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The Nervous System
Images with permission of Eric Chudler <[email protected]>
The Nervous System
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Neurons
Soma
Axon
Dendrite
Myelin sheath
Schwann cells
Synapse
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Motor neurons
Interneurons
Sensory neurons
Neurotransmitters
Action potential
Reflex arc
Nerve tissues respond to stimuli and
are composed of individual cells called
neurons and their associated support
cells.
Nerve cells can measure up to two meters
in length and are well suited to
transmitting messages. These cells convert
stimuli into electrochemical impulses and
transmit the signals down their lengths.
Parts of the Neuron
Cell Body: contains a large centrally located
nucleus. Its cytoplasm contains mitochondria,
lysosomes , Golgi body and rough
endoplasmic reticulum
Dendrites: are the primary sites for receiving
information signals from other neurons
Axon: Long cylindrical extension of the cell body.
It ranges from 1mm-1m in length. It transmits
waves of depolarization when receiving an
impulse that is strong enough
Axon Terminal: Bud like extension of the axon. It is
responsible for releasing neurotransmitters.
Myelin Sheath(Schwann cells and Nodes of Ranvier): found
in the CNS and PNS in which speed is important.
It is a fatty layer that is formed by the Schwann cells
that wrap around the axon.
Schwann cells line the length of the axon in the PNS. The gap
between the Schwann cells is called the nodes of Ranvier. The
axon is exposed in this gap and allows the impulse to jump
from one node to another thereby increasing the wave of
depolarization to 120m/s.
Schwann cells enable neurons to regenerate themselves in
situations where damage is not severe in the PNS
Neuron Function
• Nerve cell rely on aerobic respiration which
mean it requires a constant supply of oxygen
and glucose
• Mitochondria in the cell uses oxygen and
glucose to produce ATP
• ATP release energy during its breakdown into
ADP
• The energy released by ATP is used to fuel the
active transport of sodium and potassium
across the cell membrane
•Sensory neurons information from receptor cells, to
CNS and receive a stimulus
•Motor neurons information from effector cells, respond
to a stimulus.
•Interneurons are the link between sensory and motor
neurons, in the reflex arc. Receive information from other
interneuron, sensory neurons and exchanges the information
among neurons in the CAN. These are only found in the
CNS
Reflex Arc
• Sensory neuron, interneuron and motor neuron are
all involved in the reflex arc
• During the reflex our bodies react before our brain is
aware of the stimulus
• Nerve endings are the dendrites of the sensory
neuron and require a strong stimulus to activate it
• The impulse travels along the sensory neuron to the
spinal cord where the signal is passed along to the
interneuron
• The interneuron sends an impulse to the motor
neuron
• The motor neuron causes muscles to contract
and move the body away.
• Reflex action is involuntary ( but part of the
somatic nervous system).
• The interneuron sends a message to the brain
making the brain aware of what has happened
Functioning of neuron
• No actvity indicates that the neuron is at rest
• When an impulse is transmitted a wave of
depolarization is trasnmitted along the axon
• After the impulse has passed the process of
repolarization will occur and the neuron will
return to rest
Waves of Depolarization and
Repolarization
• These waves are due to the movement of
Sodium(Na) and Potassium (K) ions.
• There are specialized channels or gates within
the cell membrane to allow for the movement
of Na+, K+ and Cl- ions.
• There are larger ions that remain trapped in
the cell
Neuron At Rest
• At Resting Potential the outside of the neuron is
positively charged relative to the inside. Potential
energy is stored by holding opposite charges
• Outside the cell, the Na+ ion concentration is high
and the K+ ion concentration is low. The Clconcentration is relatively high outside also
• Inside the cell the K+ ion concentration is high and
the Na+ is low.
• At rest the membrane is 50 times more permeable to
K+ ions then to Na+ ions. The potassium ions can
flow freely out of the cell but sodium ions can not
flow into the cell
• The sodium potassium pump inside the cell
membrane helps maintain resting potential by
actively transporting three Na+ ions from the
outside of the cell to the inside and two K+
ions from the outside of the cell inside.
• This works to increase the negative charge on
the inside of cell. At resting potential the
difference in charge is -70mV
Resting Potential of Neuron
Depolarization
• If electrical stimulus are strong enough, a wave of
depolarization will be sent along the length of the axon.
This is the All or none response. The threshold potential is
about 50mV
• A stimulus causes the gates of the Na+ ions to open. Na+
ions move into the axon and reverse polarity on the axon.
The inside of the neuron becomes positive while the
outside becomes negative.
• The change in charge is known as action potential. As
depolarization occurs at one point of the axon it will initiate
depolarization of the neighboring part of the cell so that a
wave will continue along the axon.
The Action Potential explanation!
The Action Potential explanation!
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The Action Potential explanation!
Repolarization
• Immediately after depolarization the K+ channels
open and the Na+ channels close. This allows K+ ions
to move out. This repolarizes the cell so that it is
positively charged outside due to the increase in K+
ions on the outside.
• The Sodium Potassium pump actively transports Na+
ions out of the cell and the K+ ions into the cell again
and reestablishes the ion distribution for resting
potential.
• The time between the triggering of an impulse along
the axon and when it is available for the next impulse
is the refractory period and is approximately .001 s
Neurotransmitters
• Are chemicals secreted by neurons which
stimulate motor neurons. A wave of
depolarization is carried across the
presynaptic neuron until it reaches the bulb
like ends of the axon. The end of the axon
contains the neurotransmitters in its vesicles.
Depolarization causes the calcium gates to
open and trigger the release of the
neurotransmitters through exocytosis
• Neurotransmitters diffuse between the
synapse of the terminal end of the axon of the
postsynaptic neuron and the dendrites of the
post synaptic neuron. The excitory response
involves opening the sodium gates to trigger a
wave of depolarization. The inhibratory
response will open chloride channels to make
the neuron more negative inside and raise the
threshold of stimulus.
• The dendrites of the postsynaptic neuron have
specialized receptor sites to which the
neurotransmitter will attach. The
neurotransmitter will then either excite or
inhibit the neuron.
• Once a neurotransmitter has attached to the
receptor site of the postsynaptic neuron
(i) an enzyme may be released from the
pre-synaptic neuron to break down the
neurotransmitter or
(ii) the neurotransmitter may be
reabsorbed into the vesicles of the axon
terminal
A gap called the synaptic cleft separates the
presynaptic membrane from the postsynaptic
membrane (dendrite or target cell)
• The axon of one neuron synapses with the
dendrites of other neurons.
Transmitters move across the synapse
to send messages from one neuron to
the other. Receptor molecules pick up
the message.
Classes of Neurotransmitters
Excitory: acetylcholine,
epinephrine(noradrenaline),
glutamate, serotonin, and dopamine
Relaxation : GABA, dopamine and
serotonin
Acetylcholine
• Is both excitory and inhibitory.
• Involved in learning, mood and memory in the
CNs
• Stimulates skeletal muscles(somatic) but
inhibits cardiac muscles (parasympathetic)in
the PNS
• Cholinesterase breaks down acetylcholine.
• An under supply from deteriorating neurons is
linked to Alzheimer’s
Noradrenaline(norepinephrine)
• Primary neuron in the sympathetic nervous
system(stress)
• Increase s heart rate and glucose
• Reduced levels are linked to depression
Glutamate
• Used in the cerebral cortex and accounts for
75% of excitory transmissions
• Oversupply can stimulate the brain producing
migraines and seizures
• This is why people avoid MSG in food
GABA(Gamma aminobutyric acid)
• Commom inhibitory nreurotransmitter
• Undersupply linked to seizures, tremors and
insomnia
Serotonin
• Can be both excitory and inhibitory
• Involved in sleep/wake cycle, mood,
thermoregulation, attention and learning.
• Reduced levels linked to depression
• Prozac is an antidepressant commonly
prescribed to enhance the action of serotonin
Dopamine
• Excitory and inhibitory
• controls skeletal muscles and affects sleep,
cognition, attention and learning
• Involved in the reward system that regulates
behavior by inducing pleasurable effects
• Lack of dopamine is linked to Parkinson’s
disease and an increase in schizophrenia
• Drugs like cocaine operate by affecting
dopamine receptors in the brain
Nervous Disorders
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Multiple Sclerosis
Alzheimer’s disease
Parkinsons’s Disease
Meningitis
Huntington’s Disease
Epilespy
Stroke
Spinal cord injury
Stroke!
• A stroke occurs when the blood supply to the brain
is cut off by a broken blood vessel.
• Damage to the tissue of the area effected results
from lack of fresh oxygenated blood.
• Damage to the right side of the brain effects the left
side of the body!
Spinal Cord Damage
• Damage to the cord results in loss of ability to use
body parts below the injury.
• Little or no sensation occurs in effected body parts.
Alzheimer’s
• An illness that shows up in the elderly portions of our population.
• Complications include loss of nerve cells from cerebral cortex,
neurofibrilar tangles, neuritic plaques and changes in enzyme
systems in the synapse. AD brains have amyloid plaques and
neurofibrillary tangles. Scientists do not yet fully understand what
causes AD.
• This results in low levels of transmitters and fewer neurons to
transmit messages
• The most common form of dementia among older people is
Alzheimer's disease (AD), which involves the parts of the brain that
control thought, memory, and language.
• Age is the most important known risk factor for AD. The number of
people with the disease doubles every 5 years beyond age 65.
• Family history is another risk factor (Genetics).
Epilepsy
• A type of Brain disease that can result in
sudden loss of conciousness.
• Caused by non-functioning cells in the brain
that can lead to seizures.
• Usually the seizures are triggered by
something and a warning message such as a
smell precedes the event.
• In the past an object was placed in the
mouth of the victim to prevent them from
biting their tongue.
Multiple Sclerosis
• MS is a neurological disease that occurs when the insulating material
("myelin") around nerve cells in the central nervous system is
damaged.
• the immune system attacks the myelin around nerve cells in the brain
and spinal cord.
• Hard areas called "plaques" develop along a neuron's axon.
"Multiple" refers to the many different areas of the nervous system
that may have damaged myelin.
• People with MS can experience symptoms such as difficulties in
walking, visual problems, and pain.
• several environmental and genetic factors probably contribute to the
autoimmune response.
• anti-inflammatory agents and immunosuppressant drugs block or
inhibit features of the immune system.
• Glatiramer acetate is a compound that resembles myelin and the
immune system attacks this and not the body’s neurons.
Parkinsons Disease
• Nerve cells in the part of the brain that produces dopamine
begin to decrease in number (lowers dopamine levels).
• Dopamine (MAO-B) continues to deplete what little
dopamine is left.
• This throws off the normal dopamine/ acetylcholine
balance.
• The imbalance produces a lack of coordination of your
movement that often appears as tremor, stiff muscles and
joints, and/or difficulty moving.
• There is no cure for Parkinson’s however, drug therapy can
help manage the slow decline in function that occurs
Meningitis
• Meningitis is a viral or bacterial infection of the fluid of a
person's spinal cord and the fluid that surrounds the brain.
• Viral meningitis is generally less severe and resolves without
specific treatment, while bacterial meningitis can be quite
severe and may result in brain damage, hearing loss, or
learning disability
• High fever, headache, and stiff neck are common symptoms of
meningitis. As the disease progresses, patients of any age may
have seizures.
• Bacterial meningitis can be treated with antibiotics this
treatment of reduces the risk of dying from meningitis to
below 15%.
Huntingtons disease
• HD is an inherited neurological disorder: if either
parent has it, his or her child has a 50-50 chance of
getting it.
• Symptoms, which usually appear after age 30,
include dementia, chorea (jerky, random movements
of the body), poor coordination, depression, memory
loss and mood swings.
• HD damages neurons in the
area of the brain called the basal
ganglia
• There is no treatment for HD and patients often die
within 15 years of the onset of symptoms.
Spinal Cord Damage
• Damage to the cord results in loss of ability to
use body parts below the injury.
• Little or no sensation occurs in effected body
parts.