Muscle Tissue Flashcards
Skeletal muscle tissue
•Description
•Function
•Location
Description:
•long cylindrical multinucleated cells; obvious striations
Function:
•voluntary movement, locomotion, manipulation, facial expression, voluntary control
Location:
•in skeletal muscles attached to bones or occasionally to skin
3 types of muscle tissue
- skeletal
- cardiac
- smooth
Cardiac muscle tissue
•description
•function
•location
Description:
•branching, striated, generally uninucleated cells that interdigitate at specialized junctions
Function:
•as it contracts it propels blood into the circulation; involuntary control
Location
•the walls of the heart
Cardiac muscle tissue
•description
•function
•location
Description:
•branching, striated, generally uninucleated cells that interdigitate at specialized junctions
Function:
•as it contracts it propels blood into the circulation; involuntary control
Location
•the walls of the heart
Smooth muscle
•Description
•Function
•Location
Description:
•spindle shaped cells with central nuclei; no striations; cells arranged closely to form sheets
Function:
•propels substances or objects along internal passageways; involuntary control
Location:
•mostly in the walls of hallow organs
Special physical characteristics of muscle
Excitability
Contractility
Extensibility
Elasticity
Excitability
The ability to respond to certain stimuli by producing electrical signals (action potentials)
Contractility
The ability to shorten forcefully when adequately stimulated
Extensibility
The ability to be stretched or extended
Elasticity
The ability to recoil after being stretched
Muscle functions
- producing movement
- maintaining posture and body position
- stabilizing joints
- generating heat
Nerve supply
- Has a rich nerve supply
- Each muscle fibre is innervated by its own nerve ending
- allows precise neural control of the entire muscle during contraction
Blood supply
- has a rich blood supply because contracting muscle fibres uses a huge amount of energy and a more or less continuous delivery of oxygen and nutrients
- muscle capillaries are long and winding
Connective tissue sheaths
Support and reinforce the muscle as a whole
•epimysium
•perimysium
•endomysium
Epimysium
Surrounds the whole muscle
Perimysium
Surrounds the fascicles
Endomysium
Surrounds each fiber
Attachments of muscle
Skeletal muscles span joints and are attached to bones in at least two places
•during contraction the insertion (moveable bone) moves towards the origin (less/immovable bone)
Direct attachments
The muscles epimysium is fused to the bones periosteum
Indirect attachments
The muscles connective tissue sheaths are attached to the bone either as a rope like tendon or as a sheet like aponeurosis
What is the sarcolemma
What does it have
- Plasma membrane
* T tubules that help an electrical impulse to get to the cell
What is the sarcoplasm
The cytoplasm
What is different about the sarcoplasm in a skeletal muscle fiber
Contains large amounts of
•glycosomes: bags of glycogen
•myoglobin: red coloured protein stores oxygen
Specialized organelles
- myofibrils
- sarcoplasmic reticulum
- T tubules
Myofibrils
- rodlike, run the length of the cell
- myofibrils are composed of myofilaments that are organized into sarcomeres
- has repeating light and dark bands
Sarcomere
- Smallest contractile unit of muscle fibre
- the region between two Z disks
- aligned end-to-end like boxcar trains
Proteins that make up the thick and thin filaments
- Myosin
* Actin
What are the A bands thick filaments composed primarily of
Myosin protein
Myosin protein and the muscle fibres
Contains ATP binding sites as well as ATPase enzymes.
•During contraction they form cross-bridges with thin filaments that overlap with thick filaments
What are the I bands thin filaments primarily composed of
Actin protein
Actin protein and muscle fibres
- Has a myosin binding site which forms cross-bridges with to produce contraction
- Each thin filament is composed of two strands of actin molecules twisted together one end of the strand is firmly attached to the Z disk
What are two important regulatory proteins also present in thin filaments
•tropomysin
•troponin
Both help control the myosin-actin interactions involved in contraction
Tropomysin
Rod shaped protein that spirals around each thin filaments and helps stiffen and stabilize it.
•In relaxed muscle fibre it blocks the myosin binding sites on actin so that the myosin heads on the thick filaments cannot bind to the thin filaments
Troponin
Globular three polypeptide complex, •one part binds to actin •another binds to tropomysin •the third part binds to calcium ions During muscle contraction troponin is signalled by calcium ion to lift tropomysin off the thin filament exposing it's myosin binding sites to the thick filaments
Structural proteins involved in the contractile of muscles
- Titin
* dystrophin
Sarcoplasmic reticulum
And it’s major role
An elaborate network of smooth endoplasmic reticulum wrapped around each myofibril.
•to regulate intracellular levels of ionic calcium
•stores calcium and releases it on demand for contraction
T tubules
At each A band I band junction is a hole going deep into the cell coming close in contact with the sarcoplasmic reticulum called T tubules
What is the sliding filament theory of contraction
During contraction the thin filaments slide past the thick filaments.
How does the sliding filament theory work
When muscle fibres are stimulated by the nervous system, the myosin heads of the thick filaments latch onto the myosin binding sites of the actin molecules making up the thin filaments.
The sliding of filaments and the shortening of sarcomeres causes the shortening of the whole muscle fibre and the entire muscle
Overview of skeletal muscle contraction
3 events that must occur for a skeletal muscle fibre to contract
1st event is..
1) the fibre must be activated by a nerve ending causing a change in the fibres membrane potential
•action potential (AP) arrives at axon terminal at neuromuscular junction
•Ach released; binds to receptors on sacrolemma
•local change in membrane potential occurs (depolarization)
•local depolarization ignites AP in sarcolemma
Overview of skeletal muscle contraction
3 events that must occur for a skeletal muscle fibre to contract
2nd event is..
2) the fibre must generate and propagate an electrical current (action potential) along its sarcolemma and down its T tubules
Overview of skeletal muscle contraction
3 events that must occur for a skeletal muscle fibre to contract
3rd event is..
Ionic calcium must be released from the sarcoplasmic reticulum (SR)to trigger contraction
•SR releases ca2+; binds to troponin; myosin binding sites on actin exposed
•myosin heads to actin; contraction begins
Where do action potentials arise
The neuromuscular junction
What is a neuromuscular junction
The synapse between a somatic motor neuron and a skeletal muscle fiber.
What is a synapse
- Is a region of communication between two neuroma or a neuron and a target cell (ie skeletal muscle cell)
- synapses separate cells from direct physical contact, neurotransmitters bridge that gap
What neurotransmitter is at a neuromuscular junction
Acetylcholine (ACh)
What happens when a nerve impulse reaches the neuromuscular junction and acetylcholine is released?
Ach binds to sarcolemma receptors which causes a change in sarcolemma permeability leading to a change in membrane potential.
What first three events occur at the neuromuscular junction
1) action potential arrives at motor neurons axon terminal
2) voltage-gated ca channels open and ca enters the axon terminal
3) ca entry causes some synaptic vesicles to release acetylcholine by exocytosis
What last three events occur at the neuromuscular junction
4) Ach diffuses across the synaptic cleft and binds to receptors in sarcolemma
5) Ach binding opens ion channels that allow Na to diffuse into the cell and K to diffuse out of the cell; more Na enters than K leaves, producing a local change in the membrane potential (depolarization)
6) Ach effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase
What is the potential of the sarcolemma
It is polarized
A potential difference across the membrane with the inside negative.
Action potential
The act of releasing energy
Involves two steps:
•depolarization
•repolarization
Depolarization
The binding of Ach to Ach receptors in the sarcolemma opens ion channels that allow Na to diffuse into the cell and K out. More Na moves then K causing a change in the membrane potential called depolarization
Repolarization
- Depolarization quickly returning to resting state– negative on the interior relative to the exterior
- a result of the opening of membrane K channels, allowing K to diffuse rapidly out of the cell and repolarize it.
Excitation-contraction coupling
Describes the events that connect nervous excitation of a skeletal muscle fibre to its contraction.
Excitation-contraction coupling:
What happens when a nerve stimulates a muscle fibre
1)a nerve stimulating a muscle fibre sends an action potential along its sarcolemma down T tubules towards the sarcoplasmic reticulum
Excitation-contraction coupling:
What happens when the action potential reaches the SR
When the action potential reaches the sarcoplasmic reticulum’s terminal cisterns, voltage gated Ca release channels in the SR open
Excitation-contraction coupling:
What happens when the ca voltage gated channels open
Ca pours into the cytosol where it binds with troponin, causing it to change shape
Troponin moves tropomyosin away from the myosin-binding sites on actin
Myosin is now free to bind with actin-the contraction cycle begins
Excitation-contraction coupling:
Contraction cycle
Continues until Ca active transport pumps return Ca to the sarcoplasmic reticulum. When Ca2 levels drop in the cytosol, tropomyosin falls back onto actin’s myosin binding sites and the contraction cycle stops
The cross bridge cycle
A series of events during which myosin heads pull thin filaments toward the centre of the Sarcomere
Steps of the cross bridge cycle
1) myosin heads hydrolysis ATP and become reoriented and energized
2) myosin heads bind to actin, forming crossbridges
3) myosin crossbridges rotate toward centre of the Sarcomere
4) as myosin heads bind ATP the crossbridges detach from actin
The motor unit
A motor neuron and the muscle fibres it stimulates
The motor unit is the functional unit of skeletal muscle
One single motor unit may innervate..
As few as 10 or as many as 2000 muscle fibres with an average of 150 fibres being innervated by each motor neuron
The function of the motor unit
Stimulation of a single motor unit produces weak contraction of the entire muscle
Muscles that control precise movements consist of many small motor units, each consisting of a motor neuron and 10-20 muscle fibres
