Nervous system physiology Flashcards
Nerve impulses
- neurons exhibit both excitability and conductivity
- neurons initiate and conduct signals called nerve impulses
Membrane potential
All living cells (including neurons) maintain a difference in the concentration of ions across their membranes
- an excess of (+) ions on the outside of the membrane and and excess of (-) ions on the inside of the membrane
- this difference in electrical charge is a type of stored energy (membrane potential)
Resting Membrane Potential (RMP)
The membrane potential maintained by a non-conducting neuron’s plasma membrane
- the slight excess of cations on a membrane’s outer surface is produced by (1) ion transport mechanisms and (2) selective permeability of the membrane
Sodium-potassium pump
Active transport mechanism in the plasma membrane that transports 3 Na+ out and 2 K+ in the cell
- ATP molecule binds to pump, breaks apart, and energy is transferred to the pump
Local or graded potentials
A slight shift away from the RMP in a specific region of the plasma membrane is called a local or graded potential
- stimulus-gated channels are ion channels that open in response to a mechanical or chemical stimulus located in dendrites and soma
- this change in membrane potential excites or inhibits a neuron
Excitation
Stimulus triggers the opening of stimulus-gated Na+ channels
- more Na+ enters the cell allowing the potential to move towards zero (depolarization)
Inhibition
Stimulus triggers the opening of stimulus-gated K+ channels
- more K+ exits the cell allowing the potential to move away from zero (hyperpolarization)
Action potential
The membrane potential of a neuron that is conducting an impulse, aka nerve impulse
- an electrical fluctuation that travels along the surface of a neuron’s plasma membrane
- All or Nothing response! Once threshold is surpassed, the full peak of the action potential is always reached
Steps of action potential 1-4
- Adequate stimulus triggers stimulu-gated Na+ channels to open
- Na+ diffuses rapidly into the cell and produces a local depolarization
- When threshold potential is reached voltage-gated Na+ channels open and more Na+ enters the cell, causing further depolarization
- As Na+ enters the cell membrane potential moves rapidly toward 0mV. It continues in a + direction until it peaks at +30mV
Steps of action potential 5-7
- Voltage gated Na+ channels stay open for about 1ms before they automatically close
- Repolarization: voltage gated K+ channnels open when threshold is reached, however, they open slowly and do not begin to allow outward diffusion of K+ until peak is reached
- K+ channels often remain open as membrane reaches RMP. Too much K+ may rush out causing a brief period of hyperpolarization
Refractory period
Brief period during which a local area of the axon’s membrane resists restimulation
Absolute refractory period
About 0.5ms after membrane surpassses threshold potential, it will not respond to any stimulus, no matter how strong
Relative refractory period
Few ms after absolute refractory period, the time during which the membrane is repolarizing and restoring the RMP, membrane will respond only to very strong stimuli
Conduction of an action potential
Reversal of polarity from RMP to peak of an action potential causes electrical current to flow between the site of reversal and adjacent regions of membrane
- triggers voltage-gated Na+ channels in the next segment of membrane to open
- prevents restimulation of previous segments
Nodes of ranvier
- in myelinated fibers, electrical changes can only occur at gaps in the myelin sheath
- most of the current flows under insulated myelin to the next node where voltage gated channels open
- saltatory conduction: action potential seems to leap from node to node
