Lecture 4 Flashcards
Neurophysiology
Study of the life processes of neurons, which use electrical and chemical signals to communicate (study of electrical signaling and chemical signals). Information flows within a neuron via electrical signals and between neurons through chemical signals. Action potential - rapid, all/none electrical signal that travels along the axon of a neuron. Generated if summation of graded potentials brings membrane potential to threshold. Neurotransmitter is chemical messenger between neurons that binds to receptors.
Electrical signals vocabulary of nervous system
All living cells possess a charge, more negative on the inside. Nerve cells exploited this property to transmit info. Ions: electrically charged molecules. Anions are negatively charged - gain electrons. Cations are positively charged- lose electrons. Ions are dissolved in intra and extracellular fluid. Ratio across the fluid is important. Several ions critical - Na+, K+, Ca2+, Cl-
Distribution of ions across membrane
Ions dissolved in intra and extracellular fluid. Separated by cell membrane (phospholipid bilayer). Na and K are important in the voltage changes. -50 to -80mv at rest on inside. This negative charge driven by negative protein molecules, not that integral in action potentials. Ions can’t pass through the bilayer, need ion channels or pumps. Some are always open, some are gated. K channels always open, Na channels gated. The difference at rest between inside and outside is the resting membrane potential.
Ion channels and pumps
Can’t pass freely. Ion channels are tubelike pores that allow ions of a specific type to pass through the membrane. Constitutively open channels stay open all the time. They have selective permeability to certain ions - open K+ ones central to neural signaling. Gated ones like NA+ only responsive to neural signaling. Ion pumps actively use energy to transport ions across the membrane. Sodium potassium pumps central to neural signaling. Most open ion channels in neurons only allow K+ to cross, selective permeability to K+. Pump - 3 Na+ ions out for every two K+ pumped in. Ligand = molecule that binds to something and has an effect.
Electrochemical forces
Diffusion - particles move from area of high concentration to area of low concentration, move down concentration gradient. Semipermeable membranes let some things pass through and not others. Electrostatic pressure - ions flow towards oppositely charged areas along a voltage gradient. Like charges repel, opposites attract.
The resting potential
Pump pulls K+ in, and as they build up, they also diffuse out through their ion channels down the concentration gradient. As negative charge builds up, it exerts electrostatic pressure to pull K+ back in. Eventually, this reaches an equilibrium potential at about -60mv. Pump uses energy (ATP).
Action potential
Resting potential of neuron provides a baseline level of polarization found in all cells. Unlike most cells, neurons routinely undergo a brief but radical change in polarization, sending electrical signals from one end to another. Input can move it further away from firing (hyperpolarization - increase in membrane potential, further from zero) or closer (depolarization - decrease in membrane potential). -40mv is the threshold. Hyperpolarizing means influx of Cl-. Depolarizing - sodium open and influx of Na+
Effects of hyperpolarizing/depolarizing
Up to a point, depolarizing pulse application produces graded localized responses. Until threshold, then cell becomes positive.
Generation of action potential
Brief but large changes in membrane potential that originate in the axon hillock (beginning of axon, summing all the input) and propagate along the axon. Afterpotentials are a temporary dip below resting potential after action potentials. Hyperpolarized for a bit. All or none = the neuron fires at full amplitude or not at all, this is independent of the stimulus size or strength. Intense stimulus is intense because of frequency not magnitude. Stimulus intensity is encoded in changes in frequency.
Generation part 2
Local input - small number of Na+ channels opening, membrane receptor opens the channels. K+ and Na+ comes back out and overshoots resting potential. Na+ ions go back in and then gates close. 1) open K+ channels create the resting potential, 2) any depolarizing force will bring the resting potential closer to threshold 3) at threshold, voltage-gated Na+ channels open, causing a rapid change in polarity (the action potential). 4) Na+ channels are inactivated, gated K+ channels open, repolarizing and even hyperpolarizing the cell 5) all gated channels close, and the cell goes back to resting
Refractory phase
Upper limit is 1200 spikes per second. With closely spaced stimuli, only the first can trigger an action potential. The membrane becomes refractory to later stimuli. Absolute refractory - can’t generate another action potential b/c everything is already open. Relative refractory-another action potential can only be produced by stronger stimulation than normal.
Propagation
Each adjacent region is depolarized, and a new action potential is generated, propagating down the axon. Only go one way, can’t go back b/c of refractory. Saltatory conduction: in vertebrates, the action potential travels inside the myelinated axon and jumps from node to node, increasing the speed of propagation. Node of Ranvier - gap in myelin sheath.
Postsynaptic potentials and summation
PSPs: When an action reaches the end of the axon, it causes the release of a neurotransmitter into the synapse which binds to receptors. Information removes across the synapse from the axon on the presynaptic neuron towards the target postsynaptic cell. Excitatory (Na+ channels) or inhibitory (Cl- channels). Neurotransmitters either polarize or depolarize the cell. PSPs are brief changes in resting membrane potential of postsynaptic cell that occur in response to a neurotransmitter binding to receptors. When integrated, massive array of local potentials determines whether neuron will reach threshold and generate action potential of its own. These happen to cell bodies and dendrites - channels not organized in a way to do the action potentials. Local potentials determines whether neuron will reach threshold. Go from chemical back to electrical.
EPSP and IPSP
Combination of neurotransmitters and receptors. E: stimulation of an excitatory pathway, produces a small local depolarization, pushing the neuron closer to threshold. I: stimulation of an inhibitory pathway, produces a local hyperpolarization, pushing the neuron farther from threshold. IPSPs involve influx of Cl ions. The nature of the neurotransmitter released by the presynaptic cell helps determine whether synapse excites or inhibits postsynaptic cell. Some do Es and others Is. GABA does inhibitory and glutamate E. Neurons perform information processing in an axon hillock to summate or integrate the hundreds to thousands of E and I occurring at any moment. Spatial: combination of graded potentials that come from different parts of the neuron. Temporal: combination of graded potentials that arrive at the axon hillock at different times. Postsynaptic potentials are local, graded, and dissipate as they spread away from the point of origin. Potentials close to the hillock pack a bigger punch.
