Lecture 3 - Action Potentials/Synapses/Neurotransmitters Flashcards
Definition of Potential Difference
An uneven distribution of +ve and -ve charges.
Definition of Membrane Difference
Potential difference across a membrane.
Membrane Potential of undisturbed cell
+ve charge on the outside, -ve charge on the inside
Cell membrane resting potential of a neuron
-70mV
Ion content of Extracellular Fluid
High Concentration of Na+ and Cl-
Ion content of Intracellular Fluid
High Concentration of K+ and Pr- (Proteins)
Function of Selective Permeability of Membrane
Maintains differences in Extra/Intracellular Fluid
2 types of Channels in Membrane
Leak - always open
Gated - open/closed
Chemical Concentration gradient across membrane in Passive Movement
K+ moves out faster than Na+ moves in
Electrical Gradient across membrane in Passive Movement
+ve outer membrane repels K+ ions
-ve inner membrane attracts Na+ ions
Stronger Gradient in Passive Movement
Chemical Conc Gradient Stronger
Therefore K+ still moves out
Therefore net loss of +ve ions
Sodium/Potassium ion exchange pump
3 intracellular Na+ for 2 Extracellular K+
2 conditions for disturbing resting potential
Altering of Membrane permeability to Na+/K+
Altering of Na+/K+ exchange pump
Chemically gated channel mechanism (Na+)
At resting state, channel remains closed
In the presence of Acetylcholine at a binding site, conformational change occurs
Channel opens, allowing Na+ into the cell
Depolarisation Definition
Shift of membrane potential towards 0mV
Repolarisation Definition
Restoring of resting membrane potential
Hyperpolarisation Definition
Shift of Membrane potential away from 0mV
3 types of gated channels
Chemically Gated
Voltage Gated
Mechanically Gated
Voltage Gated Channel mechanism (Na+)
Gate closed at -70mV
During Depolarisation, activation (outer) gate opens
Na+ enters cell
At +30mV, inactivation (inner) gate closes
Mechanically Gated Channel mechanism (Na+)
Gate closed at resting state
When membrane is distorted, channel opens
Na+ enters cell
Once distortion ends, gate closes
Graded potential definition (2 points)
Potential produced when gated channel opens
Local to limited portion of cell membrane
6 Steps of Graded Potential Mechanism
Stimuli exposed to membrane
Sodium channel opens
Depolarisation occurs
Movement of Na+ produces local current
Depolarises local membrane
Once stimuli is removed, repolarisation occurs
Action Potential Definition
Electrical Signal that affects entire membrane surface
Change in permeability of cell membrane
Excitable Membranes Definition
Membranes in Skeletal Muscle fibres/axons that can conduct action potentials
Start point of Action Potential in neuron
Axon Hillock
Direction of Action Potentials
Along axon length towards synaptic terminals
All or None Principle
Every stimulus, no matter the size, generates identical action potential
Action Potential Threshold
-60mV
6 Steps in Action Potential Generation
Graded depolarisation brings excitable membrane to threshold (-60mV)
Voltage Gated Na+ channels open
Na+ enters cell
At +30mV, Na+ channels close, Voltage Gated K+ channels open
Repolarisation occurs
Once at resting potential, Voltage Gated K+ channels close
Propagation of Action Potential Definition
Movement of Action Potential across Axon
2 types of Propagation
Continuous
Saltatory
4 Steps of Continuous Propagation
Action Potential develops in initial segment, depolarised to +30mV
Na+ ions spread away from open voltage gated channel in initial segment, causes graded depolarisation in next segment
Action Potential develops in new segment whilst initial segment repolarises
Cycle repeats
Continuous Propagation Speed
1m/sec
Schwann Cell Definition
Cell wrapped around neuron to provide protection and insulation
Condition of Continuous Propagation
Occurs in unmyelinated axons
Condition of Saltatory Propagation
When Continuous Propagation can’t occur
Occurs in Myelinated Axons
Saltatory Propagation Speed
18-140m/sec
4 Steps of Saltatory Propagation
Action Potential developed at initial segment, depolarised to +30mV
Local current produces graded depolarisation that brings neighboring axolemma to threshold (-60mV)
Action Potential develops at next node
Cycle repeats
Synapse Definition
Site of communication between nerve cell and some other cell
Most common type of Synapse
Chemical Synapse
3 Locations of Synapses
Dendrite
On a cell body
Along length of Axon
Direction of communication across Synapses
1 direction
Presynaptic Neuron to Postsynaptic Neuron/other cell
6 Parts of a Synapse
Axon Terminal
Mitochondrion
Synaptic Vesicles
Presynaptic Membrane
Postsynaptic Membrane
Synaptic Cleft
Function of Synaptic Vesicles
Contain Neurotransmitters
Synaptic Cleft Definition
Narrow space between Pre/Postsynaptic Membranes
2 types of Neurotransmitters
Excitatory
Inhibitory
Excitatory Neurotransmitters Features
Causes depolarisation of postsynaptic membranes
Promotes action potentials
Inhibitory Neurotransmitters Features
Causes hyperpolarisation of postsynaptic membranes
Suppresses action potentials
2 Acetylcholine (ACh) Features
Most common neurotransmitter
Released at Cholinergic Synapses
Locations of Cholinergic Synapses
Common in CNS
All Neuron-Neuron synapses in PNS
All Neuromuscular/Neuroglandular junctions in Parasympathetic division of ANS
8 Events at Cholinergic Synapse
Action potential depolarises axon terminal of presynaptic neuron
Calcium ions enter axon terminal
ACh release via exocytosis
ACh diffuses across synaptic cleft
ACh binds to postsynaptic membrane Chemically gated Na+ channels
Depolarisation ends
ACh broken down by AChE and reabsorbed into axon terminal as choline
ACh resynthesized
5 Important Neurotransmitters
Acetylcholine
Norepinephrine (or noradrenaline)
Dopamine
Serotonin
Gamma aminobutyric acid (GABA)
Enzyme that breaks down Acetylcholine
Acetylcholinesterase (AChE)
4 Features of Norepinephrine
Excitatory Neurotransmitter
Used in brain/ANS
Released at adrenergic synapses
Broken down by Monoanime oxidase
4 Features of Dopamine
CNS Neurotransmitter
Both excitatory/inhibitory
Released by Dopaminergic Synapses
Dopamine deficiency linked to muscle stiffness
4 Features of Serotonin
CNS Neurotransmitter
Involved in Sleep/Wake cycles
Serotonin deficiency important in context for depression
Released by Serotonergic Synapses
2 Features of GABA
Inhibitory Neurotransmitter
Involved in anxiety reduction in CNS
