6.5 Neurons and Synapses

Neurons

Specialised cells that function to transmit electrical impulses within the nervous system

membrane potential

The unequal distribution of ions on different sides of the membrane creates a charge difference

Maintenance of a resting potential

* The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)

* It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)

* This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment

* The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)

Resting potential

The difference in charge across the membrane when a neuron is not firing

Action potential

The rapid changes in charge across the membrane that occur when a neuron is firing.

Three stages of action potential

depolarization, repolarization, refractory period

Depolarisation

Refers to a sudden change in membrane potential.

In response to signal in dendrite sodium channels opening within the membrane of the axon

As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium.

The influx of sodium causes the membrane potential to become more positive.

Repolarisation

Following an influx of sodium. Potassium channels open within the membrane of the axon

As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium.

The efflux of potassium causes the membrane potential to return to a more negative internal differential.

Refractory period

In a normal resting state, sodium ions are predominantly outside the neuron and potassium mainly inside.

Following depolarization and repolarization, this ionic distribution is largely reversed.

Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump.

All or none principle

Action potential will only occur at 50 mV

Different charges at different times.

Resting \= -70

action/depolarization \= 40

repolarization \=
little below -70

Oscilloscopes

Scientific instruments that are used to measure the membrane potential across a neuronal membrane.

Myelin

A mixture of protein and phospholipids that is produced by glial cells

The main purpose of the myelin sheath is to increase the speed of electrical transmissions via **saltatory conduction**

Saltatory conduction

In myelinated neurons, the action potentials 'hop' between the gaps in the myelin sheath called the nodes of Ranvier

Chemical transfer across synapses

* When action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels.
* Calcium ions diffuse into the cell and promote the vesicles (containing neurotransmitters) to move towards the cell membrane.
* The neurotransmitters are released from the axon by exocytosis and move across the synaptic cleft.
* The neurotransmitters bind to the receptors on the post-synaptic membrane.
* This will open ligand-gated ion channels
* These will release an electrical impulse
* The old neurotransmitters are degraded or recycled

Neurotransmitters

Chemical messengers released from neurons and functions to transmit signals across the synaptic cleft.

Three targets neurotransmitters can have:

Neuron: Stimulation or inhibition of an electrical signal

Glandular cell: Stimulation or inhibition of secretion (exocrine or endocrine)

Muscle fibre: Stimulation or inhibition of muscular contraction

What does the central nervous system exist of?

The brain and the spinal cord

What does the peripheral nervous system exist of?

All nerves

Acetylcholine (example of neurotransmitter)

* It is commonly released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction
* It is also commonly released within the autonomic nervous system to promote parasympathetic responses (‘rest and digest’)

Why does acetylcholine need to be removed?

It can lead to fatal convulsions and paralysis

Pesticide that binds to nicotinic acetylcholine receptor

Neonicotinoid pesticides

Why are neonicotinoid pesticides good?

They affect insects more than mammals.

Acetylcholine

* Created in axon terminal by combining choline with an acetyl group
* Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse

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Breakdown of acetylcholine

Acetylcholine is broken down into its two component parts by the synaptic enzyme **acetylcholinesterase** (AChE)

* AChE is either released into the synapse from the presynaptic neuron or embedded on the membrane of the post-synaptic cell
* The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine