The Nervous System I:
The Spinal Cord and Spinal Nerves
Anatomy & Physiology I
Chapter 9
Coordination and Control

endocrine and nervous system maintain
internal communication and coordination
◦ endocrine system - communicates by means
of chemical messengers (hormones) secreted
into to the blood
◦ nervous system - employs electrical and
chemical means to send messages from cell to
cell
Functions of the Nervous System
The nervous system carries out its task in three
basic steps:
1.
◦
Sensory input
Information gathered by sensory receptors about
internal and external changes
◦
2.
◦
3.
◦
Integration
Interpretation of sensory input by the CNS
Motor output
Activation of effector organs (muscles and glands)
produces a response
Sensory input
Integration
Motor output
Two Major Divisions of Nervous
System

central nervous system (CNS)
◦ brain and spinal cord
◦ Integration and command center

peripheral nervous system (PNS)
◦ all the nervous system except the brain and
spinal cord
◦ nerves and ganglia
◦ nerves carry messages to and from the CNS
PNS: Nerves and Ganglia

nerve – a cable-like bundle of axons
wrapped in connective tissue
◦ found only in the PNS

ganglion – a knot-like bundle of neuron
cell bodies wrapped in connective tissue
Dendrites
Cell body (Soma)
(receptive regions)
Typical Neuron
(nerve cell)
Axon
(impulse generating
and conducting region)
Impulse
direction
Subdivisions of Nervous System
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Spinal cord
Nerves
Ganglia
Peripheral Nervous System (PNS)

Two functional divisions:
1. Sensory (afferent) division



Transmits signal from receptors to the CNS
Somatic sensory fibers—convey impulses from
skin, skeletal muscles, bones, and joints
Visceral Sensory fibers—convey impulses from
visceral organs
2. Motor (efferent) division

Transmits impulses from the CNS to effectors
 Effectors are muscles and glands
Motor Division of PNS
Somatic motor division (voluntary)
1.
◦
Conscious control of skeletal muscles
Visceral motor division (involuntary)
2.
◦
Also called the autonomic nervous system
(ANS)
Regulates smooth muscle, cardiac muscle,
and glands
Two functional subdivisions
◦
◦


Sympathetic
Parasympathetic
Peripheral nervous system (PNS)
Central nervous system (CNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Brain and spinal cord
Integrative and control centers
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Skin
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body
systems during activity
Sympathetic motor fiber of ANS
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Bladder
Histology of Nervous Tissue

Two principal cell types
1. Neurons—excitable cells that transmit
electrical signals
Histology of Nervous Tissue
2.






Neuroglia (glial cells)—supporting cells:
Astrocytes (CNS)
Microglia (CNS)
Ependymal cells (CNS)
Oligodendrocytes (CNS)
Satellite cells (PNS)
Schwann cells (PNS)
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
Neurons (Nerve Cells)
 Special
characteristics:
◦ Long-lived ( 100 years or more)
◦ Amitotic—with few exceptions
◦ High metabolic rate—depends on
continuous supply of oxygen and
glucose
◦ Plasma membrane functions in:
 Electrical signaling
 Cell-to-cell interactions during development
Cell Body (Soma)

Biosynthetic center of a neuron

Spherical nucleus with nucleolus

Well-developed Golgi apparatus

Rough ER called Nissl bodies

Axon hillock—cone-shaped area from
which axon arises

Clusters of cell bodies are called nuclei in
the CNS, ganglia in the PNS
Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Typical Neuron
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
Impulse
direction
Node of Ranvier
Schwann cell
(one interNeurilemma
Terminal
node)
branches
Axon
terminals
(secretory
region)
Processes
Dendrites and axons
 Bundles of processes are called

◦ Tracts in the CNS
◦ Nerves in the PNS
Dendrites
Short, tapering, and diffusely branched
 Receptive (input) region of a neuron
 Convey electrical signals toward the cell
body

The Axon (nerve fiber)

One long axon per cell

Occasional branches (axon collaterals)

Numerous terminal branches

Knoblike axon terminals (synaptic knobs)
◦ Release neurotransmitters to excite or inhibit
other cells
Axons: Function

Conducting region of a neuron

Generates and transmits nerve impulses
(action potentials) away from the cell
body
Myelin

myelin sheath – an insulating layer around a
nerve fiber
◦ formed by oligodendrocytes in CNS and
Schwann cells in PNS
◦ consists of the plasma membrane of glial cells
 20% protein and 80 % lipid
Myelination

production of the myelin sheath
◦ begins the 14th week of fetal development
◦ proceeds rapidly during infancy
◦ completed in late adolescence
◦ dietary fat is important to nervous system
development
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.
Space between each
myelin sheath is the
nodes (of Ranvier).
Conduction Speed of Nerve Fibers

speed at which a nerve signal travels along
a nerve fiber depends on two factors
◦ diameter of fiber
◦ presence or absence of myelin

signal conduction occurs along the surface
of a fiber
◦ larger fibers have more surface area and
conduct signals more rapidly
◦ myelin further speeds signal conduction
Conduction Speed of Nerve Fibers

conduction speed
◦ small, unmyelinated fibers - 0.5 - 2.0 m/sec
◦ small, myelinated fibers -
3 - 15.0 m/sec
◦ large, myelinated fibers -
up to 120 m/sec

slow signals supply the stomach and dilate
pupil where speed is less of an issue

fast signals supply skeletal muscles and
transport sensory signals for vision and
balance
Diseases of Myelin Sheath

degenerative disorders of the myelin sheath
◦ multiple sclerosis
 oligodendrocytes and myelin sheaths in the
CNS deteriorate
 myelin replaced by hardened scar tissue
 nerve conduction disrupted (double vision,
tremors, numbness, speech defects)
 onset between 20 and 40 and fatal from 25 to
30 years after diagnosis
 cause may be autoimmune triggered by virus
Diseases of Myelin Sheath

degenerative disorders of the myelin sheath
◦ Tay-Sachs disease
 a hereditary disorder of infants of Eastern
European Jewish ancestry
 abnormal accumulation of glycolipid in the myelin
sheath; disrupts conduction of nerve signals
 normally decomposed by lysosomal enzyme
 enzyme missing in individuals homozygous for
Tay-Sachs allele
 blindness, loss of coordination, and dementia
 fatal before age 4
Diagram of a
motor neuron
The arrows show the
direction of the
nerve impulse.
ZOOMING IN
• How can you tell this is
motor neuron and not a
sensory neuron?
Types of Neurons

Sensory neurons (afferent neurons)
◦ Conduct impulses to spinal cord, brain

Motor neurons (efferent neurons)
◦ Conduct impulses to muscles, glands

Interneurons (association neurons)
◦ Conduct information within CNS
Functional Classes of Neurons
Peripheral nervous system
1
3
Central nervous system
Sensory (afferent)
neurons conduct
signals from
receptors to the CNS.
Motor (efferent)
neurons conduct
signals from the CNS
to effectors such as
muscles and glands.
2
Interneurons
(association
neurons) are
confined to
the CNS.
Nerves and Tracts

Nerve: fiber bundle within PNS

Tract: fiber bundle within CNS

Organized into fascicles

Connective tissue layers
◦ Endoneurium
◦ Perineurium
◦ Epineurium
Anatomy of a Nerve
Epineurium
Perineurium
Endoneurium
Nerve
fiber
Rootlets
Posterior root
Posterior root
ganglion
Anterior
root
Spinal
nerve
Fascicle
Blood
vessels
Blood
vessels
Fascicle
Epineurium
Perineurium
Unmyelinated nerve fibers
Myelinated nerve fibers
Endoneurium
Myelin
The Nervous System at Work
Electrical impulses sent along neuron fibers
and transmitted between cells at
junctions
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
The Synapse
Junction point for transmitting nerve impulse
 Axon (presynaptic cell)
 Dendrite (postsynaptic cell)
 Synaptic cleft
 Neurotransmitters
◦ Epinephrine (adrenaline)
◦ Norepinephrine (noradrenaline)
◦ Acetylcholine
 Receptors
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.
A Synapse
(A)The
end-bulb of the
presynaptic 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.
The Spinal Cord
Links PNS and brain
 Helps coordinate impulses within CNS
 Contained in and protected by vertebrae

Spinal Cord and Spinal Nerves
Nerve plexuses
(networks) are
shown.
(A) Lateral view. (B)
Posterior view.
ZOOMING IN
• 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?
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

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).
Meninges of Vertebra and Spinal Cord
Posterior
Meninges:
Dura mater (dural sheath)
Arachnoid mater
Pia mater
Spinous process of vertebra
Fat in epidural space
Subarachnoid space
Spinal cord
Denticulate ligament
Posterior root ganglion
Spinal nerve
Vertebral body
(a) Spinal cord and vertebra (cervical)
Anterior
Reflex Arc

Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to
the CNS
3. Interneuron – (Integration center) within the
CNS
4. Motor neuron—conducts efferent impulses
from the integration center to an effector organ
5. Effector—muscle fiber or gland cell that
responds to the efferent impulses by contracting
or secreting
Stimulus
Reflex Arc
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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.
ZOOMING IN
• Is this a somatic or an
autonomic reflex arc? What
type of neuron is located
between the sensory and
motor neuron in the CNS?
Reflex Activities

Simple reflex
◦ Rapid
◦ Uncomplicated
◦ Automatic

Spinal reflex
◦ Stretch reflex
Medical Procedures Involving the
Spinal Cord

Lumbar puncture (spinal tap)
◦ Cerebrospinal fluid (CSF) removed for testing

Drug administration
◦ Anesthetic (an epidural)
◦ Pain medication
Diseases and Other Disorders of the
Spinal Cord
Multiple sclerosis (MS)
 Amyotrophic lateral sclerosis
 Poliomyelitis
 Tumors
 Injuries
◦ Monoplegia
◦ Diplegia
◦ Paraplegia
◦ Hemiplegia
◦ Tetraplegia

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
Branches of the Spinal Nerves

Cervical plexus
◦ Phrenic nerve

Brachial plexus
◦ Radial nerve

Lumbosacral plexus
◦ Sciatic nerve

Dermatomes
Dermatomes
A dermatome is a
region of the skin
supplied by a
single spinal
nerve.
ZOOMING IN
• Which spinal nerves
carry impulses from
the skin of the toes?
From the anterior hand
and fingers?
Disorders of the Spinal Nerves

Peripheral neuritis

Sciatica

Herpes zoster

Guillain-Barré syndrome
Autonomic Nervous System (ANS)

The ANS consists of motor neurons that:
◦ Regulate the action of smooth and cardiac
muscle and glands
◦ Make adjustments to ensure optimal support
for body activities
◦ Operate via subconscious control
Motor Divisions: Somatic vs.
Visceral (ANS)
Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Somatic and Autonomic Nervous
Systems

The two systems differ in
◦ Effectors
◦ Efferent pathways (and their neurotransmitters)
◦ Target organ responses to neurotransmitters
Effectors

Somatic nervous system
◦ Skeletal muscles

ANS
◦ Cardiac muscle
◦ Smooth muscle
◦ Glands
Efferent Pathways
 Somatic
nervous system
◦ A, thick, heavily myelinated somatic motor
fiber makes up each pathway from the CNS
to the muscle
 ANS
pathway is a two-neuron chain
1. Preganglionic neuron (in CNS) has a thin,
lightly myelinated preganglionic axon
2. Ganglionic neuron in autonomic ganglion has
an unmyelinated postganglionic axon that
extends to the effector organ
Neurotransmitter Effects
Somatic nervous system
◦ All somatic motor neurons release
acetylcholine (ACh)
◦ Effects are always stimulatory
 ANS
◦ Preganglionic fibers release ACh
◦ Postganglionic fibers release norepinephrine
or ACh at effectors
◦ Effect is either stimulatory or inhibitory,
depending on type of receptors

SOMATIC
NERVOUS
SYSTEM
Cell bodies in central
nervous system
Peripheral nervous system
Neurotransmitter
at effector
Effector
organs
Single neuron from CNS to effector organs
Effect
+
ACh
Stimulatory
Heavily myelinated axon
Skeletal muscle
NE
SYMPATHETIC
ACh
Unmyelinated
postganglionic axon
Lightly myelinated Ganglion
Epinephrine and
preganglionic axons
norepinephrine
ACh
Adrenal medulla
PARASYMPATHETIC
AUTONOMIC NERVOUS SYSTEM
Two-neuron chain from CNS to effector organs
Acetylcholine (ACh)
Blood vessel
ACh
ACh
Lightly myelinated
preganglionic axon
Norepinephrine (NE)
Ganglion
+
Unmyelinated
postganglionic
axon
Smooth muscle
(e.g., in gut),
glands, cardiac
muscle
Stimulatory
or inhibitory,
depending
on neurotransmitter
and
receptors
on effector
organs
Divisions of the ANS
Sympathetic division
2. Parasympathetic division
 Dual innervation
1.
◦ Almost all visceral organs are served by both
divisions, but they cause opposite effects
CN III
Ciliary
ganglion
CN VII
CN IX
CN X
Pterygopalatine
ganglion
Submandibular
ganglion
Otic ganglion
Eye
Lacrimal
gland
Nasal
mucosa
Submandibular
and sublingual
glands
Parotid gland
Heart
Cardiac and
pulmonary
plexuses
Celiac
plexus
Lung
Liver and
gallbladder
Stomach
Pancreas
S2
S4
Pelvic
splanchnic
nerves
Inferior
hypogastric
plexus
Genitalia
(penis,
clitoris, and vagina)
Large
intestine
Small
intestine
Rectum
Urinary
bladder
and ureters
Preganglionic
Postganglionic
Cranial nerve
Eye
Lacrimal gland
Nasal mucosa
Pons
Sympathetic trunk
(chain) ganglia
Blood vessels;
skin (arrector pili
muscles and
sweat glands)
Superior
cervical
ganglion
T1
Middle
cervical
ganglion
Inferior
cervical
ganglion
Salivary glands
Heart
Cardiac and
pulmonary
plexuses
Lung
Greater splanchnic nerve
Lesser splanchnic nerve
Celiac ganglion
L2
Liver and
gallbladder
Stomach
White rami
communicantes
Superior
mesenteric
ganglion
Spleen
Adrenal medulla
Kidney
Sacral
splanchnic
nerves
Lumbar
splanchnic
nerves
Inferior
mesenteric
ganglion
Small
intestine
Large
intestine
Rectum
Preganglionic
Postganglionic
Genitalia (uterus, vagina, and
penis) and urinary bladder
Sympathetic nervous system

Thoracolumbar area

Adrenergic system

Activated in the four E’s: excitement,
emergency, embarassment, exercise
Role of the Sympathetic Division

Mobilizes the body during activity; is the
“fight-or-flight” system

Promotes adjustments during exercise, or
when threatened
◦ Blood flow is shunted to skeletal muscles and
heart
◦ Bronchioles dilate
◦ Liver releases glucose
Parasympathetic nervous system

Arise in craniosacral areas

Cholinergic system
Role of the Parasympathetic
Division

Promotes maintenance activities and
conserves body energy

Its activity is illustrated in a person who
relaxes, reading, after a meal
◦ Blood pressure, heart rate, and respiratory
rates are low
◦ Gastrointestinal tract activity is high
◦ Pupils are constricted and lenses are
accommodated for close vision
Sympathetic Effects…












On the iris - Pupillary dilation
On the sweat glands – secretion
On piloerector muscles – hair erection (goose bumps)
On the heart – increased heart rate and force
On blood vessels of skeletal muscle – vasodilation
On blood vessels of skin – vasoconstriction
On the bronchi and bronchioles – bronchodilation
On the kidneys – reduced urine output
On the GI Tract – decreased motility and secretion
On the Liver – glycogen breakdown
On the pancreas – decreased insulin secretion; decreased
digestive enzyme secretion
On the reproductive system – stimulation of orgasm and
relaxation of the uterus
Parasympathetic Effects...

On the iris - Pupillary constriction

On the heart – decreased heart rate and force

On blood vessels of skin – vasodilation

On the bronchi and bronchioles –
bronchoconstriction

On the bladder wall – contraction

On the GI Tract – increased motility and secretion

On the Liver – glycogen synthesis

On the pancreas – increased digestive enzyme
secretion

On the reproductive system – stimulation of
penile and clitoral erection
Autonomic Nervous System
The diagram
shows only one
side of the body
for each
division.
ZOOMING IN
• Which division of the
autonomic nervous
system has ganglia
closer to the effector
organ?
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
Drugs and the Nervous System

sympathomimetics enhance sympathetic
activity
◦ stimulate receptors or increase norepinephrine
release
 cold medicines that dilate the bronchioles or constrict nasal
blood vessels

sympatholytics suppress sympathetic activity
◦ block receptors or inhibit norepinephrine
release
 beta blockers reduce high BP interfering with effects of
epinephrine/norepinephrine on heart and blood vessels

parasympathomimetics enhance activity
while parasympatholytics suppress activity
End of Presentation