Neurons and Intro to Electrical Signaling

size and scale (from largest to smallest)

• cells

• organelles

  • nucleus, mitochondria, etc/

• viruses

• proteins (macromolecules) - membrane proteins

• molecules

  • amino acids, neurotransmitters, etc

• ions

  • Na⁺, K⁺, Ca⁺², Cl^-

parts of a neuron

• dendrites - receive signals

  • branched structures from the soma

  • each neuron has many

• axon - transfers signal (to the next neuron)

  • one large extension from the cell body

• the signal starts off as electrical within the neuron but becomes chemical when released at the synapse

pathway of signal in a neuron

• electrical signal travels through the pre-synaptic neuron.

• at the end of it, the signal changes to chemical with the vesicle releasing of a neurotransmitter (now we’re on the post-synaptic side).

• the neurotransmitter activates the next neuron making the signal electrical

synapse

• the space between the pre- and post-synaptic neurons

• where vesicles release neurotransmitters, where a neuron communicates with its partner

• can be between

  • neuron

    • axo-dendritic

    • axo-axonic

    • axo-somatic

  • glands (neuroglandular junction)

  • muscles (neuromuscular junction)

chemical signal

neurotransmitters are released at the synapse and bind to proteins on the post-synaptic cell

neurotransmitters can … or … action potentials in the recieving neuron

excite or inhibit

EPSP

• excitatory post-synaptic potential

• moves to the membrane potential CLOSER to threshold for triggering an AP

  • neurotransmitters that let Na⁺ enter chemically-gated channels helps with that

• if they’re not sufficient to reach threshold, an AP will not occur

IPSP

• inhibitory post-synaptic potential

• moves to the membrane potential FURTHER from threshold for triggering an AP

  • neurotransmitters that move K⁺ out or let Cl^- in

summation

• EPSPs and IPSPs are integrated/totaled in the receiving cell

• if these reach threshold (if there’s enough net (+) being inputted), an AP will fire

• types:

  • temporal

  • spatial

temporal summation

• a single neuron sends multiple APs close together

• ex. neuron receives multiple EPSPs from Neuron E₁ at around the same time, triggering an AP

spatial summation

• summation of EPSPs and IPSPs from multiple different neurons

  • ex. the neuron receives an EPSP from Neuron E₁ AND E₂ around the same time, triggering an AP

  • ex. the neuron receives an EPSP from Neuron E₁ and an IPSP from Neuron I around the same time

    • the EPSP and IPSP cancel each other out. no AP is triggered

explain action potential

• whether the signal is (+) or (-) depends on whether the cell itself is (+) or (-)

• Na⁺ ions rush into the cell, increasing membrane potential

• K⁺ ions rush out of the cell, decreasing membrane potential

action potential

spike in the membrane potential (voltage) that travels like a wave down the axon

membrane potential

• the cell membrane does not allow large, polar, or charged (ions) particles across the membrane

• if the inside and outside of the cell have different amounts of charged particles, then the cell has membrane potential

• this difference in charge produces an electrical potential across the membrane, which is measured in volts

potential voltage

• the difference in electrical charge between the outside and inside of the cell

• inside of the cell has higher amounts of potassium (+), proteins, DNA, and other negatively charged objects (-)

• outside of the cell has higher levels of sodium (+)

what does this mean?

according to diffusion, would Na+ want to move in or out of the cell?

• the outside is defined as 0 mV, so the inside is -70 mV compared to the outside

• Na+ would want to move in the cell because it’s (-) and follow the electrochemical gradient

resting membrane potential (RMP)

• the membrane potential of a cell at rest

  • it’s not changing (no IPSPs, EPSPs, or AP) for a resting neuron

• a neuron’s RMP is -70 mV

  • so by default it has more negative charges than positive charges

what happens to the neuron when membrane potential INCREASES

• the neuron has either gained positive charges or lost negative charges

  • ex. sodium (Na⁺) is moving into the cell due to the electrochemical gradient

what happens to the neuron when membrane potential DECREASES

• the neuron has either gained negative charges or lost positive charges

  • ex. potassium (K⁺) is moving out of the cell due to the electrochemical gradient

how do ions pass through the membrane?

• ions cannot pass directly through the membrane, but only through protein channels (border checkpoints specific to the ion) when they’re open

what happens if protein channels remain open? how do cells delay this?

• Na⁺ and K⁺ will continue to move in/out of the cell until a balance of concentration and electrical charge is achieved (equilibrium)

• this means there will be no more electrochemical gradient, which means the neuron can’t send APs

• this is why protein channels will only open for fractions of a second at a time, only letting tiny amounts of Na⁺ and K⁺ move in/out of the cell… but that still only makes equilibrium happen slower

how do cells specifically prevent concentration/electrical equilibrium

• ion pumps use ATP to physically move ions against the concentration gradient (from areas of low concentration to high concentration)

  • needs ATP because you are putting in that physical effort of going against natural flow of particles

• Na⁺/K⁺ pump move 3 Na⁺ out for every 2 K⁺ in

  • ensures the outside will be guaranteed more (+) to keep concentration gradient

Na⁺/K⁺ pump steps

1. three Na⁺ bind inside the pump

2-3. ATP provides the energy to change the shape of the protein, moving Na⁺ outside the cell

4. K⁺ binds to the protein

5-6. protein opens inside the cell and K⁺ is released into the cell

protein ion channels

• protein machines that allow specific ions to diffuse through

• can be always open (leak channels) or opened by specific triggers

• named based on the trigger and the ion allowed to through

  • “[trigger] gated [ion]”

• types of triggers

  • ligand (specific molecule binds to the receptor which opens the gate, like a key)

  • temperature (capsaicin and mint), pressure, voltage, etc

• the trigger will change the shape of the protein causing it to open

neuron

• basic unit of the nervous system

• take info in from the surroundings and respond to it

• interconnected with billions of other neurons

cell body/soma

• the part of the cell with all other organelles (nucleus, golgi apparatus, etc)

• metabolizes, takes in/uses energy, does normal cell funcitons

dendrites

• extensions from the cell body

• connected to other neurons and receive information in the form of neurotransmitters

• have dendritic spines which store special synapses and important for learning/memory

axon

• where information is transferred

• begins at axon hillock and moves to terminals

what is the direction of a signal in a neuron

• dendrite, axon, terminals

myelin sheath

• oligodendrocyte or schwann cell mylenating the axon if the signal needs to travel over a long distance

• lets information travel faster

• not in all neurons, esp in shorter distance

nerve fiber

• an axon wrapped around a myelin sheath

synapse

• connection between neurons

• neurons never physically touch

neuron structure

• ask yourself how many poles are coming off of the cell body?

• give clues to function

• multipolar (most common)

• bipolar

• unipolar

• anaxonic (no axon)

afferent/sensory neurons

• transfer information towards the CNS

efferent/motor neurons

• transfer information AWAY the CNS

interneurons

• connect afferent/efferent neurons

chemical synapse

1. AP moves down the neuron

2. triggers the release of neurotransmitters into the gap that dock with receptors on the other side

electrical synapse

1. AP moves down the neuron and is triggered by voltage gated channels

2. opens more voltage gated channels at the synapse

3. pros: fast

4. cons: no control

synaptic vesicles

carry neurotransmitters

• dendrites

• soma/cell body

• nucleus

• myelin sheaths

• axon

• axon terminals

what happens in a chemical synapse

• AP moves down the axon

• triggers the release of neurotransmitters into the synaptic cleft (what makes it chemical)

• the neurotransmitters dock to chemically gated channels/receptors on the other side which can lead to an AP on the other side

what happens in an electrical synapse

• AP moves down the axon

• travels directly to the other neuron through channel proteins, still electrical

• pros: fast

• con: no control

vesicles

filled with neurotransmitters

what happens in chemical synapse (more detailed)

• AP moves down the axon

• the depolarization of the neuron opens up the calcium (+) voltage-gated channels, letting Ca²⁺ enter and dock with chemicals in the vesicles

• this lets the vesicles dock with proteins at the end of the presynaptic side and release the neurotransmitters in the cleft where they can dock with chemically gated channels on the other side

types of channels

• leak channels

• voltage-gated channels

• chemically-gated channels

leak channels

• for sodium (Na⁺) and potassium (K⁺)

• establish resting potential

• constantly open

• help with AP transmissions

voltage-gated channels

• closed or opened by changes in membrane potential

• depends on whether potential was reached or not, or at the default -70 mV

chemically/ligand-gated channels

• when neurotransmitters gap with it, they open up (closed otherwise)

• can move the neuron towards or away from action potential (-55 mV)

long-term potentiation

• if you keep firing the same neurons (if you keep remembering something), the cell will actually build more receptors/channel proteins to be better adapted to recieve it

what the fucking kind of neuron is this

multipolar

what the fucking kind of neuron is this

unipolar

what the fucking kind of neuron is this

bipolar

what the fucking kind of neuron is this

anaxonic

also say what direction the signal is going

1. nucleus

2. soma

3. dendrites

4. axon

5. axon hillock

6. myelin sheath

7. dendrites

it is going from the dendrites to the axon collaterals

default voltage of a cell/resting membrane potential

• -70 mV compared to the outside (0 mV)

• inside of the cell has higher amounts of potassium (+), proteins, DNA, and other negatively charged objects (-)

• outside of the cell has higher levels of sodium (+)

voltage needed to reach potential

• -55 mV

why would Na+ want to flow inside a neuron

• diffusion (concentration/chemical gradient) - there is less Na+ on the inside

• make the electrical charge 0 (electrical gradiet)

  • Na⁺ goes to -70 mV cell

• electrochemical gradient

electrochemical gradient principles

• when the concentration and electrical agree, the molecule moves

• if the concentration and electrical disagree, the molecule doesn’t move

would K+ go in or out of the cell

• there is more K+ on the inside, so by the chemical gradient it would go outside

• but the inside is (-) so by the electrical gradient it would go inside

• because they conflict it doesnt move at all