SKELETAL MUSCLE
PREPARED BY MR. ABHIJIT DAS
Skeletal muscle is one of the three main
types of muscle tissue in the human body, alongside smooth muscle and cardiac
muscle. Here are some basic characteristics of skeletal muscle:
Voluntary
Muscle:
·
Skeletal muscles are
under voluntary control, meaning that their
contraction is consciously initiated and controlled by the individual.
Striated
Muscle:
·
Skeletal muscle is striated, meaning it has a striped appearance when
viewed under a microscope. This striation is due to the alternating arrangement
of dark and light bands of actin and myosin filaments.
Multinucleated:
·
Skeletal muscle cells,
also known as muscle fibers (or myofibers), are
multinucleated. This means that each muscle cell contains multiple nuclei. These nuclei are located at the
periphery of the cell.
Attached
to Bones:
·
Skeletal muscles are
typically attached to bones by tendons. When a muscle contracts, it pulls on
the attached bones, causing movement at the associated joint.
STRUCTURE OF SKELETAL MUSCLE:
Figure Credit: Jayashree Baidya
A fascicle is a bundle of muscle fibers, and the number of fascicles within one muscle varies depending on the muscle's size and function.
Figure Credit: Jayashree Baidya
Epimysium is the outermost layer of connective tissue that surrounds the entire muscle, perimysium surrounds bundles of muscle fibers known as fascicles, and endomysium surrounds individual muscle fibers within a fascicle.
Figure Credit: Jayashree Baidya
A myofibril is
a thread-like structure within a muscle fiber that contains the contractile
units responsible for muscle contraction.
Figure Credit: Jayashree Baidya
Inside a myofiber, myofibrils enable muscle contraction, the sarcoplasmic reticulum regulates calcium levels, myoglobin stores oxygen, and muscle glycogen provides a local energy source for muscular activity.
Figure Credit: Jayashree Baidya
PHYSIOLOGY OF MUSCLE CONTRACTION
Ø Muscle
Contraction Physiology:
·
Thin Filament:
Actin is a protein in the thin filament of muscle cells.
·
Thick Filament:
Myosin is a protein in the thick filament that interacts with actin.
·
Actin-Myosin Interaction:
When stimulated, myosin heads attach to actin, causing contraction.
Ø Neuromuscular
Junction (NMJ):
·
Acetylcholine Release:
Nerve signals release acetylcholine at NMJ, signaling muscle fiber activation.
Ø Intracellular
Calcium (Ca) Release:
·
Ca Release from SR:
Nerve signal triggers calcium release from sarcoplasmic reticulum (SR) in
muscle cells.
Ø Initiation
of Muscle Contraction:
·
Ca Binds to Troponin:
Released calcium binds to troponin, a protein on the thin filament.
· Muscle Contraction: This binding initiates a series of events leading to muscle contraction.
DISORDERS OF SKELETAL MUSCLES
1.
Muscular
Dystrophies: A group of genetic disorders characterized by progressive weakness and degeneration
of the skeletal muscles.
2.
Myopathies: Diseases in which the muscle
fibers do not function properly.
3.
Myasthenia
Gravis: An autoimmune
disorder where antibodies attack the neuromuscular junction, leading to muscle weakness.
4.
Myositis: Inflammation of
the muscles, which can be due to autoimmune diseases, infections, or drug side
effects.
5.
Cramps and
Spasms: Involuntary
contractions of a muscle that can be caused by overuse, dehydration, or
electrolyte imbalances.
6.
Tendinitis: Inflammation of a tendon,
which can be due to overuse or injury.
PROBABLE QUESTIONS FROM
THIS CHAPTER
1. STRUCTURE OF SKELETAL MUSCLE. (5 MARKS)
2. PHYSIOLOGY OF MUSCLE CONTRACTION. (5
MARKS)
3. WHAT IS NMJ (NEURO MUSCULAR JUNCTION)?
(1 MARK)
ANS: It is a specialized synapse or
connection between a motor neuron and a skeletal muscle fiber. At the
neuromuscular junction, nerve impulses are transmitted from the motor neuron to
the muscle fiber, leading to the initiation of muscle contraction.