PHGY 209 FINAL

what encompasses the central nervous system

brain and spinal cord

what are neurons

cells that send electrical signals to each other

what are synapses

points of contact between neurons that facilitate communication

how many synapses are on a neuron

10 thousand

what are the main components of a neuron

cell body, dendrites, axon

what are the components of an axon

initial segment, axon body, presynaptic terminals

what are dendrites and what do they do

they are large branches into the internal environment and receive inputs from other neurons. they increase surface area and contain many synapses.

what happens in the soma

nucleus, mitochondria and ribosomes are located here, this is where metabolism takes place

what does the axon do

enables the neuron to communicate with other neurons

what is the initial segment

where the initial electrical impulse originates

what is the general flow of information through neurons

dendrites to cell body to axon

what is the typical resting potential of a neuron

-60 to -70 mV

what causes the resting membrane potential

the small excess of negatively charged ions in the cell created by concentration gradients for various physiological ions and the selective permeability of the resting membrane to K+ ions

why is there an excess of negatively charged ions inside the cell

the internal environment of neurons is an aqueous environment with free floating ions. it has a slight negative voltage creating a potential difference

what is the concentration of Na, K, and Cl outside the cell

high Na, low K, high Cl

what is the concentration of Na, K, Cl, and amino acids/organic molecules inside the cell

low Na, high K, low Cl, high A

what is the relationship between membrane permeability and K+ ions

the membrane is permeable to K+ so it can easily flow out

at rest, the neuronal membrane is _______ to K+

highly permeable

what happens to K+ ions at rest

they leak out of the cell down the concentration gradient, leaving negative ions inside the cell

when do K+ ions stop flowing out of the cell

when the internal cell environment gets too negative from the anions that K+ left, so it pulls K+ back into the cells

what does the Nernst equation describe

the membrane potential at equilibrium

what is the main factor for determining the neuron resting membrane potential

equilibrium potential for K+

what happens to membrane potential if the concentration of ions outside = the concentration of ions inside

membrane potential = 0

what happens to membrane potential if the ratio of ions outside to ions inside gets closer to 1

it gets more positive and closer to 0

what is the equilibrium potential for K+ and what does it mean

-90mV; K+ leaks out of the cell until the membrane potential gets to -90mV

what causes the resting permeability of the cell membrane to K+

leak channels

what are leak channels

proteins that form K+ selective pores through the membrane

when do leak channels open

at resting membrane potential

what determines an ion’s equilibrium potential

its charge, internal, and external concentrations

what is the equilibrium potential of Na

+70mV

what is the equilibrium potential of Cl

-80mV

why is the resting membrane potential closer to the equilibrium membrane potential of K+ than Na

because Na pushes the membrane potential to +70 and K pushes the membrane potential to -90, and the membrane is more permeable to K, so it has more influence

in a neuron, is the membrane completely impermeable to other ions

no, just more permeable to K+ at rest

______ makes the greatest contribution to the membrane potential

dominant permeability

what does the sodium potassium pump do

it maintains the concentration gradients of Na and K as they constantly leak out and in of the cell

what are action potentials

brief electrical impulses that neurons send to each other to relay information

what is the path of an action potential

initial segment, down the length of the axon, presynaptic terminals

what happens at the peak of an action potential spike

the membrane potential approaches the equilibrium potential of Na

what does it mean for the membrane to depolarize

it is becoming more positive (-70 to +30)

what does it mean for the membrane to repolarize

it is becoming more negative (+30 to -70)

what does it mean for the membrane to hyperpolarize

the membrane potential drops below -70mV

when is the action potential initiated

when the membrane potential depolarizes to a threshold level

what causes the depolarizing phase of action potential

voltage gated sodium channels

what are the 3 critical properties of voltage gated sodium channels

they are closed at resting membrane potential but open when membrane depolarizes, they are selective for Na, the open channel rapidly inactivates stopping the flow of Na ions

what is the absolute refractory period

the time it takes for sodium channels to go from the inactivated state to closed state, limiting how fast neurons can fire action potentials

where are voltage gated sodium channels located along the axon

everywhere, especially at the initial segment

describe the positive feedback loop of the action potential, Na, and Na channels

initially, 10% of Na channels are open, allowing Na to flow in making the initial segment more positive. this causes more Na channels to open (60%) allowing more Na to flow in, making the axon more positive.

what happens when voltage gated sodium channels are inactivated

K+ leaks out of the cell and the membrane potential goes back to -70mV

what has higher density: voltage gated sodium channels or leak potassium channels

voltage gated sodium channels

why are axons specialized for propegating action potentials

because they have a high density of voltage gated Na channels

what happens at the peak of action potential as a result of the high density of Na channels in the axon

Na permeability overtakes the resting permeability for K

what are the 2 causes of the falling phase of action potential

Na channel inactivation and delayed activation of voltage gated potassium channels

when do voltage gated potassium channels open/close

open when membrane is depolarized and closed at resting potential

when are voltage gated potassium channels maximally open

during the falling phase of action potential (repolarizing)

why is it beneficial for voltage gated K channels to be open maximally during repolarization

so K can flow out faster to bring the membrane potential back to -70mV rapidly, allowing action potentials to be very short

why is it beneficial for action potentials to be very short

so you can send more information over time

what causes action potential propagation

the spread of electrotonic currents from the site of the action potential, exciting adjacent regions of the axon

what is the process of action potential propagation

an Na channel will release Na into a section of an axon, making it positive. the membrane potential will increase to +30mV, and the positive charge spreads to the adjacent section to depolarize it

why do action potentials move in one direction

because channels that have already released Na ions undergo their absolute refractory period, so they are still inactive

what is the relative refractory period

a longer period where voltage gated potassium channels are open and the membrane potential overshoots its resting level. axon is less excitable and less likely to fire an action potential

how do neurons send information

frequency and pattern of action potentials

what would the frequency and pattern of action potentials be if you were stimulated with a dull needle

low frequency of action potentials

what would the frequency and pattern of action potentials be if you were stimulated with a sharp needle

high frequency of action potentials

what does tetrodotoxin do

inhibits the sodium channels and can cause muscle paralysis

what does batrachonotoxin do

irreversibly open Na channels

what will happen if you take batrachotoxin

neurons will constantly fire action potentials because they are depolarized from all the Na inside the cell, and the patient can die from seizues.

what kinds of drugs can block sodium channels therapeutically

local anesthetics and antiepileptics

how does lidocane work

it is injected into the nerves of the face blocking the voltage gated Na channels, preventing action potentials from being fired

what are antiepileptics

prophylactic (preventative) drugs that block voltage gated Na channels to suppress abnormal seizure activity

when is rapid propagation useful

during survival and in situations that require rapid reflexive responses

propagation rate of the action potential is proportional to _______

axon diameter

what kinds of fibers are present in slow pain and how fast do they move

C fibers moving at ½ m/s

how has evolution solved the problem of rapid propagation

by making fatter axons

how do vertebrate neurons create an axon with high conduction velocity

they wrap the axon in myelin

what is myelin

an insulator that wraps around the outside of the axon using its extensions to allow the charge to travel farther and faster down the axon

how is myelin formed in the PNS

schwann cells

how is myelin formed in the CNS

oligodendrocytes

what are the nodes of ranvier and how large are they

the gaps in myelin wrapping with a high concentration of voltage gated Na channels approximately 1mm apart

where are there gaps in myelin

at nodes of ranvier and at initial segment

how do you regenerate signals in unmyelinated axons

you need Na channels down the length of the axon

what do the nodes of ranvier do

they enable to signal to be regenerated at periodic intervals by allowing na to move into the axon

where are there high concentration of Na channels

at the initial segment and at the nodes of ranvier

how is multiple sclerosis caused

autoimmune disorder where immune system attacks oligodendrocytes so there is loss of myelin. myelin can regrow causing episodes of MS but overtime the constant stripping of myelin damages the axons preventing them from propagating action potentials properly

what is white matter

regions of the brain and spinal cord that contain mostly myelinated axons

what is gray matter

areas of the brain that contain cell bodies, dendrites, and synapses

what types of synapses are typically excitatory

spine synapses

what types of synapses are typically inhibitory

shaft synapses

what are spines

extensions that protrude from the dendrites

what type of synapse composes majority of the brain/cerebral cortex

spine synapses

what are axodendritic synapses

synapses on the dendrites

what are axosomatic synapses

synapses on the cell body, typically inhibitory

what happens at the axoaxonic synapse

2 presynaptic terminals from 2 different axons make a synapse

what does it mean when a neuron is divergent

when a neuron makes synapses with many other neurons

what happens to the action potential as it travels down branching axons and what is the effect

a full action potential gets sent down both branches, sending the same message to many different neurons and body parts

what is the synaptic cleft

the space between the presynaptic terminal and post synaptic spine

why is the synaptic cleft so small

because the presynaptic terminal releases chemicals that have to diffuse across the cleft

what are the 2 kinds of presynaptic vesicles

floating and docked

what are floating presynaptic vesicles

randomly distributed backup vesicles floating in the presynaptic terminal

what are docked presynaptic vesicles

vesicles lined up in the active zones tethered to the membrane, involved in synaptic activity

what is postsynaptic density

the dark area under active zones that are packed with proteins involved in synaptic transmission