Download Nerve Impulses

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.

This results in a difference in electrical charge
across the plasma membrane called membrane
potential.
The Voltmeter
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

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).
The Sodium-Potassium Pump at
Work
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
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

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
Steps of the Mechanism
that Produces an Action
Potential
Step 1
A stimulus triggers
stimulus gated Na+
channels to open
and allow inward
Na+ diffusion.
This causes the
membrane to
depolarize.
Step 2
As the threshold
potential is
reached, voltage
gated Na+
channels open.
Step 3
As more Na+
enters the cell
through voltage
gated Na+
channels, the
membrane
depolarizes even
further.
Step 4
The magnitude of
the action potential
peaks (at + 30 mV)
when voltage gated
Na+ channels
close.
Step 5
Repolarization
begins when
voltage gated K+
channels open,
allowing outward
diffusion of K+.
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.
Action
Potential
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
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.
Action Potential Travels Along the
Axon
Saltatory Conduction
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.
Synaptic Transmission
Synapses

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

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.

Electrical and Chemical Synapses

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
1.
2.
3.
Calcium influx
Neruotransmitter
vesicles move to
membrane and
open
Neurotransmitter
diffuses across
synaptic cleft and
bind to receptor
molecules.