Fundamentals of the Nervous System and Nervous Tissue

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Fundamentals of the Nervous System and Nervous
Tissue

• Chapter 12

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Introduction

• The nervous system is the master controlling and
  communicating system of the body
• It is responsible for all behavior
• Along with the endocrine system it is responsible
  for regulating and maintaining body homeostasis
• Cells of the nervous system communicate by means
  of electrical signals

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Nervous System Functions

• The nervous system has three overlapping
  functions
• Gathering of sensory input
• Integration or interpretation of sensory input
• Causation of a response or motor output

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Introduction

• Sensory input
• The nervous system has millions of sensory
  receptors to monitor both internal and external
  change
• Integration
• It processes and interprets the sensory input and
  makes decisions about what should be done at each
  moment
• Motor output
• Causes a response by activating effector organs
  (muscles and glands)

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Organization of the Nervous System
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Organization

• There is only one nervous system however, for
  convenience the nervous system is divided into
  two parts
• The central nervous system
• Brain and spinal cord
• Integrative and control centers
• The peripheral nervous system
• Spinal and cranial nerves
• Communication lines between the CNS and the rest
  of the body

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Organization

• Basic divisions of the nervous system
• Central Nervous Systems
• Peripheral Nervous System

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Organization

• The peripheral nervous system has two fundamental
  subdivisions
• Sensory (afferent) division
• Somatic and visceral sensory nerve fibers
• Consists of nerve fibers carrying impulses to the
  central nervous system
• Motor (efferent) division
• Motor nerve fibers
• Conducts impulses from the CNS to effectors
• (glands and muscles)

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Organization
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Organization

• The motor division of the peripheral nervous
  system has two main subdivisions
• The somatic nervous system
• Voluntary (somatic motor)
• Conducts impulses from the CNS to skeletal muscle
• Branchial motor
• Motor innervation of pharyngeal arch muscles
• The autonomic nervous system (ANS)
• Involuntary
• Conducts impulses from the CNS to cardiac
  muscles, smooth muscles, and glands

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Organization
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Innervation of Visceral Organs
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Organization

• The autonomic nervous system has two principle
  subdivisions
• Sympathetic division
• Mobilizes body systems during emergency
  situations
• Parasympathetic division
• Conserves energy
• Promotes non-emergency functions
• The two subdivisions bring about opposite effects
  on the same visceral organs
• What one subdivision stimulates, the other
  inhibits

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Peripheral Nervous System

• Visceral organs are served by motor fibers of the
  autonomic nervous system and by visceral sensory
  fibers
• The somata (limbs and body wall) are served by
  motor fibers of the somatic nervous system and by
  sensory somatic sensory fibers
• Arrows indicate the direction of impulses

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Histology of the Nervous Tissue

• Nervous tissue is highly cellular
• Less that 20 of the CNS is extracellular space
• Cells are densely packed and tightly intertwined
• Nervous tissue is made up of two cell types
• Neurons
• Excitable cells that transmit electrical signals
• Support cells
• Smaller cells that surround and wrap the delicate
  neurons
• These same cells are found within CNS and PNS

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Supporting Cells

• All neurons associate closely with non-nervous
  support cells of which there are 6 types
• Support cells of the CNS
• Astrocytes
• Microglial
• Ependymal
• Oligodendrocyte
• Support cells of the PNS
• Schwann cells
• Satellite cells

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Supporting Cells

• While each support cell has a unique specific
  function, in general these cells provide a
  supportive scaffolding for neurons
• In addition, they all cover nonsynaptic parts of
  the neurons thereby insulating the neurons and
  keeping the electrical activities of adjacent
  neurons from interfering with each other

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Clinical Insight

• The importance of support cells insulating nerve
  fibers is illustrated in the disorder call tic
  douloureux (doo loo-roo)
• In this condition the support cells around the
  sensory nerve fibers of the trigeminal nerve
  degenerate and are lost
• Impulses that carry touch sensations proceed to
  influence and stimulate the uninsulated pain
  fibers in the same nerve

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Supporting Cells in the CNS

• The supporting cells of the CNS are collectively
  called neuroglia or simply, glial cells
• Neuroglia usually refer to the CNS but some
  authors include the PNS

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Supporting Cells in the CNS

• Like neurons, glial cells have branching
  processes and a central cell body
• Neuroglia can be distinguished from neurons by
  their much smaller size and darker staining
  nuclei
• They outnumber neurons in the CNS by a ratio of
  10 to 1
• Make up half of the mass of the brain

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Astrocytes

• Star shaped
• Most abundant type of glial cell
• Radiating projections cling to neurons and
  capillaries, bracing the neurons to their blood
  supply
• Astrocytes play a role in exchanges of ions
  between capillaries and neurons

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Astrocytes

• Astrocytes take up and release ions to control
  the environment around neurons
• Concentrations of ions must be kept within narrow
  limits for nerve impulses to be generated
  conducted
• Astrocytes recapture and recycle potassium ions
  and released neuro- transmitters

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Astrocytes

• Astrocytes contact both the neuron and the
  capillary in order to sense when the neuron are
  highly active and releasing large amounts of
  neurotransmitters (glutamate)
• Astrocytes then extract blood sugar from the
  capillaries they contact to obtain the energy
  they need to fuel the process of glutamate uptake

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Microglial

• Smallest and least abundant type of neuroglial
  cell
• The ovid cells have relatively long thorny
  processes
• Their branches touch nearby neurons to monitor
  health of the neuron

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Microglial

• These are small ovid cells with relatively long
  thorny processes
• Microglial derive from blood cells and migrate to
  the CNS during embryonic and fetal development

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Microglial

• These cells are phagocytes, the marcophages of
  the CNS
• Microglial move to and then engulf microorganisms
  and injured or dead neurons

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Microglial

• When invading micro- organisms are present or
  damaged neurons have died, the micro- glial
  transforms into a special type of macro- phage
  that protects the CNS by phagocytizing the
  microorganisms or neuronal debris
• Important because cells of the immune system can
  enter CNS

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Ependymal

• Range in shape from squamous to columnar and many
  are cilated
• Line the central cavities of the brain and spinal
  cord
• Form a fairly permeable barrier between
  cerebrospinal fluid of those cavities and the
  cells of the CNS
• Beating cilia circulates cerebrospinal fluid

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Oligodendro- cytes

• Fewer branches than astrocytes
• Cells wrap their cytoplasmic extensions tightly
  around the thicker neurons in the CNS
• Produce insulating coverings called myelin sheaths

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Supporting Cells of the PNS

• There are two supporting cells in the PNS
• Satellite cells
• Schwann cells
• These cells are similar in type and differ mainly
  in location

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Satellite Cells

• Somewhat flattened satellite cells surround cell
  bodies within ganglia
• Thought to play some role in controlling the
  chemical environment of neurons with which they
  are associated, but function is largely unknown

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Schwann Cells

• Surround and form myelin sheaths around the
  larger nerve fibers in PNS
• Similar to the oligodendrocytes of CNS
• Schwann cells are vital to peripheral nerve fiber
  regeneration

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Neurons

• Neurons are the structural units of the nervous
  system
• Neurons are highly specialized cells that conduct
  messages in the form of nerve impulses from one
  part of the body to another

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Neuron Characteristics

• Extreme longevity
• Live and function optimally for a lifetime
• Amitotic
• As neurons assume their role in the nervous
  system they lose their ability to divide
• Neurons cannot be replaced if destroyed
• High metabolic rate
• Require continuous and abundant supplies of
  oxygen and glucose
• Homeostatic deviations often first appear in
  nervous tissue which has specific needs

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Neurons

• The plasma membrane of neurons is the site of
  electrical signaling, and it plays a crucial role
  in most cell to cell interaction
• Most neurons have three functional components in
  common
• A receptive component
• A conducting component
• A secretion or output component
• Each component is associated with a particular
  region of a neurons anatomy

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Neuron structure

• Typically large, complex cells, they all have the
  following structures
• Cell body
• Nuclei
• Chromatophilic (Nissl) bodies
• Neurofibrils
• Axon hillock
• Cell processes
• Dendrites
• Axon
• Myelin sheath or neurilemma

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Neuron structure

• Cell Body
• Nuclei
• Chromatophilic (Nissl) bodies
• Neurofibrils
• Axon hillock
• Neuron Processes
• Dendrites
• Axons
• Myelin sheaths
• Axon terminals

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Neuron structure

• The cell body consists of a large, spherical
  nucleus with a prominent nucleolus surrounded by
  cytoplasm
• The cell ranges from 5 to 140?m in diameter
• The cell body is the biosynthetic center of the
  neuron

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Neuron structure

• The cell body contains the usual organelles with
  the exception of centrioles (not needed in
  amitotic cells)
• The rough endoplasmic reticulum or Nissl bodies
  is the protein and membrane making machinery of
  the cell
• The cell body is the focal point for neuron
  growth in development

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Neuron structure

• Neurofibrils are bundles of intermediate
  filaments (neurofilaments) that run in a network
  between the chromatophilic bodies
• Neurofibrils keep the cell from being pulled
  apart when it is subjected to tensile stresses

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Neuron structure

• In most neurons, the plasma membrane of the cell
  body acts as a receptive surface that receives
  signals from other neurons

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Neuron Cell Bodies

• Most neuron cell bodies are located with the CNS
  where they are protected by the bones of the
  skull and vertebral column
• Clusters of cell bodies in the CNS are called
  nuclei
• The relatively rare collection of cell bodies in
  the PNS are called ganglia

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Neuron Processes
Motor neuron

• Cytoplasmic extension called processes extend
  from the cell body of all neurons
• The CNS contain both neuron cell bodies and their
  processes
• The PNS consists chiefly of processes

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Neuron Processes
Motor neuron

• Bundles of neuron processes in the CNS are called
  tracts
• Bundles of neuron processes in the PNS are called
  nerves

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Dendrites

• Dendrites are short, tapering diffusely branching
  extensions
• Motor neurons have hundreds of dendrites
  clustering close to the cell body
• Dendrites are receptive cites and provide an
  enormous surface area for the reception of
  signals
• In many areas of the brain the finer dendrites
  are highly specialized for information collection

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Dendrites

• Dendritic spines represent areas of close contact
  with other neurons
• Dendrites convey information toward the cell body
• These electrical signals are not nerve impulses
  but are short distance signals call graded
  potentials

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Axons

• Each neuron has a single axon
• The axon arises from the cone shaped axon hillock
• It narrows to form a slender process that stays
  uniform in diameter the rest of its length
• Length varies short or absent to 3 feet in length

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Axons

• Each axon is called a nerve fiber
• Axons are impulse generators and conductors that
  transmit nerve impulses away from the cell body

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Axons

• Chromatophilic bodies and the Golgi apparatus are
  absent from the axon and the axon hillock
• The axons also lack ribosomes and all organelles
  involved in protein synthesis so they must
  receive their proteins from the cell body

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Axons

• Neurofilaments, actin microfilaments, and
  microtubules are especially evident in axons,
  where they provide structural strength

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Axons

• Neurofilaments are cytoskeleton elements that
  also aid in the transport of substances to and
  from the cell body as the axonal cytoplasm is
  continually recycled and renewed
• This movement of substances along axons is called
  axonal transport

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Axons

• Axons branch less extensively that dendrites
• Each neuron has only one axon but may possess a
  collateral branch
• All axons branches profusely at its terminal end
  to form more than 10,000 telodendria or terminal
  branches

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Axons

• The axon terminals contact other neurons to form
  specialized cell junctions called synapses
• A nerve impulse is conducted along the axon to
  the axon terminals where it causes a release of
  chemicals called neurotransmitters

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Axons

• Neurotransmitters are release into the
  extracellular space called a synaptic cleft
• The neurotransmitters excite or inhibit the
  neurons with which axon is in close contact
• Because each neuron typically receives signals
  from and sends to scores of other neurons, it
  carries on conversations with many different
  neurons at the same time

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Axons

• Axon diameter varies considerably among the
  different neurons of the body
• Axons with larger diameters conduct impulses
  faster than those of smaller diameters because of
  the basic laws of physics The resistance to the
  passage of an electrical current decreases as the
  diameter of any cable increases

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Synapses

• The site at which neurons communicate is called a
  synapse, a cell junction that mediates the
  transfer of information from one neuron to the
  next

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Synapses

• Because signals pass across most synapses in one
  direction only, synapses determine the direction
  of information flow throughout the nervous system

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Synapses

• The neuron the conducts impulses toward a synapse
  is called the presynaptic neuron

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Synapses

• The neuron that conducts impulses away from the
  synapse is called the postsynaptic neuron

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Synapses

• Most neurons function as presynaptic (information
  sending) and postsynaptic (information receiving
  neurons
• In essence they get information from some neurons
  and dispatch it to others

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Synapses

• Most synapses occur between the axon terminals of
  one neuron and the dendrites of another axons
• These are called axodendritic synapses

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Synapses

• Less common, and far less understood, are
  synapses between two axons (axoaxonic), between
  two dendrites (dendrodendritic) or between a
  dendrite and a cell body (dendosomatic)

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Synapses

• Structurally synapses are elaborate cell
  junctions
• At the typical axodendritic synapse the
  presynaptic axon terminal contain synaptic
  vesicles

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Synapses

• Synaptic vesicles are membrane bound sacs filled
  with molecular neurotransmitters
• These molecules transmit signals across the
  synapse

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Synapses

• Mitochondria are abundant in the axon terminal as
  the secretion of neurotransmitters requires a
  great deal of energy

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Synapses

• At the synapse, the plasma membranes of the two
  neurons are separated by a synaptic cleft
• On the under surfaces of the opposing cell
  membranes are dense materials the pre- and post-
  synaptic densities

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Synapses

• When an impulse travels along the axon of the
  presynaptic neuron, it signals the synaptic
  vesicles to fuse with the presynaptic membrane at
  the presynaptic density
• The released neurotransmitter molecules diffuse
  across the synaptic cleft and bind to the
  postsynaptic membrane at the post synaptic density

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Synapse

• The binding of the two membranes changes the
  membrane charge on the postsynaptic neuron,
  influencing the generation of a nerve impulse or
  action potential in that neuron

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Signals Carried by Neurons

• In review, plasma membranes of neurons conduct
  electrical signals and that synapses relay the
  signals from neuron to neuron

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Signals Carried by Neurons

• In a resting (unstimulated) neuron, the membrane
  is polarized which means that the inner
  cytoplasmic side is negatively charged with
  respect to its outer, extracellular side

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Signals Carried by Neurons

• When a neuron is stimulated the permeability of
  the plasma membrane changes at the site of the
  stimulus, allowing positive ions to rush in.
• As a result, the inner face of the membrane
  becomes less negative or depolarized

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Signals Carried by Neurons

• Any part of the neuron depolarizes if stimulated,
  but at the axon alone this can result in the
  triggering of a nerve impulse or action potential

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Signals Carried by Neurons

• When a nerve impulse or action potential develops
  the membrane is not only depolarized , but its
  polarity is completely reversed so it becomes
  negative externally and positive internally

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Signals Carried by Neurons

• Once begun, the nerve impulse travels rapidly
  down the entire length of the axon without
  decreasing in strength

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Signals Carried by Neurons

• After the impulse has passed the membrane
  repolarizes itself

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Graded Potential

• In humans, natural stimuli are not applied
  directly to axons, but to dendrites and the cell
  body which constitute the receptive zone of the
  neuron
• When the membrane of this receptive zone is
  stimulated it does not undergo a polarity
  reversal
• Instead it undergoes a local depolarization in
  which the inner surface of the membrane merely
  becomes less negative

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Graded Potential

• This local depolarization is called a graded
  potential which spreads from the receptive zone
  to the axon hillock (trigger zone) decreasing in
  strength as it travels
• If this depolarizing signal is strong enough when
  it reaches the initial segment of the axon, it
  acts as the trigger that initiates an action
  potential in the axon
• Signals from the receptive zone determine if the
  axon will fire an impulse

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Synaptic Potential

• Most neurons in the body do not receive stimuli
  directly from the environment but are stimulated
  only by signals received at synapses from other
  neurons
• Synaptic input influences impulse generation
  through either excitatory or inhibitory synapses

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Synaptic Potential

• In excitatory synapses, neurotransmitters
  released by presynaptic neurons alter the
  permeability of the postsysnaptic membrane to
  certain ions, this depolarizes the postsynapatic
  membrane and drives the postsynaptic neuron
  toward impulse generation

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Synaptic Potential

• Inhibitory synapses cause the external surface of
  the postsynaptic membrane to become even more
  positive, thereby reducing the ability of the
  postsynaptic neuron to generate an action
  potential
• Thousands of excitatory and inhibitory synapses
  act on every neuron, competing to determine
  whether or not that neuron will generate an
  impulse

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Neural Integration

• The organization of the nervous system is
  hierarchical
• The parts of the system must be integrated into a
  smoothly functioning whole
• Neuronal pools represent some of the basic
  patterns of communication with other parts of the
  nervous system

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Neuronal Pools

• Neuronal pools are functional groups of neurons
  that process and integrate incoming information
  from other sources and transmit it forward

One incoming presynaptic fiber synapses
with Several different neurons in the pool.
When Incoming fiber is excited it will excite
some Postsynaptic neurons and facilitate others.
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Neuronal Pools

• Neurons most likely to generate impulses are
  those most closely associated with the incoming
  fiber because they receive the bulk of the
  synaptic contacts
• These neurons are in the discharge zone

Discharge Zone
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Neuronal Pools

• Neurons farther away from the center are not
  excited to threshold by the incoming fiber, but
  are facilitated and can easily brought to
  threshold by stimuli from another source
• The periphery of the pool is the facilitated zone

Facilitated zone
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Neuronal Pools

• Note The illustrations presented are a gross
  oversimplification of an actual neuron pool
• Most neuron pools consist of thousands of neurons
  and include inhibitory as well as excitatory
  neurons

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Classification of Neurons

• Neurons can be classified structurally or
  functionally
• Both classifications are described in the text
• According to the structural classification system
  there are three types of neurons
• Multipolar
• Bipolar
• Unipolar

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StructuralClassification

• Multipolar - many processes extend from cell
  body, all dendrites except one axon
• Bipolar - Two processes extend from cell, one a
  fused dendrite, the other an axon
• Unipolar - One process extends from the cell body
  and forms the peripheral and central process of
  the axon

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Multipolar Neurons

• Multipolar neurons have more than two processes
• Most common type in humans
• Major neuron of the CNS
• Most have many dendrites and one axon, some
  neurons lack an axon

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Bipolar Neurons

• Bipolar neurons are rare in the human body
• Found only in special sense organs where they
  function as receptor cells
• Examples include those found in the retina of the
  eye, inner ear, and in the olfactory mucosa
• They are primarily sensory neurons

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Unipolar Neuron

• Unipolar neurons have a single process that
  emerges from the cell body
• The central process (axon) is more proximal to
  the CNS and the peripheral is closer to the PNS
• Unipolar neurons are chiefly found in the ganglia
  of the peripheral nervous system
• Function as sensory neurons

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Functional Classification

• The functional classification scheme groups
  neurons according to the direction in which the
  nerve impulse travels relative to the CNS
• Based on this criterion there are three neurons
• Sensory neurons
• Motor neurons
• Interneurons

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Functional Classification
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Sensory Neurons

• Neurons that transmit impulses from sensory
  receptors in the skin or internal organs toward
  or into the CNS are called sensory or afferent
  neurons
• Virtually all primary sensory neurons of the body
  are unipolar

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Sensory Neurons

• Sensory neurons have their ganglia outside of the
  CNS
• The single (unipolar) process is divided into the
  central process and the peripherial process

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Sensory Neuron

• The central process is clearly an axon because it
  carries a nerve impulse and carries that impulse
  away from the cell body which meet the criteria
  which define an axon
• The peripheral by contrast carries nerve impulses
  toward the cell body which suggests that it is a
  dendrite
• However, the basic convention is that the central
  process and the peripheral process are parts of a
  unipolar neuron

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Motor Neurons

• Neurons that carry impulses away from the CNS to
  effector organs (muscles and glands) are called
  motor or efferent neurons
• Upper motor neurons are in the brain
• Lower motor neurons are in PNS

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Motor Neurons

• Motor neurons are multipolar and their cell
  bodies are located in the CNS (except autonomic)
• Motor neurons form junctions with effector cells,
  signaling muscle to contract or glands to secrete

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Interneuron or Association Neuron

• These neurons lie between the motor and sensory
  neurons
• These neurons are found in pathways where
  integration occurs
• Confined to CNS
• Make up 99 of the neurons of the body and are
  the principle neuron of the CNS

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Interneuron Neurons

• Almost all interneurons are multipolar
• Interneurons show great diversity in the size and
  branching patterns of their processes

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Interneurons

• The Pyramidal cell is the large neuron found in
  the primary motor cortex of the cerebrum
• The Purkinje cell from the cerebellum

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Interneurons

• Stellate cells of the cerebellum

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Interneurons

• Granule cells of the cerebellum

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Interneurons

• Basket cells of the cerebellum

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Myelin Sheaths

• Myelin sheaths are segmented structures, each
  composed of the lipoprotein myelin
• The sheaths surround the thicker axons of the
  body

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Myelin Sheaths

• Myelin sheaths form an insulating layer that
• Prevents the leakage of electrical current from
  the axon
• Increases the speed of impulse conduction
• Makes impulse propagation more energy efficient

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Myelin Sheath

• Myelin increases the speed of transmission of
  nerve impulses
• Myelinated axons transmit nerve impulses rapidly
  150 meters/second
• Unmyelinated axons transmit quite slowly 1
  meter/second

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Myelin Sheaths

• Each segment of myelin consists of the plasma
  membrane of the supporting cell rolled in
  concentric layers around the axon

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Myelin Sheaths - PNS

• The myelin sheaths in the PNS are formed by
  Schwann cells
• Myelin develops during the fetal period and the
  first year or so of postnatal life

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Myelin Sheaths - PNS

• In forming the cells indent to receive the axon
  and then wrap themselves around the axon
  repeatedly in a jellyroll fashion
• Initially loose, the wrapping eventually squeeze
  the cytoplasm outward between cell membrane layers

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Myelin Sheaths - PNS

• The nucleus and most of the cytoplasm end up just
  external to the myelin layers

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Myelin Processes - PNS

• Myelin sheaths are associated only with axons and
  their collaterals as these are impulse conducting
  fibers and need insulation
• Dendrites which carry only graded potentials are
  always unmyelinated

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Myelin Sheaths - PNS

• When the wrapping process is complete many
  concentric layers wrap the axon
• Plasma membranes of myelinating cells have less
  protein which makes them good electrical
  insulators

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Myelin Sheaths - PNS

• Because the adjacent Schwann cells do not touch
  one another there are gaps in the myelin sheath
• These gaps, called nodes of Ranvier, occur at
  regular intervals about 1 mm apart

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Myelin Sheaths - PNS

• Since the axon is only exposed at these nodes
  nerve impulses are forced to jump from one node
  to the next which greatly increases the rate of
  impulse conduction

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Myelin Sheaths - PNS

• Schwann cells that surround but do not coil
  around peripheral fibers are considered
  unmyelinated
• A single Schwann cell can partly enclose 15 or
  more axons
• Each ends occupying a separate tubular recess

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CNS Axons

• Oligodendrocytes form the CNS myelin sheaths
• In contast to Schwann cells, oligodendrocytes can
  form the sheaths of as many as 60 processes at
  one time
• Nodes are spaced more widely than in PNS
• Axons can be myelinated or unmyelinated

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CNS Axons

• Regions of the brain containing dense collections
  of myelinated fibers are referred to as white
  matter and are primarily fiber tracts
• Gray matter contains mostly nerve cell bodies and
  unmyelinated fibers

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Types of Circuits

• Individual neurons in a neuron pool send and
  receive information and synaptic contacts may
  cause either excitation or inhibition
• The patterns of synaptic connections in neuronal
  pools are called circuits and they determine the
  functional capabilities of each type of circuit
• There are four basic types of circuits
• Diverging, converging, reverberating, and
  parallel discharge circuits

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Diverging Circuits

• In diverging circuits one incoming fiber triggers
  responses in ever-increasing numbers of neurons
  farther and farther along in the circuit
• Diverging circuits are often called amplifying
  circuits because they amplify the response

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Diverging Circuits

• These circuits are common in both sensory and
  motor systems
• Input from a single receptor may be relayed up
  the spinal cord to several different brain
  regions
• Impulses from the brain can activate a hundred
  neurons and thousands of muscle fibers

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Converging Circuits

• The pattern of converging circuits is opposite to
  that of diverging circuits
• Common in both motor and sensory pathways
• In these circuits, the pool receives inputs from
  several presynaptic neurons, and the circuit as a
  whole has a funneling or concentrating effect

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Converging Circuits

• Incoming stimuli may converge from many different
  areas or from the same source, which results in
  strong stimulation or inhibition

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Reverberating (oscillating) Circuits

• In reverberating circuits the incoming signal
  travels through a chain of neurons, each of which
  makes collateral synapses with neurons in the
  previous part of the pathway
• As a result of this positive feedback, the
  impulses reverberate through the circuit again
  and again

Reverberating circuit
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Reverberating (oscillating) Circuits

• Reverberating circuits give a continuous signal
  until one neuron in the circuit is inhibited and
  fails to fire
• These circuits are involved in the control of
  rhythmic activities such as the sleep-wake cycle
  and breathing
• The circuits may oscillate for seconds, hours, or
  years

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Parallel After-Discharge Circuits

• The incoming fiber stimulates several neurons
  arranged in parallel arrays that eventually
  stimulate a common output cell
• Impulses reach the output cell at different
  times, creating a burst of impulses called an
  after discharge that may last 15 ms after initial
  input ends

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Parallel After-Discharge Circuits

• This circuit has no positive feedback and once
  all the neurons have fired, circuit activity ends
• These circuit may be involved with complex
  problem solving activities

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Patterns of Neural Processing

• Processing of inputs in the various circuits is
  both serial and parallel
• In serial processing, the input travels along a
  single pathway to a specific destination
• In parallel processing, the input travels along
  several different pathways to be integrated in
  different CNS regions
• Each pattern has its advantages
• The brain derives its power from its ability to
  process in parallel

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Serial Processing

• In serial processing the whole system works in a
  predictable all-or-nothing manner
• One neurons stimulates the next in sequence,
  producing a specific, anticipated response
• Reflexes are examples of serial processing but
  there are others

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Parallel Processing

• In parallel processing inputs are segregated into
  many different pathways
• Information delivered by each pathway is dealt
  with simultaneously by different parts of neural
  circuitry
• During parallel processing several aspects of the
  stimulus are processed
• Barking dog
• The same stimulus can hold common or unique
  meaning to different individuals

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Parallel Processing

• Parallel processing is not repetitious because
  the circuits do different things with more
  information
• Each parallel path is decoded in relation to all
  the others to produce a total picture of the
  stimulus

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Parallel Processing

• Even simple reflex arcs do not operate in
  complete isolation
• As an arc moves through an association neuron
  this activates parallel processing of the same
  input at higher brain levels
• The reflex arc may cause you to pull away from a
  negative stimulus while parallel processing of
  the stimulus initiates problem solving about what
  need to be done

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Parallel Processing

• Parallel processing is extremely important for
  higher level mental functioning
• An integrated look at the whole problem allows
  for faster processing
• Parallel processing allows you to store a large
  amount of information in a small volume
• This allows logic systems to work much faster

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Reflexes

• Reflexes are rapid, automatic responses to
  stimuli, in which a particular stimulus always
  causes the same motor response
• Reflex activity is stereotyped and dependable
• Some your are born with and some you acquire as a
  consequence of interacting with your environment

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Reflex Arcs

• Reflex arcs are simple chains of neurons that
  explain our simplest, reflective behaviors and
  determine the basic structural plan of the
  nervous system
• Reflex arcs are responsible for reflexes, which
  are defined as rapid, automatic motor responses
  to stimuli

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Reflex Arcs

• Reflexes that involve the contraction of skeletal
  muscle are referred to as somatic reflexes
• Reflexes that involve the contraction of smooth
  muscle, cardiac muscle, or glands are referred to
  as visceral reflexes

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Serial Processing A Reflex Arc

• Reflexes occurs over neural pathways called
  reflex arcs that contain five essential
  components
• Receptor
• Sensory neuron
• CNS integration center
• Motor neuron
• Effector

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Reflex Arcs

• The receptor, sensory neuron, motor neuron, and
  effector are all relatively straightforward
  components
• When considering the integration center
  associated with reflex arcs, it is important to
  understand that the number of synapses involved
  can vary
• The simplest reflex arcs involve only one synapse
  in the CNS while others involve multiple synapses
  and interneurons

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Reflex Arcs

• At the top is a reflex arc, at the left is a
  monosynaptic reflex and on the right is a poly
  synaptic reflex

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Reflex Arcs

• The monosynaptic reflex has only one synapse and
  no interneuron, while the polysynaptic has
  multiple synapses and an interneuron

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Reflex Arcs - Monosynaptic

• This is the simple knee-jerk reflex
• The impact of the hammer on the patellar tendon
  stretches the quadriceps muscles

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Reflex Arcs - Monosynaptic

• Stretching activates a sensory neuron that
  directly activates a motor neuron in the spinal
  cord, which then signals the quadriceps
  muscle to contract
• This contraction counteracts the original
  stretching caused by the hammer

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Reflex Arcs - Monosynaptic

• Many skeletal muscles of the body can be
  activated by monosynaptic stretch reflexes
• These reflexes help maintain equilibrium and
  upright posture
• In these postural muscles, sensory neurons sense
  the stretching of muscles that occurs when the
  body begins to sway
• Motor neurons activate muscles that adjust the
  bodys position to prevent a fall

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Reflex Arcs - Monosynaptic

• Because stretch reflexes contain just one synapse
  monosynaptic reflexes are the fastest of all
  reflexes
• They are used in the body to maintain balance and
  equilibrium where speed of adjustment is
  essential to keep from falling

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Reflex Arcs - Polysynaptic

• Polysynaptic reflexes are the more common
  reflexes in the body
• In these reflexes, one or more interneurons are
  part of a reflex pathway between the sensory and
  motor neurons

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Reflex Arcs - Polysynaptic

• Most of the simple reflex arcs in the body
  contain a single interneuron and therefore have
  a total of three neurons
• Since there are two synapses joining the three
  neurons they are referred to as polysynaptic

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Reflex Arcs - Polysynaptic

• Withdrawal reflexes by which we pull away from
  danger are three-neuron reflexes
• Pricking a finger with a tack initiates an
  impulse in the sensory neuron, which activates
  the interneuron in the CNS

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Reflex Arcs - Polysynaptic

• The interneuron signals the motor neuron to
  contract the muscle that withdraws the hand from
  the negative stimulus

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Reflex Arcs - Polysynaptic

• The three neuron reflex arc are of special
  importance in the science of neuroanatomy
• Three neuron reflex arcs reveal the fundamental
  design of the entire nervous system

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Design of the Nervous System

• Three neuron reflex arcs from the basis of the
  structural plan of the nervous system

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Design of the Nervous System

• Note that the cell bodies of the sensory neurons
  lie outside the CNS in sensory ganglia and that
  their central processes enter the dorsal aspect
  of the cord

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Design of the Nervous System

• In the CNS the cell bodies of most interneurons
  lie dorsal to those of the motor neurons and the
  long axons exit the ventral aspect of the spinal
  cord

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Design of the Nervous System

• The nerves of the PNS consist of the motor axons
  plus the long peripheral process of the sensory
  neurons

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Design of the Nervous System

• These motor and sensory nerve fibers extend
  throughout the body to reach the peripheral
  effectors and receptors

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Design of the Nervous System

• Even though reflex arcs determine its basic
  organization, the human nervous system is
  obviously more complex than a series of simple
  reflex arcs
• To appreciate its complexity, we must expand our
  conception of interneurons
• Interneurons include not only the inter- mediate
  neurons of reflex arcs, but also all the neurons
  that are entirely confined within the CNS

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Design of the Nervous System

• The complexity of the CNS arises from the
  organization of the vast numbers of interneurons
  in the spinal cord and brain into complex neural
  circuits that process information
• The complexity of the CNS results from long
  chains of interneurons that are interposed
  between each sensory and motor neuron

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Design of the Nervous System

• Although tremendously oversimplified, the
  infor-mation depicted is a useful way to
  conceptualize the organization of neurons in the
  CNS

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Design of the Nervous System

• The CNS has distinct regions of gray and white
  matter that reflect the arrangement of its
  neurons
• The gray matter is a gray colored zone that
  surrounds the hollow cavity of the CNS
• It is H-shaped in the spinal cord, where its
  dorsal half contains cell bodies of interneurons
  and its ventral half contains cell bodies of
  motor neurons

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Design of the Nervous System

• Gray matter is a site where neuron cell bodies
  are clustered
• Specifically, gray matter is a mixture of neuron
  cell bodies, dendrites, and short unmyelinated
  axons

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Design of the Nervous System

• White matter which contains no neuron cell bodies
  but millions of axons
• Its white color comes from the myelin sheaths
  around many of the axons
• Most of these axons ascend from the spinal cord
  to the brain or descend from the brain to the
  spinal cord, allowing these two regions of the
  CNS to communicate with each other

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Design of the Nervous System

• White matter consists of axons running between
  different parts of the CNS
• Within the white matter, axons traveling to
  similar destinations form axon bundles called
  tracts

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Nervous Tissue Development

• During the embryonic period, which spans 8 weeks,
  the embryo goes from zygote to blastocyst, to two
  layer embryo, to three layer embryo
• The embryo upon reaching three layers begins to
  form the neural tube from which will
  differentiate the brain and spinal cord

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Nervous Tissue Development

• The nervous system develops from the dorsal
  section of the ectoderm, which invaginates to
  form the neural tube and the neural crest

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Nervous System Development

• The walls of the neural tube begin as a layer of
  neuroepithelial cells become the CNS
• These cells divide, migrate externally, and
  become neuroblasts (future neurons) which never
  again divide

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Nervous System Development

• These cells divide, migrate externally, and
  become neuroblasts (future neurons) which never
  again divide
• They cluster as future interneurons and motor
  neurons

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Nervous System Development

• Just external to the neuroepithelium, the
  neuroblasts cluster into alar and basal plates

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Nervous System Development

• Dorsally, the neurons of the alar plate become
  interneurons
• Ventrally, the neuroblasts of the basal plate
  become motor neurons and sprout axons that grow
  out to the effector organs

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Nervous System Development

• Axons that sprout from the young interneurons
  form the white matter by growing outward the
  length of the CNS
• These events occur in both the spinal cord and
  the brain

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Nervous System Development

• Most of the events described take place in the
  second month of development, but neurons continue
  to form rapidly until the about the sixth month
• At the sixth month neuron formation slows
  markedly, although it may continue at a reduced
  rate into childhood

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Nervous System Development

• Just before neuron formation slows, the
  neuroepithelium begins to produce astrocytes and
  oligiodendrocytes
• The earliest of these glial cells extend outward
  from the neuroepithelium and provide pathways
  along which young neurons migrate to reach their
  final destination
• As the division of its cells slows, the
  neuroepithelium becomes the ependymal layer

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Nervous System Development

• Sensory neurons do not arise from the neural tube
  but from the neural crest
• This explains why the cell bodies of the sensory
  neurons lie outside the CNS
• Sensory neurons also stop dividing during the
  fetal period

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Nervous System Development

• Sensory neurons cell bodies develop outside the
  CNS in the neural crest
• Sensory neurons also stop dividing during the
  fetal period

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Nervous System Development

• Neuroscientists are actively investigating how
  forming neurons hook up with each other during
  development
• As the growing axons elongate at growth cones,
  they are attached by chemical signals from other
  neurons called neurotrophins
• At the same time, the receiving dendites send out
  thin, extensions to reach the approaching axons
  to form synapses

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Nervous System Development

• Which synaptic connections are made, and which
  persist, are determined by two factors
• The amount of neurotrophin initially received
• The degree to which a synapse is used after being
  established

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Nervous System Development

• Neurons that make bad connections are signaled
  to die via apoptosis
• Of the neurons formed during the embryonic
  period, about two-thirds die before birth
• This initial overproduction of neurons ensures
  that all necessary neural connections will be
  made and that mistaken connections will be
  eliminated