2. Definition of Terms
Action potential – all-or-none change in membrane potential
in an excitable tissue that is propagated as an electrical
signal
Sarcomeres – the structural and functional unit of a muscle
Myofibrils – fine, longitudinal fibril within a skeletal muscle
consisting of thick (myosin) and thin (actin)
myofilaments
Sliding filament model – mechanism by which actin and
myosin myofilaments slide over one another during
muscle contraction
3. Sarcolemma – cell membrane of a muscle fiber
T-tubule – small tubules which run transversely through a
striated muscle fiber and through which electrical
impulses are transmitted from the sarcoplasm to the
fiber's interior
Troponin - a protein of muscle tissue that binds calcium
ions and is involved in contraction.
Tropomyosin - a protein component of sarcomere
filaments, which, together with troponin, regulate
interactions of actin and myosin in muscle
contractions.
4. Cross-bridges – when a myosin attach to the myosin
binding site
Acetylcholine – a neurotransmitter substance released
from motor neurons that innervate skeletal
muscle fibers…
Sarcoplasmic reticulum – ER of a muscle fiber
Neuromuscular junction – synaptic junction between a
nerve axon and a muscle fiber
5. The Events of Muscular Contraction
Action
potential
Neuromuscular
Junction
Ca2+ channels open
Ca2+ enter
presynaptic cleft
ACh release
ACh bind to
ACh receptors
Na2+ channels open
Na2+ diffusion
K+ exit
Initiate
postsynaptic
cleft AP
6. Ca2+ release Ca2+ bind to troponin
exposes actin
attachment site
ATP attach to
myosin head
Hydrolysis of ATP to ADP and
inorganic phosphate
Myosin head
becomes active
Myosin head bind to the
actin attachment site
Cross - bridge
formation
ADP is released
7. Power stroke
Actin slides over myosin
Another ATP binds to
myosin head
Cross-bridge detachment
Myosin head detaches
ATP is hydrolyzed
to ADP & inorganic
phosphate
Reactivation of the
myosin head
Energy released
during hydrolysis
reactivates myosin
myofilament to
return to its resting
position
8. Muscle Twitch, Summation, Tetanus,
and Recruitment
Muscle Twitch – the contraction of a muscle fiber in response
to a stimulus.
9. Three Phases of Muscle Twitch
1. Lag phase – the time between the application of a
stimulus and the beginning of contraction.
2. Contraction phase – the time which the muscle
contracts
3. Relaxation phase – the time during which the
muscle relaxes
10. Summation - occurs when a muscle fiber is exposed
to a series of stimuli of increasing
frequency, and is therefore unable to
completely relax before the next stimulus
occurs. As a result, individual twitches
begin to combine, and the contraction
becomes sustained.
In summation, the force of contraction of individual
muscle fibers is increased by rapidly stimulating them.
When stimulus frequency, which is the number of times
a motor neuron is stimulated per second, is low, there
is time for complete relaxation of muscle fibers
between muscle twitches.
11. As stimulus frequency increases, there is enough time
between contractions for muscle fibers to relax
completely. Thus, one contraction summates, or is
added onto, a previous contraction.
Tetanus – a sustained contraction that occurs when the
frequency of stimulation is so rapid that no
relaxation occurs. The increases force of
contraction produced in summation and tetanus
occurs Ca2+ builds up in myofibrils, which promotes
cross-bridge formation and cycling. The build-up 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 into the
sarcoplasmic reticulum.
12. Recruitment - The activation of additional motor neurons
in response to sustained stimulation of a given
receptor or afferent nerve.
In recruitment, the number of muscle fibers contracting is
increased by increasing the number of motor units stimulated
and the muscle contracts with more force. When only a few
motor units are stimulated, a small force of contraction is
produced because only a small number of muscle fibers are
contracting. As the number of motor units stimulated
increases, more muscle fibers are stimulated to contract, and
the force of contraction increases. Maximum force of
contraction is produced in a given muscle when all the motor
units of that muscle are stimulated.
