The Nervous
System
Multicellular Organisms Must Coordinate
• The nervous system contains cells called neurons that
can transmit signals from one part of the body to
another quickly
• The nervous system provides animals with nearly
instantaneous communication to coordinate body
functions
Nerves of
the zebra
fish tail
An Overview of the Nervous System
• The nervous system of many
invertebrates, and all vertebrates,
can be divided into the
peripheral nervous system (PNS)
and the central nervous system
(CNS)
• The PNS gathers information
from the external and internal
environment and sends it on to
the CNS
• The CNS processes it and often
generates a return signal to be
delivered by the PNS to the body
parts that will execute the signal
The Neuron
• A neuron is a specialized cell that
can receive and transmit
information from many different
types of cells
• Neurons contain the same
organelles found in any other
animal cell with the addition of:
– Dendrites: extensions which
receive signals from adjacent
cells
– Axons: transmit signals to
other cells
• Nervous system tissues also
contain a variety of support
cells, known as glial cells
The Neuron
• Myelin is an insulating
sheath made of a fatty
material which
surrounds the axons
• Myelinated axons can
carry signals more
rapidly than
unmyelinated axons
• White matter in the
CNS is due to myelin
sheaths in this area.
Glial Cells
• Nervous system tissues also
contain a variety of support
cells, known as glial cells:
– Oligodendrocytes form
myelin in the brain and
spinal cord.
– Astrocytes are near blood
vessels and support
structures, aid in
metabolism, and respond
to brain injury by filling in
spaces.
– Schwann cells are the
myelin-producing neuroglia
of the peripheral nervous
system.
The Nerve
• A nerve is made up of many individual neurons
bundled together with supporting cells, blood
vessels, and connective tissue to form a major
communication pathway
Ganglia
• Ganglia provide relay points and intermediary connections
between different neurological structures in the body, such as
the peripheral and central nervous systems.
• Cluster of nerve cell bodies that serve to integrate signals,
especially between the PNS and the CNS
The Central Nervous System of
Vertebrates: Brain and a Spinal Cord
• Vertebrates have a thick central nerve
cord called the spinal cord, which
contains large concentrations of
dendrites and axon terminals that
enable rapid information exchange
• The CNS is protected by the cranium
and the vertebrae while the
• PNS of vertebrates consists of
branching nerves that carry
information into and out of the spinal
cord
Reflex Arc
• The reflex arc consists of a
sensory neuron that sends
the message to the spinal
cord, an interneuron, and
a motor neuron that
creates a response in the
body
• A reflexive motor
response to pain does not
require the brain’s
involvement, which
requires more time to
process information
The Peripheral and Central Nervous
Systems Exchange Information
• Sensory neurons from the PNS convey sensory input to
interneurons found only in the CNS
The Peripheral and Central Nervous
Systems Exchange Information
• An interneuron may also send its output up to
the brain and out to the PNS at the same time,
in a simultaneous flow
The Peripheral and Central Nervous
Systems Exchange Information
• Interneurons process the sensory input and may
send them directly to the motor neurons for
immediate action or to the brain for further
processing
PNS: Voluntary vs. Involuntary
• PNS output that is under voluntary control is
called somatic control
• Autonomic control of PNS output is involuntary
• The sympathetic and parasympathetic divisions
of the nervous system work together to control
body functions
PNS
Autonomic
Somatic
PNS: Sympathetic vs. Parasympathetic
• The sympathetic and parasympathetic divisions
of the nervous system work together to control
body functions
PNS
Autonomic
Sympathetic
Parasympathetic
Somatic
PNS: Sympathetic vs. Parasympathetic
Opposes
Fight
or
Flight
Fight
or
Flight
Signal Transmission by Neurons
• An electrical disturbance in a neuron travels down the
length of an axon as a pulse of electrical activity known as
an action potential
• An action potential triggers the release of chemical
messengers, called neurotransmitters, that signal to the
next cell in this line of communication
Action Potential
• An action potential is a self-sustaining electrical signal that
travels away from the body of the neuron
• The action potential is dependent on the positively charged
ions moving across the plasma membrane
• The plasma membrane is in a polarized state because there is
a difference in electrical charges across the plasma membrane
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Polarize
d nerve
Na+
Na+
Na+
Na+
Na+
Na+
Na+
• The electrical charge that exists across the plasma membrane
of an unstimulated neuron is known as the resting potential.
Usually -70mV
Action Potentials
• A stimulus depolarizes a neuron if the ion flow is changed in
such a way that many more positively charged ions are able to
enter the cell
• Once the action potential has passed, the neuron returns to
its resting potential
Action Potentials
• Unmyelinated gaps between the myelin sheath are called nodes
of Ranvier and are the site of action potentials
• Action potentials are sped up by the presence of the myelin
sheath
• The action potential is regenerated at each node, allowing the
signal strength to jump rapidly from one node of Ranvier to the
next
Action Potentials Have Several
Important Features
• Action potentials move along the
axon in only one direction, keeping
the signal from being lost
• An action potential remains
consistently strong as it moves from
one end of the axon to the other and
does not weaken with distance
• A strong stimulus will initiate action
potentials more often, but any
individual action potential will be no
stronger than any other: it is an all-ornone event
Neurotransmission
at the Synapse
• The electrical signal
is converted into a
chemical message
and relayed to the
next cell at a junction
called a synapse
Neurotransmission
at the Synapse
• Electrical signals are
transformed into
chemical signals in
the form of
molecules called
neurotransmitters,
which are
transmitted across a
synaptic cleft
Neurotransmission
at the Synapse
• Neurotransmitters
excite or inhibit nonneuronal target cells,
such as muscle cells,
by binding to specific
receptor proteins on
the plasma
membrane
Neurotransmitters Transmit
Signals between Adjacent Cells
• The types of receptors on the target cell plasma
membrane determine which neurotransmitters will
activate a response
• Once released, neurotransmitters are cleared from the
synaptic cleft quickly through uptake by either the
neuron that released them or special glial cells; they
can also be destroyed or inactivated by specific
enzymes in the synaptic cleft
• Communication through the nervous system comes
from the capacity of each neuron to generate large
numbers of action potentials in a fraction of a second
and to aim those signals narrowly at specific target
cells
Selective Serotonin Reuptake Inhibitors (SSRIs)
• A class of drugs used as
antidepressants in the
treatment of depression,
anxiety disorders, and some
personality disorders.
• Believed to increase the
extracellular level of the
neurotransmitter serotonin
by inhibiting its reuptake into
the presynaptic cell
• This increases the level of
serotonin in the synaptic cleft
available to bind to the
postsynaptic receptor.
Drugs and Neurotransmission
Sensory Structures: Making Sense of the
Environment
Humans have five basic types of sensory receptors
• Chemoreceptors: Nose and tongue
• Photoreceptors: Eyes
• Mechanoreceptors detect physical stimuli inside
and outside the body
• Thermoreceptors: Skin, mouth, some internal
organs
• Pain receptors are located on just about every
tissue type inside and on the surface of the body
and detect different types of noxious stimuli