Transcript The Nervous System Ch. 12 & 13

Lindsey Bily
Austin High School
Anatomy & Physiology
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Made up of the brain, spinal cord and nerves.
The purpose of the nervous system is to
detect changes in internal and external
environment, evaluate the information, and
possibly respond by causing changes in the
muscles or glands.
Divided into the Central and Peripheral
Nervous System.
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Central Nervous System: brain and spinal
cord and nerves that lie completely within the
brain and spinal cord.
Peripheral Nervous System: nerve tissues
that lie in the “outer regions” of the nervous
system.
Cranial Nerves: nerves that originate in the
brain.
Spinal Nerves: nerves that originate in the
spinal cord.
CNS
PNS 
Obviously, signals go to
the brain and back out of
it, so we need nerves to
send messages both ways.
 Afferent Division:
incoming or sensory
pathways (usually blue in
diagrams).
 Efferent Division: outgoing
or motor pathways (usually
red in diagrams).
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Somatic Nervous System: carry information
to the skeletal muscle cells. Voluntary.
Autonomic Nervous System: carry
information to the smooth and cardiac
muscles, and glands. Involuntary.
 Sympathetic: “fight or flight”
 Parasympathetic: normal resting activities “rest
and repair”.
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Glia or Glial cells: do not conduct information
but support the function of neurons.
Neurons: excitable cells that conduct nerve
impulses.
Neurons
Glial Cells
(gray) 
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Glia literally means “glue”.
There are about 900 billion glial cells in the
body. 9 times the number of stars in the Milky
Way.
Unlike neurons, glial cells can divide
throughout their life.
Susceptible to cancer due to their ability to
divide. Most brain cancers are due to glial
cells.
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Astrocytes: “Stars of the Nervous System” They get glucose from the
blood and feed it to the neurons. Help to form the Blood Brain Barrier
(BBB).
Microglia: In CNS. They are usually small and stationary, but enlarge and
move around when they are needed to “eat” (phagocytosis)
microorganisms and cellular debris during inflamed or degenerating
nerve tissue.
Ependymal cells: form thin sheets that line fluid filled cavities in the brain
and spinal cord. Similar to epithelial cells.
Oligodendrocytes: similar to astrocytes but have fewer branches. Help
to hold nerve fibers together and produce the fatty myelin sheath around
the nerve fibers in the CNS.
Schwann Cells: found only in the PNS. Serve the same role as
oligodendrocytes.
Motor neuron (red)
astrocytes (green)
Microglia (green) 
Ependymal Cells
Oligodendrocytes
Schwann Cells
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Many Schwann Cells wrap themselves
around a single neuron.
Myelin is a white fatty substance that
insulates the neuron like plastic on a
wire.
The microscopic gaps between
Schwann Cells are called Nodes of
Ranvier.
Very important for nerve impulse
conduction.
Cells with myelin are called white fibers
and gray fibers when they are nonmyelinated.
Formed by the astrocytes that wrap their “feet” around the
capillaries in the brain.
 Regulates the passage of ions and molecules into and out of
the brain.
 Water, oxygen, carbon dioxide, glucose and small lipid
soluble molecules such as alcohol can cross the barrier easily.
 Ions (Na+ and K+) are regulated because they could disrupt
nerve impulses.
 Must be taken into consideration when developing drug
treatments for brain disorders.
 Ex. Parkinsons need dopamine but it cannot pass BBB. They
are given L-dopa which can pass and is made into dopamine
by the brain cells.
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Myelin disorder of the oligodendrocytes.
Loss of myelin and destruction of the
oligodendrocytes.
Hard plaquelike lesions replace the myelin
and causes inflammation.
Impaired nerve conduction, loss of
coordination, visual impairment and speech
disturbances.
Most common in women 20-40.
Caused by autoimmunity or a viral infection.
We have about 100 billion neurons. This is only
about 10% of all the nervous system cells in the
brain.
 Neurons are also called nerve fibers.
 Parts of the neuron:
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 Cell body
 Dendrites: branch off from the cell body. Means “tree”.
They receive stimuli and conduct electrical signals towards
the cell body and/or axon.
 Axon: a single process that comes off the cell body via the
axon hillock. They conduct impulses away from the cell.
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Multipolar: one axon, several dendrites. Most
neurons in the brain and spinal cord.
Bipolar: one axon and one highly branched
dendrite. Least common type of neuron,
found in retina, inner ear, and olfactory
pathway (nasal).
Unipolar or Psuedounipolar: single process
extending from the cell body. Always sensory
neurons that conduct information to the
CNS.
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Multipolar
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Bipolar
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Unipolar
Afferent (sensory) neurons: Transmit impulses
to the CNS.
 Efferent (motor) neurons: transmit impulses
away from CNS towards or to muscles or
glands.
 Interneurons: Transmit impulses from afferent
neurons to efferent neurons. Lie completely
within the CNS.
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Neurons are often arranged in a pattern called a
reflex arc. It’s a signal conduction route.
 Most common form is a 3-neuron arc (sensory
interneuron motor)
 2-neuron arc (sensory  motor)
 Synapse: place where nerve information is passed
from one neuron to another. Passed from the
synaptic knobs of one neuron to the dendrites of
the other.
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Nerves: bundles of nerve fibers in
the Peripheral nervous system held
together by several layers of
connective tissue. (ex. Sciatic
nerve)
 Tracts: bundles of nerve fibers in
the central nervous system. (ex.
Corticospinal tract)
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 White matter: myelinated nerve fibers
 Gray matter: unmyelinated nerve
fibers and cell bodies
Mature neurons cannot divide, damaged neurons
cannot be replaced.
 They can sometimes repair themselves in the PNS
if the damage is not too severe.
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 1. After the damage has occurred, the distal portion of the
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axon degenerates.
 2. macrophages move in and remove the debris.
3. the neurolemma (nerve sheath formed by Schwann Cells)
forms a tunnel from the point of injury to the effector.
 4. new Schwann cells grow within the tunnel to support
axon growth.
The skeletal muscle that is innervated to the
damaged nerve atrophies as it is not being
stimulated.
 If the damaged axon doesn’t repair itself,
sometimes a nearby healthy neuron will establish a
connection with the muscle.
 One damaged axon in a single neuron can shut
down an entire nerve pathway if not repaired.
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Cells in the CNS hardly ever repair themselves. They
lack a neurolemma to build a tunnel and astrocytes
fill in damaged areas and form scar tissue.
 Most spinal cord injuries involve crushing or bruising
of the nerves.
 Inflammation after the accident causes more
damage to surrounding nerves.
 Early treatment of the antiinflammatory drug,
methylprednisolone is prescribed within 8 hours of
injury to reduce the swelling.
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Neurons exhibit excitability and conductivity.
A nerve impulse is a wave of electrical
fluctuation that travels along the plasma
membrane.
Membrane potential: Cells have slightly more
(-) charges inside the cell than on the outside
(extracellular fluid is more +).
This difference in ion concentration across
the plasma membrane has potential energy.
A membrane is polarized if it has a membrane
potential.
 We can measure the potential difference between
the two sides of the polarized membrane in (V=volts
or mV= millivolts).
 -70 mV tells us that the difference in charge is 70 mV
and that the inside of the cell is negative (-).
 +30 mV tells us that the difference in charge is 30
mV and the inside of the cell is positive (+).
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A neuron is “resting” when it is not
conducting nerve impulses.
Stays about -70mV (RMP-resting membrane
potential)
There are no gates or they are closed to not
allow anions (-) in or out of the cell.
Cations (+) Na+ and K+can move in and out of
the cell through gates.
K+ gates are usually open and Na+ gates are usually
closed.
 There are also Na+ and K+ pumps that are active
transport mechanisms.
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 Pumps 3 Na+ out for every 2 K+ in and at different rates.
 If 100 K+ are pumped inside the cell, 150 Na+ are pumped
out.
 This maintains a difference in + charges inside and out of
the cell. Slightly more positive outside the cell.
Very little Na+
diffuses through
the membrane.
The pump
maintains a
imbalance of
ions inside and
outside of the
cell.
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The resting membrane potentials (RMP) of
neurons can fluctuate due to certain stimuli.
Local Potential: slight change in the RMP.
Stimulus-gated Na+ channels open in
response to a sensory stimulus or stimulus
from another neuron (excitation)
When they open, more Na+ rushes into the
cell, causing it to become more +.
Depolarization (movement of the membrane
potential to zero mV).
Stimulus-gated K+ channels open during
inhibition. Causing the outside of the cell to become
more +. Hyperpolarization (movement of the
membrane potential away from zero mV.) Now we
are below the RMP.
 Local potentials are graded potentials meaning
they can be large or small depending on the
strength of the stimulus. They are also isolated to a
particular location on the plasma membrane and do
not travel down the axon.
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Small
depolarizations or
hyperpolarizations
applied to certain
dendrites on a
neuron.
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Action Potential is the membrane potential
of a neuron that is conducting an impulse.
Also called a nerve impulse.
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There are 6 steps in conducting an action
potential.
http://www.metope.org/neuron/
1.
2.
3.
4.
5.
6.
An adequate stimulus must be applied and the stimulus-gated Na+
channels will open to allow Na+ in (depolarization).
If the level of depolarization surpasses the threshold potential (usually
-59 mV) voltage-gated Na+ channels will open allowing MORE Na+ in
the cell.
As more Na+ comes inside, the voltage inside the cell gets closer and
closer to 0 mV and will continue to +30 mV. Means we now have more
+ ions in the cell than outside of the cell.
Voltage-gated Na+ channels only stay open for about 1 millisecond
before they close. Action potentials are all-or-none, either they will
occur or not at all.
Once the peak of the action potential is reached , it starts to move
back to -70 mV (resting potential). This is called repolarization. The
reaching of the threshold potential causes voltage-gated K+ channels
to open as well, but they are slow to respond. So they don’t open until
the +30 mV potential is reached. Then K+ pours out and Na+ goes back
out.
Because so much K+ pours out of the cell, the voltage goes past -70
mV for a brief period of hyperpolarization, but then it gets back to the
resting state.
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Brief period during which a local area on the axon’s
membrane can not be restimulated.
 Absolute refractory period: ½ a millisecond after the
threshold potential is surpassed. Axon will not respond to
any stimulus.
 Relative refractory period: few milliseconds after the
absolute refractory period. Can only be stimulated if the
stimulus is really strong.
 A strong stimulus causes more action potentials vs. a weak
stimulus. However, the strength of each action potential is
the same.
The action potential causes an electrical current to
flow down segments of the axon’s membrane.
 It will never move backward due to the refractory
period of the membrane before the AP.
 In myelinated fibers, the myelin sheath prevents ion
movement, so electrical changes only occur in the
gaps between myelin (Nodes of Ranvier).
 The AP seems to “leap” from node to node. This is
called saltatory conduction. (Latin-saltare-”to leap”)
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The larger the diameter of the fiber, the
faster it conducts impulses.
Myelinated fibers conduct impulses faster
than unmyelinated fibers.
Fastest fibers innervate skeletal muscles and
can fire impulses close to 300 mph.
Slowest fibers, such as sensory receptors in
the skin, conduct impulses at less than 1 mph.
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Block pain.
Inhibit the opening of Na+ channels so the
nerve cannot conduct impulses.
 Bupivacaine (Marcaine): used in dental
procedures.
 Procaine used to block signals in sensory
pathways of the spinal cord.
 Benzocaine and phenol: found in over the counter
products that release pain associated with
teething, sore throat pain, and other ailments.
Synapse is where signals are sent from one neuron
to another (presynaptic neuron to the postsynaptic
neuron)
 Types of Synapses
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 Electrical: occur where two cells are joined at gap
junctions (cardiac muscle, some smooth muscle). The
impulse goes from one plasma membrane to the other.
 Chemical: Use chemicals (neurotransmitters) to send a
signal from the pre- to the postsynaptic cell.
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Structures of the chemical synapse.
 1. Synaptic knob- tiny bulge at the end of the presynaptic
neuron’s axon. Contains numerous small sacs or vesicles
that contain neurotransmitter.
 2. synaptic cleft- space between the synaptic knob and the
plasma membrane of the postsynaptic neuron, 1 millionth
of an inch wide!
 3. the plasma membrane of the postsynaptic neuron- has
protein receptors embedded in which neurotransmitters
bind.
Electrical Synapse
Chemical Synapse
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Action potentials cannot cross synaptic clefts
even though the spaces are so tiny.
Instead, neurotransmitters (NT) are released
in cause a response in the postsynaptic
neuron.
Excitatory NT cause depolarization and
inhibitory NT cause hyperpolarization.
1.
2.
3.
4.
5.
Action potential (AP) reaches a synaptic knob, causing
voltage-gated Ca 2+ channels to open and allow Ca 2+ to
diffuse into the knob rapidly.
Increase in Ca2+ causes NT to be released into the synaptic
cleft.
The NT binds to receptors on the postsynaptic membrane
which causes the ion gates to open.
The NT will either cause an excitatory postsynaptic
potential (EPSP) or an inhibitory postsynaptic potential
(IPSP).
Once the NT binds to the receptor its action is terminated.
Neurotransmitters are how neurons talk to one
another.
 Can be excitatory or inhibitory.
 Their affect is determined by the receptor, not the
actual NT.
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Acetylcholine- excites skeletal muscles but inhibits cardiac
muscles.
Amines- (seratonin, histamine, dopamine, epinephrine and
norepinephrine). Affect learning, motor control, emotions,
etc.
Amino Acids- (glutamate, GABA, glycine). Some are
excitatory and some are inhibitory in the CNS.
Other small molecule transmitters- (nitric oxide NO and
carbon monoxide CO).
Neuropeptides- short strands of amino acids. Include
enkephalins and endorphins. Inhibitory and block pain.
Severe psychic depression occurs when there is a
lack of norepinephrine, dopamine, serotonin and
other amines.
 Antidepressants work several different ways.
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 They may inhibit enzymes that are used to inactivate the
NT.
 They may block the reuptake of the NT by the neuron,
keeping them in the synapse longer.
 Cocaine blocks the reuptake of dopamine, giving a
temporary feeling of well-being.