Lecture 20
The Nervous System
The Nervous System
 The master controlling
and communicating
system of the body
 Functions
 Sensory input –
monitoring stimuli
occurring inside and
outside the body
 Integration –
interpretation of
sensory input
 Motor output –
response to stimuli by
activating effector
organs
Evolutionary Path to Vertebrate Nervous Systems
 Cnidarians have simplest nervous
system
 Neurons are linked to one another
through a nerve net
 No associative activity, just reflexes
 First associative activity is seen in
free-living flatworms
 Two nerve cords run down bodies
 Permit complex control of muscles
 More complex animals developed:
 More sophisticated sensory
mechanisms
 Differentiation into central and
peripheral nervous systems
 Differentiation of sensory and motor
nerves
 Increased complexity of association
 Elaboration of the brain
Organization of the vertebrate nervous system
 The nervous system links
sensory receptors & motor
effectors in all vertebrates (and
most invertebrates)
Central Nervous System
(CNS)
Association neurons (or
interneurons) are located
in the brain and spinal
cord
Peripheral Nervous System
(PNS)
Motor (or efferent) neurons carry
impulses away from CNS
Sensory (or afferent) neurons carry
impulses to CNS
Neurons Generate Nerve Impulses

All neurons have the same
basic structure
 Cell body – Enlarged
part containing the
nucleus
 Dendrites – Short,
slender input channels
extending from end of
cell body
 Axon – A single, long
output channel
extending from other
end of cell body






Most neurons require nutritional support provided by companion neuroglial cells
Schwann cells (PNS) and oligodendrocytes (CNS) envelop the axon with fatty material called
myelin which act as a electrical insulator
During development cells wrap themselves around each axon several times to form a myelin
sheath
Uninsulated gaps are called nodes of Ranvier
Nerve impulses jump from node to node
Multiple sclerosis and Tay-Sachs disease result from degeneration of the myelin sheath
Three types of neurons
The Nerve Impulse

Ionic differences are the
consequence of:
 Differential permeability of
the cell membrane to Na+
and K+
 Operation of the sodiumpotassium pump

The potential difference
(–70 mV) across the
membrane of a resting
neuron is generated by
different concentrations
of Na+, K+, and Cl
 Graded potentials are short-lived, local changes in membrane potential
 Decrease in intensity with distance
 Their magnitude varies directly with the strength of the stimulus
 Sufficiently strong graded potentials can initiate nerve impulses called
action potentials
PLAY
Action Potential
How an Action Potential Works
 An action potential forms when the membrane
potential reaches -55 to -50 mV
 The action potential results from ion movements in
and out of voltage-gated channels
 The change in membrane potential causes Na+
activation channels to open
 Sudden influx of Na+ into cell causes “depolarization”
 Local voltage change opens adjacent Na+ channels
and an action potential is produced
 When the membrane potential reaches +100 mV, K+
voltage-gated channels open
 K+ flows out of the cell
 Na+ inactivation channels snap close
 The negative charge in the cell is restored
 The Na+ channels remain closed until the membrane
potential normalizes (-70 mV), keeping the action
potential from moving backward
 The ion balance across the membrane is restored by
the action of the sodium-potassium pump
Synapses
 A junction that mediates information
transfer from one neuron:
 To another neuron
 To an effector cell
 Presynaptic neuron – conducts impulses
toward the synapse
 Postsynaptic neuron – transmits impulses
away from the synapse
PLAY
Transmission Across A Synapse
Kinds of Synapses

Excitatory synapse
 Receptor protein is a chemically-gated sodium
channel
 On binding the neurotransmitter, the
channel opens
 Na+ floods inwards
 Action potential begins

Inhibitory synapse
 Receptor protein is a chemically-gated
potassium or chloride channel
 On binding the neurotransmitter, the
channel opens
 K+ floods outwards or Cl– floods inwards
 Action potential is inhibited

An individual nerve cell can possess both kinds of
synapses

Integration (Summation)
 Various excitatory and inhibitory electrical
effects cancel or reinforce one another
 Occurs at the axon hillock
Neurotransmitters

Are chemical messengers that carry nerve impulses across synapses
 Bind to receptors in the postsynaptic cell causing chemically-gated
channels to open





Acetylcholine
 Released at the neuromuscular junction
 Have an excitatory effect on skeletal
muscle and inhibitory effect on cardiac
muscle
Glycine and GABA
 Inhibitory neurotransmitters
 Important for neural control of brain
function
Biogenic amines
 Dopamine – Control of body movements
 Serotonin – Sleep regulation and mood
Neuromodulators are chemicals that prolong
the effect of neurotransmitters by aiding their
release or preventing their reabsorption
Example: Depression may be caused by a
shortage of serotonin
 Prozac, inhibits its reabsorption
Drug Addiction
 Cells that are exposed to a chemical signal for a prolonged time, lose their
“sensitivity”
 They lose their ability to respond to the stimulus with their original intensity
 Nerve cells are particularly prone to this loss of sensitivity
 They respond to high neurotransmitter exposure by inserting fewer receptor
proteins
Drug Addiction
 Addiction occurs when chronic exposure to a drug induces the nervous
system to act physiologically
 Cocaine is a neuromodulator
 It causes large amounts of neurotransmitter to remain in synapses
for long periods of time
 Dopamine transmits pleasure messages in the body’s limbic
system
 High levels for long periods of time, cause nerve cells to lower the
number of receptors
 Tobacco
 “Nicotine receptors” normally served to bind acetylcholine
 Brain adjusts to prolonged exposure to nicotine by
 1. Making fewer nicotine receptors
 2. Altering the pattern of activation of nicotine receptors
 Addiction occurs because the brain compensates for the nicotineinduced changes by making others
 There is no easy way out
 The only way to quit is to quit!
Evolution of the Vertebrate Brain
 Brains of primitive fish, while small, already had the 3 divisions found in
contemporary vertebrate brains
 Hindbrain (Rhombencephlon)
 Major component of early
fishes, as it is today
 An extension of the spinal
cord devoted primarily to
coordinating muscle reflexes
 Most coordination is done
by the cerebellum
 Midbrain (Mesencephlon)
 Composed primarily of optic
lobes that receive and process
visual information
 Forebrain (Proencephlon)
 Devoted for processing
olfactory (smell) information
 Note:
 Brains of fishes continue growing
throughout their lives!
How the Human Brain Works
 Diencephalon
 Thalamus – Relay center between
incoming sensory information and the
cerebrum
 Hypothalamus – Coordinates nervous
and hormonal responses to many
internal stimuli and emotions
 Telencephalon
 Devoted largely to associative activity
 Cerebrum (~ 85% of the weight of the
human brain)
 Dominant part of the brain, receives
sensory data and issues motor
commands
 Cerebral cortex (Gray outer layer)
Functions in language, thought,
personality and other “thinking and
feeling” activities
Basic Geography of the Human Brain

The cerebrum is divided by a
groove into right and left halves
called cerebral hemispheres
 Linked by bundles of neurons
called tracts that serve as
information highways

In general:
 The left brain is associated with
language, speech and
mathematical abilities
 The right brain is associated
with intuitive, musical, and
artistic abilities

The Central Sulcus divides the
front and back of the cerebrum
 The front is associated with
motor functions
 The back with sensory

Higher association functions
are in the prefrontal area

Stroke
 A disorder caused by blood clots
blocking blood vessels in the brain
The Diencephalon
 Thalamus
 Major site of sensory
processing in the brain
 Controls balance
 Hypothalamus
 Integrates internal activities:
body temperature, blood
pressure, etc.
 Controls pituitary gland
secretions
 Linked to areas of cerebral
cortex via limbic system
The Brain Stem & Cerebellum
 Cerebellum
 Extends back from the base of
the brain
 Coordinates muscle movement
 Even better developed in birds
 Brain Stem
 Made up of midbrain, pons, and
medulla oblongata
 Connects rest of brain to spinal
cord
 Controls breathing, swallowing,
digestion, heart beat, and blood
vessel diameter
Memory Processing

Memory is the storage and retrieval
of information

The three principles of memory are:
1. Storage – occurs in stages and
is continually changing
2. Processing – accomplished by
the hippocampus and
surrounding structures
3. Memory traces – chemical or
structural changes that encode
memory


Short-term memory –appears
to be stored electrically in the
form of a transient neural
excitation
Long-term memory –appears
to involve structural changes in
certain neural connections
Types of Sleep
 There are two major types of sleep:
 Non-rapid eye movement (NREM)
 Rapid eye movement (REM)
 One passes through four stages of NREM during the first 30-45 minutes
of sleep
 REM sleep occurs after the fourth NREM stage has been achieved
Importance of Sleep
 Slow-wave sleep is presumed to be the restorative stage
 Those deprived of REM sleep become moody and
depressed
 REM sleep may be a reverse learning process where
superfluous information is purged from the brain
 Daily sleep requirements decline with age
 Sleep Disorders
 Narcolepsy – lapsing abruptly into sleep from the awake
state
 Insomnia – chronic inability to obtain the amount or
quality of sleep needed
 Sleep apnea – temporary cessation of breathing during
sleep
Degenerative Brain Disorders
 Alzheimer’s disease – a progressive degenerative disease
of the brain that results in dementia
 Parkinson’s disease – degeneration of the dopaminereleasing neurons of the substantia nigra
 Huntington’s disease – a fatal hereditary disorder caused
by accumulation of the protein huntingtin that leads to
degeneration of the basal nuclei
The Spinal Cord
 The spinal cord is a cable of
neurons extending from the brain
down through the backbone
 Neuron cell bodies in the center
 Gray matter
 Axons and dendrites on the
outside
 White matter
 It is surrounded and protected by
the vertebrae
 Through them spinal nerves
pass out to the body
 Motor nerves from spine
control most of the muscles
below the head
Major Nerves of Humans
Voluntary and Autonomic Nervous Systems
Are two subdivisions of vertebrate motor pathways
The Voluntary Nervous System
 Relays commands to
skeletal muscles
 Can be controlled by
conscious thought
 Reflexes are rapid
involuntary movements
 Are rapid because sensory
neuron passes information
directly to a motor neuron
 Most involve single
connecting interneuron
between sensory and motor
neurons
The Autonomic Nervous System
 Stimulates glands and relays commands to smooth muscles
 Cannot be controlled by conscious thought
 Composed of elements that act in opposition to each other
 Parasympathetic nervous
System
 Controls normal functions
 Conserves energy by slowing
down processes
 Sympathetic nervous system
 Dominates in time of stress
 Controls the “fight-or-flight”
reaction
 Increases blood pressure,
heart rate, breathing