CHAPTER 4 Flashcards
What did Galvani discover?
Galvani discovered that the muscles of dead frogs legs twitched when struck by electricity
Electricity:
A flow of electrons from a body that contains a higher charge (more electrons) to a body that contains a lower charge (fewer electrons)
Electrical Potential:
An electrical charge measured in volts; the ability to do work through the use of stored potential electrical energy
Hermann von Helmholtz
Measuring electric nerve impulses in frog leg nerves
Conclusion: Flow of information in the nervous system is too slow to be a flow of electricity!
Measuring the electric potentials (i.e. membrane voltage
–Recording electrode is inside the nerve fiber
–Reference electrode is outside the fiber
–Difference in charge between them is about -70 to -80 mV in the steady state (resting membrane potential ,RMP)
Cations
Positively charged ions
Examples: Sodium (Na+),
potassium (K+)
How do the movement of ions create electrical potentials in neurons
Diffusion & Concentration Gradient:
Anions
Negatively charged ions Examples: Chloride (Cl-), protein molecules (A-)
Diffusion
Movement of ions from an area of higher concentration to an area of lower concentration
How do the movement of ions create electrical potentials in neurons - cont.
Electric Potential: Difference in electrical charge between two regions – Oppositechargesattract
– Similarchargesrepel
Concentration Gradient:
Differences in concentration of a substance from an area of higher concentration to an area of lower concentration
The Movement of Ions is caused by two major forces:
– Chemical force (Diffusion): the tendency for molecules to distribute themselves equally within a medium
– Electric potential: the difference in electrical charge across the membrane
what do these forces make together
electrochemical gradient
Net effect:
high concentration (potential) of Na+ ions outside and negativeproteinsA-andK+ inside.Theresultingmembranepotential is about -70 mV
Action Potentials are triggered at the
Axon Hillock
Voltage-Sensitive (gated) Ion Channels
- Gated protein channel that opens or closes only at specific membrane voltages (typically at about -65 mV
- Sodium (Na+) and potassium (K+)
- Closed at membrane’s resting potential
Action potentials occur when
Permeability of the membrane changes (e.g. due to opening of touch sensitive sodium channels in a receptor neuron)
* If the excitation (i.e., the depolarization of the membrane) is strong enough it reaches a threshold which activates voltage sensitive sodium (Na+) channels
* The Neuron will then further depolarize very fast due to the influx of more and more sodium ions
Continuous conduction
action potential propagation in unmyelinated axon
All-or-none law
the strength of the action potential is independent of the intensity of the stimulus that elicits it
How does the action potential encode information about the intensity of a stimulus?
All-or-none law & (rate law)
Saltatory Conduction:
Action potential propagation in myelinated axon
rate law
Coding of intensity is by the firing rate (rate law) of a neuron and by the number of neurons firing.
Neural impulse
The propagation of an action potential along an axon.
The axon depolarizes in a sequential fashion from the axon hillock to the presynaptic terminal.
The neural impulse occurs only one way because of the absolute refractory period.
Speed of transmission varies due to thickness of the axon, presence or absence of myelination, and number of synapses.
Properties of action potentials
Action potentials remain the same size
* The only coding possible on a neuron is by variation firing
rate (AP/s)
* Increase in stimulus intensity can increase the firing rate of neurons
* Refractory period is 1 ms - upper firing rate is 500 to 800 impulses per second
* In some neurons, spontaneous activity of action potentials occurs without stimulation
Graded Potentials
Small voltage fluctuations in the cell membrane where ion concentrations change
Hyperpolarization
– Increase in electrical charge across a membrane (more
negative)
– Usually due to the inward flow of chloride ions or outward flow of potassium ions
– Can be blocked: Tetraethylammonium (TEA), Curare
Depolarization
– Decrease in electrical charge across a membrane (more
positive)
– Usually due to the inward flow of sodium
– Tetrodotoxin
Excitatory Postsynaptic Potential (EPSP)
– Brief depolarization of a neuron membrane in response to an input
– Neuron is more likely to produce an action potential
Inhibitory Postsynaptic Potential (IPSP)
– Brief hyperpolarization of a neuron membrane in
response to an input
– Neuron is less likely to produce an action potential
- Temporal Summation
– Pulses that occur at approximately the same time on a membrane are summed
The Axon Hillock
- Junction of cell body and axon
- Rich in voltage-sensitive
channels - Where EPSPs and IPSPs are
integrated - Where action potentials are
initiated
- Spatial Summation
– Pulses that occur at approximately the same location on a membrane are summed
How Nerve Impulses Produce Movement
Acetylcholine
Ligand (Transmitter)-sensitive channel
* End plate
Acetylcholine
– The first neurotransmitter discovered in the peripheral and central nervous systems
– Activates skeletal muscles
Ligand (Transmitter)-sensitive channel
– Receptor complex that has both a receptor site for a chemical and a pore through which ions can flow
- End plate
– On a muscle, the receptor–ion complex that is activated
by the release of the neurotransmitter acetylcholine from the terminal of a motor neuron
