Exam II Questions / Answers

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Exam II Questions / Answers
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Nervous System
Nervous System
The master controlling and communicating system
of the body
3 overlapping Functions
Sensory receptors to monitor changes
inside/outside the body
Processes or interprets sensory input,
e.g. sensory input
e.g. integration
Motor output – response to stimuli
Nervous System
Figure 11.1
Organization of the Nervous System
Divided into 2 parts: CNS and PNS
Central nervous system (CNS)
Consists of brain and spinal cord
Integration and command center of the nervous system
Peripheral nervous system (PNS)
Consists of nerves (bundles of axons) that extend from the
brain & spinal cord.
Spinal nerves: carry impulses to and from spinal cord
Cranial nerves: carry impulses to and from brain
Serve as lines of communication
Peripheral Nervous System (PNS):
Two Functional Divisions
Sensory (afferent) division (“carrying toward”)
E.g. Sensory afferent fibers – carry impulses from
skin, skeletal muscles, and joints to the brain
E.g. Visceral afferent fibers – transmit impulses
from visceral organs to the brain
Motor (efferent) division (“carrying away”)
Transmits impulses from the CNS to effector
organs (muscles and glands)
Motor Division: Two Main Parts
Somatic nervous system
Somatic motor nerve fibers (axons) that conduct
impulses from the CNS to skeletal muscles
Voluntary nervous system (conscious control
of skeletal muscle)
Motor Division: Two Main Parts
Autonomic nervous system (ANS)
Visceral motor nerve fibers that regulate the activity of
smooth muscle, cardiac muscle, and glands
Involuntary nervous system
Consists of 2 subdivisions:
i) sympathetic
ii) parasympathetic
i and ii usually work in opposition of each other,
e.g. one stimulates, while the other inhibits
Histology of Nervous Tissue
The two principal cell types of the nervous system
are:
Neurons – the excitable nerve cells that transmit
electrical signals
Supporting cells – smaller cells that surround and
wrap the more delicate neurons
Supporting Cells: Neuroglia (“Nerve Glue”)
Often called glial cells
Tend to have a smaller size and dark staining nuclei
Outnumber neurons in the CNS 10:1
Make up ½ the mass of the brain
6 types of glial cells: 4 in CNS, 2 in PNS
All 6 types;
Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to their proper connections
Promote health and growth of neurons
Neuroglia in the CNS
Astrocytes
Most abundant, versatile, and highly branched glial
cells
Radiating processes cling to neurons and their
synaptic endings, and cover capillaries
Astrocytes
Functionally, they:
Support and brace neurons
Anchor neurons to their nutrient supplies and supply
nutrients themselves to neurons
Guide migration of young neurons
Aid synapse formation
“Mop up” leaked K+ and recapture/recycle released
neurotransmitters
Connected by gap junctions, astrocytes can themselves
respond to nearby nerve impulses.
Astrocytes: Nutrient Supply
Figure 11.3a
Microglia Cells: Immune defense
Microglia – small, ovoid cells with spiny processes
Processes touch nearby neurons
Migrate toward neurons when they are in trouble and
“transform” into macrophage-like cell that phagocytize
microorgansims and neural debris
Important because cells of the immune system are denied access
to the CNS
Ependymal Cells: Cerebral Spinal Fluid
Produce cerebral spinal fluid
Line the ventricles of the brain and the spinal cord canal
Often ciliated
Range in shape from squamous to columnar
Oligodendrocytes, Schwann Cells, and
Satellite Cells
Oligodendrocytes – branched cells that wrap
around CNS nerve fibers producing insulating
coverings called myelin sheaths.
Schwann cells (neurolemmocytes) – surround
fibers of the PNS forming myelin sheaths as well
Satellite cells are flattened schwann cells found in
the PNS. Surround neuron cell bodies within
ganglia. Function is similar to that of astrocytes
(2 types of neuroglia in the PNS)
Oligodendrocytes, Schwann Cells, and
Satellite Cells
Figure 11.3d, e
Neurons (Nerve Cells)
Structural units of the nervous system
Conduct nerve impulses from one part of the body to another
Characteristics:
Extreme longevity- can function for up to 100+ years!
Amitotic- lose ability to divide. Can not be replaced
Exceptions: olfactory and hippocampal (memory)
cells
High metabolic rate. Require abundant supplies of O2 &
glucose
Neurons all have cell bodies that project processes and the
plasma membrane is the site of electrical signaling
Neurons (Nerve Cells)
Figure 11.4b
Nerve Cell Body (Perikaryon or Soma)
Contains the usual organelles (e.g. nucleus, nucleolus, etc…) but has no
centrioles (hence its amitotic nature)
Is the major biosynthetic center
Membrane synthesis and free ribosomes & rough E.R. are most
developed in the cell body
Golgi apparatus is highly developed
Some neuron cell bodies are pigmented (lipofuscin)
Has well-developed Nissl bodies (rough ER)
Most neuron cell bodies are located in the CNS where they are protected
by the bones of the skull and vertebral column
Contains an axon hillock – cone-shaped area from which axons arise
Processes
Armlike processes extend form the cell body
CNS contain both neuron cell bodies and processes
PNS contain only neuron processes
Bundles of processes are called tracts in the CNS
and nerves in the PNS
There are two types of neuron processes: axons
and dendrites
Axons and dendrites differ in their structure and
function of their plasma membranes
Dendrites of Motor Neurons
Short, tapering, and diffusely branched processes
Hundreds of dendrites cluster close to cell body
Most organelles found in the cell body are also found in the
dendrites
They are the main receptive, or input, regions of the neuron
Large surface area to receive signals from other neurons
Convey in coming messages toward the cell body
Message are usually not action potentials (nerve
impulses) but rather short distance graded potentials
Axons: Structure
Each neuron has a single axon which arises from the cell body at the
axon hillock then narrows to form a process w/ consistent diameter the
rest of its length
Some axons are 3-4’ long (e.g. lumbar region to great toe)
Long axons are called nerve fibers
If an axon branches, the branches are called axon collaterals and extend
at right angles to the axon
Axons usually branch profusely at their terminus (e.g. 10,000 branches is
common per neuron)
Ends of each branch are called axon terminals, synaptic knobs, or
boutons
The axon is the conducting region of the neuron
It generates nerve impulses and transmits them away from the cell body
Axons: Function
In motor neurons, the nerve impulse is generated at the junction of the
axon hillock and axon (trigger zone) and conducted along the axon to the
axon terminals which are the secretory regions of the neuron
When impulses reach the terminals, neurotransmitters are released into
the extracellular space
Neurtotransmitters either excite or inhibit neurons (effector cells) with
which the axon is in close contact
Axons have the same organelles in the cell body as the dendrites, but it
lacks Nissel bodies and Golgi apparatus (protein synthesis)
Thus,
1) axon depends on cell body for protein and membrane renewal
2) Efficient transport mechanisms to transport these items
Axons: Function
Movement along axons occurs in two ways
Anterograde movement:
Movement toward axonal terminal
Include replenishing sources from the cell body to the
outlying axon
Retrograde movement:
Movement toward the cell body
E.g. organelles moving back to the cell body for
recycling, communication to the cell body of axon
health, signaling molecules being brought to the cell
body
Myelin Sheath
Whitish, fatty (protein-lipoid), segmented sheath
found around long axons
It functions to:
Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission
Axons are always myelinated
Dendrites are always unmyelinated
Myelin Sheath and Neurilemma: Formation
Formed from Schwann cells in the PNS
Schwann cells:
Cytoplasm is squeezed from between the membrane layers
Possess few proteins in their membranes (good insulators)
Envelopes an axon in a trough
Encloses the axon with its plasma membrane
Has concentric layers of membrane that make up the myelin
sheath
Neurilemma – remaining nucleus and cytoplasm of a
Schwann cell
Myelin Sheath and Neurilemma: Formation
Figure 11.5a–c
Nodes of Ranvier (Neurofibral Nodes)
Gaps in the myelin sheath between adjacent
Schwann cells
They are the sites where axon collaterals can
emerge
Unmyelinated Axons
A Schwann cell surrounds nerve fibers but coiling
does not take place
Schwann cells partially enclose 15 or more axons
Axons of the CNS
Both myelinated and unmyelinated fibers are
present
Myelin sheaths are formed by oligodendrocytes
Nodes of Ranvier are widely spaced
There is no neurolemma
(neurolemma is associated with the outermost
layer of the Schwann cell sheaths)
Regions of the Brain and Spinal Cord
White matter – dense collections of myelinated
fibers
Gray matter – mostly soma and unmyelinated
fibers
Neuron Classification
Classified structurally & functionally
Structural classifications: (grouped according to
the # of processes extending from the cell body)
Structural:
Multipolar: three or more processes
Most common neuron type (99%)
Major type in the CNS
Bipolar: two processes (axon and dendrite)
Axon and dendrite extend from opposite sides of the cell
body
Rare, found in special sense organs, e.g. retina, olfactory
mucosa
Unipolar: single, short process emerging from cell body
Process divides “T”-like into central & peripheral
processes
Found mainly in ganglia of the PNS
Functional: (neurons are grouped according to the direction
in which the nerve impulse travels relative to the CNS)
Sensory (afferent): transmit impulses from sensory
receptors in the skin or internal organs toward or into the
CNS
All are unipolar
Cell bodies are located in sensory ganglia outside of the
CNS
Only the most distal parts act as receptor sites, with long
peripheral processes (e.g. again, the great toe)
Motor (efferent): carry impulses away from the
CNS to the effector organs (e.g. muscles, glands)
Multipolar
Cell bodies are located in the CNS
Interneurons (association neurons): lie between
sensory & motor neurons in neural pathways
Confined to the CNS
99% of neurons in the body
multipolar
Comparison of Structural Classes of Neurons
Table 11.1.1
Comparison of Structural Classes of Neurons
Table 11.1.2
Neurophysiology
Neurons are highly irritable (responsive to stimuli)
Action potentials, or nerve impulses, are:
Electrical impulses carried along the length of
axons
Always the same regardless of stimulus
The underlying functional feature of the nervous
system
…and now, for something completely different.
Serratus Anterior…in Action!
Basic Principles of Electricity:
Opposite charges attract
The coming together of opposite charges liberates
(releases) energy
Thus, situations in which there are separated (by a
membrane) electrical charges of opposite sign (+, -),
there will exist potential energy.
The potential, or possibility, to release energy.
Electricity Definitions
Voltage (V):
Measure of potential energy (volts, millivolts)
Measured between 2 points (called the potential difference) or
simply potential
The greater the difference in charge between 2 points, the greater
the voltage
Current (I):
The flow of electrical charge between two points that can be used to
do work
The amount of charge that can travel between 2 points depends on:
Resistance (R) – the hindrance to charge flow (e.g. insulators
have high hindrance, conductors have low hindrance)
Voltage (V)
Electrical Current and the Body
Reflects the flow of ions rather than electrons
There is a potential on either side of membranes
when:
The number of ions is different across the
membrane
The membrane provides a resistance to ion flow
Ohm’s Law
Current (I) = voltage (V)/Resistance (R)
Here:
I is proportional to V
I is inversely proportional to R
No net voltage (0 V) means no net current
In the body, instead of electrons moving down a
copper wire, we have ions passing thru membranes
Role of Membrane Ion Channels
Large proteins in membranes that allow ions to
pass
Gated channels change shape and open and close
in response to a specific signal
Types of plasma membrane ion channels:
Passive, or leakage, channels – always open but
selective to the ion they let in
Chemically gated/ligand gated channels – open with
binding of a specific chemical (neurotransmitter)
Voltage-gated channels – open and close in
response to changes in membrane potential
Mechanically gated channels – open and close in
response to physical deformation of receptors (e.g.
touch or pressure receptors)
Operation of a Gated Channel
When gated ion channels are open, ions diffuse quickly across the
plasma membrane in the direction of their electro-chemical gradient,
creating electrical currents and voltage changes across the membrane
according to Ohm’s law: V= I x R
Thus, electro-chemical gradients underlie all electrical phenomena
in neurons
Example: Na+-K+ gated channel
Closed when a neurotransmitter is not bound to the extracellular receptor
Na+ cannot enter the cell and K+ cannot exit the cell
Open when a neurotransmitter is attached to the receptor
Na+ enters the cell and K+ exits the cell
Operation of a Gated Channel
Figure 11.6a
Operation of a Voltage-Gated Channel
Example: Na+ channel
Closed when the intracellular environment is
negative
Na+ cannot enter the cell
Open when the intracellular environment is
positive
Na+ can enter the cell
Operation of a Voltage-Gated Channel
Lys, Arg, His:
All positively charged amino acids
Figure 11.6b
KU Game Day!! (courtesy of PE “Game Face”
Wed.
7 pm
Sun.
1 pm
Electrochemical Gradient
Ions flow along their chemical gradient when they
move from an area of high concentration to an area
of low concentration
Ions flow along their electrical gradient when they
move toward an area of opposite charge
Electrochemical gradient – the electrical and
chemical gradients taken together
Measuring Membrane Potential
Figure 11.7
Resting Membrane Potential (Vr)
The potential difference (–70 mV) across the membrane of
a resting neuron
It is generated by different concentrations of Na+, K+, Cl−,
and protein anions (A−)
Ionic differences are the consequence of:
Differential permeability of the neurilemma to Na+ and K+
Operation of the sodium-potassium pump maintaining Na+
and K+ concentrations
At Rest…
Membrane is impermeable to large anionic
cytoplasmic proteins
Membrane is slightly permeable to Na+
Membrane is 75x more permeable to K+
Membrane is freely permeable to Cl- (which
balances the Na+ & K+ charge)
Resting Membrane Potential (Vr)
Figure 11.8
Membrane Potentials: Signals
Changes in membrane potential is used to communicate
signals and send information about environment by
neurons
Membrane potential changes are produced by:
Anything that changes membrane permeability to ions
Anything that alters ion concentrations across the
membrane
2 types of signals are produced by changes in membrane
potential:
Graded potentials (operate over short distance)
Action potentials (operate over long distance,
e.g. axons)
Changes in Membrane Potential
Changes are caused by three events
Depolarization: reduction in membrane potential, e.g. inside
of membrane side becomes less negative (moves closer to 0
mV) than the resting potential.
-45mV
Repolarization – the membrane returns to its resting
membrane potential
E.g. -70mV
E.g. -45mV
-70mV
Hyperpolarization – the inside of the membrane side
becomes more negative (moves further from 0mV) than the
resting potential
E.g. -70mV
-90mV
Changes in Membrane Potential
Figure 11.9
Graded Potentials
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Magnitude varies directly with the strength of the
stimulus
Triggered by change in environment (stimulus)
that causes gated ion channels to open
Essential for initiating action potentials
Sufficiently strong graded potentials can initiate
action potentials
Graded Potentials
Shu
f
Inward flow of ions
Shu
fflin
fl i n
g of
g of
cha
rge
s ou
t si d
e th
e ce
ll
cha
rges
insi
de t
he c
ell
Figure 11.10
Graded Potentials
Voltage changes are decremental
Current is quickly dissipated due to the leaky
plasma membrane
Only travel over short distances
ESSENTIAL FOR INITIATING ACTION
POTENTIALS!!
Graded Potentials
Figure 11.11
Action Potentials (APs)
Long distance signalling by neurons
Only excitable membranes, e.g. neurons and muscle cells, can
generate action potentials.
A brief reversal of membrane potential with a total amplitude of
100 mV
Unlike graded potentials, APs do not decrease in strength over
distance
They are the principal means of neural communication
An action potential in the axon of a neuron is a nerve impulse
Action Potentials (APs)
In neurons, AP is generated in the axon
In summary, stimulus changes the permeability of
the neuron’s membrane by opening specific
voltage gated ion channels on the axon
Thus, VGICs are activated by local currents
(Graded Potentials) that spread toward the axon
along the dendritic cell body membranes
Often, the transition from GP to AP occurs at the
axon hillock
Action Potential: Resting State
Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates
Activation gates –
closed in the resting
state
Inactivation gates –
open in the resting
state
Figure 11.12.1
Action Potential: Resting State
Maintaining -70mV
Depolarization opens and then inactivates Na+
channels
Both must be open for Na+ to enter
Only one must be closed
to close Na+ channels
Action Potential: Depolarization Phase
Na+ permeability increases; membrane potential reverses
Na+ gates are opened; K+ gates are closed
Threshold – a critical level of depolarization
(-55 to -50 mV)
At threshold, depolarization becomes self-generating and
progresses by positive feedback
Figure 11.12.2
Action Potential: Repolarization Phase
Slow Na+inactivation gates close
A) Membrane permeability to Na+ declines to
resting levels
AP spike stops rising
B) As sodium gates close, slow voltage-sensitive
K+ gates open
K+ exits the cell and
internal negativity
of the resting neuron
is restored (-70mV)
Figure 11.12.3
Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive
efflux of K+
This efflux causes hyperpolarization of the
membrane (undershoot)
The neuron is
insensitive to
stimulus and
depolarization
during this time
Figure 11.12.4
Action Potential:
Role of the Sodium-Potassium Pump
Repolarization
Restores the resting electrical conditions of the
neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is
restored by the sodium-potassium pump
Phases of the Action Potential
1 – resting state
2 – depolarization
phase
3 – repolarization
phase
4 – hyperpolarization
Figure 11.12
Propagation of an Action Potential
(Time = 0ms)
Na+ influx causes a patch of the axonal membrane
to depolarize
Positive ions in the axoplasm move toward the
polarized (negative) portion of the membrane
Sodium gates are shown as closing, open, or closed
(Time = 0ms)
Figure 11.13a
(Time = 2ms)
Ions of the extracellular fluid move toward the area
of greatest negative charge
A current is created that depolarizes the adjacent
membrane in a forward direction
The impulse propagates away from its point of
origin
(Time = 2ms)
Figure 11.13b
(Time = 4ms)
The action potential moves away from the stimulus
Where sodium gates are closing, potassium gates
are open and create a current flow
(Time = 4ms)
Figure 11.13c
Threshold and Action Potentials
Threshold – membrane is depolarized by 15 to 20 mV
Established by the total amount of current flowing through
the membrane
Weak (subthreshold) stimuli are not relayed into action
potentials
Strong (threshold) stimuli are relayed into action potentials
All-or-none phenomenon – action potentials either happen
completely, or not at all
Coding for Stimulus Intensity
All action potentials are alike and are independent
of stimulus intensity
Strong stimuli can generate an action potential
more often than weaker stimuli
The CNS determines stimulus intensity by the
frequency of impulse transmission
Stimulus Strength and AP Frequency
Figure 11.14
Absolute Refractory Period
Time from the opening of the Na+ activation gates
until the closing of inactivation gates
The absolute refractory period:
Prevents the neuron from generating an action
potential
Ensures that each action potential is separate
Enforces one-way transmission of nerve impulses
Absolute and Relative Refractory Periods
Figure 11.15
Relative Refractory Period
The interval following the absolute refractory
period when:
Sodium gates are closed
Potassium gates are open
Repolarization is occurring
The threshold level is elevated, allowing strong
stimuli to increase the frequency of action potential
events
Conduction Velocities of Axons
Conduction velocities vary widely among neurons
Rate of impulse propagation is determined by:
Axon diameter – the larger the diameter, the faster
the impulse
Presence of a myelin sheath – myelination
dramatically increases impulse speed
Saltatory Conduction
Current passes through a myelinated axon only at
the nodes of Ranvier
Voltage-gated Na+ channels are concentrated at
these nodes
Action potentials are triggered only at the nodes
and jump from one node to the next
Much faster than conduction along unmyelinated
axons
Saltatory Conduction
Figure 11.16
Multiple Sclerosis (MS)
An autoimmune disease that mainly affects young
adults
Symptoms: visual disturbances, weakness, loss of
muscular control, and urinary incontinence
Nerve fibers are severed and myelin sheaths in the
CNS become nonfunctional scleroses
Shunting and short-circuiting of nerve impulses
occurs
Multiple Sclerosis: Treatment
The advent of disease-modifying drugs including
interferon beta-1a and -1b, Avonex, Betaseran, and
Copazone:
Hold symptoms at bay
Reduce complications
Reduce disability

Exam II Questions / Answers

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