Transcript Chapter 7 - Faculty Web Sites

Chapter 7
Neurons: The
Matter of the
Mind
Lecture Presentation
Betty McGuire
Cornell University
Copyright © 2012 Pearson Education, Inc.
Neurons: The Matter of the Mind




Cells of the nervous system
Structure of neurons
Nerve impulses
Synaptic transmission
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 Overview of the nervous system
 Function
 Integrates and coordinates the body’s
activities
 Divisions
 Central nervous system (CNS)
 Brain and spinal cord
 Peripheral nervous system (PNS)
 All of the nervous tissue outside the
brain and spinal cord
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 The nervous system is composed of two
types of specialized cells
 Neurons
 Neuroglial cells
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 Neurons (nerve cells)
 Excitable cells that generate and transmit
messages
 Neuroglial cells (glial cells)
 Outnumber neurons 10 to 1
 Several types, each with a specific job
 Provide structural support, growth factors,
and insulating sheaths around axons
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 Three categories of neurons
 Sensory (or afferent) neurons
 Motor (or efferent) neurons
 Interneurons (or association neurons)
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 Sensory neurons
 Carry information toward the CNS from
sensory receptors
 Motor neurons
 Carry information away from the CNS to an
effector (muscle or gland)
Copyright © 2012 Pearson Education, Inc.
Cells of the Nervous System
 Interneurons
 Located between sensory and motor
neurons within the CNS
 Found only in the brain and spinal cord
 Integrate and interpret sensory signals
 Account for more than 99% of the body’s
neurons
Copyright © 2012 Pearson Education, Inc.
Sensory
receptor
for pain
Muscle
(effector)
Impulse direction
Cell
body
Sensory
neuron
Interneuron
Copyright © 2012 Pearson Education, Inc.
Motor
neuron
Structure of Neurons
 The shape of a typical neuron is
specialized for communicating with other
cells
 Parts of a neuron
 Dendrites = many short, branching
projections
 Axon = a single long extension
 Cell body
 Contains nucleus and other organelles
 Functions to maintain the neuron
Copyright © 2012 Pearson Education, Inc.
Structure of Neurons
 Dendrites
 Receive signals from other cells
 Carry information toward the cell body of a
neuron
 Axon
 Carries information away from the cell body
to either another neuron or an effector
Copyright © 2012 Pearson Education, Inc.
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.
An axon conducts the
nerve impulse away
from the cell body.
Receiving portion of neuron
Copyright © 2012 Pearson Education, Inc.
Axon
endings
Sending portion of neuron
Structure of Neurons
 Nerves
 Consist of parallel axons, dendrites, or both
from many neurons
 Covered with tough connective tissue
 Classified as sensory, motor, or mixed
(sensory and motor together) depending on
the type of neurons they contain
Copyright © 2012 Pearson Education, Inc.
Structure of Neurons
 Myelin sheath
 Found on most axons outside the CNS and
some of those within the CNS
 Provides electrical insulation that increases
the rate of conduction of a nerve impulse
 Composed of the plasma membranes of
glial cells
Copyright © 2012 Pearson Education, Inc.
Structure of Neurons
 Myelin sheath (cont.)
 In the PNS, Schwann cells (a type of glial
cell) form the myelin sheath
 Gaps between adjacent Schwann cells are
called nodes of Ranvier
 Messages travel faster as they jump from
one node of Ranvier to the next in a type of
transmission called saltatory conduction
Copyright © 2012 Pearson Education, Inc.
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)
Axon
Myelin sheath
(c) Myelin sheath surrounding cut end
of axon
Schwann
cell
(b)
Nucleus
Myelin
sheath
Copyright © 2012 Pearson Education, Inc.
Nucleus of
Schwann cell
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)
Myelin sheath
Copyright © 2012 Pearson Education, Inc.
Axon
(b)
Schwann
cell
Nucleus
Myelin
sheath
Copyright © 2012 Pearson Education, Inc.
Nucleus of
Schwann cell
(c) Myelin sheath
surrounding cut end of axon
Copyright © 2012 Pearson Education, Inc.
Structure of Neurons
 Multiple sclerosis (MS)
 Results from the destruction of the myelin
sheath that surrounds axons in the CNS
 The resulting scars (scleroses) interfere with
the transmission of nerve impulses
 Can result in paralysis and loss of sensation,
including loss of vision
Copyright © 2012 Pearson Education, Inc.
Structure of Neurons
Web Activity: Myelinated Neurons
and Saltatory Conduction
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 A nerve impulse, or action potential, is an
electrochemical signal involving sodium
ions (Na+) and potassium ions (K+) that
cross the cell membrane through ion
channels
 Ions pass through channels without using
cellular energy
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 Each ion channel is designed to allow only
certain ions to pass through it
 Sodium channels permit sodium ions to pass
 Potassium channels permit potassium ions
to pass
 Ion channels may be permanently open or
regulated by a “gate,” which is a protein
that changes shape and opens or closes a
channel
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 Ions also are transported across the
membrane by the sodium-potassium pump
 Special proteins in the cell membrane that
actively transport sodium and potassium ions
across the membrane
 These pumps use cellular energy to eject
sodium ions from within the cell and to bring
potassium ions into the cell
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 When a neuron is not conducting a nerve
impulse, it is in a resting state
 There is a slight difference in charge across the
membrane, which is called the resting potential
 The inner surface of the membrane is about
70 mV more negative than the outer surface
 There are more sodium ions outside the
membrane than inside
 There are more potassium ions inside the
membrane than outside
 This state is maintained by the sodiumpotassium pump
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 When the neuron is stimulated, there is a
sudden reversal of charge across the
membrane because the sodium gates
open and sodium ions enter the cell
 Threshold
 Minimum charge that causes the sodium
gates to open
 Depolarization
 Reduction of the charge difference across
the membrane
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 Next, the potassium gates open and
potassium ions rush out of the cell
 This causes the cell to return to its original
state (i.e., for the interior of the neuron to
become more negative relative to the
outside)
 Repolarization
 Restoration of the charge difference
across the membrane
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 Action potential
 Sudden reversal of the charge across the
membrane followed immediately by its
restoration
 These changes occur in a wave along
the axon
Copyright © 2012 Pearson Education, Inc.
Resting Neuron
Plasma membrane is
charged, with the inside
negative relative to the
outside.
Action Potential
The charge difference
across the membrane
reverses and then is
restored.
Neuron cytoplasm
Cytoplasm
Plasma membrane
Step 1: The loss of the charge
difference across the
membrane (depolarization)
occurs as sodium ions (Na+)
enter the axon.
Na+
Cytoplasm
Na+ flows inward
Step 2: The return of the membrane potential to
near its resting value (repolarization) occurs as
potassium (K+) ions leave the axon.
K+
Na+
Cytoplasm
K+
K+ flows outward
Restoration of Original
Ion Distribution
The sodium-potassium
pump restores the original
distribution of ions.
K+
Na+
K+
Cytoplasm
Na-K Pearson
pump restores
the original
Copyright © 2012
Education,
Inc. ion distribution
Action
Potential
Resting Neuron
Restoration of Original
Ion Distribution
Membrane potential
+30
0
Step 1:
Sodium ions
enter neuron.
Step 2: Potassium
ions leave neuron.
Sodium-potassium
pump is active.
Threshold
Resting potential
–70
0
1
2
3
Time (milliseconds)
Copyright © 2012 Pearson Education, Inc.
4
5
Nerve Impulses
 An action potential
 Does not diminish, once started
 Does not vary in intensity with the strength of
the stimulus that triggered it
 Is “all-or-nothing”
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
 For a very brief period following an action
potential, the neuron cannot be stimulated
again
 This is called the refractory period
 It occurs because the sodium channels are
closed and cannot be reopened
Copyright © 2012 Pearson Education, Inc.
Nerve Impulses
Web Activity: Nerve Impulse
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 Communication between a neuron and an
adjacent cell occurs by neurotransmitters
 Synapse
 Junction between a neuron and another cell
 Synaptic cleft
 Gap between two cells
 Neurotransmitters diffuse across the gap
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 In the case of two neurons, the presynaptic
neuron sends a message to the postsynaptic
neuron
 Synaptic knob
 Swelling at the end of the axon of the
presynaptic neuron
Copyright © 2012 Pearson Education, Inc.
Plasma membrane of an axon
ending of a sending (presynaptic)
neuron
Synaptic
vesicle
Synaptic
knob
Plasma membrane
of a receiving
(postsynaptic)
neuron
Synaptic
cleft
Ion channel
Receptor for
neurotransmitter
(a)
Axon of presynaptic neuron
(b)
Synaptic vesicles
Copyright © 2012 Pearson Education, Inc.
Plasma membrane of an
axon ending of a sending
(presynaptic) neuron
Synaptic
vesicle
Synaptic
knob
Synaptic
cleft
(a)
Plasma membrane of a
receiving (postsynaptic)
neuron
Ion channel
Receptor for
neurotransmitter
Copyright © 2012 Pearson Education, Inc.
Axon of presynaptic neuron
(b)
Synaptic vesicles
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 Specific steps
 The nerve impulse reaches the synaptic knob of the
presynaptic neuron
 The synaptic knob releases neurotransmitter into the
synaptic cleft
 Prompted by calcium ions moving into the knob
 Membranes of synaptic vesicles (packets of
neurotransmitter) fuse with plasma membrane at
the synaptic knob, spilling contents into the cleft
 The neurotransmitter diffuses across the synaptic
cleft and binds with receptors on the membrane of
the postsynaptic neuron, causing an ion channel
to open
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 At an excitatory synapse, binding of the
neurotransmitter to the receptor causes
sodium channels to open
 Sodium ions enter the postsynaptic cell,
increasing the likelihood that an action
potential will begin
Copyright © 2012 Pearson Education, Inc.
Dendrites
Nucleus
Axon
Impulse
Cell
body
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
Synaptic
vesicle
Synaptic
vesicle
Membrane of
postsynaptic neuron
Neurotransmitter
Step 3: Neurotransmitter
diffuses across synaptic cleft.
Receptor (of sodium ion
channel) on postsynaptic membrane
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.
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 At an inhibitory synapse, binding of the
neurotransmitter to the receptor opens
different ion channels
 The postsynaptic cell’s interior becomes
more negatively charged than usual,
reducing the likelihood that an action
potential will begin
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 A neuron may have as many as 10,000 synapses
with other neurons at the same time
 Some synapses have excitatory effects and
some have inhibitory effects
 Summation
 Combined effects of excitatory and inhibitory effects
at any given moment
 Determines whether an action potential is generated
 This level of integration provides fine control over
neuronal responses
Copyright © 2012 Pearson Education, Inc.
Axon
Myelin Receiving Excitatory
sheath cell body synapse
Inhibitory
synapse
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 Neurotransmitters have temporary effects
 Once released into a synapse,
neurotransmitters are quickly removed
 Some are deactivated by enzymes
 The enzyme acetylcholinesterase
removes acetylcholine from synapses
 Others are pumped back into the synaptic
knob of the presynaptic axon
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
Web Activity: The Synapse
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 There are dozens of neurotransmitters
 Some neurotransmitters produce different
effects on different types of cells
 Example: Acetylcholine
 Acts in both the PNS and the CNS
 Released at every neuromuscular junction
 Myasthenia gravis
 Autoimmune disease that attacks the
acetylcholine receptors at
neuromuscular junctions, resulting in
little muscle strength
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 In the CNS, different neurotransmitters are
associated with different behavioral
systems
 Norepinephrine regulates mood, hunger,
thirst, and sex drive
 Serotonin promotes a feeling of well-being
 Dopamine regulates emotions and complex
movements
Copyright © 2012 Pearson Education, Inc.
Synaptic Transmission
 Changes in the levels of neurotransmitters
cause disorders
 Alzheimer’s disease
 Associated with decreased levels of
acetylcholine
 Clinical depression
 Associated with decreased levels of
serotonin, dopamine, and norepinephrine
 Parkinson’s disease
 Associated with decreased levels of
dopamine
Copyright © 2012 Pearson Education, Inc.