Nervous Coordination Flashcards
Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.
- Myelination provides insulation
- Myelination allows saltatory conduction at nodes of Ranvier
- Non-myelinated needs conduction across entire length of axon
How is a resting potential maintained?
- Sodium potassium pump actively transports 2K+ in, 3NA+ out
- This establishes a electrochemical gradient
- The neuronal membrane is more permeable to K+ ions so K+ moves back out the membrane by facilitated diffusion
- More K+ channels than Na+
- K+ channels are mainly open and Na+ channels are closed ( until there is a high enough voltage to trigger them to open)
How are action potentials generated?
- Sodium ion channels in the axon membrane open
- Sodium ions pass into the axon down the electrochemical gradient
- This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – depolarisation
- Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation
- If the potential difference reaches -55mV (threshold potential), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of +40mV
- An action potential is generated
What is repolarisation?
- About 1 ms after an action potential, all the sodium ion channels in this section close.
- Potassium ion channels in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon
- This returns the potential difference to normal - repolarisation
- There is actually a short period of hyperpolarisation where the potential difference briefly becomes more negative than the normal resting potential- refractory period
- The potassium ion channels then close and the sodium ion channels in this section of membrane become responsive to depolarisation again
What is the refractory period?
Until Na+ channels become responsive again this section of the axon membrane is in a period of recovery and is unresponsive - refractory period
What is the importance of the refractory period?
- ensures that ‘new’ action potentials are generated ahead rather than behind the original action potential
- action potentials are discrete events
- unidirectional
- limits the number of impulse transmission (limits overreaction to a stimulus)
How does axon diameter affect speed of conduction?
- thicker axons have a higher speed of conduction
- greater SA → increased rate of diffusion
- fewer leakage of ions SO action potentials are generated faster
How does temperature affect speed of conduction?
-ions diffuse faster
- enzymes involvedin respiration work faster SO more ATP for active transport of Na+/K+ pump
- warmer conditions speed up conduction
- more kinetic energy for the facilitated diffusion of K+ and Na+
Describe transmission across a cholinergic synapse.
- Action potential arrives at pre-synaptic neuron
- Ca2+ channels open so diffuses into pre-synaptic knob
- Cause the exocytosis of vesicles
- Vesicles fuse with pre-synaptic membrane
- Acetyl choline diffuses across synaptic cleft
- Binds to receptors on post-synaptic membrane
- Causes a conformational change
- Na+ channels open and Na+ diffues into post synaptic knob
- Depolarisation of post-synaptic membrane
- If threshold reached action potential generated
- The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline
- The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives
What is temporal summation?
Multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential.
What is spatial summation?
Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential.
Describe inhibition by inhibitory synapses.
- opening the gated potassium ion channels in the membrane so that potassium ions are able to diffuse out
- opening chloride ion channels
Describe transmission across a neuromuscular junction.
- Depolarisation of pre-synaptic neuron
- Ca2+ channels open so diffuses into pre-synaptic knob
- Cause the exocytosis of vesicles
- Vesicles fuse with pre-synaptic membrane
- Acetyl choline diffuses across the NMJ
- ACh binds to sarcolemma receptors
- Na+ channels open and Na+ diffuses into sarcolemma
- Action potential passes along sarcolemma and down T-tubules
- Ca2+ ions diffuse out of sarcoplasmic reticulum into sarcoplasm
- Ca2+ binds with troponin, initiating muscle contraction
