Types of Muscle Tissue
• Under voluntary control
• Skeletal muscles
• The muscular system
• Under involuntary control
• Cardiac muscle
• Heart wall
• Smooth muscle
• Visceral organs
Muscular System Functions
• Skeletal muscles attach to bones directly or indirectly
• Perform five functions
• Produce movement of skeleton
• Maintain posture and body position
• Support soft tissues
• Guard entrances and exits
• Maintain body temperature
Anatomy of Skeletal Muscles
Gross Anatomy
• Connective tissue organization
• Epimysium
• Fibrous covering of whole muscle
• Perimysium
• Fibrous covering of fascicle
• Endomysium
• Fibrous covering of a single cell (a muscle fiber)
• Tendons (or aponeurosis)
Anatomy of Skeletal Muscles
Microanatomy of a Muscle Fiber
• Sarcolemma
• Muscle cell membrane
• Sarcoplasm
• Muscle cell cytoplasm
• Sarcoplasmic reticulum (SR)
• Like smooth ER
• Transverse tubules (T tubules)
• Myofibrils (contraction organelle)
• Sarcomeres
Anatomy of Skeletal Muscles
Sarcomere—Repeating structural unit of the myofibril
•
Components of a sarcomere
• Myofilaments
• Thin filaments (mostly actin with troponin and tropomyosin)
• Thick filaments (mostly myosin)
• Z lines at each end
• Anchor for thin filaments
Sarcomere:
A band: thick and thin filaments;
M line: clearer area with thick filaments only
I band: only thin filaments; Z line: darker area
Sarcomere: area between two consecutive Z lines
The Neuromuscular Junction
• Synaptic terminal
• Acetylcholine release
• Synaptic cleft
• Motor end plate
• Acetylcholine receptors
• Acetylcholine binding
• Acetylcholinesterase
• Acetylcholine removal
Motor end plate: folding of the sarcolemma at the meeting point between motor neuron and
muscle fiber
Neuromuscular junction: terminal end pf motor neuron + motor end plate + narrow place in
between
In cytoplasm of motor end plate at its terminal end, there are synaptic vesicles containing a
neurotransmitter: acetylcholine (Ach)
NERVE SUPPLY
Resting membrane potential
+++++++++++++++++++
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- - - - -- - - - - - - - -- - - - - - -
•
•
It a small voltage differential across the plasma membrane
When nerve cell or muscular fiber stimulated, R.M.P. is briefly changed: Na+ rushes into cell,
the inside becomes positive, then Na+ goes back out restoring R.M.P.
The brief reversal is called action potential
Some nerve cells carry the action potential to skeletal muscle fibers. They are called motor
neurons.
Motor unit: motor unit with the group of muscle fibers that is stimulates
Synaptic cleft: meeting point of motor neuron with muscular fiber( many mitochondria there)
SLIDING FILAMENT MECHANISM
•
•
The muscle contracts when the thin filaments slide across the thick filaments towards the
center of each sarcomere.
For this to happen, the following events must take place:
1-THE FIBER AT REST
o Ca++ stored in sarcoplasmic reticulum
o ATP bound to thick filament
o Thin filaments intact
2- ROLE OF THE STIMULUS
o ACh is released into synaptic cleft
o ACh binds to receptor molecules in the motor end plate of the muscle fiber
o Action potential is started in sarcolemma and passes along the membrane until it reaches the
sarcoplasmic reticulum
o Ca++ is released from the sarcoplasmic reticulum
3- MUSCLE CONTRACTION
o Ca++ diffuses in sarcoplasm and binds to thin filaments
o Binding sites on actin molecules are freed
o The heads of myosin (thick filaments) attach to binding sites: cross-bridges connections
o Cross-bride shift their angle causing the thin filaments to slide over thick filaments.
o
Energy ATP is needed for the last step. ATP is bound to myosin; Ca++ activates ATP
breakdown into ADP and PO4 -2
o Part of the energy produced is used to move bridges, the other part is released as heat,
increasing the body temperature.
o More ATP breaks the bond between thick and thin filaments in the cross bridge.
RIGOR MORTIS
o When a person dies, there is no more ATP available
o the cross-bridges are not released
o the muscles remain contracted causing a muscular rigidity.
Sliding-Filament Theory of Muscle Contraction
http://3dotstudio.com/zz.html
http://www.blackwellpublishing.com/matthews/myosin.html
4-RETURN TO REST
o Ca++ returns to sarcoplasmic reticulum
o Thin filaments return to their original position, closing off binding site to cross bridges
5- ENERGY FOR CONTRACTION
o Provided by ATP which is used up quickly.
o Glucose, creatine phosphate, glycogen and fats release additional ATP after they are
metabolically converted.
6-OXYGEN DEBT
o O2 is involved to synthesize ATP.
o Strenuous exercise needs more energy than what can be provided in the presence of the
available oxygen.
o The muscle will go into anaerobic fermentation producing lactic acid that accumulates in
muscle.
o Rapid breathing will get rid of lactic acid
o Lactic acid could lead to muscle fatigue leading to the inability of the muscle to contract
normally: cramp
MUSCULAR RESPONSE
All or none response
TYPES OF MUSCULAR CONTRACTIONS
TWITCH CONTRACTION
THE BASIC UNIT OF MUSCLE CONTRACTION.
Rapid response to a single stimulus slightly over the threshold.
It has three phases: a latent period, contraction period and relaxation period.
TREPPE CONTRACTION/STAIRCASE EFFECT
Single twitches progressively increasing in strength.
Enables muscle to warm up
WAVE SUMMATION
MUSCLE RECEIVES A SECOND STIMULUS BEFORE THE FIRST CONTRACTION
CYCLE IS COMPLETED
THE SECOND CONTRACTION IS STRONGER THAN THE FIRST ONE ETC…
TETANUS
INCOMPLETE TETANUS
20-30 stimuli/ second. It is the maximum contraction strength that can be sustained
Partial relaxation occurs between stimuli
COMPLETE TETANUS
35-50 stimuli/second
No more relaxation between stimuli
Twitches fuse together
Produces usual means of body movement
Provides for muscle tone: sustained contraction by a small number of fibers tightening muscle
without body movement, it maintains posture.
Measuring the force
PRODUCTION OF MOVEMENT
ORIGIN AND INSERTION
TENDONS ATTACH MUSCLE TO BONE
 THE ORIGIN IS THE MORE STATIONARY BONE
THE INSERTION IS THE MORE MOVABLE BONE.
MOVEMENT IS PRODUCED WHEN THE INSERTION IS PULLED TOWARDS THE
ORIGIN
GROUP ACTION
The coordinated response of a group of muscles is known as a group action. The group action
produces the movement
Includes: prime mover, antagonist, synergist and fixator
Prime movers
Cause the desired action
Ex. Biceps brachii contracts
ANTAGONISTS
Relaxe during the action
Ex. Triceps brachii relaxing
SYNERGISTS
Steady the movement
Ex. Other muscles of forearm contracting
FIXATORS
Stabilize the origin of the prime mover
Ex. Muscles of neck, back and shoulders
Muscle Tone
•
Muscle tone—Tension in a “resting” muscle produced by a low level of spontaneous motor
neuron activity. Distinct from resting tension produced by passive stretching.
•
Function of muscle tone
• Stabilizes bones, joints
• Prevents atrophy (muscle wasting )
Types of Contractions
•
Isotonic contraction
The tension (load) on a muscle stays constant (iso = same, tonic = tension) during a
movement. (Example: lifting a baby)
• Isometric contraction
The length of a muscle stays constant (iso = same, metric = length) during a
“contraction” (Example: holding a baby at arms length)
Muscle Elongation
•
Muscle contracts actively
• Muscles can only pull
• Muscles never push
•
Muscle elongates passively
• Elastic forces
• Contraction of opposing muscles
• Effects of gravity
Muscle Performance
Two Types of Skeletal Muscle Fibers
• Fast fibers
Large diameter, abundant myofibrils, ample glycogen, scant mitochondria. Produce
powerful, brief contractions
• Slow fibers
Smaller diameter, rich capillary supply, many mitochondria, much myoglobin. Produce
slow, steady contractions
Physical Conditioning
•
Anaerobic endurance
Time over which a muscle can contract effectively under anerobic conditions.
• Hypertrophy
Increase in muscle bulk. Can result from anerobic training.
• Aerobic endurance
Time over which a muscle can contract supported by mitochondria.
Muscle Performance
Key Note
What you don’t use, you lose. When motor units are inactive for days or weeks, muscle fibers
break down their contractile proteins and grow smaller and weaker. If inactive for long periods,
muscle fibers may be replaced by fibrous tissue.
Cardiac Muscle
Cardiac Muscle Tissue
• Small cells
• Single nucleus/cell
• Aerobic metabolism
• Intercalated discs
• Long contraction time
• Self-exciting (automaticity)
• No tetanic contraction
Smooth Muscle
• Nonstriated cells (no sarcomeres)
• Calcium control of contraction different from striated muscle
• Wide range of operating lengths
• Involuntary muscle
• Under hormonal or local control
• Pacesetter cells
• Motor neurons often unneeded
Aging and the Muscular System
Age-Related Reductions
• Muscle size
• Muscle elasticity
• Muscle strength
• Exercise tolerance
• Injury recovery ability