PSYCH 202: Physiology of the Neuron

Structure and Function of the Neuron & Synapse What are the three main structural components of a neuron and their functions?

• Dendrites: receive information via synapses

• Soma: contains nucleus and metabolic machinery

• Axon: sends information to other neurons via APs

Structure and Function of the Neuron & Synapse What is the function of an axon

Propagates APs away from the cell body to communicate with other neurons

Structure and Function of the Neuron & Synapse What is myelin and what is its function

A fatty insulating sheath around axons that:

• increases conduction speed

• prevents current leakage

• improves efficiency of neural transmission

Structure and Function of the Neuron & Synapse Which glial cells produce myelin in the CNS

Ogliodendrocytes

Structure and Function of the Neuron & Synapse What are the major functions of astrocytes

• structural support

• nutrient delivery

• formation and maintenance of the blood-brain barrier

• regulation of the extracellular environment

Structure and Function of the Neuron & Synapse What is the blood-brain barrier

A protective barrier formed largely by astrocytes that regulates which substances can enter the brain from the bloodstream

Structure and Function of the Neuron & Synapse What occurs at a synapse

Neurotransmitters are released from one neuron and bind to receptors on another neuron, influencing its activity

Resting Membrane Potential

The electrical charge difference across the membrane of a resting neuron

-70 mV

Resting Membrane Potential What does a resting membrane potential of −70 mV mean?

The inside of the neuron is 70 millivolts more negative than the outside

Resting Membrane Potential Which membrane potential must typically be reached to trigger an AP

~ -60 mV (threshold)

Resting Membrane Potential Which ions are most important in generating the resting membrane potential

• Na+

• K+

• Cl-

Resting Membrane Potential Why is the inside of a resting neuron negatively charged

Because negatively charged proteins and anions are trapped inside the cell and cannot cross the membrane

Resting Membrane Potential What two forces act on ions across the neuronal membrane

• Diffusion pressure

• Electrostatic pressure

Resting Membrane Potential What two forces act on ions across the neuronal membrane: diffusion

movement down concentration gradient

Resting Membrane Potential What two forces act on ions across the neuronal membrane: electrostatic pressure

attraction/repulsion by electrical charge

Resting Membrane Potential What forces act on K+ at rest

• Diffusion pushes K+ out of the cell

• Electrostatic pressure pulls K+ into the cell

Resting Membrane Potential What forces act on Na+ at rest

• Diffusion pushes Na+ into the cell

• Electrical attraction also pulls Na+ into the cell

Both forces favour Na+ entry

Resting Membrane Potential Why does Cl- act as a regulator of excitation

Because diffusion and electrostatic forces are approximately balanced, making Cl- effective at stabilising membrane potential

Resting Membrane Potential What is the role of the sodium-potassium pump

Restores ion concentrations by pumping:

• 3 Na+ out

• 2 K+ in

using ATP

Resting Membrane Potential Why is the sodium-potassium pump important

It maintains the ion gradients necessary for resting membrane potential and APs

Post-Synaptic Potentials What are ligand-gated ion channels

Ion channels that open when neurotransmitters bind to their receptors

Post-Synaptic Potentials EPSP

Excitatory Post-Synaptic Potential

• depolarisation that brings neuron closer to threshold and increases the likelihood of firing

Post-Synaptic Potentials How does Na+ produce EPSP

Opening Na+ channels allows Na+ to enter the neuron, making the membrane potential more positive

Post-Synaptic Potentials What is an IPSP

Inhibitory Post-Synaptic Potential

• a change in membrane potential that makes firing less likely

Post-Synaptic Potentials How does K+ produce an IPSP

Opening K+ channels allows K+ to leave the neuron, making the inside more negative

Post-Synaptic PotentialsHow does Cl- produce an IPSP

Opening Cl- channels allows Cl- to enter the neuron, increasing negativity and opposing excitation

Post-Synaptic Potentials What is neural integration

The process by which a neuron combines all excitatory and inhibitory inputs to determine whether it will fire

Post-Synaptic Potentials What is the axon hillock

The region where summed inputs are evaluated and APs are initiated

Post-Synaptic Potentials Why is the axon hillock important

It acts as the neuron’s decision point for firing

Post-Synaptic Potentials What is spatial summation

The combination of EPSPs and IPSPs from multiple synapses occurring simultaneously

Post-Synaptic Potentials When does spatial summation occur most effectively

When active synapses are close together on the neuron

Post-Synaptic Potentials What determines whether a neuron fires

The balance between excitatory and inhibitory inputs reaching axon hillock

The Action Potential

A rapid, all-or-none electrical signal that travels down the axon

The Action Potential: what triggers it

When EPSPs depolarise the axon hillock threshold (~ -60 mV)

The Action Potential: Why are EPSPs called graded potentials

Because they vary in size depending on the amount of synaptic input

The Action Potential: What type of ion channels initiate an AP

Voltage-gated Na+ channels

The Action Potential: what happens during depolarisation

Voltage-gated Na+ channels open and Na+ rushes into the cell, making the inside more positive

The Action Potential: what happens to Na+ channels during repolarisation

They close (become inactivated)

The Action Potential: why must neurons repolarise after firing

To return to resting membrane potential and prepare for another AP

The Action Potential: why is the AP considered “all-or-none”

Once threshold is reached, the AP occurs at full strength regardless of stimulus size

The Action Potential: what is the sequence of events in one AP

1) Resting Potential

2) Thresholf Reached

3) Na+ channels open (depolarisation)

4) K+ channels open

5) Na+ channels close

6) K+ exits (repolarisation)

7) Return to resting state

Axonal Conduction

The propagation of an AP down an axon

Axonal Conduction Why can unmyelinated axons conduct more slowly

Current can leak across the membrane, reducing efficiency

Axonal Conduction How does myelin improve conduction

It insulates the axon and prevents current leakage

Axonal Conduction What is saltatory conduction

The process by which APs appear to jump between noded of Ranvier

Axonal Conduction Why is saltatory conduction faster than continuous conduction

The AP only needs to be regenerated at the nodes rather than every point along the axon

Axonal Conduction Where is the AP regenerated during saltatory conduction

at the Nodes of Ranvier

Axonal Conduction What is the overall flow on info through a neuron

Dendrites —> Soma —> Axon Hillock —> Synapse —→ Next Neuron

Summarise how neurons communicate from start to finish

1) Neurotransmitter binds to ligand-gated channels

2) EPSPs and IPSPs are generated

3) Inputs summate at the axon hillock

4) Threshold reached —> AP generated

5) AP travels down axon

6) Saltatory conduction speeds transmission in myelinated axons

7) Neurotransmitter released at synapse

8 ) Signal passed to next neuron