Transcript Supporting Cells - Net Start Class

Nervous System Divisions
► Central
Nervous System and Peripheral NS
► Afferent Division and Efferent Division
► Somatic and Autonomic NS
Autonomic:
- sympathetic
- parasympathetic
Types of neurons
►
Sensory (afferent)

Receives stimulus and sends info to brain
►
Motor (efferent)

Carries signal from brain to effector muscles
►
Interneuron

Connects sensory neuron with motor neuron; found in brain and spinal cord
Supporting Cells
► Reinforce, protect insulate and assist neurons.
► Do not conduct nerve impulses
► Out-number neurons.
► Unlike neurons, go through mitosis entire life
Supporting cells of the CNS are called glial cells
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Astrocytes
Microglia
Ependymal
Oligodendrocytes
Schwann cells which produce the myelin sheath for
peripheral neurons.
Astrocytes: largest & most numerous
Function:
BBB, control of environment
structural framework & repairs
regulation of ions, nutrients, gases
Oligodendrocyte
Smaller than astrocyte
Cover neurons with myelin in CNS (white
matter vs. gray matter!)
Myelin improves the rate of impulse
conduction
Microglial cells
► Smallest
► Phagocytosis
►
 # during
infection or injury
Ependymal
cells
Lining of ventricles & central canal
Some regions ciliated
Some specialized to produce and
monitor CSF
Typical Neuron Structure
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Cell body or Soma with Perikaryon
Dendrites
Axon with axon hillock
Synaptic terminals
Fig 12.4
Cells of the Nervous System
►
Neurons
 Excitable cells
 Transmit chemical and electrical messages from one part of the
body to another.
 Cell Body: nucleus, cytoplasm, organelles
► Dendrites
 Convey signals to the cell body
 Short, numerous and extensively branched
► Axons
 Conducts signals away from the cell body –larger the diameter, the faster
the conduciton
 Wrapped in Schwann cells that create an insulating covering called the
myelin sheath.
 Extend from the axon hillock
 Often have one or more side branches called axon collaterals
 Distal tips of axons form branches called teledendria that each terminate
in synaptic terminals that release neurotransmitters
►
Myelin Sheath
► insulates the neuron
► fatty covering formed by Schwann cells
► Nodes of Ranvier
 gap between Schwann cells
 serves as points along the neuron for generating a signal
 signals jumping from node to node travel hundreds of times
faster than signals traveling along the surface of the axon.
 allows your brain to communicate with your toes in a few
thousandths of a second.
► Insulation permits the nervous system to exercise fine control over
muscles.
► The reason that babies cannot smile or move precisely at birth is
that the insulation for their nerve fibers is not completely
developed. As the insulation does develop in a child, they can smile
and move with greater coordination and precision.
► Synapse
– gap between the synaptic
terminals and dendrites of another neuron.
► Neurotransmitter – chemical that relays the
nerve impulse across the synapse.
► Gray matter: parts of a neuron not covered
with myelin (cell body, dendrites)
► White matter: covered with myelin sheath
(axons, etc)
Nerve Cell
Neurons
Nerves: bundles of nerve fibers of axons held together by
several layers of connective tissue. (Nerves are called
tracts in the central nervous system.)
- endoneurium: fibrous connective tissue around EACH nerve
fiber
- perineurium: connective tissue that holds a fascicle (bundle
of nerve fibers) together
- epineurium: fibrous coat that holds together numerous
fascicles and the blood vessels that supply them
►
Nerve structure
Fig 12.16, p 333
•A nerve is USUALLY both
sensory and motor
•Similar to muscle terminology
•Epineurium
•Covers the nerve
•Perineurium
•Covers a fascicle
•Endoneurium
•Covers an axon
Classification of Neurons
► Classified
according to number of
extensions from the cell body:
1.Multipolar
2.Bipolar
3. Unipolar
Structural Neuron Classification
cont. . .
Bipolar
Unmyelinated
Rare, but
important in
special senses
Multipolar
Most common
All motor
neurons
Structural Neuron Classification
Unipolar
Also called
pseudounipolar
Sensory neurons
See fig. 13-10
Reflex Arc
►
Sensory neurons convey information from the external
environment to the CNS.
 Presynaptic cell - cell transmitting the signal (affector cell)
►
Interneurons – integrate sensory input and motor output.
 Located within the CNS
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Motor neurons convey impulses from the CNS to effector
cells.
Postsynaptic cell - target cell (effector cell)
 Effector cells – cells that actually carry out the response.
► Muscle
or glands cells
The circuit
fig 12.11
Membrane Potential
► Signal
transmission occurs when the membrane
potential is changed within a neuron.
► A stimulus causes the membrane to become
permeable to sodium thus changing the
membrane potential.
► A charge separation between the outside of the
cell and the cytoplasm creates voltage across the
membrane.
 This voltage is maintained by the sodium- potassium
pump.
►A
solute potential also exists across the neural
membrane.
 Na+ has a tendency to slowly diffuse into the cell.
 K+ has a tendency to rapidly diffuse out of the cell.
 In general the cytoplasm of the cell is much more
negative than it’s exterior.
► Resting
Potential
 Voltage across the membrane of a neuron that is not
transmitting a signal.
 It is about -70mV
► Negative
sign means the cytoplasm is negative relative to the
cell’s surroundings
Action Potential
►A
rapid reversible depolarization of a neuron’s
membrane near the point of stimulation that
generates a nerve impulse.
► Originate from the axon hillock
► Is an “all or none” event
 Magnitude of the voltage change is the same at each
”firing” regardless of stimuli strength.
 Stronger stimulus increases the frequency of action
potentials but NOT the strength of the response.
How is Action Potential (an impulse)
Generated?
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Stimuli increase the membrane’s permeability to Na+.
Depolarization
 Some of the sodium channels open and Na+ rushes into the cell
causing the cytoplasm to become less negative.
 This is known as depolarization.
 If enough depolarization occurs then the cell will reach a
threshold potential and additional Na+ will open.
 If the threshold potential is reached then action potential is
initiated and the impulse will travel down the axon towards the
synaptic terminals.
 During action potential the interior of the cell becomes positive.
Repolarization
cell has to be repolarized to prepare
for another action potential.
► The
► The
K+ channels slowly open allowing K+ to
leave the cell and the Na+ channels close
preventing more Na+ from entering the cell.
► The
cytoplasm becomes more negative as
positive charges increase outside the cell.
Propagation of the Nerve Impulse
► Saltatory
Conduction
 The action potential ”jumps” from one node of
Ranvier to the next , skipping myelinated
regions of the membrane.
 Results in faster transmission
Saltatory Conduction