Excitatory Tissue- Neurons Flashcards
Relative Ion Concentrations
Extracellular
[Na]: 150mM, [K]:5mM
Intracellular
[Na]: 15mM, [K]: 100mM.
How is resting membrane potential maintained?
Permeability of K+ ions is 40x the permeability of Na+ ions due to 40x more K leak channels. More permeable to outward diffusion of cations.
NaKATPase actively transports 3 Na out for every 2K in- net movement of cation out of the axon.
Equilibrium potential
The membrane potential at which the movement of ions down its concentration is completely cancelled out by the movement of ions down its electrical gradient in the opposite direction.
Nernst Equation
61.5 x log([ion outside]/[ion inside])
Calculating the resting membrane potential
Vm=61.5log(PK [K+]o + Pna[Na+]o/PK[K+]i + Pna[NA+]i)
Why is the RMP at -65mV
Both Na+ and K+ will diffuse to attempt to push the potential towards their own equilibrium potentials. Since the membrane is more permeable to the diffusion of K+ ions, it will be closer to the equilibrium potential of K+.
Mechanism of Action potential
Stimulus potential depolarises membrane to threshold and transiently opens Na+ channels. Ration of Na+: K+ permeability rises to 20:1. Causes the RMP to shift towards ENa. Membrane depolarises to 30mV and Na+ channels start to close. K+ channels start to open. Transient opening of K+ channels causes relative K+/ Na+ permeability ratio to become 100:1. shifts RMP towards Ek.
K+ channels are slow to close to lead to hyperpolarisation. After they close only leak channels remain, and it returns to the RMP.
Artificial Stimulation of Potentials
Placing a cathode and an anode outside the neuron. Positive charge will flow from the anode to the cathode both outside and inside the neuron. Accumulation of cations inside the neuron where the cation is causes depolarization.
If the current is flowing out of the axon at a region then it’s depolarised-region is usually positive.
Formation and conduction of AP by stimulus
Stimulus on the axon causes Na+ channels to open. If the stimulus is large enough, enough Na+ channels will open to give a suprathreshold potential that turns into an AP. The cations will diffuse to nearby polarised regions to stimulate opening of Na+ channels there. AP conducted thus, but slow as it must be regenerated at every point, and significant extent of ion loss.
Saltatory conduction
Myelin formed from membranes of oligodendrocytes/ Schwann cells. Reduce ion permeability of axon except at Nodes of Ranvier. 100x faster conduction as there is less loss of potential and less regeneration of potential.
Receptor Potential
Receptor act as transducer.
In muscle spindles, the change of length of the spindle opens stretch-gated Na+ channels to stimulate depolarisation. Potential reaches trigger zone to stimulate formation of an action potential if it’s large enough.
Increasing intensity means more neurons are activated, so more APs reach the CNS. Also reflected in higher frequency of APs in a single neuron, as the relative refractory period is overcome.
Neurotransmitters
Excitatory: Acetylcholine , glutamate (brain).
Inhibitory: GABA (brain), glycine.
Can be amino acids and fatty acids.
Effect can vary dependent on the type of receptor it binds to.
Excitatory opens channels permeable to both Na+ and K+ (but increased diffusion of Na+ due to potential being closer to EK). Inhibitory only opens K+ channels.
Neuropeptides
Slower acting, large molecules which regulate neurotransmitter and receptor activity.
Can bind to G-proteins to stimulate cascade in postsynaptic neuron.
Synaptic Integrations
Individual synaptic inputs insufficient to depolarise the membrane to threshold. Many dendrites are found on soma to allow summation of action potentials.
Temporal: Addition of two potentials arriving in quick succession.
Spatial: Many arriving at a node simutaneously. Summation of EPSP with IPSP speeds up decay of EPSP.
