muscles Flashcards
3 types of muscles :
smooth :
attached to hair follicles in skin
in walls of hollow organs- blood vessels
non striated appearance
skeletal muscle :
attaches to bone, skin or or fascia
striated with light and dark bands visible with scope
Voluntary control of contraction and relaxation
Cardiac muscle tissue
is only found in your heart, where it performs coordinated contractions that allow your heart to pump blood through your circulatory system.
action potentials in nerve muscles
entire muscle cell membrane vs on the axon of the neuron is involved
Resting membrane potential :
nerve : - 70mv
skeletal and cardiac muscle is closer to 90mV
duration:
nerve impuse is 1/2 to 2 msecs
muscle action potential lasts 1-5 msec for skeletal and 10-300 msecs for cardiac and smooth
fastest nerve conduction
transverse tubules
t tubules are navigation of the sarcolemma into the centre of the cell :
filled with extracellular fluid
carry muscle action potentials down the cell
Mitochondria lie in rows throughout the cell
near the muscle proteins that use ATP during contraction
sarcolemma , t tubules and sarcoplasm
skeletal muscle consists of fibres (cells) covered by a sarcolemma
the fibres contain t tubules and sarcoplasmic
t tubules are tiny navigations of the sarcolemma that quickly spread the muscle action potential
sarcoplasm in the muscle cell cytoplasm and contains a large amount of glycogen for energy production and myoglobin
sarcoplasmic reticulum (SR)
they system of tubular sacs similar to the smooth ER non-muscle cells
stores Ca2+ in a relaxed muscle
release of Ca2+ triggers a muscle contraction
events occurring after a nerve signal
arrival of nerve impulses at nerve terminal causes release of acetylcholine (ACh) from NMJ synaptic vesicles
ACh binds to receptors AChRon a muscle motor end plate opening the gated ion channels so that No can rush into can rush into the muscle cell
inside the muscle cell become ore positive triggering a muscle action potential that travels over the cell and down the T-tubules
the release of Ca2+ from the SR is triggered and the muscle cell will shorten and generate force
Acetylcholinervse breaks down the ACh attached to the receptors on the motor end plate so the muscle action potential will cease and the muscle will relax
proteins of a muscle
myofibrils are built 3 kinds of proteins :
contractile proteins
myosin and actin
regulatory proteins which turn contraction on and off
Troponin (consists of 3 subunits I,C,T that interact with calcium, actin and tropomyosin)
MYOSIN :
thick filaments are composed of myosin
each molecule resembles 2 golf clubs twisted together
myosin heads (cross bridges )extend towards the thin filaments
ACTIN:
thin filaments are made up of calcium, actin and tropomyosin
the myosin - binding binding site on each actin molecule is covered by tropomyosin in relaxed muscle
the thin filaments are held in place by Z lines - from one Z line to the next is a sarcomere
how does contraction begin ?
- nerve impulse reaches an axon terminal and synaptic vesicles release acetylcholine
- ACh diffuses to receptors (AChR) on the sarcolemma and Na+ channels open and Na+ rushes into the cell
- a muscle action potential spreads over sarcolemma and down into the transverse tubules
- SR - Ca2+ into the sarcoplasm
- Ca2+ binds to tropinin and causes a tropinin-tropomyosin complex to move and reveal myosin binding sites on the actin -
THE CONTRACTION CYCLE BEGINS
In a relaxed skeletal muscle , tropomyosin blocks the cross-bridge binding site on actin
Contraction occurs when calcium ions bind to troponin; this complex then pulls tropomyosin away from the cross bridge binding site
passage of an action potential along the transverse tubule opens nearby voltage gated calcium channels
the RYANODINE RECEPTOR located on the SR and calcium ions released into the cytosol bind to troponin
the calcium- troponin complex pulls tropomyosin off the myosin binding site of actin filament
contraction cycle
repeating sequence of events that cause thick and thin filaments to move past each other
4 steps to the contraction cycle :
ATP hydrolysis
Attachment of myosin to actin to form cross bridges Power stroke
Detachment of myosin from actin
cycle keeps repeating as long as there is ATP available and there is high ca2+ level near the filaments
relaxation
Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft
muscle action potential ceases
Ca2+ release channels close
the active transport pumps ca2+ back into storage in the SR
calcium binding protein (calsequesterin) helps to hold Ca2+ in the SR
tropomyosin- troponin complex recovers binding site on the actin
functions of ATP in skeletal muscle contraction
- Hydrolisis of ATP by myosin energises the cross-bridges providing the energy for force generation
- binding of ATP to myosin dissociates cross bridges bound to actin allowing the bridges to repeat their cycle of activity
- hydrolysis of ATP by Ca2+ :
ATPase in the SR provides the energy for the active transport of calcium ions into the reticulum lowering the cytosolic calcium to release level ending contraction and allowing muscle fibre to relax
pharmacology of the NMJ (neuromuscular junction synapse)
botulinum toxin blocks release of neurotransmitter at the NMJ so muscle contraction can’t occur:
bacteria found in improperly canned food
death occurs form paralysis of the diaphragm
curare (plant poison from arrows):
causes muscle paralysis by blocking the ACh receptors
used to relax muscle during surgery
neostigmine (anti-cholinesterase agent):
blocks the removal of ACh rom receptors so strengthens weak muscle contractions of the Myasthenia Gravis (MG)
MG:long term neuromuscular disease that leads to varying degrees of skeletal muscle weakness - its most commonly affected muscles are the eyelids, trouble talking and trouble walking
neurons - resting potential
the sodium potassium pump concentrates K+ ions on the inside and Na+ on the outside of the neuron
3Na+ out for every 2K+ in leading to a negative charge on the interior (inside)membrane
the interior of the membrane is made more negative due to the presence of PASSIVE K+ ION CHANNELS
these allow K+ ions to leak out leaving behind unbalances negative changes which thus leading to the resting potential
the resting membrane potential
A POTASSIUM EQUILIBRIUM POTENTAIL exists when electric charge that develops across the membrane is sufficient to prevent net diffusion of potassium ions down their conc gradient
at this equilibrium the inside of the cell is negative
THIS IS THE RESTING POTENTIAL OF THE NERVE CELL (APPROX- 60mV)
