Transcript File

The Nervous System
A system of the body that coordinates & regulates
the activities of the body
Control System
Maintains homeostasis
 homeo/stasis (same/changing): changes in
order to keep a balance


Feedback
◦ Negative feedback: to reverse a current trend
◦ Positive feedback: amplifies a current trend
5 major components
Stimulus
receptor
-highly specific
-receive stimuli
Sensory Pathway
modulator/regulator
-Selects appropriate
Response (spinal cord
or brain)
Motor Pathway
effector
-carries out the response
(muscle or gland)
Action
The Nervous System & Homeostasis
◦ Organization of the Nervous
System
◦ Structure of a neuron
◦ Action Potential
◦ Synaptic Transmission
◦ Structure of the Brain
◦ Senses: The Eye & Ear
Nervous System
Organizational Tree!
Nervous System
Central Nervous System
(CNS)
Decision maker
Brain & Spinal Cord
Peripheral Nervous
System (PNS)
Feeds into & out of CNS
Sensory Pathway
Somatic Pathway
(Voluntary)
under conscious control
Examples?
NS Overview
Sympathetic
(Stimulatory)
Speeds you up!
Excites you!
Motor Pathway
Autonomic Pathway
(Involuntary)
unconscious control
Examples?
Parasympathetic
(Restores to normal)
Restores balance!
Restores Homeostasis!
Anatomy of a Nerve Cell

Two different types of cells are found in
the nervous system:
◦ Glial Cells: non-conducting; important for
support and metabolism of nerve cells
◦ Neurons: function units of the nervous
system (conduct nerve impulses)
The Neuron: Wires within a nerve!
See diagram on p.410 of text
The Neuron
Dendrite – receives information from receptors or other neurons and conducts
nerve impulses toward the cell body.
Synaptic Knob – aids in nerve impulse transmission
Cell Body – contains nucleus and organelles
Nodes of Ranvier – gaps within the myelin sheath
Impulses jump from node-to-node therefore speeding
up the impulses
Neurillemma – delicate membrane that promotes regeneration of
damaged neurons
Only found in myelinated neurons
Myelin
– a fatty
that
Axon – conducts nerve
impulses
away protein
from the
cellcovers
body the axon
Composed of Schwann cells, which help regenerate
damaged neurons
Insulate the axon allowing nerve impulses to travel faster
Myelination is only found outside the brain and spinal cord
Structure of a Neuron
From Nelson Biology
The Job of Schwaan
cells = Wrap Axons!
Schwaan cells:
 Nourish the
axon
 Provide
insulation
 Repair axon
damage
 Found mainly in
PNS

Neurilemma =
•concentric rings around axon
•created by the Schwaan cell
• repair mechanism
Job of Myelin!
Provide insulation
like the covering on
speaker wire
 Prevent loss of signal
down axon!
 Damaged myelin
results in a loss of
signal down the
axon! (Results in)
 Multiple Sclerosis
(MS)

Types of Neurons – see your handout!

Motor Neuron
◦ Connects the central nervous system to a
muscle or a gland (also called efferent neurons)

Sensory Neuron
◦ Connects a sensory receptor to the central
nervous system (also called afferent neurons)

Interneuron (or Association Neuron)
◦ Connects two or more neurons
Types of Neurons
Sensory Neurons
(Afferent Neurons) –
conducts nerve impulse
from sense organs to the
brain and spinal cord
(CNS)
Interneuron
(Association Neuron) –
found within the CNS
No myelination
Intergrates and
interprets sensory
information and relays
information to
outgoing neurons
Motor Neuron
(Efferent Neurons) –
conducts nerve impulses
from CNS to muscle fiber
or glands (effectors)
To Do:
Complete Sections A thru C in your
Notes Package
 Color and Label Neuron Diagram

Action Potential
how do nerves work???

In 1900 Bernstein hypothesized that
nerve impulses where electrochemical in
nature.
◦ Future experimentation proved this.

Giant Squid Experiment:
◦ Cole and Curtis placed two tiny electrodes –
one inside the large axon of a squid and the
second across from the first outside the axon.
Giant Squid Experiment
Squid Axon
•Cole and Curtis measured the electrical potential
across the membrane.
•The resting potential was found to be about –
70mV.
When stimulated, the action potential
jumped to about +40 mV.
 The action potential only lasted for a few
milliseconds before the nerve cell returned
to the resting potential.

+40
threshold
mV
-70
1
2 ms
3
4
Definitions:

Action Potential:
◦ the voltage difference across a nerve cell
membrane when the nerve is excited
(~40 mV)

Resting Potential:
◦ Voltage difference across a nerve membrane
when it is NOT transmitting a nerve impulse
(almost always -70 mV)
Maintaining Resting Potential
Caused by an uneven distribution of positively
charged ions across the membrane
 Set up and maintained by a Sodium-Potassium
pump.
 3 Na+ are pumped out of the cell, 2 K+ ions are
pumped into the cell.

sodium/potassium ion pump
sodium/potassium pump 2
These positive ions want to move with
their concentration gradient by diffusion.
 Sodium moves out faster than potassium
moves in leaving a “relative” negative charge
inside the cell.
 The cell is polarized.


resting potential clip
Action Potential
A Nerve impulse is an Action Potential
 When a neuron receives a stimulus it becomes
more permeable to sodium than potassium

◦ When stimulated the ion gates for potassium close and
the ion gates for sodium open up.

Positive ions flood into the cell making it positive.
This rapid inflow is referred to as
depolarization.

After the impulse, the Na+ channels close
and the K+ channels open. This is called
repolarization.
◦ The flow of potassium ions out of the cell (with
their concentration gradient) restores the resting
potential.


The potassium gates close relatively slowly
which makes the inside of the neuron slightly
more negative then resting potential
(hyperpolarization)
The Na+/K+ pump continues to pump the
sodium and potassium across the membrane
against the concentration gradient to restore
the resting potential.
Refractory Period:
Repolarization takes about 0.001 seconds.
 Once stimulated, the membrane cannot be
depolarized until after the refractory
period.

◦ Recovery time required before a neuron can
produce another action potential.
Summary of Impulse.
1. At rest – Na+/K+ pump moving
2. Stimulation – sodium gates open
3. The flood of sodium into the cytoplasm stimulate
adjacent areas
4. Refractory – potassium gates open – sodium gates
close
5. At rest – Na+/K+ pump moving ions – potassium
gates open
Action potential overview
Electropotential graph
Movement of Action Potential
Many action potentials are generated one after
another along the cell membrane, causing a wave
of depolarization (similar to falling dominos).
 When axons are myelinated, nerve impulses travel
by saltatory conduction

◦ Gated ion channels are concentrated at the nodes of
Ranvier
◦ Flow of ions across cell membrane can only happen at
the nodes so action potentials “jump” from node to
node
◦ This causes the signal to be transmitted down an
axon much faster.
myelinated vs. unmyelinated impuse
To Do:
Read pages 415 – 419 in your textbook
 Complete Section D in your notes
package
 Nerve impulse coloring diagram


http://www.mcgrawhill.ca/school/applets/abbio/ch11/actionpotential_action.swf
Reflex Arc
A reflex that does
not require the brain
 Interneuron sends
message back on the
motor neuron at the
same time as it sends
the message to the
brain
 Reflexes may be
innate or acquired

Reflex Arc

Reflexes are autonomic responses to
certain stimuli

They are not under conscious control,
they are involuntary

They pathway that a nerve impulse takes
is called a reflex arc
◦ We need to identify the stimulus, receptor,
sensory neuron, motor neuron, effector, and
the response.
Anatomy of a Reflex Arc
How the Reflex
Arc Functions:
1) Sensory organs
(receptors) detect
dangerous stimuli!
2) Impulse is passed
from the sensory
organ to a sensory
neuron!
3) Sensory Neuron
transfers the impulse
to the Association
neuron in the spinal
cord!
How the Reflex
Arc Functions…
4) The INTERNEURON
links the SENSORY
to the MOTOR
neuron!
5) The MOTOR neuron
takes the impulse to
the EFFECTOR!
6) The effector (usually
a muscle) reacts.
7) Simultaneously,
interneurons send
the signal up to the
BRAIN for
interpretation!
Reflex Arc Video
To Do:
Reflex Lab
 Section E in your notes package

◦ P. 414 Questions # 2-6
Synaptic Transmission
Neurons are not
directly connected to
each other.
 The electrochemical
action potential cannot
jump the synaptic cleft
(or synapse).
 Synaptic transmission
is entirely chemical in
nature.

Synapse movie clip
Synapse
At the end of axons, tiny synaptic vesicles
contain neurotransmitters
 When an impulse reaches the end of an
axon, these synaptic vesicles migrate
toward the end of the axon
 They then release their neurotransmitter
and it diffuses across the synaptic cleft

Synapse
Neurotransmitters attach to specific
receptor sites and causes sodium
channels to open resulting in a
depolarization in the membrane.
 An action potential is created and the
impulse travels down the neuron.
 Diffusion takes time, so the more
synapses involved, the slower the
response.

Synapse



Synaptic transmission can only occur in one
direction.
Since only presynaptic neurons contain
synaptic vesicles, and only post synaptic
neurons have receptor sites for them, the
messages cant be sent in the other direction
This explains why impulses can only travel
from sensory neuron to interneuron to
motor neuron and never in the other
direction
Figure 10(b), pg. 420
Synaptic vesicles in the end
plate of the presynaptic
neuron release
neurotransmitters into the
synaptic cleft. The
neurotransmitters attach
themselves to receptors
on the postsynaptic
membrane, causing it to
depolarize. The action
potential continues along
the postsynaptic neuron.
Synapse flash animation
 Synapse action at muscular juction
 Action Potential Detail


To Do:
◦ Relflex/Synpase Worksheet
Neurotransmitters

Acetylcholine (Ach):
(excitatory – passes message along)
◦ neurotransmitter produced in the presynaptic knob and stored in vesicles.
◦ when an action potential reaches the presynaptic knob the vesicles rupture releasing
their contents (acetylcholine) into the
synaptic cleft
◦ The acetylcholine diffuses across the synapse
and binds to receptor sites on the postsynaptic knob
Neurotransmitters

How do we stop the message?
◦ Before another message can cross the cleft, it
must be cleaned (remove the
neurotransmitter)
◦ The enzyme acetyl cholinesterase removes
acetylcholine from the receptor sites and
breaks it into acetic acid & choline
◦ the acetic acid & choline are reabsorbed into
the presynaptic knob to be reused
Neurotransmitters
• Not all neurons cause depolarization in the post
synaptic membrane. Some neurons are inhibitory.

Neurotransmitters can be:
◦ excitatory - passes along message to the next
neuron, or
◦ Inhibitory – binds to the next neuron and
inhibits the message from being passed on
See figure 11 p. 422
Neurotransmitters

Often it takes more than one neuron
releasing its neurotransmitter into the
synaptic cleft to elicit a response in the
post synaptic neuron.
◦ This is referred to as SUMMATION
Figure 11, pg. 422
Action potentials must occur
simultaneously in A and B to
reach the threshold in D.
Types of Neurotransmitters:

See handout
13.2 Summary Electrochemical Impulse
• Nerves conduct electrochemical impulses from
the dendrites along the axon to the end plates of
the neuron.
• Active transport and diffusion of sodium and
potassium ions establish a polarized membrane.
• An action potential is caused by the inflow of
sodium ions.
• Nerve cells exhibit an all-or-none response.
• Neurotransmitters allow the nerve message to
move across synapses.
To Do:
Reflex/Synapse Worksheet
 Complete section F in your notes package
 Case Study “Drugs and the Synapse” p.
423-424 of text


For Extra Practice:
◦ Pg. 418 # 1-4
◦ Pg. 420 # 5-7
◦ Pg. 425 #3-7