11.2 Movement Flashcards
Skeletal system
Skeletons are a rigid framework that function to provide support and protection for body organs
Skeletons can be internal (endoskeletons) or external (exoskeletons) depending on the organism
Endoskeletons typically consist of numerous bones, while exoskeletons are comprised of connected segments
Skeletons provide a surface for muscle attachment and thus facilitate the movement of an organism
Bones and exoskeletons act as levers, moving in response to muscular contraction
Bones are connected to other bones by ligaments, and bones are connected to muscles by tendons
Muscular system
muscles deliver the force required to move one bone in relation to another
Joints
joints are the areas where two bones meet
most joints are mobile, allowing the bones to move
Ball-and-socket joint
such as the shoulder and hip joint
allow backward, forward, sideways, and rotating movements
Hinge joints
such as fingers, knees, elbows and toes
allowing only bending and straightening movements
Pivot joints
such as neck joints
allow limited rotating movements
Ellipsoidal joints
such as wrist joints
allow all types of movement except pivotal motion
Synovial joints
Synovial joints are capsules that surround the articulating surfaces of two bones (i.e. where the bones connect)
Joints function to maintain structural stability by allowing certain movements but not others
What are synovial joints composed of?
Synovial joints consist of three main components:
Joint capsule – Seals the joint space and provides stability by restricting the range of possible movements
Cartilage – Lines the bone surface to facilitate smoother movement, as well as absorbing shock and distributing load
Synovial fluid – Provides oxygen and nutrition to the cartilage, as well as lubrication (reduces friction)
as well as synovial membrane, ligaments, tendons, bursas and meniscus
Elbow joint structures
humerus - anchors muscle
radius - acts as forearm lever for bicep
ulna - acts as forearm lever for tricep
biceps - bends the forearm (flexion)
triceps - straightens the forearm (extension)
joint capsule - seals joint space and limits range of movement to promote stability
synovial fluid - provides food, oxygen and lubrication to the cartilage
cartilage - allows smooth movement, absorbs shock and distributes load
Muscles
Muscles connect to bones (via tendons) and contract to provide the force required to produce movement
The muscle connects a static bone (point of origin) to a moving bone (point of insertion)
Skeletal muscles exist in antagonistic pairs (when one contracts, the other relaxes) to enable opposing movements
Opposing movements may include: flexion vs extension, abduction vs adduction, protraction vs retraction, etc.
Antagonistic pairs in insect legs
Many types of insects (including grasshoppers and praying mantises) have hind legs that are specialised for jumping
The jointed exoskeleton of the hind leg is divided into three parts: femur (upper leg), tibia (middle leg) and tarsus (lower leg)
The femur and tibia are connected by two antagonistic muscles: flexor tibiae muscle and extensor tibiae muscle
When the flexor muscle contracts, the extensor muscle relaxes and the tibia and femur are brought closer together
This retracts the hind quarters in preparation for pushing off the ground
When the extensor muscle contracts, the flexor muscle relaxes and the tibia is pushed away from the femur
This extends the hind quarters and causes the insect to jump
Organisation of skeletal muscles
Skeletal muscles consist of tightly packaged muscular bundles (fascicles) surrounded by connective tissue (perimysium)
Each bundle contains multiple muscle fibres, which are formed when individual muscle cells fuse together
Muscle fibres contain tubular myofibrils that run the length of the fibre and are responsible for muscular contraction
The myofibrils can be divided into repeating sections called sarcomeres, each of which represent a single contractile unit
Muscle fibre structure
Each individual muscle fibre has the following specialised features designed to facilitate muscle contraction:
They are multinucleate (fibres form from the fusion of individual muscle cells and hence have many nuclei)
They have a large number of mitochondria (muscle contraction requires ATP hydrolysis)
They have a specialised endoplasmic reticulum (it is called the sarcoplasmic reticulum and stores calcium ions)
They contain tubular myofibrils made up of two different myofilaments – thin filament (actin) and thick filament (myosin)
The continuous membrane surrounding the muscle fibre is called the sarcolemma and contains invaginations called T tubules
Sarcomeres
Myofibrils consist of repeating contractile units called sarcomeres, which are made of two protein myofilaments
The thick filament (myosin) contains small protruding heads which bind to regions of the thin filament (actin)
Movement of these two filaments relative to one another causes the lengthening and shortening of the sarcomere
Each individual sarcomere is flanked by dense protein discs called Z lines, which hold the myofilaments in place
The actin filaments radiate out from the Z discs and help to anchor the central myosin filaments in place
The recurring sarcomeres produce a striated (striped) pattern along the length of the skeletal muscle fibres
The centre of the sarcomere appears darker due to the overlap of both actin and myosin filaments (A band)
The peripheries of the sarcomere appear lighter as only actin is present in this region (I band)
The dark A band may also contain a slightly lighter central region where only the myosin is present (H zone)
Drawing sarcomeres
The myosin filaments are the thick filaments and should be represented as being thicker than the actin filaments
The myosin filaments should include protruding heads (myosin heads form cross-bridge attachments with actin)
The striated banding pattern should be identified (A band = dark region ; I band = light region)
Process of muscular contractions
Depolarisation and calcium ion release
Actin and myosin cross-bridge formation
Sliding mechanism of actin and myosin filaments
Sarcomere shortening (muscle contraction)
Muscular contractions - depolarisation and Ca2+ release
An action potential from a motor neuron triggers the release of acetylcholine into the motor end plate
Acetylcholine initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules
Depolarisation causes the sarcoplasmic reticulum to release stores of calcium ions (Ca2+)
Calcium ions play a pivotal role in initiating muscular contractions
Muscular contraction - Actin and Myosin Cross-Bridge Formation
On actin, the binding sites for the myosin heads are covered by a blocking complex (troponin and tropomyosin)
Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads
The myosin heads then form a cross-bridge with the actin filaments
Muscular contraction - Sliding Mechanism of Actin and Myosin
ATP binds to the myosin head, breaking the cross-bridge between actin and myosin
ATP hydrolysis causes the myosin heads to change position and swivel, moving them towards the next actin binding site
The myosin heads bind to the new actin sites and return to their original conformation
This reorientation drags the actin along the myosin in a sliding mechanism
The myosin heads move the actin filaments in a similar fashion to the way in which an oar propels a row boat
Muscular contraction - Sarcomere Shortening
The repeated reorientation of the myosin heads drags the actin filaments along the length of the myosin
As actin filaments are anchored to Z lines, the dragging of actin pulls the Z lines closer together, shortening the sarcomere
As the individual sarcomeres become shorter in length, the muscle fibres as a whole contracts
Summary of muscular contractions
Action potential in a motor neuron triggers the release of Ca2+ ions from the sarcoplasmic reticulum
Calcium ions bind to troponin (on actin) and cause tropomyosin to move, exposing binding sites for the myosin heads
The actin filaments and myosin heads form a cross-bridge that is broken by ATP
ATP hydrolysis causes the myosin heads to swivel and change orientation
Swiveled myosin heads bind to the actin filament before returning to their original conformation (releasing ADP + Pi)
The repositioning of the myosin heads move the actin filaments towards the centre of the sarcomere
The sliding of actin along myosin therefore shortens the sarcomere, causing muscle contraction
State of contraction
The arrangement of myofilaments within a sarcomere give a skeletal muscle fibre a striated appearance
A sarcomere has a central darker region (A band) where actin and myosin filaments overlap
A sarcomere has peripheral lighter regions (I bands) where actin is present, but not myosin
When muscle fibres contract, actin filaments slide along the myosin, reducing the length of the lighter I bands
The movement of the actin filaments also reduces the width of the H zone, however the length of A bands do not change
actin
thin, thread-like proteins that are anchored to Z lines of sarcomeres
A band
region of sarcomere that contains thick (myosin and actin) filaments
Antagonistic pair
muscle pairs arranged to work against each other to move a joint
Tendons
attach muscle to bone
Ligaments
connect bone to bone, restricting movement at joints and helping prevent dislocation
Nerves function and types
stimulate muscles to contract at a precise time and extent, so that movement is co-ordinated.
There are two types of nerves:
- sensory neurons that take information from muscles and skin to the brain
- motor neurons that send electrical signals to the muscles to cause muscle contraction
Cartilage
at the joint the bones are covered with cartilage which is made up of cells and fibers and is wear-resistant
cartilage helps reduce the friction of movement
Synovial membrane
a tissue that lines the joint and seals it into a joint capsule
the synovial membrane secretes synovial fluid around the joint to lubricate it
Bursas
fluid-filled sacs between bones, ligaments of other adjacent structures help cushion the friction in a joint
Meniscus
a curved part of cartilage in the knees and other joints
Cardiac muscle cells
located in the walls of the heart, appear striated, and are under involuntary control
Smooth muscle fibers
are located in walls of hollow visceral organs, except the heart, appear spindle-shaped and are also under involuntary control
Skeletal muscle fibers
occur in muscles which are attached to the skeleton
they are striated in appearance and are under voluntary control
Dark bands
regions where actin and myosin filaments overlap and where cross-bridges are formed
H Zone
region of a sarcomere - middle of A band; thick filaments (myosin) only
I Band
region of sarcomere that contains thin filaments (actin only)
Light bands
the zone of thin filaments in the sarcomere that is not superimposed by thick filaments
Myofibrils
thick, thread-like proteins with heads that can bind to specialised site on actin filaments
