Chapter 17
The nervous system
The Nervous System
• Nerves communicate with each other by
sending chemical and electrical signals to each
other.
• Nerve receptors (part of your peripheral
nervous system PNS) collect information and
send it to your central nervous system (CNS)
(which is your brain and spinal cord). This is
then sent back to the PNS so your body can
respond
Neuron structure
• Dendrites  send signals toward the cell body
• Cell body  contains all the cell organelles and the
nucleus
• Axon  conducts nerve impulses along its length
• Information always moves
dendrites cell body  axon
• Axons are filled with a fluid called axoplasm. It
is slightly different from normal cytoplasm.
Axoplasm is filled with molecules that conduct
electricity.
• Axons are also surrounded by an
axomembrane. Very similar to a regular cell
membrane, but it has a lot of sodium and
potassium pumps.
3 Types of Neurons
• Sensory Neuron  part of PNS. Has receptors. This
sends a signal to the interneuron
• Interneuron  part of CNS. Receives information
from the PNS or from other parts of the CNS. It will
transmit information to other CNS parts or to motor
neurons.
• Motor Neuron  part of PNS. Receives messages
from the CNS to perform an action.
Myelin Sheath
• Axons are covered by a protective myelin sheath.
• The myelin sheath is actually made from other cells
called Schwann Cells.
• This protects the axon and also makes signals travel
faster down the axon.
• Between the myelin sheaths are spaces. These
spaces are called the Nodes of Ranvier
Nerve Impulse
• A nerve impulse is how neurons transmit
information
• Step 1. when the axon is not sending a
message the inside of the axon is negative
compared to the outside the axon. This is
called the Resting Potential
• The potassium K+ has greater concentrations
inside the axon than outside the axon.
• The sodium Na+ has a greater concentrations
outside the axon than inside the axon.
• The reason why there is a difference between the
inside and the outside of an axon is because of
sodium-potassium pumps. These pumps use ATP to
and actively move potassium and sodium against a
concentration gradient
• Resting potential
Step 2: Action Potential
• This is a rapid change in polarity across the
axomembrane.
• Sodium gates open first and Na+ flows into the
axon. This causes the axon to become depolarized.
Going from negative to positive.
• This changes the charge of the axon from -65mV to
+40mV
• Potassium gates then open and K+ flows from the
inside to the outside of the axon. This causes it to
become repolarized. Or for the axon to go back to
being -65mV
Step 3: Propagation of an action
potential
• This means how to make it keep moving down the axon
• When an action potential reaches a large enough
depolarization it is called a threshold
• This will make the next section undergo depolarization.
• After a section has done its action potential then there
is a short period of time for the sodium and potassium
pumps to reset. This is called the refractory period. It
makes sure that the action potential can only move in
one direction.
• This only occurs at the nodes of Ranvier, and
when this action potential reaches the end of
the node of Ranvier it is able to induce an
action potential at the next node of ranvier.
• This is how the mylenated sheath increases
the speed of a signal.
• The electricity that is generated at one node
of Ranvier is enough to excite the next node of
Ranvier. This is how mylenated sheaths speed
up the signals.
• This is called saltatory transmission
Transmission across a Synapse
• Axon branches and ends in an axon bulb.
• The axon bulbs come into close contact with the
dendrites of the next neuron.
• The membrane of the axon bulb is referred to as the
presynaptic membrane.
• The membrane of the dendrite is referred to as the
postsynaptic membrane.
• The space between the axon bulb and the dendrite
is called the synaptic cleft.
The Threshold
• If a nerve impulse is above the threshold then
the impulse and the message will be sent. If it
is below the threshold then it won’t be sent.
• This is called the “all or none” response
– If its above the threshold then it is “ALL”
– If its below the threshold then it is “None”
How does a message cross the
synapse?
Step 1: Nerve impulse reaches the axon bulb, and this
causes the calcium gates to open and Ca2+ ions
move into the axon bulb.
Step 2: This causes vesicles carrying a chemical called
neurotransmitters to merge with the presynaptic
cleft and release its neurotransmitter in the
synaptic cleft.
Step 3: the neurotransmitter moves across the
synaptic cleft and binds with a receptor on the
dendrite.
Neurotransmitters
• These are chemicals that cross the synaptic cleft.
• There are many types of neurotransmitters.
• Acetylcholine and norepinephrine are two
examples of neurotransmitters.
• These bind with the receptor on the post-synaptic
membrane. An enzyme takes the neurotransmitter
out of the receptor.
• Acetylcholinesterase is an enzyme that removes
acetylcholine
The Reflex Arc
• This refers to the reaction to a stimulus that is
painful or scary or that surprises you.
• Sensory neuron receives a stimulus and sends the
signal to the interneuron in the spinal cord. This
makes a decision and sends a message down your
motor neuron to an effector muscle.
Drugs. Are they a Disease or a Choice?
• What is a disease?
• A disease is any disturbance or anomaly in the
normal functioning of the body that probably has a
specific cause and identifiable symptoms.
• People who take drugs can quit anytime, or can
they?