Chapter 13 Anatomy

Compression of which nerve produces the sensation that your leg has "fallen asleep"?

Compression of the sciatic nerve produces the sensation that your leg has "fallen asleep."

The synapsing of several neurons on the same postsynaptic neuron is called

Convergence

The reflexes that control the most rapid, stereotyped motor responses to stimuli are

Monosynaptic reflexes

An example of a stretch reflex triggered by passive muscle movement is the

Patellar reflex

The contraction of flexor muscles and the relaxation of extensor muscles illustrate the principle of

Reciprocal inhibition

Define reinforcement as it pertains to spinal reflexes.

Reinforcement is an enhancement of spinal reflexes. It occurs when the postsynaptic neuron enters a state of generalized facilitation caused by chronically active excitatory synapses.

Mary complains that when she wakes up in the morning, her thumb and forefinger are always "asleep." She mentions this condition to her physician, who asks Mary whether she sleeps with her wrists flexed. She replies that she does. The physician tells Mary that sleeping in that position may compress a portion of one of her peripheral nerves, producing her symptoms. Which nerve is involved?

The median nerve is involved.

Why do cervical nerves outnumber cervical vertebrae?

The first cervical nerve exits superior to vertebra C1 (between the skull and vertebra); the last cervical nerve exits inferior to vertebra C7 (between the last cervical vertebra and the first thoracic vertebra). So, there are 8 cervical nerves but only 7 cervical vertebrae.

What is the minimum number of neurons in a reflex arc?

The minimum number of neurons required for a reflex arc is two.

One of the first somatic reflexes to develop is the suckling reflex. Which type of reflex is this?

The suckling reflex is an innate reflex.

Identify the three spinal meninges.

The three spinal meninges are the dura mater, arachnoid mater, and pia mater.

Explain the anatomical significance of the fact that spinal cord growth ceases at age 4.

The vertebral column continues to grow, extending beyond the spinal cord. The end of the spinal cord is visible as the conus medullaris near L1, and the cauda equina extends the remainder of the vertebral column.

A weight lifter is straining to lift a 200-kg barbell above his head. Shortly after he lifts it to chest height, his muscles appear to relax and he drops the barbell. Which reflex has occurred?

This response is the tendon reflex.

List the five circuit patterns found in neuronal pools.

The five neuronal pool circuit patterns are divergence, convergence, serial processing, parallel processing, and reverberation.

Bowel and urinary bladder control involve spinal reflex arcs that are located in the sacral region of the spinal cord. In both instances, two sphincter muscles—an inner sphincter of smooth muscle and an outer sphincter of skeletal muscle—control the passage of wastes (feces and urine) out of the body. How would a transection of the spinal cord at the L1 segment level affect an individual's bowel and bladder control?

The individual would still exhibit a defecation (bowel) and urination (urinary bladder) reflex because these spinal reflexes are processed at the level of the spinal cord. Efferent impulses from the organs would stimulate specific interneurons in the sacral region that synapse with the motor neurons controlling the sphincters, thus bringing about emptying when the organs began to fill. (This is the same situation that exists in a newborn infant who has not yet fully developed the descending tracts required for conscious control.) However, an individual with the spinal cord transection at L1 would lose voluntary control of the bowel and bladder because these functions rely on impulses carried by motor neurons in the brain that must travel down the cord and synapse with the interneurons and motor neurons involved in the reflex.

Identify the major networks of nerves known as plexuses.

The major nerve plexuses are the cervical, brachial, lumbar, and sacral.

Predict the effects on the body of a spinal cord transection at segment C7. How would these effects differ from those of a spinal cord transection at segment T10?

Transection of the spinal cord at segment C7 would most likely result in paralysis from the neck down. Transection at segment T10 would produce paralysis and eliminate sensory input in the lower half of the body only.

The following are the steps involved in a neural reflex.

arrival of a stimulus and activation of a receptor,activation of a sensory neuron,information processing,activation of a motor neuron, response by an effector.

Spinal nerves are called mixed nerves because they

contain sensory and motor fibers

The ventral roots of each spinal segment

control peripheral effectors, and contain the axons of somatic motor and visceral motor neurons.

Reflex arcs in which the sensory stimulus and the motor response occur on the same side of the body are

ipsilateral

List, in sequence, the five steps involved in a neural reflex.

(1) arrival of stimulus and activation of receptor; (2) activation of sensory neuron; (3) information processing; (4) activation of a motor neuron; and (5) response by an effector (muscle, gland, or adipose tissue)

What five characteristics are common to all polysynaptic reflexes?

(1) involvement of pools of interneurons; (2) intersegmental distribution; (3) involvement of reciprocal innervation; (4) motor response prolonged by reverberating circuits; and (5) cooperation of reflexes to produce a coordinated, controlled response

Proceeding deep from the most superficial structure, number the fol-lowing in the correct sequence: (a) walls of vertebral canal (c) dura mater (e) subdural space (g) epidural space (b) pia mater (d) arachnoid membrane (f) subarachnoid space (h) spinal cord

(a) 1 (b)7 (c) 3 (d) 5 (e) 4 (f) 6 (g) 2 (h) 8

A disease that damages myelin sheaths would affect which portion of the spinal cord?

A disease that damages myelin sheaths would affect the columns in the white matter of the spinal cord, because the columns are composed of bundles of myelinated axons.

Define neuronal pool.

A neuronal pool is a functional group of organized interneurons within the CNS.

After injuring her back, Tina exhibits a positive Babinski reflex. What does this imply about Tina's injury?

A positive Babinski reflex is abnormal in adults. It indicates possible damage to descending tracts in the spinal cord.

Define reflex.

A reflex is a rapid, automatic response to a specific stimulus. It is an important mechanism for maintaining homeostasis.

Define spinal reflex.

A spinal reflex is a rapid, automatic response triggered by specific stimuli that is controlled in the spinal cord.

Identify the basic characteristics of polysynaptic reflexes.

All polysynaptic reflexes involve pools of interneurons; are intersegmental in distribution; and involve reciprocal inhibition, reverberating circuits, and the cooperation of several reflexes.

An anesthetic blocks the function of the dorsal rami of the cervical spinal nerves. Which areas of the body will be affected?

An anesthetic that blocks the function of the dorsal rami of the cervical spinal nerves would affect the skin and muscles of the back of the neck and of the shoulders.

Injury to which of the nerve plexuses would interfere with the ability to breathe?

Damage to the cervical plexus—or more specifically to the left and right phrenic nerves, which originate in this plexus and innervate the diaphragm—would greatly interfere with the ability to breathe and might even be fatal.

Damage to which root of a spinal nerve would interfere with motor function?

Damage to the ventral root of a spinal nerve, which contains both visceral and somatic motor fibers, would interfere with mo-tor function.

During a withdrawal reflex of the foot, what happens to the limb on the side opposite the stimulus? What is this response called?

During a withdrawal reflex, the limb on the opposite side is extended. This response is called a crossed extensor reflex.

Which of the following statements is false concerning the gray matter of the spinal cord?

It is primarily involved in relaying information to the brain

Differentiate between sensory nuclei and motor nuclei.

Sensory nuclei receive and relay sensory information from peripheral receptors. Motor nuclei issue motor commands to peripheral effectors.

What effect does the stimulation of a sensory neuron that innervates an intrafusal muscle fiber have on muscle tone?

Stimulation of the sensory neuron will in-crease muscle tone.

Karen falls down a flight of stairs and suffers lumbar and sacral spinal cord damage due to hyperextension of her back. The injury resulted in edema around the central canal that compressed the anterior horn of the lumbar region. What signs would you expect to observe as a result of this injury?

The anterior horn in the lumbar region of the spinal cord contains somatic motor neurons that direct the activity of skeletal muscles of the hip, lower limb, and foot. As a result of the injury, Karen would be expected to have poor control of most muscles of the lower limbs, causing difficulty walking (if she could walk at all) and problems maintaining balance (if she could stand).

If the anterior gray horns of the spinal cord were damaged, what type of control would be affected?

The cell bodies of spinal motor neurons are located in the anterior gray horns, so damage to these horns would result in a loss of motor control.

Name the components of the central nervous system (CNS) and of the peripheral nervous system (PNS).

The central nervous system (CNS) is made up of the brain and spinal cord, while cranial nerves and spinal nerves make up the peripheral nervous system (PNS).

Where is the cerebrospinal fluid that surrounds the spinal cord located?

The cerebrospinal fluid that surrounds the spi-nal cord is located in the subarachnoid space, which lies beneath the epithelium of the arachnoid mater and superficial to the pia mater.

A person with polio has lost the use of his leg muscles. In which area of his spinal cord would you expect the virus-infected motor neurons to be?

The polio virus-infected neurons would be in the anterior gray horns of the spinal cord, where the cell bodies of somatic motor neurons are located.

The improper use of crutches can produce a condition known as "crutch paralysis," characterized by a lack of response by the extensor muscles of the arm, and a condition known as "wrist drop," consisting of an inability to extend the fingers and wrist. Which nerve is involved?

The radial nerve is involved.

For the patellar (knee-jerk) reflex, how would the stimulation of the muscle spindle by gamma motor neurons affect the speed of the reflex?

When intrafusal fibers are stimulated by gamma motor neurons, the muscle spindles become more sensitive. As a result, little if any stretching stimulus would be needed to stimulate the contraction of the quadriceps muscles in the patellar reflex. For this reason, the reflex response would appear more quickly.

List all of the CNS sites where cerebrospinal fluid (CSF) is located. What are the functions of CSF?

Within the CNS, cerebrospinal fluid fills the central canal in the spinal cord, the ventricles of the brain, and the subarachnoid space between the arachnoid mater and the pia mater. CSF acts as a shock absorber and a diffusion medium for dissolved gases, nutrients, chemicals, and wastes.

A sensory region monitored by the dorsal rami of a single spinal segment is

a dermatome

Ascending tracts

carry sensory information to the brain

Side-to-side movements of the spinal cord are prevented by the

denticulate ligaments

The genitofemoral, femoral, and lateral femoral cutaneous nerves are major nerves of the

lumbar plexus

The major nerve of the cervical plexus that innervates the diaphragm is the

phrenic nerve

The adult spinal cord extends only to

the first or second lumbar vertebra

Polysynaptic reflexes can produce far more complicated responses than can monosynaptic reflexes because

the interneurons involved can control several muscle groups.