Download Figure 12.15b

Nervous System
•
Master control and communication system
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Nervous System: Functions
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Three overlapping functions
• Sensory receptors monitor changes inside and
outside the body
• Change – a stimulus
• Gathered information – sensory input
• CNS Processes and interprets sensory input
• Makes decisions – integration
• Dictates a response by activating effector organs
• Response – motor output
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Basic Divisions of the Nervous System: CNS
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Central nervous system
(CNS)
• Brain and spinal cord
• Integrating and
command center
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Basic Divisions of the Nervous System: PNS
•
Peripheral nervous
system (PNS)
• Outside the CNS
• Nerves extending
from brain and spinal
cord
• Cranial nerves
• Spinal nerves
• Link all regions of the
body to the CNS
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Sensory Input and Motor Output
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Sensory signals picked up by sensory receptors
• Carried by afferent nerve fibers of PNS to the CNS
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Motor signals are carried away from the CNS
• Carried by efferent nerve fibers of PNS to effectors
• Innervate muscles and glands
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Sensory Input and Motor Output
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Divided according to region they serve
• Somatic body region
• Visceral body region
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Results in four main subdivisions
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Somatic sensory
Visceral sensory
Somatic motor
Visceral motor
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Somatic Sensory
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Somatic sensory
• General somatic senses – receptors are widely
spread
• Touch, pain, vibration, pressure, and temperature
• Proprioceptive senses – detect stretch in tendons and
•
muscle
Body sense – position and movement of body in
space
• Special somatic senses
• Hearing, balance, vision, and smell
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Visceral Sensory
•
Visceral sensory
• General visceral senses – stretch, pain, temperature,
nausea, and hunger
• Widely felt in digestive and urinary tracts,
reproductive organs
• Special visceral senses – taste
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Somatic Motor
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Somatic motor
• General somatic motor – signals contraction of
skeletal muscles
• Under voluntary control
• Often called “voluntary nervous system”
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Visceral Motor
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Visceral motor
• Regulates the contraction of smooth and cardiac
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•
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muscle and gland secretion
Makes up autonomic nervous system
Controls function of visceral organs
Often called “involuntary nervous system”
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Peripheral Nervous System Summary
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Figure 12.3
Types of Sensory and Motor Information
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Figure 12.3
Types of Sensory and Motor Information
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Figure 12.3
Nervous Tissue
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Cells are densely packed and intertwined
• Two main cell types
• Neurons – transmit electrical signals
• Support cells (neuroglial cells) – nonexcitable
• Surround and wrap neurons
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The Neuron
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The human body contains billions of neurons
• Basic structural unit of the nervous system
• Specialized cells conduct electrical impulses along
the plasma membrane
• Graded potentials
• Action potentials
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The Neuron: Special Characteristics
• Longevity – can live and function for a lifetime
• Do not divide – fetal neurons lose their ability to
•
undergo mitosis; neural stem cells are an exception
High metabolic rate – require abundant oxygen and
glucose
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Neuron Structure
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The Cell Body or Soma (also called Perikaryon)
• Size varies from 5–140µm
• Contains nucleus, organelles plus other structures
• Chromatophilic bodies (Nissl bodies)
• Clusters of rough ER and free ribosomes
• Stain darkly and renew membranes of the cell
• Neurofibrils – bundles of intermediate filaments
• Form a network between chromatophilic bodies
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Nissl Body Staining
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The Cell Body
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Most neuronal cell bodies
• Located within the CNS (clustered in nuclei)
• Protected by bones of the skull and vertebral
column
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Ganglia – clusters of cell bodies in PNS
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Cell Body Structure
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Figure 12.4
Neuron Processes: Dendrites
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Dendrites
• Extensively branching from
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the cell body
Transmit electrical signals
(graded potentials) toward the
cell body
Chromatophilic bodies – only
extend into the basal part of
dendrites
Function as receptive sites
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Dendritic Spines
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Neuron Processes: Axons
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Axons (nerve fibers)
• Neuron has only one, but it can
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branch
Impulse generator and conductor
Transmits action potentials away
from the cell body
Chromatophilic bodies absent
No protein synthesis in axon
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Neuron Processes: Axons
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Axons
• Neurofilaments, actin
microfilaments, and
microtubules
• Provide strength along
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length of axon
Aid in the transport of
substances to and
from the cell body
• Axonal transport
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Neuron Processes
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Axons
• Branches along length are
infrequent
• Axon collaterals
• Multiple branches at end of axon
• Terminal branches (telodendria)
• End in knobs called axon
terminals (also called end bulbs
or boutons)
Neuron Structure
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Neuron Processes: Action Potentials
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Nerve impulse (action potential)
• Generated at the initial segment of the
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axon
Conducted along the axon
Releases neurotransmitters at axon
terminals
Neurotransmitters – excite or inhibit
neurons
Neuron receives and sends signals
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Synapses
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•
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Site at which neurons communicate
Signals pass across synapse in one direction
Presynaptic neuron
• Conducts signal toward a synapse
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Postsynaptic neuron
• Transmits electrical activity away from a synapse
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Two Neurons Communicating at a Synapse
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Figure 12.6
Structure of a Synapses
PLAY
Synapse
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Figure 12.8a, b
Signals Carried by Neurons: Resting Membrane Potential
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Plasma membranes of neurons
conduct electrical signals
Resting neuron – membrane is
polarized
Inner, cytoplasmic side is
negatively charged
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Changes in Membrane Potential
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Signals occur as changes in membrane potential
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Directional Signals
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Stimulation of the neuron  depolarization
Inhibition of the neuron  hyperpolarization
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Action Potentials
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Figure 12.9a, b
Action Potentials on Axons
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Strong depolarizing stimulus applied to the axon
hillock triggers
• Action potential
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Membrane becomes positive internally
Action potential travels the length of the axon
Membrane repolarizes itself
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Action Potentials on Axons
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Figure 12.9c–e
Graded Potentials on Dendrites and the Cell Body
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Natural stimuli applied to dendrites and the cell
body
• Receptive zone of the neuron
Membrane stimulation causes local depolarization
• A graded potential – inner surface becomes less
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negative
Depolarization spreads from receptive zone to the
axon hillock
• Acts as the trigger that initiates an action potential in
the axon
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Synaptic Potentials
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Excitatory synapses
• Neurotransmitters alter the permeability of the
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postsynaptic membrane
Leads to an inflow of positive ions
• Depolarizes the postsynaptic membrane
• Drives the postsynaptic neuron toward impulse
generation
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Synaptic Potentials
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Inhibitory synapses
• The external surface of the postsynaptic membrane
becomes more positive
• Reduces the ability of the postsynaptic neuron to
generate an action potential
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Classification of Neurons
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Structural Classification
Functional Classification
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Structural Classification of Neurons
Classification based on number of processes
• Multipolar
• Bipolar
• Unipolar (pseudounipolar)
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Multipolar Neurons
Possess more than two
processes
Numerous dendrites and
one axon
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Figure 12.10a–c
Bipolar Neurons
Possess two processes
Rare neurons – found
in some special
sensory organs
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Figure 12.10a–c
Unipolar (Pseudounipolar) Neurons
Possess one single process
Start as bipolar neurons
during development
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Figure 12.10a–c
Afferent neurons
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Afferent (sensory) neurons –
transmit impulses toward the CNS
• Virtually all are pseudounipolar neurons (some true
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bipolar)
Cell bodies in ganglia outside the CNS
• Short, single process divides into
• The central process – runs centrally into the
•
CNS
The peripheral process – extends peripherally to
the receptors
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Afferent Neurons
Periphery
Sensory receptors
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CNS
Axon terminals
Efferent Neurons
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Efferent (motor) neurons
• Carry impulses away from the CNS to effector
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organs
Most efferent neurons are multipolar
Cell bodies are within the CNS
Form junctions with effector cells
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Interneurons
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Interneurons (association neurons) – most are
multipolar
• Lie between afferent and efferent neurons
• Confined to the CNS
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Neurons Classified by Function
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Figure 12.11
Variety of Interneurons
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Purkinje cell, stellate cell, granule cell, and basket
cell
• Located in the cerebellum
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Pyramidal cell – located in the cerebral cortex
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Variety of Interneurons
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Glial Cells (Supporting Cells)
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Six types of glial cells
• Four in the CNS
• Two in the PNS
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Provide supportive functions for neurons
Cover nonsynaptic regions of the neurons
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Supporting Cells (Neuroglial Cells) in the CNS
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Neuroglia – usually only refers to supporting cells
in the CNS, but can be used for PNS
• Glial cells have branching processes and a central
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•
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cell body
Outnumber neurons 10 to 1
Make up half the mass of the brain
Can divide throughout life
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Types of Glial Cells in the CNS
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Astrocytes
Microglia
Ependymal Cells
Oligodendrocytes
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Astrocytes
• Astrocytes – most abundant glial cell type
• Take up and release ions to control the environment
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around neurons
Recapture and recycle neurotransmitters
Involved with synapse formation in developing
neural tissue
Produce molecules necessary for neural growth
(BDTF)
Propagate calcium signals that may be involved in
memory
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Astrocytes
Necessary for development and maintenance of
theblood brain barrier
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Figure 12.12a
Microglia
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Microglia – smallest and
least abundant
• Phagocytes –
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•
the macrophages
of the CNS
Engulf invading
microorganisms and dead
neurons
Derived from blood cells
called monocytes
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Figure 12.12b
Ependymal Cells
• Ependymal cells
• Line the central cavity of the spinal cord and brain
• Bear cilia – help circulate the cerebrospinal fluid
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Oligodendrocytes
• Oligodendrocytes – have few branches
• Wrap their cell processes around axons in CNS
• Produce myelin sheaths
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Supporting Cells in the PNS
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Satellite cells – surround neuron cell bodies within
ganglia
Schwann cells (neurolemmocytes) – surround
axons in the PNS
• Form myelin sheath around axons of the PNS
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Figure 12.13
Myelin Sheaths
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Segmented structures composed of the lipoprotein
myelin
Surround thicker axons
Form an insulating layer
• Prevent leakage of electrical current
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Increase the speed of impulse conduction
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Myelin Sheaths in the PNS
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Formed by Schwann cells
Develop during fetal period and in the first year of
postnatal life
Schwann cells wrap in concentric layers around
the axon
• Cover the axon in a tightly packed coil of
membranes
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Myelin Sheaths in the PNS
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Nodes of Ranvier – gaps along axon
Allow current exchange across axon membrane
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Myelin Sheaths in the PNS
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Thick axons are myelinated
• Fast conduction velocity
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Thin axons are unmyelinated
• Slow conduction velocity
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Myelin Sheaths in the PNS
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Figure 12.14a
Myelin Sheaths in the PNS – myelinated axon
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Figure 12.15b
Myelin Sheaths in the PNS – unmyelinated axons
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Figure 12.15b
Myelin Sheaths in the CNS
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Oligodendrocytes form the
myelin sheaths in the CNS
• Have multiple processes
• Coil around several
different axons
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Oligodendrocytes
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Nerves
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Nerves – cordlike organs in the PNS
Consists of numerous axons wrapped in
connective tissue
Axon is surrounded by Schwann cells
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Nerves
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Endoneurium – layer of delicate
connective tissue surrounding the
axon
Nerve fascicles – groups of axons
bound into bundles
Perineurium – connective tissue
wrapping surrounding a nerve
fascicle
Epineurium – whole nerve is
surrounded by tough fibrous
sheath
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Simplified Design of the Nervous System
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Sensory neurons – located dorsally
• Cell bodies outside the CNS in sensory ganglia
• Central processes enter dorsal aspect of the spinal
cord
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Motor neurons – located ventrally
• Axons exit the ventral aspect of the spinal cord
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Interneurons – located centrally
• Provide communication between sensory and
motor neurons and between levels of the CNS
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Example of Neuronal Organization: Reflexes
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Reflex arcs – simple neural pathways
• Responsible for reflexes
• Rapid, autonomic motor responses
• Can be visceral or somatic
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Five Essential Components to the Reflex Arc
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Receptor – detects the stimulus
Afferent (sensory neuron) – transmits impulses to
the CNS
Integration center – consists of one or more
synapses in the CNS
Efferent (motor neuron) – conducts impulses from
integration center to an effector
Effector – muscle or gland cell
• Responds to efferent impulses
• Contraction or secretion
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Example of the Five Components to the Reflex Arc
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Figure 12.17
Reflex Classification
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Monosynaptic or polysynaptic
Spinal or cranial
Somatic or autonomic
Innate or learned
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Types of Reflexes: Number of Classes
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Monosynaptic reflex – simplest of all reflexes
• Just one synapse
• The fastest of all reflexes
• Example – knee-jerk reflex
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Polysynaptic reflex – more common type of reflex
• Most have a single interneuron between the sensory
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and motor neuron
Example – withdrawal reflexes
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Monosynaptic Reflex
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Figure 12.18a, b
Polysynaptic Reflex
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Figure 12.18a, b
Spinal vs Cranial Reflexes
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Spinal = spinal cord integration center
• Ex. Knee-jerk reflex
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Cranial = brain as integration center
• Ex. Pupillary light reflex
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Somatic vs Autonomic Reflexes
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Somatic = motor neurons to skeletal muscles
• Ex. Knee-jerk reflex
•
Autonomic = autonomic neurons to smooth
muscle and glands
• Ex. Pupillary light reflex
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Innate vs Learned Reflexes
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Innate = born-with
• Knee-jerk reflex, pupillary reflex
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Learned = develops based on experiences
• Pavlov’s dogs salivation in response to bell
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Gray versus White Matter in the Central Nervous System
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Gray matter
• Cell bodies
• Dendrites
• Synapses
•White matter
•Axons (myelin)
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Gray Matter in the Spinal Cord
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Gray matter in the spinal cord
• H-shaped (butterfly) region – surrounds central cavity
• Dorsal half contains cell bodies of interneurons
• Ventral half contains cell bodies of motor neurons
• Cell bodies are clustered in the gray matter
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White Matter in the Spinal Cord
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White matter in the spinal cord
• Located externally to the gray matter
• Contains no neuronal cell bodies, but millions of axons
• Myelin sheath – white color
• Consists of axons running between different parts of
the CNS
• Tracts – bundles of axons traveling to similar
destinations
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Gray Matter in Brain
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Cortex and nuclei
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White Matter in Brain
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Pathways, tracts and commissures
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Disorders of the Nervous System
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Multiple sclerosis – common cause of neural
disability
• Varies widely in intensity among those affected
• Cause is incompletely understood
• An autoimmune disease
• Immune system attacks the myelin around axons in
the CNS
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