Neural Circuits
There a two ways that a stimulus can be
processed by the central nervous system.
 One is a reflex arc where information that
has to be interpreted quickly is processed
in the spinal cord.
 The other is when you have to think about
things and the information is sent to the
brain to be interpreted.
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The Reflex Arc
The simplest nerve pathway is the reflex
arc. Most reflexes occur without brain
coordination. It is a predictable and
involuntary response to a stimulus.
 Reflex arcs contain 5 essential
components: the receptor, the sensory
neuron, the interneuron in the spinal cord,
the motor neuron and the effector.
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The Pathway of the Reflex Arc
1. Receptor cells detect a stimulus.
 2. The receptor cell passes information to
a sensory neuron. The axon of the
sensory neuron enters the dorsal root of
the spinal cord.
 3. Information passes from sensory
neuron to interneuron.
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4. Interneuron passes information to motor
neuron. The axon of the motor neuron
exits the ventral root of the spinal cord.
 5. Information passes along the motor
neuron axon to the target cells, such as a
muscle.
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How information travels in the
nervous system
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When nerves get excited or stimulated,
they cause a nerve impulse. This is how
information travels from nerve to nerve
and allows information to get to/from the
brain and spinal cord.
Nerve Impulse
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A nerve impulse is the electrochemical response
to a stimulus.
When a nerve cell is stimulated, it sends an
impulse along the neuron and the impulse is
passed to another neuron until it reaches its
destination.
The electrochemical response is caused by a
reversal in charge between the inside of a
neuron and the outside of a neuron.
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The differences in charge are caused by a
distribution of ions on each side of the
membrane. Normally, there is a greater
distribution of positive ions outside the
membrane because sodium (Na+) ions are
positively charged. Inside the membrane
there are negative protein ions and
positive potassium (K+) ions.
Inside the cell is more negative
How it works – Resting Potential
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At any given time a nerve cell has electrical potential
across its cell membrane because of a difference in the
number of positively and negatively charged ions on
each side of the cell membrane.
When there is no impulse, the nerve is polarized and is
said to be at resting potential.
A charge of -70mV indicates the difference between the
number of positive charges found on the inside of the
nerve cell relative to the outside.
A neuron stays at resting potential until it is
stimulated.
 The neuron stays at resting potential by
using potassium – sodium pumps, which
pumps sodium ions out of the cell.
(Potassium can leak out without a pump).
 This makes the inside of the cell more
negative creating a potential.
 If the nerve is stimulated, an impulse
moves along the nerve.
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The Impulse – Action Potential
1. There is an impulse
2. The impulse opens sodium gates.
3. Sodium ions (Na+) rush into the nerve, which make the
nerve positive. The membrane becomes depolarized.
5. Depolarization causes Na+ gates to close and K+ gates
to open. Potassium floods out of the cell making the
nerve negative again (repolarization)
6. The sodium-potassium pump then pumps sodium back
out of the nerve and potassium back in (refractory period
- takes about 1 millisecond)
Cool animation
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http://www.biology4all.com/resources_libra
ry/source/63.swf
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During an impulse the dendrites receive
stimulation from the external environment
or from within the body.
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An impulse is sent along the axon to:
A
muscle
 A gland
 A dendrite
 An axon
Threshold Levels and the All-orNone Response
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Threshold Level – minimum level of a stimulus required
to produce a response.
Different neurons have different threshold levels.
Increasing the intensity of stimuli above the critical
threshold value will not produce an increased response –
the intensity of the nerve impulse and speed of
transmission remain the same.
- 50mV/-55mV is the threshold – the minimum voltage
required for a neuron to send an impulse or “fire”
All-or-none response – a nerve or muscle fiber
responds completely or not at all to a stimulus.
Say it again!!
1. Resting Potential -70mV
- more Na+ outside the nerve making the nerve polarized
2. Action Potential – impulse opens pumps allowing Na+ into the nerve
- Nerve becomes depolarized (positive instead of negative)
- It is between 40mV and 50mV
3. Refractory Period – repolarization takes place (potassium leaves the
cell making it negative on the inside again, then cell goes back to
normal state)
- No action potential until membrane is back to resting potential
Questions
Do nerve impulses move faster along
myelinated or unmyelinated nerve fibers?
 What is the all-or-none response of a
nerve?
 What happens when an impulse reaches
the end of an axon?
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Assigned Readings:
 P. 416, 417 Summary, 418, 419, 420, 421
and 422
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