Muscle Function
• Muscle plays four important roles in the body:
– Produces movement
– Maintains posture
– Stabilizes joints
– Generates heat
Producing Movement
• Movements of the human body are a result of
muscle contraction
• Mobility of the body reflects the activity of
skeletal muscles
• Skeletal muscle is responsible for all
locomotion
• Allows for response to stimuli and emotional
expression
Maintaining Posture/Stabilizing Joints
• Skeletal muscle has to function continuously
in or to maintain posture.
• Similar to involuntary action
• Stabilizing joints
– As skeletal muscle contract and pull on bones to
cause movement, they also stabilizes joints
– Tendons are important in stabilizing joints with
poor articulating surfaces (shoulder)
Generating Heat
• A by-product of muscle activity
• About 75% of energy during muscle
contraction escapes as heat
• Skeletal muscle is most responsible for
generating heat (40% body mass)
Skeletal Muscle Activity
• Irritability
– the ability to receive and respond to a stimulus
• Contractility
– The ability to shorten (forcibly) when an adequate
stimulus is received
• Skeletal muscle cells must be stimulated by nerve
impulses in order to contract.
• A single motor neuron (nerve cell) may stimulate
anywhere from a few to hundreds of cells
• One neuron and all the skeletal muscle cells it
stimulates are a motor unit
• Neuron (nerve fiber/axon) reaches the muscle
and branches into axonal terminals that form
junctions with the sarcolemma of different
cells (neuromuscular joints).
• The nerve endings and cells never touch, the
gap between the two are called a synaptic
cleft and is filled with fluid (interstitial)
• Neurotransmitter
– A chemical that is released when a nerve impulse reaches
the axonal terminals
– Acetylcholine – neurotransmitter that stimulates skeletal
muscle cells ACh
• ACh diffuses across the synaptic cleft and attaches to
receptors (membrane proteins) and on part of the
sarcolemma
• If enough ACh is released the sarcolemma becomes
temporarily permeable to sodium ions that rush into
the muscle cell.
• This causes an excess of positive ions inside the cell
and generates an electrical current (action potential)
• The action potential travels over the sarcolemma to
either end of the cell and results is muscle contraction
Oxygen Debt
• Not being able to take in enough oxygen to keep the
muscles supplied when they are working strenuously
• In this case, blood supply aids in reducing fatigue
• A lack of oxygen causes lactic acid begins to
accumulate and ATP levels decrease
• This combination results in less effective contractions
and finally stop all together
• True muscle fatigue is rare
• Oxygen debt always occurs to some extent during
vigorous muscle activity
Muscle Fatigue and Oxygen Debt
• Muscle Fatigue
– Occurs when a muscle is unable to contract, even
when it’s being stimulated
– Without rest, active/working muscles begins to
tire and contracts more weakly until it finally fails
to contract
– Results from oxygen debt that occurs during
prolonged muscle activity
Types of Muscle Contractions
• Isotonic – same tone/tension
– Most familiar
– Myofilaments successful in sliding movements, muscle
shortens and movements occurs
– Examples: bending of elbow, knee; rotating ankle,
wrist, smiling
• Isometric – same measurement/length
– Contractions in which the muscles do not shorten
– Myofilaments can’t slide and move due to
immoveable object, therefore the tension builds in
the muscle