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Summary Neurochemistry
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Extensive summary of all exam material of the course Neurochemistry (Brain and Cognition).
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Summary Exam NeurochemistryClass 1 – Introduction and Basics of Neurochemistry
Lecture 1
Goals
• Understanding the main ways of neuronal communication (electrical & chemical)
• Recalling the basic characteristics of an action potential
• Identifying the main components of a synapse
• Understanding the basics of chemical communication between neurons
• Describing the mechanisms of excitation and inhibition at the receptor level
The neuron
• Anatomy:
- Soma = cell body
- Dendrite (input)
- Axon (output)
- Neurites = Dendrite + Axon
• Classifications:
- Number of neurites or polarity:
▪ Multipolar
▪ Bipolar
▪ Unipolar
- Dendritic shape:
▪ Stellate
▪ Pyramidal
▪ Purkinje
▪ Granule
- Connections:
▪ Primary sensory (from sensory surfaces to brain)
▪ Motor (from brain to muscles)
▪ Interneurons (within nervous system)
- Function
- Neurotransmitter
• Neuronal connections:
- Sensory neuron > Interneuron > Motor neurons
- Thus: From senses > CSN > To muscle
- Longest axon in human is the sciatic nerve (~1 meter long)
Neural communication
• Mechanical → Electricity & chemical signal
→ E.g. Sensory transduction in the ear
• Electric communication within neurons: The Action Potential:
- The neuron has an excitable membrane
- Elevate the membrane potential above a threshold, and an all-or-nothing response
(principle) will appear (= Action Potential)
- AP can travel along the cell membrane without attenuation
• The Resting Membrane Potential:
, - In neurons a voltage difference is present across the cell membrane
- Outside the cell is more positive than inside the cell
- This voltage difference (= Resting Membrane Potential) is ~-65 mV
- At equilibrium, an equal number of potassium (K+) enters and leaves the cell
• The Action Potential:
- Equilibrium potassium ion (K+): Ek = -70 mV
- Equilibrium sodium ion (Na+): ENa = 62 mV
- Lowest threshold area of each neuron = the axon hillock
- Step-by-step:
▪ A: Threshold reached (due to electrical effect of receptors through multiple
other neurons)
▪ B: Sodium channels open; Potassium channel opens
▪ C: Sodium channels close; Potassium flows out due to charge difference
▪ D: Potassium closes; Winds back to usual flow
, - Blocking of Sodium channels can be lethal (e.g. by Tetrodotoxin (TTX) in pufferfish)
- AP through the axon:
▪ Signal travels only one way due to:
• Proteins close to the hillock have a concentration that direct the
signal in a specific direction
• Downtime of where the signal came from
▪ On its way to the next neuron:
▪ Myelin sheaths fasten the signal:
- Electricity and chemistry:
▪ Within neurons: Electric conduction → AP
▪ Between neurons: Chemical (and also little bit electric) conduction →
Neurotransmission
Synapses
• Synapse = Junction between presynaptic terminals and postsynaptic membrane of the next
neuron
• Each axon may have synapses up to 1000 other neurons (sometimes even more)
• Electrical or chemical communication
• Note that there are multiple synapses between the two neurons
• There is also feedback (auto-receptors): Often as a negative feedback loop
→ Inhibition so you will stop releasing a NT if it is too much
→ Epilepsy: Negative feedback loop is out of control
, • Main parts:
- Presynaptic membrane (part of the axon)
- Synaptic cleft
→ ~30 nM wide in chemical synapses (electrical: ~3 nM)
- Postsynaptic membrane (mostly on dendrite, but not always)
• Synaptic arrangements:
- Axosecretory = Axon terminal secretes directly into bloodstream
- Axoaxonic = Axon terminal secretes into another axon
- Axodendritic = Axon terminal ends on a dendrite spine
- Axoextracellular = Axon with no connection secretes into extracellular fluid
- Axosomatic = Axon terminal ends on soma
- Axosynaptic = Axon terminal ends on another axon terminal
• The chemical synapse:
- Arrival of AP at terminal will trigger the release of neurotransmitter (= synaptic
transmission)
- Neurotransmitters:
→ Peptides: Massive chunks of chemicals
→ Amines: Modified amino acids
- Presynaptic axon terminal contains synaptic vesicles (~50 nM In diameter)
- Synaptic vesicles contain neurotransmitters
- Dense-core vesicles contain peptides
- Vesicles around presynaptic active zones
- Postsynaptic densities contain receptors
Synaptic neurotransmission
• AP induces release of neurotransmitter from presynaptic membrane
• Neurotransmitter diffuses across synaptic cleft
• Neurotransmitter induces change in postsynaptic membrane
• Central factors in chemical synaptic transmission: Neurotransmitters release:
- Synthesis of neurotransmitter
- Packing/storage in vesicles
- Release
• Neurotransmitter synthesis:
- Different neurotransmitters are synthesized in different ways
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