Download BIo 218 Lecture Outline Tortora Ch18

Biology 218 – Human Anatomy
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Chapter 18 Adapted form Tortora 10th ed.
LECTURE OUTLINE Chapter 18
A. Spinal Cord Anatomy (p. 554)
1. Surrounding and protecting the delicate nervous tissue of the spinal cord (and, in a
similar way, the brain) are:
i.
vertebral column, including:
a. vertebrae
b. vertebral ligaments
ii. three spinal meninges (surrounding the brain and continuous with the spinal
meninges are the cranial meninges), which include:
a. outermost meninx is the dura mater
- it is composed of dense, irregular connective tissue
- it extends from the level of the foramen magnum to the second
sacral vertebra where it is close-ended
- surrounding it is the epidural space filled with fat and connective
tissue which provide additional protection to the spinal cord
b.
middle meninx is the avascular arachnoid mater
- it consists of connective tissue with a spider web-like arrangement
of collagen fibers and some elastic fibers
- surrounding it is the subdural space filled with interstitial fluid
c. innermost meninx is the pia mater
- it is attached to the surface of the spinal cord (and brain)
- it is a layer of connective tissue that contains collagen fibers and
some elastic fibers as well as many blood vessels that provide
nutrients and oxygen to the spinal cord (and brain)
- surrounding it is the subarachnoid space filled with cerebrospinal
fluid
All three spinal meninges cover the spinal nerves up to the point of exit
from the vertebral column through the intervertebral foramina.
Membranous extensions of the pia mater called denticulate ligaments
suspend the spinal cord in the middle of its dural sheath to provide
protection against shock and sudden displacement.
iii. cushion of cerebrospinal fluid (produced in the brain)
2. External Anatomy of the Spinal Cord: (p. 554)
i.
It is roughly cylindrical and extends from the medulla oblongata to the
superior border of the second lumbar vertebra in an adult.
ii. It has two enlargements which are sites where nerves supplying the limbs
emerge:
a. cervical enlargement extends from the fourth cervical to the first
thoracic vertebra
b. lumbar enlargement extends from the ninth to the twelfth thoracic
vertebra
iii. At its lower end, the spinal cord has a tapering, cone-shaped portion called
the conus medullaris which ends at the level of the intervertebral disc
between the first and second lumbar vertebrae in an adult.
iv. An extension of the pia mater called the filum terminale extends inferiorly
from the conus medullaris to attach the spinal cord to the coccyx.
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v.
The roots of spinal nerves emerging from the lower part of the spinal cord
travel inferiorly to form the cauda equina.
vi. Each of the 31 pairs of spinal nerves emerge from a spinal segment; the
spinal nerves are named and numbered according to the region and level of
the spinal cord from which they emerge:
a. 8 pairs of cervical nerves (the first pair emerge between the atlas and
the occipital bone) represented as C1-C8
b. 12 pairs of thoracic nerves represented as T1-T12
c. 5 pairs of lumbar nerves represented as L1-L5
d. 5 pairs of sacral nerves represented as S1-S5
e. 1 pair of coccygeal nerves (represented as Co1)
vii. Each spinal nerve is attached to a spinal segment by two bundles of axons
called roots:
a. posterior or dorsal root contains sensory nerve fibers which transmit
nerve impulses from the periphery into the spinal cord; it has an
enlargement called the posterior or dorsal root ganglion which
contains the cell bodies of the sensory neurons
b. anterior or ventral root contains motor neuron axons which transmit
nerve impulses from the spinal cord to effector organs and cells
3. Internal Anatomy of the Spinal Cord: (p. 558)
i.
Two grooves divide the spinal cord into right and left sides:
a. anterior median fissure is a deep, wide groove on the ventral side
b. posterior median sulcus is a shallow, narrow groove on the dorsal
side
ii.
iii.
iv.
v.
vi.
The spinal cord contains a core of gray matter, shaped like a butterfly when
viewed in cross section, that is surrounded by white matter:
a. the gray matter consists primarily of cell bodies of neurons, neuroglia,
unmyelinated axons, and dendrites of interneurons and motor neurons
b. the white matter consists of bundles of myelinated and unmyelinated
axons of motor neurons, interneurons, and sensory neurons
The gray commissure is a region of gray matter that connects the two wings
of the butterfly.
At the center of the gray commissure is the central canal which extends
throughout the entire length of the spinal cord; it is continuous with the fourth
ventricle of the brain.
Anterior to the gray commissure is the anterior (ventral) white commissure
which connects the white matter of the left and right sides of the spinal cord.
The gray matter contains sensory nuclei and motor nuclei; the gray matter on
each side of the spinal cord is subdivided into regions called horns:
a. anterior (ventral) gray horns contain cell bodies of somatic motor
neurons and motor nuclei which provide nerve impulses for contraction
of skeletal muscles
b. posterior (dorsal) gray horns contain somatic and autonomic
sensory nuclei
c. lateral gray horns (which are present only in the thoracic, upper
lumbar, and sacral segments of the spinal cord) contain cell bodies of
autonomic motor neurons that regulate activities of involuntary
effectors
Biology 218 – Human Anatomy
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vii. The white matter is subdivided by the anterior and posterior gray horns into
regions called columns:
a. anterior (ventral) white columns
b. posterior (dorsal) white columns
c. lateral white columns
viii. Each column contains bundles of nerve axons called tracts:
a. ascending (sensory) tracts transmit nerve impulses upward to the
brain
b. descending (motor) tracts transmit nerve impulses downward from
the brain
Spinal cord tracts are continuous with tracts in the brain.
ix. The various spinal cord segments vary in size, shape, relative amounts of
gray and white matter, and distribution and shape of gray matter; these
features are summarized in Table 18.1.
B. Spinal Cord Functions (p. 560)
1. The spinal cord has two major functions in maintaining homeostasis:
i.
The tracts in the white matter of the spinal cord transmit nerve impulses
between the brain and the periphery.
ii. The gray matter of the spinal cord receives and integrates the incoming and
outgoing information.
2. Sensory and Motor Tracts:
i.
A tract is often named according to its position in the white matter, where it
begins and ends, and the direction of nerve impulse transmission; e.g., the
anterior spinothalamic tract is located in the anterior white column, and it
begins in the spinal cord and ends in the thalamus - since it transmits nerve
impulses upward, it is an ascending (sensory) tract. The tracts are
summarized in Table 21.3 and 21.4.
ii. Sensory information from receptors is transmitted up the spinal cord to the
brain via two main ascending routes on each side of the spinal cord:
a. spinothalamic tracts transmit nerve impulses for sensing:
- pain
- temperature
- deep pressure
- itching and tickling
- crude, poorly localized touch
b. posterior columns transmit nerve impulses for sensing:
- proprioception
- discriminative touch
- two-point discrimination
iii.
- light pressure
- vibrations
Motor output from the brain to skeletal muscles travels down the spinal cord
via two types of descending pathways on each side of the spinal cord:
a. direct (pyramidal) pathways (the lateral corticospinal, anterior
corticospinal, and corticobulbar tracts) transmit nerve impulses
destined to cause precise, voluntary movements of skeletal muscles
b. indirect (extrapyramidal) pathways (the rubrospinal, tectospinal, and
vestibulospinal tracts) transmit nerve impulses that:
- govern automatic movements
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- help coordinate body movements with visual stimuli
- maintain skeletal muscle tone and posture
- play a major role in equilibrium by regulating muscle tone in
response to movements of the head
3. Reflexes and Reflex Arcs: (p. 562)
i.
The gray matter in the spinal cord promotes homeostasis by serving as the
integrating center for spinal reflexes (the brain stem is the integrating center
for cranial reflexes).
ii. Reflexes are fast, predictable, automatic responses to changes in the
environment that help maintain homeostasis:
a. somatic reflexes involve contraction of skeletal muscles
b. autonomic (visceral) reflexes involve responses of smooth muscle,
cardiac muscle, and glands
iii. The pathway followed by nerve impulses that produce a reflex response is
called a reflex arc (reflex circuit), which includes five functional
components:
a. sensory receptor (dendrite or associated sensory structure) responds
to a specific stimulus and may trigger one or more nerve impulses
b. sensory neuron transmits nerve impulses from the receptor to the
gray matter of the spinal cord or brain stem
c. integrating center is a region of gray matter within the CNS:
- it may have a single synapse between the sensory neuron and a
motor neuron to form a monosynaptic reflex arc
- more commonly, it has one or more interneurons in the pathway to
the motor neuron resulting in a polysynaptic reflex arc
d. motor neuron transmits nerve impulses from the integrating center to
a part of the body that will respond
e.
effector is a muscle or gland that will respond to the motor nerve
impulse; its action is called a reflex:
- in a somatic reflex, the effector is skeletal muscle
- in an autonomic (visceral) reflex, the effector is smooth muscle,
cardiac muscle, or a gland
C. Spinal Nerves (p. 564)
1. A spinal nerve is formed by merger of a posterior root (containing sensory axons) and
an anterior root (containing motor axons) at an intervertebral foramen; therefore, every
spinal nerve is a mixed nerve.
2. Every spinal nerve (and cranial nerve) is surrounded and protected by connective
tissue coverings:
i.
each axon is wrapped in a layer called the endoneurium
ii. groups of axons with their endoneuria are arranged in bundles called
fascicles, and each fascicle is wrapped in a layer called the perineurium
iii. groups of fascicles collectively form a nerve which is covered by a layer called
the epineurium
The dura mater of the spinal meninges fuses with the epineurium as the nerve
exits through the intervertebral foramen.
3. Shortly after passing through its intervertebral foramen, a spinal nerve divides into
several branches called rami:
i.
posterior (dorsal) ramus serves the deep muscles and skin of the posterior
surface of the trunk
Biology 218 – Human Anatomy
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ii.
anterior (ventral) ramus serves the muscles and structures of the limbs and
the skin of the lateral and anterior surfaces of the trunk
iii. meningeal branch reenters the vertebral canal via the intervertebral foramen
and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal
cord, and the meninges
iv. rami communicantes are components of the autonomic nervous system
4. The anterior rami of spinal nerves, except for thoracic nerves T2-T12, form networks
on both right and left sides of the body that are called plexuses; emerging from the
plexuses are nerves that bear names which often describe the general regions they
serve or the routes that they follow.
5. The major pairs of plexuses (see Exhibits 18.1–18.4) are:
i.
cervical plexuses
ii. brachial plexuses
iii. lumbar plexuses
iv. sacral plexuses (two small coccygeal plexuses are also present)
6. The anterior rami of spinal nerves T2-T12 do not enter into the formation of plexuses
and are called intercostal or thoracic nerves; these nerves travel directly to the
intercostal regions and the nearby muscles and skin regions that they innervate.
7. Dermatomes: (p. 577)
i.
The skin over the entire body is supplied by somatic sensory neurons that
carry nerve impulses into the spinal cord and brain stem.
ii. The underlying skeletal muscles receive signals from somatic motor neurons
that carry impulses out of the spinal cord.
iii. Each spinal nerve supplies a specific, predictable segment of the body; the
area of the skin that provides sensory input to one pair of spinal nerves or to
cranial nerve V (for the face and scalp) is called a dermatome.
iv. The nerve supply in adjacent dermatomes overlaps slightly or, in some cases,
more extensively.
D. Key Medical Terms Associated with the Nervous Tissue (p. 579)
1. Students should familiarize themselves with the glossary of key medical terms.