Download Biology 30: Unit A - County Central High School

Biology
30: Unit A
The Nervous and Endocrine
Systems
Learning Objectives
 Formation
and transmission of action
potentials along an axon
 All-or-none
 Threshold
 Intensity
response
level
of response
The Action Potential
What is it?
An action potential occurs when there is a
reversal of charge along the axon
membrane of a neuron
Resting Potential
 As
with all cells, the neuron contains a
plasma membrane
 Looking
closely at the membrane, we see
that there is a high concentration of
sodium (Na+) outside the membrane and
a high concentration of potassium (K+)
inside the membrane
Resting Potential
Diffusion
 The
plasma membrane of the neuron is
selectively permeable – ions cannot just
simply pass through the membrane, they
need assistance.
 This
assistance is met through the use of
Na+ and K+ gated ion channels
Diffusion
 There
is a larger number of K+ ion
channels along the membrane than Na+
channels
 More K+ is leaving the cell than Na+
entering which gives the membrane a
more positive charge outside and a more
negative charge inside
The Resting Membrane
 The
resting membrane is also called a
polarized membrane because it is
charged by unequal distribution of
positive ions along the membrane
 The
membrane is considered to be resting
when it shows an electrical charge of
-70mV
The Action Potential
 There




are 4 stages to an action potential:
Depolarization
Repolarization
Hyperpolarization
Refractory Period
Depolarization
 This
is the process by which there is a rush
of sodium ions coming into the cell which
causes a change in the charge of the
cell.
 The
inside of the cell membrane is now
more positive than the outside
Repolarization
 As
soon as the charge changes, the Na+
gates close and the K+ gates open
 This
eventually allows the charges to
reverse again and bring the membrane
back to its original state
Hyperpolarization
 K+
gates take longer to close though which causes
even more ions to move out of the cell leading to
an even more positive extracellular environment
 The
Na+/K+ pump uses active transport to bring the
proper amount of K+ back into the cell
Refractory Period
 This
is the time it takes to for the cell to reach its
resting membrane state
 This
can last 1 – 10ms
How does an Action Potential
Move?
A
single action potential does not move on its own
down the axon of a nerve cell.
 There
are multiple action potentials occurring one
after another to transmit the information
 There
is a kind of domino effect
How does an Action Potential
Move?
 Remember
that the nerve impulse goes from one
end of the cell to the other and never backwards
 This
 If
is all due to the action potential
you look behind the action potential, the axon is
in the depolarization stage and therefore cannot
begin a new action potential
Axons and Impulses
 When
an axon is myelinated, the impulses travel by
saltatory conduction
 This
means that action potentials only occur at the
nodes of Ranvier
 This
greatly increases the speed of the nerve
impulse along the axon because there is less space
for the action potentials to happen
Threshold Level
 Before
an action potential can occur, it needs to
reach the membrane’s threshold level
 This
is the minimum level of stimulation required to
get a response
 If
the stimulus is too low, there will be no action
potential
All – or – None Response
 Once
a membrane has reached its threshold level,
the action potential either will happen completely
or not at all
 It
doesn’t matter how much you increase the
stimulus, the action potential will give the exact
same response
All – or – None Response
Threshold Levels Con’t
 Not
all threshold levels are the same for every
nerve cell
 This
allows for us to sense many different stimuli in
our environment
 Example:
warm vs. hot objects
Learning Objectives
 Transmission
 Main
of a signal across a synapse
chemicals and transmitters involved
in synaptic transmission
Synaptic Transmission
 The
area between the end plate of one
neuron and the cell bodies or dendrites of
another neuron is called a synapse
 This
is where neurons transmit information
to one another through the use of
chemicals called neurotransmitters
Synaptic Transmission
 The
presynaptic neuron carries the nerve
impulse to the synapse
 The
postsynaptic neuron carries the nerve
impulse away from the synapse
Synaptic Transmission
 Neurotransmitters
are released from the
presynaptic neuron into the synaptic cleft
 These
neurotransmitters diffuse across the
synapse to receptor sites on the
postsynaptic neuron
 This
creates a depolarization effect
leading to an action potential in this
neuron
Synaptic Transmission
 Diffusion
is a slow process so that is why
certain neurons have very few synapses
(ones involved in a reflex arc) while others
have many more (problem solving
neurons)
Neurotransmitters
What is a neurotransmitter?
What is an example of a neurotransmitter
and what does it do?
Talk to the person beside you.
Neurotransmitters
 These
are the chemicals that are released
from the presynaptic neuron
 They
are the messengers that carry
information from one neuron to the next
 There
are many different kinds of
neurotransmitters that each have their
own ‘job’ to do
Excitatory Neurotransmitters
 Acetylcholine
is an excitatory
neurotransmitter
 This
means that it opens sodium ion
channels stimulating an action potential
 One
major effect of acetylcholine is the
contraction of skeletal muscles
Cholinesterase
 Cholinesterase
acetylcholine
 This
is an enzyme that destroys
allows the sodium channels to close
and the neuron to enter the repolarization
stage
Other Excitatory
Neurotransmitters
 Norepinephrine

wakefulness
 Dopamine

Voluntary movements and emotion
Inhibitory Neurotransmitters
 These
neurotransmitters prevent action
potentials from occurring by opening
potassium gates leading to a state of
hyperpolarization
Inhibitory Neurotransmitters
 Norepinephrine

Wakefulness
 Serotonin

Sleep
 GABA

Motor behaviour
Inhibitory Neurotransmitters
 These
neurotransmitters allow you to
prioritize information
 This
allows you to focus on important
information and ignore other stimuli
around you
Summation
 Remember
back to our action potential
lecture and think about threshold levels.
 Each
neuron needs to reach a certain
voltage before an action potential can
occur
 Some
neurotransmitters will not achieve
this threshold level when acting alone
Summation
 To
reach the threshold level, there needs
to be a summation of the
neurotransmitters
 Just
as the word sum means, you add the
reactions of two or more neurotransmitters
to achieve the appropriate threshold
level