Psyc 255 : Neural communication

4 Key ions

• Potassium (K+)

• Sodium (Na+)

• Chloride (Cl-)

• Calcium (Ca2+)

These are distributed unevenly inside vs. outside a cell

There are also large negatively charged proteins that stay inside the cell (anions)

Diffusion and electromagnetic attraction/repulsion

Diffusion : Ions move from an area of high concentration to low concentration

Electromagnetic attraction/repulsion : Ions move towards their opposites and away from their likes

Gradient

• A gradient is a measure of how much one quantity changed in relation to another e.g., high vs. low concentration, positive vs negative ions

If ions want to move, why don’t they simply move across the neuron ?

• Because the cell membrane is a selectively permeable barrier

• It has channels and pumps that maintain the gradient of ions

Cell membrane - membrane potential

• The voltage difference between the inside and outside of the neuron

• This voltage difference occurs due to the different electrical charges of the ions inside vs outside of the cell

Cell membrane - resting potential

When a neuron is at rest, there are more negatively charged ions inside the cell relative to outside the cell

• This is the resting potential, the inside of the cell is 70mV more negative than the outside of the cell

Changes in membrane potential - Graded potentials

• The membrane has channels, some of which can open and close to let ions in and out

• What happens to the membrane potential when positive ions enter the neuron?

  • It becomes less negative → depolarization

• What happens when positive charge leaves ?

  • The membrane becomes more negative → hyperpolarization

Inhibitory Post-Synaptic Potential (IPSP)

Hyperpolarisation makes the cell less likely to fire

Excitatory Post-Synaptic Potential (EPSP)

Depolarisation makes the cell more likely to fire

Summation of inputs

• EPSPs and IPSPs last only a few milliseconds at most before they decay and the resting potential is restored

However, EPSPs and IPSPs can be summed

• Temporal summation

  • Pulses that occur at approximately the same time on a membrane are summed

• Spatial summation

  • Pulses that occur at approximately the same place on a membrane are summed

Voltage-gated ion channels : closed at rest

• Some channels open their gates due to chemical activity

• Other channels open their gates due to changes in voltage

• At rest, voltage-gated Na+ and K+ channels are closed

Voltage-gated ion channels : threshold of approx. -55mV

• If the membrane potential reaches a certain threshold (-55mV) then the voltage-gated ion channels detect this and change shape!

• This is the start of an action potential : a neuron “firing” an electrical signal down the axon

Threshold

• The axon Hillock is where EPSPs and IPSPs are summated

• If the addition and subtraction of charges ultimately reaches -55mV, this pushes a neuron past the threshold

• Thus, action potentials are initiated at the axon hillock

Action Potential

• A sequence of events occurs when an action potential is triggered

  • Neuron is at rest (1)

  • Rapid depolarisation (2)

  • Repolarisation (3,4)

  • Hyperpolarisation (5,6)

  • Back to rest (1)

Rapid depolarisation (rising phase)

• When the cell reaches threshold, gate 1 of voltage-gated Na+ channels open quickly

• Na+ floods in, triggering even more Na+ channels to open

• K+ channels remain closed ; slowpokes

• This rapidly makes the membrane potential less negative and more positive

Repolarisation (falling phase)

• Voltage-gated Na+ channels clog and are inactivated

  • Na+ can no longer enter the neuron, stopping the increase in membrane potential

• Voltage-gated K+ channels finally opens

  • K+ can exit the neuron, allowing the membrane potential to decrease again

  • Unlike Na+, these channels cannot inactivate; they are only open or closed

Hyperpolarisation (undershoot)

• When the resting potential is reached, the K+ channels take a few extra ms to swivel closed

• During this time K+ continues to leave the neuron, making the neuron more negative than usual

• This undershoot helps unclog voltage-gated Na+ channels, allowing them to close

• Na+/K+ pumps then help restore the neuron back to its resting potential of -70mV

  • 2 potassium ions are brought inside, and 3 sodium ions are pumped out

All or none law

• A neuron will either fire, or it won’t

  • There are no stronger or weaker signals, it always fire at maximum strength

• When you record APs overtime, you can see that what matters is how frequently they occur. Not how strong or weak they are, since they are all the same strength

  • Stronger stimuli produce more rapid spikes

Refractory periods

• Absolute refractory period

  • During repolarization a new action potential cannot be elicited because gate 2 of sodium channels, which are not voltage-activated is closed

• Relative refractory period

  • During hyperpolarization, a stronger electrical current is required to produce another action potential

  • Potassium channels are still open

Saltatory conduction

• Some neurons have myelinated axons

  • Myelin help insulate the neuron and speed up action potentials

• Each piece of myelin is seperated by a gap: node of ranvier

• APs jump from node to node, a process called saltatory conduction

Transduction

• Sensory receptors within sense organs detect energy and convert that energy into nerve impulses — called transduction

• All sense receptors have a mechanism that opens ion channels on sensory neurons

• This allows the cell to reach threshold potential, thus opening voltage-senstive channels

Muscle movement

• Motor neurons send nerve impulses to synapses on muscle cells

• Axon terminals/boutons of motor neurons release acetylcholine onto the end plate of a muscle fiber

  • Acetylcholine is a neurotransmitter, it attaches to transmitter-activated channels

  • These transmitter sensitive channels open, bringing muscle fibers to threshold potential, initiating an AP thus contracting the muscle