Muscle and Neuron Function Flashcards
Structure of muscle
A muscle is a hierarchy of longitudinal units • 1 muscle = several fibre bundles • 1 bundle = several muscle fibres • 1 muscle fibre has several myofibrils • 1 myofibril has several actin and myosin filaments, arranged in end to-end sarcomeres
What happens when muscle contracts ?
thick filaments (myosin) move past thin filaments (actin) = Sliding-filament model • filaments remain the same length, but the sarcomeres shorten • muscle contraction cycle needs • actin binding site on myosin • myosin binding site on actin • ATP • Ca2+
Sliding filament model
proposed by Andrew Huxley and Ralph Niedergerke, and by Hugh Huxley and
Jean Hanson
• lengths of thick and thin filaments do not change during muscle contraction
• length of sarcomere decreases during contraction because two types of filaments
overlap more; thick and thin filaments slide past each other in contraction
• force of contraction generated by process that actively moves one type of filament
past neighbouring filaments of other type
Control of the contraction cycle
• at rest: myosin binding sites on actin are blocked by tropomyosin
• an action potential in a motor neuron induces the release of
acetylcholine into the neuromuscular junction
• voltage-gated Ca2+ channels in the sarcoplasmic reticulum release Ca2+
into the cytosol
• Ca2+ binds to troponin; tropomyosin is pulled off myosin binding sites
• Ca2+ is pumped back into the sarcoplasmic reticulum
Motor unit
- motor neuron and all the muscle fibres it controls • each muscle fibre (cell) synapses with only one motor neuron, but each motor neuron can synapse with many muscle fibres
Energetics ATP is needed ?
during the contraction cycle
• to pump Ca2+ from the cytosol back into the SR
Energetics source of ATP
ATP pool in muscle fibre
• only enough for a few contractions
Creatine phosphate
• phosphagen (energy storage) of vertebrates
• supplies phosphate group to ADP
Glycolysis
• splitting of glucose into pyruvate
• glucose stored as glycogen
Oxydative phosphorylation
• production of ATP using energy derived from the redox
reactions of an electron transport chain
• in mitochondria
Transmission of signals in neurons - Ion channels
• allow passive movement of ions across a membrane down the
electrochemical gradient
• specific for one ion
• may be gated: mechanical, ligand, voltage
• fast
Transmission of signals in neurons - Membrane potential
-membrane potential is the separation of charge across a
membrane
• electrochemical gradient – determines the direction of passive
movement of an ion
• [specific ion] = chemical gradient
• relative electrical charge = electrical gradient
Transmission of signals in neurons - Resting potential
typically between -60 mV and -80 mV
• a K+ leak channel (non-gated) allows K+ almost to equilibrate
across the membrane
• a Na+ leakage inwards tends to destroy the resting potential
• the Na+/K+ pump corrects for the Na+ leakage
Transmission of signals in neurons - Action potential
(nerve signal transmission) • non-graded, all-or-nothing event • needs two voltage-gated channels • a sodium channel • a potassium channel
How the voltage gated channels work ? the sodium channel
activation gate
• closed in resting state
• opens rapidly in response to depolarization
Inactivation gate
• open in resting state
• closes slowly in response to depolarization
How the voltage gated channels work ? the potassuim channel
single gate
• closed in resting state
• opens slowly in response to depolarization
Transmission of signals in neurons
• transmission is unidirectional,
because the inactivation gate
on the Na+ channel is slow to
reopen
Nodes of Ranvier
• breaks in the myelin sheath
• increase the speed of conduction
- Conduction velocity also increases with axon diameter
