Neuron Physiology (Exam 3, 4 of 4)

How do neurons send signals down their length?

Electrophysiology

Electrophysiology

cellular mechanisms for producing electrical potentials and currents

Charge

an electrical property that repel and opposite attract

Voltage

a difference in electrical charge between one point and another (results in potential energy)

Pumps

membrane proteins that maintain a concentration gradient by moving substances against their concentration gradient; require cellular energy

Neurons have what pumps in their membranes?

Sodium-potassium and calcium

Channels

protein pores in the membrane that allow ions to move down their concentration gradients (into or out of cell)

Leak (passive) channels

are always open for continuous diffusion

Chemically gated channels

are normally closed, but open when neurotransmitter binds

Voltage-gated channels

are normally closed, but open when membrane charge changes

3 states of voltage-gated na+ channels

resting state
activation state

inactivation state

Resting state

activation gate closed; inactivation gate open; entry of Na+ prevented

Activation State

activation gate opened (due to voltage change); inactivation gate open; Na+ moves through channel

Inactivation state

activation gate open, inactivation gate closed; entry of Na+ prevented; this state lasts a short time

Modality gated channels

normally closed, but open in response to a stimulus other than a chemical or voltage change. found in membranes of sensory neurons

Plasma membrane of functional segments in a neuron

receptive segment

initial segment

conductive segment

transmissive segment

Receptive segment

dendrite and cell body; chemically gated channels

Initial segment

axon hillock; voltage gated Na+ and K+ channels

Conductive segment

axon and its branches; voltage gated Na+ and K+ channels

Transmissive segment

synaptic knobs; voltage gated Ca2+ channels and Ca2+ pumps

Electrical energy

movement of charged particals

Voltage (potential energy) is measured in…

volts or millivolts

Current

movement of charged particles across barrier separating them; can be harnessed to do work

Resistance

opposition to movement of charged particles

An increase in resistance lowers the…

current

Ohm’s law

current = voltage/resistance; current increases with larger voltage and smaller resistance

Resistance decreases when…

channels open

Characteristics of resting neurons

ions are unevenly distributed across membrane (because of pumps)

more K+ inside of cell
more Na+, Cl-, Ca2+, outside of cell

gated channels are closed

inside of cell is more negative than outside

Resting membrane potential (RMP)

charge difference between inside and outside of cell, usually about -70mV

RMP is measured by..

electrodes + voltmeter

__ ______ is the most important factor in setting RMP

K+ diffusion

K+ diffusion steps

K+ moves out of the cell because of concentration gradient

leaving K+ makes inside more negative

Electrical gradient pulls K+ back in

If there was only K+, RMP would be…

-90mV

Na+ effect

Na+ leaks into cell
moves due to concentration and electrical gradient
makes cell less negative (why its -70mV and not -90mV)

Na+/K+ Pump

pumps 3 Na+ out and 2 K+ in
adds about - 3mV to RMP
maintains ion gradients

Graded potentials

small, short-lived changes in the RMP

Graded potentials characteristics

local (stay in one area)
short-lived
can be different sizes
can change RMP

Where do graded potentials happen?

occur in the receptive segment (dendrites/soma)

What causes graded potentials

chemically gated ion channels opening

Types of changes for graded potentials

depolarization and hyperpolarization

Depolarization is…

less negative

Hyperpolarization is…

more negative

Postsynaptic potentials

graded potentials in a postsynaptic neuron

Excitatory Postsynaptic Potential (EPSP)

postsynaptic potential resulting in depolarization; caused by Na+ entry

Inhibitory Postsynaptic Potential (IPSP)

postsynaptic potential resulting in hyperpolarization; caused by K+ exit or Cl- entry

Summation of EPSPs and IPSPs occur at…

axon hillock

The sum may or may not reach…

threshold membrane potential for initiating an action potential

Threshold membrane potential is equal to…

about -55mV

If threshold is reach at the axon hillock, voltage-gated channels…

open and an action potential is generated

Spatial Summation

multiple summation on cell’s receptive regions receive neurotransmitter simultaneously and generate postsynaptic potentials

Temporal Summation

a single presynaptic neuron repeatedly releases neurotransmitter and produces multiple EPSPs within a very short period of time

All or none law

action potential either happens fully or not at all

If threshold is met…

action potential is generated and travels down axon without losing strength

If threshold is not reached…

no action potential and volatge-gated channels stay closed

____ conducts action potentials

the axon

Action potential involves…

depolarization and repolarization

Depolarization

gain of positive charges as Na+ enters through voltage-gated Na+ channels

Repolarization

return to negative potential as K+ exits through voltage-gated K+ channels

Action potential is propagated down axon to…

synaptic knob

Propagation is called a…

nerve impulse/signal

Action Potential Depolarization Steps

1. At resting membrane potential (RMP) → voltage-gated channels are closed

2. Na⁺ enters from nearby area → Na⁺ channels open

3. More Na⁺ rushes in → inside becomes positive

4. Na⁺ channels close (inactivate)

Action Potential Repolarization Steps

5. K⁺ channels open → K⁺ leaves cell → membrane becomes negative

6. K⁺ leaves too long → hyperpolarization (more negative than RMP)

7. K⁺ channels close → return to RMP

Refractory period

period of time after start of action potential when it is impossible or difficult to fire another action potential

Absolute refractory period

No stimulus can initiate another action potential; Na+ channels are open, then inactivated

Relative refractory period

another action potential is possible but the minimum stimulus strength is now greater; some K+ channels are still open

Continuous conduction

occurs on unmyelinated axons

Saltatory conduction

occurs on myelinated axons

Neurotransmitter is released by…

exocytosis

Graded potential occur in the…

receptive region

Graded potentials vs Action potenitals

Graded can be depolarization or hyperpolarization

Action can be depolarization or repolarization

Conduction speed depends on…

axon diameter and myelination

__ fibers conduct faster than _ ones

thicker; thin / myelinated; unmyelinated

Nerve fiber

an axon and its myelin sheath

Impulse frequency…

(Action potentials per second) varies

Neurotransmitters

synthesized by neurons and stored within vesicles in synaptic knobs

are released from the vesicles when action potential tiggers calcium entry

bind to receptor in a target cell

trigger a physiologic response in the target cell

4 main chemical classes on neurotransmitters

acetylcholine

biogenic amines (monoamines)

amino acids

neuropeptides

Acetylcholine

structure differs substantially from other transmitters

Biogenic amines (monoamines)

an amino acid is slightly modified to synthesize the transmitter (catecholamines and indolamines)

Amino acids

include common transmitters glutamate, glycine, GABA

Neuropeptides

chains of amino acids including endorphins, substance P

__ transmitters causes EPSPs; __ transmitters cause IPSPs

excitatory; inhibitory

Direct transmitters bind to receptors that…

are chemically gated channels

Indirect transmitters bind to receptors that…

involve G-proteins and second messengers

Neuromodulators

chemicals that alter responses of local neurons

Facilitation

modulation that causes greater response in postsynaptic neuron; may increase amount of neurotransmitter in cleft or number of postsynaptic receptors

Inhibition

Modulation that causes weaker response; May decrease amount of neurotransmitter in cleft or number of postsynaptic receptors

Nitric oxide

acts as a neurotransmitter, short-lived gas made by postsynaptic neurons. helps strengthen memory

Endocannabinoids

affect same receptors as marijuana, small nonpolar molecules made by postsynaptic neurons. control neurotransmitter release, influence memory, appetite and mood.

Neuronal pools

groups of neurons arranged in specific patterns

4 types of circuits

converging

diverging

reverberating

parallel-after-discharge

Converging circuit

input converges at a single postysynaptic neuron

Diverging circuit

spreads information from one presynaptic neuron to several postsynaptic neurons

Reverbating circuit

use feedback to produce repeated, cyclical activity. once started, it stays active until there is an inhibitory stimulus or synaptic fatigue

Parallel-after-discharge circuit

Input transmitted simultaneously alongseveral paths to a postsynaptic cell.

Since paths vary in numberof synapses, signal arrives at postsynaptic cell at various times.

Believed to be involved in higher-order thinking.