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Chapter 10
Nervous System I
Divisions of the Nervous System
• The organs of the
nervous system are
divided into two major
groups:
– Central Nervous System
(CNS) = brain & spinal
cord
– Peripheral Nervous
System (PNS) = nerves
that extend from the brain
(cranial nerves) and
spinal cord (spinal nerves)
Divisions of PNS
• Sensory Division
– picks up sensory information and delivers it to the
CNS
• Motor Division
– carries information to muscles and glands
• Divisions of the Motor Division
– Somatic – carries information to skeletal muscle
– Autonomic – carries information to smooth
muscle, cardiac muscle, and glands
Three Major Functions of
the Nervous System
Functions of the Nervous System
• Sensory Function
– sensory receptors (located at the ends of
peripheral neurons) gather information
– information is carried to the CNS
– A sensory impulse is carried on a sensory
neuron
Functions of the Nervous System
• Motor Function
– decisions are acted upon
– impulses are carried to effectors
– Motor impulses are carried from CNS to
responsive body parts called effectors
– A motor impulse is carried on a motor neuron
– Effectors = 2 types:
• muscles (that contract)
• glands (that secrete a hormone)
Functions of the Nervous System
• Integrative Function
– Can involve CNS or PNS
– sensory information used to create
•
•
•
•
sensations
memory
thoughts
decisions
Neuron Structure
• Each neuron is composed of a cell body and
many extensions from the cell body called
neuron processes or nerve fibers
• Cell Body = central portion of neuron
– contains usual organelles, except centrioles
• identify: nucleus, prominent nucleolus, and
many Nissl bodies = RER
• Neuron Processes/ Nerve Fibers =
extensions from cell body
Neuron Structure cont.
• Dendrites:
– many per neuron
– short and branched
– receptive portion of a neuron
– carry impulses toward cell body
• Axons:
– one per neuron
– long, thin process
– carry impulses away from cell body
Figure 10.01
Neuron Structure cont.
• Note terminations of axon branch = axonal
terminals (synaptic knobs)
• Axons in PNS:
• Large axons are surrounded by a myelin sheath
produced by many layers of Schwann Cells
(neuroglial cell)
– "myelinated nerve fiber"
– myelin = lipoprotein
– Interruptions in the myelin sheath between Schwann cells
= Nodes of Ranvier
• Small axons do not have a myelin sheath
– "unmyelinated nerve fibers"
– however all axons (in PNS) are associated with Schwann
cells
Figure 10.04c
Neuron Structure cont.
• Neuron = the structural & functional unit of the
nervous system
– a nerve cell
• Neuron Structure
–
–
–
–
Nerve Fibers
Axons (continued)
Axons in CNS (i.e. in brain & spinal cord)
Myelin is produced by an oligodendrocyte rather than
Schwann Cells
• A bundle of myelinated nerve fibers = "White
Matter"
• This is in contrast to CNS "Gray Matter" = A bundle
of cell bodies (or unmyelinated nerve fibers)
Structural Classification
• Bipolar Neurons
– two extensions
– one fused dendrite leads toward cell body, one axon leads
away from cell body
• Unipolar Neurons
– one process from cell body
– forms central and peripheral processes
– only distal ends are dendrites
• Multipolar Neurons
– many extensions
– many dendrites lead toward cell body, one axon leads
away from cell body
Functional Classification
• Sensory neurons
– afferent neurons
– carry sensory impulses from sensory receptors to
CNS
– input information to CNS
– Location of receptors = skin & sense organs
• Interneurons (Association)
– CNS
– link other neurons together (i.e. sensory neuron
to interneuron to motor neuron)
Functional Classification cont.
• Motor Neurons
– efferent neurons
– carry motor impulses away from CNS and to
effectors
– output information from CNS
– Effectors = muscles & glands
Neuroglial Cells
• Neuroglial Cells = accessory cells of nervous
system form supporting network for neurons
PNS = 2 Types
• Schwann cells
– produces myelin (in the PNS)
• Satellite Cells
– support clusters of neuron cell bodies (ganglia)
Neuroglial Cells cont.
CNS =4 types:
– provide bulk of brain and
spinal cord tissue
• Astrocyte
– scar tissue
– star-shaped
Function:
– mop up excess ions, etc
– induce synapse formation
– connect neurons to blood
vessels
• Oligodendrocyte
– looks like eyeball
– Function: produces
myelin
• Microglia
– looks like spider
– Function: phagocytosis
• Ependyma
– epithelial-like layer
– Function: lines spaces
in CNS
– brain = ventricles
Nerve Repair
• Regeneration of Nerve Axons
– Cell body injury = death of neuron
– Damage to an axon may allow for regeneration
The Synapse
•Nerve
impulses pass
from neuron to
neuron at
synapses
Synaptic Transmission
•Neurotransmitter
s are released
when impulse
reaches synaptic
knob
Distribution of Ions
Potential Difference
• A resting neuron's cell membrane is said to be
polarized = electrically charged:
– Consequently, a potential difference (PD) exists
across this resting cell membrane
• Potential Difference (PD) = the difference in
electrical charge between 2 points (i.e. across
a cell membrane)
Resting Membrane Potential
• The resting membrane potential (RMP) of a
neuron results from the distribution of ions
across the cell membrane
– K+= high inside
– Na+= high outside
– Cl-= high outside
– Negatively charged proteins or Anions-; high
inside
• Recall that these ion concentrations are
maintained by active transport mechanisms
– mainly the Na+/K+ pump
Resting Membrane Potential cont.
• Resting Potential
– The RMP of a nerve cell is measured to be -70
mV or millivolts (inside / outside)
– As long as the RMP in a nerve cell is undisturbed,
it remains polarized.
– In order for a nerve impulse to be started or
propagated in a nerve cell, this resting potential
must be disturbed
Local Potential Changes
• caused
by various stimuli
• temperature changes
• light
• pressure
• environmental changes affect the membrane
potential by opening a gated ion channel
Development of resting membrane potential
Slide number: 1
Nerve axon
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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diffusion
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Development of resting membrane potential
Slide number: 2
Nerve axon
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A
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
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Development of resting membrane potential
Slide number: 3
Nerve axon
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Intracellular fluid
Extracellular fluid
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A
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K+
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Development of resting membrane potential
Slide number: 4
Nerve axon
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Intracellular fluid
Extracellular fluid
Na+ diffusion
K+
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A
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
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Development of resting membrane potential
Slide number: 5
Nerve axon
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A
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
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Development of resting membrane potential
Slide number: 6
Nerve axon
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Extracellular fluid
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diffusion
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
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Development of resting membrane potential
Slide number: 7
Nerve axon
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pump Na+
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
K+
Na+
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Development of resting membrane potential
Slide number: 8
Na+
+
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High Na+ Low K+
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pump K+
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pump Na+ K diffusion
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Na+ diffusion
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Na+
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K+
K+ diffusion
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Na+
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B
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Na+
–
Local Potential Changes (Graded
Potentials)
• The RMP of - 70 mV can be disrupted or
changed in one of two directions:
• more negative = "hyperpolarization"
• less negative (i.e. towards zero) =
"depolarization“
• summation can lead to threshold stimulus
that starts an action potential
Figure 10.15
Action Potential
• When the resting membrane potential (RMP)
of a neuron is depolarized to -55mV,
threshold potential is reached
– The threshold potential for a neuron is -55mV
– Therefore, a threshold stimulus = +15 mV
• When threshold potential is reached, the rapid
opening of Na+ channels results in rapid
depolarization (and even reversal of the
membrane potential [MP] to +30mV)
– This event is called the action potential
– The action potential represents the start of the
nerve impulse on a neuron.
Action Potential cont.
• Then K+ channels open, (while Na+
channels close), and repolarization occurs =
recovery of the RMP to -70mV
• This all occurs very quickly = 1/1000 sec
• An action potential represents the start of a
nerve impulse in one small portion of the
neuron's membrane
Figure 10.15
Action potential
Na+
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Threshold
stimulus K+
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B Region of depolarization
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C Region of repolarization
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 1
Action potential
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Na+
Na+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Slide number: 2
Action potential
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A
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Na+
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Na+
Na+
Na+
Na+
B Region of depolarization
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Slide number: 3
Action potential
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Threshold
stimulus K+
K+
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A
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Na+
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Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
B Region of depolarization
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Slide number: 4
Action potential
Slide number: 5
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Threshold
stimulus K+
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B Region of depolarization
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C Region of repolarization
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Action Potential cont.
• Nerve Impulse (NI) = the propagation of
action potentials (AP) along a nerve fiber; (i.e.
the entire length of the neuron)
– The NI is an electrical impulse
• An NI is similar to a row of dominos falling
(i.e. once the first domino falls, the entire row
will fall)
• A nerve impulse begins on a dendrite (or cell
body of a neuron), runs toward the cell body,
through the cell body, and then down the axon
Characteristics of a Nerve Impulse
(NI)
• All or Nothing Response = if a nerve cell responds
at all, it responds completely.
– sub threshold stimulus (5mV) = no AP/no NI
– threshold stimulus (15mV) = yes AP/yes NI
• > threshold stimulus (20mV) = yes AP
– yes NI, but no greater intensity than above
• Refractory Period = the period following a NI when
a threshold stimulus cannot produce another NI
– The RMP has to be restored before it can be depolarized
again
– (i.e. dominos must be set up in order to be knocked down
again)
Impulse Conduction Review
Characteristics NI cont.
• Impulse Conduction = the manner in which
the NI runs down the neuron/nerve fiber
• unmyelinated nerve fibers: NI must travel
the length of the nerve fiber
– slow
• myelinated nerve fiber: "Saltatory
Conduction"
– NI jumps from node of Ranvier to node of Ranvier
– Very fast transmission
THE SYNAPSE
Nerve impulses are transferred from one
neuron to the next through synaptic
transmission.
The Synapse
• Synapse = the junction between two neurons
where a nerve impulse is transmitted
• occurs between the axon of one neuron and
dendrite or cell body of a second neuron
• Note that the two neurons do not touch
There is a gap between them = synaptic cleft
Synaptic Transmission
• NI reaches axonal terminal of pre-synaptic
neuron causing depolarization of synaptic
knob
– Ca++ channels open and calcium ions rush into
axonal terminal
– synaptic vesicles (filled with neurotransmitter/NT)
to release NT via exocytosis into the synaptic
cleft
– NT diffuses across synaptic cleft and depolarizes
the post-synaptic neuron's membrane.
– An action potential (AP) is triggered and a NI
begins in the post-synaptic neuron
Neurotransmitters
Synaptic Potentials
EPSP
• excitatory postsynaptic potential
• graded
• depolarizes membrane of postsynaptic neuron
• action potential of postsynaptic neuron
becomes more likely
IPSP
• inhibitory postsynaptic potential
• graded
• hyperpolarizes membrane of postsynaptic
neuron
• action potential of postsynaptic neuron
becomes less likely
Summation of EPSPs and IPSPs
EPSPs and IPSPs
are added together in
a process called
summation
• More EPSPs lead to
greater probability of
action potential
•
Neurotransmitters (NT)
• at least 30 different produced by CNS
• some neurons produce/release only one while
release many;
• Most typical NT is Acetylcholine (ACh)
– ACh is released by all motor neurons (i.e. those
that stimulate skeletal muscle)
– some CNS neurons
Other NTs
• monoamines (modified
amino acids)
• are widely distributed in
the brain where they
play a role in:
– emotional behavior and
– circadian rhythm
• are present in some
motor neurons of the
ANS.
• include:
–
–
–
–
•
•
•
•
•
epinephrine
norepinephrine
dopamine
serotonin
histamine
unmodified amino acids
glutamate
aspartate
GABA (gamma
aminobutyric acid)
• glycine
Fate of Neurotransmitter in Synaptic
Cleft
• Destruction of Neurotransmitter:
– Enzymes that are present in the synaptic cleft
destroy NT
– For example, acetylcholinesterase destroys ACh
• Reuptake of Neurotransmitter:
– NT is transported back into pre-synaptic knob
• Both of the above processes prevent
continual stimulation of the post-synaptic
membrane!
Neuropeptides
• synthesized by CNS neurons
• act as NTs or neuromodulators that either:
– alter a neuron's response to a NT
– block the release of a NT
• include enkephalins:
– synthesis is increased during painful stress
– bind to the same receptors in the brain as the narcotic
morphine
– relieve pain
• include endorphines:
– same as above, but with a more potent and longer lasting
effect
IMPULSE PROCESSING
Impulse Processing
• Neuronal Pools – neurons that synapse and
work together
– Working together results in facilitation – a
general excitation that makes stimulation easier to
achieve
• Convergence
– many neurons come together on fewer neurons
(summation occurs)
– typical of motor pathways
– many inputs from brain, but usually only one
motor response
Impulse Processing cont.
• Divergence
– fewer neurons spread out to signal many neurons
(signal amplifies)
– typical of sensory pathways
– reason that a stimulus (i.e. odor) can cause many
responses