Chapter 3 - Neurons: Extended Cut

What contributes to the resting membrane potential?

Sodium Potassium Pumps and Passive Diffusion

Sodium Potassium Pump

Establishes the difference in electrochemical gradient;

Continual “pumping” of 3 sodium (Na+) out, 2 potassium (K+) in

Passive Diffusion

Diffusion of potassium (K+) along the cell’s semipermeable membrane;

Sodium (Na+) can NOT diffuse into the cell

When neurons receive input from other neurons, the input either

1. Drives their membrane potential UP (depolarization)

2. Drives their membrane potential DOWN (hyperpolarization)

Excitatory Post-Synaptic Potential (EPSP)

Occurs when positive ions flow into the cell ➡ Causes depolarization

Inhibitory Post-Synaptic Potential (IPSP)

Occurs when positive ions flow out or negative ions flow in ➡ Causes hyperpolarization

Anatomically, where is the decision to fire made?

The axon hillock;

If the membrane potential reaches approx. -40 mV, then an action potential is produced;

Post-synaptic potentials are summed together here

Action Potential Process

1. At rest, sodium (Na+) ions are “waiting” for their chance to diffuse into the cell

2. At threshold, voltage-gated sodium (Na+) channels open, allowing ions to flow into the cell

3. Sodium (Na+) ion influx causes depolarization ➡ Triggers the opening of potassium (K+) channels

4. After the polarization change, the voltage-gated sodium (Na+) channels need a few milliseconds to re-open again ➡ Neuron has a refractory period where it cannot fire an action potential

Spatial Summation

The closer an input is to the cell body/axon hillock, the larger the impact on the membrane potential

Temporal Summation

Inputs closer in time are added together non-linearly

Unidirectional Action Potential Movement

When a neuron fires an action potential, sodium (Na+) enters the nodes of Ranvier, causing enough depolarization to open the next channel

Saltatory Conduction

The rapid “jumping” of action potentials between gaps in myelin sheath

Synaptic Transmission

Process in which a chemical signal is released from one neuron to another

Synaptic Transmission Process

1. Depolarization at axon terminal causes voltage-sensitive calcium (Ca2+) channels to open and flow into the neuron

2. Calcium (Ca2+) entry causes the synaptic vesicles to dock and empty their content into the synapse

3. When released, the neurotransmitters bind to post-synaptic receptors

Not all neurotransmitters bind

1. Re-uptake: channels on the pre-synaptic neuron “suck back in” the neurotransmitter to be repackaged and used again

2. Degradation: enzymes sitting in the synaptic cleft break down neurotransmitters

Synaptic Vesicles

Protected packages in the terminal filled with neurotransmitters

Neurotransmitter Receptors

Sit on the neuron’s membrane waiting for a neurotransmitters to bind to

Two types of Neurotransmitter Receptors

1. Ionotropic: binding of neurotransmitter causes an ion channel to open ➡ Ions enter the post-synaptic cell

2. Metabotropic: binding of neurotransmitter activates a G-protein coupled receptor ➡ Causes complex effects that indirectly change neuron excitability

What are the most abundant neurotransmitters in the brain?

1. Glutamate: almost always excitatory, causes depolarization of the post-synaptic neuron

2. GABA: almost always inhibitory, causes hyperpolarization of the post-synaptic neuron