SUSAN, EMILY AND HANNAH
INTRODUCTION
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Nervous system helps
coordinate body functions to
maintain homeostasis
Enables body to respond to
changing conditions
Nerve cells are called
neurons- specialized to react
to physical and chemical
changes in surroundings
Nerve impulses are
electrochemical changes that
transmit info between
neurons
TYPICAL NEURON
MOTOR NEURON
NEURONS
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Neurons consist of:
-Cell body- rounded area with nucleus
-Dendrites- extensions that receive electrochemical
messages
-Axons- extensions that send info in nerve impulses
- axon hillock- conical region of axon where it joins the cell
body
- Schwann cell- a glial or supporting cell that forms myelin
sheaths around the axons of neurons in the PNS
- myelin sheath- fatty lipoprotein protection of axon that
provides electrical insulation
- neurons with myelin sheath= white matter
- neurons without myelin= gray matter
- nodes of Ranvier- gaps between adjacent Schwann cells
PNS AND CNS
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Organs can be split into two
groups:
- Central nervous system
(CNS)- brain and spinal cord
-Peripheral nervous system
(PNS)- nerves that connect the
CNS to other body parts
Together they provide 3 general
functions: sensory, integrative,
motor
NEURON GROUPS
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Sensory neurons:
-Carry nerve impulses from PNS to
CNS
-Either receptor ends at tips of
dendrites or dendrites closely
associated with receptor cells in skin
or sensory organs
-Changes inside or outside body
stimulate receptors; trigger sensory
nerve impulses
- impulses travel along axons,
leading to one of CNS organs; other
neurons process the impulses
- internal and external environment
ex) light, sound intensity,
temperature, oxygen levels
For example, this man’s
sensory neurons allow him
to feel the heat of the stove
NEURON GROUPS
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Interneurons
- lie in the brain and spinal cord
- link other neurons
- transmit impulses from one part
of brain/spinal cord to another;
direct incoming sensory impulses
 Motor Neurons
- carry nerve impulses out of CNS
to effectors
- effectors- responsive structures
that include muscles that contract
and glands that secrete
- motor impulses stimulate the
effectors
This man touched the hot stove.
His sensory neurons sent an
impulse to the CNS, which sent
a responding impulse to his
motor neurons and effectors, in
this case his muscles pulling
his hand back.
CELL MEMBRANE POTENTIAL
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Resting potential
- it is the potential difference (or difference in electrical
charge) between region inside membrane and region outside
membrane in neurons not transmitting signals
- depends on the ionic gradients; net movement of K+ and
Na+ ions from higher to lower concentration
- in a resting cell membrane K+ ions diffuse out of the cell
more rapidly than Na+ ions can diffuse in ; outside gains “+”
surplus and inside gains “-” surplus
- meanwhile, Na+/K+ pumps actively transport ions in
opposite directions to maintain this concentration gradient
CELL MEMBRANE POTENTIAL
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Potential changes:
- nerve cells are excitable; respond to changes in
environment
- changes affect resting potential in a particular region of
nerve cell membrane
- depolarization- membrane’s resting potential decreases
(inside less negative than outside); opening of gated
Na+ channels and K+ channels remain closed
- Changes in resting potential are graded: change in
potential is directly proportional to the intensity of the
stimulations
-threshold potential- summation of depolarizations to a
certain membrane voltage, after which an action
CELL MEMBRANE POTENTIAL
Action Potential:
- rapid sequence of
depolarization and repolarization
- at threshold potential, channels
in membrane open and Na+ ions
begin to diffuse freely inward and
membrane loses its negative
charge; becomes depolarized
- then, the Na+ channels close
and channels open that allow K+
ions to pass out, and inside
becomes negatively charged
again; it is repolarized and
membrane returns to resting
potential
- this process forms the nerve
impulse that is propagated along
the axon
Action potential animation
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NERVE IMPULSES
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Nerve impulse- movement of action potentials along a nerve
axon
Local current: when action potential occurs in one region of
nerve cell membrane, causes bioelectric current to flow to
adjacent portions;
stimulates to threshold level and trigger another action
potential= chain reaction!
Wave of action potentials moves down the axon
IMPULSE CONDUCTION
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An unmyelinated axon conducts an impulse over its
entire surface
Myelinated axon is different b/c myelin insulates and
prevents almost all ion flow through the membrane it
encloses
Therefore it is not continuous; nodes interrupt the sheath
and action potentials occur at these nodes; so impulse
appears to jump from node to node
Termed saltatory and is much faster than unmyelinated
Speed of nerve impulse conduction is proportional to
diameter of axon; greater diameter=faster impulse
NERVE IMPULSES
IMPULSE CONDUCTION
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Nerve impulse conduction is an “all-or-none-response”
If neuron responds at all, it responds completely
A greater intensity of stimulation does not produce a
stronger impulse, but rather more impulses per second
Refractory period- short time following a nerve impulse
when a threshold stimulus will not trigger another
impulse on an axon; limits the frequency of impulses
Calcium’s role:
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- too little Ca + = spasms
- too much Ca+ = neuron is hard to fire, impulse hard to trigger
THE SYNAPSE
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How the impulse passes from neuron to neuron
Synapse- junction between any two communicating neurons; not in
direct physical contact (axon to dendrite, or axon to cell body)
- electrical synapse- gap junctions that allow current to flow directly
from cell to cell; synchronize activity of neurons for certain rapid,
stereotypical behaviors
- chemical synapse- majority are this; release of chemical
neurotransmitters
Synaptic cleft- gap between neurons; communication along a
nerve pathway must cross these gaps
Presynaptic neuron- the sender; the neuron carrying the impulse
into the synapse
Postsynaptic neuron- the receiver; receives the impulse
Synaptic transmission- process of crossing the synaptic gap
Neurotransmitters- biochemicals that carry out synaptic
transmission; it diffuses across the synaptic cleft and reacts with
specific receptors on the postsynaptic neuron membrane
SYNAPSE
Synaptic knob- extension
from axon
Synaptic vesicle- membranous sac
that releases neurotransmitters
postsynaptic
THE SYNAPSE: THE BASIC PROCESS
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When an action potential reaches a synaptic knob (terminal),
it depolarizes the membrane, opening voltage-gated calcium
channels.
 Calcium ions (Ca 2+ ) then diffuse into the knob, and the
increase in ion concentration causes some of the synaptic
vesicles to fuse with the membrane
 The vesicles then release their neurotransmitters, which
diffuse across the synaptic cleft; subsequent affect on
postsynaptic neuron may be:
 Direct synaptic transmission- neurotransmitters bind to
ligand-gated ion channels in membrane of postsynaptic cell;
result is change in membrane potential
 Indirect synaptic transmission- neurotransmitter binds to a
receptor that is not part of an ion channel; activates a signal
transduction pathway; slower but effects last longer
THE SYNAPSE
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Excitatory neurotransmitters- increase postsynaptic
membrane permeability to sodium ions which will bring it
closer to the threshold and may trigger nerve impulses
Inhibitory neurotransmitters- decrease the potential that
threshold will be reached; less chance that nerve
impulse will occur
Synaptic knobs of 1000 or more neurons could
potentially interact with one postsynaptic neuron; some
knobs will release excitatory, some inhibitory; effect on
the neuron depends on the ratio of each
synapse animation
NEUROTRANSMITTERS
Type/Name
Functional Class
Secretion Sites
Acetylcholine
Excitatory to vertebrate
skeletal muscles; either at
other sites
CNS; PNS; vertebrate
neuromuscular junction
Biogenic Amines:
Dopamine
Serotonin
Generally excitatory
Amino Acids
GABA
Glutamate
Inhibitory
excitatory
Neuropeptides
Substance P
Excitatory
Met-enkephalin
(an endorphin)
Generally inhibitory
Released in brain and affects
sleep, mood, attention,
learning
At most inhibitory synapses in
brain
CNS
CNS; PNS; mediates our
perception of pain
Decreases pain perception
KNEE-JERK REFLEX