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Chapter 7
Neurons: The Matter of the Mind
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Cells of the Nervous System
Sensory (or afferent) neurons carry
information toward the CNS from
sensory receptors
Motor (or efferent) neurons carry
information away from the CNS to an
effector
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Cells of the Nervous System
Association neurons (or interneurons)
are located between sensory and motor
neurons within the CNS where they
integrate and interpret sensory signals
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Types of Neurons in
the Nervous System
• Sensory
• Motor
• Interneuron
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Cells of the Nervous System
• Glial cells (also called neuroglial cells)
are more numerous than neurons and
provide structural support, growth
factors, and insulating sheaths around
the nerves
– They are able to reproduce, unlike neurons
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Neurons (in red)
and Glial Cells
(in green) in
muscle tissue
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Function of Glial Cells
• Protection/selective uptake - provides blood
brain barrier
• Guides neurons to proper destination
(synapse)
– Developing embryo
– Adult
• Nurture - Secrete nerve growth factors
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Function of Glial Cells (cont.)
• Clean-up - Mop-up chemicals &
neurotransmitters
• Insulation of neurons
– Schwann cells in PNS
– Oligodendrocyte in CNS
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Neurons Have Dendrites, a Cell
Body, and an Axon
Dendrites receive information from
other neurons or from the
environment
Dendrites carry information toward the
cell body of a neuron
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Neurons Have Dendrites, a Cell
Body, and an Axon
A single long axon carries information
away from the cell body
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Dendrites, Cell Body, and Axon
A nerve consists of parallel axons,
dendrites, or both from many neurons
Nerves are covered with tough
connective tissue
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Dendrites, Cell Body, and Axon
Most axons outside of the CNS are
electrically insulated by a myelin
sheath
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Dendrites, Cell Body, and Axon
Schwann cells (glial cells) form the
myelin sheath, insulating it and allowing
messages to travel faster as they jump
from one node of Ranvier to the next
in a type of transmission called
saltatory conduction
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How a Schwann cell wraps around an axon to create a myelin sheath
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Cross section of an axon surrounded by myelin
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Dendrites, Cell Body, and Axon
The myelin sheath also facilitates nerve
repair outside the CNS
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Myelin Sheath on Neuron
– Multiple sclerosis involves
progressive destruction of myelin
sheaths within the brain and spinal
cord.
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Depending on where the nerve damage occurs, MS can
affect vision, sensation, coordination, movement, and
bladder and bowel control.
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The part of the neuron that carries impulses
(info) away from the cell body is called ….
A. Dendrite
B. Axon
C. Synapse
D. Impulsive
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Dendrites ….
A. Carry information away from the cell
body
B. Carry information toward the cell body
C. Receive information from other
neurons
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The myelin sheath
A. Slows the conduction of nerve impulses
B. Is formed by Schwann cells wrapping
around axons
C. Make saltatory conduction possible
D. Has gaps in it called “Nodes of Ranvier”
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The Nerve Impulse Is an
Electrochemical Signal
A nerve impulse, or action potential, is
a bioelectrical signal involving sodium
ions (Na+) and potassium ions (K+)
that cross the cell membrane through
the ion channels
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The Nerve Impulse
• Na+ and K+ are transported across the
membrane by the sodium-potassium
pump
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Resting Membrane Potential
• When a neuron is not conducting a nerve impulse, it is in a resting state
• The sodium-potassium pump uses ATP to transport sodium ions out of and potassium ions into the neuron
• The inside of a resting neuron has a negative charge relative to the outside
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The Nerve Impulse
• When the neuron is stimulated the
sodium gates open and sodium ions
enter the cell
– The minimum charge that causes the
sodium gates to open is called the
threshold
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• Na+ channels open: sodium moves into the axon
• K+ channels open: potassium moves out of the
axon
• Re-establishment of the Na+ and K+ gradients by
the normal activity of the sodium-potassium pump
Action Potential (aka, nerve impulse)
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The first and second events of an
action potential are:
A. Ca++ gated channel opens then closes
B. Na+ pumped out and K+ pumped into the
axon by the Na+/K+ pump
C. Na+ moves in then K+ moves out of the
axon
D. Ca++ moves out of the SER and then is
pumped back in
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Axons….
A. Conduct impulses away from the cell
body
B. Conduct information toward the cell
body
C. Carry information both toward and
away from the cell body
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• Action potentials are “all-or-none”
responses
• Action potentials are “self-propagating”
– Electrical current reaches threshold
throughout axon during spread of the action
potential
All-or-None and Self-PropagatingAction Potentials
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Synaptic Transmission Is
Communication between Neurons
• Communication between neurons is by
neurotransmitters; chemicals that
cross the gap between two neurons
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Communication between
Neurons
• A synapse is the junction between the
presynaptic neuron, which sends a
message to the postsynaptic neuron
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Communication between
Neurons
• When an action potential (impulse)
reaches the axon ending of the
presynaptic neuron----
• This causes the membrane of the
synaptic vesicles to fuse with the
plasma membrane and to release the
neurotransmitter substances
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Synaptic Vesicles ….
A. Contain mitochondria
B. Contain DNA
C. Contain neurotransmitters
D. Release neurotransmitters into the
synaptic cleft
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Communication between
Neurons
• Neurotransmitters diffuse across the
synaptic cleft to the other side where
they bind with receptors on the
postsynaptic cell causing the ion
channels to open
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Presynaptic neurons signal postsynaptic
neurons across the synaptic cleft by:
A. Electrons
B. Ions
C. ICBMs
D. Neruotransmitters
E. Texting
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Communication between
Neurons
• After the neurotransmitter crosses the
membrane, it is quickly broken down or
pumped back into the synaptic knob of
the presynaptic axon
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Removal of Neurotransmitter from
Synapse
1. Enzymatic breakdown
2. Re-uptake of neurotransmitter
– Example: Dopamine transporter (re-uptake
protein) knockout mouse is hyperactive and
unresponsive to cocaine and amphetamines
3. Many drugs (legal and illegal), toxins, and
venoms work at the level of the synapse
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Communication between
Neurons
• If neurotransmission occurs at an
excitatory synapse and enough
receptor sites bind with
neurotransmitter substances to cause
depolarization to threshold value, an
action potential is generated in the
postsynaptic cell
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Communication between
Neurons
• However, in an inhibitory synapse,
the postsynaptic cell becomes more
negatively charged and there is no
action potential generated
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Resting Membrane Potential, Graded Potential And Action Potential
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• Post-Synaptic Response
–Release of neurotransmitter: graded potential
achieved
• Effects of neurotransmitter is either:
– Excitatory: depolarize postsynaptic cell
– Inhibitory: hyperpolarize postsynaptic cell
–Role of postsynaptic neuron: integrates the
relative amount of excitatory and inhibitory input
Transfer of Information from Neuron to Post-Synaptic Membrane
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Points of Synaptic Regulation
Compensation (up or down regulation)
occurs at several levels:
• Release of Neurotransmitter
• Reuptake of Neurotransmitter
• Enzymatic Breakdown of Neurotransmitter
• Post-synaptic Receptor for Nt
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What would you expect to happen in the synapses of a person who chronically takes cocaine, a drug that causes
excess amounts of dopamine (DA) to collect in the synapse by inhibiting DA reuptake by the pre-synaptic neuron.
A. There would be an increase in DA receptors on
the post-synaptic membrane.
B. There would be a decrease in DA receptors on
the post-synaptic membrane
C. There would be an increase in the enzymes that
breakdown DA
D. There would be a decrease in the enzymes that
breakdown DA
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Stimulus Quality
• How do we distinguish light from
sound?
– Different stimuli (e.g, light and sound) are
recognized as qualitatively different
because they are sent to different areas of
the brain.
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Stimulus Intensity
• How do we perceive different signal
intensities?
1. Increased frequency of impulse signals
increased intensity
2. The number of neurons that fire increases
when strength of stimulus increases
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Neurotransmitters
• Different neurotransmitters play different
roles
– Alzheimer’s disease - Acetylcholine
– Depression – Serotonin, Dopamine, Norepineph
– Parkinson’s disease - Dopamine
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Alzheimer’s disease - Acetylcholine
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Parkinson’s disease - Dopamine
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Inhibitors that work at the post-
synapse
• Curare (plant bark)
– Compet. Inhibitor of Ach for its receptor on
muscle cell
• Alpha-bungerotoxin (black mamba
venom)
– Similar mechanism to curare
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Inhibitors that work at the pre-
synapse
• Tetnus toxin
– Blocks inhibitory input to muscle; result is
excess muscle contraction
• BOTOX (Botulism toxin)
– Blocks Ach neurotransmitter release
(exocytosis) at neuromuscular synapse
• Antidepressants (SSRIs)
– Selective Serotonin Reuptake Inhibitors
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Neurotransmitters
• Dopamine (DA)
– Generally excitatory; emotional responses,
euphoria
– Re-uptake blocked by cocaine and
amphetamines (see KO mouse)
– Parkinson’s disease: degeneration of
substantia nigra area of brain leads to DA
– Schizophrenia: Thorazine and clozapine
are drugs that inhibit DA release from pre-
synaptic terminal
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Neurotransmitters (cont)
• Serotonin
– Involved in sleep and control of mood (eg,
depression)
– Antidepressants/SSRIs (eg, prozac)
inhibit re-uptake from synaptic cleft.
– LSD mimics serotonin
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Neurotransmitters (cont)
• Acetylcholine
– Neurotransmitter in neuromuscular
synapses
– Blocked by curare; a-bungarotoxin (snake
venom), BOTOX, and nerve gas
• Norepinephrine (NE)
– Neurotransmitter used in pathways
involved in arousal, regulation of moods
such as pleasure and feeling good
– Cocaine augments the effects of NE
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Neurotransmitters (cont)
• Substance P
– Signals perception of pain in sensory
nerves
• Endorphin
– Binds to opiate receptor (same one that
morphine binds to) & inhibits pain by
inhibiting substance P release
– Thought to be responsible for “runners
high”?
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Neurotransmitters (cont)
• Gamma amino butyric acid (GABA)
– Tends to inhibit or slow down other
pathways
– Target for anti-anxiety drugs such as
Valium, which enhance the action of
GABA
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Action potentials are all-or-none. Which of the
following statements summarizes this principle?
A. An action potential can vary in both strength and duration
B. If a threshold stimulus is reached the action potential will
always be of the same strength
C. Several stimuli will cause a stronger response
D. The effects of several stimuli are combined until the
neuron fires
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Which of the following carry impulses toward the
cell body?
1. axon
2. dendrite
3. glial cells
4. Schwann cells