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Homeostasis


External environment changes drastically
Internal environment (inside our bodies)
must stay the same
Homeostasis is the
ability of an
organism to
maintain its internal
make-up.

For your internal environment to remain
constant you must have a
1. Monitor (detect the problem)
2. Control (fix it)

Example 1)

Oxygen concentration in blood.
 Monitor = Oxygen chemoreceptor in aortic arch
 Control = Medulla Oblongata

If not enough O2 in blood,
 chemoreceptor detects problem
 Medulla oblongata commands diaphragm &
intercostals to work harder
 Oxygen debt is soon solved
 chemoreceptor stops detecting problem
 Medulla Oblongata stops trying to fix it

our bodies use 2 mechanisms for
controlling our internal make-up.
1.
Nerves
 and the nervous system
2.
Hormones
 and the endocrine system
Nervous System

The human body is made up of two nervous
systems.

Central Nervous System
 CNS

Peripheral Nervous System
 PNS
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
Central Nervous System

Yellow

Brain and spinal cord.
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



Peripheral Nervous
System
Blue Green Red Orange
Connects the CNS with
the rest of the body
nerves that leave the
spine
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
PNS can be divided into:
Somatic
Nervous System
 voluntary control
 Nerves leading to skeletal muscles
 fast nerve transmission (myelinated neurons)
 Autonomic
System
Nervous
 Involuntary
 Automatic functions
 slower nerve transmission (unmyelinated neurons)
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 Autonomic


Nervous System
Controls things like heart rate, blood pressure,
breathing rate, digestion
Divided into two halves:
 Sympathetic
 Parasympathetic
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



Sympathetic
Nerves which
transmit impulses
during times of stress
Speed up functions
like blood pressure
and heart rate…
Nerves from spinal
cord




Parasympathetic.
Nerves which
transmit impulses
when the body is
attempting to return
back to normal after a
time of stress.
Slows heart rate and
blood pressure…
Nerves from brain.
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
General Properties of all Neurons
The basic unit of the nervous system is the neuron.
 A nerve is a bundle of these neurons.
 Neurons are specialized to conduct an electrical impulse.


All neurons have the same basic components:



Dendrite -- receive stimuli
Cell body -- contains nucleus and cell organelles
Axon -- long cylinder carrying impulse to next neuron or
to effector.
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

Axons can be up to 1 meter in length.
Sometimes, the axon of a neuron can have an
insulating cover called a myelin sheath
white
 made up of Schwann Cells
 speeds up impulse transmission
 appears like sausages
 the naked spaces of axon in between myelinated sections
(sausages) are called NODES OF RANVIER

 every 1 mm along the axon.
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

Neurons can be categorized based on their function
Sensory Neurons



Interneurons



Brings information to the CNS.
Located in ganglia next to the spinal cord in the dorsal
root.
Found only in brain and spinal cord (CNS).
Form link between sensory and motor neurons.
Motor Neurons

Carries impulses from CNS to effectors (muscles or
glands)
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



The somatic nervous system controls all voluntary
systems within the body except for reflex arcs.
These are what we call reflexes.
They protect the body quickly when presented with
a stimulus that the body perceives as being
dangerous.
Although reflex arcs send secondary signals to the
brain during the reflex action, the primary response
is "hard wired" through the spinal cord.
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
Certain stimuli, such as touching a hot surface,
cause a reflex arc

the nerve impulse travels
 up the sensory nerve
 through an interneuron in the spine
 down the appropriate motor nerves


result = jerk the hand away from the hot surface.
This automatic response system allows for
extremely fast reaction times.
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
A reflex arc must always include 5
components:
1.
Receptor (pain receptor…)
2.
Sensory neuron
3.
Interneuron
4.
Motor neuron
5.
Effector (muscles)
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



A nerve impulse can actually be thought of as an
electrical signal from one place to another.
This can be achieved by pumping charged particles
in and out of the neuron.
Sodium and Potassium play important roles.
There is lots of sodium and potassium around the
neuron.
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 In
reality, there is a proton
pump that will transport
potassium into the neuron
and sodium out of the
neuron.
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 There
are also channels that
will allow certain ions to
diffuse back across the
membrane of the neuron.
 Potassium
Channels
 Sodium Channels
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 The
Goal…
 To have a charge difference across the
membrane.
 The electrical potential
 This is achieved by having one
channel open and the other closed.
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
At rest, the inside of the neuron is negative relative
to the outside




inside is said to be -70 millivolts (mv)
The neuron is POLARIZED.
It is polarized because the inside of the neuron and
the extracellular fluid are oppositely charged.
When electrical charges are separated in this way,
they have the potential to do work should they be
permitted to come together.
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Resting Potential Summary


Polarization occurs because of the unequal ion
distribution.
Polarization is mainly due to 3 main factors:
1.
2.
3.
The outward diffusion of potassium ions
The sodium pump is actually slightly more efficient than
the potassium pump (more sodiums out than
potassiums in – approximately 3:2)
The presence of large diameter negatively charged
anions that are stuck inside the neuron.
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
Electrical, chemical, or mechanical stimulus will
alter the resting potential by causing sodium to leak
back into the neuron.




“Sneaks” through the sodium channel.
This changes the polarity slightly
If the stimulus is strong enough to bring the inside
to about -55 mv, a THRESHOLD has been reached.
Once this occurs, the sodium channels immediately
open wide and potassium channels close.
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
The rapid influx of sodium causes a momentary
reversal in polarity .





Depolarization
Membrane potential shoots to about +40 mv.
This sharp rise and fall of action potential is called a
spike.
This could be described as a slight electrical
disturbance in the neuron.
The action potential is an electric current strong
enough to induce the collapse of the resting
potential in the adjacent area of the neuron.
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
As the wave of depolarization moves
along the axon, the normal polarized
state is quickly reestablished behind it.
 Must
get back to Positive outside and
Negative inside.
 Sodium channels close, and potassium
channels open.
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The
sodium and
potassium pumps
soon reestablishes
resting potential ion
separation.
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
The amount of time it takes to repolarize is called
the refractory period



approximately 1 ms (millisecond)
During this time the axon cannot transmit an action
potential no matter how great the stimulus.
During repolarization, the rapid pumping of
sodium causes a momentary hyperpolarization.



Too much positive outside
Inside is about - 85mv
Resting state is established when potassium re-enters the
neuron.
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+ 40
0
-55
-70
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+ 40
0
-55
-70
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
Neurons and Impulse Transmission

Follow the “all-or-none” principle
 a stimulus above the threshold level, whether strong or
VERY strong produces the same strength of signal
transmission.
More stimulus (i.e. more painful) = more impulses
generated, NOT a stronger impulse.
 An impulse does not diminish in strength as it travels
along a neuron.

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

We already know that having a myelin insulation
on an axon will speed its impulse transmission.
This is because the impulse will jump from node to
node.


In this way, sodium and potassium do not have to
undergo exchanges along the entire length of the axon
Sodium and potassium pumps and channels are active
only at each NODE OF RANVIER.
 This is where the axon can actually exchange ions with the
extra cellular fluid.
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Getting the impulse
from one neuron to the next.

Adjacent neurons in a nerve fiber do not actually
touch end to end.
The junction between them is called a synapse.

The gap is called a synaptic cleft.

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
The gap between the terminal axon of one neuron
and the dendrite of the next is about 0.02 m.



one millionth of an inch
When the nerve impulse (depolarizing wave)
reaches the synaptic knob, it must jump to the next
neuron.
impulse moves from axon of a pre-synaptic neuron
to dendrite of a post-synaptic neuron.
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
Each synaptic knob (terminal axon) has vesicles
that will produce and secrete neurotransmitters.

When the impulse reaches the synaptic
knob, the membrane surrounding the knob
becomes permeable to calcium.

The calcium causes the vesicles to fuse to
the membrane of the knob emptying their
neurotransmitters into the synaptic cleft.
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

A neurotransmitter is a chemical that will signal the
next neuron’s dendrites to send an impulse down
its axon.
Important neurotransmitters are…

acetylcholine -- most common
 neuromuscular junctions, brain, internal organs
 usually has an excitatory effect on post-synaptic dendrite.

noradrenalin (norepinephrine)
 same thing but involved in the sympathetic nervous system
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HOW DOES IT REALLY HAPPEN??
 Acetylcholine is released from
vesicles in the pre-synaptic axon

 it
diffuses across the synaptic cleft
 it lands on receptor sites on the postsynaptic dendrite
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 Its
purpose is to partially
depolarize the membrane of
the post-synaptic dendrite.
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
This excites the post-synaptic neuron
 sodium channels on post-synaptic dendrites open
 depolarization (charge reversal)
 action potential is achieved
 wave of depolarization spreads across post-synaptic neuron

Problem: If acetylcholine remains in the receptor site, the
sodium channels will remain open
 repeated stimulation of muscle

Solution: Cholinesterase (an enzyme released into
synaptic cleft) breaks down acetylcholine.
 Once sodium channels close, the neuron begins recovery.
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
Troubles…
 Nerve gas inactivates cholinesterase.
 the amount of acetylcholine in synaptic
cleft increases with each successive nerve
impulse
 repeated stimulation of muscle
 life-threatening spasms
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
The acetylcholine from one axon
terminal is usually not enough to
cause depolarization of the postsynaptic neuron.
 Usually,
neurotransmitters from a few
different pre-synaptic knobs are
needed to induce an action potential
 This is known as summation.
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
Some neurotransmitters are not excitatory but rather
inhibitory.

These cause post-synaptic potassium channels to open
fully

Hyper-polarization
 more potassium will flow out (diffusion)
 more positive charges outside neuron
 Resting potential is now even more negative
 Need higher stimulus to overcome threshold and initiate
action potential
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
M.S.
Multiple Sclerosis
 Deterioration of the myelin sheath
 scar tissue on axon
 no impulse transmission
 impaired neural function

 loss of coordination
 tremor
 paralysis
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
Nerve Damage due to injury



Parkinson’s Disease



If damaged neurons are covered by the thin membrane
called neurilemma, regeneration is likely (only in
peripheral nervous system)
No neurilemma = no chance of regeneration
Involuntary muscle contractions
Insufficient production of dopamine (a neurotransmitter)
Alzheimer’s Disease


loss of memory
decreased production of acetylcholine
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The Central Nervous
System
The Peripheral Nervous
System
Ear
Eye
-Identify the principal structures of the CNS
and PNS
-Explain their functions in regulating
somatic and autonomic systems

Spinal Cord




runs down neck and
back inside the spine
receives information
from skin & muscles
sends motor
commands for
movement
controls reflex
activities

Brain


more complex
functions
coordination of
 homeostasis
 perception
 movement
 intellect
 emotions
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
Spinal Cord

Structure
 White matter
 outer layer
 consists of motor and sensory axons
 myelinated
 Grey matter
 inner layer
 contains cell bodies of motor neurons
and interneurons.
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 Central cavity
 contains cerebrospinal fluid
 Sensory Neurons
 pass through dorsal root
 have cell bodies in the dorsal root
ganglion
 Motor Neurons
 leave the spinal cord through the ventral
root.
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 31 pairs of mixed nerves
nerves that contain both
motor and sensory neurons
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



3 pounds
one of the largest organs in body
soft
squishy
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
Protected by 3 layers of membranes
called Meninges
Dura Mater -- outer (next to skull)
 Arachnoid -- middle
 Pia Mater -- inner


Infection (and swelling) of meninges
is called …
MENINGITIS
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

Surrounds the brain
Lies in between the Pia Mater and the
Arachnoid layer.




Shock absorption
brings nutrients, hormones and WBC to
parts of brain
Circulates between meninges and central
canal of spinal cord
Drains into veins
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 Doctors
drill into spine
 Extract cerebrospinal
fluid
 Test for infection
 Meningitis
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Divided
into
Forebrain
Midbrain
Hindbrain
Hindbrain

Forebrain
 Controls
 pattern & image formation
 memory
 learning
 emotions
 Includes:
 Thalamus
 Hypothalamus
 Cerebrum

Relay center for information on its
way to cerebrum

sensory information is sorted out

sent to appropriate higher brain
centers

Regulation of homeostasis
 source of hormones (ADH, Oxytocin - uterine
contractions)
 pituitary gland is connected to hypothalamus
thermostat
 hunger
 thirst
 sexual response
 mating behaviors
 fight-or-flight response
 pleasure / rage
 biological clock

 when we sleep
 when our sex drive peaks
 2 cerebral hemispheres
 left -- controls the right side of body
 Logical thought
 right -- controls the left side of body
 Creative thought
 The two halves of the brain
communicate with each other
through the corpus callosum
 a thick white band of fibers
 Outer gray matter is called cerebral
cortex
 2 cerebral hemispheres
 left -- controls the right side of body
 right -- controls the left side of body
 communicate with each other through the
corpus callosum
 Outer gray matter is called cerebral cortex
 each hemisphere is divided into 4 lobes

4 lobes in each hemisphere
 frontal -- near the forehead
 speech, personality, precise movements
 temporal -- by your ears
 hearing
 smell
 parietal-- top of your head
 taste
 reading
 body position
 occipital -- back of your head
 vision
Largest part
of the brain
 most complex
 different parts of the cortex are
in charge of different parts of
the body.
Hindbrain

Small region

relay center between
 forebrain & hindbrain
 forebrain & eyes

Vision


vision is controlled in the forebrain
vision reflexes & some perceptual functions are
controlled here
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Hindbrain

Called the “brain stem”


lower brain
Coordinates large-scale body movements like
walking.
 Medulla Oblongata
 Pons
 Cerebellum

Autonomic & homeostatic functions
breathing, heart rate, blood pressure,
vasoconstriction/vasodilatation,
swallowing, digestion, vomiting …
 Here, motor axons from the mid and
forebrain cross from one side of the
CNS to the other SIDE!!!.

 as a result, right side of brain controls left
side of body, and visa versa.


just above medulla oblongata
relay center between the
cerebellum and the cerebral
cortex




Smooth coordination of movement
Hand-eye coordination
Balance
Organizes information about



position of joints
length of muscles
visual & auditory activity

Nerves that leave the brain and spine

Cranial nerves
 nerves that leave the brain
 Lead to organs of the head & upper body
 parasympathetic system
 Vagus Nerve = important cranial nerve

Spinal nerves
 nerves that leave the spine
 Lead to the whole body
 sympathetic system
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
When both the sympathetic and
parasympathetic systems innervate the
same effector, they are called
antagonistic.

I.e. Heart
 Sympathetic = speed up
 Parasympathetic = slow down
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