Muscular System High Yield Notes Flashcards
Three types of muscle
- smooth muscle,
- cardiac muscle
- skeletal muscle
Striated
means the muscle contains sarcomeres .
Cardiac muscle
contains intercalated discs, which are made of desmosomes (hold cells together) and gap junctions that connect the cytoplasm of cells together to allow ion exchange and electrical impulse propagation.
Skeletal muscle
is composed of many bundles
within bundles .
sarcolemma
is the muscle fiber’s cellular membrane, and it protects each muscle fiber.
sarcoplasm
is the cytoplasm of the muscle fiber and holds the myofibrils
Sarcomeres
inside of myofibrils are the functional unit of muscle fibers and shorten to cause muscle contraction.
Myofilaments
are contained within sarcomeres, divided into thin actin filaments and thick myosin filaments . These filaments slide past each other to shorten sarcomeres through the sliding filament model of muscle contraction .
Stimulation of a muscle contraction (4 steps)
- Action potential propagation reaches the end of a motor neuron’s axon.
- Acetylcholine is released as a neurotransmitter between the presynaptic motor neuron and postsynaptic skeletal muscle fiber at the neuromuscular junction.
- Acetylcholine binds to ligand - gated sodium channels , causing sodium to enter
the cell, which creates graded potentials on the muscle fibers. - The graded potentials trigger opening of
voltage-gated sodium channels , which may
produce action potentials on the muscle if the stimulus is large enough.
sarcolemma
is the cell membrane of striated muscle and contains T-tubules invaginations that quicken action potential propagation on the
muscle.
sarcoplasmic reticulum
is the endoplasmic reticulum of muscle fibers that releases stored calcium ions into the sarcoplasm through voltage-gated calcium channels when triggered by the depolarization of the muscle cell.
calcium ions
bind to troponin , which removes tropomyosin from the myosin-binding-sites on actin , allowing myosin to interact with actin and cause sarcomere shortening, via sliding filaments.
Cross bridge cycling
- Initiation: Calcium ions expose the
myosin-binding-sites on actin. - A cocked back, high energy myosin head
(containing ADP and P i ) forms a cross bridge with the actin. - The myosin head contracts and the power
stroke occurs, bringing the myosin head back to a low energy state and releasing ADP and P i . As a result, the sarcomere shortens. - A new ATP molecule binds to myosin, causing detachment of the myosin head from the actin filament.
- The myosin head is an ATPase , and it
hydrolyzes the ATP into ADP and P i . This
causes the myosin head to re-enter a cocked back, high energy state . (Return to Step 2 if calcium ions present). - Termination: Neuronal signaling from motor neurons ends. The sarcoplasmic reticulum pumps calcium back into itself, and troponin brings tropomyosin back to cover
Rigor mortis
occurs in dead animals when there is no ATP available to release myosin from the actin.
Z lines
are the ends of the sarcomeres.
Thin actin filaments
branch from the Z lines towards the middle of the sarcomere.
M lines
are the midpoints of the sarcomeres
Thick myosin filaments
branch from the M lines towards the ends of the sarcomere.
I band
is the area in the sarcomere where only actin filaments are present. (Mnemonic : “I” is a thin letter, representing thin actin filaments)
A band
is the area in the sarcomere where actin and myosin overlap.
H zone
is the area in the sarcomere where only myosin is present. (Mnemonic : “H” is a thick letter, representing thick myosin filaments)
Motor units
make up muscles; a motor unit refers to all the muscle fibers innervated by a single neuron .
Small motor units
include only a few muscle fibers and are used in precision movement.
Large motor units
include many muscle fibers that are innervated by a single neuron and are used in powerful movements.
twitch contraction
is the contraction of a muscle fiber through motor unit stimulation. Each twitch has the same size and duration .
all-or-none principle
which states that a depolarization will cause all the muscle fibers to twitch if it is above threshold potential but will not cause any twitching if the depolarization is below threshold potential.
Three phases of a twitch:
- Latent: action potential spreads over sarcolemma and T-tubules, signaling to
sarcoplasmic reticulum to release calcium. - Contraction: formation of cross bridges as a result of calcium ions binding to troponin . H zones shrink and muscle tension increases.
- Relaxation: calcium is pumped back into the sarcoplasmic reticulum, ending cross bridge cycling and decreasing muscle tension.
Summation
is the process by which twitches add up to create a larger overall contraction .
There are two types of summation:
- Wave summation (temporal summation)
- Motor unit summation
Wave summation (temporal summation)
depolarizing a motor unit again during the
relaxation phase. May cause tetanus , which
is when the muscle fibers cannot be further
stimulated due to a lack of relaxation.
Twitches blend together during tetany,
eventually causing fatigue (loss of muscle
contraction).
Motor unit summation
different motor units are stimulated at different times to produce the intended amount of muscle contraction. This is also known as the size principle of motor unit recruitment because smaller motor units are stimulated first before larger motor units come in to help.
muscle tone (muscle tonus) .
Weak and involuntary twitches in small motor unit groups contribute to maintaining muscle tone (muscle tonus). Fatigue is never reached because different motor units are stimulated at different times.
