Biology 121
Anatomy and physiology
Lecture 5
Chapter 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 nerves
3. Each individual muscle fiber (cell) within a fascicle are bound to
adjacent fibers with endomysium
Epimysium,
perimysium,
endomysium
Come 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
sarcoplasm =cytoplasm
Transverse tubules (T tubules)
Sarcolemma
= cell membrane
Sarcoplasmic reticulum
Calcium store: releases
calcium when stimulated
causes myofilaments to
interact
Transmit nervous stimulus (action
potential) from sarcolemma through cell
Allows entire muscle fiber to contract
simultaneously
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
Myofilaments
Thin 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
Myosin
Actin
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
Myosin binds
actin
Figure 7-5
Molecular Events of the Contraction Process
Figure 7-5
Myosinactin
releases
ADP
causes
myosin
head to
pull on
actin
Molecular Events of the Contraction Process
ATP binds to
Myosin enabling
myosin to
release actin
Figure 7-5
NOTE: ATP IS
REQUIRED TO
RELEASE
MYOSIN
CROSSBRIDGE
Molecular Events of the Contraction Process
ATP is split
causing
myosin to
“cock”
Figure 7-5
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
Nerve fiber
1 nerve
controls a few
muscle cells
1 motor nerve
controls 100’s of
muscle cells
Fine control
No fine control ut
lots of force
Common in
eye
Motor endplate
Common in
muscles of the
back
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
3. Impulse spreads
from t tubule into SR
causing Ca release
2. Impulse
enters t tubule
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 O is lacking, pyruvate
lactic acid
causing muscle fatigue, burning, soreness
2.) To recover you stop activity, deliver O to cells
to make ATP. Lactic acid leaves muscles and
may be converted back to pyruvate, glucose or
glycogen in the liver, (reverse glycolysis).
2
2
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.