Download Nervous System - Neurons

Nervous System - Neurons
Biol 105
Lecture 9
Chapter 7
Outline
I.
II.
III.
IV.
V.
VI.
Nervous system function
Central and peripheral nervous system
Nervous system cells
Myelinated neurons
Nerve signal transmission
Nerve Synapse
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Nervous Tissues
 Nervous tissue functions to conduct
messages throughout the body.
 When nerve cells are stimulated, an electrical
signal quickly travels through the nerve cell to
the nerve ending, triggering events
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Nervous System
 Includes nerve tissue and sense organs
 Nervous system functions to:
 Senses environment – receives
information from both outside and inside
the body
 Processes the information it receives
 Respond to information – sends out
orders
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Two Parts of the Nervous System
1. Central Nervous System (CNS)
 Brain and Spinal Cord
2. Peripheral Nervous System (PNS)
 Nervous tissue outside brain and
spine
 Sense organs
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Central Nervous System
Peripheral
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Nervous System Cells
 Two types of nervous tissue cells
 Neurons – the cells that are responsible for
transmitting messages
 Neuroglial cells – cells that support the
neurons
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Neuroglial cells
 Microglia – immune system cells, engulf
bacteria and cellular debris
 Astrocytes – provide nutrients to neurons
 Oligodenrocytes and Schwann cells – form
myelin sheaths
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Figure 4.6
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Parts of a Neuron
 Cell body – contains the nucleus, main body
of cell
 Dendrites – projections from the cell body
that carry messages to the cell body
 Axons – one large projection that carry
messages away from the cell body
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Neuron
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Neurons Have Dendrites, a Cell Body, and an
Axon
Dendrites receive
information from
other neurons or
from the environment.
The cell body controls
the cell’s metabolic
activities.
Axon endings release
chemicals called
neurotransmitters that
affect the activity of
nearby neurons or an
effector (muscle or gland).
Nucleus
Cell
body
The cell body
integrates input
from other neurons.
Receiving portion of
neuron
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An axon conducts the
nerve impulse away
from the cell body.
Axon
endings
Sending portion of neuron
Figure 7.2
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12-12
Neurons of the Peripheral Nervous System
 Neurons in the PNS are either carrying
messages to or from the CNS
 Afferent = Sensory neurons = Neurons carrying
messages to the CNS
 Efferent = Motor neurons = Neurons carrying
messages from the CNS
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Interneurons in the Central Nervous System
 Located between sensory and motor
neurons within the CNS
 Interneurons integrate and interpret
sensory signals
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Sensory Neurons
 The afferent or sensory neuron cell bodies
are located in dorsal root ganglion.
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Motor Neurons
 The efferent or motor neuron cell bodies are
located in the gray matter of the spinal cord.
 Their axons leave the CNS and go to the
skeletal muscles
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The cell bodies of these neurons are located in the dorsal
root ganglion
1. Motor
2. Sensory
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Neurons of the Nervous System
Muscle
(effector)
Sensory
receptor
for pain
Impulse direction
Sensory
neuron
Cell
body
Motor
neuron
Interneuron
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Figure 7.1
12-5
These neuroglial cells provide nutrients to neurons
1.
2.
3.
4.
Microglia
Astrocytes
Oligodenrocytes
Schwann cells
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These are projections of the neuron cell body that carry
messages to the cell body
1. Axons
2. Dendrites
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Which of the following type of neuron would alert the brain
that you had touched a hot object?
1. efferent neuron
2. afferent neuron
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What type of neuron is the arrow pointing to?
1. Sensory
2. Motor
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Myelinated neurons
 Neurons that have axons covered with
neuroglial cells that contain the protein myelin
are called myelinated neurons
 Myelinated neurons are able to carry messages
faster than non-myelinated neurons
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Functions of Myelin Sheaths
1. The main benefit of myelin sheaths is that
myelinated neurons are able to carry
messages faster than non-myelinated neurons
2. Myelin sheaths from Schwann cells also help
regenerate injured PNS neuron axons
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Two Types of Cells that Myelinate neurons
 Schwann cells and Oligodenrocytes are
wrapped around neuronal axons
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Myelinated neurons
 Schwann cells are found in the PNS
 Oligodendrocytes are found in the CNS
 Nodes of Ranvier are spaces on the axon
between the glial cells
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Myelinated Neurons
http://www.youtube.com/watch?v=mOgHC5G8LuI
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Myelinated Neurons
Nucleus
Dendrites
Cell
body
In saltatory conduction, the
nerve impulses jump from one
node of Ranvier to the next.
Node of
Ranvier
Schwann cell
(a)
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Myelin sheath
Figure 7.3a
Myelin Sheath
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Figure 7.3b
Myelin Sheath
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Figure 7.3c
Multiple Sclerosis (MS)
 Caused by the destruction of the myelin
sheath that surrounds axons found in the
CNS
 Can result in paralysis and loss of
sensation, including loss of vision
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Nerve
 Nerve contain Neuron axons are bundled
together
 These bundles contain
 Axons
 Blood vessels
 Connective tissue
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Nerve
Connective tissue surrounding one nerve
Blood supply
Axons within a connective
tissue sheath
One axon
(d) The anatomy of a nerve
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Figure 8.9d
An Ion is an atom that has gained or lost a
1. Neutron
2. Proton
3. Electron
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How can an ion pass through a membrane
1.
2.
3.
4.
5.
Simple diffusion
Facilitated diffusion
Active transport
Both 2 and 3
All of the above
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The Nerve Impulse Is an Electrochemical
Signal
 A nerve impulse, or action potential, involves
sodium ions (Na+) and potassium ions (K+)
that cross the cell membrane through the ion
channels

 Each ion channel is designed to allow only
certain ions to pass through
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Action Potential
Cross section
Axon membrane
Neuron plasma
membrane
Extracellular
fluid
Cytoplasm
Continually open ion channels
“Gated” ion channels
Ion channels
Ion channels can be open continuously or opened and
closed by a molecular gate
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Sodium-potassium pump
Sodium-potassium pump
The sodium-potassium pump
uses cellular energy (ATP) to
pump sodium ions out of the
cell and potassium ions into
the cell
Figure 7.4
Membrane Potential
 The difference in charge between the inside
and outside of the neuron is the membrane
potential
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Resting Membrane Potential
 A neuron that is not conducting a message is
said to be “Resting”
 When a neuron is resting there is more
sodium (Na+) outside the neuron cell and
more potassium (K+) inside the cell
 The inside of the cell has a negative charge
compared to the outside the cell
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Resting Membrane Potential
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The Nerve Impulse
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Figure 7.5 (1 of 4)
Sodium Potassium Pump
 To maintain this resting membrane potential
the neuron pumps Na+ out of the cell and K+
into the cell.
 The transport proteins take 3 Na+ ions out for
every 2 K+ ions into the cell = Na+/K+ pump
 This is Active Transport – requiring ATP
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Action Potential
 An electrochemical signal conducted along
an axon. It is a wave of depolarization
followed by repolarization
 Depolarization is caused by sodium ions
entering the axon
 Repolarization is caused by potassium
ions leaving axon
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Steps of an Action Potential
1. The axon is depolarized when voltage gated
sodium ion channels open and Na+ comes
rushing in, causing the inside of the neuron to
become positively charged
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Action Potential
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Figure 7.5 (2 of 4)
Steps of an Action Potential
2. The axon is repolarized when voltage gated
potassium ion channels open up and allow K+
to go out of the axon

This returns the membrane potential to be
negative on the inside of the neuron

The action potential travels down the axon
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Action Potential
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Figure 7.5 (3 of 4)
Action Potential
 After the action potential, the sodium
potassium pump restores the original
conditions by pumping sodium (Na+) out of
the cell and potassium (K+) back into the cell
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The Nerve Impulse
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Figure 7.5 (4 of 4)
The Nerve Impulse
PLAY
Animation—The Nerve Impulse
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Figure 7.6
Action Potentials
 It is an all or nothing response – if it is not
a great enough stimulation the channels
won’t open. The level of the action
potential is always the same.
 The direction is always one way down the
axon. The sodium channels are
inactivated for awhile after the action
potential passes = refractory period.
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When a neuron is resting, sodium ions have a greater
concentration:
1. inside the neuron cell
2. outside the neuron cell
3. concentration is the
same
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When a neuron is depolarizing, which ions come into the
neuron?
1.
2.
3.
4.
Calcium (Ca++)
Sodium (Na+)
Potassium (K+)
Chlorine (Cl-)
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When a neuron is depolarizing, the inside of the neuron cell
becomes
1. Positively charged
2. Negatively charged
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Nerve Synapse
 How are messages passed from one nerve to
the next or from the nerve to a muscle?
 The junction between two neurons or
between a neuron and a muscle is called a
synapse
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Components of the Synapse
1. Presynaptic neuron is the transmitting
neuron
2. postsynaptic neuron is the receiving neuron
or the muscle
3. And the gap in between them = synaptic
cleft
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Presynaptic neuron
 Presynaptic neuron has synaptic
vesicles that contain neurotransmitters
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Synaptic Transmission
Dendrites
Axon
Nucleus
Cell
body
Impulse
Impulse
Step 1: The impulse reaches
the axon ending of the
presynaptic membrane.
Synaptic
knob
Step 2: Synaptic
vesicles release
neurotransmitter
into the synaptic
cleft.
Synaptic
cleft
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Synaptic
vesicle
Membrane of
postsynaptic neuron
Figure 7.8 (1 of 3)
Synaptic Transmission
Neurotransmitter
Synaptic
vesicle
Step 3: Neurotransmitter
diffuses across synaptic cleft.
Receptor (of sodium ion
channel) on postsynaptic membrane
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Figure 7.8 (2 of 3)
Synaptic Transmission
Step 4: Neurotransmitter
molecules bind to
receptors on the
postsynaptic neuron.
Step 5: Sodium
ion channels
open.
Step 6: Sodium
ions enter the
postsynaptic
neuron, causing
depolarization and
possible action
potential.
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Figure 7.8 (3 of 3)
Transmission across synaptic cleft
1. The action potential gets to the end of the
presynaptic axon
2. The action potential triggers Ca2+ to enter
the presynaptic axon terminal
3. The Ca2+ triggers synaptic vesicles located
at the axon terminal to merge with the
neural membrane
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Transmission across synaptic cleft
4. The synaptic vesicles release the
neurotransmitters into the synaptic cleft
5. These neurotransmitters travel across the
synaptic cleft to the postsynaptic neuron
(or the muscle)
6. Neurotransmitter binds to receptors on the
postsynaptic neuron (or muscle)
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Transmission across synaptic cleft
7. These receptors are ligand gated sodium ion
channels which allow Na+ to enter the
postsynaptic neuron (or muscle) and triggers
an action potential in the postsynaptic neuron
(or muscle contraction)
8. Once the neurotransmitters are released they
need to be destroyed or contained quickly or
they will continue to stimulate the nerve
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Synapse
PLAY
Animation—The Synapse
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Neurotransmtters
 Acetylcholine
 Acts in both the PNS and the CNS as a
neurotransmitter
 Causes voluntary muscles to contract
 Acetylcholinesterase
 Myasthenia gravis is an autoimmune
disease that attacks the acetylcholine
receptors, resulting in reduced muscle
strength
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Important Concepts
 Read Chapter 8
 What are the functions of nervous system
 What are the two types of cells in nervous tissue
(neuroglial cells and neurons).
 What are the three types of neuroglial cells and
their functions
 What are the two main divisions of nervous
system (CNS, PNS) and where each is found
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Important Concepts
 What are the parts and functions of a neuron
 What are the three types of neurons (sensory,
interneuron and motor neurons) and their
functions, and where are they located
 Where are the cell bodies are located for motor
and sensory nerve cells
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Important Concepts
 What are schwann cells and oligodendrocytes and
what are their function
 Where Schwann vs oligodendrocytes are found
 What is the cause and effects of multiple sclerosis
 What are the parts of a nerve
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Important Concepts
 How do ions pass through membranes
 What is the function of the sodium potassium
pump
 What are the steps of messages being conducted
through a neuron, starting with the resting stage
and ending with the next neuron or muscle being
stimulated.
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Important Concepts
 What ions enter and the leave the neuron during
the depolarization and repolarization steps of
action potential, what is the relative charge of the
inside vs the outside of the neuron during these
events, what is the order of events.
 Components of the synapse
 Function of neurotransmitters, how do they work,
where do they work, know the ions involved and
their functions.
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Important Concepts
 What is acetylcholine, where is it found, what effect
does it have, how is acetylcholine removed from
the synaptic cleft
 What is the cause and effect of Myasthenia gravis
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Definitions
 Afferent neurons, efferent neurons, dendrites,
axons, sensory neurons, interneuron, motor
neurons, myelin, myelin sheath, myelinated
neurons, schwann cells, oligodendrocytes,
nodes of ranvier, nerve, ions, ion channels,
ligand gated ion channels, voltage gated ion
channels, action potential, repolarization,
depolarization, membrane potential, resting
potential, sodium potassium pump, refractory
period, synapse, synaptic cleft, synaptic
vesicles, neurotransmitters,
acetylcholinesterase, presynaptic neuron,
postsynaptic neuron, stimulate, inhibit
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