nervous coordination Flashcards
label this neurone + give functions of everything
how is resting potential maintained
Before a nerve can transmit an electrical impulse its membrane must be made polarised to create a
potential difference (voltage) of about -70mV. This is call the resting potential.
1. The sodium potassium pump actively transports about 3 Na+ out of the neurone and 2 K+ ions into the neurone.
2. The membrane is more permeable to K+
than Na+ so some K+ diffuse back out down their electrochemical gradient.
(Most sodium ion channels are closed so Na+
cannot diffuse back in)
3. Overall, there is a higher concentration of Na+ outside and a higher concentration of
K+inside the neurone.
how is an action potential created?
- When the neurone is stimulated voltage gated sodium ion channels open in the axon membrane.
- Na+ diffuse into the axon causing the membrane to become more positive. This is called depolarisation.
- If threshold is reached, more voltage gated sodium ion channels open via positive feedback and the
neurone depolarises to about +40mV. - The sodium ion channels then close preventing Na+ entering the axon and more potassium ion channels open causing K+ to diffuse out the axon.
- The neurone becomes more negative-repolarisation.
- Too many K+ diffuse out causing the neurone
to become too negative. This is called hyperpolarisation. - Potassium ion channels close and the sodium potassium pump returns the
neurone back to resting potential (3Na+ out and 2K+
in via active transport).
what is the refractory period?
The time during which
no new action potential can be produced- due to the sodium ion channels
briefly staying closed, preventing depolarisation, as Na+ cannot enter the axon.
3 functions of refractory period
- Produces discrete impulses – each action potential is
separate and cannot overlap on top of each other. - Ensure action potentials travel in one direction (uni directional) – they cannot move backwards, due to
the membrane behind the action potential not being able to conduct. - Limits the maximum frequency of impulses – there is a limit to the number of maximum impulses that
can be sent down a neurone per second, limiting the strength of the stimulus.
describe the all or nothing principle
action potential either does or doesn’t occur.
* If the depolarisation DOES NOT reach threshold is, there is NO action potential.
* If threshold IS reached THERE IS an action potential and it is always the same size.
The strength of a stimulus is determined from the frequency of action potentials. The stronger the stimulus the greater the frequency of action potentials NOT the greater the depolarisation, as all action potentials are the same size
action potential transmission along unmyelinated axon
- The neurone is at rest, and polarised, An action potential occurs at a section of axon membrane
- membrane adjacently in front of the action potential detects the depolarisation and voltage gated sodium
ion channels open, also causing depolarisation and an action potential - action potential moves forward. The refractory period behind the action
potential prevents the impulse being conducted backwards (as Na+channels
stay closed)
action potential transmission alone a myelinated axon
In a myelinated axon, the myelin insulates the axon preventing ion movements.
* Depolarisations only occur at the Nodes of Ranvier, so action potentials ‘jump’ from node to node (saltatory conduction). The action potential is conducted faster down a myelinated axon because in an non-myelinated neurone depolarisations have to occur
along the whole length of the membrane)
factors affecting conduction speed
- Myelination - This increases the speed of conduction as action potentials only have
to occur at the nodes of ranvier (saltatory conduction) instead of whole length. - Axon diameter – axons with a greater diameter conduct faster due to less resistance to ion flow.
- Temperature –the higher the temperature, the faster the conduction. Ions have
more kinetic energy so diffuse faster through the channels in the membrane.
what is a synapse?
the junction between 2 neurones or a neurone and an effector.
describe transmission along a cholinergic synapse
- An action potential depolarises the pre-synaptic membrane, voltage gated calcium ion channels open and calcium ions diffuse in.
- Calcium ions cause synaptic vesicles to fuse with the pre-synaptic membrane and release acetylcholine into the synaptic cleft.
- Acetylcholine diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane.
- Sodium ion channels open and sodium ions diffuse into the postsynaptic neurone,
depolarising it, causing action potentials - Acetylcholinesterase hydrolyses the acetylcholine. The sodium ion channel closes and no more action potentials are generated.
- The products are taken up by active transport via the re-uptake pump in the pre synaptic neurone and resynthesised back into acetylcholine using ATP.
why can transmission of an impulse only be in one direction?
Only the presynaptic neurone releases neurotransmitter.
Only the postsynaptic neurone has neurotransmitter reeptors (receptors are not present on the presynaptic neurone).
how are some synapses inhibitory?
Some neurotransmitters open Cl-channels instead of Na+ channels. The influx of Cl- ions hyperpolarises the post synaptic neurone which
becomes more negative.
Therefore more sodium ions are needed to reach threshold for an action potential.
describe 2 types of summation
Summation increases the amount of neurotransmitter released, increasing depolarisation, so it is more likely reach threshold.
Spatial summation – more than one presynaptic
neurone releasing neurotransmitter onto one post synaptic neurone. (Retinal convergence in rod cells is an example of spatial summation).
Temporal summation –Several action potentials
arrive in a short time from ONE presynaptic neurone
to ONE post synaptic neurone
The insecticide DDT inhibits the enzyme acetylcholinesterase, how does this lead to muscle spasms + death?
Preventing acetylcholine breakdown, causing a build up of acetylcholine in the synaptic cleft of motor neurones, which binds acetylcholine receptors, causing sodium ion channels to open and sodium ions to diffuse in, which leads to continuous action potentials and therfore muscle spasms and death.
