The Nervous System
Nancy G. Morris
Volunteer State Community College
Campbell Chapter 48
Nervous System
Endocrine System
Complexity
More structurally complex; can
integrate vast amounts of
information & stimulate a wide
range of responses
Less structurally complex; evolved
from the nervous system
Structure
System of neurons that branch
throughout the body
Endocrine glands secrete
hormones into bloodstream where
they are carried to the target
organ
Communication
Neurons conduct electrical
signals directly to and from
specific targets; allows fine pinpoint control; transmission is
hormonal: acetylcholine,
epinepherin, norepinephrin, etc.
Hormones circulate as chemical
messengers via the bloodstream;
most cells are exposed but only
target cells with receptors
respond
Response Time
Fast transmission of nerve
impulses up to 100 m/sec
May take minutes, hours or days
for hormones to be produced,
diffuse to target organ, & for
response to occur
Effect
Acts at the cellular level;
Immediate and short-lived
Acts a the cellular level; Occur
over time and are long-lived
Organization of nervous systems
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There is great diversity among animals.
All phyla have a nervous “system” except
sponges.
In the Hydra’s nerve net, impulses are
conducted in both directions causing
movement of entire body.
Some cnidarians & echinoderms have
modified nerve nets with rudimentary
centralization.
Organization of nervous systems

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Cephalization = evolutionary trend for concentration of
sensory & feeding organs on the anterior end of a moving
animal (Bilaterial symmetry).
Most bilateral animals have a PNS & a CNS (brain & one
or more nerve cords).
Flatworms have a simple “brain” containing large
interneurons.
Annelids & arthropods have a well-defined ventral nerve
cord & prominent brain. Often coordinate ganglia in each
segment to coordinate action.
Cephalopods have the most sophisticated invertebrate
nervous system containing a large brain & giant axons.
Fig. 48.13 Diversity in Nervous Systems
Three overlapping functions of
the Nervous System:

Sensory input is the conduction of signals from sensory

Integration is a process by which information from

Motor output is the conduction of signals from the
receptors to integration centers of the nervous system.
sensory receptors is interpreted & associated with
appropriate responses of the body.
processing center to effector cells (muscle & gland cells)
that actually carry out the body’s response to stimuli.
Fig. 48.1
Overview: Vertebrate Nervous System
Three majors classes of neurons

Sensory neurons – convey information
about the external & internal
environments from sensory receptors to
CNS; most synapse with interneurons.
 Interneurons – integrate sensory input
and motor input; located within the CNS;
synapse only with other neurons
 Motor neurons – convey impulses form
the CNS to effector cells
Fig. 48.4
Structural Diversity
of
Neurons
A single neuron
on the surface
of a
microprocessor.
A cm3 of the
human brain
will contain
more than 50
million
neurons.
Signals are conducted by nerves with many axons
coming from many different neurons surrounded
by connective tissue, the perineurium.


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Found in both parts of the nervous system:
1) Central Nervous System (CNS) = comprised of
brain & spinal cord; responsible for integration of sensory
input & associating stimuli with appropriate motor output
2) Peripheral Nervous System (PNS) = consists of a
network of nerves extending into different parts of the
body that carry sensory input to the CNS & motor output
away form the CNS
Composition of Nervous System

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The Nervous System contains two types of cells:
1) Neurons – cells specialized for transmitting
chemical & electrical signals form one location to
another
2) Glia or supporting cells – structurally
reinforce, protect, insulate, & generally assist
neurons
Neurons


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Possess a large cell body located either in the
CNS or a ganglion
Possess two fingerlike extensions (processes)
that conduct messages:
1) Dendrites – convey signals to the neurons cell
body Numerous, short, extensively branched to increase
surface area

2) Axons – conduct impulses away from the cell
body. Single, long process
Fig. 48.2
Structure
of
Vertebrate Neuron
AXONS

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Vertebrate axons in the PNS are wrapped in
concentric layers of Schwann cells, which
form an insulating myelin sheath.
In the CNS, the myelin sheath is formed by
ogliodendrites.
Extend from the neuron cell body to many
branches (arborization of the axon) which are
tipped with synaptic terminals that release
neurotransmitters.
Transmission of the impulse

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Must cross the synapse, the gap between a
synaptic terminal and a target cell (either
another neuron or an effector cell).
Neurotransmitters are chemicals that cross
the synapse to relay the impulse
Table 48.1: acetylcholine, norepinephrine,
dopamine, serotonin, neuropeptides:
endorphines
Neurons are arranged in circuits


Simple circuit: synapse between sensory
neurons & motor neurons, resulting in a simple
reflex.
Complex circuit: such as those associated
with most behaviors, involve integration by
interneurons in the CNS
 Convergent circuits
 Divergent circuits
 Reverberating circuits (memory storage)
Figure 48.3
The kneejerk reflex
Coordination by cluster

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Nerve cell bodies are often arranged into
clusters; these clusters allow coordination of
activities by only a art of the nervous system
A nucleus is a cluster of nerve cell bodies within
the brain
A ganglion is a cluster of nerve cell bodies in the
peripheral nervous system
Supporting cells

Do not conduct impulses
Outnumber neurons by 10- 50- fold

Several types of glia cells:

1) astrocytes – encircle capillaries of the brain

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2) oligodendrocytes form the myelin sheaths
that insulate the CNS nerve processes
3) Schwann cells form the insulating myelin
sheath around axons in the PNS
Myelination of neurons

Occurs when Schwann cells or oligodenrocytes
grow around an axon so their plasma membranes
form concentric layers

Provides electrical insulation

Increases speed of nerve impulse propagation

In MS, myelin sheaths deteriorate causing a
disruption of nerve impulse transmission &
consequent loss of coordination
Nature of Neural Signals

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Signal transmission along a neuron depends on
voltages created by ionic fluxes across neuron
plasma membranes.
Membrane potentials arise from differences in ion
concentrations between a cell’s contents and the
extracellular fluid.
All cells have an electrical potential or voltage
across their plasma membrane.
The charge outside is designated as zero, so the
minus sign indicates that the cytoplasm inside is
negatively charged compared to the extracellular
fluid.
Nature of Neural Signals

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Ion channel = integral transmembrane protein
that allows a specific ion to cross the membrane.
May be passive all the time or it may be gated,
requiring stimulus to change into an open
conformation.
Is selective for a specific ion, such as Na+, K+,
and ClA shift in ionic gradients is prevented by
sodium-potassium pumps which maintain the
concentration gradient.
Action Potential


A rapid change in the membrane
potential of an excitable cell,
caused by stimulus-triggered
selective opening & closing of
voltage-gated ion channels.
There are four stages.
Four stages of an Action Potential
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1) resting stage; no channels open.
2) depolarizing phase – membrane reverses polarity
(cell interior becomes + relative to exterior); the Na+
activation gates open, Na+ rushes in, potassium gates
remain closed.
3) repolarizing phase - returns the membrane
potential to resting level; inactivation gates close Na+
channels & K+ channels open.
4) undershoot phase – membrane potential is
temporarily more negative than the resting stage
(hyperpolarized); Na+ channels remain closed but K+
channels remain open since the inactivation gates have not
had time to respond to repolarization of the membrane.
Figure
48.9
Role of
gated
ion
channels
in the
action
potential
Action Potential….

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Refractory Period occurs during the undershoot
phase.
During this period, the neuron is insensitive to
depolarizing stimuli.
Limits the maximum rate at which action
potentials can be stimulated in a neuron.
Action potentials are all-or-none events. The
nervous system distinguishes between strong &
weak stimuli based on the frequency of action
potentials generated.
Action potential “travel”
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Action potentials “travel” along the axon because
they are self-propagating.
A neuron is stimulated at its dendrites or cell
body and the action potential travels along the
axon.
The signal travels in a perpendicular direction
along the axon regenerating the action potential.
Figure 48.7
Propagation
of the action
potential
Action potential “travel”

Saltatory conduction – the action

Figure 48.11
potential “jumps” from one node of
Ranvier to the next, skipping
myelinated regions of membranes
Figure 48.11 Saltatory Conduction
Communication

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Communication between cells happens across the
synapse
Synapse – tiny gap between a synaptic terminal
of an axon & a signal- receiving portion of another
neuron or effector cell
Presynaptic cell is the transmitting cell;
postsynaptic cell is the receiving cell
There are two types of synapses:
 1) electrical
 2) chemical
Electrical Synapses


Allow action potentials to spread directly
from pre- to postsynaptic cells via gap
junctions (intercellular channels)
Allow impulse travel without delay or loss
of signal strength

Less common than chemical synapses

Common in crustaceans
Chemical Synapses
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Synaptic vesicles containing thousands
of neurotransmitter molecules are present
in the cytoplasm of the synaptic terminal
of the presynaptic neuron
Chemical synapses allow transmission in
one direction only
Receptors for neurotransmitters are
located only on postsynaptic membranes
Figure 48.12 A Chemical Synapse
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One neuron may receive information
from thousands of synapses. Some
synapses are excitatory, others are
inhibitory (Fig. 48.11).
The same neurotransmitter can
produce different effects on different
types of cells
(Table 48.2)
Figure 48.13
Integration of
multiple
synaptic inputs
Numerous
synaptic
terminals
communicating
with a single
postsynaptic cell
(SEM).
Fig. 48.17
Functional
hierarchy
of the
peripheral
nervous
system
Figure 48.18
Parasympathetic
– maintains
normal activity
- slows down
- acetylcholine
Sympathetic
-”flight or fight”
- speeds up
- norepinephrine
Integration & Control
CNS
Autonomic
NS
(internal organsjoints)
Parasympathetic
(normal activity)
&
PNS
Somatic
NS
(muscles, skin, viscera)
Sympathetic
(“flight or fight”)
CNS: Meninges

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Meninges series of 3 membrane
layers which surround brain &
spinal cord:
1) dura mater (outer)
2) arachnoid membrane
3) pia mater (inner)
CNS: Spinal Cord

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connecting mechanism between the
body & the brain
part of the CNS
enclosed in vertebral column
31 pairs of spinal nerves
fluid-filled cavity – cerebrospinal
fluid
Fig. 48.16
The nervous
system of a
vertebrate:
31 pairs of spinal
nerves each
containing one
sensory neuron
(afferent dorsal
route) & one
motor neuron
(efferent ventral
route)
Vertebrate Brain

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100 billion neurons in human brain
10 billion in the cerebral cortex
weights about 3 lbs
composed of gray matter (cell bodies)
& white matter (axons & dendrites)
developed from the 3 bulges at the
anterior end of the dorsal tubular
nerve cord: hind – mid–fore brain)
Fig.48.19
Embryonic Development of the Vertebrate Brain
Fig.48.19
Embryonic
Development of
the brain
Major parts of the human brain.
Vertebrate Brain (Hindbrain)

Medulla –(upper spinal cord)
center for respiratory, cardiac
function; vomiting, sweating, gastric
secretion, heartbeat
 Cerebellum – regulate & controls
bodily muscular contractions;
coordination, balance, equilibrium

Pons – bridge between two halves of
the cerebellum; carries fibers that
coordinate activity of muscles on two
sides of the body
Vertebrate Brain (Mid & Fore)

Midbrain – relay center; visual &

Thalamus – relay center for sensory

Hypothalamus – (floor of thalmus)
auditory reflexes
impulses going to cerebrum control
center for external manifestations of
emotion (laughing, crying, etc.)
regulates hunger, thirst, body temp,
CHO & fat metabolism, blood pressure,
sleep; regulates the pitiutary
Vertebrate Brain (Forebrain)

Cerebral hemispheres –
Cerebrum – Controls learned behavior
& memory; makes up about 80% of
brain mass
1) frontal (speech, motor cortex)
2) parietal (taste, reading,)
3) temporal (smell, hearing)
4) occipital (vision)
 Corpus Collosum – junction between
two hemispheres
Figure 48.24
Structure and Functional areas of the cerebrum
Figure 48.26 Mapping Language areas of
the cerebral cortex.
Figure 49.16
Neural pathways
for vision
Sensory transduction by a taste receptor.