Common in eye

Motor endplate

Nerve fiber

The Neuromuscular Junction• Action potential (electrical signal) nervous impulse

– Travels along nerve axon

– Ends at synaptic terminal:

• Synaptic terminal:

– releases neurotransmitter (acetylcholine or ACh)

– into the synaptic cleft (gap between synaptic

terminal and motor end plate)

Structure and Function of the Neuromuscular Junction

Figure 7-4 b

Structure and Function of the Neuromuscular Junction

Figure 7-4

Structure and Function of the Neuromuscular Junction

Figure 7-4

Structure and Function of the Neuromuscular Junction

Figure 7-4

4. Mechanism of contraction a. Impulse and T-tubules b. Calcium release and its role c. ATP and its role with actin and myosin d. Shortening of sarcomere

1. Impulse spreads from nerve across sarcolemma

2. Impulse enters t tubule

3. Impulse spreads from t tubule into SR causing Ca release

5. Chemistry of contraction a. Source of ATP 1.) The liver stores glucose as animal starch or glycogen 2.) The Kreb’s cycle and ETS may provide sustained energy as ATP if sufficient oxygen is available. 3.) Glycolysis may anaerobically supply small bursts of ATP energy quickly 4.) Phosphocreatine (PC) may recharge ATP for 5-10 second bursts of energy PC + ADP + P ATP + creatine (C)

b. Lactic acid and oxygen debt 1.) If O2 is lacking, pyruvate lactic acid causing muscle fatigue, burning, soreness 2.) To recover you stop activity, deliver O2 to cells to make ATP. Lactic acid leaves muscles and may be converted back to pyruvate, glucose or glycogen in the liver, (reverse glycolysis).

E. Types of Muscle Contraction 1. Tonic – partially contracted muscle, different motor groups contracting at different times. 2. Isotonic – shortening of cells, no increase in tension. 3. Isometric – no shortening, but increase in tension. 4. Tetanic – maximal, sustained contraction due to rapid-firing stimuli. Muscle never gets to relax.

5. Twitch – contraction due to a single stimulus.

F. Origin, Insertion, and action of muscle 1. Origin – attached end of muscle that does not move. 2. Insertion – attached end of muscle that moves. 3. Action – movement produced by the muscle. a. Prime mover (agonist) – muscle mostly responsible for movement. b. Antagonist - muscle whose action opposes another. c. Synergist – muscle that helps a prime mover. May add pull to prime mover near insertion. 1.) Fixator – a synergist that stabilizes the origin of a prime mover.

4. FLEXION OF ELBOW: a. Prime Mover – biceps brachii b. Antagonist – triceps brachii c. Syngergist – brachialis

5. EXTENSION OF ELBOW: a. Prime Mover – triceps brachii b. Antagonist – biceps brachii c. Fixator - brachialis

G. Naming Muscles 1. Action – adductor, abductor, extensor, flexor. 2. Shape – Deltoid, orbicularis, serratus. 3. Location – anterior, external, rectus. 4. Origin or Insertion points – biceps, triceps, quadriceps. 5. Number of units makeup – (same as above). 6. Size – maximus, minimus. 7. Direction of fibers – oblique, rectus, transverse.