1.2 Neurons and Synapses

Created: March 17, 2021 9:23 PM Status: Open Updated: March 17, 2021 9:33 PM

Neurons

Neurons are specialised cells that function to transmit electrical impulses within the nervous system

• The nervous system converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli

While neurons may differ according to role (sensory, relay or motor), most share three basic components:

• Dendrites – Short-branched fibers that convert chemical information from other neurons or receptor cells into electrical signals
• Axon – An elongated fiber that transmits electrical signals to terminal regions for communication with other neurons or effectors
• Soma – A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

In some neurons, the axon may be surrounded by an insulating layer known as a myelin sheath

• The myelin sheath improves the conduction speed of electrical impulses along the axon, but require additional space and energy

Neuron Doctrine:

• The neuron is the fundamental structural and functional unit of the brain
• Neurons are discrete cells and not continuous with other cells
• Information flows from the dendrites to the axon via the cell body

Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane

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

A resting potential is the difference in charge across the membrane when a neuron is not firing

• In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV)

The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps

• The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions
• It expels 3 Na+ ions for every 2 K+ ions admitted
• This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)
• The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)

Synapses

Synapses are the physical gaps that separate neurons from other cells (other neurons and receptor or effector cells)

• Neurons transmit information across synapses by converting the electrical signal into a chemical signal

Neurotransmitters bind to neuroreceptors on the post-synaptic membrane of target cells and open ligand-gated ion channels

• The opening of these channels cause small changes in membrane potential known as graded potentials

A nerve impulse is only initiated if a threshold potential is reached, so as to open the voltage-gated ion channels within the axon

• Excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation by opening ligand-gated sodium or calcium channels
• Inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation by opening ligand-gated potassium or chlorine channels

The combined action of all neurotransmitters acting on a target neuron determines whether a threshold potential is reached

• If overall there is more depolarization than hyperpolarization and a threshold potential is reached, the neuron will fire
• If overall there is more hyperpolarization than depolarization and a threshold potential is not reached, the neuron will not fire

For a typical neuron, the threshold potential (required to open voltage-gated ion channels) is approximately –55 mV

Presynaptic neurons release neurotransmitters into the synapse to trigger graded potentials in post-synaptic neurons

• Some generate excitatory post-synaptic potentials (EPSPs) and others produce inhibitory post-synaptic potentials (IPSPs)
• EPSPs trigger depolarization in the post-synaptic membrane, IPSPs trigger hyperpolarization in the post-synaptic membrane
• If the combination of signals reaches a threshold level, an action potential will be triggered in the post-synaptic neuron

The combination of graded potentials in the post-synaptic neuron is known as summation

• Cancellation occurs when excitatory and inhibitory graded potentials cancel each other out (no threshold potential reached)
• Spatial summation occurs when EPSPs are generated from multiple presynaptic neurons simultaneously to reach threshold
• Temporal summation occurs when multiple EPSPs are generated from a single presynaptic neuron in quick succession

These summative effects determine which nerve pathways are activated and hence lead to alternate decision-making processes

• The Synapse Doctrine: Synapses are the basis for memory and learning
• The mechanism which allows learning in the brain is called Synaptic Plasticity and one such example is Hebbian Plasticity:
  • If neuron A repeatedly takes part in firing neuron B, then the synapse from the A to B is strengthened
• Evidence of Hebbian Plasticity is seen in LTP and LTD

Long Term Potentiation:

Experimentally observed increase in synaptic strength that lasts for hours or days

Long Term Depression:

Experimentally observed decrease in synaptic strength that lasts for hours or days

• Synaptic Plasticity depends on spike timing!
• Whether LTP or LTD occurs is dependant on the relative timing of input and output spikes