Chapter 12– Nervous Tissue
12-1
Ch. 12 Nervous Tissue–
Study Guide
1. Critically read Chapter 12 pp. 442-461 before
12.5 Synapses.
2. Comprehend Terminology (those in bold in
the textbook) within the reading scope above
3. Study-- Figure questions, Think About It
questions, and Before You Go On (sectionending) questions (within the reading scope
above) . Before You Go On Questions.
4. Do end-of-chapter questions—
–
–
Testing Your Recall— 1-4, 7, 11-17
True or False– 1-4, 6, 8
10-26-2
12.1 Overview of the
nervous system
12-3
§ Maintaining internal coordination
by two body systems
1. Homeostasis? How?
2. Endocrine and nervous systems-A. Endocrine sys. – slower
– chemical messengers (hormones) delivered
to the bloodstream
– Example-- insulin
B. Nervous sys. – quicker
– chemical and electrical means
– Example– in cold environment,
vasoconstriction/shivering
12-4
§ Subdivisions of Nervous System
Two major ANATOMICAL subdivisions:
• Central nervous system (CNS)
– brain and spinal cord enclosed in bony
coverings
• Peripheral nervous system (PNS)– all
nervous tissue outside the CNS; made up of:
– Nerves-- bundles of axons in connective
tissue; emerge from the CNS; carry signals
– Ganglia-- knotlike swellings in nerves
Fig. @12.1
12-5
Part of PNS (next slide)
§ Functional divisions of PNS (SAME)
• Sensory (Afferent) divisions (receptors to
CNS)– carry signals to the CNS
– somatic division—Ex.
– visceral sensory division—Ex.
• Motor (Efferent) division (CNS to effectors)
1. somatic motor division
Effectors: skeletal muscles
2. visceral motor division (also called ANS)
Effectors: glands and cardiac/smooth muscles
• sympathetic division/parasympathetic
division
Fig. 12.2
12-7
PNS
CNS
Brain
Sensory
division
Spinal
cord
Visceral
sensory
division
Somatic
sensory
division
Motor
division
Visceral
motor
division
Sympathetic
division
Somatic
motor
division
Parasympathetic
division
12.2 Properties of neurons
12-9
§ Universal properties of neurons
Nerves made up of nerve cells (neurons);
neurons’ properties include:
• Excitability
– ability to respond to stimuli by producing
action potential
• Conductivity
– produce traveling electrical signals
• Secretion
– Where? Why?
– What is secreted?
12-10
§ Functional Classes of Neurons
1. Sensory (afferent) neurons
– detect changes in body and external environment
– Ex.
2. Interneurons (association neurons)
– Confine ENTIRELY in CNS
– 90% of our neurons are interneurons
– process, store and retrieve information
3. Motor (efferent) neuron
– send signals out to muscles and gland cells
(effectors carry out body responses)
– Ex.
Fig. 12.3
12-11
Fig.12.3
Three
Classes of
Neurons
Example-Detecting
your own
pulse at
wrist
12-12
§ Neuron
• Cell body = soma
– Nucleus
• Dendrites (1-many)
– Function
• Axon (single; nerve
fiber)
– Function
Fig. 12.4 c-d-e
12-13
Neurofibrils
(d)
Axon
Figure 12.4d
12-15
Axoplasm
Axolemma
(c)
Schwann cell
nucleus
Neurilemma
Myelin sheath
§ Variation in
Neuron Structure–
No. of processes from the
soma: (Fig. 16.34)
• Multipolar neuron
– most common
• Bipolar neuron
– one dendrite/one axon
• Unipolar neuron
– Ex. sensory from skin to
spinal cord directly
• Anaxonic neuron
– many dendrites/no axon
– Ex. help in visual
processes
12-17
12.3 Supportive cells
(Neuroglia)
12-19
§ Introduction
• How many?! Neurons are outnumbered
by neuroglia (1:50) in the nervous sys.
• Functions- protect the neurons and
help them function
• Example– in the fetus, guide young
migrating neurons to their destinations
12-20
§ 4 Types of Neuroglial Cells (CNS)
1. Astrocytes (star-shaped)
– most abundant glial cells - form framework of CNS
– contribute to blood-brain barrier and regulate
composition of brain tissue fluid
2. Oligodendrocytes form myelin sheaths in
CNS; distinguish these from Schwann cells
3. Ependymal cells (epithelial cells) line
ventricles of the brain and central canal of the
spinal cord; produce CSF
4. Microglia formed from monocytes; engulf
invading microbes
– in areas of infection, trauma or stroke
Fig. 12.6
12-21
Neuroglial Cells of CNS
12-22
§ 2 Types of Neuroglial Cells (PNS)
1. Schwann cells -- myelinate fibers of
PNS; assist in the regeneration of
damaged fibers
2. Satellite cells – surround cell bodies
in ganglia; regulate the chemical
environment of the neurons
12-23
§ Myelin
• Insulating layer around a nerve fiber;
analogy– the rubber insulation on a wire
• In CNS,
– Each oligodendrocyte myelinate several
fibers (Fig. 12.7a)
• In PNS,
– The ___________ cell wraps the nerve fiber
– outermost coil is called neurilemma
containing bulging body of the Schwann cell
(nucleus and most of its cytoplasm) (Fig.
12.7b)
12-24
Figure 12.7b
Myelin
Sheath in
CNS
Myelin Sheath in PNS
Schwann cell
Axon
Neurilemma
Myelin
sheath
Myelin Sheath in PNS
Node of Ranvier
(gaps)-between
Schwann cells
(also in CNS)
Internodes–
from one gap to
the next
12-27
§ Unmyelinated nerve fibers
• Locations?
CNS, PNS, both (circle one)
• One Schwann cell harbors ______ small
fibers
• The Schwann cell– folds once around
each fiber
Fig. 12.8
12-28
Unmyelinated
nerve fibers
Schwann cell
Basal lamina
§ Myelination and Speed of
Nerve Signal
• Diameter of fiber and presence of myelin
– large fibers have more surface area for signal
conduction
• Speeds
– 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
• Functions
– slow signals supply the stomach and dilate pupil
– fast signals supply skeletal muscles and transport
sensory signals for vision and balance
12-30
12.4A Electrophysiology of
neurons
KEY issues–
1.How does a neuron generate an
electrical signal?
2.Cellular mechanisms for producing
electrical potential and currents
12-31
§ Electrical Potentials & Currents
1. Electrical potential – a difference in the
concentration of charged particles
between one point and another
2. Electrical current– flow of charged
particles from one point to another
3. Living cells have electrical potentials (are
polarized)
– resting membrane potential is -70 mV with a
negative charge on the _______ of membrane;
why? (next slide)
12-32
§ Resting Membrane Potential of
all cells (RMP; -70 mV)
Factors contribute to RMP: unequal
distribution of electrolytes in ECF & ICF
1. Diffusion of ions down their conc. gradient
2. Selective permeability of the cell mem.
3. Cations and anions attract to each other
Details-• Membrane very permeable to K+
• Membrane much less permeable to Na+
• Cytoplasmic anions can not escape— EX.
Proteins, phosphates etc.
Fig. 12.11
12-33
Fig. 12.11--Ion basis of the resting membrane potential
Negative charge (-70 mV)
• Na+ more concentrated in the ECF
• K+ more concentrated in the ICF
12-34
§ When a neuron is stimulated
• Local potentials– changes in membrane
potential when a neuron is stimulated
– Causes– How?
– Results– depolarization
– Ionic bases-• Na+ rushes into/out of (circle one) the cell
• Na+ diffuses for short distance inside
membrane producing a local potential
Fig. 12.12 and X
12-35
For
example– a
chemical
(pain signal;
ligand)
stimulates a
neuron
12-36
Local potential-- Local, graded,
and decremental
Time
Resting
mem.
Potential
(-70 mV)
Magnitude
of stimulus
A
B
C
D Stimuli
12-37
§ Action potentials if stimulus is strong
(Fig. 12.13)
1. Threshold reached
2. Depolarization
(sodium channels
open)
3. Repolarization
(Sodium channels
close and K+ gates
fully open)
4. Hyperpolarization
5. Resting membrane
potential restores
12-38
12-38
Figure 12.13a
12-39
§ Action potentials vs. local
potentials (Table 12.5)
Local Potentials
Action Potentials
 Local
?
 Graded +
reversible
 Decremental
?
?
12-40
Ionic
base
12-41
§ The refractory period of the
action potential (AP)
Period of resistance to
stimulation for another AP
• Absolute refractory period
– as long as Na+ gates are open
– no stimulus will trigger AP
• Relative refractory period
– as long as K+ gates are open
– only especially strong
stimulus will trigger new AP
12-42
12.4B--Conduction of a
nerve signal in an
unmyelinated fiber
12-43
• Where are ion
gates?
• First action
potential
occurs at?
• The next action
potential
occurs at?
• Chain
reactions
continue until
_____
12-44
12.4C--Conduction of a
nerve signal in a myelinated
fiber
12-45
§ Saltatory conduction (Fig. 12.17a-b)
A. Most (99%) of the voltage-regulated
ion gates are at the _______________.
– Slow but nondecremental
B. At the internodes–
– nerve signals travel very ______ (diffusion
of ions) and decremental.
C. Most of the axon is covered with
myelin (internodes)
– nerve signal is faster at 120m/sec (than
unmyelinated ones (up to 2 m/sec)
12-46
•Action potentials occurs only at the _________________
•It is called saltatory conduction meaning _____________.
12-47
12-48