Chapter 11 Physiology of the Muscular System

1
Chapter 11Physiology of the Muscular System
2
Introduction

• Muscular system is responsible for moving the
  framework of the body
• In addition to movement, muscle tissue performs
  various other functions

3
General Functions

• Movement of the body as a whole or of its parts
• Heat production
• Posture

4
Function of Skeletal Muscle Tissue

• Characteristics of skeletal muscle cells
• Excitability (irritability)ability to be
  stimulated
• Contractilityability to contract, or shorten,
  and produce body movement
• Extensibilityability to extend, or stretch,
  allowing muscles to return to their resting
  length

5
Overview of the muscle cell

• Muscle cells are called fibers because of their
  threadlike shape
• Sarcolemmaplasma membrane of muscle fibers
• Sarcoplasmic reticulum (SR)
• Network of tubules and sacs found within muscle
  fibers
• Membrane of the sarcoplasmic reticulum
  continually pumps calcium ions from the
  sarcoplasm and stores the ions within its sacs
  for later release

6
Overview of the muscle cell

• Muscle fibers contain many mitochondria and
  several nuclei
• Myofibrilsnumerous fine fibers packed close
  together in sarcoplasm
• Sarcomere
• Segment of myofibril between two successive Z
  lines
• Each myofibril consists of many sarcomeres
• Contractile unit of muscle fibers

7
(No Transcript)
8
Overview of the muscle cell

• Striated muscle Dark stripes called A bands
  light H zone runs across midsection of each dark
  A band
• Light stripes called I bands dark Z line extends
  across center of each light I band
• T tubules
• Transverse tubules extend across sarcoplasm at
  right angles to long axis of muscle fiber
• Formed by inward extensions of sarcolemma
• Membrane has ion pumps that continually transport
  Ca ions inward from sarcoplasm
• Allow electrical impulses traveling along
  sarcolemma to move deeper into cell

9
Overview of the muscle cell

• Triad
• Triplet of tubules a T tubule sandwiched between
  two sacs of sarcoplasmic reticulum allows an
  electrical impulse traveling along a T tubule to
  stimulate the membranes of adjacent sacs of the
  sarcoplasmic reticulum

10
(No Transcript)
11
Myofilaments

• Each myofibril contains thousands of thick and
  thin myofilaments
• Four different kinds of protein molecules make up
  myofilaments
• Myosin
• Makes up almost all the thick filament
• Myosin heads are chemically attracted to actin
  molecules
• Myosin heads are known as cross bridges when
  attached to actin
• Actinglobular protein that forms two fibrous
  strands that twist around each other to form bulk
  of thin filament
• Tropomyosinprotein that blocks the active sites
  on actin molecules
• Troponinprotein that holds tropomyosin molecules
  in place

12
Myofilaments (cont.)

• Thin filaments attach to both Z lines (Z disks)
  of a sarcomere and extend partway toward the
  center
• Thick myosin filaments do not attach to the Z
  lines

13
The mechanism of contraction

• Excitation and contraction
• A skeletal muscle fiber remains at rest until
  stimulated by a motor neuron
• Neuromuscular junctionmotor neurons connect to
  sarcolemma at motor endplate (Figure 11-7)
• Neuromuscular junction is a synapse where
  neurotransmitter molecules transmit signals

14
Excitation and contraction

• Acetylcholineneurotransmitter released into
  synaptic cleft that diffuses across gap,
  stimulates receptors, and initiates impulse in
  sarcolemma
• Nerve impulse travels over sarcolemma and inward
  along T tubules, which triggers release of
  calcium ions
• Calcium binds to troponin, causing tropomyosin to
  shift and expose active sites on actin

15
Excitation and contraction

• Sliding filament model
• When active sites on actin are exposed, myosin
  heads bind to them
• Myosin heads bend, pulling the thin filaments
  past them
• Each head releases, binds to next active site,
  and pulls again
• Entire myofibril shortens

16
The mechanism of contraction

• Relaxation
• Immediately after Ca ions are released,
  sarcoplasmic reticulum begins actively pumping
  them back into sacs (Figure 11-3)
• Ca ions are removed from troponin molecules,
  shutting down contraction

17
Energy sources for muscle contraction

• Hydrolysis of ATP yields energy required for
  muscular contraction
• Adenosine triphosphate (ATP) binds to myosin head
  and then transfers its energy to myosin head to
  perform work of pulling thin filament during
  contraction
• Muscle fibers continually resynthesize ATP from
  breakdown of creatine phosphate (CP)

18
Energy sources for muscle contraction

• Catabolism by muscle fibers requires glucose and
  oxygen
• At rest, excess O2 in the sarcoplasm is bound to
  myoglobin
• Red fibersmuscle fibers with high levels of
  myoglobin
• White fibersmuscle fibers with little myoglobin
• Aerobic respiration occurs when adequate O2 is
  available

19
(No Transcript)
20
(No Transcript)
21
Energy sources for muscle contraction

• Anaerobic respiration occurs when low levels of
  O2 are available and results in formation of
  lactic acid
• Glucose and oxygen supplied to muscle fibers by
  blood capillaries
• Skeletal muscle contraction produces waste heat
  that can be used to help maintain set point body
  temperature

22
Twitch contraction

• A quick jerk of a muscle that is produced as a
  result of a single, brief threshold stimulus
  (generally occurs only in experimental
  situations)
• The twitch contraction has three phases
• Latent phasenerve impulse travels to the
  sarcoplasmic reticulum to trigger release of Ca
• Contraction phaseCa binds to troponin and
  sliding of filaments occurs
• Relaxation phasesliding of filaments ceases

23
Treppethe staircase phenomenon

• Gradual, steplike increase in the strength of
  contractions seen in a series of twitch
  contractions that occur 1 second apart
• Eventually, the muscle responds with less
  forceful contractions, and relaxation phase
  becomes shorter
• If relaxation phase disappears completely, a
  contracture occurs

24
Tetanussmooth, sustained contractions

• Multiple wave summationmultiple twitch waves are
  added together to sustain muscle tension for a
  longer time
• Incomplete tetanusvery short periods of
  relaxation occur between peaks of tension
• Complete tetanusthe stimulation is such that
  twitch waves fuse into a single, sustained peak

25
Muscle tone

• Tonic contractioncontinual, partial contraction
  of a muscle
• At any one time, a small number of muscle fibers
  within a muscle contract, producing a tightness
  or muscle tone
• Muscles with less tone than normal are flaccid
• Muscles with more tone than normal are spastic
• Muscle tone is maintained by negative feedback
  mechanisms

26
Graded strength principle

• Skeletal muscles contract with varying degrees
  of strength at different times
• Factors that contribute to the phenomenon of
  graded strength
• Metabolic condition of individual fibers
• Number of muscle fibers contracting
  simultaneously the greater the number of fibers
  contracting, the stronger the contraction
• Number of motor units recruited

27
Isotonic and isometric contractions

• Isotonic contraction
• Contraction in which the tone or tension within a
  muscle remains the same as the length of the
  muscle changes
• Concentricmuscle shortens as it contracts
• Eccentricmuscle lengthens while contracting
• Isotonicliterally means same tension
• All of the energy of contraction is used to pull
  on thin myofilaments and thereby change the
  length of a fibers sarcomeres

28
Isotonic and isometric contractions

• Isometric contraction
• Contraction in which muscle length remains the
  same while the muscle tension increases
• Isometricliterally means same length
• Most body movements occur as a result of both
  types of contractions

29
Cardiac Muscle Tissue

• Cardiac muscle
• Found only in the heart, forming the bulk of the
  wall of each chamber
• Also known as striated involuntary muscle
• Contracts rhythmically and continuously to
  provide the pumping action needed to maintain a
  constant blood flow

30
Cardiac Muscle Tissue

• Cardiac muscle resembles skeletal muscle but has
  specialized features related to its role in
  continuously pumping blood
• Each cardiac muscle contains parallel myofibrils
• Cardiac muscle fibers form strong, electrically
  coupled junctions (intercalated disks) with other
  fibers individual cells also exhibit branching
• Syncytiumcontinuous, electrically coupled mass
• Cardiac muscle fibers form a continuous,
  contractile band around the heart chambers that
  conducts a single impulse across a virtually
  continuous sarcolemma

31
Cardiac Muscle

• Cardiac muscle
• T tubules are larger and form diads with a rather
  sparse sarcoplasmic reticulum
• Cardiac muscle sustains each impulse longer than
  in skeletal muscle therefore, impulses cannot
  come rapidly enough to produce tetanus
• Cardiac muscle does not run low on ATP and does
  not experience fatigue
• Cardiac muscle is self-stimulating

32
Smooth Muscle Tissue

• Smooth muscle
• Smooth muscle is composed of small, tapered cells
  with single nuclei
• No T tubules are present, and only a loosely
  organized sarcoplasmic reticulum is present
• Ca comes from outside the cell and binds to
  calmodulin instead of troponin to trigger a
  contraction
• No striations, because thick and thin
  myofilaments are arranged differently than in
  skeletal or cardiac muscle fibers myofilaments
  are not organized into sarcomeres

33
Smooth Muscle Tissue

• Two types of smooth muscle tissue
• Single-unit (visceral)
• Gap junctions join smooth muscle fibers into
  large, continuous sheets
• Most common type forms a muscular layer in the
  walls of hollow structures such as the digestive,
  urinary, and reproductive tracts
• Exhibits autorhythmicity, producing peristalsis
• Multiunit
• Does not act as a single unit but is composed of
  many independent cell units
• Each fiber responds only to nervous input