Transcript The Nervous System - biology-rocks

Quick Review
What’s another name for neurons?
Can you name the parts of a neuron?
Review Questions
How do nerve cells communicate with each
other?
Nerve cells are also called ____________
How many types of neurons are there? Which
nervous system are they related to?
What is a resting potential?
Taking a closer look at
the AXON
nerve impulses
-- unmyelinated axons  1 m/s
- myelinated axons  100 m/s.
Myelin
Sheath
A protective layer that wraps around the axon
Allows impulse to travel faster
Made by Schwann cells
“Schwann cells myelinate parts of an axon”
Gaps are called Nodes of Ranvier
Myelin Sheath
in PNS
Gives fibers their white glistening appearance
Myelin serves as an excellent insulator
Regenerate nerves

Damaged axon may use myelin sheath as a passageway
to regenerate new fiber
in CNS
Nerve regeneration does not occur
Myelinated Axon – in CNS
2 types of nervous tissue (Made up of many neurons):
Gray matter
White matter
-gray because it contains
cell bodies + short
-Not myelinated
-white because it contains
myelinated axons
-Carries messages from
one part of the CNS to
another
-Lies deep within the gray
matter (in the brain)
The Nerve Impulse
Conveys messages in our body
Unit: millivolts (mV)
Measures the electrical potential difference
(polarity) between 2 points
Saltatory Impulse Transmission
Flow is due to ion movements at the nodes
IONS: Electrically charged chemicals
Na + (outside axon)
electrical potential
K+
difference
(Inside axon)
Cl –
Therefore, the voltage (mV) is created by the
polarity (difference of charge) of the inside
and outside of an axon
Saltatory Impulse Transmission
Neurons have a semi-permeable
membrane for ions to pass through
Axomembrane: membrane of the axon
- contain Na/K pumps and gates
Axoplasm: cytoplasm of the axon
(fluid inside the axon)
Saltatory Impulse Transmission
“jumping” transmission
Saltatory Impulse Transmission
When neurons (axon) are at rest – not sending a
nerve impulse it is called the RESTING
POTENTIAL
A neuron in the resting state is polarized
Resting Potential is always negative on the inside
of the axon (-65 mV)
The Resting Potential
At rest, axomembrane is NOT permeable to
these ions because the gates are closed.
Due to natural unequal distribution of ions a
slight negative charge results
outside
[Na+] > [K+]
The Resting Potential
Na/K pumps (proteins) are found on the
axomembrane
pumps Na+ out and K+ into the axon
Keeps the unequal distribution of the ions
Membrane is permeable
to K+ therefore, it leaks
out (outside is always
more positive than
inside)
The Action Potential
Only takes 2 milliseconds
Rapid change in polarity (electrical difference)
Occurs when there is a intense stimulus that
reach the threshold value
ALL-OR-NONE (yes-impulse or none at all)
The Action Potential
What happens next:
1. DEPOLARIZATION
Na+ gates (in the axomembrane) becomes permeable to Na+
-
Na+ moves down its concentration gradient into the axon
Membrane potential -65mV  +40mV
2. REPOLARIZATION
K+ gates open
-
K+ moves down its concentration gradient out of the axon
Membrane potential reverts back to +40mV  -65mV
Since there is more negative ions inside compared to positive ions
Refractory Period
Prevents the action potential from moving backwards
During this period, Na+ gates can’t be opened
The Na+/K+ pump is busy restoring the resting
potential by pumping Na+ ions out and K+ ions in
through the axomembrane.
Stop here