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NERVOUS
SYSTEM
NERVOUS FUNCTIONS
• Body’s master controlling and communicating
system
• Three functions
– Sensory input
• Gathers information
from sensory receptors
– Integration
• Processes and interprets
sensory input
– Motor output
• Activates effector organs to cause a response
Nervous System Organization
ORGANIZATION
Two Principal Parts of the System
• Central nervous system (CNS)
– Brain and spinal cord
– Integrating and command center
• Interprets sensory input
• Dictates motor responses
• Peripheral nervous system (PNS)
– Nerves extending from brain and spinal cord
– Carry impulses to and from the CNS
PERIPHERAL DIVISIONS
Two Functional Subdivisions of the PNS
• Sensory division
– “afferent division”
– Nerve fibers conveying impulses to the CNS
• Somatic afferent fibers convey impulses from the skin,
muscles, and joints
• Visceral afferent fibers convey impulses from visceral organs
• Motor division
– , “efferent division”
– Nerve fibers conveying impulses from the CNS
ORGANIZATION
HISTOLOGY
• Nervous system consists mainly of nervous tissue
• Highly cellular
– e.g., <20% extracellular space in CNS
• Two principal cell types
– Neurons
• Excitable nerve cells that transmit electrical signals
– Supporting cells
• Smaller cells surrounding and wrapping neurons
• “Neuroglia”
NEURONS
• Nerve cells
• Structural units of nervous system
– Billions are present in nervous system
• Conduct messages throughout body
– Nerve impulses
• Extreme longevity
– Can function optimally for entire lifetime
• Amitotic
– Ability to divide is lost in mature cells
– Cannot be replaced if destroyed
• Some (very few) exceptions
• e.g., stem cells present in olfactory epithelium can produce new neurons
– Stem cell research shows great promise in repairing damaged neurons
• High metabolic rate
– Require large amounts of oxygen and glucose
Neurons
Dendrites
Cell Body
Myelin
Sheath
Axon of another
neuron
Axon
Dendrites of
another neuron
Collins I
4 lines
• Based on the
diagram, what do you
think each part does
to receive and pass
along an impulse
toward the brain
Agenda
11/3/11 – Day 1
• Take more notes
• HW- vocab
NEURONS
• Generally large, complex cells
• Structures vary, but all neurons have the same basic
structure
– Cell body
– Slender processes
extending from cell
body
– Plasma membrane
is site of signaling
NEURON CELL BODY
• Most neuron cell bodies are located in the CNS
– Protected by bones of skull or vertebral column
• Clusters of cell bodies in the CNS are termed
“nuclei”
• Clusters of cell
bodies in the
PNS are
termed
“ganglia”
NEURON CELL BODY
• Major biosynthetic (control) center of neuron
• Other usual organelles present except CENTRIOLES
-Why?
• What do centrioles
do?
NEURON PROCESSES
• Extend from the neuron’s cell body
• Two types of neuron processes
– Dendrites
– Axons
NEURON PROCESSES
Typical Dendrite
• Short, slender, branching extensions of cell body
– Generally hundreds clustering close to cell body
– Most cell body organelles also present in dendrites
• Main receptive / input regions
– Large surface area for
receiving signals from
other neurons
– Convey incoming messages
toward cell body
NEURON PROCESSES
Typical Axon
• Single axon per neuron
• The axon forms from the narrowing of the cell body.
The region between the large cell body and the axon is
the “axon hillock”
• Sometimes very short
• Sometimes very long
– e.g., axons controlling
big toe are 3 – 4 feet
long
NEURON PROCESSES
Typical Axon
• Single axon may branch along length
• “Axon collaterals” extend from neurons at ~ 90o angles
• Usually branches
profusely at end
– 10,000 or more
terminal branches
is common
– Distal endings
termed “axonal
terminals”
NEURON PROCESSES
Typical Axon
• Conducting component of neuron
• Generates nerve impulse
• Transmits nerve
impulses away from
cell body towards the
axonal terminals
NEURON PROCESSES
Typical Axon terminal
• Axonal terminals are secretory component of neuron
• Sequence of events
– Signal reaches terminals
– Membranes of vesicles fuse with
plasma membrane
– Neurotransmitters released
– Neurotransmitters interact
with either other neurons
or effector cells
• Excite or inhibit
Vocabulary
Either in flash card form OR in list
• CNS
• PNS
• Neuron
• Stimulus
• Afferent division
• Efferent division
• neuroglia
•
•
•
•
•
•
•
Amitotic
Dendrite
Cell body
Axon
Axon terminal
Ganglia
Nuclei (in terms of
clusters)
Collins I
2 lines
• What is the difference between the PNS and
the CNS?
Agenda
11/4/11 -- Day 2
•
•
•
•
Remember quiz 11/9
Take notes
Complete labeling and coloring of neuroglia
HW-complete ALL vocab terms
MYELIN SHEATH
• Whitish, fatty covering the axons of many
neurons
• Protects and electrically insulates fibers
• Increases speed of nerve impulse transmission
– Some axons and all dendrites are unmyelinated
MYELIN SHEATH
• In PNS, Schwann cells Continually wrap around the axon
of a neuron
– Result is many concentric layers of plasma membrane
surrounding the axon
– Thickness depends on number of wrappings
• Nucleus and most of
cytoplasm exist as a bulge
external to the myelin sheath
Myelin sheath and schwann cells
Node of
Ranvier
Schwann Cells
MYELIN SHEATH
• Adjacent Schwann cells on axon do not touch
each other
– Gaps in sheath occur at regular intervals
• “Nodes of Ranvier”
– Axon collaterals
can emerge at
these nodes
MYELIN SHEATH
• In CNS, there are both myelinated and
unmyelinated axons
• Oligodendrocytes, not Schwann cells, form
CNS myelin sheaths
– Numerous processes that can coil around
numerous (up to 60) axons at once
NEUROGLIA
• “Nerve glue”
• Six types of small cells associated with neurons
– 4 in CNS
– 2 in PNS
• Several functions
– Supportive scaffolding for neurons
– Electrical isolation of neurons
– Neuron health and growth
CNS NEUROGLIA
•
•
•
•
Astrocytes
Microglia
Ependymal cells
Oligodendrocytes
CNS NEUROGLIA
Astrocytes
• Anchor neurons to capillary blood supply
• Facilitate nutrient delivery to neurons
– (blood  astrocyte  neuron)
CNS NEUROGLIA
Microglia
• Small ovoid cells; thorny looking
• Transform into macrophage
– Phagocytize microorganisms, debris
– (Cells of immune system cannot enter the CNS)
CNS NEUROGLIA
Oligodendrocytes
• Wrap processes tightly around thicker neuron
fibers in CNS
– Makes “Myelin sheath”
– Insulating covering
CNS NEUROGLIA
Ependymal Cells
• Line central cavities of brain and spinal cord
• Many are ciliated
– Beating helps circulate cerebrospinal fluid cushioning brain
and spinal cord
PNS NEUROGLIA
Schwann cells
• Surround and form myelin sheaths around larger
neurons of PNS
– Functionally similar to oligodendrocytes
PNS NEUROGLIA
• Satellite cells
– Surround cell bodies of PNS ganglia
HW- Vocab Terms
•
•
•
•
•
•
•
•
•
Myelin sheath
Schwann cells
Nodes of ranvier
Oligodendrocytes
Neuroglea
Astrocyte
Microglia
Ependymal cell
Satalite cell
MYELIN SHEATH
• White matter
– Regions of the brain and spinal cord containing
dense collections of myelinated fibers
• Gray matter
– Regions of the brain and spinal cord containing
mostly nerve cell bodies and unmyelinated fibers
NEURON CLASSIFICATION
• Structural classification based upon number of
processes
– Multipolar neurons
– Bipolar neurons
– Unipolar neurons
• Functional classification based upon direction
nerve impulse travels
– Sensory (afferent) neurons
– Motor (efferent) neurons
– Interneurons (association neurons)
NEURON CLASSIFICATION
Structural Classification • Unipolar neurons
– Single short process
• Multipolar neurons
– Three or more processes
– Most common neuron
type in humans
• (> 99% of neurons)
• Bipolar neurons
– Two processes – axon
and dendrite
– Found only in some
special sense organs
• e.g., retina of eye
– Act as receptor cells
– Process divides into proximal
and distal branches
• Distal process often associated
with a sensory receptor
– “Peripheral process”
• Central process enters CNS
– Most are sensory neurons in
PNS
Classification of neurons by
shape
NEURON CLASSIFICATION
Functional Classification
• Sensory (afferent) neurons
– Transmit impulses toward CNS
• From sensory receptors or internal
organs
– Most are unipolar
– Cell bodies are located outside
CNS
• Motor (efferent) neurons
– Carry impulses away from CNS
• Toward effector organs
– Multipolar
– Cell bodies generally located in
the CNS
• Interneurons
– a.k.a., association
neurons
– Lie between motor
and sensory neurons in
neural pathways
– Shuttle signals through
CNS pathways where
integration occurs
– > 99% of neurons in
body
– Most are multipolar
– Most are confined
within the CNS
NEUROPHYSIOLOGY
• Neurons are highly irritable
– Responsive to stimuli
• Response to stimulus is action potential
– Electrical impulse carried along length of axon
– Always the same regardless of stimulus
– The underlying functional feature of the nervous
system
ION CHANNELS
Plasma membranes contain various ion channels
• Passive channels (leakage channels)
– Always open
• Active channels (gated channels)
– Ligand-gated channels
• Open when specific chemical binds
– Voltage-gated channels
• Open and close in response to membrane potential
– Mechanically-gated channels
• Open in response to physical deformation of receptor
MEMBRANE POTENTIALS
• A voltage exists across the plasma membrane
– Due to separation of oppositely charged ions
• Potential difference in a resting membrane is
termed its “resting membrane potential”
– ~ -70 mV in a resting
neuron
– Membrane is
“polarized”
MEMBRANE POTENTIALS
• Neurons use changes in membrane potentials
as signals
– Used to receive, integrate, and send signals
• Changes in membrane potentials produced by
– Anything changing membrane permeability to ions
– Anything altering ion concentrations
• Two types of signals
– Graded potentials
• Short-distance signals
– Action potentials
• Long-distance signals
MEMBRANE POTENTIALS
Graded Potentials
• Short-lived local changes in membrane
potential
– Either depolarizations or hyperpolarizations
• Cause current flows that decrease in
magnitude with distance
• Magnitude of potential dependent upon
stimulus strength
– Stronger stimulus  larger voltage change
– Larger voltage change  farther current flows
MEMBRANE POTENTIALS
Graded Potentials
• Triggered by change in neuron’s environment
– Change causes gated ion channels to open
• Small area of neuron’s plasma membrane becomes
depolarized (by this stimulus)
• Current flows on both sides of the membrane
– + moves toward – and vise versa
MEMBRANE POTENTIALS
Graded Potentials
• Inside cell: + ions move away from depolarized area
• Outside cell: + ions move toward depolarized area
– (+ and – ions switch places)
• Membrane is leaky
– Most of the charge is quickly lost through membrane
– Current dies out after traveling a short distance
MEMBRANE POTENTIALS
Graded Potentials
• Act as signals over very short distances
• Important in initiating action potentials
MEMBRANE POTENTIALS
Action Potentials
• Principal means by which neurons
communicate
– Brief reversal of membrane potential
• Total amplitude of ~ 100 mV (-70  +30)
– Depolarization followed by repolarization, then
brief period of hyperpolarization
– Time for entire event is only a few milliseconds
• Events in generation and transmission of an
action potential identical between neurons
ACTION POTENTIALS
ACTION POTENTIALS
• Not all local depolarizations produce action
potentials
• Depolarization must reach threshold values
– Brief, weak stimuli produce sub threshold
depolarizations that are not translated into nerve
impulses
– Stronger threshold stimuli produce depolarizing
events
ACTION POTENTIALS
• Action potential is all-or-nothing phenomenon
– Happens completely or doesn’t happen
• Independent of stimulus strength once
generated
– Strong stimuli generate more impulses of the
same strength per unit time
– Intensity is determined by number of impulses per
unit time
ACTION POTENTIALS
Multiple Sclerosis (MS)
• Autoimmune disease mainly affecting young
adults
• Myelin sheaths in CNS are gradually destroyed
• Interferes with impulse conduction
– Visual disturbances, muscle control problems,
speech disturbances, etc.
• Some modern treatments showing some
promise in delaying problems
SYNAPSE
• Nerve impulse reaches axonal terminal
• Voltage-gated Ca2+ channels open in axon
– Ca2+ enters presynaptic neuron
• Neurotransmitter is released via exocytosis
– Vesicles fuse with axonal membrane
• Neurotransmitter binds to postsynaptic receptors
• Ion channels open in
postsynaptic membrane
– Result is excitation or
inhibition
SYNAPSE
• Binding of neurotransmitter to its receptor is
reversible
• Permeability affected as long as
neurotransmitter is bound to its receptor
• Neurotransmitters do not persist in the
synaptic cleft
– Degraded by enzymes associated with
postsynaptic membrane
– Reuptake by astrocytes or presynaptic terminal
– Diffusion of neurotransmitters away from synapse
NEUROTRANSMITTERS
• More than fifty neurotransmitters identified
• Most neurons make two or more
– Can be released singly or together
Classification by Structure
• Acetylcholine (ACh)
• Biogenic amines
• Amino acids
• Peptides
• ATP
• Dissolved gases
Classification by Function
• Excitatory/Inhibitory
• Direct/Indirect