Neuroscience Lecture 3 2024.pdf

Neuroscience Lecture 3: The anatomy & physiology of neurons (understanding how neurons work) (Carlson pp 90-101) 86 billion neurons in human brain Understanding how neurons work can help us understand what information they carry – Turns out to be ver...

Neuroscience Lecture 3: The anatomy & physiology of neurons (understanding how neurons work) (Carlson pp 90-101) 86 billion neurons in human brain Understanding how neurons work can help us understand what information they carry – Turns out to be very simple, and not unlike electric circuits Anatomy of a neuron Dendrites Soma (cell body) Axon Anatomy of a neuron Terminals Myelin Soma (cell body) Axon Axons can be very long (eg, length of your leg, length of giraffe’s neck) Dendrites  Convey signals from point A to point B Direction of transmission A B What signals do neurons convey ?  Convey signals from point A to point B Direction of transmission A B What signals do neurons convey ? Binary (“on” / “off”) signals. How?....  Neurons covered in lipid (fatty) membrane: semipermeable (control ion concentration) Inside membrane Outside In resting state, inside negatively charged (vs outside). + + + + + + - + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - -- - + - - + - - + + + + + + + + - - - + + - - - - - - + + + + + + + + + Neuron is polarised  The Action Potential (all or none) + Ion channels open, letting ions enter and exit; + + + + + - ++ + + + + + + + Neuron is depolarised + - - - - + - -- -- - - - -- - - -- -- - - - -- - - - + - - + + + + + + + + -- - - + + - - - - - + Thus two states: + + +++ + + + either “off” (polarized, at rest), or - “on” when flips into depolarised state - - - - - + - - - - - - - - - - + ++ + - + + + ++++ ++++++++++ ++++ ++ + - + + - - - - - - - - + +- - ++ + ++ + +- - - - - - - - - Neuron depolarized for only a few msec, but can “spike” (switch on and off) many times a second  The Action Potential (“all or none”) One spike Average of 500 spikes 1 msec Real time Half Speed  Neurons are digital = convey only one bit of information analog digital  Propagation of Action Potential: AP is localised to small segment of membrane and spreads along membrane + + + + + + + + + + + + + + + + - - - - - - + - - + - - - - - - - - + + + + + + + + + + + + + + + + + + + + + Hyperpolarisation after AP  Myelin Myelin prevents depolarization (except at gaps) gaps Myelin Axon Myelin speeds up propagation of AP Neurons with myelin are faster Eg. 2 types of pain fibres.  What we have learned from knowledge of neuronal function? Neurons are digital (convey only one bit of information) Sophistication of brain function due to: (1) high speed of information transmission, (2) enormous number of neurons, and (3) complexity of connections between neurons (circuitry). (Can we build artificial brains with computers?) Certain drugs (anaesthetics and alcohol) work by interfering with action potentials.  How do neurons interact? Neurons form small junctions called “synapses” Neurons communicate across synapses  Chemical Synapses The vast majority of neurons communicate via chemical transmission across synapse. Synaptic cleft is 10-20 nm* wide. Therefore, transmission is very fast. [* millionth of a mm]  Axon Neurotransmission Neurotransmitter (NT) Pre-synaptic released into synapse neuron from terminal. NT Vesicles Terminal NT NT Synapse Post-synaptic neuron  Binding between NT and receptor is highly specific (like lock and key) Pre-synaptic neuron Changes in 2nd neuron NT Eg, opens ion channel (induce or prevent AP Terminal in 2nd neuron ) NT Synapse NT NTs can be excitatory or inhibitory Receptor Ion channel  Neurotransmission Pre-synaptic neuron Effect of NTs must be brief NT Therefore, effect ended by: Enzymes Enzymes in synapse NT & Re-uptake NT NT Re-uptake NT Post-synaptic neuron  Neuropharmacology Pre-synaptic neuron Psychoactive drugs affect functioning of neurons: Enzymes NT Many mimic or block NTs NT Can influence the NT NT Re-uptake release, re-uptake, NT enzymatic destruction, or receptor-binding of NTs Post-synaptic neuron As agonists or antagonists  Psychoactive drugs “Recreational” drugs (& drugs of abuse): 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 Therapeutic drugs: Benzodiazepines (eg valium) enhance inhibitory effects of GABA (as do barbiturates and alcohol). Most anti-schizophrenic drugs block dopamine Antidepressant drugs enhance serotonin and noradreline transmission (eg, by blocking re-uptake).