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Structures of the Nervous System

Chapter 4
Nervous Tissue
Central Nervous System (CNS)
Autonomic Nervous System (ANS)

Structures of the Nervous System

Brain: neurons enclosed within skull
Spinal cord: connects to brain and enclosed
within spinal cavity
Nerves: bundles of many axons of neurons
Cranial nerves (12 pairs) emerge from brain
Spinal nerves (31 pairs) emerge from spinal cord

Ganglia: groups of neuron cell bodies located
outside of brain and spinal cord
Enteric plexuses: networks in digestive tract
Sensory receptors: monitor changes in internal
or external environments

Functions of the Nervous System
Sensory receptors and sensory nerves
Carry information into brain and spinal cord

Integration: information processing
Perception = awareness of sensory input
Analyzing and storing information to help lead to
appropriate responses

Motor activity: efferent nerves
Signals to muscles and glands (effectors)

Organization of the Nervous System
Central Nervous System (CNS)
Brain and spinal cord

Peripheral Nervous System (PNS)
All nervous system structures outside of the CNS

Histology of the Nervous System
Neurons
Can respond to stimuli and convert stimuli to
electrical signals (nerve impulses) that travel along
neurons

Neuroglia cells: support, nourish and protect
neurons
Neuroglia - critical for homeostasis of interstitial
fluid around neurons

Neuronal Structure
Cell body: nucleus, cytoplasm
with typical organelles
Dendrites: highly branched
structures that carry impulses
to the cell body
Axon: conducts away from cell
body toward another neuron,
muscle or gland
Emerges at cone-shaped axon
hillock

Axon terminals: contain
synaptic vesicles that can
release neurotransmitters

Structural Classes of Neurons
Multipolar
Have several or many dendrites and one axon
Most common type in brain and spinal cord

Bipolar
Have one dendrite and one axon
Example: in retina of eye and inner ear

Unipolar
Have fused dendrite and axon
Sensory neurons of spinal nerves

Functional Classes of Neurons
Sensory (afferent)
Convey impulses into CNS (brain or spinal cord)

Motor (efferent)
Convey impulses from brain or spinal cord out
through the PNS to effectors (muscles or glands)

Interneurons (association neurons)
Most are within the CNS
Transmit impulses between neurons, such as
between sensory and motor neurons

Neuroglia
Cells smaller but much more numerous than
neurons
Can multiply and divide and fill in brain areas
Gliomas: brain tumors derived from neuroglia
Functions
Do not conduct nerve impulses
Do support, nourish and protect neurons

Neuroglia
Astrocytes: help form blood brain barrier
Oligodendrocytes: produce myelin in CNS
Microglia: protect CNS cells from disease
Ependymal cells: form CSF in ventricles
Schwann: produce myelin around PNS
neurons; help to regenerate PNS axons
Satellite cells: support neurons in PNS ganglia

Fig
...
Seizures, blind,
demented, uncoordinated, die before 5)

Tay Sachs

Collections of Nervous Tissue
Clusters of neuron cell bodies
Ganglion: cluster of cell bodies in PNS
Nucleus: cluster of cell bodies in CNS

Bundles of axons
Nerve: bundle of axons in PNS
Tract: bundle to axons in CNS

Gray and White Matter
Gray matter forms “H” (or “butterfly”)
Three horns on each side; sites of cell bodies,
dendrites, unmyelinated axons, axon terminals,
neuroglia
Posterior gray horns: contain sensory neurons
Anterior gray horns: contain somatic motor neurons
Lateral: contain autonomic motor neurons

White matter (surrounds gray “H”)
Consists of white columns; primarily myelinated
axons
Posterior, anterior, and lateral columns
Contain tracts (bundles of axons)
Sensory tracts: ascending to brain
Motor tracts: descending from brain

Gray and White Matter
White matter: primarily myelinated axons
Gray matter: cell bodies, dendrites,
unmyelinated axons, axon terminals, neuroglia
Locations of gray and white matter
Spinal cord: white matter (tracts) surround centrally
located gray matter “H” or “butterfly”
Brain: gray matter in thin cortex surrounds white
matter (tracts)

Neuron Regeneration

Organization of the Nervous System

Regeneration of PNS neurons
Axons and dendrite in the PNS can be repaired if
cell body is intact and Schwann cells functional
...


Resting Membrane Potential
Typically –70 mV
Inside of membrane more negative than outside

Caused by presence of ions:
Inside (more negative) because cytosol has:
Many negative ions (too large to leak out): amino acids
(in cellular proteins) and phosphates (as in ATP)
K+ that easily leaks out through many K+ channels

Outside (more positive) because interstitial fluid
has:
Few negative ions
Na+ that does not leak out of cell: few Na+ channels
Membrane “pumps” that quickly pump out Na+ that
does leak (diffuse) into cell

Open and close on command
Respond to changes in membrane so can generate and
conduct action potentials

Resting Membrane Potential

Action Potential

Action Potential

Series of events that activate cell membrane
in neuron or muscle fiber
An initial event (stimulus) is required

Depolarizing phase

Triggers resting membrane to become more
permeable to Na+
Causes enough Na+ to enter cell so that cell
membrane reaches threshold (~ –55 mv)
If so, the following events occur: action potential
which spreads along neuron or muscle fiber

Repolarizing phase

Action Potential

Na+ channels open
as more Na+ enters cell,
membrane potential rises and becomes positive
0
+ 30 mv)
(–70
K+ channels open
as more K+ leave cell,
membrane potential is returned to resting value
(+ 30
0
–70 mv)
May overshoot: hyperpolarizing phase

Typically depolarization and repolarization take
place in about 1 millisecond (1/1000 sec)

Action Potential
Recovery
Levels of ions back to normal by action of Na+/K+
pump
Refractory period (brief): even with adequate
stimulus, cell cannot be activated

All-or-none principle
If a stimulus is strong enough to cause
depolarization to threshold level, the impulse will
travel the entire length of the neuron at a constant
and maximum strength
...


Conduction
of Nerve
Impulses

Conduction
of Nerve
Impulses

Synaptic Transmission
Similar sequence of events occurs at
Synapse (neuron-neuron)
Neuromuscular junction (neuron-muscle fiber)
Neuroglandular junction (neuron-gland)

Triggered by action potential (nerve impulse)
Components of synapse:
Sending neuron: presynaptic neuron (releases
neurotransmitter)
Space between neurons: synaptic cleft
Receiving neuron: postsynaptic neuron

Synaptic Transmission
Action potential arrives at presynaptic
neuron’s end bulb
Opens voltage gated Ca2+ channels
Ca2+
flows into presynaptic cytosol
Increased Ca2+ concentration
exocytosis
of synaptic vesicles
Neurotransmitter (NT) released into cleft
NT diffuses across cleft and binds to
receptors in postsynaptic cell membrane

Synaptic Transmission
NT serves as chemical trigger (stimulus) of
ion channels
Postsynaptic cell membrane may be
depolarized or hyperpolarized
Depends on type of NT and type of postsynaptic
cell
1000+ neurons converge on synapse; the sum of
all of their NTs determines effect

If threshold reached, then postsynaptic cell
action potential results

Synaptic Transmission
One-way transmission only because

Signal Transmission at the Chemical
Synapse

Only presynaptic cells release NT
Only postsynaptic cells have receptors for NT
binding

Finally, NT must be removed from the cleft
...


Neurotransmitters
Acetylcholine (ACh): common in PNS
Stimulatory (on skeletal muscles)
Inhibitory (on cardiac muscle)

Amino acids
Glutamate, aspartate, gamma aminobutyric acid
(GABA), glycine

Modified amino acids
Norepinephrine (NE), dopamine (DA), serotonin

Neuropeptides such as endorphins
Nitric oxide (NO)

Reflex Arc
1
...
Sensory neuron: through dorsal root
ganglion and root
posterior horn
3
...
Motor neuron: from anterior horn
ventral root
spinal nerve
5
...


2
...

4
...


Brain: Major Parts

Sensory receptor is
stimulated by tap on
patellar tendon
Sensory neuron: through
dorsal root
spinal
cord
Integrating center: single
synapse in spinal cord
Motor neuron: through
ventral root
spinal
nerve
femoral nerve
Effector: quads contract,
extend leg

Brain Blood Supply and Blood-Brain
Barrier
Requires 20% of the body’s O2 supply
4 min lack

permanent damage

Requires continuous glucose supply
Protected by blood-brain barrier
Allows passage of lipid soluble materials: O2,
CO2, alcohol, anesthetic agents
But controls entry of most harmful materials

Created by tight capillaries and astrocytes

Memory
Process for storing and retrieving information
Involves structural and functional changes
Involves association areas, parts of limbic
system, and diencephalon
Skill memory also involves cerebellum and
basal ganglia

Aging
Rapid brain growth during first few years of
life
Due to increase in size of neurons and
proliferation of neuroglia
Increase in development of dendritic branches
and synaptic contacts

Somatic Senses and Special Senses

From early adulthood through old age:
Decline in brain mass
Fewer synaptic contacts brain function
Some decrease in brain function

Special Senses
Smell (olfaction)
Taste (gustation)
Vision
Balance
Hearing

General Senses: Somatic and Visceral
Somatic
Tactile: touch, pressure, vibration
Thermal (warm, cold)
Pain
Proprioception (joint, muscle position sense;
movements of limbs, head)

Visceral: internal organ conditions

Definition of Sensation
Conscious or subconscious awareness of
change in external or internal environment
Requires
1
...

3
...


Stimulus
Sensory receptor
Neural pathway
Brain region for integration

Characteristics
Perception: conscious awareness
Occurs in cerebral cortex

Adaptation: decreased receptor response
during prolonged stimulation
Decreased perception
Adaptation speed varies with receptor
Rapid adaptation: pressure, touch, smell
Slow adaptation: pain, body position, chemical levels in
blood

Sensory Receptors: Structural Types
Free nerve endings
Pain, thermal, tickle, itch, some touch receptors

Encapsulated nerve endings
Touch pressure, and vibration

Separate, specialized cells
Hair cells in inner ear
Photoreceptors in retina of eye

Copyright 2010, John Wiley & Sons, Inc
...
Rare!

Smell:
Olfaction

Stimulation of Receptors
Genetic evidence: 100’s of primary odors
exist
Binding of chemical odorants stimulates
receptor
Recognition of 10,000 odors from
combination of primary receptor input
Rapid adaptation by 50% in 1 second

Olfactory Pathway
First-order neurons
Olfactory receptors are neurons in
nasal mucosa
Axons form olfactory nerves
(cranial nerve I)
Extend through cribriform plate
into cranium to olfactory bulb

Second-order neurons
Neuron cell bodies in olfactory
bulb
Olfactory tract: axons extend from
olfactory bulb to cerebral cortex
(temporal lobe)

Limbic system: emotional
response to odors

Taste: Gustation

Taste: Gustation
Five primary tastes: salt,
sweet, sour, bitter, and
umami
Perception of what is
called “taste” includes
olfactory input
Receptors in 10,000
taste buds
Located on tongue,
pharynx, epiglottis
In structures called papillae
Vallate (posterior)
Fungiform (all over)
Filiform: touch receptors
only

Structure of Taste Bud
Contains 3 types of
epithelial cells
Supporting cells that
surround
Gustatory receptor
cells
Gustatory hair projects
from receptor through
taste pore

Basal cells
Stem cells that produce
supporting cells that
develop into receptor
cells (10-day life span)

Stimulation of Taste Receptors
Sequence of events
Tastant dissolves in saliva
Enters taste pore
contacts gustatory hair
Electrical signal produced
Causes gustatory cell to release neurotransmitter
That activates dendrites of first-order neurons

Adaptation occurs within minutes
Different tastes arise from activation of
different groups of taste neurons



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