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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 potentiail 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
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 entiepileptics
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
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