DATE:
NAME:
BLOCK:
NERVOUS SYSTEM – NERVES & IMPULSES
(A) INTRODUCTION
 The nervous system is our ____________________ system
 keeps us in contact with the outside world
 tells us that we exist
 along with the muscles, allows us to move and react
to stimuli
 our consciousness resides in our nervous systems,
as do our thoughts and emotions.
 In short, the roles of the nervous system are:
(1) coordination of _________________
(2) response to environmental _________________
(3) ______________________
(4) self-awareness
(5) thought
(6) emotion
(B) ORGANIZATION OF THE HUMAN NERVOUS SYSTEM
 Nervous system has ____ major divisions:
NERVOUS SYSTEM
(1) Central Nervous System (CNS)
 includes spinal cord and ___________
Cenral Nervous System
Peripheral Nervous System
 In the "_______________" of the body.
Brain
(2) Peripheral Nervous System (PNS)
Spinal Chord
Autonomic Nervous System
 the rest of the nervous system:
To Smooth Muscles
 further divided into the:
(a) Somatic Nervous System
Sympathetic Nervous System
 connects to ________________ muscle
“Fight or Flight”
Somatic Nervous System
To Skeletal Muscles,
excterior sensory organs
Parasympathetic Nervous System
Non-emergency Situations
(b) Autonomic Nervous System
 connects to _______________ (involuntary) muscles
 further divided into the:
(i) Sympathetic Nervous System - usually causes effects associated with emergency situations
(ii) Parasympathetic Nervous System - promotes activities associated with a normal state
 the two systems (CNS & PNS) work together and are connected to one another
 the Nervous System is composed of nerve cells called ______________, which are specialized to carry
nerve _______________
(B) STRUCTURE OF NEURONS
 All neurons have THREE parts:
(1) __________________ - conduct nerve impulses towards the cell body
(2) _____________________ - produces all the _______________ for the neuron and contains the
____________________ such as the nucleus, mitochondria, golgi, ER, and ribosomes.
(3) __________ - conduct nerve impulses away from the cell body
PLEASE LABEL THIS DIAGRAM
A
STRUCTURE NAME
A
B
E
C
D
E
B
D
F
G
C
 dendrites and axons are sometimes called _______________
H
F
G
H
 most long, larger diameter fibers are covered by a _______________ sheath
 the sheath has spaces in it exposing the axon called _______________________________
 the sheath is secreted by ________________ Cells, each of which has a nucleus.
PLEASE LABEL
 there are three types of neurons:
(1) Sensory neuron(W) (afferent neuron)
T
 takes a message from a sense organ(U) to CNS
 has long dendrite(V), cell body(T), and short axon
(2) Motor neuron (efferent neuron)
 takes message away from CNS to a muscle fiber
or gland(Z)
 short dendrites, long axon(Y)
(3) Interneuron(X) (association neuron/connector neuron)
 conveys messages between parts of the nervous system
 completely contained within CNS (usually in spinal cord)
 dendrites, axons, may be long or short.
(C) GENERATION AND TRANSMISSION OF NERVE IMPULSES
 Scientists used giant axons in _____________ to figure out how nerve impulses are generated.
 Nerve Conduction is an __________________________ CHANGE that moves in one direction along the
length of a nerve fiber
 it involves changes in _______________ and changes in the _____________________ of certain ions
 Since it is electric, we can use an ________________________ (a type of voltmeter that shows a graph of
voltage changes) to _________________ potential differences (voltages).
Na+
 There are THREE distinct phases in the generation
of a nerve impulse along an axon
K
K
(1) RESTING PHASE
 the potential difference across the membrane of the
axon when it is not conducting an impulse
 normally equals ________ mV.
 This negative polarity is caused by the presence of
large negative proteins and chlorine ions in the
______________ (the cytoplasm inside an axon).
+
+
Na+
K
+
K+
Na+
Na+
Na+
K
K+
+
K+
K
+
Na+
K
+
K
+
K
Na
Na+
+
+
Na+
+
Na+
Na
K
+
K+
K+
+
Na+
Na
K+
K
K+
+
K
+
K+
K+
+
Na+
Na+
+
Na
K
+
Na+
Na+
K
Na
+
Na
K
K
+
+
K
K+
+
Na
K
+
+
Na+
+
K+
K+
K+
Na+
Na+
K
K+
+
Na+
+
Na
Resting Potential
When not conducting impulses, there is a voltage difference
across the membrane of the axon of about -60 mV. The
negative charge on the inside of the axon is due to the
presence of large negative ions. There are more sodium ions
on the outside of the axon compared to the inside of the axon,
and more potassium ions on the inside compared to the
outside.
 During the resting potential, _____ ions are more concentrated on the outside of the membrane and
_____ ions are more concentrated on the inside of the axon
 This uneven distribution of Na+ and K+ ions is maintained by _____________ transport across Na+/K+
pumps which operate whenever the neuron is not conducting an impulse (pumps require ATP)
(2) ACTION PHASE
 if nerve is stimulated by electric shock, pH change, or mechanical stimulation, a nerve impulse is
generated, and a change in electrical potential can be measured/detected.
 This nerve impulse is called the ____________ potential
 On the oscilloscope, the action potential can be
broken into an ____________ and ________________
 During the upswing (-60 mV to +40 mV), the axon
membrane becomes permeable to _____ ions.
V = RESTING PHASE
W = DEPOLARIZATION
(where Na+ moves into the axon)
X = PEAK
(Na+ gates close/K+ gates open)
Y = REPOLARIZATION
(where K+ moves out of the axon)
Z = RECOVERY PHASE
(where the [Na+] and [K+] are returned
to their original concentrations)
 Na+ ions move from the outside to the inside of the axon
 "_____________________" occurs -- the inside of the
axon becomes positive
 In the downswing (+40 mV to -60 mV), the membrane
becomes permeable to _____
 K+ moves from the inside to the outside of the axon
 “_______________________” occurs -- the inside of
axon becomes negative again
(3) RECOVERY PHASE
 between transmissions, K+ ions are returned to the ____________ of axon, Na+ to the ____________
 this is done _________________ (requires ATP) using a Na+/K+ pump
 A summary of all three phases is shown on the last page of this note package
 the speed of nerve impulses is quite ____________, due to the _________________ of the nerves:
 have a _____________ sheath which is formed by tightly packed spirals of the cell membrane
of ______________ cells.
 have interruptions or ________ of the sheath called the _______________________________
 the speed of transmission is ~_______ m/s in myelinated fibers, but only 0.5 m/s in non-myelinated fibers.
 the reason is that the nerve impulse "________" from node to node in myelinated fibers (In nonmyelinated fiber, the nerve impulse must depolarize and repolarize each point along the nerve fiber)
INTERESTING NOTE
Destruction of large patches of Myelin characterize a disease called Multiple Sclerosis. In multiple
sclerosis, small, hard plaques appear throughout the myelin. Normal nerve function is impaired, causing
symptoms such as double vision, muscular weakness, loss of memory, and paralysis.
(D) TRANSMISSION OF IMPULSES ACROSS SYNAPSES
 What happens to a nerve impulse once it reaches the end of an axon?
 How does one nerve communicate with another?
ANSWER = SYNAPSES
(1)
(2)
(3)
(4)
(5)
(6)
Synapse: the region between end of an axon and the cell body or dendrite to which it is attached.
Synaptic Endings: swollen terminal knobs on the ends of axon terminal branches.
Presynaptic Membrane: the membrane of the axon synaptic ending.
Postsynaptic Membrane: the membrane of the dendrite on the next neuron.
Synaptic Cleft: the space between the presynaptic and the postsynaptic membranes
Neurotransmitter Substances (neurotransmitters): chemicals that transmit the nerve impulses across
a synaptic cleft.
(7) Synaptic Vesicles: contain the neurotransmitters. Contained near surface of synaptic endings.
 Acetylcholine (Ach), Noradrenalin (NA), Serotonin, Adrenalin (epinephrine) are some important
neurotransmitters.
a)
Z
b)
W
c)
Y
X
d)
e)
f)
 Transmission across a synapse is _____________ because only the ends of axons have synaptic vesicles
that are able to ______________ neurotransmitters to affect the potential of the next neuron(s).
 __________________ or _________________ of postsynaptic membranes can occur.
 another nerve is usually on the receiving end of many synapses -- some may be give inhibitory and some
may give stimulatory impulses. Whether or not the neuron they are attached to fires depends on the
_____________________ effect of all the neurotransmitters received.
 If amount of excitatory neurotransmitters received is sufficient
to overcome the amount of inhibitory neurotransmitters received,
the neuron fires. If not, only local membrane excitation occurs. This
total process allows neurons to fine-tune to their activity.
SOMETHING TO KNOW
The MINIMUM LEVEL of a STIMULUS that is REQUIRED to Activate a neuron is
called the THRESHOLD. If the THRESHOLD is reached, an impulse (ACTION
POTENTIAL) begins in the second cell.
Carefully examine this diagram that has appeared on several
provincial exams. What do you think structure Y is? Which direction
Is the impulse moving?
Sequence of events:
1. Nerve impulse travel
along axon, reach a
synaptic ending.
2. Arrival of nerve
impulse at synaptic
ending changes
membrane ----> Ca2+
flows into ending
3. Ca2+ ions cause
contractile proteins
to pull synaptic
vesicles to inner
surface of the
presynaptic
membrane.
4. Vesicle fuses with
presynaptic
membrane, releasing
neurotransmitters
into synapse.
(exocytosis).
5. Neurotransmitters
DIFFUSE across
synaptic cleft to
receptors on the
postsynaptic
membrane. The
receptors control
selective ion
channels; binding of
a neurotransmitter to
its specific receptors
opens the ion
channels.
6. The resulting ion flux
(not shown on
diagram) changes the
voltage of the
postsynaptic
membrane. This
either:
K+
Na+
Ca+
Ca+
Ca+
Ca+
(1) moves the
membrane voltage
closer to the
‘threshold voltage’
required for an action
potential (an excitatory
synapse), the
neurotransmitters
binding to receptors on
the dendrite causes
the nerve impulse to
be transmitted down
the dendrite of the
second neuron. The
nerve impulse has
now been transmitted
from the first neuron to
the second neuron.
K+
Na+
(2) or hyperpolarizes
the membrane (an
inhibitory synapse).
Moves voltage away
from threshold voltage.
7. Neurotransmitters are
quickly deactivated to
prevent them from
continually acting on
postsynaptic
membrane. This can
occur when:
a) neurotransmitter is
degraded by
enzymes (e.g.,
acetylcholinesterase
(= “cholinesterase”)
breaks down
acetycholine).
b) synaptic ending
reabsorbs the
neurotransmitter. e.g.
this is what happens to
Serotonin.
K+
Na+
INTERESTING NOTE
NERVE GAS prevents
enzymes from breaking down
neurotransmitters, as a result
muscles in the respiratory and
nervous system becomes
paralyzed. They can never
recover back to resting phase
 ___________________________ take nerve impulses across synapses.
 Neurotransmiters are __________ molecules. They can be single amino acids, short chains of amino
acids, or derivatives of protein (eg. Seratonin)
HO
 proper brain and nervous system function depends on the proper __________
of excitatory and inhibitory synaptic transmitters.
C
 _________________ transmitters: include acetylcholine (ACh), adrenalin
(epinephrine), noradrenalin (norepinephrine), serotonin (derived
from the amino acid tryptophan), and dopamine.
 _________________ transmitters: include GABA (gamma aminobutyric acid - a
type of amino acid), glycine (an amino acid).
CH2
CH2
NH2
CH
N
H
Structure of Serotonin
 neurotransmitters include endorphins and enkephalins (polypeptide neurotransmitters which function as a
natural pain relievers in the brain). (Opium and heroin (drugs) mimic the action of natural endorphins/enkephalins).
 A single neuron may receive information from _______________
Axons from
nearby neurons
of neighbouring neurons through thousands of synapses.
 Some of the messages are excitatory (i.e. they tell the neuron to “fire”)
 others may be inhibitory (i.e. they tell the neuron not to fire).
 Whether or not a neuron “fires” off an action potential at any
particular instant depends on its ability to integrate these multiple
positive and negative inputs.
SUMMARY OF NERVE IMPULSE GENERATION AND TRANSMISSION
Step 1: Sodium moves in
+
Na
Step 2: Depolarization
+
Na
+++++++
Sodium channels open, Na+ ions diffuse into axon.
Step 3: Na+ channels close, K+ open
K+
The inside of the axon has become positive in that
region. This is called depolarization.
Step 4: Repolarization
K+
Potassium channels open, K+ ions diffuse out of axon.
Step 5: Recovery Phase
K+
The movement of K+ ions counters the depolarization.
The voltage differerence across the membrane returns to
the resting potential level (-60mV).
Step 6: Depolarization of adjacent part of axon
Na+ Na+
Na+
Na+ and K+ actively transported back across
membrane until they are distributed in the same
concentrations as before the impulse was sent.
The impulse will continue to move down the axon until
it reaches the synapse.
Sodium channels open, Na+ ions diffuse into axon.
++++++++++++++++++++++++++
++++
++++
++++++++++++++++++++++++++
++++++ ++++++++++++++++++++
 ++++

 ++++

++++++++++++++++++++++++++
++++++++++ ++++++++++++++++
++++ 
++++
++++++++++ ++++++++++++++++
++++++++++++++ ++++++++++++
++++
++++
++++++++++++++ ++++++++++++
?