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Neurons and Synapses
Your brain is made of approximately
100-billion nerve cells, called neurons.
 Neurons have the amazing ability to
gather and transmit electrochemical
signals

Cell body - This main part has all of the necessary
components of the cell, such as the nucleus, ER
and ribosomes (for building proteins) and
mitochondria (for making energy).
 Axon - This long, cable-like projection of the cell
carries the electrochemical message (nerve
impulse or action potential) along the length of
the cell.
 Dendrites - These small, branch-like projections
of the cell make connections to other cells and
allow the neuron to talk with other cells or
perceive the environment.

Neurons also vary with respect to their functions:
 Sensory neurons carry signals from the outer parts
of your body (periphery) into the central nervous
system.
 Motor neurons (motoneurons) carry signals from the
central nervous system to the outer parts (muscles,
skin, glands) of your body.
 Interneurons connect various neurons within the
brain and spinal cord.

Reflex Arc

The simplest type of neural pathway is a monosynaptic
(single connection) reflex arc, like the knee-jerk reflex.
When the doctor taps the right spot on your knee with a
rubber hammer, receptors send a signal into the spinal cord
through a sensory neuron. The sensory neuron passes the
message to a motor neuron that controls your leg muscles.
Nerve impulses travel down the motor neuron and stimulate
the appropriate leg muscle to contract. The response is a
muscular jerk that happens quickly and does not involve your
brain.
Nerve Impulse
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A nerve impulse is an impulse from another nerve
or a stimulus from a nerve receptor. A nerve
impulse causes:
The permeability of the membrane to sodium ions
suddenly increases.
Sodium ions diffuse rapidly from the outside to
the inside of the membrane.
This reverses the polarity of the cell membrane
(inside positive and outside negative).
Nerve Impulse Continued
This reversal occurs in a small area of the
membrane and results in a flow of electrical
current that affects the permeability of the
adjacent areas of the membrane
 The reversal of polarization is the nerve impulse
and it travels the length of the axon.
 High permeability of the membrane to sodium ions
last only a fraction of a second and then returns
to normal.
 The sodium pump and potassium diffusion allow
normal distribution of ions to be restored.

Nerve Impulse Continued
A brief recovery period occurs during which the
nerve cell membrane cannot be stimulated to carry
impulses. This refractory period lasts a few
thousandths of a second.
 The rate at which an impulse travels depends on
the size of the nerve and whether or not it is
myelinated (unmyelinated = 2 m/s and myelinated =
100 m/s).
 In myelinated fibers the signal jumps from one
node of Ranvier to the next. This is saltatory
conduction and occurs because the membrane at
the node is highly sensitive and this uses less
energy due to polarization only at the nodes.

Nerve Impulse Continued
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For a nerve impulse to be transmitted, the
stimulus must be at least a certain minimum
strength or must reach a threshold.
The impulses transmitted by a given neuron are all
alike, a neuron operates on a “all-or-none” basis.
The strength of the stimulus is measured by two
effects: 1. A stronger stimulus causes more
impulses to be transmitted each second.
2.
Different neurons have different thresholds.
A large number of neurons fire when a stimulus is
stronger.
Synapse
Another Synapse
Transmission at the Synapse
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The transmission of the impulse across the
synaptic cleft is a chemical process.
Within the synaptic knob, the synaptic vesicles
contain neurotransmitters (which are chemicals
such as acetylcholine and norepinephrine).
When an impulse reaches the synaptic knob, the
synaptic vesicles fuse with the membrane of the
synaptic knob and release their contents into the
synaptic cleft.
Special receptor proteins in the membrane of the
neighboring dendrite attach to these
neurotransmitters.
Transmission Continued
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When the impulses are arriving at a faster rate
(representing a stronger initial stimulus), more
neurotransmitter is released into the synaptic
cleft and more impulses per second are sent.
When the neurotransmitter has done its work, it is
removed from the synaptic cleft by an enzyme
that breaks down the molecules. The transmission
of the impulse across the synaptic cleft is a
chemical process.
Neurotransmitters
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Excitatory neurotransmitters are chemicals that
initiate impulses in adjacent neurons. Examples
include: acetylcholine, norepinephrine, histamine,
and glutamic acid (an amino acid)
Inhibitory neurotransmitters are chemicals that
inhibit the firing of impulses. Examples include:
serotonin, epinephrine, and glycine
If the overall results are excitatory, impulses are
transmitted down the axon to the next set of
synapses. If the results are inhibitory, no impulses
are transmitted.
Neuromuscular Junction
The passage of impulses from motor neurons to
muscles occur at special points of contact called
neuromuscular junctions.
 The motor end plates contain synaptic vesicles
which release acetylcholine. The acetylcholine
combines with receptor molecules on the muscle
cell membrane, thus sending an impulse to the
muscle.
 The acetylcholine causes muscle cell membrane to
become more permeable to sodium, causing an
impulse to travel the membrane and the muscle
cell to contract.

Neuromuscular Junction
Drugs and Synapses
Many poisons and drugs affect the activity of chemical
neurotransmitters at the synapses. Nerve gas, curare,
botulin toxin, and some poisonous insecticides can
interfere with the functioning of acetylcholine and
cause muscle paralysis (death for respiratory
paralysis).
 Stimulants cause a feeling of well-being, alertness, and
excitement such as amphetamines (mimic
norepinephrine by binding to receptors) and caffeine
(aids in synaptic transmissions).
 Depressants slow the body activity or cause depression
such as barbiturates (block the formation of
norepinephrine).
 Hallucingens such as LSD or mescaline interfere with
the effect of the inhibitory transmitter serotonin.

Taken from:
1. Text book (Biology: The Study of Life)
2. John Broida, PhD.
http://www.usm.maine.edu/psy/broida/101/neuron.JPG
3. How Stuff Works
http://health.howstuffworks.com/brain1.htm
4. Airline Safety.Com
http://www.airlinesafety.com/editorials/PilotsAndMemory.htm
5. Neruoscience Glossary
http://shp.by.ru/spravka/neurosci/
5. Muscel Physiology
http://fig.cox.miami.edu/~cmallery/150/neuro/neuromuscularsml.jpg