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Chapter 43
Nervous System





irritability – ability to respond to environmental
stimuli
neural mechanisms – relatively rapid communication
hormonal mechanisms – relatively less rapid or
long-term adjustments in animals

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2 The Peripheral Nervous System Collects
information and carries out responses


Inputs
o excitatory – causing a signal to be generated
and propagated
o inhibitory – making signal generation and
propagation less likely

Peripheral nervous system
o Sensory neurons
o Motor neurons

Somatic nervous system – motor neurons
that stimulate skeletal muscles to contract

Autonomic nervous system – regulates
the activity of the smooth muscles, cardiac
muscles, and glands
 Sympathetic
 Parasympathetic

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dendron, tree) – processes that are
often profusely branched

Nerve cell’s receptive apparatus

designed to receive information from several
different sources at once

dendritic spines – extensions from
dendrites that increase the surface area
available to receive stimuli
o axon (Gr
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

Resting Membrane Potential: balance between
two forces
o Arises due to the action of the Na-K pump and
+
the greater permeability of membrane to K
o (many vertebrate neurons) – (-40 to 90 mV)
o Average resting membrane potential – (-70mV)
o Equilibrium potential – produced by the
balance between diffusional force and electrical
force

Nernst equation – action of a single ion
 EK = 58 mV log([K+]out/[K+]min)

TABLE 43
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2 Mechanism of Nerve Impulse Transmission


Signaling depends on the properties of a variety of
specialized membrane transport proteins:
o Channel – open or close – passive
o Carriers – conformational change – active
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1 Membrane Potential



Membrane Potential – difference in electrical charge
between the inside and the outside of the cell
o Negative pole – cytoplasmic side
o Positive pole – extracellular side



The Ionic Composition of Cytoplasm and Extracellular Fluid (ECF)
Concentration
Ratio
Equilibrium
(mM)
in ECF (mM)
(ECF:cytoplasm)
Potential (mV)
150
15
10:1
+60
5
150
1:30
-90
110
7
15:1
-70

Electrode and voltmeter – measures resting
membrane potential of a neuron

Contributors to Membrane Potential:
o Sodium-potassium pumps

o



Ion channels – membrane proteins that allows
diffusion of specific ions

more numerous for K+ than Na2+
+

membrane – more permeable to K
+

K diffuses out of cell

Leakage channels – open all time

Gated channels – open or close in
response to a stimulus

Two kinds of changes in membrane potential:
o Graded potential – small continuous changes
o Action potential – transient disruptions

Actual signs that propagate along an axon

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3 Action Potentials Result when
Depolarization reaches a Threshold


Threshold Potential – level of depolarization needed
to produce an action potential
o -55 mV – in mammalian axon



Voltage-gated ion channels – open and close in
response to changes in membrane potential
o found in neurons and muscle cells



sodium and potassium voltage-gated channels
+
o voltage-gated Na channels

two gates:
 activation gate
 inactivation gate

resting state:
 activation gate – closed
 inactivation gate – open

threshold voltage is reached:
+
 activation gate – open : influx of Na
 inactivation gate – closes after a short
period
o voltage-gated K+ channels

single activation gate
 resting state: closed
 threshold voltage: opens slowly – efflux
of K+



tracing an action potential changes
o three phases of action potential

rising phase

falling phase

undershoot phase

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2 Changes in Membrane Potential are Due to
Changes in Membrane Permeability


Gated channels
o Two types of gated channels:

Ligand-gated (chemically gated) – respond
to chemical signal
 Lead to graded potentials that determine
whether the axon will fire

Voltage-gated – respond to changes in
membrane potential
 Produce action potentials



Graded Potentials: Depolarizing or
Hyperpolarizing
o Depolarizing – makes the membrane less
negative (more positive)

- 70mV to - 65mV
o Hyperpolarizing – makes the membrane
potential more negative

– 70mV to -75mV

A chemically gated ion channel
...
Binding the ACh causes
+
the channel to open allowing Na ions to flow into the cell by diffusion
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3 Synapses: Where Neurons Communicate with other
cells




Synapses – at the end of an axon
Presynaptic cell – neuron that transmits action
potentials to the synapse
Postsynaptic cell – cell that receives the signal on
the other side of the synapse
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1 two types of synapses are electrical and
chemical



Propagation of an action potential in an unmyelinated axon
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Morphine and heroin) – have
analgesic effect by binding in the receptors
for enkephalins and endorphins
o nitric oxide (NO) – first gas known to act as a
regulatory molecule

produced from argentine

appears to participate in the process of
learning and memory

(PNS) some neurons that innervate GIT,
penis, respiratory passages and cerebral
blood vessels
 Releases NO
 Causes smooth-muscle relaxation –
produce the engorgement of spongy
tissue of the penis with blood
 Sildenafil (Viagra) – inceases the
release of NO in the penis
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3 A postsynaptic Neuron must Integrate input
from many synapses



Biogenic amines
o Catecholamines – derived from tyrosine

Epinephrine (Adrenaline) – hormonal
secretion

Noripinephrine - released at synapses of
neurons in SNS

Both for fight or flight response
o Dopamine – controlling body movements and
other functions

Resting muscle tremors of Parkinson’s
disease – produced by degeneration of
particular dopamine-releasing neurons
 L-dopa (L-3,4dihydroxyphenylalanine) – a precursor
from which a dopamine can be
produced



Synaptic integration - EPSP (depolarization) and
IPSP (hyperpolarization) interact with each other
when they reach the cell body of the neuron
o Small EPSPs add together to bring the
membrane potential closer to the threshold
o IPSPs subtract from the depolarizing effect of
EPSPs to deter the membrane potential from
reaching the threshold



2 ways the membrane can reach threshold voltage:
o Spatial summation – by many different dendrite
producing EPSPs that sum to the threshold
voltage
o Temporal summation – by one dendrite
producing repeated EPSPs that sum to the
threshold voltage in less than 15ms

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2 Three basic divisions of vertebrate Brains

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4 Neurotransmitters play a role in drug
addiction


Habituation –
losing the
ability to
respond to a
stimulus due to
frequent
exposure



Cocaine –
affects
neurons in the
brain’s
pleasure
pathway
(limbic system)



Nicotine – binds directly to a specific receptor on
postsynaptic neurons of the brain
o Nicotine receptors – class of receptors that
normally bind Ach
o brain adjusts to prolonged exposure to nicotine
by “turning down the volume” in two ways:

By making fewer receptor proteins to which
nicotine can bind

By altering the pattern of activation of the
nicotine receptors

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4
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4
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The
other toes fan out
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5 the Peripheral Nervous system: Sensory and Motor
Neurons


PNS – receive information from the environment,
convey it to the CNS and carry responses to effectors
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1 PNS has Somatic and Autonomic Systems

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5 Spinal cord conveys messages and
controls some responses directly


Spinal cord (information highway) – cable of neurons
extending from the brain through the backbone
o Meninges – covers the spinal cord and the brain
o Two zones of the spinal cord:

Inner zone (gray matter) – cell bodies of
interneurons, motor neurons and neuroglia

Outer zone (white matter) – contains cables
of sensory axons in the dorsal columns and
motor axons in the ventral columns



Reflexes – sudden, involuntary movement of muscles
o Produces rapid motor response to a stimulus
o Monosynaptic reflex arc – direct



Spinal nerves
o Dorsal root – sensory neurons that enter the
dorsal surface of the spinal cord
o Ventral root – motor axons that leave from the
ventral surface of the spinal cord
o Dorsal root ganglia – group of sensory neurons
outside each level of spinal cord

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2 Somatic nervous system controls
movements

Walls



Somatic motor neurons – stimulate the skeletal
muscles of the body to contract in response to
conscious commands and as part of reflexes that do
not require conscious control

Gall Bladder
Muscle



Medulla oblongata – coordinate the sympathetic and
the parasympathetic division

Increased motility

Relaxation

Contraction

Relaxation

Sphincter

Lungs

Contraction

Contraction
Increased rate and
strength
Dilation of Bronchioles

Relaxation

Constriction of Bronchioles

Dilation

None

Decrease rate

Blood vessels
In muscles



Parasympathetic
stimulation

Urinary Bladder

Heart muscle

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3 Autonomic nervous System controls
involuntary Functions Through Two Divisions

Sympathetic Stimulation
Decreased tone

Target Tissue

In skin

Constriction

None

In viscera

Constriction

dilation

Two neurons of the Efferent motor pathway:
o Preganglionic neuron – has its cell body in the
CNS and sends an axon to an autonomic
ganglion

Release ACh at their synapse
o Postganglionic neuron – has its cell body in the
autonomic ganglion and sends its axon to
synapse with smooth muscle, cardiac muscle or
gland cell

Parasympathetic: release Ach

Sympathetic: release norepinephrine

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4 G proteins mediate cell responses to
autonomic signals


TABLE 43

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