What is a cell body?
input zone, organelles and nucleus inside
What are dendrites?
input zone, plasma membrane contains receptors binding to chemical messengers
Describe an axon.
-one of the longest cells in the body -still cellular size, not that thick -conducting zone
What is the axon hillcock?
-trigger zone -particularly sensitive, where graded potentials trigger AP - at the beginning of the axon
What are the axon terminals?
What is the difference between a nerve and a fibre tract?
nerve= bundle of axons outside the CNS fibre tract= bundle of axons inside the CNS
What do voltage-gated ion channels do?
open or close in response to changes in membrane potential, crucial to action potentials
What do chemically-gated channels do?
- (ligand gated) change conformation allosterically (= binding of a regulatory molecule to the allosteric site=binding site) in response to the binding of chemical messenger with a membrane receptor associated with the channel
What do mechanically gated channels do?
respond to stretching or other mechanical deformation such as touch, important in sensory transduction
What do thermally gated channels do?
respond to local changes in temperature, important in sensory transduction
What makes the channels open?
Channel opening results from graded potentials (more primitive) or action potentials.
What are graded potentials?
-local changes in membrane potential -the stronger the triggering event the stronger the graded potential, since more gated channels open -trigger event causes gated ion channels to open -the ions flow in, depolarizing or hyperpolarizing the small region of the plasma membrane (usually K+ so it depolarizes the axon)
Does spread of graded potential happen locally or does it affect the rest of the membrane?
-graded potential happens locally, the rest of the membrane remains at resting potential
What is the temporarily depolarised region in graded potential called?
How is the graded potential spread?
-there is a constant current flow – ions in the depolarized area will spread to the neighbouring inactive areas along the membrane this causes the potential in the other areas to change -results from the imbalance of ion pumps
Which direction does graded potential spread to and how far-reaching is its influence?
-the potential spreads in both directions from the initial area but the ions spread out and the potential weakens evens with short distance (a few millimeters) then dies out due to the dilution and movement through open channels= this type of spread of potential is called passive
Why is graded potential termed graded?
-graded= depends on how much stimulation there is
How long does graded potential last?
What two factors can affect graded potential?
-electrical field change- can also change the potential -chemical messengers too
What values of membrane potential are there during an action potential?
-70mV resting up to +20/+30 (depolarisation) down to -80mV (hyperpolarisation)
What is the spread of AP called?
-spread unlike graded is nondecremental (no diminishment in strength during propagation) that’s why they’re good for long distance signals
How much of the membrane is excited when an AP takes place?
-like graded, small portion of membrane excited
What property must portion of a membrane have to be able to generate AP?
-can only be generated in portions of the membrane with abundance of Na+ channels, graded potential brings these more sensitive areas to threshold
-brief, rapid, large changes in the membrane potential when the inside becomes more positive than the outside
What triggers an AP?
-if graded potential is of sufficient magnitude it can initiate AP before dying off (normally the membrane bit that was in graded potential doesn’t experience AP- the adjacent regions do)
What is an action potential (AP)?
-action potential is the entire rapid change from threshold to peak back to resting
How long does an action potential last?
-duration is mostly fixed= 1msec (0.001sec) (bit longer in muscle)
Will stronger stimulus generate largere APs?
-the strength of a stimulus is coded by the frequency of AP, stronger stimulus will trigger more APs not bigger APs
What must be achieved to have an AP?
- must reach threshold, all or nothing event
Which ion is the plasma membrane more permeable to K+ or Na+?
-plasma membrane much more permeable to K+ than Na+ so K+ contributes most to establishing resting potential
-many leaky K+ channels so membrane more permeable to it than Na+ (20 to 30 times more)
What effect does an AP have on the permeability of the plasma membrane?
-during AP changes to permeability of K+ and Na+ because of voltage gated channels (Na+ and K+ ones) opening and allowing ions in (positive feedback- the ions carry the current responsible for the voltage gated channels opening, reinforcing the effect)= some Na+ open further depolarizing the membrane which leads to more channels opening etc…