Download Answers to WHAT DID YOU LEARN questions

CHAPTER 16
Answers to “What Did You Learn?”
1.
The cervical enlargement is located in the inferior cervical part of the spinal
cord. It contains the neurons that innervate the upper limbs. The
lumbosacral enlargement extends through the lumbar and sacral parts of the
spinal cord and contains neurons that innervate the lower limbs.
2.
There are 31 pairs of spinal nerves: 8 cervical nerves (C1–C8), 12 thoracic
nerves (T1–T12), 5 lumbar nerves (L1–L5), 5 sacral nerves (S1–S5), and 1
coccygeal nerve (Co1).
3.
The denticulate ligaments are paired, lateral triangular extensions of the spinal pia
mater that attach to the dura mater. They help suspend and anchor the spinal cord
within the middle of the vertebral canal to prevent potential lateral displacement
of the spinal cord.
4.
The gray matter in the spinal cord is centrally located and its shape resembles
an H or a butterfly. Bilaterally symmetrical right and left regions of gray
matter are connected in the midline by a thin, horizontal bar of gray matter,
called the gray commissure, which surrounds a narrow central canal. Gray
matter regions on both sides are artificially separated into three projections,
called horns (anterior, lateral, and posterior horns). The white matter is
external to the gray matter and is partitioned into three regions, each called a
funiculus.
5.
The anterior horns are left and right anterior masses of gray matter. They
primarily house the cell bodies of somatic motor neurons which innervate skeletal
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muscle. The posterior horns are the left and right posterior masses of gray matter.
The axons of sensory neurons and the cell bodies of interneurons are located in
the posterior horns.
6.
White matter is partitioned into three regions, each called a funiculus. The
posterior funiculus lies between the posterior gray horns, the lateral funiculus
is the white matter region on each lateral side of the spinal cord, and the
anterior funiculus are the white matter tracts that are on each anterior side of
the cord between the anterior gray horns and the anterior median fissure of the
spinal cord. Each funiculus region contains both ascending and descending
tracts. Thus, each funiculus contains both motor and sensory axons.
7.
Cell bodies of the sensory neurons are housed in the posterior root ganglion that is
attached to the posterior root. Each posterior root ganglion is located medial to
the pedicles of the adjacent vertebrae.
8.
The principal nerve plexuses, from superior to inferior, are the cervical, brachial,
lumbar, and sacral.
9.
The brachial plexus is formed from the anterior rami of spinal nerves C5 through T1.
10.
The main nerves of the lumbar plexus are the femoral nerve and the obturator
nerve. The main nerve of the sacral plexus is the sciatic nerve, which divides into
the tibial and common fibular nerves.
11.
The five steps in a reflex arc are: (1) activation of a receptor by a stimulus; (2)
nerve impulse travels through a sensory neuron to the spinal cord; (3) the nerve
impulse is processed in the integration center by interneurons; (4) a motor neuron
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transmits a nerve impulse to an effector; and (5) the effector responds to the nerve
impulse from the motor neuron.
12.
In a monosynaptic reflex, sensory axons synapse directly on the motor neurons
whose axons then project to the effector. Interneurons do not function in this type
of reflex. More complex neural pathways are observed in polysynaptic reflexes,
which may have a number of synapses involving interneurons within the reflex
arc.
13.
Most components of the cranial and spinal nerves form from neural crest cells that
have split off from the developing neural tube.
14.
The alar plates form the posterior horns of the gray matter and the posterior part
of the gray commissure.
Answers to “Content Review”
1.
The major parts of the spinal cord, from superior to inferior, are the cervical part,
thoracic part, lumbar part, sacral part, and coccygeal part. Each part of the spinal
cord contains motor neuron cell bodies and sensory axons that contribute to spinal
nerves of the same name. For example, the cervical part of the spinal cord
contains the motor neurons whose axons contribute to the cervical spinal nerves
and receives input from sensory neurons through these spinal nerves. The
different parts of the spinal cord do not match up exactly with the vertebrae of the
same name, because the spinal cord proper ends at the level of the L1 vertebra.
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2.
The epidural space lies between the dura mater and the periosteum covering the inner
walls of the vertebrae. This space houses areolar connective tissue, blood vessels, and
adipose connective tissue.
3.
Anterior horns are the left and right anterior masses of gray matter. The anterior
horns primarily house cell bodies of somatic motor neurons that innervate skeletal
muscle. Lateral horns are found only in the T1–L2 regions of the spinal cord.
They contain the cell bodies of autonomic motor neurons, which innervate cardiac
muscle, smooth muscle, and glands. Posterior horns are posterior masses of gray
matter. The axons of sensory neurons and the cell bodies of interneurons are
located in the posterior horns.
4.
The cervical plexuses are located deep on each side of the neck immediately
lateral to cervical vertebra C1–C4. Branches of the cervical plexuses innervate
the anterior neck muscles, as well as the skin of the neck, and portions of the head
and shoulders.
5.
The five major terminal branches of the brachial plexus are the axillary nerve,
median nerve, musculocutaneous nerve, radial nerve, and ulnar nerve. (1) The
axillary nerve supplies the deltoid and teres minor muscles. (2) The median nerve
innervates most of the anterior forearm muscles, the thenar muscles, and the
lateral two lumbricals. (3) The musculocutaneous nerve innervates the anterior
arm muscles (coracobrachialis, biceps brachii, and radialis). (4) The radial nerve
arises from the posterior cord of the brachial plexus. It innervates the posterior
arm and posterior forearm muscles, as well as brachioradialis. (5) The ulnar
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nerve arises from the medial cord of the brachial plexus. It innervates some of the
anterior forearm muscles. It also innervates most of the intrinsic hand muscles.
6.
The lumbar plexus is formed from the anterior rami of L1–L4. The femoral and
obturator nerves are formed from this plexus.
7.
The tibial nerve innervates most of the hamstring muscles (except the short head
of the biceps femoris), the posterior leg muscles, and the muscles on the plantar
side of the foot. The common fibular nerve innervates the short head of the
biceps femoris, the anterior leg muscles, lateral leg muscles, and the muscles on
the dorsum of the foot.
8.
Reflexes are rapid, automatic, involuntary reactions of muscles or glands to a
stimulus. Usually a conscious movement is somewhat slower and not
predictable.
9.
A withdrawal reflex is a polysynaptic reflex that is initiated by a painful
stimulus causing the movement of a traumatized body part away from the
source of harm. A stretch reflex is a simple, monosynaptic reflex that
monitors and regulates skeletal muscle length. When a muscle is stretched, it
reflexively contracts to shorten. A Golgi tendon reflex is a polysynaptic
reflex that prevents skeletal muscles from tensing excessively.
10.
The basal plates lie anterior to the sulcus limitans. The basal plates develop into
the anterior and lateral horns, motor structures of the gray matter. They also will
form the anterior part of the gray commissure.
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