Salient features and functions of bones of axial and appendicular skeletal
system:
The skeletal system can be divided into two major
components: the axial skeleton and the appendicular skeleton. Each component
has its own set of bones with distinct features and functions. Here are the
salient features and functions of the bones in the axial and appendicular
skeletal systems:
Axial Skeleton:
- Skull: The skull protects the brain and houses
the sensory organs of the head, including the eyes, ears, and nose. It
consists of various bones, such as the cranium and facial bones.
- Vertebral Column: The vertebral column, or
spine, provides support, stability, and flexibility to the body. It
consists of individual vertebrae separated by intervertebral discs, which
allow for movement and shock absorption.
- Rib Cage: The rib cage is composed of the
ribs, sternum, and thoracic vertebrae. It encloses and protects the organs
of the thoracic cavity, including the heart and lungs. The ribs also play
a role in respiration.
- Hyoid Bone: The hyoid bone is a U-shaped bone
located in the neck. It supports the tongue and provides attachment points
for various muscles involved in swallowing and speech.
- Sacrum and Coccyx: The sacrum and coccyx are
fused bones at the base of the vertebral column. They provide support and
stability to the pelvis.
Appendicular Skeleton:
- Upper Limbs: The bones of the upper limbs
include the humerus (upper arm bone), radius and ulna (forearm bones),
carpals (wrist bones), metacarpals (hand bones), and phalanges (finger
bones). They provide mobility and dexterity to the arms and hands.
- Lower Limbs: The bones of the lower limbs
consist of the femur (thigh bone), tibia and fibula (leg bones), tarsals
(ankle bones), metatarsals (foot bones), and phalanges (toe bones). They
support the weight of the body, provide stability, and allow for movement
and locomotion.
- Pectoral Girdle: The pectoral girdle consists
of the scapulae (shoulder blades) and clavicles (collarbones). It connects
the upper limbs to the axial skeleton and allows for movement and
flexibility of the arms.
- Pelvic Girdle: The pelvic girdle is formed by
the hip bones (ilium, ischium, and pubis) and sacrum. It supports the
weight of the body, protects the pelvic organs, and provides attachment
points for muscles involved in walking and standing.
- Joints: Both the axial and appendicular
skeletons have joints, which are points where bones come together. Joints
allow for movement and flexibility, and they can be classified as fibrous,
cartilaginous, or synovial joints depending on their structure and
function.
These are some of the salient features and functions of the bones in the
axial and appendicular skeletal systems. Each bone and region plays a crucial
role in providing support, protection, and enabling various movements and
functions in the human body.
Organization of skeletal muscle:
Skeletal muscles are organized in a hierarchical
manner, consisting of various levels of structural organization. Here is the
organization of skeletal muscle from the largest to the smallest components:
- Muscle: The entire muscle is the largest unit
and is composed of bundles of muscle fibers, connective tissues, blood
vessels, and nerves.
- Fascicles: Muscle fibers are grouped together
into fascicles, which are cylindrical bundles surrounded by connective
tissue called perimysium. Fascicles contain multiple muscle fibers that
work together to generate force.
- Muscle Fibers: Muscle fibers, also known as
muscle cells or myofibers, are the individual contractile units within the
muscle. They are long, multinucleated cells that span the length of the
muscle. Each muscle fiber is surrounded by connective tissue called
endomysium.
- Myofibrils: Within each muscle fiber,
myofibrils are densely packed, cylindrical structures that extend the
entire length of the fiber. Myofibrils are composed of repeating units
called sarcomeres, which are responsible for muscle contraction.
Sarcomeres contain actin and myosin filaments arranged in a highly
organized pattern.
- Sarcomeres: Sarcomeres are the basic
contractile units of muscle. They are defined by the Z-lines at each end
and contain actin and myosin filaments. The arrangement of these filaments
gives skeletal muscle its characteristic striped or striated appearance.
- Myofilaments: Actin and myosin are the two
types of myofilaments present within the sarcomeres. Actin filaments are
thin and anchored to the Z-lines, while myosin filaments are thicker and
located in the center of the sarcomere. The interaction between actin and
myosin filaments allows for muscle contraction.
- Proteins: Numerous proteins are involved in
the structure and function of skeletal muscle. Some notable proteins
include titin, which provides elasticity to muscle fibers, and troponin
and tropomyosin, which regulate the interaction between actin and myosin
during muscle contraction.
- Myofibrillar Proteins: Within the sarcomeres,
myofibrillar proteins are responsible for the organization and stability
of the contractile apparatus. These include myosin, actin, tropomyosin,
troponin, and other associated proteins.
This hierarchical organization of skeletal muscle allows for efficient
force generation and coordinated muscle contraction. The interaction between
actin and myosin filaments, regulated by various proteins, enables the sliding
of filaments, resulting in muscle contraction and movement.
Physiology of muscle contraction:
Muscle contraction is a complex physiological
process that involves the activation of muscle fibers, the generation of force,
and the shortening of muscle length. It relies on the interaction between actin
and myosin filaments within the muscle cells. Here is a simplified explanation
of the physiology of muscle contraction:
- Neuromuscular Junction: The process of muscle
contraction is initiated by a nerve impulse that travels from the central
nervous system to the muscle. At the neuromuscular junction, the nerve
ending releases a neurotransmitter called acetylcholine, which binds to
receptors on the muscle fiber.
- Excitation-Contraction Coupling: When
acetylcholine binds to the receptors on the muscle fiber, it triggers an
electrical impulse that spreads along the muscle cell membrane
(sarcolemma) and into the T-tubules. The T-tubules are invaginations of
the sarcolemma that allow the electrical impulse to reach deep into the
muscle fiber.
- Calcium Release: The electrical impulse in the
T-tubules causes the sarcoplasmic reticulum (a specialized type of
endoplasmic reticulum in muscle cells) to release calcium ions (Ca2+) into
the muscle cell's cytoplasm (sarcoplasm). Calcium ions are essential for
muscle contraction.
- Cross-Bridge Formation: When calcium ions are
present in the sarcoplasm, they bind to the regulatory proteins (troponin
and tropomyosin) associated with the actin filaments. This binding causes
a conformational change in the regulatory proteins, exposing binding sites
on the actin filaments.
- Sliding Filament Mechanism: With the binding
sites exposed, myosin heads on the thick myosin filaments can form
cross-bridges with the actin filaments. The myosin heads undergo a series
of conformational changes, pulling the actin filaments toward the center
of the sarcomere. This sliding filament mechanism results in the shortening
of the sarcomeres and the overall muscle contraction.
- ATP Hydrolysis: The energy required for muscle
contraction comes from adenosine triphosphate (ATP). ATP binds to the
myosin heads, which energizes them and allows them to attach to the actin
filaments. ATP is then hydrolyzed (broken down) into adenosine diphosphate
(ADP) and inorganic phosphate (Pi), releasing energy that powers the
myosin heads to perform the power stroke and generate force.
- Relaxation: When the nerve impulse stops,
acetylcholine is broken down, and the electrical stimulation ceases.
Calcium ions are actively pumped back into the sarcoplasmic reticulum,
reducing their concentration in the sarcoplasm. This causes the regulatory
proteins to reposition and block the binding sites on the actin filaments,
preventing further cross-bridge formation. As a result, the muscle
relaxes, and the actin and myosin filaments slide back to their original
positions.
This process of neuromuscular activation, calcium release, cross-bridge
formation, and sliding filament mechanism repeats rapidly to generate sustained
muscle contraction. The regulation of calcium ions, ATP availability, and the
interaction between actin and myosin are crucial for the physiology of muscle
contraction.
Neuromuscular junction :
The neuromuscular junction (NMJ) is a specialized
synapse where a motor neuron communicates with a muscle fiber. It plays a
critical role in transmitting signals from the nervous system to the muscle,
initiating muscle contraction. Here is an overview of the neuromuscular
junction and its key components:
- Motor Neuron: A motor neuron is a nerve cell
that carries signals from the central nervous system (brain or spinal
cord) to the muscle. Each motor neuron branches out to innervate multiple
muscle fibers, forming a motor unit.
- Synaptic Terminal: At the end of the motor
neuron, there is a specialized structure called the synaptic terminal,
also known as the presynaptic terminal or motor end plate. It contains
numerous small vesicles filled with a neurotransmitter called
acetylcholine.
- Synaptic Cleft: The synaptic cleft is a narrow
space that separates the synaptic terminal of the motor neuron from the
muscle fiber. It acts as a communication gap where chemical signals are
transmitted.
- Motor End Plate: The motor end plate is the
specialized region on the surface of the muscle fiber that lies directly
across the synaptic cleft from the synaptic terminal. It contains
receptors called acetylcholine receptors, which are specific proteins that
bind to acetylcholine.
- Acetylcholine (ACh): Acetylcholine is the
neurotransmitter released by the motor neuron at the neuromuscular
junction. When an action potential reaches the synaptic terminal,
acetylcholine is released into the synaptic cleft and diffuses across to
the motor end plate.
- Acetylcholine Receptors: Acetylcholine
receptors are located on the motor end plate of the muscle fiber. They are
ligand-gated ion channels that, when bound by acetylcholine, open up and
allow the flow of ions across the muscle cell membrane.
- Action Potential: When the acetylcholine
receptors are activated by acetylcholine binding, they allow sodium ions
(Na+) to enter the muscle fiber and potassium ions (K+) to exit. This
influx of positively charged ions generates an electrical impulse called an
action potential in the muscle fiber.
- Muscle Contraction: The action potential
spreads along the muscle fiber's membrane, eventually reaching the
sarcoplasmic reticulum (a specialized endoplasmic reticulum). This
triggers the release of calcium ions (Ca2+) into the sarcoplasm,
initiating the process of muscle contraction.
The neuromuscular junction is a crucial site for the transmission of
signals from the nervous system to the muscle, enabling the precise control and
coordination of muscle contractions. Disruptions in the function of the
neuromuscular junction can lead to various neuromuscular disorders, such as
myasthenia gravis, where the acetylcholine receptors are targeted by the immune
system, causing muscle weakness and fatigue.
