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The Nervous System
Introduction
• The nervous system is the communication and control system of an
animal’s body.
• Function:
• _____________ (senses changes within or outside the body and sends
info to the spinal cord and brain)
• _____________ (brain and spinal cord receive, analyze, store, and
integrate the info to produce a response)
• _____________ (an instruction of the body to do something)
• Composed of two main divisions:
• _______________ Nervous System (CNS) - brain and spinal cord
• _______________ Nervous System (PNS) - nerves that arise from the
CNS and innervate rest of body
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• Neuron: basic functional units of
nervous system
• High requirement for
____________.
• Cannot ___________ but can
________________ if the cell body
remains intact
• Neuroglia (AKA glial cells):
provide structural/functional
support and protection to neurons
Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.
• Soma
• Central cell body
• Cell processes
• _____________ – receive stimuli/impulses
and transmit the information ___________
soma.
• Short and branched
• _____________ - conducts nerve impulses
________ from soma towards another
neuron or an effector cell (muscle, gland)
• Long, single process
Neuron
Myelin
• Axons are often covered in myelin
• Nervous tissue containing myelinated
axons is called ___________ matter.
• Myelin: cell membrane of glial cells tightly
wrapped around the axon
• _____________________ are glial
cells of the brain and spinal cord
• ____________ ______ are glial
cells of the peripheral nerves
• Myelinated axons conduct impulses
faster than unmyelinated ones.
Nodes of Ranvier
• Multiple Schwann cells or
oligodendrocytes cover the entire
length of the axon
• Nodes of Ranvier are ______ in the
myelin sheath between adjacent glial
cells
• Myelin sheath and nodes of Ranvier
work together to enhance the
___________ of conduction of
nerve impulses along the axon.
Organization of Nervous System
1. Anatomical
• ____________ nervous system (CNS)
• Brain and spinal cord
• ____________ nervous system (PNS)
• Nerves extending outward from the central
axis toward the periphery of the body
• Cranial nerves originate directly from the
brain
• Spinal nerves emerge from the spinal cord
2.
Direction of Impulses
•
_____________ nerves - conduct
impulses TOWARD CNS
• a.k.a. sensory nerves - conduct
sensations from sensory
receptors in skin and other
locations to CNS
•
______________ nerves conduct impulses AWAY from
CNS
• a.k.a. motor nerves – cause
muscle contraction/movement or
glandular secretion
•
Some nerve fibers are sensory
(optic), motor (oculomotor), or
Organization of
Nervous System
Organization of Nervous System
3.
Function: Somatic vs. Autonomic
• _________________ nervous system - actions under conscious, or
voluntary control
• Motor nerves lead to skeletal muscle and cause limb or body movement
• Example: turning your head when your name is called
• _________________ nervous system - controls and coordinates automatic
functions
• Motor nerves lead to smooth muscle, cardiac muscle, and glands
• Example: increasing of the heart rate after a car crash, stomach
releasing HCl when food is present
• ______________________ Division (Stressed state of mind: fight/flight)
• ______________________ Division (Relaxed state of mind: feed/breed
rest/digest)
Neuron Function: DEPOLARIZATION AND
REPOLARIZATION
• Resting __________ - when neuron is not being stimulated to
transmit information
• But, the cell membrane is not truly at rest (Na/K pump is very active)
• Resting __________ ____________- difference in electrical
charge across neuronal membrane
• Due to differences in distribution of positive and negative
charges from sodium, potassium, and other ions on either side
of neuronal membrane
• Resting membrane potential is a negative number (-70 mV), indicating
the negative charge inside the cell
Neuron Function
• Na+/K+ pump: specialized molecules located in the neuron’s cell
membrane that maintain the resting membrane potential.
• Pumps ____ (Na+) out of neuron
• Pumps ____(K+) into neuron
Cell membrane becomes
________________
Depolarization
STEPS:
• Neuron receives external stimulus
• ______ channel opens on cell membrane
• Na+ flow into cell by passive _____________
• Down the concentration gradient
• Electrical gradient (opposites attract)
Depolarization
STEPS:
• During depolarization, inside of neuron goes from being
____________ to _____________ charged due to influx of Na+
ions
• This change in electrical charge is also known as the
______________ POTENTIAL
Neuron Function: Depolarization
STEPS:
• Na+ channels close
• ______ channels open
• K+ diffuses out of cell
Repolarization
• Electrical and concentration gradient, just like Na
• Resting membrane potential restored
• Cell is ___________________
• But, Na+ is inside, K+ outside
Repolarization
AFTER Repolarization
• Na+/K+ pump moves ions back to
their original sides
• Resting state restored
• Stimulus must be sufficient to exceed threshold and make the neuron respond. Will
cause complete depolarization. __________ stimuli do not cause depolarization.
• “_________________________ principle” - neuron depolarizes to its maximum
strength or not at all
• Strong stimuli cause numerous sodium channels open, which creates a WAVE OF
DEPOLARIZATION
• Conduction of the Action Potential —spreading wave of opening Na+ channels
in sufficient numbers to allow sodium influx and depolarization
• Wave of Depolarization = Conduction of Action Potential = Nerve Impulse
Threshold Stimulus
Neuron Function: Action Potential Propagation
• Time period when sensitivity
of neuron is reduced
—Cell is still in depolarization/early
repolarization
• _____________ refractory period
- during Na+ influx and K+ outflow
• No amount of stimulus can cause
depolarization
• ____________ refractory period -
during end of repolarization period
• Strong stimulus can cause
depolarization
Refractory Period
Saltatory Conduction
• RAPID means of conducting an action potential (more rapid than
in unmyelinated axons)
• Depolarization in myelinated axons can only take place at
_______________________________
• Myelin sheath prevents sodium ions from entering the cell.
• The depolarization wave is able to skip from one NOR to the next,
accelerating the rate at which the wave moves from the cell body to the
end of the axon
Local Anesthetics
• Work by blocking the
propagation of the action
potential
• Blocks _______ channels
• Sodium cannot flow into the
cell, so threshold is not
achieved
• Example: Lidocaine
• ____________ – junction between two neurons or a neuron and target
cell
• ___________ ________– space between adjacent neurons/effector
cell
• Presynaptic neuron - neuron bringing the depolarization wave to the
synapse
• Releases _______________________
• Postsynaptic neuron - contains receptors for the neurotransmitter
Synaptic
Transmission
Neurons: synapses
Synaptic Transmission
• The axon of the presynaptic neuron ends as a branched structure
(_____________________).
• Synaptic knob- slightly enlarged bulb at end of axon
• Vesicles contain neurotransmitter (a chemical messenger)
• When depolarization wave reaches axon terminal, calcium channels open and
cause vesicles to fuse with cellular membrane and release
neurotransmitter into the synapse
Synaptic Transmission
• Neurotransmitters diffuse across
synaptic cleft toward
postsynaptic membrane
• Receptors on postsynaptic
membrane bind
neurotransmitter
• Receptors are VERY
specific for each
neurotransmitter
(similar to a lock and
key)
Types of Neurotransmitters
• ______________ neurotransmitters
(acetylcholine, norepinephrine, dopamine)
• Usually cause an influx of Na+;
postsynaptic membrane moves toward
threshold (more positive)
• _______________ neurotransmitters
(acetylcholine, norepinephrine, dopamine,
GABA, glycine)
• Move the charge of postsynaptic cell
farther away from threshold (more
negative)
• May open K+ channels/Cl- channels
Recycling the Neurotransmitter
• Acetylcholinesterase -
found on postsynaptic
membrane; breaks down
acetylcholine
• Monoamine oxidase
(MAO) - breaks down
reabsorbed
epi/norepinephrine in the
synaptic knob
• Catechol-O-methyl
transferase (COMT) breaks down
epi/norepinephrine that is
not reabsorbed