The Nervous System - PLCNeTanatomy.plcnet.org/files/Lectures/English_1_2/2018...Basic Functions of...
Transcript of The Nervous System - PLCNeTanatomy.plcnet.org/files/Lectures/English_1_2/2018...Basic Functions of...
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The Nervous System
• A network of billions of nerve cells linked
together in a highly organized fashion to
form the rapid control center of the body.
• Functions include:
– Integrating center for homeostasis,
movement, and almost all other body
functions.
– The mysterious source of those traits that we
think of as setting humans apart from animals
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Basic Functions of the Nervous System
1. Sensation • Monitors changes/events occurring in and outside the
body. Such changes are known as stimuli and the cells
that monitor them are receptors.
2. Integration • The parallel processing and interpretation of sensory
information to determine the appropriate response
3. Reaction • Motor output.
– The activation of muscles or glands (typically via the release
of neurotransmitters (NTs))
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Organization of the
Nervous System
• 2 big initial divisions: 1. Central Nervous System
• The brain + the spinal cord
– The center of integration and control
2. Peripheral Nervous System • The nervous system outside of the
brain and spinal cord
• Consists of:
– 31 Spinal nerves
» Carry info to and from the spinal cord
– 12 Cranial nerves
» Carry info to and from the brain
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Peripheral Nervous System
• Responsible for communication btwn the CNS and the rest of the body.
• Can be divided into: – Sensory Division
• Afferent division – Conducts impulses from receptors to the CNS
– Informs the CNS of the state of the body interior and exterior
– Sensory nerve fibers can be somatic (from skin, skeletal muscles or joints) or visceral (from organs w/i the ventral body cavity)
– Motor Division • Efferent division
– Conducts impulses from CNS to effectors (muscles/glands)
– Motor nerve fibers
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Motor Efferent Division
• Can be divided further:
– Somatic nervous system
• VOLUNTARY (generally)
• Somatic nerve fibers that conduct impulses from
the CNS to skeletal muscles
– Autonomic nervous system
• INVOLUNTARY (generally)
• Conducts impulses from the CNS to smooth
muscle, cardiac muscle, and glands.
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Autonomic Nervous System
• Can be divided into:
– Sympathetic Nervous
System
• “Fight or Flight”
– Parasympathetic
Nervous System
• “Rest and Digest”
These 2 systems are antagonistic.
Typically, we balance these 2 to keep ourselves in a
state of dynamic balance.
We’ll go further into the difference btwn these 2
later!
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Nervous Tissue
• Highly cellular
– How does this compare to the other 3 tissue types?
• 2 cell types
1. Neurons • Functional, signal
conducting cells
2. Neuroglia • Supporting cells
1.
2.
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Neuroglia
• Outnumber neurons by about
10 to 1 (the guy on the right had an inordinate amount of them).
• 6 types of supporting cells
– 4 are found in the CNS:
1. Astrocytes
• Star-shaped, abundant, and versatile
• Guide the migration of developing neurons
• Act as K+ and NT buffers
• Involved in the formation of the blood brain barrier
• Function in nutrient transfer
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Neuroglia
2. Microglia • Specialized immune cells that act
as the macrophages of the CNS
• Why is it important for the CNS to have its own army of immune cells?
3. Ependymal Cells • Low columnar epithelial-esque
cells that line the ventricles of the brain and the central canal of the spinal cord
• Some are ciliated which facilitates the movement of cerebrospinal fluid
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Neuroglia
4. Oligodendrocytes
• Produce the myelin sheath which provides the electrical insulation for certain neurons in the CNS
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• 2 types of glia in the
PNS
1. Satellite cells • Surround clusters of
neuronal cell bodies in the
PNS
• Unknown function
2. Schwann cells • Form myelin sheaths
around the larger nerve
fibers in the PNS.
• Vital to neuronal
regeneration
Neuroglia
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Neurons • The functional and structural unit
of the nervous system
• Specialized to conduct information from one part of the
body to another
• There are many, many different types of neurons but most
have certain structural and functional characteristics in
common:
- Cell body (soma)
- One or more specialized, slender processes (axons/dendrites)
- An input region (dendrites/soma)
- A conducting component (axon)
- A secretory (output) region (axon terminal)
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Soma
• Contains nucleus plus most normal organelles.
• Biosynthetic center of the neuron.
• Contains a very active and developed rough endoplasmic reticulum which is responsible
for the synthesis of ________.
– The neuronal rough ER is referred to as the Nissl body.
• Contains many bundles of protein filaments (neurofibrils) which help maintain the shape, structure, and integrity of the cell.
In the soma above, notice the small
black circle. It is the nucleolus, the site
of ribosome synthesis. The light
circular area around it is the nucleus.
The mottled dark areas found
throughout the cytoplasm are the Nissl
substance.
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Neuronal Processes
• Armlike extensions emanating from every neuron.
• The CNS consists of both somata and processes whereas
the bulk of the PNS consists of processes.
• Tracts = Bundles of processes in the CNS (red arrow)
Nerves = Bundles of processes in the PNS
• 2 types of processes that differ in structure and function:
– Dendrites and Axons
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• Dendrites are thin, branched processes whose main function is to receive incoming signals.
• They effectively increase the surface area of a neuron to increase its ability to communicate with other neurons.
• Small, mushroom-shaped dendritic spines further increase the SA
• Convey info towards the soma thru the use of graded potentials – which are somewhat similar to action potentials.
Notice the multiple
processes extending
from the neuron on the
right. Also notice the
multiple dark circular
dots in the slide. They’re
not neurons, so they
must be…
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• Most neurons have a single axon – a long (up to 1m) process designed to convey info away from the cell body.
• Originates from a special region of the cell body called the axon hillock.
• Transmit APs from the soma toward the end of the axon where they cause NT release.
• Often branch sparsely, forming collaterals.
• Each collateral may split into telodendria which end in a synaptic knob, which contains synaptic vesicles – membranous bags of NTs.
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Axons
• Axolemma = axon plasma membrane.
• Surrounded by a myelin sheath, a wrapping of lipid which: – Protects the axon and electrically isolates it
– Increases the rate of AP transmission
• The myelin sheath is made by ________ in the CNS and by _________ in the PNS.
• This wrapping is never complete. Interspersed along the axon are gaps where there is no myelin – these are nodes of Ranvier.
• In the PNS, the exterior of the Schwann cell surrounding an axon is the neurilemma
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Myelination in the CNS
Myelination in the PNS
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• A bundle of processes in the PNS is a nerve.
• Within a nerve, each axon is surrounded by an endoneurium (too small to see on the photomicrograph) – a layer of loose CT.
• Groups of fibers
are bound
together into
bundles
(fascicles) by a
perineurium (red
arrow).
• All the fascicles
of a nerve are
enclosed by a
epineurium
(black arrow).
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Action Potential Conduction
• If an AP is generated at the axon hillock, it will
travel all the way down to the synaptic knob.
• The manner in which it travels depends on
whether the neuron is myelinated or
unmyelinated.
• Unmyelinated neurons undergo the continuous
conduction of an AP whereas myelinated
neurons undergo saltatory conduction of an AP.
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Continuous Conduction • Occurs in unmyelinated axons.
• In this situation, the wave of de- and repolarization simply travels from one patch of membrane to the next adjacent patch.
• APs moved in this fashion along the sarcolemma of a muscle fiber as well.
• Analogous to dominoes falling.
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Saltatory Conduction
• Occurs in myelinated axons.
• Saltare is a Latin word meaning “to leap.”
• Recall that the myelin sheath is not completed. There exist
myelin free regions along the axon, the nodes of Ranvier.
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Types of Nerve Fibers
1. Group A – Axons of the somatic sensory neurons and motor neurons
serving the skin, skeletal muscles, and joints.
– Large diameters and thick myelin sheaths.
• How does this influence their AP conduction?
2. Group B – Type B are lightly myelinated and of intermediate diameter.
3. Group C – Type C are unmyelinated and have the smallest diameter.
– Autonomic nervous system fibers serving the visceral organs, visceral sensory fibers, and small somatic sensory fibers are Type B and Type C fibers.
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Synaptic
Transmission • An AP reaches the
axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open.
• Ca2+ rushes in, binds to regulatory proteins & initiates NT exocytosis.
• NTs diffuse across the synaptic cleft and then bind to receptors on the postsynaptic membrane and initiate some sort of response on the postsynaptic cell.
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Development of the Central Nervous
System
an ongoing process, through adolescence and maybe even adult hood ?
the nervous system is “plastic”
Experience plays a key role
Dire consequences when something goes wrong
- “teratogens” - Drugs of abuse, industrial chemicals, caffeine?,
household chemicals
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Stages of Development
Phase Approximate Age Highlight
Prenatal Conception - birth Rapid physical
growth
Infancy Birth - 2 yrs Motor development
Childhood 2 - 12 yrs Abstract reasoning
Adolescence 13 - 20 yrs Identity creation,
“Judgement”
Directly related to maturation of
the “Prefrontal Cortex”
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Phases of Prenatal
Development
• Ovum + sperm
– zygote
• Once zygote
implants in uterus
– embryo
• Week 8 until birth
– fetus
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The University of South Wales, Dr. Mark Hill
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a. Consists of 3 layers
of cells: endoderm,
mesoderm, and
ectoderm.
Thickening of the
ectoderm leads to the
development of the
neural plate
b. The neural groove
begins to develop at 20
days.
At about 18 days after conception the embryo begins to implant in the uterine
wall.
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c. At 22 days the
neural groove closes
along the length of
the embryo making
the neural tube.
d. A few days later 4
major divisions of the
brain are observable
– the telencephalon,
diencephalon,
mesencephalon, and
rhombencephalon.
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Phases of brain development
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
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Induction of the Neural Plate
• 2-3 weeks after conception
• A patch of tissue on the dorsal surface of the
embryo that will become the nervous system
• Development induced by chemical signals
“growth factors”: several chemicals produced in developing
and mature brain that stimulate neuron development and
help neurons respond to injury
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Neural Plate
Totipotent (zygote) –
Fertilized ovum has ability to divide and produce all cells
of the body (brain, kidney, liver, skin, bone etc.)
Can produce a whole animal
Pluripotent: 5 days after fertilization = blastocyst forms, some of
these cells are embryonic“stem cells”. Can be taken and
differentiated into any organ ?
With the development of the neural tube, cells become
multipotent –
able to develop into any type of mature nervous system cell
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Phases of brain development
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
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Proliferation – Generation of new cells
3 swellings at the anterior
end in humans will become the forebrain, midbrain, and hindbrain
2. Mitosis/Proliferation
•Occurs in ventricular zone
•Rate can be 250,000/min
•After mitosis “daughter” cells
become “fixed” post mitotic
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3. Migration: slow movement to the “right place”
Only a soma and immature axon at this
point
-undifferentiated at start of migration.
Differentiation begins as neurons migrate.
They develop neurotransmitter
making ability, action potential
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3. Migration
Radial glial cells act as guide wires
for the migration of neurons
Migrating cells are immature,
lacking dendrites
Cells that are done migrating align
themselves with others cells and
form structures (Aggregation)
Radial Glia
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Growth Cones: tips of axons on migrating, immature neurons
Growth cones crawl forward as they
elaborate the axons training behind
them. Their extension is controlled by
chemical cues in their outside
environment that ultimately direct them
toward their appropriate targets.
Chemoattractants
Vs
Chemorepellants
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5 Phases of Neurodevelopment
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
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4. Axon Growth/Synaptogenesis
Once migration is complete and structures have formed (aggregation), axons and dendrites begin to grow to their “mature” size/shape.
Axons (with growth cones on end)
and dendrites form a synapse with
other neurons or tissue (e.g. muscle)
Growth cones and chemo-attractants
are critical for this.
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Synaptogenesis
• Formation of new synapses
• Depends on the presence of glial cells – especially astrocytes
• Chemical signal exchange between pre- and postsynaptic neurons is needed
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5 Phases of Neurodevelopment
– Neural plate induction
– Neural proliferation
– Migration & Aggregation
– Axon growth & Synapse formation
– Cell death & Synapse rearrangement
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5. Neuronal Death
Between 40-75% neurons made, will die after
migration – death is normal and necessary !!
Neurons die due to failure to compete for chemicals provided by targets
Neurotrophins –
promote growth and survival
guide axons
stimulate synaptogenesis
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Release and
uptake of
neurotrophic
factors
Neurons receiving
insufficient
neurotropic factor die
Axonal processes
complete for limited
neurotrophic factor
Synaptic rearrangment
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Synaptic rearrangment, cont’d:
Myelination
Time after synaptogenesis
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Postnatal Cerebral Development
Human Infants
• Postnatal growth is a consequence of
– Synaptogenesis
– Increased dendritic branches
– Myelination (prefrontal cortex continues into
adolescence)
• Overproduction of synapses may underlie the
greater “plasticity” of the young brain
• Young brain more able to recover function after injury,
as compared to older brain
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Early Studies of Experience and Brain
Development
• Early visual deprivation
– fewer synapses and dendritic spines in visual cortex
– deficits in depth and pattern vision
• “Enriched” environment
– thicker cortices
– greater dendritic development
– more synapses per neuron
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Development of the Prefrontal Cortex
• Believed to underlie age-related changes in cognitive function, judgement, decision-making
• No single theory explains the function of this area
• Prefrontal cortex plays a role in working memory, planning and carrying out sequences of actions, and inhibiting inappropriate responses
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Where is your Prefrontal Cortex ?
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Postnatal Cerebral Development:
Adolescence
• The prefrontal lobe is the last to fully
develop
• http://www.youtube.com/watch?v=4-
9sjvitKWA
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Neuroplasticity in Adults ?
• Mature brain changes and adapts
• Neurogenesis (birth of new neurons)
– seen only in olfactory bulb and
hippocampus of adult mammals
– Not clear if this is critical for “normal” adult
behavior
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Effects of Experience on the
Reorganization of the Adult Cortex
Skill training leads to reorganization of motor cortex
Adult musicians who play instruments have an enlarged representation of the hand in somatosensory cortex
Reorganization is synaptogenesis or pruning of
unused synapses…
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Neurodevelopmental Disorders
• Autism Spectrum Disorders
– 1/91 live births in U.S.)
• Fetal Alcohol Spectrum Disorders
– (1/100 live births in North America ?)
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Autism
• http://health.yahoo.com/nervous-videos/what-is-autism/healthination--
HNB10051_autism_1.html
• 3 core symptoms:
– Reduced ability to communicate
– Reduced capacity for social interaction
– Preoccupation with a single subject or activity
• Heterogenous – level of brain damage and
dysfunction varies (Autism Spectrum Disorder)
– Probably no single cause
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Autism
• Most have some abilities preserved
Savants – intellectually
handicapped individuals
who display specific
cognitive or artistic abilities ~1/10 autistic individuals display
savant abilities
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Neural Basis of Autism
• Genetic basis – Siblings of the autistic have a 5% chance of being
autistic
– 60% concordance rate for monozygotic twins
• Several genes interacting with the environment
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50+% of women who could become
pregnant are drinking
2% of women drink significantly during
pregnancy, 10% drink some
Glass of wine, bottle of beer, shot of liquor are equal
approximately 0.5 oz absolute alcohol
Fetal brain damage occurs at regular doses of 1-2 oz/day (2-4 drinks)
Source: National Institute on Alcohol
Abuse and Alcoholism
Fetal Alcohol Spectrum
Disorders
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Infant: Problems with sleep, feeding, milestones, muscle tone,
sensory information processing
Child: Hyperactive, poorly coordinated, learning delays
Adolescent/Adult: poor judgment, attention, problems with arithmetic,
memory, abstraction, frustration/anger
Symptoms of FASD
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Neural Basis of Fetal Alcohol
Spectrum Disorders
When is alcohol exposure most dangerous ???
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Neural Basis of Fetal Alcohol
Spectrum Disorders
Alcohol inhibits all stages of brain development, except
neuronal death, which it promotes.
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Brain damage resulting from prenatal
alcohol
Photo courtesy of Sterling Clarren, MD
Brain of baby with Brain of baby with heavy no exposure to
alcohol prenatal exposure to alcohol