Memmler’s
The Human Body in Health and Disease
11th edition
Chapter 9
The Nervous System: The
Spinal Cord and Spinal Nerves
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Role of the Nervous System
Nervous system coordinates all body systems
•Detects and responds to stimuli
•Brain and spinal cord act as switching centers
•Nerves carry messages to and from centers
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Structural Divisions
•Central nervous system (CNS)
– Brain
– Spinal cord
•Peripheral nervous system (PNS)
– Cranial nerves
– Spinal nerves
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Anatomic
divisions of
the nervous
system.
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Functional Divisions
Somatic nervous system
•Controlled voluntarily
•Effectors are skeletal muscles
•No further subdivisions
Autonomic (or visceral) nervous system (ANS)
•Controlled involuntarily
•Effectors are smooth muscle, cardiac muscle, and glands
•Subdivided into
–
Sympathetic nervous system
–
Parasympathetic nervous system
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Brain
Somatic
Skeletal;
Voluntary control
Autonomic
Glands; Cardiac & Smooth Muscle;
Involuntary
Sympathetic
“Fight/ flight”
Parasympathetic
“Rest/ digest”
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Checkpoint 9-1: What are the two divisions of the
nervous system based on structure?
Checkpoint 9-2: The nervous system can be divided
functionally into two divisions based on type of control and
effectors. What division is voluntary and controls skeletal
muscle, and what division is involuntary and controls
involuntary muscles and glands?
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Neurons and Their Functions
Neurons
•Functional cells of nervous system
•Highly specialized
•Unique structure
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Diagram of a motor
neuron. The break in
the axon denotes
length. The arrows
show the direction of
the nerve impulse.
• Is the neuron shown
here a sensory or a motor
neuron?
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Structure of a Neuron
Cell body
•Nucleus
•Other organelles
Cell fibers
•Dendrites
•Axons
– Some are protected by myelin sheath
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What is a dendrite?
a. A neuron fiber that conducts impulses away
from the cell body
b. A neuron fiber that conducts impulses to the
cell body
c. A gap between myelin-covered axons
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Answer:
b. A neuron fiber that conducts impulses to the cell body
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Formation of a
myelin sheath.
(A)Schwann cells
wrap around the
axon, creating a
myelin coating.
(B) The outermost
layer of the Schwann
cell forms the
neurilemma.
Spaces between the
cells are the nodes
(of Ranvier).
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ACTIVITY
•Make your own myelin covered nerve
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Checkpoint 9-3: The neuron, the functional unit of the
nervous system, has long fibers extending from the cell
body. What is the name of the fiber that carries impulses
toward the cell body and what is the name of the fiber that
carries impulses away from the cell body?
Checkpoint 9-4: Myelin is a substance that covers and
protects some axons. What color describes myelinated
fibers, and what color describes unmyelinated tissue of the
nervous system?
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Types of Neurons
•Sensory neurons (afferent neurons)
– Conduct impulses to spinal cord, brain
•Motor neurons (efferent neurons)
– Conduct impulses to muscles, glands
•Interneurons (central or association neurons)
– Conduct information within CNS
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Nerves and Tracts
•Nerve: fiber bundle within PNS
•Tract: fiber bundle within CNS
•Organized into fascicles
•Connective tissue layers
– Endoneurium
– Perineurium
– Epineurium
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Checkpoint 9-5: Nerves are bundles of neuron fibers in
the PNS. These nerves may be carrying impulses either
toward or away from the CNS. What name is given to
nerves that convey impulses toward the CNS, and what
name is given to nerves that transport away from the CNS?
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Neuroglia
Neuroglia (glial cells)
•Protect and nourish nervous tissue
•Support nervous tissue
•Aid in cell repair
•Remove pathogens and impurities
•Regulation composition of fluids around and between cells
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Checkpoint 9-6: The nervous system’s nonconducting
cells protect, nourish and support the neurons. What are
these cells called?
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The Nervous System at Work
Electrical impulses sent along neuron fibers and
transmitted between cells at junctions
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The Nerve Impulse
•Plasma membrane carries electrical charge (potential)
•Plasma membrane is polarized (negative charge)
•Membrane potential reverses, generates electrical charge
(action potential)
– Resting state
– Depolarization
– Repolarization
• Sodium/potassium (Na+/K+) pump
•Myelin sheath speeds conduction
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Question:
Which ions are involved in the action
potential?
a. Potassium and calcium
b. Sodium and oxygen
c. Sodium and potassium
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Answer:
c. Sodium and potassium
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Checkpoint 9-7: An action potential occurs in two stages.
In the first stage, the charge on the membrane reverses,
and in the second stage, it returns to the resting state.
What are the names of these two stages?
Checkpoint 9-8: What ions are involved in generating an
action potential?
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The Synapse
Junction point for transmitting nerve impulse
•Axon (presynaptic cell)
•Dendrite (postsynaptic cell)
•Synaptic cleft
•Neurotransmitters
– Epinephrine (adrenaline)
– Norepinephrine (noradrenaline)
– Acetylcholine
•Receptors
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Neurotransmitters and Psychoactive Drugs
•Psychoactive drugs affect neurotransmitter activity in the
brain
•Used to treat depression, anxiety, obsessive-compulsive
disorder (OCD)
•Selective serotonin reuptake inhibitors (Example: Prozac)
– Block serotonin uptake
•Others block norepinephrine, dopamine.
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A synapse.
(A)The end-bulb of the
presynaptic
(transmitting) axon
has vesicles containing
neurotransmitter,
which is released into
the synaptic cleft to
the membrane of the
postsynaptic
(receiving) cell.
(B) Close-up of a
synapse showing
receptors for
neurotransmitter in
the postsynaptic cell
membrane.
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Question:
The point of junction for transmitting a
nerve impulse is called what?
a. axon
b. synapse
c. vesicle
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Answer:
b. synapse
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Checkpoint 9-9: Chemicals are needed to carry
information across the synaptic cleft at a synapse. As a
group, what are all these chemicals called?
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The Spinal Cord
•Links PNS and brain
•Helps coordinate impulses within CNS
•Contained in and protected by vertebrae
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Spinal cord and
spinal nerves. Nerve
plexuses
(networks) are
shown.
(A) Lateral view.
(B) Posterior view.
• Is the spinal cord
the same length as
the spinal column?
How does the number
of cervical vertebrae
compare with the
number of cervical
spinal nerves?
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Structure of the Spinal Cord
•Unmyelinated tissue (gray matter)
– Dorsal horn
– Ventral horn
– Gray commissure
– Central canal
•Myelinated axons (white matter)
– Posterior median sulcus
– Anterior median fissure
– Ascending and descending tracts
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Checkpoint 9-10: The spinal cord contains both gray and
white matter. How is this tissue arranged in the spinal
cord?
Checkpoint 9-11: What is the purpose of the tracts in the
white matter of the spinal cord?
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The spinal cord.
(A) Cross-section of the
spinal cord showing the
organization of the gray and
white matter. The roots of
the spinal nerves are also
shown.
(B) Microscopic view of the
spinal cord in cross-section
(x5).
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The Reflex Arc
•Receptor detects stimulus
•Sensory neuron transmits impulses to CNS
•CNS coordinates impulses and organizes response
•Motor neuron carries impulses away from CNS
•Effector carries out response
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Typical reflex arc. Numbers
show the sequence of
impulses through the spinal
cord (solid arrows).
Contraction of the biceps
brachii results in flexion of
the arm at the elbow.
• Is this a somatic or an
autonomic reflex arc? What type
of neuron is located between
the sensory and motor neuron
in the CNS?
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Checkpoint 9-12: What name is given to a pathway
through the nervous system from a stimulus to an effector?
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Reflex Activities
•Simple reflex
– Rapid
– Uncomplicated
– Automatic
•Spinal reflex
– Stretch reflex
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Medical Procedures Involving the
Spinal Cord
•Lumbar puncture (spinal tap)
– Cerebrospinal fluid (CSF) removed for testing
•Drug administration
– Anesthetic (an epidural or spinal anesthesia)
– Pain medication
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Diseases and Other Disorders of the
Spinal Cord
•Multiple sclerosis (MS)
•Amyotrophic lateral sclerosis
•Poliomyelitis
•Tumors
•Injuries
–
Monoplegia
–
Diplegia
–
Paraplegia
–
Hemiplegia
–
Tetraplegia (Quadriplegia)
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Question:
In a lumbar tap, what is removed from
the body for testing?
a. cerebrospinal fluid
b. lymph
c. bone marrow
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Answer:
a. cerebrospinal fluid
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The Spinal Nerves
•31 pairs
•Each nerve attached to spinal cord by two roots
– Dorsal root
• Dorsal root ganglion
– Ventral root
•Nerves near end of cord travel together in the cord until
each exits from its respective intervertebral foramen
•Mixed nerves
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Branches of the Spinal Nerves
•Cervical plexus
– Phrenic nerve
•Brachial plexus
– Radial nerve
•Lumbosacral plexus
– Sciatic nerve
•Dermatomes
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Dermatomes. A
dermatome is a
region of the skin
supplied by a single
spinal nerve.
• Which spinal nerves
carry impulses from
the skin of the toes?
From the anterior hand
and fingers?
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Checkpoint 9-13: How many pairs of spinal nerves are
there?
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Disorders of the Spinal Nerves
•Peripheral neuritis
•Sciatica
•Herpes zoster
•Guillain-Barré syndrome
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The Autonomic Nervous
System (ANS)
Regulates the action of glands, smooth muscles of hollow
organs and vessels, and heart muscle
•Preganglionic neuron connects spinal cord to ganglion
•Postganglionic neuron connects ganglion to effector
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Checkpoint 9-14: How many neurons are there in each
motor pathway of the ANS?
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Divisions of the Autonomic Nervous
System
•Sympathetic nervous system
•Parasympathetic nervous system
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Sympathetic nervous system
•Thoracolumbar area
•Collateral ganglia
– Celiac ganglion
– Superior mesenteric ganglion
– Inferior mesenteric ganglion
•Adrenergic system
•Activated in the four E’s: excitement, emergency,
embarassment, exercise
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Parasympathetic nervous system
•Arise in craniosacral areas
•Terminal ganglia
•Cholinergic system
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Question:
Any group of nerve cell bodies located
outside the central nervous system is
known as what?
a. a ganglion
b. a plexus
c. a horn
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Answer:
a. a ganglion
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Autonomic nervous
system. The
diagram shows
only one side of
the body for each
division.
• Which division of
the autonomic
nervous system has
ganglia closer to the
effector organ?
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Cellular Receptors
•“Docking sites” on postsynaptic cell membranes
Two types:
•Cholinergic receptors
– Nicotinic (bind nicotine) on skeletal muscle cells
– Muscarinic (bind muscarine, a poison) on effector cells of
PNS
•Adrenergic receptors
– Found on receptor cells of sympathetic nervous system
– Bind norepinephrine, epinephrine
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Functions of the Autonomic Nervous
System
•Sympathetic nervous system
– Fight-or-flight response
•Parasympathetic nervous system
– Returns body to normal
•Systems generally have opposite effects on organ
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Checkpoint 9-15: Which division of the ANS stimulates a
stress response, and which division reverses the stress
response?
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Question:
What is the technical name of a “docking
site” on a postsynaptic cell membrane?
a. neurotransmitter
b. cellular receptor
c. dendrite
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Answer:
b. cellular receptor
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QUESTIONS?
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