Action potentials, synapses and neurotransmitters Flashcards
Define potential difference
An uneven distribution of charges
Why is a membranes resting potential negative?
Greater positive charge on the outside of the cell more negative inside
What is the resting potential of a neuron?
-70mV
What is the difference between extracellular and intracellular fluid?
Extracellular contains a high concn of Na+ and Cl- ions
Intracellular contains a high concn of K+ and Pr- (proteins)
What is a leak channel?
Always open
What is a gated channel?
Open/closed depending on circumstances
How is resting potential maintained?
The sodium/potassium pump moves 3 Na+ ions out and 2K+ ions in
A chemical concn gradient moves K+ out and an electrical gradient attracts Na+ ions
What does a stimulus do? (Resting potential)
Opens gated ion channels which accelerates movement of ions.
What happens when a stimulus opens chemically gated channels?
Increases Na+ entry to cell causing depolarisation when the stimulus is removed we get hyperpolarisation
Define depolarisation
When the membrane potential shifts towards 0mV- more +ve ions on inner surface off the cell
Define hyperpolarisation
Opens K+ channels and membrane potential shifts past -70mV to -80mV
Define repolarisation
Stimulus is removed and restored to resting potential
What is a graded potential?
Any stimuli that opens a gated channel affects a limited portion of the cell membrane
Cant affect enormous cells and isn’t limited to the nervous system
What are the steps of graded potentials
- Resting membrane exposed to chemical which opens Na+ ion channels causing depolarisation
- Local current - depolarises nearby cell membranes
- Stimulus is removed and repolarisation occurs
What is an action potential?
Change in the permeability of entire excitable membrane
Begins at the axon hillock then along the axon if it reaches the threshold
Outline the steps of the generation of an action potential
- Depolarisation of membrane to threshold
- Na+ channels open and rapid depolarisation
- At 30mV Na+ channels are closed and K+ channels are opened
- K+ channels close and we get a brief hyperpolarisation and then a return to normal permeability
Continuous propagation of an action potential
- AP in segment 1 depolarises membrane to +30mV
- Local current formed as Na+ ions spread and depolarises the second segment
- Second segment develops an action potential as first enters the refractory period
4 Cycle repeats travels in 1 direction at 1m/s
Define refractory period
Period of time of recovery where an action potential cant be generated. Allows APs to be discrete and unidirectional
Define saltatory propagation
Action potentials of a myelinated neuron jump from node to node at 18-140 m/s
Compare graded potentials and action potentials
GP - depolarising and hyperpolarising dependent on stimulus intensity
AP- always depolarising - all or none law
GP- No threshold
AP- threshold
GP- No refractory period
AP- refractory period
What is the all or none law?
For every stimulus that brings the membrane over the threshold then an identical action potential is generated
Define synapse
Site of communication between a nerve cell and another cell
What does the synaptic terminal contain?
Mitochondria, vesicles with a specific neurotransmitter, endoplasmic reticulum
Define neurotransmitter
Chemical compounds released by 1 neuron to affect the membrane potential of another
Define excitatory neurotransmitters
Cause depolarisation of the postsynaptic membrane and promote AP
Define inhibitory neurotransmitters
Cause hyperpolarisation of post synaptic membranes and suppress AP
What is the most common synapse?
Cholinergic - all neuron to neuron synapses in the PNS and all NMJs and NGJs in parasympathetic ANS
Outline the events at a cholinergic synapse
- AP arrives depolarises the presynaptic neuron
- Ca2+ ions enter the cystol of the axon terminal. ACh is released from vesicles via exocytosis
- Diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane opening Na+ channels leading to graded depolarisation
- AChE breaks down ACh into acetate and choline which is reabsorbed and resynthesized to form ACh in the vesicles
Outline Norepinephrine
Excitatory
Brain and ANS
Adrenergic synapses
Broken down by monoamine oxidase
Outline dopamine
Both excitatory and inhibitory (Precise control)
CNS - brain
Dopaminergic synapses
Lack = muscle stiffness
Outline serotonin
CNS
Interacts with other neurotransmitters e.g. NE for regulation of sleep wake cycles
Serotonergic synapses
Outline Gamma Aminobutyric Acid (GABA)
Inhibitory
Incompletely understood
CNS - reduces anxiety
