Neuroscience Lecture 3: Anatomy and Physiology of Neurons

Neurons

• primary cell type in nervous system

• processes information 

• 86 billion cells in the human brain that transmit information.

• filaments extending out from the cell body

• convey signals from point A to point B

Anatomy of a neuron

• centre is soma (with nucleus etc)

• filaments — fibres extending out — Axon is the thickest filament

• complex web of fibres around neuron

Anatomy of a neuron

• dendrites - thin filaments

• axon extending out 

• length of the axon varies between neurons — can be very long, varies a lot in length 

• end of axon is terminal — forms connections to other neurons

What do neurons do?

• neurons pick up signals at the end where dendrites are and they transmit those signals down the axon to the end of the axon where those signals are sent to other neurons 

• conveys signals from point A to point B 

What signals do neurons convey?

binary signals

• signalling whether it is on or off

Cellular makeup of a neuron

• neurons covered in lipid membrane

• doesn’t really let water pass through

• semipermeable (control ion concentration) - can control the flux of ions across the membrane

• neuron can control the charge of ions inside it

• 

Electrical charge of neurons

• in resting state — inside is negatively charged (vs outside which is positively charged)

• sodium potassium pump in the membrane — swaps out 3 sodium ions for every 2 potassium ions it lets in

• net result build up of positively charged sodium ions outside the membrane

• electrical difference between inside and outside of neuron — form of stored energy which neuron uses in its signalling mechanism 

• neuron is polarised

Action potential (or not)

• Moments when neuron acc sending signal - neuron becomes depolarised 

• Channels within the membrane that are normally closed – open up for brief moments and let the sodium ions sitting outside flow through – flood into neuron 

• Becomes positively charged 

• Triggers large scale change that drives neuron into flipped polarity 

• Flips electrical difference 

• If enough positive electrical charge introduced inside the neuron – starts opening up sodium channels everywhere else in surrounding membrane 

• Sudden cascade of response called action potential – flips from being negatively charged to positively charged 

• Only lasts a few milliseconds – repeats it over and over  

Myelin 

• Insulating material which covers ion channels underneath it 

• When they open up sodium ions can't come in 

• Stops action potential happening where covered by myelin 

• Small gaps between them – action potential is able to jump from one gap to another

• Long axons use myelin to speed up the transmission of action potential  

• Prevents depolarisation 

Knowledge of neuronal function? 

• Neurons are digital – convey only one bit of info 

• Sophistication of brain function due to 

  • High speed of info transmission 

  • Enormous number of neurons 

  • Complexity of connections between neurons (circuitry) 

• certain drugs (anesthetics and alcohol) work by interfering with action potential

How do neurons interact?

• form small junctions called synapses (space between neurons)

• they communicate across synapses

• 10-20 nanometers 

chemical synapses

• vast majority of neurons communicate via chemical transmission across the synapse 

Neurotransmission

• end of axon is swollen - contains vesicles full of neurotransmitter chemical 

• calcium ions also entered during action potential which triggers NT vesicles — causes them to move from inside to fuse onto the wall of the membrane and spill the NT content out into synapse 

• once it is in synapse - immediately finds a receptor site in the membrane of the other neuron  

NT and Receptor

• right receptor on the synapse for it to work - highly specific (lock and key process) 

• receptor is big protein molecule that has ion channel within it — normally closed

• when NT binds w receptor — electrochemical force of the interaction changes the structure of protein slightly 

• opens the ion channel 

• most common transmitter — glutamate e.g when it bind w glutamate receptor which has sodium channel within it and lets sodium ions it — depolarises the membrane — can trigger action potential in this neuron

Excitatory transmission

when one neuron has excited another to have action potential

Inhibitory transmission

Gaba has opposite effect — inhibits the neuron

• gaba receptor has chloride ion channel (-vely charged) - so when let in you hyperpolarise the membrane

• makes it more negative than before and stops it from having action potential

Re-uptake

when released NT into synapse — only want it to be there for short amount of time 

• enzymes located in the tissue which target and destroy NT when come in contact w it

• pre-synaptic terminal has pumps in membrane that sucks up the NT as soon as released

• makes sure process it brief and synapse is ready for new signal to be sent

Neuropharmacology

• psychoactive drugs affect functioning of neurons 

• many mimic or block NTs

• can influence the release, re-uptake, enzymatic destruction or receptor binding of NTs

• work as agonists (increase effect of NT) or antagonists (decrease effect of NT)

Psychoactive drugs — recreational 

• opiates (heroin, morphine, codeine) mimic brain’s opioid NTs

• cocaine, amphetamines and ecstasy promote transmission of dopamine, noradrenaline and serotonin 

• nicotine - stimulates acetylcholine receptors 

• caffeine - blocks adenosine receptors 

Psychoactive drugs — therapeutic drugs  

• benzodiazepines (e.g. valium) — enhances inhibitory effects of GABA (same as barbiturates and alcohol)

• most anti-schizophrenic drugs block dopamine

• antidepressant drugs enhance serotonin and noradrenaline transmission by blocking re-uptake