Membrane Potential, Synapses, and Neurotransmitters Flashcards
What are excitable cells?
Cells that can be electrically excited resulting in the generation of action potentials
Examples = neurons, muscle cells (skeletal, cardiac, smooth), some endocrine
What is action potential?
Brief reversal of electric polarisation of the cell membrane
The dynamic changes in the membrane potential in response to stimulus
Also called nerve impulse
After action potential, cell membrane returns to its resting state
What are nerve impulses?
Nerve impulses are the commuincation between cells and the cause of action potential
Very brief electrical impulses that travel along the nerve fibre (axon), this electrical signal is converted to chemical signal via neurotransmitters
What are the key advantages of nerve impulses?
Fastest process in the cell
Energy efficient by applying energy gradients that already exist
Dependable over long distances
What is membrane potential and what is its function?
Electrical potential is the difference between the inside of the cells and the surrounding extracellular fluid
Present in all cells and especially important in nerve and msucle cells
Function = code and transmit sensory and motor information
What is resting membrane potential?
When the cell is in its “rest state”, determined by the concentration gradient of ions across the membrane
Neuron = -70mV (polarised)
The inside of the cell is negative with respect to the surrounding extracellular fluid
What is an ion?
An atom or molecule in which the total number of electrons is not equal to the total number of protons (charged)
Cation = positive charge Anion = negative charge
How is resting membrane potential achieved?
Higher levels of sodium (Na+) and chloride (Cl-) outside of cells
Higher levels of potassium (K+) and anions inside the cell
Inside neuron is more negatively charged than outside, resting membrane potential = -70mV
This is maintained through transport proteins such as 3Na / 2K + ATP-ase pumps and ion channels which allow K+ or Na+ to move fairly easily through the cell membrane
Many more K+ channels than Na+ channels
How does potassium use ion channels to maintain resting membrane potential?
Less potassium outside the cell which creates concentration gradient encourgaing K+ from inside to cross the membrane
As K+ diffuse out of cell, electrical potential created by the remaining anions (remember that -ve charge) pull K+ back in
Resting membrane potential is achieved when the concentration gradient and electrical potential are equalise (-70mV)
How are action potentials generated?
Ligand-gated channels open with stimulating allowing inflow of cations (Na+ and Ca+) which lowers the membrane potential
Lowered membrane potential then triggers voltage-gated Na+ channels to open making the cell ~ 600 times more permeable to Na+
This achieves local depolarisation of cell membrane (membrane potential mecomes less negative +30mV and spreads depolarisation down the axon)
This is action potential
What is a voltage-gated Na+ channel?
Ion selective (Na+ only)
Gate controlled by a voltage sensor which responds to the level of membrane potential unlike ligand-gated channels which respond to neurotransmitter signals
Normally closed and open only when prompted by gating agent - voltage above the threshold
What are the three phases of actoin potential?
Phase 1 = Depolarisation
Phase 2 = Repolarisation
Phase 3 = Hyper-polarisation and back to resting membrane potential
How does repolarisation occur?
Inactivation of voltage-gated NA+ cahnnels after 0.5 - 1 msec
Opening of voltage-gated K+ channels and K+ efflux starts to re-polarise the cell
How does hyper-polarisation and achieving resting membrane potential happen?
Increased K+ permeability from voltage-gated K+ channels which has a delayed closure process causing an icnrease in polarisation
Voltage-gated Na+ cahnnels return to active but closed status
The Na+/K+ pump returns the membrane potential to resting state
What is the purpose of hyper-polarisation?
When voltage-gated Na+ channels are in their inactive state, they cannot be opened again until they have a returned to a closed state
This ensures a refactory period when the membrane is hyper-polarised and not as excitable
Ensures one direction of current flow
How does axon resistance impact the speed of action potential propagation?
Larger diameter = less resistance
Myelin provides additional isolation on the axon, maintains the initial stimulus
Action potentials are only produced in the exposed spaces of ranvier’s nodes
Current spreads under myelin sheath = appears to jump across
Therefore, unmyelinated nerve has a slower conduction
What is synaptic signalling?
It’s a special case of paracrine signalling
Can only occur between cells linked with synapses between the originating cell and receiving cell
It’s the communication between neurons or between neurons and effectors
At synapses a chemical signal (transmitter) is released from one neuron and diffuses into another neuron or target cell to generate a signal
What is a synapse?
A junction between two nerve cells consistinng of a tiny gap across whcih impulses pass by diffusion of a neurotransmitter
What is a neurotransmitter?
Biological active chemicals that transmit signals from one neuron to another or to a target cell across a synapse
There are two types: Excitatory NTs and Inhibitory NTs
What’s the difference between excitatory NTs and inhibitory NTs?
Excitatory NTs generates an action potential in the receiving neuron
(influx of NA+ > depolarisation > action potential)
Inhibatory NTs prevent action potentials
(influc of Cl- > hyper-polarisation > difficult to reach threshold)
How do A-delta fibres and C fibres transmit tooth pain?
A-delta fibres are myelinated and thicker (fast transmitting information, sharp pain, well localised)
C fibres are unmyelinnated and thinner (slow pain, burning sensations and lingering pain, chronic pain that is poorly localised)
How does LA impact pain transmission?
LA - a drug which reversibly prevents transmission of the nerve impulse in the region where it is applied, without affectingn consciousness
Lignocaine binds to voltage-gated Na+ channels in the peripheral nerve cell membrane and blocks the influx of Na+
