Download SBI 4U Homeostasis 2

Homeostasis 2:
The Electrical Nature of Nerves
Electrical Nature of Nerves
• Neurons use electrical signals to
communicate with other neurons, muscles
and glands.
• When microelectrodes are placed on either
side of the membrane of an inactive neuron,
measurements from a voltmeter indicate an
electrical potential difference of -70mV
(millivolts)
• The charge of the inside of the neuron cell is
negative in relation to the outside
• This charge separation across the membrane
is known as the membrane potential.
Electrical Nature of Nerves
Sodium Potassium Pump
• A system that uses ATP in order to keep the electrical
potential difference across the membrane.
• For every three sodium ions transported out of the
cell, two potassium ions are transported into the cell.
• An overall positive charge is going to accumulate on
the outside of the cell membrane and a negative
charge on the inside.
• This ensures the resting potential of the neuron is at
-70mV so that the cell is ready for an impulse to
happen.
The Sodium Potassium Pump
Action Potential
• A nerve cell is polarized because it has a negative
charge.
• It will depolarize during an action potential because
the inside becomes less negative.
• An action potential is the movement of an electrical
impulse along the membrane of a nerve cell’s
axon.
• Action potentials are an all-or-none phenomenon
where the strength of the action potential is always
the same as long as there is enough depolarization
to trigger the potential. (usually around -50mV and
is called the threshold potential).
Action Potential Across the
Membrane
Steps of an Action Potential
• Action potential triggered when threshold potential is met.
• Voltage gated sodium channels open and make
membrane more permeable to sodium ions and they rush
into the cell, making it depolarize. Now the membrane
potential is +40mV
• Sodium channels close, potassium channels open,
potassium moves down the concentration gradient out of
the cell, which makes the membrane repolarize and
actually becomes even hyperpolarized to about -90mV.
• Potassium channels close and the sodium-potassium
pump continues to work so that the resting potential is
restored.
• The next few milliseconds the membrane cannot be
stimulated again as the membrane goes through a
refractory period.
Myelinated Nerve Impulse
• At regular intervals along the axon, the nerve has
nodes of Ranvier that are parts of exposed nerves
between glial cells of the myelin sheath.
• These nodes contain many voltage-gated sodium
channels.
• When the sodium ions move into the cell, the charge
travels through the cell to the next node.
• This occurs at each node along the axon until it
reaches the end of the neuron.
• Because the action potentials are forced to jump from
one node to the next due to the myelin sheath, the
conduction of an impulse along a myelinated neuron is
called saltatory conduction (saltatory means to jump in
latin)
• Saltatory conduction: 120 m/s, unmyelinated: 0.5m/s
Myelinated Propagation
Synapse
• A synapse is a junction between two neurons
or between a neuron and an effector (muscle
or gland)
• A neuromuscular junction is a synapse
between a motor neuron and a muscle cell.
• Neurons are not directly connected. They
have a small gap between them called the
synaptic cleft.
Signal Transmission Across a
Synapse
• When an impulse reaches the far end (called
the synaptic terminal), the impulse must travel
from the presynaptic neuron to the
postsynaptic neuron.
• Chemical messengers called
neurotransmitters carry the neural signal from
one neuron to the next neuron or effector.
• Neurotransmitters are found in the
presynaptic terminal inside synaptic vesicles.
• Once the impulse reaches the synaptic
terminal the synaptic vesicles move towards
and fuse with the presynaptic membrane.
• Neurotransmitters are released into the
synaptic cleft.
• Neurotransmitters bind to receptor proteins
and affect the postsynaptic neuron.
• An enzyme comes in and breaks up the
neurotransmitter and its components will be
reabsorbed by the presynaptic neuron.
Synapse
Neurotransmitters
• Can have either an excitatory or an inhibitory
effect on the postsynaptic membrane.
• If the effect is excitatory, the receptor proteins
will allow positive ions, such as sodium to
flow into the postsynaptic neuron and the
membrane will depolarize.
• If the neurotransmitter is inhibitory, the
receptor will trigger potassium ions to open,
allowing potassium ions to flow out,
hyperpolarizing the membrane.
Examples of Neurotransmitters
• Acetylcholine: crosses a neuromuscular junction and
excites the muscle causing depolarization and
contraction of the muscle.
• Dopamine: affects brain synapses in control of body
movements, linked to sensations of pleasure (eating)
• Serotonin: regulates sensory and temperature
perception, involved in mood control
• Endorphins: act as natural pain killers, emotional
areas of the brain.
• Norepinephrine: complements the actions of the
hormone epinephrine that ready the body to respond
to danger or stress.
Practice:
• P. 362 # 6, 7, 9, 10, 11.