Nervous System - Westminster College

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Transcript Nervous System - Westminster College

The Nervous System
• Neurons are main cells in nervous system, as
well as some of the largest cells in your body
(one extends from base of your spine to tip of
your big toe!)
• Function: to carry electrical signals from brain to
all parts of your body
• Brain is the “control panel” – Neurons are the
“wires” to external devices
Types of Neurons
• A neuron that carries signals from tissue to
the brain is a sensory neuron or afferent
• A neuron that carries signals from the
brain to tissue is a motor neuron or
efferent neuron.
Neuron Geometry
• The dendrite receives signals from other cells
• The axon relays signals from the neuron to the next
• The spaces between neurons where signals are
transferred from cell to cell are called synapses
The Brain
• Contains about 100 billion neurons – these
are your brain cells
• These neurons each have connections to
thousands of neighboring neurons
(through synapses)
• Average adult brain has between 100 –
500 trillion synapses
• A child’s brain has about 1 quadrillion
Why Does the Signal Travel?
• Motion of electrical impulse along a
neuron is called an action potential
• For this to happen, there must be a
difference in charge accumulation
between inside and outside of cell
(a.k.a. a voltage difference)
Ion Separation in Cells
• In most cells, ion pumps in the cell membrane
distribute ions differentially between the inside
and outside of the cell.
• Ion pumps: For every two positively charged
potassium ions pumped into cell, three
positively charged sodium ions are pumped out
• This creates an voltage difference of 70 mV
across cell membrane (more positive charges
are outside than inside)
So How Do Neurons Signal?
Neuron receives impulse
Sodium gates open on cell
Positively charged sodium ions flood into cell (since the inside is
already less positively charged and since cell pumps them in)
As the charge evens out, voltage difference decreases, and more
sodium channels open up allowing in even more positive charge –
positive feedback loop.
Voltage changes from -70 mV to +40 mV.
At +40 mV sodium channels close – negative feedback loop
All the sodium ions flooding into the cell creates a force that pushes the
sodium ions away to adjacent parts of the cell (like charges repel)
 wave of positivity flies down neuron cell
 this then signals the next cell
Unfortunately, nerve cells
are not very resilient…
Nerve cells in the central nervous system
(brain and spinal cord) do not grow back if
damaged (at least not currently–we’ll talk
about the potential for stem cells to help
with this next time) – unlike most other
types of cells
Spinal Cord Function
Each portion of the spinal cord
innervates a different part of the
• Cervical – diaphragm &
• Thoracic – chest muscles &
abdominal muscles
• Lumbar – Legs
• Sacral – Bowel & Bladder
If the spinal cord is damaged,
some signals will not get sent
and received
Nerve Injury
Myelin (insulator
around nerve cell)
damage – nerves intact
but signals impaired.
Examples: Multiple
sclerosis, GuillanBarre, leukodystrophy.
Spinal Cord Injury
Physical break or compression in spinal cord. Level of injury indicates level
of function.
And again, nerve cells in the central nervous system do not grow back if
damaged (at least not currently) – unlike most other types of cells
Next time…
• We’ll talk about the basics of stem cells
• Debate and human cloning papers due
next Wednesday