Download sensory, motor, and integrative systems

SENSORY, MOTOR, AND INTEGRATIVE SYSTEMS
A.
SENSATION
Identify the three essential functions of the nervous system.
1.
2
3.
receive sensory information
integrate and interpret the information
initiate motor responses, if necessary
What is the basis for selecting some things for consciousness?
The CNS requires a continuous flow of sensory information to regulate
homeostasis and initiate appropriate responses to changes in the internal
and external environments. The CNS selects for consciousness only
those pieces of information that are of consequence at a given moment.
All other information remains subconscious or unconscious.
Define the concept of a sensation.
In its broadest sense, a sensation is the conscious, subconscious, or
unconscious awareness of external and internal stimuli.
1.
LEVELS OF SENSATION
What are the levels of CNS translation?
The nature of the sensation and the type of reaction generated by
the CNS vary with the level of the CNS at which the stimulus is
translated:
a.
spinal reflexes
b.
visceral reflexes (brainstem)
c.
crude identification (thalamus)
d.
precise identification (cerebral cortex)
2.
MODALITY
What is modality?
Each specific type of sensation is called a sensory modality. The
distinct quality that makes one sensation different from another is
its modality.
A given sensory neuron carries only one modality.
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3.
COMPONENTS OF A SENSATION
In the proper order, list and describe the four processes that must occur in
order to produce a sensation.
4.
1.
stimulation -- a stimulus (change in the environment)
capable of activating a sensory neuron must be present
2.
transduction -- a sensory receptor or sensory organ must
respond to the stimulus and transduce it to a generator
potential
3.
conduction -- at threshold, an action potential must be
conducted along a sensory neuron
4.
translation -- a region of the CNS must translate the nerve
impulse into a sensation
GENERATOR POTENTIALS AND RECEPTOR POTENTIALS
Define the concept of generator potential.
A generator potential is the graded depolarization that results in a
change in resting membrane potential in a receptor (specialized
neuronal ending).
Describe some of the characteristics of a generator potential.
1.
2.
3.
always produces depolarization
will initiate an action potential in a sensory neuron if the
stimulus is strong enough to form threshold depolarization
all receptors, except those for vision, hearing, equilibrium,
and taste, create generator potential.
Define the concept of a receptor potential.
A receptor potential is depolarization or hyperpolarization of the
plasma membrane of a receptor cell that alters release of
neurotransmitter from the cell.
Describe some of the characteristics of a receptor potential.
1.
2.
3.
may produce depolarization or hyperpolarization
never directly initiates an action potential in a sensory
neuron
directly regulates release of neurotransmitter onto a sensory
neuron that may initiate threshold depolarization.
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5.
ADAPTATION OF SENSORY RECEPTORS
What is adaptation of a receptor?
Adaptation is a change in sensitivity, usually a decrease, to a longlasting stimulus.
Receptors vary in their ability to adapt. What is the difference between
fast-adapting and slow-adapting receptors?
Fast-adapting (phasic) receptors -- pressure, touch, and smell
receptors adapt quickly and play a major role in signaling changes
in a stimulus.
Slow-adapting (tonic) receptors -- pain, proprioceptors, and
chemoreceptors adapt slowly. They are important in signaling
information regarding the steady states of the body.
B.
GENERAL SENSES
1.
CUTANEOUS SENSATIONS
Name the cutaneous sensations?
Cutaneous sensations include tactile (touch, pressure, vibration),
thermal (hot, cold) and pain.
Where are these receptors located?
The receptors for these sensations are located in the skin,
connective tissues under the skin, mucous membranes, mouth, and
anus. These receptors are distributed across the body such that
some areas are heavily populated (very sensitive) while other areas
contain only a few receptors (poorly sensitive).
In general, describe the structure of cutaneous receptors.
Cutaneous receptors consist of the dendritic ends of sensory
neurons that may be enclosed in a specialized encapsulated
structure or have no special structure at all. Pain receptors, for
instance, are really just bare dendrites.
2.
PROPRIOCEPTIVE SENSATIONS
What is the proprioceptive sense?
An awareness of the activities of the muscles, joints, and tendons,
and of balance is provided by the proprioceptive (kinesthetic)
sense.
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Where are proprioceptors located?
Skeletal muscles, joint capsules, and tendons
What is the role of proprioceptors in homeostasis?
They apprise the CNS of muscle and tendon tension, change of
position, and the orientation of the head. They adapt very slowly,
allowing the CNS to be constantly aware of body position so that
fine adjustments can be made.
C.
PHYSIOLOGY OF SENSORY PATHWAYS
Most information from somatic receptor decussates. What does this mean?
Most input from somatic receptors on one side of the body crosses over
(decussates) to the opposite side of the CNS before ascending to the
thalamus.
From the thalamus, where does somatic sensory information go?
From the thalamus, the sensory information proceeds to the
somatosensory (primary sensory cortex) cortex of the postcentral gyrus in
the parietal lobe of the cerebrum. It is in this cortical region that the
sensory input becomes conscious (perception).
Three neurons are required to complete the sensory pathway from somatic
receptor to the brain. Name and describe them.
First-order neuron -- The first-order neuron is always the sensory neuron
bearing the receptor on its dendritic end. Its cell body is in a
sensory ganglion.
Second-order neuron -- The second-order neuron arises in the gray
matter. Its axon travels in an ascending tract and decussates prior
to entering the thalamus.
Third-order neuron -- The third-order neuron arises in the thalamus. Its
axon passes to the somatosensory cortex where it synapses on a
cortical neuron.
1.
POSTERIOR COLUMN-MEDIAL LEMNISCUS PATHWAY
Describe the posterior column-medial lemniscus pathway. What are the
names of the tracts involved and what modalities do they convey?
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Fasciculus gracilis and fasciculus cuneatus carry sensory
information related to discriminative touch, stereognosis,
proprioception, vibration, and weight discrimination.
The axon of the first-order neuron enters the dorsal horn, passes
into either fasciculus gracilis or cuneatus, then ascends ipsilaterally
to the white matter of the medulla.
In the medulla, the axon enters either nucleus gracilis or nucleus
cuneatus, where it synapses with the second-order neuron.
The axon of the second-order neuron decussates in the medulla
and enters a projection tract known as the medial lemniscus to
ascend to the thalamus.
The second-order neuron synapses in the thalamus with its thirdorder neuron, the axon of which passes into the internal capsule to
reach the somatosensory cortex, where consciousness of the
sensation occurs.
2.
ANTEROLATERAL (SPINOTHALAMIC) PATHWAYS
Describe the anterolateral (spinothalamic) pathways. What are the names
of the tracts involved and what modalities do they convey?
Anterolateral (spinothalamic) pathways are the lateral
spinothalamic tract (pain and temperature) and the anterior
(ventral) spinothalamic tract (crude touch, tickle, pressure, itch).
For both tracts, the first-order neuron enters the dorsal gray horn
and immediately synapses with the second-order neuron.
The axon of the second-order neuron decussates through the gray
commissure and enters either the lateral or anterior tract,
depending upon the modality being carried.
Each tract passes through the brainstem to the thalamus where the
axon synapses with the third-order neuron.
The axon of the third-order neuron passes through the internal
capsule to the somatosensory cortex and synapses with the
appropriate cortical neuron so that perception of the modality
occurs.
3.
SOMATOSENSORY CORTEX
Describe the somatosensory cortex. What is a homunculus?
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Areas of the somatosensory cortex in the postcentral gyrus of the
parietal lobe have been mapped to form a homunculus, the areas
of representation of body parts in the cerebral cortex.
Some areas of representations are huge compared to other areas.
In particular the hands and face have a tremendous amount of
representation in the cortex.
The size of cortical areas given to a particular structure is indicative
of the number of receptors located in that body part.
The size of somatosensory cortex and the number of receptors in a
given area are directly related to the functional importance of that
part.
For each cutaneous receptor, there is a corresponding first-,
second-, and third-order neuron in the somatosensory cortex.
D.
PHYSIOLOGY OF MOTOR PATHWAYS
Describe the motor cortex?
To direct voluntary motion there is the motor cortex, neurons found in the
precentral gyrus of the frontal lobe of the cerebrum. The functional
importance of an area is indicated by the amount of motor cortex devoted
to movement of that body part. As with the somatosensory cortex, the
hands, face, and mouth have the largest area of cortex given to them.
What is the difference between an upper motor neuron and a lower motor
neuron?
Each motor unit of skeletal muscle in the body has a corresponding
neuron in the motor cortex. Such a neuron is called an upper motor
neuron. The axons of upper motor neurons descend through the CNS to
synapse on lower motor neurons located in the brainstem gray matter or
the ventral horns of the spinal cord. Lower motor neurons, in turn, leave
the CNS to stimulate skeletal muscles.
1.
DIRECT (PYRAMIDAL) PATHWAYS
Name the tracts of the pyramidal pathways and describe the means by
which voluntary motor information reaches skeletal muscles.
Voluntary motor impulses are propagated from the motor cortex via
upper motor neurons in the direct (pyramidal) tracts, so called
because the axons pass through the pyramids of the medulla.
There are three primary pyramidal tracts:
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1.
2.
3.
lateral corticospinal tracts
anterior corticospinal tracts
corticobulbar tracts
Upper motor neurons for all tracts originate in motor cortex. Their
axons pass through the internal capsule then through the brainstem.
Axons of the lateral tracts and corticobulbar tracts decussate in the
medulla, while those of the anterior tracts decussate at the level of
appropriate lower motor neurons in the spinal cord.
The upper motor neurons synapse on the lower motor neurons
located in the ventral horns. Axons of all lower motor neurons pass
into the periphery stimulate the appropriate motor units.
Lower motor neurons associated with the lateral and anterior
corticospinal tracts pass into the periphery via spinal nerves while
those associated with the corticobulbar tracts pass via the cranial
nerves.
2.
INDIRECT (EXTRA-PYRAMIDAL) PATHWAYS
Give a brief description of the indirect (extrapyramidal) pathways.
The indirect pathways include all motor tracts other than the
corticospinal and corticobulbar tracts. The upper motor neurons
carry information from the motor cortex, basal ganglia, thalamus,
cerebellum, reticular formation, and brainstem nuclei to influence
the activity of the lower motor neurons.
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