Neuron graded potential

resting neurons have stable charge seperation across memberane. cations on outside, anions on inside. we call the outside 0. resting potential of inside anions is around -60 mV.

input from stimuli can increase or decrease membrane potential temporarily → graded potentials

  • transient

  • occur in dendrites and soma

  • size and duration proportional to size and duration of inputs

  • do not pass into axons, axons have action potentials triggered when graded potentials combine brings membrane of trigger zone across the threshold potential.

  • threshold potential varies, but around -50 mV is common.

  • decay with both time and distance. brief and local.

  • if two graded potentials occur at different times, they wont effect each other.

Summation: adding of graded potentials. summation at trigger zone is how neurons porcess information.

temporal summation: closely timed graded potentials combine for an additive effect

spatial summation: closely distanced graded potentials combine for additive effect.

neurotransmitter molecules are released at synapses. nuerotransmitters are the most common input a neuron will receive.

synaptic/post synaptic potentials: graded potentials produced from a synapse, tend to be very small

receptor potentials are generated by receptors.

moves membrane potential to less negative number = depolarization or excitatory

moves membrane potential to more negative number = hyperpolarization or inhibitorty

synapse closer to trigger zone has greater influence on action potentials.

neuron resting potential

resting neurons have stable charge seperation across memberane. cations on outside, anions on inside. we call the outside 0. resting potential of inside anions is around -60 mV.

resting potentials are related to concentration differences/gradients of ions across cell membrane.

Cations - Potassium (K+), Sodium (NA+), Calcium (CA2+)

Anions - Chloride (Cl-), Organic Anions (OA-)

Higher concentration inside than out - Potassium, Organic Anions

Higher Concentration outside than out - Sodium, Chloride, Calcium

electrical force from membrane potentials - opposite charges attract, anions attracted to outside, cations attracted to inside.

diffusion force/chemical force - higher concentration to lower concentration. potassium and chloride have opposing electrical and diffusion forces.

neuron resting potential mechanism

all steps occur at same time.

  1. neuron creates organic anions, adding to cytoplasm → neg charges inside create small membrane potential. slightly negative inside.

  2. electrical force drives organic anions outside, but they cant leave because membrane doesn’t let them.

leak channels allow other ions to cross membrane. they are open all the time

sodium potassium pump - active ion transporter.

  1. sodium potassium pump uses 1 ATP molecules energy to transport three sodium ions outside and 2 potassium ions inside. adds to membrane potential, slightly more negative inside. higher concentration of potassium inside than outside. much smaller concentration of sodium inside than outside.

  2. potassium’s electrical force drives it into cell, diffusion forces drive it out from higher to lower concentration. the diffusion force is much larger than the electrical force. the potassium ions move out of the neuron through leak channels, carrying positive charges out. this makes the inside of the membrane more negative. this continues until the electrical force is the same size as the diffusion forces, which cancels out the forces. this could occur around -70 mV.

  3. for sodium ions, the electrical and diffusion forces draw it into the membrane. pernmiability of membrane to sodium is much less than that of potassium. equilibrium potential around -60mV. when membrane is permeable to multiple ions that have electrochemical driving forces, the resulting membrane potential is weighted avg of equilibrium potentials weighted by permeability.

  4. resting membrane has intermediate permeability to chloride. membrane potential drives chloride out of neuron until its conc. gradient is big enough to balance it. small concentration inside compared to outside. equilibrium potential is around -60mV.

chloride potassium symporter drives chloride out by allowing potassium out, which chloride is attracted to due to charge. makes resting potential slightly more neagtive.

  1. sodium calcium exchanger drives calcium out by harnessing energy of sodium to repel calcium out. creates strong electrical and diffusion force driving calcium into neuron. resting membrane permeability to calcium is quite small.

equilibrium potential: ion has balanced electrical and diffusion forces to result in no net movement of ion.

Neuron action potential

if graded potentials push energy over threshold potential, action potentials usually occur.

  • action potentials have the same size and duration for any particular neuron (all or none property)

  • action potentials go down the entire neuron

  • action potentials rise to very positive membrane potential, then it plateuas for a short time, then goes down to more negative than resting potential then slowly returns.

  • action potentials don’t decay as they travel. they travel very fast.

  • large diameter conducts faster, myelinated conduct faster

saltatory conduction: the process of nerve impulse transmission in myelinated axons where the action potential "jumps" from one node of Ranvier to the next, rather than traveling continuously along the axon

neuron action potential mechanism

leak channels: open all the time

voltage-gated channels: in membrane of axon. open when the membrane potential crosses the threshold value. they are sodium channels. sodium’s electrical and diffusion forces drive them into neuron. membrane depolarizes from positive charges, causing chain reaction and triggering the next voltage gated channels, depolarizing membrane and repeating process. they open very quick and trigger each other in the wave. they are highest in density in trigger zone. membrane permeability to sodium is very increased. membrane potential rise (rising phase). now more positive inside membrane. voltage gated channels close so that sodium stops flowing in. then, membrane potential dips down below resting potential, then levels out as potassium starts to exit neuron through leak channels because the membrane potential is positive. also exitin through voltage gated channels, which are slower for potassium than sodium.

inactivated state: briefly cant open

after hyperpolarizatoin or refractory period: difficult to trigger another action potential. absolute refractory period is when the voltage gated sodium channels are in inactivated state. relative refractory period is when voltage gated sodium channels can be activated, but membrane potential is hyperpolarized.

effects of axon diameter and myelination

Diameter:

larger diameter = less resistance = ions move faster = action potential faster

myelin sheath = faster action potential. less ions = less time to depolarize membrane.

action potential patterns

some neurons don’t fire until they are excited enough, then they fire in a burst

some neurons fire consistently and then they fire in a burst of activity when they receive excitatory inpurt. then, when they receive inhibitory input, the frequency of firing slows.

some neurons fire in bursts with constant spacing, but the bursts become more frequent or larger when the neuron is excited, and less frequent when the neuron is inhibited.