Download Nerve Signals

SBI 4U
January 17 th , 2012
 Luigi Galvani: muscles of dead frog twitch (partially correct)
 Continued research began to reveal differences between
electrical and neural transmission
 Nerve impulses more slow than electric current
 Nerve impulses do not undergo loss of strength during
transmission
 Synapse: site of connection between neurons/neuron
and effector
 Pre-synaptic and post-synaptic
 Chemical vs. Electrical synapse
 Neurotransmitter released by terminal at synapse
 Synaptic cleft: separation of plasma membranes by a
25 nm wide gap
 Presynaptic cell makes direct contact with the postsynaptic cell,
allowing current to flow via gap junctions between the cells
 Unbroken transmission
 Rapid transmission and synchronous activity
 Outside of cell – positive
 Inside of cell – negative
 Charge separation -> voltage, or electrical potential difference 
Membrane potential
 Plasma membrane  selectively permeable  select ions move
through ion channels
 Some ion channels closed during resting state, others
open upon the binding of a specific substance
 Neurons and muscle cells are exciteable
 Na+/K+ ion active transport pump, uses energy from
ATP to pump 3 Na+ out of the cell for every 2 K+
pumped in
-more Na+ outside and more K+ inside
-inside of cell more negative due to anions. Outside
more positive
 -70 mV – resting potential. Unstimulated neuron
 Polarized
 Action potential: the voltage difference across a nerve
cell membrane when the nerve is excited
 Causes the interior to be less negative
 Incoming positive ions raise membrane potential to
less negative  depolarization!
 If depolarization continues, membrane potential
reaches the threshold potential (~ -50 to -55 mv)
 Na+ channels open
 Na+ channels remain open  flows across
concentration gradient  action potential fires!
 Action potential rises high, inside of plasma
membrane positive
 Action potential reaches peak, reaching +30 mV or
more
 Na+ channels close and K+ channels open
 K+ channels open  membrane potential falls during
the process of repolarization
 Voltage-gated K+ channels begin to close slowly
 Membrane repolarizes
 Undershoot of membrane potential
 Final phase
 Membrane potential stabilizes at the resting value
 Ready for a new action potential!
 Many stimuli can cause some degree of depolarization
 All-or-nothing principle
 Propagation of action potential  does not need
further triggering events
 At the peak of an action potential, membrane enters a
resting period, the refractory period (few
milliseconds)
 During refractory period, threshold required for action
potential much higher than normal
 Lasts until membrane has stabilized
 Intensity not dependent upon the magnitude of
stimulus