Sensory, Motor, and Integrative Systems

Question 1

Identify the three essential functions of the nervous system.

Answer 1

1. receive sensory information
   2 integrate and interpret the information
2. initiate motor responses, if necessary

Question 2

What is the basis for selecting some things for consciousness?

Answer 2

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.

Question 3

Define the concept of a sensation.

Answer 3

In its broadest sense, a sensation is the conscious, subconscious, or unconscious awareness of external and internal stimuli.

Question 4

What are the levels of CNS translation?

Answer 4

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)

Question 5

What is modality?

Answer 5

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.

Question 6

stimulation

Answer 6

– a stimulus (change in the environment)

capable of activating a sensory neuron must be present

Question 7

transduction

Answer 7

a sensory receptor or sensory organ must

respond to the stimulus and transduce it to a generator potential

Question 8

conduction

Answer 8

at threshold, an action potential must be

conducted along a sensory neuron

Question 9

translation

Answer 9

a region of the CNS must translate the nerve

impulse into a sensation

Question 10

Define the concept of generator potential.

Answer 10

A generator potential is the graded depolarization that results in a change in resting membrane potential in a receptor (specialized neuronal ending).

Question 11

Describe some of the characteristics of a generator potential.

Answer 11

1. always produces depolarization
2. will initiate an action potential in a sensory neuron if the
   stimulus is strong enough to form threshold depolarization
3. all receptors, except those for vision, hearing, equilibrium,
   and taste, create generator potential.

Question 12

Define the concept of a receptor potential.

Answer 12

A receptor potential is depolarization or hyperpolarization of the plasma membrane of a receptor

Question 13

Describe some of the characteristics of a receptor potential.

Answer 13

1. may produce depolarization or hyperpolarization
2. never directly initiates an action potential in a sensory
   neuron
3. directly regulates release of neurotransmitter onto a sensory
   neuron that may initiate threshold depolarization.

Question 14

What is adaptation of a receptor?

Answer 14

Adaptation is a change in sensitivity, usually a decrease, to a long-lasting stimulus.

Question 15

Fast-adapting (phasic) receptors

Answer 15

pressure, touch, and smell receptors adapt quickly and play a major role in signaling changes in a stimulus.

Question 16

Slow-adapting (tonic) receptors

Answer 16

– pain, proprioceptors, and chemoreceptors adapt slowly. They are important in signaling information regarding the steady states of the body

Question 17

Name the cutaneous sensations?

Answer 17

Cutaneous sensations include tactile (touch, pressure, vibration), thermal (hot, cold) and pain

Question 18

Where are these receptors located?

Answer 18

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).

Question 19

In general, describe the structure of cutaneous receptors.

Answer 19

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.

Question 20

What is the proprioceptive sense?

Answer 20

An awareness of the activities of the muscles, joints, and tendons, and of balance is provided by the proprioceptive (kinesthetic) sense.

Question 21

Where are proprioceptors located?

Answer 21

Skeletal muscles, joint capsules, and tendons

Question 22

What is the role of proprioceptors in homeostasis?

Answer 22

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.

Question 23

Most information from somatic receptor decussates. What does this mean?

Answer 23

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.

Question 24

From the thalamus, where does somatic sensory information go?

Answer 24

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).

Question 25

First-order neuron

Answer 25

The first-order neuron is always the sensory neuron

bearing the receptor on its dendritic end. Its cell body is in a sensory ganglion.

Question 26

Second-order neuron

Answer 26

The second-order neuron arises in the gray

matter. Its axon travels in an ascending tract and decussates prior to entering the thalamus.

Question 27

Third-order neuron

Answer 27

The third-order neuron arises in the thalamus. Its

axon passes to the somatosensory cortex where it synapses on a cortical neuron.

Question 28

Describe the posterior column-medial lemniscus pathway. What are the names of the tracts involved and what modalities do they convey?

Answer 28

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 third-order neuron, the axon of which passes into the internal capsule to reach the somatosensory cortex, where consciousness of the sensation occurs.

Question 29

Describe the anterolateral (spinothalamic) pathways. What are the names of the tracts involved and what modalities do they convey?

Answer 29

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.

Question 30

Describe the somatosensory cortex. What is a homunculus?

Answer 30

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.

Question 31

Describe the motor cortex?

Answer 31

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

Question 32

What is the difference between an upper motor neuron and a lower motor neuron?

Answer 32

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.

Question 33

Name the tracts of the pyramidal pathways and describe the means by which voluntary motor information reaches skeletal muscles.

Answer 33

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:

1. lateral corticospinal tracts
2. anterior corticospinal tracts
3. corticobulbar tracts

Upper motor neurons for all tracts originate in motor cortex. Their axons pass through the internal capsule then through the brain-stem.

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.

Question 34

Give a brief description of the indirect (extrapyramidal) pathways.

Answer 34

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.