Neurophysiology

AP = long distance

GP = short 

Membrane potential = electrical potential difference

Ions down gradient - high to low concentration 

Ion channel 

• leakage = non gated = open 
• membrane more permeable to K
• all type neuron
• ligand gated
• chemical ligand = neurotransmitter, hormone, ions
• dendrite 
• voltage gated 
• respond to direct change in MP 
• states:
• resting = inactivation gate open, activation gate close = no Na cross
• activated = both gate open = initiate AP = Na cross axolemma
• inactivated = inactivation gate close, activation gate open = no Na can pass = AP finish, return resting 
• mechanically gated 
• respond to stretch/touch, pressure, vibration

Resting MP = -ve voltage - common -70Mv 

Polarized = voltage difference aross PM not at 0Mv 

• negative ions along inside CM, positive ion outside 
• ECF rich NaCl 
• membrane more permeable to K 
• factors allow membrane return resting 
• unequal distribution of ions in ECF
• inability of most anions to leave cells
• electrogenic nature of Na K ATPase

GP 

• hyperpolarize = more polarize, inside more -ve 
• depolarize
• vary in amplitude due to length of stimulation, strength of stimulus, no of open channel & localized 
• reversible 
• dendrite & cell body 
• decremental conduction = short distance 
• summation = GP add together 
• greater depolarizing GP = 2 depolarizing GP
• GP disappear = 2 opp GP 

AP 

• sudden & transient change in the MP at 1 spot on membrane surface 
• a seq of rapidly occurring events that decrease & reverse MP & then restore to resting 
• all-or-none = stimulus reach threshold, AP always same/ a given stimulus either trigger AP or not 
• events:
• local potential depolarize axolemma of trigger zone to threshold -55Mv
• voltage gated Na activate, Na enter, axon depolarize
• Na inactivate, voltage gated K activate, Na stop enter K exit axon, repolarize
• Na channel return resting, repolarization continue 
• axolemma may hyperpolarize b4 K channel return resting 
• axolemma return resting MP 
• Rising phase - rapid depolarization of membrane - reach threshold 
• Overshoot - inside neuron is positively charged 
• Falling phase - rapid repolarization til membrane more negative than resting potential 
• Undershoot/after hyperpolarization 

Refractory  = period of time during which neuron cannot generate other AP in response to threshold stimulus

• absolute
• inactivated Na must return resting b4 reopen
• large fiber 0.4mses, 1000 impulse/sec 
• smaller axon 4msec, 250 impulse/sec
• GP do not exhibit refractory period 
• Na open til inactive, membrane cannot respond 
• relative 
• suprathreshold start AP 
• K channel still open 
• Na close
• stronger stimuli trigger more frequent AP
• Begin when Na regain normal resting 

Propagation = mode of conduction of AP in neuron  

Continuous conduction

• unmyelinated fiber
• step-by-step depolarization
• ions flow thro voltage gated channel in adjacent segment of membrane

Saltatory conduction

• myelinated
• AP propagate more rapidly
• special mode due to uneven distribution of voltage gated channel
• depolarization only at nodes of Ranvier 
• current carried by ions flows thro ECF from node-node
• B fiber medium size - longer refractory 

Factors affect speed of propagation

• amount of myelination
• axon diameter
• temp - cold, slow

Synapse

• electrical
• occurs between cells/synapse electrically coupled via gap junctions 
• faster, 2 way information transfer = bidirectional synaptic transmission 
• nearly instantaneous synaptic transmission = change in potential is extraordinary fast 
• in brain 
• chemical
• one way information transfer 
• axons houses synaptic vesicles with neurotransmitter 
• synaptic cleft occur
• involve receptors for neurotransmitter linked directly or indirectly to ion channels 
• in nervous system

Spatial summation = summation of effects of neurotransmitter released from several end bulbs onto one neuron

Temporal summation = released from >2 firings of same end bulb in rapid succession onto second neuron