Lectures 4 & 5 (membrane potentials + neurotransmission) Flashcards
Nerve conduction vs neurotransmission
movement of nerve impulses down neurons vs transmission of nerve impulses across a synapse
Anatomy of a neuron
message in
- dendrites (branch from cell body)
- perikaryon/soma (cell body)
- axon
- axon terminal
- contain vesicles (carry neurotransmitter)
message out
Types of synapses (contact-wise)
- axosomatic (terminal to cell body)
- axodendritic (terminal to dendrites)
- axoaxonic (terminal to axon)
Conduction of nerve impulses (basic)
wave of depolarization along axon
- axon has positive outside, negative inside
- stimulus (increase in voltage) triggers action potential
- wave initiated at axon hillock (where axon meets soma)
- membrane depolarizes down axon (positives in, voltage increase)
- repolarizes quickly behind (refractory period prevents reverse conduction)
conduction of nerve impulses (Na+ & K+ channels)
in unmyelinated axons (slower, < 10m/s)
resting membrane potential ~ -80mV
- +ive stimulus increases V (eg: up to -55mV)
- +ive increase opens VG Na+ channels
- Na+ enters axon (moves ∆V down axon, eg: up to +20mV)
- +ive V triggers opening of K+ channels
- K+ leaves axon (-ive outside, resets ∆V)
- V dips below RMP (eg: -90mV)
back to -80mV
*axons cannot send message until repolarized
why is the refractory period important
- transmission is one way
- each action potential is a separate event (allows for encoding of message)
- allows cell to recover (prevents charge/ion accumulation + energy shortage)
Saltatory conduction
in myelinated axons only (much faster, ~150m/s)
- exactly the same conduction method BUT
- each node of Ranvier (unmyelinated section) gets depolarized down axon
- effectively ‘skips’ myelinated sections
- only have to repolarize nodes (area of polarity reversal)
important for pain signals, muscle control…
Electrical synapse
depolarization along cell membrane
- signal transmitted across synapse via gap junctions (hole between two cells)
- +ive ions flow through ion channels
- allows transmission between, eg: adjacent muscle cells
eg: synchronized contractions down muscle (heart, uterus)
key concepts about K+ ions
- high concentration in the cell
- wants to leave the cell (efflux, down concentration gradient)
- potential difference increases when they leave (V more negative, inside more -ive, hyperpolarized)
key concepts about sodium ions
- less concentrated inside the cell
- wants to enter the cell (influx, down concentration AND charge gradient)
- potential difference decreases when they enter (V less -ive, inside more +ive, depolarized)
factors maintaining resting membrane potential
- leaky membrane (sodium leaks in much slower than potassium leaks out, less +ive in than +ive out, net -ive)
-
sodium-potassium pump pumps out 3 sodium for every 2 potassium (more +ive out than +ive in, net -ive)
*also fixes Na+ and K+ concentrations - more -ive charged proteins inside than Cl- outside
Graded Potentials (types + traits)
- short lived
- local changes
- depolarizing or hyper-polarizing
- can be summed (spatial and temporal summation)
- have -ive feedback mechanisms to reset RMP
Action potential (traits)
- occurs in muscles and axons
- can be triggered by summed graded potentials
- caused a brief reversal of the voltage (+ive voltage)
- can never be summed
- works by +ive feedback
Action potential (phases)
-
depolarizing phase:
- stimulus increases voltage (less -ive, small spike)
- voltage gated Na+ channels open, Na+ in, further increases ∆V (becomes +ive, large spike)
- repolarizing phase: voltage gated K+ channels open, Na+ close (V down to RMP)
- hyperpolarizing phase: K+ briefly stay open (V < RMP)
- resting phase (flat line)
electromyography (what it’s for, expected results + meaning)
- used to measure electrical activity in muscles
- theres a voltage spike from stimulation and a second from contraction
- distance between spikes is the conduction speed
(we can also stimulate muscles with electrical currents)
