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cell lecture 10
Electrical Signalling (Basics)
All cells use active transport of ions to move ions (charged particles) across their membranes.
This creates an imbalance of ions on either side of the membrane.
Since ions carry charge, this imbalance leads to an electrical potential (a difference in charge across the membrane).
This electrical potential is like stored energy — we call it voltage.
Electrical Signalling in Neurons
Neurons are specialized cells that are really good at using this electrical potential.
They use it to send rapid signals over long distances — this is how the nervous system works.
What makes neurons special is that they can create an action potential — a quick, temporary electrical charge that travels down the neuron in response to a stimulus.
Parts of the Nervous System
The nervous system is divided into two main parts:
1. Central Nervous System (CNS)
Brain
Spinal cord
2. Peripheral Nervous System (PNS)
Peripheral nerves (nerves outside the brain and spinal cord)
Nerves = bundles of neurons working together.
Types of Neurons
There are four main kinds of neurons:
Motor Neurons → These control muscle movement. They carry information from your brain to your body.
Sensory Neurons → These detect things like touch, smell, sound, taste, and pain. They carry information from your body to your brain.
Interneurons → These are found only in the brain and spinal cord. They connect and process information between neurons.
Autonomic Neurons → These control things you don’t think about, like your heartbeat, blood pressure, digestion, and body temperature.
Neuron Structure (What a Neuron Looks Like)
Neurons have a specific shape and structure so that information flows in one direction. This is called being polarized.
Here are the key parts:
Dendrites → These are like branches that receive information from other neurons.
Cell Body (Soma) → This part contains the nucleus. It collects information from the dendrites and decides if it should send a signal.
Axon → This is a long extension that carries the electrical signal (action potential) away from the cell body toward other neurons or muscles.
Axon Hillock → This is the part where the action potential starts.
Synapse → This is the small gap at the end of the axon where the neuron passes the signal to the next cell using chemicals.
Principles of Ion Transport
Diffusion (Principles of Ion Transport)
Diffusion is when particles move from an area where there are lots of them (high concentration) to an area where there are fewer of them (low concentration) — basically, they spread out naturally.
In cells:
Inside the cell, there are a lot of potassium ions (K⁺).
Outside the cell, there are fewer potassium ions.
This creates what we call a potassium ion gradient (a difference in potassium ion concentration).
Because of this gradient, potassium ions will naturally move (diffuse) out of the cell to where there are fewer potassium ions.
Electroneutrality (Principles of Ion Transport)
Electroneutrality means that in any solution, the total positive charges and total negative charges must be balanced — the solution can't have extra positive or negative charges floating around by themselves.
So, every ion has a counterion — an ion with the opposite charge.
In the cell:
Inside the cell (in the cytosol), there are lots of potassium ions (K⁺), which are positive.
These K⁺ ions balance out the negative ions (anions) trapped inside the cell.
Outside the cell:
The main positive ion is sodium (Na⁺).
The counterion (the negative one) is chloride (Cl⁻).
So, both inside and outside the cell, positive and negative ions are balanced to maintain electroneutrality.
What is Membrane Potential?
The membrane potential (Vm) is the difference in electrical charge between the inside and outside of a cell.
All cells have a membrane potential, but it’s especially important in nerve and muscle cells.
When the cell is at rest:
The inside of the cell is more negative.
The outside of the cell is more positive.
This charge difference is called the resting membrane potential.
In many cells, it is around -60 mV (millivolts).
The minus (-) sign means the inside is more negative than the outside.
What creates this Membrane Potential?
The membrane potential is mainly created by:
Different concentrations of ions inside and outside the cell.
Trapped negatively charged molecules inside the cell.
What happens to these ions?
Potassium (K⁺): There’s a lot of K⁺ inside the cell. It tends to diffuse out of the cell because it wants to move from high to low concentration.
Sodium (Na⁺) and Chloride (Cl⁻): There’s a lot of Na⁺ and Cl⁻ outside the cell. They tend to diffuse into the cell.
Why is the inside negative?
The inside is negative because:
More positive ions (like K⁺) leave the cell.
Negative molecules (like proteins, DNA, RNA) stay trapped inside — they can’t leave.
This leads to a net negative charge inside and positive charge outside.
Electroneutrality and Potential
Electroneutrality:
In any solution, the total positive charge equals the total negative charge. Ions always appear with an oppositely charged counterion.Inside vs. Outside the Cell:
Outside (Extracellular):
High Na⁺ (sodium) and Cl⁻ (chloride), low K⁺ (potassium).Inside (Cytoplasm):
High K⁺, low Na⁺ and Cl⁻, plus trapped anions like proteins, DNA, RNA (which are negatively charged).The overall charge inside is -60 mV compared to outside.
→ This is the membrane potential.
Why is there a Membrane Potential?
The system is balanced overall, but there’s an unequal distribution of ions across the membrane.
This creates a voltage difference (electrical potential).
Membrane potential = the voltage difference due to this uneven ion distribution.
Effect of Ions on Membrane Potential
K⁺ (Potassium):
Moves out of the cell → makes the inside more negative.Na⁺ (Sodium):
Moves into the cell → makes the inside more positive (causes depolarization).Cl⁻ (Chloride):
Wants to move into the cell but is pushed back by the negative inside, so it usually only enters when positive ions also enter.
Electrical Excitability
Neurons and muscle cells can change their membrane potential quickly.
This is called electrical excitability.When a stimulus occurs, it causes an action potential:
Membrane potential shifts from negative → positive → back to negative.
This happens in milliseconds.
Resting Membrane Potential
When the cell is not excited, the membrane potential stays around -60 mV.
This is called the resting membrane potential.
It is maintained by:
Leak channels: allow some ions to move passively.
Na⁺/K⁺ ATPase pump: uses energy (ATP) to push Na⁺ out and K⁺ in, keeping the concentration gradient.
What is an Action Potential?
An action potential is a quick electrical signal that travels along the membrane of neurons and muscle cells.
It is caused by the movement of Na⁺ (sodium) and K⁺ (potassium) ions through voltage-gated channels.
1. Resting Potential (about -60 mV)
At rest, the inside of the cell is more negative.
All Na⁺ and K⁺ channels are closed.
2. Rapid Depolarization (to +40 mV)
A stimulus opens voltage-gated Na⁺ channels.
Only open a little at first and then when +40mV is reached they open completely
Na+ keeps flowing in until +40mV is reached and then the Na+ channel closes
3. Slow Repolarization (to -75 mV, undershoot)
At +40 mV, Na⁺ channels close.
Voltage-gated K⁺ channels open.
K⁺ exits the cell → brings the potential back down.
It overshoots to -75 mV because K⁺ channels are slow to close.
4. Stabilization (Return to -60 mV)
K+ channels are still open and the Na+ channels are still closed.
Often times the membrane potential becomes more negative than the resting potential but eventually the membrane returns to its resting state.
The membrane potential is now restored, but the ion concentrations not yet.
Ion pumps (Na/K ATPase) restore the original balance of ions, so that a new action potential can develop
What is Propagation?
Once an action potential is triggered at one part of the membrane, it doesn't just stay there.
It travels down the membrane so the signal can move along the entire cell.
This movement is called propagation.
1. Stimulation & Depolarization Starts
A stimulus (like a neurotransmitter) causes the membrane at one spot to depolarize.
Na⁺ channels open → Na⁺ rushes into the cell.
The inside of the membrane at that point becomes positive.
2. Membrane Polarity Reversed
That area that depolarized is now positive inside and negative outside.
The positive charge spreads sideways along the membrane.
3. Threshold in Adjacent Area
The positive charge reaching the next part of the membrane causes it to reach threshold.
When threshold is reached → Na⁺ channels open in the next section → Na⁺ rushes in there too → Depolarization spreads.
4. Repolarization of Original Area
The first area now repolarizes:
K⁺ channels open → K⁺ leaves the cell → inside becomes negative again.
So while the action potential moves forward, the old area returns to rest.
5. The Chain Reaction
This process continues:
Depolarization at one spot triggers depolarization in the next.
Behind it, the membrane repolarizes and resets.
This is how the signal travels down the axon or membrane.
What is Myelination?
Myelin is a fatty insulating layer that wraps around the axons of neurons.
Its main job is to: ✅ Increase the speed of electrical signals (action potentials).
✅ Improve efficiency (less energy used, faster communication).
Who makes the myelin?
In the Peripheral Nervous System (PNS):
→ Made by Schwann cellsIn the Central Nervous System (CNS):
→ Made by Oligodendrocytes
Structure:
Myelin doesn’t cover the entire axon like a sleeve.
There are small gaps between the myelinated sections called Nodes of Ranvier.
These nodes are the only places where depolarization (Na⁺ influx) happens in a myelinated neuron.
How does myelination help?
The myelin sheath acts like an insulator.
No depolarization happens where there’s myelin.
This saves energy and prevents ion leakage.
Action potential jumps from one Node of Ranvier to the next.
This is called saltatory conduction (from Latin "saltare" = to jump).
It’s much faster than unmyelinated conduction.
Because the signal jumps, transmission is:
Fast
Efficient
Can travel long distances without losing strength.
What is Saltatory Conduction?
Saltatory conduction is how an action potential "jumps" from one Node of Ranvier to the next in a myelinated neuron.
How does it work?
1. Where does the action potential happen?
Only at the Nodes of Ranvier.
The myelin sheath blocks depolarization — no Na⁺ channels under the myelin.
At the node:
Na⁺ channels open → Na⁺ rushes in → action potential generated.
2. What happens between the nodes?
No new action potential.
The positive charge from the Na⁺ entry at one node spreads passively inside the axon toward the next node.
This passive spread is fast because:
Myelin prevents ion leakage.
Less resistance.
3. Renewal at the next node
When the charge reaches the next node, it brings the membrane potential to threshold.
Na⁺ channels at this next node open → new action potential starts there.
The process repeats all the way down the axon.
What is a Synapse?
A synapse is the connection point where a neuron communicates with:
Another neuron (axon → dendrite, cell body, or axon).
A non-neuronal cell (like a muscle cell at a neuromuscular junction).
Types of Synapses (Based on Transmission Mode)
1. Electrical Synapse
Connected by Gap Junctions (channels that connect two cells).
Signal passes directly from one cell to the next.
Very fast, but less regulation (cannot change signal strength).
Found in places where speed is essential → e.g., reflexes.
No neurotransmitters needed.
➡ Disadvantage:
Electrical synapses lack gain → signal strength cannot be increased.
2. Chemical Synapse
There is a small gap (Synaptic Cleft) between the two cells.
Signal is passed using neurotransmitters.
Process:
Action potential arrives at axon terminal.
Voltage-gated Ca²⁺ channels open → Ca²⁺ enters presynaptic terminal.
Ca²⁺ triggers vesicles full of neurotransmitter to fuse with the membrane and release neurotransmitterinto synaptic cleft (exocytosis).
Neurotransmitters bind to receptors on the postsynaptic membrane.
This can depolarize the postsynaptic cell and trigger a new action potential.
➡ Chemical synapses are slower but can be regulated.
Kiss and Run
A form of rapid chemical transmission where the vesicle doesn’t fully fuse but quickly releases neurotransmitter and pulls back — for quick, repeatable signaling.
Role of Ca²⁺ at the Synapse
Ca²⁺ influx happens because of voltage-gated Ca²⁺ channels.
Action potential arrives → Depolarization → Ca²⁺ channels open → Ca²⁺ enters.
Ca²⁺ triggers vesicle fusion via SNARE proteins:
v-SNARE (on vesicle) + t-SNARE (on membrane) = fusion
Neurotransmitters are released and bind to receptors.
This can trigger another action potential in the postsynaptic neuron.
Types of Neurotransmitters
Neurons can be classified by the type of neurotransmitter they use:
Glutamate → Major excitatory neurotransmitter.
GABA (γ-aminobutyric acid) → Major inhibitory neurotransmitter.
Acetylcholine → Important for muscle movement (neuromuscular junction), alertness, cognition.
Norepinephrine → Controls wakefulness, attention, autonomic functions.
Dopamine → Involved in motivation, reward, and motor control.
Serotonin → Affects stress, emotion, digestion.
Neurotransmitters can also be:
Neurotransmitters (direct action)
Neuromodulators (indirect, modify signal strength)
GABA (Inhibitory Neurotransmitter)
GABA receptor is a ligand-gated Cl⁻ channel.
[Cl⁻] is higher outside the cell.
When GABA is activated → Cl⁻ enters the cell → inside becomes more negative (hyperpolarization) → inhibition.
Benzodiazepines (Valium, Xanax) and alcohol enhance GABA activity → increase inhibition in the CNS.
Inactivation of Neurotransmission
After neurotransmitter release, the signal must be quickly turned off to prevent overstimulation.
There are two ways to inactivate neurotransmitters:
Direct Reuptake
Neurotransmitter is taken back up into the presynaptic neuron.
Example: Serotonin reuptake inhibitors (antidepressants), Cocaine inhibits dopamine reuptake.
Catalytic Breakdown
Neurotransmitter is broken down by enzymes.
Example: Acetylcholine is broken down by acetylcholinesterase into acetate + choline.
Inhibitors of acetylcholinesterase include nerve gases and pesticides → dangerous because they cause overstimulation of muscles and nerves.
Neurons Receive Both Excitatory & Inhibitory Signals
A single neuron can receive thousands of inputs from other neurons.
Some signals are excitatory (depolarizing) → e.g., Glutamate.
Some signals are inhibitory (hyperpolarizing) → e.g., GABA.
The sum of these inputs determines if the neuron will fire an action potential.