The Nervous System - Linn–Benton Community...
Transcript of The Nervous System - Linn–Benton Community...
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Tips
Use the Study Guide (SG) to follow the lectures
Lectures will be posted after class
Reading the SG before class will be helpful
Use the textbook to supplement lectures/SG
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N E U R O N S A N D
T H E N E R V O U S I M P U L S E
The Nervous System
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The Nervous System
Cooperates with endocrine system
Senses environment
Responds to changes in environment
Maintains homeostasis
The nervous system is responsible for all our behaviors, memories, and movements
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Functions of the Nervous System
1. Sensory input
Information gathered by sensory receptors about internal and external changes
2. Integration
Interpretation of sensory input
3. Motor output
Activation of effector organs (muscles and glands) produces a response
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Figure 11.1
Sensory input
Motor output
Integration
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Organization
Central Nervous System
1. Brain
2. Spinal Cord
Peripheral Nervous System
1. Afferent (sensory)
2. Efferent (motor)
a. Somatic
b. Autonomic
i. Sympathetic
ii. Parasympathetic
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Copyright 2009 John Wiley & Sons, Inc. 7
Major Structures of the Nervous System
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Figure 11.2
Central nervous system (CNS)
Brain and spinal cord
Integrative and control centers
Peripheral nervous system (PNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Parasympathetic
division
Conserves energy
Promotes house-
keeping functions
during rest
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers Conducts impulses from
receptors to the CNS
Somatic nervous
system
Somatic motor
(voluntary) Conducts impulses
from the CNS to
skeletal muscles
Sympathetic division
Mobilizes body
systems during activity
Autonomic nervous
system (ANS)
Visceral motor
(involuntary) Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Structure
Function
Sensory (afferent)
division of PNS Motor (efferent)
division of PNS
Somatic sensory
fiber
Visceral sensory fiber
Motor fiber of somatic nervous system
Skin
Stomach Skeletal
muscle
Heart
Bladder Parasympathetic motor fiber of ANS
Sympathetic motor fiber of ANS
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Histology of Nervous Tissue
Two principal cell types
1. Neurons = excitable cells that transmit electrical signals
2. Accessory cells (Neuroglia) = non-excitable supporting cells
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Neurons
Individual nerve cells
Nerves are parallel bundles of neurons
carrying impulses
Types:
Sensory (afferent)
Motor (efferent)
Association (interneurons)
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Neuron Structure
Body
Dendrites
Axons
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Copyright © 2010 Pearson Education, Inc.
Figure 11.4 Structure of a motor neuron.
Dendrites
(receptive regions)
Cell body
(biosynthetic center and receptive region)
Nucleolus
Nucleus
Nissl bodies
Axon
(impulse generating and conducting region)
Axon hillock
Neurilemma Terminal branches
Node of Ranvier
Impulse direction
Schwann cell (one inter- node)
Axon terminals (secretory region)
Dendritic spine
Neuron cell body
(a)
(b)
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Special Characteristics of Neurons
Long lived
Amitotic
High metabolic rate
Excitable
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Neuroglia
Neuroglia
About %50 of cellular mass of nervous system
Do not conduct impulses
Most retain capacity to divide
Different types found in PNS and CNS
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Neuroglia of Central Nervous System
Astrocytes
Oligodendrocytes
Microglial cells
Ependymal cells
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Astrocytes
Most abundant of CNS neuroglia
Contact blood vessels
Blood Brain Barrier regulates passage of molecules
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Copyright © 2010 Pearson Education, Inc. Figure 11.3a
Astrocytes are the most abundant of
CNS neuroglia. Provide most structural support
Capillary
Neuron
Astrocyte
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Astrocytes
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Oligodendrocytes
Branched cells
Processes wrap CNS nerve fibers, forming insulating myelin sheaths
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Copyright © 2010 Pearson Education, Inc. Figure 11.3d
Oligodendrocytes have processes that form
myelin sheaths around CNS nerve fibers.
Nerve
fibers
Myelin sheath
Process of
oligodendrocyte
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Oligodendrocytes
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Ependymal Cells
Line the central cavities of the brain and spinal column
Form cerebrospinal fluid
Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities
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Copyright © 2010 Pearson Education, Inc. Figure 11.3c
Brain or
spinal cord
tissue
Ependymal
cells
Fluid-filled cavity
Ependymal cells line cerebrospinal
fluid-filled cavities.
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Microglia
Small, ovoid cells with thorny processes
Migrate toward injured neurons
Phagocytize microorganisms and neuronal debris
Add these to your list!
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Copyright © 2010 Pearson Education, Inc. Figure 11.3b
(b) Microglial cells are defensive cells in
the CNS.
Neuron
Microglial
cell
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Neuroglia of PNS
Satellite cells
Surround neuron cell bodies in the PNS
Schwann cells
Surround peripheral nerve fibers and form myelin sheaths
Vital to regeneration of damaged peripheral nerve fibers
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Copyright © 2010 Pearson Education, Inc. Figure 11.3e
Satellite cells and Schwann cells (which
form myelin) surround neurons in the PNS.
Schwann cells
(forming myelin sheath)
Cell body of neuron Satellite
cells
Nerve fiber
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More on Schwann cells…
Myelin Sheath
Neurilemma
Nodes of Ranvier
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Copyright 2009 John Wiley & Sons, Inc. 30
Structure of a Multipolar Neuron
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Copyright © 2010 Pearson Education, Inc. Figure 11.5a
(a) Myelination of a nerve
fiber (axon)
Schwann cell
cytoplasm
Axon
Neurilemma
Myelin sheath
Schwann cell nucleus
Schwann cell
plasma membrane
1
2
3
A Schwann cell
envelopes an axon.
The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
The Schwann cell
cytoplasm is forced from
between the membranes.
The tight membrane
wrappings surrounding
the axon form the myelin
sheath.
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The Nervous Impulse
Dependent upon a resting potential across the cell membrane
Active transport carriers (Sodium/Potassium pump)
Polarization of neuron
Impulse results from depolarization
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Copyright 2009 John Wiley & Sons, Inc. 33
Factors that contribute to resting
membrane potential
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Copyright © 2010 Pearson Education, Inc. Figure 11.8
Finally, let’s add a pump to compensate
for leaking ions.
Na+-K
+ ATPases (pumps) maintain the
concentration gradients, resulting in the
resting membrane potential.
Suppose a cell has only K+ channels...
K+ loss through abundant leakage
channels establishes a negative
membrane potential.
Now, let’s add some Na+ channels to our cell...
Na+ entry through leakage channels reduces
the negative membrane potential slightly.
The permeabilities of Na+ and K
+ across the
membrane are different.
The concentrations of Na+ and K
+ on each side of the membrane are different.
Na+
(140 mM ) K+
(5 mM )
K+ leakage channels
Cell interior
–90 mV
Cell interior
–70 mV
Cell interior
–70 mV
K+
Na+
Na+-K+ pump
K+
K+ K+
K+
Na+
K+
K+ K
Na+
K+ K+ Na+
K+ K+
Outside cell
Inside cell Na+-K+ ATPases (pumps)
maintain the concentration
gradients of Na+ and K+
across the membrane.
The Na+ concentration
is higher outside the
cell.
The K+ concentration
is higher inside the
cell.
K+
(140 mM ) Na+
(15 mM )
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Copyright © 2010 Pearson Education, Inc. Figure 11.7
Voltmeter
Microelectrode
inside cell
Plasma
membrane Ground electrode
outside cell
Neuron
Axon
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The Nervous Impulse
Polarization
Depolarization
Hyperpolarization
Polarity is dependent upon channels within the membrane (K+ and Na+)
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Copyright © 2010 Pearson Education, Inc. Figure 11.9a
Depolarizing stimulus
Time (ms)
Inside
positive
Inside
negative
Resting
potential
Depolarization
(a) Depolarization: The membrane potential
moves toward 0 mV, the inside becoming
less negative (more positive). This increases the
probability of nerve impulse production.
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Copyright © 2010 Pearson Education, Inc. Figure 11.9b
Hyperpolarizing stimulus
Time (ms)
Resting
potential
Hyper-
polarization
(b) Hyperpolarization: The membrane
potential increases, the inside becoming more negative. This decreases the probability
of nerve impulse production.
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Membrane Channels
Chemically gated (ligand gated)
Voltage gated
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Copyright © 2010 Pearson Education, Inc. Figure 11.6
(b) Voltage-gated ion channels open and close in response
to changes in membrane voltage.
Na+
Na+
Closed Open
Receptor
(a) Chemically (ligand) gated ion channels open when the
appropriate neurotransmitter binds to the receptor,
allowing (in this case) simultaneous movement of
Na+ and K
+.
Na+
K+
K+
Na+
Neurotransmitter chemical
attached to receptor
Chemical
binds
Closed Open
Membrane
voltage
changes
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The Graded Potential
Short lived, localized changes in membrane potential due to stimulation
Channels open
Ions flow
Short distance signals
May be depolarization or hyperpolarization events
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Copyright © 2010 Pearson Education, Inc. Figure 11.10a
Depolarized region
Stimulus
Plasma
membrane
(a) Depolarization: A small patch of the
membrane (red area) has become depolarized.
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Copyright © 2010 Pearson Education, Inc. Figure 11.10b
(b) Spread of depolarization: The local currents
(black arrows) that are created depolarize
adjacent membrane areas and allow the wave of
depolarization to spread.
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Copyright © 2010 Pearson Education, Inc. Figure 11.10c
Distance (a few mm)
–70
Resting potential
Active area
(site of initial
depolarization)
(c) Decay of membrane potential with distance: Because current
is lost through the “leaky” plasma membrane, the voltage declines
with distance from the stimulus (the voltage is decremental ).
Consequently, graded potentials are short-distance signals.
Mem
bra
ne
p
oten
tia
l (m
V)
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Action Potential
Long distance signal
Initiated by sufficient depolarization at site of graded potential
Involves opening of voltage gated channels
Does not decrease in strength with distance
All or none phenomenon
Also called a nerve impulse
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Copyright © 2010 Pearson Education, Inc.
Figure 11.4 Structure of a motor neuron.
Dendrites
(receptive regions)
Cell body
(biosynthetic center and receptive region)
Nucleolus
Nucleus
Nissl bodies
Axon
(impulse generating and conducting region)
Axon hillock
Neurilemma Terminal branches
Node of Ranvier
Impulse direction
Schwann cell (one inter- node)
Axon terminals (secretory region)
Dendritic spine
Neuron cell body
(a)
(b)
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Figure 11.11 Action Potential (5 of 5)
Na+
Na+
Potassium
channel
Sodium
channel
1 Resting state
2 Depolarization
3 Repolarization
4 Hyperpolarization
The events
Activation
gates
Inactivation gate K+
K+
Na+
K+
Na+
K+
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Action
potential
1 2 3
4
Resting state Depolarization Repolarization
Hyperpolarization
The big picture
1 1
2
3
4
Time (ms)
Threshold Me
mb
ra
ne
p
ote
ntia
l (m
V)
Figure 11.11 (1 of 5)
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Conduction Velocity
Axon diameter
Degree of myelination
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Myelinated Neurons
Nodes of Ranvier
Saltatory conduction
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Copyright © 2010 Pearson Education, Inc. Figure 11.15
Size of voltage
Voltage-gated
ion channel
Stimulus
Myelin
sheath
Stimulus
Stimulus
Node of Ranvier
Myelin sheath
(a) In a bare plasma membrane (without voltage-gated
channels), as on a dendrite, voltage decays because current leaks across the membrane.
(b) In an unmyelinated axon, voltage-gated Na+ and K+
channels regenerate the action potential at each point
along the axon, so voltage does not decay. Conduction
is slow because movements of ions and of the gates
of channel proteins take time and must occur before voltage regeneration occurs.
(c) In a myelinated axon, myelin keeps current in axons
(voltage doesn’t decay much). APs are generated only
in the nodes of Ranvier and appear to jump rapidly
from node to node.
1 mm
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Study Guide: Nerves 8-5
2b. should read….
“…and also use considerably LESS energy.”
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Refractory Period
Neuron cannot respond to a second stimulus
During repolarization
Limits number of impulses per second
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Copyright © 2010 Pearson Education, Inc. Figure 11.14
Stimulus
Absolute refractory
period
Relative refractory
period
Time (ms)
Depolarization
(Na+ enters)
Repolarization
(K+ leaves)
After-hyperpolarization
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Stimuli that Initiate Action Potentials
Light
Heat
Chemicals
Mechanical energy
Chemical stimuli
from other neurons
Sensory Neurons Motor and Association
Neurons
Threshold stimulus always required
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Coding for Stimulus Intensity
All Action Potentials are independent of stimulus strength
CNS must discern strong from weak signals to initiate appropriate response
Stimulus intensity is coded for by frequency of action potentials
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Questions?
Homework due in lab
Lab Exercise 17 pg.’s 265-266 #1-10
Please bring your Physio-Ex CD to lab