Physiology of the Muscular
System Chapter 11
Anatomy & Physiology
Functions
• Movement
• Heat production
• Posture
Typical cells
Muscle cell=fiber
Plasma membrane Sarcolemma
Cytoplasm
Sarcoplasm
Endoplasmic
reticulum
Sarcoplasmic
reticulum (SR)
Many mitochondria
Multiple nuclei
Muscle cell structures not found
in other cells
• Myofibrils: bundles of very fine fibers
• Thick and thin myofilaments: very fine
fibers that make up myofibrils
• Sarcomere: segment of myofibril
between two Z lines; contractile unit
• T tubules: run transversely across
sarcoplasm at right angle to long axis of
the cell; transmit electrical impulses thru
cell
Myofilaments
• 4 protein molecules that make up
myofilaments: Myosin, actin, tropomyosin,
troponin
• Thin filaments: actin, tropomyosin,
troponin
• Thick filaments: mostly myosin
Contraction
• Motor neuron: nerve cell that
stimulates muscle cells
• Neuromuscular junction: motor
neuron connects to sarcolemma
• This connection is a synapse in
which there is a narrow gap across
which the neurotransmitter,
acetylcholine, is released
Neuromuscular Junction
Mechanism of contraction
Motor neuron  acetylcholine binds to
receptors on sarcolemma  impulse
travels along sarcolemma thru T tubules
to SR  Ca released from SR binds to
troponin exposing actin molecules in
thin filaments  myosin in thick
filaments bind to actin and pull thin
filaments to center of sarcomere
Mechanism of Relaxation
• Ca pumped back into SR  shuts
down the contraction  troponin can
again block actin’s active site
Energy for Muscle
Contractions
• ATP: adenosine triphosphate
• CP: creatine phosphate
Glucose & Oxygen
• Glucose stored in form of
glycogen in muscle
• Excess oxygen molecules in
sarcoplasm bound to myoglobin
Anaerobic respiration
• Allows body to avoid use of
oxygen in short term
• Produces lactic acid
• Accumulation of lactic acid in
muscles causes burning
sensation
Motor unit
• One motor neuron plus the
muscle fibers it attaches to
• The fewer the number of fibers
supplied by one motor neuron the
more precise the movements that
can be produced.
Myography
• Force from the contraction of a
muscle is recorded as a line that
rises & falls as muscle contracts
& relaxes
• For a muscle to contract, an
electrical stimulus of enough
intensity (threshold stimulus) is
applied to muscle
Twitch contraction
• Quick jerk of
muscle
• M. doesn’t contract
at moment of
stimulus
• 3 phases:
– Latent period
– Contraction phase
– Relaxation phase
Treppe: Staircase
Phenomenon
• Gradual step like increase in
strength of contractions that can
be observed in a series of twitch
contractions
Tetanus
• If a series of stimuli come in a rapid
enough succession, muscle doesn’t
have time to relax completely
• Smooth, sustained contractions
Tonic contraction
(or muscle tone)
• Continual partial contraction in a
muscle
• Important for maintaining posture
• Flaccid: less tone than normal
• Spastic: more tone than normal
Isotonic contraction
“equal tension”
• Tone or tension
within a muscle
remains the
same, length of
muscle changes
• The muscle
shortens
Isometric contraction
“same length”
• Muscle length
remains the same
while muscle
tension increases
• The muscle is
unable to shorten
Cardiac muscle
• Striated involuntary
• Cardiac m. fibers
form strong
electrically coupled
junctions: intercalated
discs
• Branching of
individual fibers
• T tubules are larger
than skeletal muscle
Smooth muscle
• Single nuclei, no T tubules
• Calcium for contraction comes from
outside the cell
• 2 types: visceral & multiunit
• Visceral: found in digestive, urinary,
reproductive tracts in hollow organs
• Multiunit: found in vessels, arrector pili