The Nervous System
NERVOUS SYSTEM - CONCEPTS
1.
2.
3.
4.
Homeostasis is maintained in the
human body by various parts of the
nervous system
Neural transmission occurs along axons,
due to an action potential that causes
depolarization of the neuron
Electrochemical communication occurs
between cells at the synapse
The central nervous system is the body’s
control center. It consists of the brain
and spinal cord
NS - CONCEPTS (CONT.)
5.
6.
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8.
The brain includes centers that control
involuntary responses and voluntary responses
The cerebrum is the largest part of the brain.
It contains four pairs of lobes, each of which is
associated with particular functions
The peripheral nervous system is composed of
the somatic (voluntary) and autonomic
(involuntary) system
The autonomic nervous system is divided into
the sympathetic and parasympathetic nervous
systems
STRUCTURES AND PROCESSES
OF THE NERVOUS SYSTEM


The nervous system regulates the human body
It coordinates with the endocrine system to
maintain homeostasis
DIVISIONS OF VERTEBRATE NERVOUS
SYSTEMS
Nervous System
CNS
PNS
CELLS IN THE NERVOUS SYSTEM

Cells within the nervous system are either:

Neurons

Glial Cells
NERVE FIBRES


Neurons and glial cells are packed together to
form nerve fibres that extend throughout the
nervous system
Neurons come in three types – sensory,
interneurons, and motor neurons
NEURAL CIRCUITS


Messages from sensory neurons sometimes will
not travel to the brain before action is taken
This is because we have reflex arcs that are used
for quick responses to stimuli
THE REFLEX ARC
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THE PURPOSE OF REFLEX ARCS
 The
purpose of a reflex arc is to prevent serious
injury
 For
example, if you touch a hot object, you will
often move your finger before feeling pain
 This
is because the reflex arc sends the pain
message to the spinal cord interneurons, which
redirect the message instantly to the motor
neurons


Without this reflex arc, we would have to receive
the pain signal, send it to the brain, have it
interpreted, and then formulate the correct
response
Within this time, a relatively minor burn would
become a very serious one
THE NEURON
COMPONENTS OF THE NEURON
 Dendrites:
Receive information from
adjoining cells or receptors and pass the
information along the neuron
 Cell Body: Contains organelles and
processes the input from dendrites
 Axon: Extension of the cytoplasm
through which nerve impulses move
 Myelin Sheath: Insulating covering
surrounding the axon
COMPONENTS OF THE NEURON



Schwann Cells: Structures that produce the
myelin sheath. These are a type of glial cell
Nodes of Ranvier: Junctions between myelin
sections
Axon Terminal: Passes nerve impulse on to the
next neuron in line
FACTORS AFFECTING NERVE IMPULSE
SPEED


The diameter of the axon – in general, the
smaller it is, the faster the impulse
Presence of myelin sheath – unmyelinated
neurons transmit much slower than myelinated
ones
MULTIPLE SCLEROSIS (MS)
 Caused
by destruction of the myelin sheath
 Myelinated
neurons are destroyed as the sheath
turns into scar tissue
 Produces
a “short circuit” within the neuron
 Symptoms
include double-vision, speech
difficulty, jerky limb movements, and partial
paralysis of voluntary muscles
THE NEURILEMMA


This is a special membrane found in the cells of
the PNS
It surrounds the axon and promotes regeneration
of damaged tissue
WHITE & GREY MATTER
White matter consists of myelinated neurons
 It is these neurons that contain the neurilemma
as well
 Grey matter is unmyelinated
 Therefore, damage to these neurons is permanent

A CROSS-SECTION OF THE SPINAL CORD
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ELECTROCHEMICAL IMPULSES

1.
2.
3.
4.
The nerve impulses produced by neurons differ
from conventional electricity in several ways:
It moves much slower than conventional
current
Cells would provide a high resistance to
conventional current
The strength of electrical currents diminish as
they move along a circuit
Conventional current requires an external
source of energy
PRODUCTION OF THE IMPULSE
1. Sodium-potassium exchange pumps use ATP to
move Na+ out of the cytoplasm of the cell and K+
into the cytoplasm. For every 2 K+ that move
into the cell, 3 Na+ move out. This creates high
concentration gradients across the cell
membrane.
Sodium-potassium Pump Animation
2. As a result of the concentration gradients, K+
begins to diffuse out of the cytoplasm and Na+
diffuses in. However, there are more available K+
ion channels in the resting membrane, so this
produces a positively charged region outside the
membrane. This is called a polarized membrane
or a resting membrane. There is a charge
difference of about -70 mV inside the axon (there
are more negative charges inside the axon than
outside)
3. As an impulse is triggered, the nerve cell
becomes more permeable to sodium than
potassium, and the sodium rushes into the
neuron. This causes a rapid reversal of charge
known as depolarization. Once the charge inside
the axon is positive, the sodium gates close.
Depolarization Animation – Sodium &
Potassium Channels
4. The potassium gates open again and K+ begins to
move back out of the nerve cell. When this
occurs, the Na+ and K+ are on the opposite side of
the membrane when compared to their position
before depolarization. However, an excess of K+
move outside of the membrane, causing brief
hyperpolarization.
5. The sodium & potassium pumps reactivate and
transport Na+ out of the cytoplasm and K+ into
the cytoplasm to return to the resting membrane
state. This return to the original polarity is
known as repolarization.
Because a neuron cannot fire again before it is
repolarized, there is a time known as the
refractory period where the nerve is unable to act
 This refractory period takes 1 to 10 ms
 Action potentials in myelinated neurons only
occur at the Nodes of Ranvier

THE ENTIRE PROCESS:
MOVEMENT OF AN IMPULSE
The nerve impulse must move along the axon
 This is achieved through the attraction of
positive and negative charges along the nerve
membrane

 The
positively charged ions moving into
the cell when an action potential is
produced are attracted to the negative ions
in the neighboring regions of the
cytoplasm
 These positive ions begin to migrate,
triggering the opening of sodium channels
in that next region, causing depolarization
 As a wave of depolarization moves along
the membrane, it causes the potassium
gates behind it to open, creating
repolarization
THE MOVEMENT OF AN IMPULSE
Action Potential Propagation Animation
ENERGY AND IMPULSES

Because active transport is used to create the
concentration gradients needed for a resting
membrane to form, ATP must be used
THRESHOLD LEVELS
Early studies with nerve cells using electrical
currents indicated that neurons will not produce
a signal if a stimulus is below a certain level
 This lowest level that produces a response is
known as the threshold level

Therefore stimuli below threshold levels will not
produce a response
 As well, these experiments indicated that the
response is often an all-or-none response
 In other words, either the response (such as
muscle contraction) would either not be present
(when the threshold level had not been reached)
or at maximum intensity (at any level above the
threshold level)

DETECTING INTENSITY OF STIMULI
This information seems to contradict what we
know from experience – stimuli can be
experienced from low to very high intensities
 For instance, we can distinguish very cold objects
from very hot objects, but we also can feel a range
of temperatures in between

 This
occurs because our brain interprets
the intensity of a stimulus based on the
frequency of the impulses it produces
 Attached to each receptor are a number of
neurons, each with a different threshold
level
 A low intensity message would be
produced when only the most sensitive
neurons fire, while high intensity
messages occur as most or all of the
neurons are actively sending impulses
THE SYNAPSE
A synapse or synaptic cleft is the space that
exists between the axon terminal of one neuron
and the dendrites of another neuron
 Neurotransmitter chemicals leave the axon
terminals through vesicles in the presynaptic
neuron and travel to receptors in the
postsynaptic neuron

The distance across the synapse is small (about
20 nm), but neurotransmitters must move via
diffusion
 This becomes the slowest part of the transmission
of a nerve impulse (again, this explains the
quickness of a reflex arc when compared to the
message being sent to the brain)

THE SYNAPSE
http://kvhs.nbed.nb.ca
Synapse Animation
TRANSMISSION AT THE SYNAPSE
 Excitatory
transmitters trigger a nerve
impulse in a neuron
 These neurotransmitters are released
from vesicles within the axon endplate
and diffuse across the synapse
 As the neurotransmitter attaches to its
receptor site, it opens sodium channels on
the postsynaptic neuron
 This initiates an action potential in the
neuron
There are also neurotransmitters that are
inhibitory – they prevent the production of a
nerve impulse in the postsynaptic neuron
 These most often open potassium gates, allowing
the neuron to become hyperpolarized
 As a result, the postsynaptic neuron cannot
produce the action potential required for an
impulse to occur

BREAKDOWN OF NEUROTRANSMITTERS
If a neurotransmitter remains in place on a
receptor, it will prevent repolarization of the
neuron
 Therefore, these neurotransmitters must be
broken down
 This is often accomplished through the action of
enzymes

ACETYLCHOLINE
A good example of a neurotransmitter and its
enzyme are acetylcholine and cholinesterase
 Acetylcholine is an excitatory neurotransmitter
 Just after acetylcholine is released, the
cholinesterase enzyme is released into the
synapse

The cholinesterase enzymes seek out
acetylcholine molecules and break them down
 As a result, there is no more acetylcholine present
and the postsynaptic neuron can repolarize
 Of course, like most enzymes, inhibitors can be
used to block their function

A number of insecticides and the nerve gas sarin
are cholinesterase inhibitors which bind with
cholinesterase and prevent it from breaking down
acetylcholine
 As a result, the muscles of the insect’s heart
remain contracted and will not relax (which
prevents it from beating)
 Cholinesterase inhibitors have also been
considered as treatments for Alzheimer’s Disease

Alzheimer’s Disease is related to a lowered
production of acetylcholine
 In patients with the disease, the cholinesterase
often breaks down the low levels of acetylcholine
before it has time to act
 Cholinesterase inhibitors would then prevent the
premature breakdown of acetylcholine by
inhibiting the action of the enzymes

COMMON NEUROTRANSMITTERS
Neurotransmitter
Function
Effects of Abnormal
Production
Acetylcholine
Excitatory
Inadequate –
Alzheimer’s Disease
Dopamine
Control of body
movements and
sensations of
pleasure
Excessive –
schizophrenia
Inadequate –
Parkinson’s Disease
Serotonin
Temperature control, Inadequate sensory perception & depression
mood
Norepinephrine
Prepares body for
stress
Excessive – anxiety,
insomnia
Inadequate – hunger,
exhaustion
SUMMATION
 In
many cases, a number of neurons come
together at a junction
 Often, when this occurs, more than one of
the neurons bringing a message into the
junction must be active to produce an
action potential in the neuron leaving the
junction
 Summation is the effect produced by the
accumulation of neurotransmitters from
two or more neurons
 As
you can see
here, both neurons
A and B must fire
at the same time to
exceed the
threshold level to
activate D (A and
B are not able to
exceed the
threshold levels
individually)
 Neuron C in this
case is producing
an inhibitory
neurotransmitter
http://www.biologymad.com
THE CENTRAL NERVOUS SYSTEM
The brain and spinal cord make up the CNS
 The brain itself is supported by three layers of
membranes known as meninges
 Between the inner and middle meninges exists a
layer of fluid known as cerebralspinal fluid (CSF)
 This fluid is also found in the central canal of the
spinal cord
 This fluid acts as a shock absorber and as a
transport medium for nutrients and waste to and
from the brain cells

CSF AND ILLNESSES
The CSF can carry bacteria and viruses
 These may cause inflammations of the meninges
or areas of the spinal cord
 The typical method of diagnosis for diseases such
as meningitis is to remove CSF from the spinal
cord and check it for pathogens

THE SPINAL CORD
The spinal cord consists of neurons and is
approximately the diameter of a pencil
 The grey matter of the spinal cord contains
unmyelinated neurons and the cell bodies of
motor neurons
 The white matter consists of myelinated
interneurons

THE SPINAL CORD
 The
dorsal nerve
tract brings sensory
information back
into the spinal cord,
while the ventral
nerve carries motor
information to
peripheral muscles
and organs
THE BRAIN
The human brain has a far more advanced
forebrain than other animal species
 The brain consists three sections – the forebrain,
the midbrain, and the hindbrain

BRAIN STRUCTURES
THE HINDBRAIN
The hindbrain is located posterior to the
midbrain and connects to the spinal cord
 It consists of three main regions: the cerebellum,
the pons, and the medulla oblongata

THE CEREBELLUM
 This
is the largest portion of the
hindbrain
 It controls limb movements, balance, and
muscle tone
 The cerebellum also receives information
from proprioceptors that keep track of the
location and position of the body’s limbs
 This is the part of the brain that
ultimately controls excitatory and
inhibitory nerve impulses
THE PONS

The Pons serves as a relay station that connects
the two halves of the cerebellum, and the
cerebellum to the medulla oblongata
THE MEDULLA OBLONGATA
This is the lowest part of the hindbrain
 It acts as a connection between the CNS and the
PNS
 It regulates involuntary muscle action (heart
rate, breathing, swallowing, coughing, etc.)
 The medulla oblongata also acts as a
coordinating center for the ANS

THE MIDBRAIN
The midbrain consists of four small spheres of
grey matter
 It relays visual and auditory information
between areas of the forebrain and the hindbrain
 It also plays a role in eye movement and the
control of skeletal muscles

THE FOREBRAIN
The forebrain contains a number of different
parts
 The olfactory lobes, which detect smell are part of
the forebrain
 The majority of the forebrain consists of the
cerebrum, which stores and interprets sensory
information and initiates voluntary motor
activities

SUPPLYING THE BRAIN
Blood is separated from the brain by a bloodbrain barrier
 The blood that travels to the brain never enters
the nervous tissue itself
 The capillaries in the brain are made up of
tightly-fused cells
 This blocks the passage of many toxins and
infectious agents

TRANSPORT & THE BLOOD-BRAIN
BARRIER
 Substances
such as glucose and oxygen
are supplied to the brain through special
transport mechanisms
 However, lipid-based molecules move
across the lipid bilayer of the capillary
cells
 Therefore, lipid-soluble materials
(caffeine, nicotine, alcohol, heroin) have
rapid effects on brain function
PARTS OF THE FOREBRAIN
AND FUNCTIONS
Lobe PARTS
Function
Frontal Lobe
Associated with conscious thought, intelligence, memory,
personality; controls voluntary muscle movement
Temporal
Lobe
Involved in auditory reception
Parietal Lobe Receive sensory information from the skin, processes
information about body position
Occipital
Lobe
Processes visual information
Mirror Neurons
HEMISPHERES OF THE BRAIN
The brain consists of a right and left hemisphere
 These two hemispheres are connected by a
bundle of nerves known as the corpus callosum

RIGHT VS. LEFT BRAIN…
Left
Brain
uses logic, detail oriented, facts rule, words and
language, present and past, math and science, can
comprehend, knowing, acknowledges, order/pattern
perception, knows object name, reality based, forms
strategies, practical, safe.
Right
Brain
uses feeling, "big picture" oriented, imagination
rules, symbols and images, present and future,
philosophy & religion, can "get it" (i.e. meaning),
believes, appreciates, spatial perception, knows
object function, fantasy based, presents possibilities,
impetuous, risk taking.
The right side of the brain is associated with
visual patterns and spatial awareness, while the
left side is associated with verbal skills
 The ability of a person to learn, and the learning
style that suits them, may be partially dictated by
which side of the brain is dominant
 However, not all people have a dominant
hemisphere of their brain

BROCA’S AREA & WERNICKE’S AREA
On the left side of the cerebral cortex are found
Broca’s area (Frontal lobe) and Wernicke’s area
(Temporal lobe)
 Broca’s area coordinates the muscles for speaking
and translates thought into speech
 Wernicke’s area stores the information involved
in language comprehension

Speech in Birds & Humans
OTHER PARTS OF THE FOREBRAIN
The forebrain also contains the thalamus and the
hypothalamus
 The thalamus, which is directly below the
cerebrum, coordinates and interprets sensory
information
 The hypothalamus is connected to the pituitary
and regulates a number of the body’s responses
such as blood pressure, heart rate, temperature,
basic drives (thirst & hunger) and emotions
 Damage to the hypothalamus can lead to a person
demonstrating unusual or violent behaviour

MAPPING BRAIN FUNCTIONS
 Early
information on the function of
various parts of the brain was gathered
from patients who recevied brain injuries
or diseases
 Later, Canadian Nobel Prize winner
Wilder Penfield mapped the motor areas
of the cerebral cortex by stimulating
different parts of the brain through
probing
NON-INTRUSIVE MAPPING
 PET
(positron-emission tomography) and
MRI (magnetic resonance imaging) are
now used to study and map the brain
 The PET can track glucose consumption
in a brain during particular activities
 MRIs can produce high-detail images of
the brain structure in three dimensions
THE PERIPHERAL NERVOUS SYSTEM
 The
peripheral nervous system includes
all nerves outside of the central nervous
system
 The somatic nervous system, which is
mostly under voluntary control, controls
movement and receives information about
the environment
 This system contains 12 pairs of cranial
nerves and 31 pairs of spinal nerves, all of
which are myelinated
CRANIAL NERVES
Some nerves exit the brain itself – these are
known as cranial nerves
 One of the most important of these cranial nerves
is the Vagus nerve
 This nerve regulates the heart, the bronchi of the
lungs, the liver, pancreas and digestive tract

THE AUTONOMIC NERVOUS SYSTEM
 The
Autonomic Nervous System controls
involuntary functions within our body
 This system helps to maintain
homeostasis despite a changing internal
environment
 It consists of sympathetic and
parasympathetic nerves, which are
controlled by the hypothalamus and the
medulla oblongata
SYMPATHETIC VS. PARASYMPATHETIC
NERVES
 Sympathetic
nerves prepare the body for
stress, while parasympathetic nerves
return the body to its normal state
 Sympathetic nerves use norepinephrine as
an excitatory neurotransmitter which
activates muscles
 A number of different organs and organ
systems are involved in ANS responses:
E
FFECTS OF THE ANS
Organ
Sympathetic
Parasympathetic
Heart
Increases heart rate
Decreases heart rate
Digestive
Decreases peristalsis
Increases peristalsis
Liver
Increases release of
glucose
Stores glucose
Eye
Dilates pupil
Constricts pupil
Bladder
Relaxes sphincter
Contracts sphincter
Skin
Increases blood flow
Decreases blood flow
Adrenal Gland
Released epinephrine
No effect
NEURON ANATOMY
Sympathetic nerves have a short preganglion and
a long postganglion
 Parasympathetic nerves have a long preganglion
and a short postganglion
 Sympathetic nerves originate from the thoracic
and lumbar vertebrae
 Parasympathetic nerves originate from the
cervical and caudal vertebrae

NATURAL AND ARTIFICIAL PAINKILLERS
The body produces its own natural painkillers in
response to injury
 Endorphins and enkephalins are manufactured
in the brain
 Specialized cells called SG (substantia gelanosa)
cells produce a transmitter chemical that signals
that damage or injury has occurred

 The
endorphins and
enkephalins fit into
receptor sites on the SG
cells, reducing the
amount of transmitter
that is produced
 Opitates such as heroin,
morphine and its
derivatives have a shape
that is similar to the
body’s nautral painkillers
Endorphin structure
Morphine structure
www.bio.davidson.edu
 As
a result, opiates can also fit into the
receptor sites that are usually used by
endorphins
 However, the use of opiates reduces the
body’s production of the natural
endorphins
 Therefore, after the opiate breaks down,
there is little or none of the natural
painkiller being produced
 This results in a return of pain, often
perceived as being greater than the pain
associated with the original injury
ACTIVATING YOUR NATURAL PAINKILLERS
A
number of different stimuli (not
necessarily all extremely painful) will
release endorphins and other similar
chemicals:
 Acupuncture
 Consumption of capsaicin (the active
ingredient in chili peppers – this is
probably why I have hot sauce on
everything…)
 Strenuous exercise (although the chemical
released is actually anandamide – which
is related to the THC found in marijuana)
OTHER DRUGS…
 Depressants
such as Valium and Librium
will enhance the action of inhibitory
synapses
 This increases the production of the
inhibitory neurotransmitter, GABA
 Alcohol actually changes the neuron
membrane, and does not act as a
neurotransmitter – it increases the effect
of GABA
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