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Nerve Impulses. Membrane Potentials. All living cells maintain a difference in the concentration of ions across their membranes. There is a slight excess of positives on the outside and a slight excess of negatives on the inside.

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Nerve Impulses

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membrane potentials
Membrane Potentials
  • All living cells maintain a difference in the concentration of ions across their membranes.
  • There is a slight excess of positives on the outside and a slight excess of negatives on the inside.
  • This results in a difference in electrical charge across the plasma membrane called membrane potential.
resting membrane potentials
Resting Membrane Potentials
  • When a neuron is not conducting electrical signals, it is said to be “resting.”
  • At rest, a neuron’s membrane potential is typically maintained at about -70 mV.
sodium potassium pump
Sodium-Potassium Pump
  • Active transport mechanisms in the plasma membrane that transports sodium ions (Na+) and potassium ions (K+) in opposite directions at different rates.
  • Three Na+ out for every two K+ in
  • Maintains an imbalance in the distribution of positive ions, thus maintaining a difference in electrical charge—the inside becomes slightly less positive (slightly negative).
role of channels in membrane
Some K+ channels are open when at rest

K+ diffuses down its concentration gradient

Adds to the positive on the outside of the cell

Na+ channels are closed

Role of Channels in Membrane
local potentials
Local Potentials
  • In neurons, membrane potentials can fluctuate above or below the resting membrane potential in response to certain stimuli.
  • A slight shift away from the RMP in a specific region of the plasma membrane is often called a local potential.
Excitation of a neuron occurs when a stimulus triggers the opening of stimulus-gated channels allows Na+ to enter the cell
  • Depolarization – movement of the membrane potential towards zero
  • Inhibition occurs when a stimulus triggers the opening of stimulus-gated K+ channels. As K+ diffuses out of the cell the positive ions outside the cell increases
  • Hyperpolarization – movement of the membrane potential away form zero (thus below the usual RMP)
action potential
Action Potential
  • An action potential is a nerve impulse in which an electrical fluctuation travels along the surface of a neuron’s plasma membrane.
  • Voltage Gated Channels (-59 mV= threshold potential)
  • All or nothing response
step 1
A stimulus triggers stimulus gated Na+ channels to open and allow inward Na+ diffusion. This causes the membrane to depolarize.Step 1
step 3
As more Na+ enters the cell through voltage gated Na+ channels, the membrane depolarizes even further. Step 3
step 6
After a brief period of hyperpolarization, the resting potential is restored by the sodium-potassium pump and the return of ion channels to their resting state.Step 6
refractory period
Absolute – can not respond to any stimulus no matter how strong

Relative -- the membrane is repolarizing and can only respond to very strong stimuli

Refractory Period
conduction of the action potential
Conduction of the Action Potential
  • The action potential causes voltage gated channels to open in adjacent areas of the axon membrane causing the action potential to move down the length of the axon.
  • In myelinated fibers, electrical changes in the membrane can only occur at gaps in the sheath (nodes of Ranvier). This is called saltatory conduction.
conduction speed
Conduction Speed
  • The speed of conduction of a nerve fiber is proportional to its diameter. The larger the diameter, the faster it conducts impulses.
  • Myelinated fibers conduct impulses more rapidly than unmyelinated fibers.
  • A synapse is where signals are transmitted between neurons.
  • Presynaptic and postsynaptic neurons.
  • Electrical synapse – occur when two cells are joined end to end by gap junctions. The action potential just continues on between cells (cardiac cells).
chemical synapse
Chemical Synapse
  • Three structures make up a chemical synapse:
    • A synaptic knob – contains vesicles with neurotransmitters
    • A synaptic cleft – space in between (20-30 nm)
    • The plasma membrane of a postsynaptic neruon.
Action potential can not cross synaptic cleft
  • Neurotransmitters are released from the synaptic knob where they travel across the cleft to the receptors on the plasma membrane of the postsynaptic neuron.
  • Neurotransmitters cause either depolarization (excitatory) or hyperpolarization (inhibitory) of the postsynaptic membrane.
the chemical synapse
The Chemical Synapse
  • Calcium influx
  • Neruotransmitter vesicles move to membrane and open
  • Neurotransmitter diffuses across synaptic cleft and bind to receptor molecules.