Class 20 - Nervous System pt. 2 Flashcards
Electrical potential, current,
+ resting membrane potential
Electrical potential - Concentration difference between one point and another
Current - Flow of charged particles from one point to another caused by electrical potential
Resting membrane potential - Charge difference across the plasma membrane
3 factors affecting RMP
- Diffusion of ions down their concentration gradients through the membrane
- Selective permeability of membrane, allowing some ions to pass more easily than others
- Electrical attraction of cations and anions to each other
What has the greatest influence of RMP?
K+
More concentrated in ICF, cell membrane more permeable to it (leak channels), resulting electrical attraction brings it back in after leaking out
Equilibrium = no net movement of K+ due to equal electrical and concentration gradients
What spends the most energy in the nervous system?
Na+K+ pumps working to compensate for the continual leakage of Na+ and K+.
Moves the ions against their concentration gradients to maintain resting membrane potential for the cell
Local potential + 4 characteristics
Temporary, short-range change in voltage.
- Graded - Vary in magnitude with stimulus strength
Stronger stimuli open more Na+ channels and they stay open longer - Decremental - Get weaker the farther they spread from origin point
- Reversible - If stimulation ceases, membrane voltage quickly returns to normal resting potential
- Can be excitatory or inhibitory
Action potential + 3 characteristics
Rapid up-and down change in voltage produced by coordinated opening and closing of voltage-gated ion channels.
Excitatory local potential must be strong enough (open enough channels) to reach the trigger zone to create an action potential
- All-or-none law - If threshold reached, neuron fires up to maximum voltage; if threshold not reached, it does not fire
- Non decremental - Does not get weaker with distance
- Irreversible - Once started, travels all the way down the axon and cannot be stopped
Steps of an action potential (6)
- Local potential spreads to axon hillock; voltage must reach threshold (minimum voltage to open channels)
- Voltage-gated Na+ channels open quickly, K+ channels open more slowly
- Na+ enters and depolarizes the membrane
- Voltage peaks, inactivating and closing Na+ channels. Membrane now more positive inside
- Voltage-gated K+ channels are now fully open, and K+ flows out, repolarizing the membrane
- K+ continues to exit and produces a negative overshoot—hyperpolarization—to account for Na+ leaking into cell
Refractory period + 2 phases
Period of resistance to stimulation.
- Absolute refractory period - No stimulus of any strength can trigger another action potential; voltage-gated Na+ channels inactivated
- Relative refractory period - An unusually strong stimulus needed to trigger a new action potential; occurs during hyperpolarization of last action potential
Continuous vs. saltatory conduction
Continuous - Unmyelinated axons. Have voltage-gated channels along entire length
Saltatory - Myelinated axons. Voltage-gated ion channels concentrated at nodes. Action “jumps” from node to node. Faster than continuous, despite “stop and go” action
Synapse + 3 steps of neuron-to-neuron communication
Point where an axon terminal meets the next cell.
- Action potential arrives at end of axon of presynaptic neuron
- Presynaptic neuron releases neurotransmitter
- Postsynaptic neuron responds to it
Axodendritic, axosomatic, and axoaxonic synapses
Axodendritic - Synapse where presynaptic cell meets dendrite of postsynaptic
axosomatic - Synapse where presynaptic cell meets cell body of postsynaptic
axoaxonic - Synapse where presynaptic cell meets axon of postsynaptic
How do electrical synapses communicate?
Via gap junctions. Electrical signals spread directly from cell to cell.
Much faster than neurotransmitters, but cannot integrate info as well.
Structure of a chemical synapse (4)
- Synaptic cleft - Gap between presynaptic neuron and postsynaptic neuron
- Cell-adhesion molecules - Link two neurons together
- Synaptic vesicles - Contained in axon terminal of presynaptic neuron; hold and release neurotransmitters
- Postsynaptic density - Neurotransmitter receptors and ion channels in the membrane of the postsynaptic neuron
6 major categories of neurotransmitters
- Acetylcholine
- Amino acids - Includes GABA
- Monoamines - Synthesized from amino acids by removal of -COOH group; includes epinephrine, dopamine, serotonin, histamine
- Purines - Adenosine, ATP
- Gases - Nitric oxide and carbon monoxide
- Neuropeptides - chains of amino acids; endorphins
Steps of synaptic transmission at an excitatory cholinergic synapse (5)
ACh is the neurotransmitter.
- Action potential depolarizes axon terminal, opens voltage-gated Ca2+ channels
- Ca2+ enters, triggering release of ACh
- ACh diffuses across cleft, binds to postsynaptic receptors
- ACh receptors are ligand-gated ion channels which open to allow Na+ and K+ across membrane
- Na+ entry causes depolarizing synaptic potential, which if strong enough will spread to trigger zone, reach threshold and cause action potential
