Nerve Impulses Flashcards
What is the Central Nervous System (CNS)?
made of brain and spinal cord
brain = analyses and coordinates response to stimuli
spinal cord = connects brain to sensory and motor neurones
What is the Peripheral Nervous System (PNS)?
Made of the sensory and motor neurone
A neurone transmits a nerve impulse
Sensory neurone takes nerve impulse from receptor to CNS
motor neurone takes nerve impulse from CNS to effector
sensory neurone has its cell body in the middle and has a dendron and axon
motor neurone has its cell body at the start and only has a long axon
What is a Nerve Impulse?
Movement of an action potential along a neurone
Action potential = change in membrane potential (charge’ in one section of the neurone changes from negative (polarised) to positive (depolarised) back to negative (repolarised/hyperpolarised)
What is Resting Potential?
Membrane potential of neurone at rest
is -70mV
polarised
caused by having more positive ions outside neurone compared to inside
Involves Na+/K+ pump, pumping 3 Na+ ions out, 2 K+ ions in
K+ channel allowing K+ ions to diffuse out
(K+ ions will eventually stop diffusing out due to a positive potential outside)
What happens during an Action Potential?
Stimuli causes Na+ ions to enter the start of the neuron
Makes membrane potential less negative
If it reaches threshold (-50mV), Na+ channels open
Therefore more Na+ ions diffuse into the neurone, therefore membrane potential becomes positive (depolarised)
The membrane potential reaches +40mV
then the Na+ channels close, the K+ channels open
Therefore K+ ions diffuse out, therefore membrane potential becomes negative (repolarised)
Too many K+ ions move out, so the membrane potential becomes more negative than normal (hyperpolarised)
one action potential = depolarisation, repolarisation, hyperpolarisation
How does an Action Potential move along a Neurone?
By local currents
If the stimuli energy is large enough and enough Na+ ions enter the start of the neurone, the threshold will be reached and an AP will occur
(the 1st AP is called a Generator Potential)
Na+ ions that move in during depolarisation of the generator potential diffuse along the neurone causing the next section to reach the threshold and an AP to occur
This process continues along the neurone
Why does AP not move back?
Because previous section has just finished an AP, therefore it is in a refractory period (Na+ channels cannot be opened) and is hyperpolarised (therefore threshold cannot be reached)
How does the Size of Stimuli Affect a Nerve Impulse?
Does not affect size of AP
(AP is all or nothing – reach threshold = get AP [all]
do not reach threshold = no AP [nothing])
Larger stimuli increase the frequency (number) of APs
What effects the Speed of Nerve Impulse?
Temperature = higher temp, higher kinetic energy, a faster rate of diffusion of ions (faster nerve impulse)
Axon diameter = wider diameter, neuron less leaky (faster nerve impulse)
Myelination = Schwann cells wrap around axon, insulate axon preventing AP, therefore AP only occurs in gaps – called node of Ranvier, so AP jumps from node to node = saltatory conduction (faster nerve impulse)
