Unit 7 - The Nervous System

What is membrane potential?

charge difference across membrane

What is membrane potential the result of? Which ions are attracted and which are not permeable?

in part from presence of large anions being trapped inside the cell. cations like K+ attracted inside by anions and Na+ not permeable and actively transported out

What is the RMP and what does it depend on? What is it most affected by and why?

resting membrane potential is -70 mV. depends on concentrations + permeability of ions inside and out of cell. affected mostly by K+ bec it’s most permeable

1 V = ? mV. Why is RMP -70 mV?

1000 mV. Use -70 mV because of constant moving of Na+ and K+ and Na+/K+ pump

What do electrodes attached to a voltmeter record? What does -70 mV refer to?

Electrodes record charge difference, the -70 mV means the inside of the cell is more negative than the outside by -70 mV

What is potential and what is equilibrium potential?

potential - charge distribution across a capacitor, equilibrium potential = voltage across cell membrane if only 1 ion could diffuse

Equilibrium potential (Ek) for K+ and Na+

K+ = -90 mV and Na+ = +66 mV

Explain how Ek is achieved and what does this mean

K+ (cations) attracted inside by trapped anions but also driven out by its concentration gradient. electrical and diffusion forces are equal and opposite

What is the concentration for these ions (which is greater outside vs inside)? Na+, K+, Ca2+, Cl-

Na+, Ca2+, Cl- = greater concen. outside and K+ = greater inside

Eflux vs influx and what are diffusion channels specific to?

Outward movement of ions across the cell membrane vs. inward movement of ions into the cell. Specific to the ion they let in

What are Na+/K+ pumps described as? How much mV does it add to the RMP and why is this charge negative / positive?

described as electrogenic because 3 Na+ pumped out for every 2 K+ taken in. Adds -3mV to RMP and it’s negative because more positive (3 Na+) leaving and gradually decreases

Cells have channels that are ____ open

always

Why is the membrane more permeable to K+ and what does this mean?

Bec of leakage channels and this means contributes more to RMP of -70 mV

Fixed anions aren’t able to do what and why?

Fixed anions can’t cross membrane bec most are proteins meaning too big to diffuse, charged molecule, no transporters

What are the divisions of the nervous system? What do they consist of?

CNS = integration, brain + spinal cord. PNS = cranial + spinal nerves

What kinds of cell types are in the nervous system (NS)?

Neurons and glial cells (neuroglia)

What are neurons and glial cells?

neurons are functional units of nervous system + glial cells are supporting cells that maintain homeostasis (5x more common than neurons)

4 ways neurons gather and transmit information

1. gather info through receptors

2. responding to stimuli

3. producing and sending electrochemical impulses

4. releasing chemical messages

How are neurons named?

Named based off # nerve fibers coming out of cell body

What are the anatomical classifications of neurons?

1. multipolar = many nerve fibers

2. bipolar = 2 fibers from cell body

3. pseudo = unipolar bec neither fake axon/dendrite

Functional classification of neurons

1. sensory = towards CNS

2. somatic motor = away from CNS → target

3. autonomic motor = anything other than skeletal muscle, divides into sympathetic and parasympathetic

Function of myelin

increase speed at which AP travels along axon + AP conduction activity increases if axon is myelinated

Difference between Schwann and oligodendrocyte

Schwann -

1. only myelinating in PNS (PNS also has satellite cells)

2. entire cell wraps around axon

3. can only make 1 myelin sheath

oligodendrocyte -

1. myelin around axons in CNS

2. sends out processes / fibers that form myelin sheath

3. can make 1+ myelin sheath

At rest, all cells have a _____ internal charge and ______ distribution of ions

negative, unequal

What does a negative internal charge and unequal distribution of ions?

1. large cations being trapped inside the cell

2. Na+/K+ pump and limited permeability keep Na+ high outside cell

   1. K+ is very permeable and is high inside cell - attracted by negative charges inside

Explain excitability. Why do neurons and muscles do this?

excitable - neuron’s ability to produce AP and excitable cells can produce an AP quickly by rapid changes in ion permeability. to generate and conduct impulses

How are Membrane potential (MP) changes measured?

Placing 1 electrode inside cell and 1 outside

Explain depolarization and hyperpolarization and provide an example

depolarization - MP becomes more pos.

hyperpolarization - MP becomes more neg. than RMP

Cl- ion → hyperpolarizes bec Cl- concen is greater outside and flows to low inside cell

What is repolarization and when does it occur?

MP returns to RMP and only happens if depolarization / hyperpolarization has occurred

What are membrane ion channels?

MP changes occur by ion flow through membrane channels

Some channels are normally open and some closed. What’s the difference between leakage and VG channels?

leakage - always open, VG channels - opened by depolarization + have molecular gates that can be opened

Which VG channels are closed in resting cells at -70 mV? Which one has 2 gates?

Na+ (2 gates), K+, and Ca+

What is the state of the activation and inactivation channels at -70mV (RMP)? What about -60 mV?

at -70 mV = Activation channel - closed and inactivation channel - open

at -60 mV = pos. enters activation gates and both are opened, sudden Na+ influx

What happens at +30 mV to the activation and inactivation gates? Why?

inactivation gate closes + activation gate opened no more Na+ → bounces because needs to reset and this depends on timing about 2ms. after this time period, activation gate closes and inactivation gate opens

Where does the AP begin and where does it go?

begins at axon hillock transmit to axon terminal where neuron communicates w/ post-synaptic cell via NT

What is AP and how is it formed?

wave of MP change that sweeps along the axon from soma to synapse and wave formed by rapid depolarization of membrane by Na+ influx; followed by rapid repolarization by K+ efflux

What’s the initial stimulus/depolarizing event in the graph the result of? What’s the max amplitude for Na+

Na+ channel opening and max amplitude = +30 mV

How long does the VG K+ channel stay open for and why? How is this restored to RMP?

stays open until equilibrium is reached at -90 mV (min MP) because there’s no inactiv gate for it. Na+/K+ pump restores gradient

Describe depolarization as it relates to AP and the feedback loop involved

at threshold, VG Na+ channels open and it’s driven inward by electrochemical gradient opening more channels and this is positive feedback

Describe repolarization as it relates to AP

VG Na+ channels close; VG K+ channels open and electrochemical gradient drives K+ outward and repolarizes axon back to RMP

How do depolarization and repolarization occur? Do they require active transport?

diffusion, no

Why don’t we always see the Na+/K+ pump working?

Because of the rapid influx of Na+ and efflux of K+ when VG Na+ / K+ channel opens and overpowers the pump

Do all APs have the same amp and why?

yes because follow all-or-none

What does the AP have to be depolarized to in the body and give an example on how this works in the body?

to -60 mV in the body, stimuli depends on what’s touching the body like a fly, if it depolarizes touch receptors to -60 mV than you will feel something

What happens when an MP reaches threshold and bec of pos fdbck what happens

AP irreversibly fired and more and more Na+ channels opened. then shortly after opening, Na+ channels close and become inactivated until repolarization

How does the brain know the stimulus strength if the amplitude is the same?

Based on the frequency of AP / time (sec) = Hertz, 1 Hz = 1 AP/sec

Increased stimulus intensity causes ____ APs to be fired and does no AP mean no depolarization occured?

more APs, not true just means it wasn’t depolarized to -60 mV

refractory period

time in which no AP can be generated at the segment of the axon

Absolute vs relative refractory period

absolute = membrane cannot produce another AP bec Na+ VG channels are inactivated

relative = VG K+ channels open making it harder to depolarize to threshold

axon conduction

AP traveling along axon from hillock to terminal

What’s the AP threshold for opening a VG Na+ channel?

-60 mV AP

Explain what happens in this diagram and once an AP is started it is _____

self-propagating

1. Any VG Na+ channel in segment 2 is open

2. Na+ goes back toward segment 1

3. depolarizes #2 resulting in AP

4. no AP in segment 1 bec refractory period

5. segment 2 is refractory → goes to #3 depolarizes

Explain conduction in an unmyelinated axon

After axon hillock reaches threshold and fires AP, Na+ influx depolarizes adjacent regions to threshold → generating new AP → process repeats along axon → AP amp is always the same → conduction is slow

Why can’t dendrites and cell bodies produce AP?

because no VG Na+ / K+ channels only axons can generate and conduct AP

What are 2 factors affecting the speed of AP?

1. diameter (size), small = slow AP velocity, large = fast AP

2. if there are bare axons, no myelin = Nodes of Ranvier

Why are AP not generated in axons w myelin?

bec they don’t have Na+/K+ VG channels

AP begins at ____ and travels in ___ direction

axon hillock, 1

What happens when ions can’t flow across a myelinated membrane and no AP generated?

• Saltatory conduction occurs

• Because voltage-gated Na⁺ channels are only at unmyelinated Nodes of Ranvier, AP not produced at every axon segment

• Instead, Na⁺ influx at one node creates a passive electrical current that flows rapidly under the myelin sheath

• This skips myelinated areas to depolarize the next node, allowing the signal to travel significantly faster to the axon terminal

Why do humans have a small diameter?

bec not enough space in the body so have myelin sheath

What’s a synapse?

connection between a neuron (presynaptic) and another cell (postsynaptic)

What are the types of synapses and which is the most common and rare?

chemical (most common) and electrical synapses (rare in NS)

How does synaptic transmission at chemical synapses occur?

via NT, AP reaches axon terminal and reaches dead end → can’t jump over synaptic cleft so NT is needed

What happens in an electrical synapse?

physical connection via gap junctions between presynaptic neuron and post synaptic cell

What are graded potentials (GP)?

are produced when a ligand (stimulus) opens a ligand-regulated / chemical channel and causes a voltage which dissipate with distance from stimulus

What are synaptic potentials/post synaptic potential (PSP)?

occur at a synapse between a neuron and another cell

What are generator potentials?

occur when a sensory receptor is stimulated

What causes voltage change / MP?

ions going in and out of the cell

Why are gap junctions produced? How does this relate to AP?

each of 2 cells have transmembrane proteins with channels / pore allowing anything in 1 cell to go through the next cell and share material. AP goes thru 1 cell via gap jxn to another one w/o using NT

synaptic transmission

process by which a neuron communicates with a cell across a synapse

Steps of synaptic transmission - leading up to NT release

1. AP reach axon terminal at -60 mV

2. Ca2+ enters axon terminal via VG channels opened by Na+ that is trapped and as AP travels down axon, axon terminal is depolarized

3. Ca2+ bind to sensor protein in cytoplasm

4. Ca2+ protein complex + flood of Ca2+ mobilizes synaptic vesicles to move

5. vesicles fuse w/ axon terminal and via exocytosis NT (can be ACh) diffuses and binds to receptors on postsynaptic cell

Steps of synaptic transmission - leading up to post synaptic potential (PSP)

1. NT diffuses across cleft

2. binds to receptor proteins on postsynaptic membrane

3. causes opening ligand/chemically-regulated ion channels

4. PSP produced as ions diffuse across membrane via ligand-regulated ion channel

What do depolarizing and hyperpolarizing channels cause? Why is depolarizing like that?

depolarizing - EPSPs (excitatory postynaptic potentials) because of the probability of AP threshold being reached + exciting the cell

hyperpolarizing - IPSPs (inhibitory postsynaptic potentials)

EPSP and IPSP depends on what? What happens if the ion channel closes before reaching -60 mV?

depends on how much ions flow through. MP goes back to rest

To elicit an AP, axon hillock has to be what?

reached

Summation happens between ____ and _____? What has to be achieved for a new AP to be generated?

between EPSPs and IPSPs. MP in postsynaptic cell has to reach threshold

Where does the signal in the AP arrive?

brain - sensory receptor → brain or effector / target organ - brain → motor output

Does a nerve have a synapse and what does this mean for curare?

no it doesn’t so curare can’t inhibit anything but it can inhibit the synapse

AP vs GP/EPSP: stimulus for opening of ionic gates

AP: depolarization to -60 mV, GP: ACh or other excitatory NT

AP vs GP/EPSP: initial effect of stimulus

AP: VG Na+ channels open and, GP: common channels for Na+ and K+ open

AP vs GP/EPSP: cause of repolarization

AP: VG opening of K+ gates, GP: loss of intracellular pos charges w/ time and distance (moving of Na+ further away from entry point that caused it to come in)

AP vs GP/EPSP: conduction distance

AP: regenerated over length of the axon, GP: 1-2 mm; a localized potential (move further away from ion = smaller potential)

AP vs GP/EPSP: pos fdbck (self-propagating) between depolarization and opening of Na+ gates

AP: yes, GP: no

AP vs GP/EPSP: max depolarization

AP: +30 mV means VG Na+ channels inactivation gate closes + no more Na+ allowed, GP: -70 to 0

AP vs GP/EPSP: summation

AP: no summation - all-or-none event, GP: summation of EPSPs producing graded depolarizations

AP vs GP/EPSP: refractory period

AP: yes, GP: no

AP vs GP/EPSP: effect of drugs

AP: ACh effects inhibited by tetrodotoxin NOT by curare, GP: ACh effects inhibited by curare, not by tetrodotoxin

Why does GP have no threshold?

bec direct proportional responses

What is the most widely used NT and where is it used?

ACh in the brain and ANS at all neuromuscular junctions

What are the 2 subtypes of ACh?

nicotinic - allow Na+ in and depol, excitatory

muscarinic - K+ channel and hyper, inhibitory

Which is involved in muscle contractions and what does the binding of ACh to nicotinic receptors do?

nicotonic involved in muscle contractions and APs, binding of ACh opens Na+ channel + EPSP

What forms nicotinic ACh channel and what does it contain?

formed by 5 polypeptide subunits and contains 2 subunits w/ ACh binding sites

Nicotinic is a ligand-gated channel and how is it opened? What does this produce?

opens when 2 AChs bind to receptor channel allowing influx of Na+ and efflux of K+ out of postsynaptic cell and produces EPSPs

Why does the inward flow of Na+ dominate in a nicotinic ACh channel?

the influx is fast and the channel is more permeable to it so it acts like an Na+ channel but still allows K+ in just slower

Are the muscarinic receptor and channel separate?

Yes

NT in G-protein coupled channels: is the NT receptor part of the ion channel? What is it and what does it activate?

No, it’s a 1 subunit membrane polypeptide and it activates ion channel indirectly through G-proteins

For muscarinic ACh channel, what does the binding of 1 ACh mean?

activation of G-protein cascade which affects gated K+ channels. opens some causing hyperpolarization and closes some causing depolarization

What does acetylcholinesterase do? Where is it located?

inactivates ACh (NT), terminating its action and no longer stimulating the receptor. located in cleft

What are neurons that use acetylcholine as a NT referred to and often found?

cholinergic neurons, often found @ neuromuscular junction

What are large synapses on skeletal muscle called?

end plates or neuromuscular junctions (NMJ)

What do end plates / NMJ produce, open, and cause?

produce large EPSPs called end-plate potentials, open VG channels beneath end plate, cause muscle contraction