Download 6.5 Nerves, Hormones and Homeostasis part 1

Outcomes… Part 1…
6.5 Nerves,
Hormones and
Homeostasis
IB Biology SL
Part 1 - Nerves
Imagine
you are
on a beach. What
do you feel? What
do you see?
Smell?
You can recreate
the experience of
being at the beach
without any
external stimuli.
How is this
possible?
the brain, and from the brain only,
arise our pleasures, joys, laughter and
jests, as well as our sorrows, pains griefs
and tears. Through it, in particular, we
think, see, hear... Eyes, ears,
tongue, hands and feet act in accordance
with the discernment of the brain.”
6.5.1State that the nervous system consists of the
central nervous system (CNS) and peripheral nerves,
and is composed of cells called neurons that can carry
rapid electrical impulses.
6.5.2Draw and label a diagram of the structure of a
motor neuron.
6.5.3State that nerve impulses are conducted from
receptors to the CNS by sensory neurons, within the
CNS by relay neurons, and from the CNS to effectors by
motor neurons.
6.5.4Define resting potential and action potential
(depolarization and repolarization).
6.5.5Explain how a nerve impulse passes along a nonmyelinated neuron.
6.5.6Explain the principles of synaptic transmission.
What
is pain? What is pleasure? What are
thoughts?
We know the brain is made up of cells but
how does the miracle of the mind emerge
from this mass of cells?
The human Nervous System is a whole
that is far greater than the sum of its parts.
“...from
Hippocrates
Much
of what we know about the
brain is drawn from inferences.
There remain many unanswered
questions... This makes
neuroscience so fascinating!
1
The Central Nervous System
The
nervous system consists of two parts, the
central nervous system and peripheral
nervous system.
The Central Nervous System
The
central nervous system (CNS) consists
of the brain and spinal cord.
Both structures receive sensory information
from receptors all over the body and they
interpret the information, process it and
decide if a response is required.
A response by the
brain or spinal cord
is known as a
motor response.
The
The Peripheral Nervous System
The
peripheral nervous system includes all
other nerves that connect to the CNS.
Peripheral nerves are composed of sensory
neurons and motor neurons.
Neurons
The
nervous system is made up of special
nerve cells called neurons.
These cells are very different in shape from
other eukaryotic cells and they transmit
messages in the form of electrical impulses at
incredible speed.
Many neurons grouped together form a nerve.
peripheral system has two categories
of peripheral nerves.
There are 31 pairs of spinal nerves which
emerge directly from the spinal cord and
are a mix of sensory neurons and motor
neurons.
It also has 12 pairs
of cranial nerves
which emerge from
the brain stem.
Structure of a Motor Neuron
2
Cell
body: Contains the nucleus, rough ER
and other organelles.
Axon:
Dendrites:
Myelin
Schwann
Nodes
much smaller cytoplasm
extensions which carry impulses to the cell
body.
Cells: special type of glial cells
that produce the myelin sheath.
a very long and thin extension of
cytoplasm from the cell body. Carries
impulses away from the cell body.
Sheath: a type of insulation that is
wrapped around the axons of neurons;
composed of Schwann cells. Helps reduce
signal loss.
of Ranvier: regularly occurring
gaps between sections of myelin sheath
where the axon is “naked”.
3
Relay
neurons carry impulses within the
CNS itself from neuron to neuron; also
known as interneurons or association
neurons.
Nerve Impulses
Sensory
neurons carry impulses
(information) from sensory cells in the body
(receptors) to the CNS; also known as
afferent neurons.
Motor
neurons carry impulses away from
the CNS to the effectors (muscles and
gland cells); also known as efferent
neurons.
Reflex Arc
neurons, relay neurons and motor
neurons are involved in a neural pathway
known as a reflex arc which allows for a quick
reaction.
Resting Potential
Sensory
All cells have an electrical potential difference (voltage)
across their plasma membrane that is measured in mV
and the difference is known as the membrane potential.
Neurons typically have
a membrane potential
between -60 and -80 mV
(millivolts) when the cell
is not transmitting a
signal and this is called
the resting potential.
4
Resting
potential can be defined as the
electrical potential across the plasma
membrane of a cell that is not sending an
impulse.
The resting potential of all neurons
depends on the ionic gradients that exist
across the plasma membrane of the cell.
NOTE: Sodium ions are highly concentrated outside
of the cell and they have a tendency to diffuse inside
the cell. Potassium ions diffuse out of the cell as
sodium diffuses in but the membrane is about 50
times more permeable to potassium than sodium so
the movement is unequal. The sodium-potassium
pumps use active transport to control the movement
of these ions.
When an impulse is passing along a neuron the
potassium and sodium ions are allowed to diffuse
(passive transport) across the membrane through
proteins known as voltage-gated ion channels.
This reverses the
electrical potential
of the neuron but it
is quickly restored
and this is the action
potential.
The neuron is
depolarized and
then quickly
Neurons use active transport to maintain a balance
of ions across their membranes.
Sodium ions are pumped out and potassium ions
are pumped in.
There are chloride ions, DNA and other negatively
charged ions inside the neuron that are fairly large
and have a tendency to stay inside which creates a
net negative charge inside the neuron as compared
with the more positive external environment.
This creates the resting potential and the membrane
is said to be polarized.
Action Potential
Action potential can be defined as the reversal and
restoration of the electrical potential across the plasma
membrane of a cell as an electrical impulse passes along
it.
It is also measured in mV
Depolarization
Depolarization is the diffusion of sodium ions into the
nerve cell resulting in a charge reversal (the inside
becomes more positive).
repolarized.
5
Repolarization
Repolarization
is
the process of
restoring the original
polarity of the nerve
membrane.
Nerve Impulses
Understanding of how an action potential works is the
key to understanding how a nerve impulse passes along
the axon of a neuron.
An action potential in one part of a neuron will cause the
development of an action potential in the next section of
the neuron.
This can occur because sodium ions flow from a region
with an action potential to a region with a resting
potential.
As the ions move the resting potential is reduced which
results in the opening of the voltage-gated channels.
When a neuron is excited the membrane becomes
more permeable to sodium than potassium.
Scientists believe this occurs because sodium gates
open while potassium gates close.
The membrane potential is reduced and more sodium
channels open.
As sodium ions flow into the neuron via diffusion and
charge attraction the inside of the membrane
becomes positive (charge reversal) and
depolarization occurs.
The membrane potential has been reversed.
6
Once depolarization occurs the sodium gates are
closed and the potassium channels open.
Potassium diffuses out of the neuron in the direction
of the concentration gradient.
The loss of the positive potassium ions causes the
internal environment of the neuron to become
negative once again and the potential across the
membrane is restored.
This is known as repolarization; the return to the
original polarity of the nerve membrane.
The action potential moves along the membrane of the
neuron creating a wave of depolarization and
repolarization.
As the impulse moves along the axon of the neuron it
moves from a depolarized region and initiates
depolarization in the next region.
Sodium-potassium
pumps are used to
restore the concentration gradients of the
ions via active transport.
Sodium is pumped back out of the neuron
while potassium is pumped back in.
The resting potential
of the neuron is now
restored and can
now conduct
another impulse.
Interactive
Action Potential
Animation:
http://outreach.mcb.harvard.
edu/animations/actionpotenti
al.swf
Review
of Action
Potential:
http://www.youtube.com/
watch?v=HnKMB11ih2o
&feature=related
7
Synaptic Transmission
At the end of the axons
of neurons there are
swollen membranous
areas called terminal
buttons.
Inside the terminal
buttons are small
vesicles filled with
chemicals known as
neurotransmitters.
Synapse
A synapse is a junction between two neurons and the
plasma membranes of those neurons are separated by a
narrow fluid-filled gap known as the synaptic cleft.
Synaptic transmission is the transmission of an action
potential across the synapse from the presynaptic neuron
to the postsynaptic neuron.
The following sequence
of events outlines the
steps in synaptic
These
chemicals are used for synaptic
transmission and they always pass in the
same direction; from the presynaptic
neuron to the postsynaptic neuron.
1.
The nerve impulse (action potential)
arrives at the end of the presynaptic
neuron.
transmission.
2.
Depolarization of the membrane
occurs and the voltage gated calcium
channels open allowing calcium ions
diffuse into the terminal button.
3.
The movement of the calcium ions in
causes the vesicles containing the
neurotransmitter to fuse with the plasma
membrane of the neuron and release the
neurotransmitter into the synaptic cleft.
8
4.
The neurotransmitter diffuses across
the cleft from the presynaptic neuron and
binds to receptors (transmitter-gated ion
channels) in the postsynaptic neuron.
neurotransmitter
in the cleft is quickly
broken down by
enzymes to prevent
continuous synaptic
transmission and the
calcium ions are
pumped out of the
presynaptic neuron
and into the synaptic
cleft. The ions
channels close to
sodium ions.
5. The binding of the neurotransmitter results in the
opening of the ion channels and sodium ions along
with other positively charged ions diffuse into the
postsynaptic neuron.
6. The movement of the ions causes the
depolarization of the postsynaptic membrane and this
initiates the movement of the action potential down
the neuron.
The
Let’s have a look…
8.
The fragments
of the broken down
neurotransmitters
diffuse back across
the synaptic cleft and
into the vesicles
where they can be
reassembled.
And now for some practice IB
questions…
Synaptic Transmission animation
http://www.sumanasinc.com/webcontent/animati
ons/content/synaptictransmission.html
Another one
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapte
r45/animations.html#
9
Draw and label a motor neuron showing the direction of
nerve impulse propagation (3)
Explain how a nerve impulse travels along an unmyelinated
neuron (8)
You could also draw a diagram to support your answer and help
you to explain!
Explain the principles of synaptic transmission (8)
Outline the use of four methods of membrane transport in
nerves and synapses (8)
10