Nervous System
Neural organization and
structure reflect function in
information transfer. The
neuron is the functional unit
of the nervous system. It is
composed of a cell body, which
contains the nucleus and
organelles; dendrites which
are cell extensions that
receive incoming messages
from other cells; and axons
which transmit messages to
other cells. Axons are
bundled together to form nerves. Many axons are covered by an insulating fatty myelin
sheath. This speeds the
rate of impulse
transmission. Schwann
cells in the peripheral
nervous system have
myelin in their cell
membrane. Gaps between
Schwann cells are called
nodes of Ranvier.
.
Overview of the Nervous System
StimulusReceptorSensory NerveCNSMotor NerveEffectorAction
There are many types of
receptors – photoreceptors
respond to light,
mechanoreceptors respond to
physical touch, chemoreceptors
respond to the presence of
various chemicals. The stimulus
alters the membrane potential of
the receptor cell and if a certain
threshold of change is achieved,
an action potential occurs. Energy
is converted from one form to
another in the process of
transduction.
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Ion pumps and ion channels maintain the resting potential of a neuron. Membrane
potential is the difference in electrical charge across a cell membrane.
The membrane potential of a nerve cell at rest is called its resting potential. It exists
because of the differences in the ionic
composition of the extracellular and
intracellular fluids across the plasma
membrane. The average resting membrane
potential is about -70mV.
The concentration of Na+ is higher outside
the cell, whereas the concentration of K+ is
higher inside the cell. Changes in the
membrane potential of a neuron are what give
rise to nerve impulses.
Action potentials are the signals conducted by axons. An action potential (nerve
impulse) is an all-or-none response to depolarization of the nerve cell. A stimulus
opens voltage-gated sodium channels and Na+ ions enter the cell, bringing the
membrane potential to a positive value. In order to generate an action potential, a
certain level of depolarization must be achieved, known as the threshold. The
membrane potential is restored to its normal resting
value by the inactivation of the Na+ channels and by
opening voltage-gated K+ channels, which increases K+
leaving the cell. Action potentials are propagated along
myelinated axons by saltatory conduction, which is the
jumping of the nerve impulse between nodes of
Ranvier. This speeds up the conduction of the nerve
impulse.
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Neurons communicate with other
cells at synapses. The signal is
conducted from the axon of a
presynaptic cell to the dendrite
of a postsynaptic cell via an
electrical or chemical synapse.
The synapse is a junction between
two neurons (or a neuron and a
muscle fiber or gland).
Neurotransmitters are released
by the presynaptic neuron into
the synaptic cleft. They bind to
receptors on the postsynaptic
membrane and are then broken
down by enzymes or taken back up
into surrounding cells. There are
two categories of
neurotransmitters; excitatory and inhibitory. Excitatory causes depolarization of the
postsynaptic membrane, whereas inhibitory causes hyperpolarization of the postsynaptic
membrane.
Neurotransmitters are chemical messengers released from vesicles in the synaptic
terminals into the synapse. They will diffuse across the synapse and bond to receptors on
the neuron, muscle fiber or gland, causing a change in the second cell. Acetylcholine is a
very common neurotransmitter; it can be inhibitory (on cardiac muscle) or excitatory (on
skeletal muscle). Other common neurotransmitters are epinephrine, norepinephrine,
dopamine and serotonin.
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ORGANIZATION OF THE NERVOUS SYSTEM
The central nervous system consists of the brain and
spinal cord; the peripheral nervous system consists of the
nerves that communicate motor and sensory signals
throughout the rest of the body. The peripheral nervous
system consists of nerves and ganglia; it has two major
branches- voluntary (somatic) and involuntary
(autonomic). The voluntary division carries messages to
skeletal muscles; the involuntary division carries messages
to cardiac muscle, smooth muscle and glands.
Cerebrum
The largest part of the brain; divided into two hemispheres that are
connected by a mass of white matter called the corpus callosum. The
surface of the cerebrum consists mostly of gray matter (neuron cell
bodies) referred to as the cerebral cortex. The cerebral cortex is
folded into ridges called gyri and grooves called sulci. This allows
more surface area to fit into a confined space. See the picture of
the cerebrum for a summary of the functions of the cerebrum.
Diencephalon
Epithalamus- contains the pineal gland (an endocrine gland) that
(has 3 parts
secretes melatonin. This hormone probably regulates the sleep-wake
cycle in humans. In other animals it has more involvement in
regulating seasonal cycles.
Thalamus- this part of the diencephalon has at least a dozen nuclei
(clusters of neurons responsible for a certain function). The
thalamus serves as a gateway for messages going to and from the
cerebral cortex.
Hypothalamus- also contains about a dozen nuclei. It regulates many
body functions such as 1) control of the autonomic nervous system
2) emotions 3) regulation of temperature 4) sleep-wake cycles
5) hunger and thirst 6) control of the endocrine system
The limbic system is a functional system containing cerebral and diencenphalon structures
that mediate emotional response.
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Midbrain
Pons
Medulla oblongata
Cerebellum
Contains centers for visual and auditory reflexes
Acts as a “bridge” for messages passing to and from the cerebrum;
also relays messages to the cerebellum. It also contains centers
involved in respiration.
This is the part of the brain that is continuous with the spinal cord.
It contains a cardiac center which adjusts the force and rate of
heartbeat, a vasomotor center that regulates blood pressure and the
medullary respiratory center that controls the basic pattern and
rate of breathing.
The cerebellum receives messages from sensory organs and functions
in balance and equilibrium; it helps to fine tune muscular action
The sympathetic division of
the autonomic nervous
system is the “fight or
flight” system. Its activity is
evident during vigorous
exercise, excitement or
emergency situations. The
major neurotransmitter of
this system is norepinephrine.
One of the target organs of
the sympathetic nervous
systems is the adrenal
medulla, an endocrine gland
which releases norepinephrine
and epinephrine into the
bloodstream. The
parasympathetic division is
most active when the body is
at rest. This division is
chiefly concerned with
conserving energy even as it
directs “housekeeping”
activities such as digestion of food and elimination of waste. The neurotransmitter of the
parasympathetic nervous system is acetylcholine.
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The “all-or-none” law, as it applies to impulse transmission, states which of the following?
a.
If a stimulus is applied, either all neurons fire or none do.
b.
A stimulus causes either all the sodium to leak into the neuronal membrane or none of it.
c.
Either all neurons develop an action potential upon stimulation or none do.
d.
If a stimulus is at or above the threshold, an action potential will be generated; if not,
then the neuron will not fire.
If a stimulus on the receptor portion of a sensory neuron is above the threshold level, a further
increase in the intensity of this stimulus will most likely cause the
a.
frequency of impulse production to increase
b.
strength of the neuronal impulse to increase
c.
impulse to move faster
d.
neuron membrane to become less permeable to sodium
Which of the following would result if the sodium-potassium pump of a neuron were
inoperative?
A)
The movement of chloride ions would produce an action potential.
B)
An impulse would travel from the axon to the dendrites of the neuron.
C)
The rate of transmission of the impulse would greatly increase.
D)
An action potential would not occur.
Which of the following best describes the function of cholinesterase in neurotransmission?
a.
It decomposes, providing a source of choline for muscular contraction.
b.
It binds acetylcholine to receptors on the post-synaptic membrane.
c.
It blocks acetylcholine reception on the post-synaptic membrane.
d.
It breaks down acetylcholine, preventing continual neurotransmission.
Lidocaine is a local anesthetic that works as a temporary pain reliever. Lidocaine binds to
voltage sensitive Na+ gates on pain receptors and prevents the the gates from opening. Which
of the following best describes why Lidocaine is an effective anesthetic?
a.
Lidocaine limits sodium influx, which increases the production of presynaptic vesicles
b.
Lidocaine increases sodium influx, which stimulates the propagation of the action
potential
c.
Lidocaine decreases sodium influx, which decreases the propagation of the action
potential
d.
Lidocaine increases sodium influx, which decreases the release of neurotransmitters
Mechanoreceptors within the skin respond to physical touch. Chemoreceptors on the taste buds
respond to salt, sweet, bitter and sour.Rods and cones within the retina respond to varying
wavelengths of light. In each example the stimulus:
a.
alters membrane permeability
b.
uses acetylcholine as a neurotransmitter
c.
influences the strength of the action potential based on the source
d.
opens voltage sensitive gates in the axon hillock of the effector cell
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The dendrites of olfactory receptor cells bind specific odor molecules present in the environment.
The olfactory cells are located in the nasal cavity and have axons that connect to the olfactory
bulb of the brain. Which of the following best describes the role of olfactory cells?
a.
they generate a response to the odor molecule
b.
they convert sensory stimuli into changes in membrane potential
c.
they process and discriminate among odors detected in the environment
d.
they integrate and interpret the sensory stimuli
All of the following statements about neurotransmitters are generally true EXCEPT:
a.
They bind to specific receptors on the postsynaptic membrane.
b.
They are actively transported across the synaptic cleft.
c.
They are released from membrane-bound packets called vesicles.
d.
They may be enzymatically degraded at the postsynaptic membrane.
Which of the following would occur as a result of increased activity of the sympathetic nervous
system?
a.
Accelerated digestion
b.
Decreased heart rate
c.
Increased epinephrine (adrenalin) secretion
d.
Increased salivation
A part of the brain used in the transfer of information from one cerebral hemisphere to
the other is the
a.
medulla oblongata c.
cerebellum
b.
corpus callosum
d.
hypothalamus
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