Phys Lecture 14 Impulse Transmision #1

Graded potentials

Changing membrane potentials, can vary on distance

Graded potentials pt.2

Generally involve and INPUT via neurotransmitter receptors

• receptors are on dendrites and cell body

• Amplitude decreases with distance from activation

Can be depolarizing or hyperpolarizing

• DEPOLARIZING: activate action potential

• HYPERPOLARIZING: restrict action potential

• Axon hillock decides

2 primary neuro-receptors

Ionotropic receptors: ligand-gated channels

Metabotrophic receptors: coupled to ion channels

Voltage gated channels

Na+, K+, and Ca2+ -calcium influx

Action potentials

Generated by changes in membrane

Propagation of the action potential

Propagation of a DEPOLARIZING electric potential along the plasma membrane (neurons, muscle, and some endocrine cells)

• an action potential is SELF PROPAGATING (continues like a row of falling dominos)

Threshold of depolarization

Activates at -55 mV - NA+ is depolarizer

Propagation of depolarization

Depolarization: rising phase of Action potential

Voltage gated sodium channels open

1) Influx of NA+ ions depolarizes atonal membrane (graded potential)

2) if depolarization reaches threshold:

• voltage gated Na+ channels open :INFLUX of Na+ ions (faster)

Positive feedback loop: depolarization

Membrane REPOLARIZATION

REPOLARIZATION: falling phase of action potential

1) voltage gated Na+ channels inactivate (around 1ms)

• stops influx of Na+ ions

• Na+ channels now inactive: channel plugged

2) voltage gated K+ channels open K+ rushes out of cell (membrane potential becomes more negative) Repolarization the membrane potential (slower)

Voltage gated K+ channels open slowly

Reaching resting membrane potentials

HYPERPOLARIZATION (undershoot phase of AP)

1) with Na+ channels closed and more K+ channels opened the membrane potential will become more negitive then the resting potential of -70mV

2) voltage gated K+ channels close moving membrane potentials back to rest

Action potentials are all or none

For an action potential to occur, axon must be depolarized by about 15mv (-70mV —> -55mV)

• if too few Na+ ions enter the cell NO action potential will occur

Absolute refractory period

Neurons can’t respond to another stimulus when Na+ channels are either open or inactivated

• Ensures that each AP is a discrete event

• Enforces one way transmission of AP

Relative refractory period

Immediately after the absolute refractory period

• Na+ channels reactivate ready to be opened for release of Na+ intracellular to extracellular

How do neurons code for stimulus intensity?

The amplitude of all action potentials is the same and is independent of stimulus intensity (all or none)

Stimulus intensity is encoded in FREQUENCY

• the more intense the higher the frequency

Synapses (presynaptic neuron)

• conducts impulses towards synapse

• Synaptic vesicles

• Stores NEUROTRANSMITTER

Synapses (synaptic cleft)

• fluid filled space between pre and post synaptic membranes

Synapses (postsynaptic neuron)

-Neurotransmitter receptors

Synaptic transmission

Ca2+ in synaptic transmission

Triggers the fusion pore opening

Termination of the neurotransmitter signal

Remove Ca2+ from the cytosol (stops neurotransmitter release)

• Ca2+ ATPase

• Na+/Ca2+ exchanger

Neurotransmitter removal

1) diffuses from synaptic cleft

2) degraded by an enzyme

3) taken up by presynaptic terminal or astrocyte

Recovery of resting membrane potential and ionic concentration

1) K+ leak channels

2) sodium/potassium pump

• helps re-establish the Na+ and K+ ion concentrations inside and outside neuron

• Helps maintain internally negative resting membrane potential