Skeletal muscles contain connective tissue, blood vessels, and nerves. There are three layers of connective tissue: epimysium, perimysium, and endomysium. Skeletal muscle fibers are organized into groups called fascicles. Blood vessels and nerves enter the connective tissue and branch in the cell. Muscles attach to bones directly or through tendons or aponeuroses. Skeletal muscles maintain posture, stabilize bones and joints, control internal movement, and generate heat.
Skeletal muscle fibers are long, multinucleated cells. The membrane of the cell is the sarcolemma; the cytoplasm of the cell is the sarcoplasm. The sarcoplasmic reticulum (SR) is a form of endoplasmic reticulum. Muscle fibers are composed of myofibrils. The striations are created by the organization of actin and myosin resulting in the banding pattern of myofibrils.
acetylcholine (ACh)
neurotransmitter that binds at a motor end-plate to trigger depolarization
actin
protein that makes up most of the thin myofilaments in a sarcomere muscle fiber
action potential
change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
aponeurosis
broad, tendon-like sheet of connective tissue that attaches a skeletal muscle to another skeletal muscle or to a bone
depolarize
to reduce the voltage difference between the inside and outside of a cell’s plasma membrane (the sarcolemma for a muscle fiber), making the inside less negative than at rest
endomysium
loose, and well-hydrated connective tissue covering each muscle fiber in a skeletal muscle
epimysium
outer layer of connective tissue around a skeletal muscle
excitation-contraction coupling
sequence of events from motor neuron signaling to a skeletal muscle fiber to contraction of the fiber’s sarcomeres
fascicle
bundle of muscle fibers within a skeletal muscle
motor end-plate
sarcolemma of muscle fiber at the neuromuscular junction, with receptors for the neurotransmitter acetylcholine
myofibril
long, cylindrical organelle that runs parallel within the muscle fiber and contains the sarcomeres
myosin
protein that makes up most of the thick cylindrical myofilament within a sarcomere muscle fiber
neuromuscular junction (NMJ)
synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle fiber with receptors for the acetylcholine released by the terminal
neurotransmitter
signaling chemical released by nerve terminals that bind to and activate receptors on target cells
perimysium
connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle
sarcomere
longitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associated proteins involved in contraction
sarcolemma
plasma membrane of a skeletal muscle fiber
sarcoplasm
cytoplasm of a muscle cell
sarcoplasmic reticulum (SR)
specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca++
synaptic cleft
space between a nerve (axon) terminal and a motor end-plate
T-tubule
projection of the sarcolemma into the interior of the cell
thick filament
the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs
thin filament
thin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere
triad
the grouping of one T-tubule and two terminal cisternae
troponin
regulatory protein that binds to actin, tropomyosin, and calcium
tropomyosin
regulatory protein that covers myosin-binding sites to prevent actin from binding to myosin
voltage-gated sodium channels
membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na+ enters through the channel
Watch this video to learn more about macro- and microstructures of skeletal muscles. (a) What are the names of the “junction points” between sarcomeres? (b) What are the names of the “subunits” within the myofibrils that run the length of skeletal muscle fibers? (c) What is the “double strand of pearls” described in the video? (d) What gives a skeletal muscle fiber its striated appearance?
(a) Z-lines. (b) Sarcomeres. (c) This is the arrangement of the actin and myosin filaments in a sarcomere. (d) The alternating strands of actin and myosin filaments.
Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this video to learn more about what happens at the neuromuscular junction. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? Can you give an example of each? (c) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?
(a) It is the number of skeletal muscle fibers supplied by a single motor neuron. (b) A large motor unit has one neuron supplying many skeletal muscle fibers for gross movements, like the Temporalis muscle, where 1000 fibers are supplied by one neuron. A small motor has one neuron supplying few skeletal muscle fibers for very fine movements, like the extraocular eye muscles, where six fibers are supplied by one neuron. (c) To avoid prolongation of muscle contraction.
1. The correct order for the smallest to the largest unit of organization in muscle tissue is ________.
A) fascicle, filament, muscle fiber, myofibril
B) filament, myofibril, muscle fiber, fascicle
C) muscle fiber, fascicle, filament, myofibril
D) myofibril, muscle fiber, filament, fascicle
B
2. Depolarization of the sarcolemma means ________.
A) the inside of the membrane has become less negative as sodium ions accumulate
B) the outside of the membrane has become less negative as sodium ions accumulate
C) the inside of the membrane has become more negative as sodium ions accumulate
D) the sarcolemma has completely lost any electrical charge
A
1. What would happen to skeletal muscle if the epimysium were destroyed?
Muscles would lose their integrity during powerful movements, resulting in muscle damage.
2. Describe how tendons facilitate body movement.
When a muscle contracts, the force of movement is transmitted through the tendon, which pulls on the bone to produce skeletal movement.
3. What are the five primary functions of skeletal muscle?
Produce movement of the skeleton, maintain posture and body position, support soft tissues, encircle openings of the digestive, urinary, and other tracts, and maintain body temperature.
4. What are the opposite roles of voltage-gated sodium channels and voltage-gated potassium channels?
The opening of voltage-gated sodium channels, followed by the influx of Na+, transmits an Action Potential after the membrane has sufficiently depolarized. The delayed opening of potassium channels allows K+ to exit the cell, to repolarize the membrane.