Chapter 10
Nervous System I
Cell Types of Neural Tissue
• neurons
• neuroglial cells
1
• Composed of some blood vessels and
connective tissue but mostly neural tissue
(2 cell types)
neurons
neuroglia
• Neurons transmit information as nerve
impulses along nerve fibers
• Nerves are bundles of nerve fibers
2
• Neuroglia surround, support, and nourish
neurons and might even send and receive
messages
• Synapses are spaces between neurons
• Neurotransmitters are biological messengers
• Central nervous system (CNS) consists of
the brain and spinal cord
3
• Peripheral nervous system (PNS) consists of
the peripheral nerves that connect the CNS
to other body parts
• CNS and PNS provide 3 general functions:
sensory, integrative, and motor
4
• Sensory receptors at the ends of peripheral
neurons gather information:
Info  nerve impulse  CNS 
integration  decision made  motor
neurons  muscles or glands (effectors)
Divisions of the
Nervous System
• Central Nervous System
• brain
• spinal cord
• Peripheral Nervous System
• nerves
• cranial nerves
• spinal nerves
6
Divisions of Peripheral
Nervous System
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
7
Divisions Nervous System
8
Functions of Nervous System
Sensory Function
• sensory receptors gather
information
• information is carried to the
CNS
Integrative Function
• sensory information used to
create
• sensations
• memory
• thoughts
• decisions
Motor Function
• decisions are acted
upon
• impulses are
carried to effectors
9
Neuron Structure
10
• Mature neurons generally do not reproduce
• All neurons have a cell body and nerve
fibers
• Cell body contains: granular cytoplasm,
mitochondria, lysosomes, a Golgi apparatus,
microtubules
• Neurofibrils (fine threads) extends into and
supports fibers
11
• Chromatophilic substance (nissl bodies)
made of rough endoplasmic reticulum is
scattered in cytoplasm
• Cytoplasmic inclusions include glycogen,
lipids, and pigments
• Large nucleus near the center with a
nucleolus
• Nerve fibers, dendrites and axons extend
from the cell body
12
• Dendrites are usually branched and
communicate with other neurons
• Axons carry nerve impulses away from the
cell body
• Axons also convey biochemicals produced
in the cell body (axonal transport)
• Schwann cells wind around axons in layers
called myelin
13
• Myelin has higher lipids than other surface
membranes and forms a myelin sheath on
the outside of axons
• A neurilemmal sheath made of Schwann
cells that have most of the cytoplasm and
nuclei forms outside the myelin sheath
• Nodes of Ranvier are gaps in myelin sheath
between Schwann cells
14
• Myelinated fibers appear white (white
matter in brain and spinal cord)
• Small axons don’t have myelin
(unmyelinated) and appears gray (gray
matter in brain and spinal cord)
15
Myelination of Axons
White Matter
• contains myelinated
axons
Gray Matter
• contains
unmyelinated
structures
• cell bodies, dendrites
16
Classification of Neurons –
Structural Differences
Bipolar
• two processes
• eyes, ears, nose
Unipolar
• one process
• ganglia
Multipolar
• many processes
• most neurons of
CNS
18
Neurons
• Vary in size and shape
• May differ in length and size of axons and
dendrites
• Vary in the number of processes by which
they communicate with other neurons
• Vary in function
19
• 3 types classified by structure: bipolar,
unipolar, multipolar (Table 10.1)
• 3 types classified by function: sensory,
interneuron, motor (Table 10.1)
20
Classification of Neurons –
Functional Differences
Sensory Neurons
• afferent
• carry impulse to CNS
• most are unipolar
• some are bipolar
Interneurons
• link neurons
• multipolar
• in CNS
Motor Neurons
• multipolar
• carry impulses away
from CNS
• carry impulses to
effectors
21
Types of Neuroglial Cells
in the PNS
Schwann Cells
• produce myelin found on peripheral
myelinated neurons
• speed neurotransmission
Satellite Cells
• support clusters of neuron cell bodies (ganglia)
23
Neuroglia
• Guide neurons in the embryo to their
positions and may stimulate them to
specialize
• Produce growth factors that nourish neurons
• Remove ions and neurotransmitters that
build up in between neurons, allowing
continued information transmission
24
• Some may communicate with neurons
• Schwann cells are the neuroglia of the PNS
• CNS neuroglia: astrocytes,
oligodendrocytes, microglia, and ependyma
25
• Neuroglia form more than half the volume
of the brain
• Most brain tumors are neuroglia that
multiply too often
26
Types of Neuroglial Cells
in the CNS
Astrocytes
• CNS
• scar tissue
• mop up excess ions,
etc
• induce synapse
formation
• connect neurons to
blood vessels
Oligodendrocytes
• CNS
• myelinating cell
Microglia
• CNS
• phagocytic cell
Ependyma
• CNS
• ciliated
• line central canal of spinal cord
• line ventricles of brain
27
Types of Neuroglial Cells
28
Regeneration of A Nerve Axon
29
• Mature neurons do not reproduce
• Injury to a neuron cell body usually kills it
• Damaged axons in a peripheral nerve may
regenerate (3-4mm /day), but may end up in
the wrong place so full function often does
not return
30
• Neuroglial cells assist in regeneration
(nerve growth factors)
• Damaged axons in the CNS are unable to
produce myelin and regeneration is unlikely
31
The Synapse
Nerve impulses pass
from neuron to neuron
at synapses
32
Synaptic Transmission
Neurotransmit
ters are
released when
impulse
reaches
synaptic knob
33
Cell Membrane Potential
• A cell membrane is polarized (electrically
charged) due to unequal distribution of ions.
The inside is negative with respect to the
outside.
34
Distribution of Ions
• K+ are the major ions inside the cell
(intracellular)
• Na+ are the major extracellular ions
• Channels in the membrane allow movement
in and out
• Chemical and electrical factors affect the
opening and closing of these gatelike
channels
35
• Resting nerve cell is not being stimulated to
send an impulse
• K+ pass through resting cell membranes
much easier than Na+
• Ca2+ less able to cross a resting cell
membrane than K+ or Na+
36
Resting Membrane Potential
• inside is
negative
relative to the
outside
• polarized
membrane
• due to
distribution of
ions
• Na+/K+ pump
37
• Active transport keeps a greater
concentration of K+ inside the cell and a
greater concentration of Na+ outside the cell
• Cytoplasm contains anions: (PO4-2), (SO4-2),
and proteins that can’t diffuse through the
cell membrane
38
• Na+ and K+ follow the laws of diffusion
(high to low)
• more K+ diffuses out than Na+ can move in
• more + charges leave the cell than enter
• outside of the cell has a positive charge and
the inside has a negative charge
•
39
• Difference in electrical charge = potential
difference (measured in volts)
• Difference between inside and outside = -70
millivolts
• Resting potential = separation of charge
• Action potential = work it may do to send a
nerve impulse
40
Local Potential Changes
• caused by various stimuli
• temperature changes
• light
• pressure
• environmental changes affect the
membrane potential by opening a
gated ion channel
41
Local Potential Changes
•if membrane potential becomes more negative, it has
hyperpolarized
• if membrane potential becomes less negative, it has
depolarized
• graded
• summation can lead to threshold stimulus that starts an
action potential
42
Local Potential Changes
43
• Nerve cells can respond to changes in their
surroundings
• Changes affect the resting potential of the
membrane
• Hyperpolarized = membrane potential
becomes more negative than resting
potential
44
• Depolarized = membrane potential becomes
more positive than resting potential (Na+
move in)
• Threshold potential = strong enough
depolarization to conduct a nerve impulse
(action potential
45
Action Potentials
• at rest membrane is
polarized
• threshold stimulus
reached
• sodium channels
open and membrane
depolarizes
• potassium leaves
cytoplasm and
membrane repolarizes
46
Action Potentials
• Reached when the membrane potential
becomes positive
• When the threshold potential is reached Na+
move into the cell causing the membrane
potential to become more positive (as high
as +30 mV)  depolarization
47
• At the same time K channels open and
allow K+ to move out
• causing the inside to become negative again
 repolarization
• Rapid sequence of depolarization and
repolarization causes an electric current to
move down the nerve fiber  nerve
impulse
48
Action Potentials
49
Action Potentials
50
All-or None Response
• If a nerve fiber responds, it responds
completely
• All impulses are the same strength
• Greater intensity of stimulation produces
more impulses per second
51
Refractory Period
• Short time after a nerve impulse that a
threshold stimulus will not trigger another
impulse
• Limits the rate of nerve conduction
• about one /millisecond
52
Impulse Conduction
53
Impulse conduction
• Unmyelinated nerve fiber conducts an
impulse over the entire surface
• Myelinated fibers conduct impulses at only
the nodes of ranvier
• myelin insulates and prevents the flow of
ions through the membrane
54
• myelin made of lipids that are insoluble to
water soluble substances
• action potentials jump from node to node
(saltatory conduction)
• Myelinated axons conduct impulses much
faster than unmyelinated axons
55
• The larger the diameter of the fiber the
faster the impulse is conducted
Ex: skeletal muscle motor fiber = 120m /s
unmyelinated sensory fiber = 0.5m/s
56
Saltatory Conduction
57
The Synapse
•
•
•
•
Junction between two cells
Synaptic cleft is a space between two cells
Synaptic knobs are at the end of axons
contain vesicles that contain
neurotransmitters
• Neurotransmitters (NT) are released when a
nerve impulse reaches the end of the axon –
they diffuse across the synaptic cleft
58
Synaptic Potentials
• Some NT depolarize membranes causing an
action potential  excitatory postsynaptic
potential (EPSP)
• Some NT hyperpolarize membranes
inhibiting an action potential  inhibitory
postsynaptic potential (IPSP)
• EPSPs and IPSPs are summed at the trigger
zone (axon area close to cell body) 
59
decision making part of the axon
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
60
Neurotransmitters
61
Neurotransmitters
62
Neurotransmitters
• At least 30 different types
• Neurons release 1-3 kinds
• Synthesized in synaptic knobs and stored in
synaptic vesicles
63
• Ca2+ cause NT to be released
• Enzymes decompose some NT to keep the
impulse short
• Reuptake transports some NT back into
synaptic knobs
• Removal of NT prevents continuous
stimulation
64
Neuropeptides
• Act as NT or neuromodulators
• Alter a neurons response to a NT or block
its release
• Enkephalins – bind to opiate receptors in
brain and relieve pain
• Beta endorphin – pain reliever
65
• Substance P – transmits pain impulses into
the spinal cord and then to the brain
• enkephalins and endorphins may relieve
pain by inhibiting the release of substance P
66
Impulse Processing
• Neuronal Pools
• Groups of neurons in the CNS
• Each receive impulses from afferent nerve
fibers
• Each input fiber divides and branches
• Impulses are conducted away from CNS on
efferent output fibers
67
Facilitation
• If incoming impulses do not reach threshold
the neuron becomes more excitable to
incoming stimulation
68
Convergence
• neuron receives input from
several neurons
• incoming impulses represent
information from different
types of sensory receptors
• allows nervous system to
collect, process, and respond to
information
• makes it possible for a neuron
to sum impulses from different
sources
69
Divergence
• one neuron sends
impulses to several
neurons
• can amplify an
impulse
• impulse from a single
neuron in CNS may be
amplified to activate
enough motor units
needed for muscle
contraction
70
Clinical Application
Drug Addiction
• occurs because of the complex interaction of
neurons, drugs, and individual behaviors
• understanding how neurotransmitters fit receptors
can help explain the actions of certain drugs
• drugs have different mechanisms of action
• several questions remain about the biological effects
of addiction, such as why some individuals become
addicted and others do not
71