Lecture 17/18 - How do neurons communicate with other cells? Flashcards
3 levels of communication based on how far/close
- Juxtacrine signalling
- Paracrine signalling
- Endocrine signalling
Juxtacrine signalling
- signal targets adjacent cells
- direct contact
- fast
- gap junction - electrical synapse
- between 2 neurons
- 2 cardiac cells
- 2 astrocytes
Paracrine signalling
- signal targets cells in the vicinity
- short distances
- proximal
- chemical synapses involving a neurotransmitter
- between 2 neurons
- between a neuron and a muscle (neuromuscular junction)
Endocrine signalling
- signal targets distant cells
- long distant
- hormones traveling in blood
- eg ADH (antidiuretic hormone) produced by the hydrophyse - targets kidneys and blood vessels
2 types of synapses
- chemical synapse
* electrical synapse
Chemical synapse
- short distance
- uses neurotransmitter
- paracrine signalling
- neuron-neuron communication
- neuron-muscle communication
Electrical synapse
- direct contact
- juxtacrine signalling
- neuron-neuron communication
- muscle-muscle (cardiac cells)
- astrocyte-astrocyte
Presynaptic
axon
• transmits info
Postsynaptic
dendrite
• gets info from many presynaptic
The membrane potential is
electric
Measuring the membrane potential
• inside axon more (-) than outside = already imbalance • glass pipette to conduct electric charge • 1 in axon, 1 just outside = measure membrane voltage
Membrane potential
the electrical charge difference across a membrane
Resting potential
the steady state membrane potential of a neuron
• -60 to -70 mV
• membrane potential when not transmitting a signal
• electric difference = voltage
Voltage
electric potential difference
• force that causes charged particles to move between 2 points
Major charged particles (ions) that carry electric current in neurons
- sodium (Na+)
- potassium (K+)
- calcium (Ca2+)
- chloride (Cl-)
• there is 1 specific channel for each ion
(ions move to opposite charge -
most –> least concentrated)
Ions can diffuse in both directions depending on 2 gradients
- electrical gradient
* chemical gradient
Electrical gradient
voltage difference across the membrane
Chemical gradient
concentration difference across the membrane
The net movement of ions depends on
- the electrochemical gradient
* whether the gates for this ion are open or not
The sodium-potassium pump
constantly moves Na+ to the outside and K+ to the inside
(requires energy)
the pump establishes concentration gradients
in a resting neuron
• K+ more concentrated inside
• Na+ more concentrated outside
In a resting neuron, … channels are the most common opened channels
K+ channels are the most common opened channels
–> leak of K+ outside
• follows the concentration gradient created by the pump
• if too much K+ going outside, there will be an electrical imbalance (more negative inside) pushing the K+ back inside
[less (+) in = more (-) in]
Potassium equilibrium potential
EK
the membrane potential at which the net diffusion of K+ out of the cell ceases
• the point at which K+ diffusion out due to the concentration gradient, is balanced by its movement in due to the negative electric potential
Leak channels
like the K+ channel are ALWAYS OPEN and mainly create the resting membrane potential
Na+ channels
opened mainly by chemical stimulation
mostly closed at rest
Nernst equation
calculates the value of K+ on both sides of the membrane
• WHEN 1 TYPE OF ION CAN CROSS A MEMBRANE
Eion = 2.3 RT/zF log (ion out/ion in)
If the membrane was permeable to only K+, the membrane potential calculated from the Nernst equation would be
-75mV
If actual mV from the Nernst equation doesn’t match predicted (-75mV) assume
other ions moving
permeable to many
In reality, the measured membrane potential int he squid axon is
-66mV
• the resting potential of the axon is not due solely to leak K+ channels
• the neuronal membrane is slightly permeable to other ions, especially Na+ and Cl-, and movements of these ions influence the resting potential
To calculate the real membrane potential, use the
Goldman equation
The Goldman equation takes into account
- all the ions that can diffuse across a membrane
* the relative permeability of the membrane to those ions
The resting potential is mainly due to
K+
Relative permeabilities (p)
• are expressed as percentagegs
Membrane’s permeability to ions
pK = 1.0 pNa = 0.05 pCl = 0.45
membranes permeability to potassium ions is the highest
Goldman equation
Vm = RT / F ln [(pk ion1 out/pk ion1 in) (ion 2) etc]
Which equation do you use to calculate the membrane potential
Goldman equation
should be around -60mV
• closer to actual bc account for all ions and permeability of each
The membrane potential at rest =
-60 mV
Some ion channels are “gated”
- voltage-gated channels
- chemically-gated channels
- mechanically-gated channels
WHEN THEY OPEN AND CLOSE, GATED ION CHANNELS CHANGE THE RESTING POTENTIAL
Voltage-gated channels
respond to change in voltage across a membrane
Chemically-gated channels
depend on specific molecules that bind are alter the protein channel
Mechanically-gated channels
respond to force applied to membrane
Deplozarized plasma membrane
if voltage-gated Na+ channels open
Na+ diffuses in (electrochemical gradient)
and the inside of the cell becomes less negative
(or more positive)
in comparison to its resting condition
inside more (+) = depolarized
Hyperpolarized plasma membrane
if gated K+ channels open and
K+ efflux increases over the normal leak rate
the membrane potential becomes
even more negative
inside more (-) = hyperpolarized
Neurons have many dendrites that can form
synapses with axons of other neurons
Graded membrane potentials
small changes from the resting potential
• the mix of excitatory and inhibitory activity determines whether the graded membrane potential is more positive or more negative than resting
Summation of excitatory and inhibitory postysynaptic potentials is how the nervous system
integrates information
Each neuron receives … or more synaptic inputs
but has … output
1,000 synaptic inputs
only 1 output
an action potential in a single axon, or no action potential
Excitatory and inhibitory potentials are summed over
space and time
too slow = accumulation of info fades, doesn’t result in action potential
binary information
Spatial summation
adds up messages at different synaptic sites
Temporal summation
adds up potentials at the same site in a rapid sequence
The action potential is integrated at
the axon hillock
Positive feedback
Na+ channel opens
more open
–> depolarized above resting potential
= action potential
(big rush of Na+ inside)
Graded membrane potential is a
gradual change in the membrane potential proportional to the inputs received
• travels just locally
Action potentials are
sudden, transient, large changes in membrane potential
• travel far
At resting potential, the voltage gated channels are
closed
• SLIGHT DEPOLARIZATION causes them to open
Voltage-gated Na+ channels are concentrated in the
hillock
Na+ channels open and Na+ rushes into the axon the influx of positive ions causes more voltage-gated channels to open and then more depolarization = positive feedback effect
When enough voltage-gated Na+ channels are opened, the membrane is
depolarized about 5-10 mV above resting potential
THRESHOLD is reached
–> an action potential is generated
How the axon comes back to resting potential
- voltage-gated Na+ channels close
2. voltage-gated K+ channels open
Both Na+ and K+ voltage-gated channels are voltage sensitive, but their dynamic is different
• Na+ gated channels open faster than K+
(goes in = more + to make action potential)
• K+ channels stay open longer
(goes out = more -)
The magnitude of the action potential
does not change between 2 - even far away - recording sites
An action potential is an
all-or-none event
• positive feedback to voltage-gated Na+ channels ensures the maximum action potential
An action potential is self-regenerating because
it spreads to adjacent membrane regions
• when an action potential is stimulated in one region of the membrane,
electric current flows to adjacent areas of membrane
and depolarizes them
• the advancing wave of depolarization causes more Na+ channels to open
and the action potential is generated in the next section of the membrane
• meanwhile, in the region where the action potential has just fired,
the Na+ channels are inactivated and
the voltage-gated K+ channels are still open
rendering this section of the axon incapable of generating an action potential
• hence the action potential cannot back up
but continuously moves forward
regenerating itself as it goes
The action potential moves in 1 direction because of the
refractory period
Refractory period
- K+ still open
- Na+ closed
- action potential not possible (requires delay)
Schwann cells make
myelin
Gap in myelin
nodes of Ranvier
• regularly spaced gaps in the myelin along an axon
• voltage-gated channels are clustered at the node
thus an action potential can only be generated at the nodes
• action potential moves 1 gap to another, travelling along the axon
Action potentials travel faster in
myelinated cells (100m/s) than non myelinated (2m/s)
Invertebrates have
large axons
Vertebrates rely on
myelin
Voltage-gated channels are clustered at
thus node
• thus the action potential can only be generated at the nodes
When positive current reaches the next node
the membrane is depolarized -
another action potential is generated
Action potentials appear to jump from node to node, a form of propagation called
saltatory conduction
Much faster with myelin because
ionic electric current flows much faster through the cytoplasm than ion channels can open and close
• no channel where myelin is
