Chapter 15 - Nervous coordination and muscles Flashcards
Characteristics of the nervous system
Nerve cells transmit electrical impulses along their length
Impulse stimulates the secretion of neurotransmitters onto target cells
Short-lived, affect a small area
Characteristics of the hormonal system
Hormones transported in blood plasma to target cells
Slow, widespread, long-lasting effect
What is the cell body?
Contains organelles, produces neurotransmitters
What are dendrons?
Extensions of cell body
Divide into dendrites
Carry impulse TO cell body
What is an axon?
A long fibre that carries impulses FROM the cell body
Function of Schwann cells
Electrical insulation
Function of myelin sheath
Covers the axon
What are the nodes of Ranvier?
No myelin sheath
How is a resting potential established?
The phospholipid bilayer prevents Na+ and K+ ions passing through by simple diffusion
Channel proteins form a sodium-potassium pump - they actively transport sodium ions out of the axon and potassium ions in
How is the membrane polarised?
Na+ transported out
K+ transported in
3 Na+ out for every 2 K+ in
An electrochemical gradient is established - there are more Na+ ions in the tissue fluid around the axon and more K+ ions in the cytoplasm
Na+ diffuse back in and K+ diffuse out
The Na+ gates are closed but the K+ gates are open
How is the membrane depolarised and repolarised?
Some K+ gates are open but the Na+ gates are closed
The stimulus causes some Na+ gates to open so Na+ ions can diffuse into the axon
As more diffuse into the axon, more voltage gates open
At a limit, the gates close and the K+ gates open
This means K+ can diffuse out of the axon and causes more K+ gates to open - the membrane is repolarised
This action causes a temporary overshoot where the inside of the axon is more negative than the outside so the K+ gates close and Na+ is pumped in
Which three factors affect the speed of an impulse?
Temperature, diameter of the axon, myelin sheath
How does the myelin sheath impact the speed of the impulse?
Saltatory conduction (impulse jumps between nodes of Ranvier)
How does temperature impact the speed of an impulse?
Rate of diffusion of ions
Enzymes
How does the diameter of the axon impact the speed of an impulse?
Big diameter = less leakage = faster
What is the all-or-nothing principle?
Above the threshold value, an action potential is triggered
Below the threshold value, no action potential is triggered
What is the refractory period?
When an action potential is generated, there is a time delay when Na+ ions can’t enter the axon because the gates are closed
What are the three purposes served by the refractory period?
Limits the number of action potentials
Produced discrete impulses
Action potentials only go in one direction
How does an action potential pass alone the neurone?
Saltatory conduction
‘Jumps’ between unmyelinated nodes of Ranvier
How is the axon depolarised?
Sudden influx of Na+ ions
In what direction will muscles work?
They can only pull, not push
Why do skeletal muscles only work in antagonistic pairs?
The pairs work in opposite directions - when one is relaxed, the other contracts
Where will myofibrils be darker in colour?
Where the actin and myosin filaments overlap
What changes happen to the sarcomere when the muscle contracts?
The I-band becomes narrower
The sarcomere shortens
The H-zone becomes narrower
What is myosin?
Made of the tail (fibrous proteins) and the head (bulbous structures)
What is actin?
A long, helical strand of protein
What is tropomyosin?
Wound around actin to block binding sites
What is the sliding filament theory of muscle contraction?
The layers of actin and myosin slide past each other to contract the muscle
How are muscles stimulated to contract?
An action potential reaches neuromuscular junctions
Ca2+ protein channels open and Ca2+ diffuses into synaptic knob
Ca2+ causes vesicles to release acetylcholine into the cleft
Acetylcholine diffuses across the cleft and binds with receptors on neighbouring muscles, depolarising them
How do muscles relax?
Hydrolysis of ATP provides the energy to actively transport Ca2+ into the endoplasmic reticulum
This makes tropomyosin block the binding sites again
Myosin can’t attach so muscles relax
What is energy needed for during muscle contraction?
Movement of myosin heads
Active transport of Ca2+
How is energy provided for muscle contraction?
ATP -> ADP
What does phosphocreatine do?
Acts as a source of phosphate
Phosphate + ADP -> ATP
Demand for oxygen outweighs supply - another way to form ATP must be used
How is the phosphate supply regenerated?
When muscle relaxes