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mUSCLE STRUCTURE
aCTION POTENTIAL
1. Resting Membrane Potential – state where the cell membranes have a negative charge on the
inside relative to positive charge outside. This exists because of the following: NIPO
higher concentration of K+ on the inside of cell membrane
higher concentration of Na+ on the outside of cell membrane
many negatively charged molecules inside the cell
cell membrane is more permeable to K+ than Na+
ACTION POTENTIAL
2. Action Potential – brief reversal of the membrane charge that is carried rapidly along the cell
membrane. NOPI
Depolarization: change in charges where the Na+ channels open causing the sodium ions to
enter the inside of the cell membrane (becomes more positive). This increase in positive charge
changes the membrane potential to a value called threshold (weakest stimulus needed to
produce a response).
ACTION POTENTIAL
3. Repolarization: returns the cell to its resting membrane conditions & action potential ends. This
occurs due to the exit of K+ from the cell & closing of Na+ channels that stops the sodium
movement (NIPO).
NERVE SUPPLY AND MUSCLE FIBER
Motor Neurons – are specialized nerve
cells that carry action potentials to
skeletal muscles & stimulate to contract.
Motor Unit – group of muscle fibers that
motor neuron stimulates.
Neuromuscular Junction / Synapse –
where the neuron and muscle fibers
meet; consists of several enlarged axon
terminals (presynaptic terminal) resting
in indentations of muscle fiber’s
sarcolemma (postsynaptic membrane)
Synaptic Vesicle – located in each
presynaptic terminal that contains the
neurotransmitter called acetylcholine
(ACh) which stimulates or inhibits
postsynaptic cells.
NEUROMUSCULAR
JUNCTION
Acetylcholinesterase – an enzyme that breaks
down the ACh to prevent overstimulation of
muscle fibers.
mUSCULAR CONTRACTION
uSE OF atp FOR CONTRACTION
Adenosine Triphosphate (ATP) – a high-energy molecule produced
from the energy that is released during the metabolism of food which
is used for muscle contraction; breaks down to adenosine diphosphate
(ADP) & phosphate.
ATP is stored in myosin heads
ATP helps form cross-bridge formation between myosin and actin
New ATP must bind to myosin before cross-bridge is released
Rigor Mortis – stiffening of muscle fibers that occurs after death
resulting from the influx of calcium ions into the sarcomere (no ATP
is available to release cross-bridges)
mUSCLE TWITCH
a single contraction of muscle fiber in response to a stimulus and
has 3 phases:
Lag Phase / Latent Phase: time between the application of
stimulus and beginning of contraction
Contraction Phase: time during which the muscle contracts
Tetany : sustained contraction, no relaxation due to
constant stimulation
Recruitment : number of muscle fibers stimulated
increased by increasing number of motor units stimulated
more force
Relaxation Phase: time during which the muscle relaxes
→
2 Ways in Increasing the
Force of Muscle Contraction
1. Summation – individual muscles contract more forcefully
through rapid stimulation of muscle fibers preventing
relaxation or detachment of cross-bridges
Tetanus: a sustained contraction that occurs when the
frequency of stimulation is so rapid that no relaxation
occurs
2. Recruitment – more motor units are stimulated which
increases the total number of muscle fibers contracting
All or None Law: no in between in muscle contraction,
either the muscle contracts or does not
With each subsequent
stimulation, more crossbridges form due to Ca2+
buildup in myofibrils. The
buildup of Ca2+ occurs
because the rapid production
of action potentials in muscle
fibers causes Ca2+ to be
released from the
sarcoplasmic reticulum faster
than it is actively transported
back.
One contraction summates, or
is added onto, a previous
contraction, increasing the
overall force of contraction.
2 types of muscle fibers
Humans have both types of fibers.
Distribution of fibers is genetically determined. An active, healthy adult has an equal number of fastand slow-twitch fibers.
Neither type can be converted but capacity can be increased through intense exercise.
Energy Requirements for Muscle Contraction
a. This energy comes from either aerobic (requires oxygen) or anaerobic (no need of oxygen) ATP
production.
4 sources of atp
1. Aerobic production of ATP during most
exercise and normal conditions
breakdown of glucose in presence of oxygen to
produce CO2, H2O, and approx. 36 ATP molecules.
Lactate - produced by skeletal muscles all the time,
especially during exercise (alternate chemical form of
lactic acid)
e.g. TCA/Krebs Cycle, Glycolysis, Citric Acid Cycle
2. Anaerobic production of ATP during intensive
short-term work
does NOT require oxygen; breaks down glucose to
make 2 ATP & lactate (less efficient and accumulates
toxins)
4 sources of atp
3. Conversion of a molecule called creatine
phosphate to ATP
1 Creatine phosphate = 1 ATP + 1 Creatine
– an energy-storage molecule from excess ATP
which is broken down quickly to directly
synthesize ATP in contracting muscle fibers
4. Conversion of two ADP to one ATP and one
AMP (adenosine monophosphate) during heavy
exercise
FQ time!
The ______ is the functional unit of skeletal
muscle because it is the smallest portion capable
of ___________.
a. muscle fiber, receiving a stimulus
b. muscle fiber, contracting
c. sarcomere, contracting
d. sarcomere, receiving a stimulus
FQ time!
All of the following describe the characteristics of
acetylcholine, except:
a. is the neurotransmitter at the
neuromuscular junction.
b. is broken down by acetylcholinesterase.
c. binds to receptors on the presynaptic
terminal.
d. causes an influx of sodium ions into the
muscle cell when bound to its receptor.
FQ time!
1. The myosin myofilaments are anchored into
the cytoskeleton of the cell by the __________.
a. M line
b. Z disks
c. A band
d. I band
e. H zone
FQ time!
1. Muscle fibers: (Select all that apply)
a. have a relatively high metabolic rate
b. depend upon large amounts of oxygen and
carbon dioxide
c. contain small numbers of mitochondria
d. preferentially use aerobic respiration
FQ time!
Which of the following statements is FALSE?
a. The resting potential of a cell develops because the cell
membrane is more permeable to K+ than it is to most other
ions.
b. The concentration of Na+ is higher inside the cell than outside.
c. Potassium ions only diffuse down their gradient until the
charge difference across the membrane is great enough
(electrical equilibrium) to prevent additional diffusion.
d. Potassium's tendency to diffuse out is opposed by the
attractive forces of the negative charge inside the cell.
e. The sodium-potassium exchange pump maintains the normal
resting concentrations of ions on either side of the cell
membrane.
FQ time!
In a resting cell, K+ ions have a higher concentration
______ the cell, which results in the net movement
of K+ _______ of the cell.
a. inside; out
b. outside; into
c. inside; into
d. outside; out
FQ time!
In skeletal muscle contraction and relaxation, calcium
ions:
a. bind to myosin myofilaments.
b. are returned to the sarcoplasmic reticulum by
active transport.
c. are released from the T tubules by active transport.
d. provide the energy for muscle contraction.
e. have both bind to myosin myofilaments and are
returned to the sarcoplasmic reticulum by active
transport as normal properties.
FQ time!
Pepper has been exerting effort to push the wall
as part of her exercise regimen in achieving good
muscle tone. As a student, she knows that there
is an increase in tension to muscles without
changing its length. This type of contraction is:
a. concentric
b. isotonic
c. eccentric
d. isometric
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