Download CHAPTER 10: NERVOUS SYSTEM I

UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
LEARNING OUTCOMES:
10.1
10.2
10.3
10.4
10.5
Introduction
1.
Describe the general functions of the nervous system.
2.
Identify the two types of cells that comprise nervous tissue.
3.
Identify the two major groups of nervous system organs.
General Functions of the Nervous System
4.
List the functions of sensory receptors.
5.
Describe how the nervous system responds to stimuli.
Description of Cells of the Nervous System
6.
Describe the parts of a neuron.
7.
Describe the relationships among myelin, the neurilemma, and nodes of
Ranvier.
8.
Distinguish between the sources of white matter and gray matter.
Classification of Cells of the Nervous System
9.
Identify structural and functional differences among neurons.
10.
Identify the types of neuroglia in the central nervous system and their
functions.
11.
Describe the role of Schwann cells in the peripheral nervous system.
The Synapse
12.
10.6
Explain how information passes from a presynaptic neuron to a postsynaptic cell.
Cell Membrane Potential
13.
Explain how a cell membrane becomes polarized.
14.
Describe the events leading to the generation of an action potential.
15.
Explain how action potentials move down the axon.
10-1
UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
LEARNING OUTCOMES:
16.
10.7
10.8
Compare nerve impulse conduction in myelinated and unmyelinated
neurons.
Synaptic Transmission
17.
Identify the changes in membrane potential associated with excitatory
and inhibitory neurotransmitters.
18.
Explain what prevents a postsynaptic cell from being continuously stimulated.
Impulse Processing
19.
Describe the basic ways in which the nervous system processes
information.
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.1
INTRODUCTION
A.
The nervous system is a network of cells that sense and respond to stimuli in ways
that maintain homeostasis.
B.
The nervous system is composed of:
1.
2.
3.
C.
Neurons have processes that:
1.
2.
D.
receive information (dendrites) and
send (axons) bioelectric signals (neurotransmitters) that cross spaces
(synapses) between them.
Organs of the nervous system are divided into:
1.
2.
10.2
neural tissue, including
a.
neurons and
b.
neuroglial cells
blood vessels
connective tissue.
central nervous system
peripheral nervous system
GENERAL FUNCTIONS OF THE NERVOUS SYSTEM
The general function of the nervous system is to coordinate all body systems! This is
accomplished by the transmission of (electrochemical) signals from body parts to the
brain and back to the body parts.
A.
The organs of the nervous system are divided into two major groups:
See Figure 10.2, page 363.
1.
Central Nervous System (CNS) = brain & spinal cord.
2.
Peripheral Nervous System (PNS) = nerves that extend from the brain
(cranial nerves) and spinal cord (spinal nerves).
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.2
GENERAL FUNCTIONS OF THE NERVOUS SYSTEM
B.
Three Major Functions
1.
2.
3.
Sensory Input Function
a.
PNS
b.
Sensory receptors (located at the ends of peripheral neurons)
detect changes occurring in their surroundings (i.e. are stimulated).
c.
Once stimulated, sensory receptors transmit a sensory impulse to
the CNS.
d.
A sensory impulse is carried on a sensory neuron.
Integrative Function
a.
CNS (brain and/or spinal cord)
b.
involves interpretation of an incoming sensory impulse (i.e.
decision is made concerning what's going to happen next, based on
sensory impulse).
c.
Integration occurs in interneurons.
d.
A motor impulse begins...
Motor Function
a.
PNS
b.
involves the response of a body part
c.
Motor impulses are carried from CNS to responsive body parts
called effectors.
d.
A motor impulse is carried on a motor neuron.
e.
Effectors = 2 types:
o
o
muscles (that contract)
glands
1. that secrete a hormone; endocrine
2. that secrete a substance; exocrine
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.2
GENERAL FUNCTIONS OF THE NERVOUS SYSTEM
C.
Levels of Organization of Nervous System (most will be discussed in Ch 11)
NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM
(BRAIN & SPINAL CORD)
(INTERNEURONS)
PERIPHERAL NERVOUS SYSTEM
(CRANIAL NERVES & SPINAL NERVES)
SENSORY
(INPUT INTO CNS)
(AFFERENT NEURONS)
SOMATIC
(EFFECTORS: SKELETAL MUSCLE)
(CONSCIOUS CONTROL)
PARASYMPATHETIC
(HOMEOSTASIS)
(NT: ACETYLCHOLINE)
MOTOR
(OUTPUT FROM CNS)
(EFFERENT NEURONS)
AUTONOMIC
(EFFECTORS: SMOOTH MUSCLE,
CARDIAC MUSCLE, GLANDS)
(UNCONSCIOUS CONTROL)
SYMPATHETIC
(FIGHT-OR-FLIGHT)
(NT: NOREPINEPHRINE)
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.3
DESCRIPTION OF CELLS OF THE NERVOUS SYSTEM
A.
Neuron = the structural and functional unit of the nervous system; a nerve cell.
1.
Neuron Structure
See Fig 10.3, page 364.
Each neuron is composed of a cell body, dendrites, and an axon.
a.
Cell Body (soma or perikaryon) = central portion of neuron
contains usual organelles, except centrioles
o
identify: nucleus, prominent nucleolus, and many Nissl
bodies (chromatophilic substance) = RER.
o
A network of fine threads called neurofibrils extends into
the axons and supports them.
b. Neuron Processes/ Nerve Fibers = extensions arising from cell body.
Two types:
o
Dendrites:
o
1.
many per neuron
2.
short and branched
3.
receptive portion of a neuron
4.
carry impulses toward cell body
Axons:
1.
2.
3.
4.
5.
one per neuron
long, thin process
carry impulses away from cell body
Note terminations of axon branch = axonal
terminals or synaptic knobs.
Axons in CNS
(i.e. in brain & spinal cord)
a.
b.
c.
Myelin is produced by oligodendrocytes
rather than Schwann Cells.
A bundle of myelinated nerve fibers =
"White Matter".
This is in contrast to CNS "Gray Matter" =
A bundle of cell bodies (or unmyelinated
nerve fibers).
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.3
DESCRIPTION OF CELLS OF THE NERVOUS SYSTEM
D.
Neuron = the structural and functional unit of the nervous system; a nerve cell.
1.
Neuron Structure Fig 10.3, page 364.
b.
Nerve Fibers
o
Axons (continued)
6.
Axons in PNS:
See Fig 10.4, page 365.
a.
Large axons are surrounded by a myelin
See Fig 10.5, pg 366.
sheath produced by many layers of
Schwann Cells (neuroglial cells).
 "myelinated nerve fiber"
 myelin = lipoprotein
 Interruptions in the myelin sheath
between Schwann cells = Nodes of
Ranvier.
 The neurilemma (or neurolemmal
sheath) composed of Schwann cell
cytoplasm and nuclei surround the
myelin sheath.
See Fig 10.5, page 366.
b.
Small axons do not have a myelin sheath.

"unmyelinated nerve fibers"

however all axons (in PNS) are
associated with Schwann cells.
10.4
CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM
A.
Classification of Neurons
1.
Neuron Structural Classification:
10.1A, page 368.
a.
b.
c.
See Figure 10.6, page 367 and Table
Bipolar Neurons
o
two extensions
o
one fused dendrite leads toward cell body, one axon leads
away from cell body
o
specialized parts of eyes, nose, ears (i.e. sensory)
Unipolar Neurons
o
one process from cell body
o
forms central and peripheral processes
o
only distal ends are dendrites
o
cell bodies in ganglia outside brain and spinal cord
Multipolar Neurons
o
many extensions
o
Many dendrites lead toward cell body; one axon leads away
from cell body.
o
most common neuron in brain and spinal cord
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.4
CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM
A.
Classification of Neurons
2.
B.
Neuron Functional Classification: See Table 10.1B, page 368.
a.
Sensory neurons
o
PNS
o
afferent neurons
o
carry sensory impulses from sensory receptors to CNS
o
input information to CNS
o
Location of receptors = skin & sense organs.
b.
Interneurons (Association)
o
CNS
o
link other neurons together (i.e. sensory neuron to
interneuron to motor neuron)
c
Motor Neurons
o
PNS
o
efferent neurons
o
carry motor impulses away from CNS and to effectors
o
output information from CNS
o
Effectors = muscles & glands.
Classification of Neuroglia
Recall that neuroglia are accessory cells of the nervous system; they form a
supporting network for neurons; "nerve glue".
1.
2.
Neuroglia of the CNS: 4 types provide bulk of brain and spinal cord
tissue: See Table 10.2, page 370, and Fig 10.8, page 369.
a.
Astrocyte
o
star-shaped
o
Function: nourishes neurons and provide structural support
b.
Oligodendrocyte
o
looks like eyeball
o
Function: produces myelin
c.
Microglia
o
looks like spider
o
Function: phagocytosis
d.
Ependyma
o
epithelial-like layer with cilia
o
Function: lines spaces in CNS
Neuroglia of the PNS: 2 types
a.
Schwann cells produces myelin. See Fig 10.4, page 365.
b.
Satellite cells surround and support cell bodies (in ganglia).
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.4 CLASSIFICATION OF CELLS OF THE NERVOUS SYSTEM
C.
10.5
Neuroglia and Axonal Regeneration:
See Fig 10.10, page 371.
1.
Cell body injury = death of neuron
2.
Damage to an axon may allow for regeneration.
THE SYNAPSE
(Nerve) impulses are transferred from one neuron to the next through synaptic
transmission.
A.
Synapse = the junction between two neurons where an impulse is transmitted
See Fig 10.11, page 372 and Fig 10.12, page 373.
1.
2.
10.6
occurs between the axon of one neuron (the presynaptic neuron) and
dendrite or cell body of a second neuron (the postsynaptic neuron).
Note that the two neurons do not touch. There is a gap between them =
synaptic cleft.
CELL MEMBRANE POTENTIAL
A.
Distribution of Ions
1.
A resting neuron's cell membrane is polarized = electrically charged (i.e.
the charge inside the cell is different than the charge outside):
a.
b.
2.
Consequently, a potential difference (PD) exists across this resting
cell membrane.
Definition: Potential Difference (PD) = the difference in electrical
charge between 2 points (i.e. across a cell membrane).
The resting membrane potential (RMP) of a neuron results from the
distribution of ions across the cell membrane. See Fig 10.14, page 374.
a.
b.
c.
d.
K+ high inside
Na+ high outside
Cl- high outside
Negatively charged proteins or Anions- high inside.
Recall that these ion concentrations are maintained by active transport
mechanisms (i.e. mainly the Na+K+-ATPase pump, Chapter 3).
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.6
CELL MEMBRANE POTENTIAL
B.
C.
Resting Potential
1.
The RMP of a nerve cell is measured to be -70 mV or millivolts (inside /
outside).
2.
As long as the RMP in a nerve cell is undisturbed, it remains polarized.
However, in order for an impulse to be started or propagated in a nerve
cell, this resting potential must be disturbed.
Local Potential Change
The RMP of - 70 mV can be disrupted or changed in one of two directions:
1.
more negative = "hyperpolarization"
2.
D.
less negative (i.e. towards zero) = "depolarization"
a.
The cell membrane of a neuron must be depolarized (to
approximately -55mV) in order for certain protein ion channels to
open and therefore start an action potential/impulse. See Fig
10.13, page 367.
Action Potentials:
See Figs 10.15 -10.18, pages 376- 377.
1.
When the resting membrane potential (RMP) of a neuron is depolarized to
-55mV, threshold potential is reached.
a.
The threshold potential for a neuron is -55mV.
b.
Therefore, a threshold stimulus = +15 mV.
2.
When threshold potential is reached, the rapid opening of Na+ channels
results in rapid depolarization (and even reversal of the membrane
potential [MP] to +30mV).
a.
This event is called the action potential.
b.
The action potential represents the start of an impulse on a neuron.
3.
Then K+ channels open, (while Na+ channels close), and repolarization
occurs = recovery of the RMP to -70mV.
4.
This all occurs very quickly = 1/1000 sec.
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.6
CELL MEMBRANE POTENTIAL
D.
Action Potentials:
5.
Summary:
(Nerve) Impulse Conduction
See Table 10.3, page 377.
RMP of neuron = -70 mV.
+ 15 mV stimulus
(threshold stimulus)
MP of neuron falls to -55mV = Threshold Potential
Na+ channels open (rapid depolarization);
Action Potential (impulse) is produced
MP of neuron reaches +30mV = Reversal of MP
Na+ channels close;
K+ channels open
(repolarization)
RMP of neuron returns to -70mV.
*
**
An action potential represents the start of an impulse in one small portion of the
neuron's membrane.
How do you think it is transmitted throughout the entire neuron?
6.
Impulse Conduction
a.
An impulse is the propagation of action potentials (AP) along a
nerve fiber (i.e. the entire length of the neuron).
See Fig 10.18, page 377.
b.
The impulse is an electrical impulse.
c.
An impulse is similar to a row of dominos falling (i.e. once the first
domino falls, the entire row will fall).
d.
An impulse begins on a dendrite (or cell body of a neuron), runs
toward the cell body, through the cell body, and then down the
axon.
See Clinical Application 10.3, page 379: Factors Affecting
Impulse Conduction.
*
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.6
CELL MEMBRANE POTENTIAL
E.
10.7
Characteristics of an Impulse
1.
All-or-None Response = if a nerve cell responds at all, it responds
completely.
a.
subthreshold stimulus (5mV) = no AP; no impulse.
b.
threshold stimulus (15mV) = yes AP; yes impulse.
c.
> threshold stimulus (20mV) = yes AP; yes impulse, but no greater
intensity than above.
2.
Refractory Period = the period following an impulse when a threshold
stimulus cannot produce another impulse.
a.
The RMP has to be restored before it can be depolarized again (i.e.
dominos must be set back up, in order to be knocked down again)
3.
Impulse Conduction = the manner in which the impulse runs down the
neuron/nerve fiber
a.
unmyelinated nerve fibers: impulse must travel the length of the
nerve fiber; slow. See Fig 10.18, page 377.
b.
myelinated nerve fiber: "Saltatory Conduction".
o
Impulse appears to jump from node of Ranvier to node of
Ranvier.
o
Very fast transmission. See Fig 10.19, page 378.
SYNAPTIC TRANSMISSION
A.
Steps in Synaptic Transmission
page to draw illustration.
1.
2.
3.
4.
5.
Presynaptic Neuron
(axon)
Fig 10.12, page 373. Use space at bottom of
The impulse reaches axonal terminal of presynaptic neuron causing
depolarization of axonal terminal/synaptic knob.
Ca2+ channels open and calcium ions rush into axonal terminal causing
synaptic vesicles (filled with neurotransmitter/NT) to release NT via
exocytosis into the synaptic cleft.
NT diffuses across synaptic cleft and depolarizes the postsynaptic
neuron's membrane.
An action potential (AP) is triggered and an Impulse begins in the
postsynaptic neuron.
Postsynaptic Neuron
(cell body or dendrite)
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.7
SYNAPTIC TRANSMISSION
B.
Synaptic Potentials
1.
2.
C.
Postsynaptic neurons respond to neurotransmitter binding.
a.
May be depolarization = Excitatory Post-Synaptic Potential
(EPSP)
b.
May be hyperpolarization = Inhibitory Post-Synaptic Potential
(IPSP)
Summation = many subthreshold stimuli received one after another may
allow threshold potential to be reached, and trigger an AP, which in turn
begins an impulse on a neuron.
a.
+15 mV = threshold = AP = impulse
b.
+5, +5, +5, = +15 mV = threshold = AP = impulse.
Neurotransmitters (NT) See Table 10.4 page 380.
1.
at least 30 different produced by CNS
2.
Some neurons produce/release only one, while others release many.
3.
Most typical NT is Acetylcholine (ACh).
a.
ACh is released by
o
all motor neurons (i.e. those that stimulate skeletal muscle)
o
some CNS neurons
4.
Other NTs include:
a.
monoamines (modified amino acids)
o
are widely distributed in the brain where they play a role in
1.
emotional behavior and
2.
circadian rhythm
o
are present in some motor neurons of the ANS
o
include:
1.
epinephrine
2.
norepinephrine
3.
dopamine
4.
serotonin
5.
histamine
b.
unmodified amino acids
o
glutamate
o
aspartate
o
GABA (gamma aminobutyric acid)
o
glycine
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.7
SYNAPTIC TRANSMISSION
C.
Neurotransmitters (NT)
5.
Fate of Neurotransmitter in Synaptic Cleft:
a.
Destruction of Neurotransmitter:
o
Enzymes that are present in the synaptic cleft destroy NT.
1.
b.
Reuptake of Neurotransmitter:
o
o
*
D.
For example, acetylcholinesterase destroys ACh.
NT is transported back into presynaptic knob.
The enzyme monoamine oxidase inactivates epinephrine
and norepineprine after reuptake.
1.
Monoamine Oxidase (MAO) Inhibitors are used to
treat some psychological disorders.
Both of the above processes prevent continual stimulation of the
postsynaptic membrane!
Neuropeptides
1.
2.
synthesized by CNS neurons
act as NTs or neuromodulators that either:
a.
b.
3.
include enkephalins:
a.
b.
c.
4.
alter a neuron's response to a NT
block the release of a NT
synthesis is increased during painful stress
bind to the same receptors in the brain as the narcotic morphine
relieve pain
include endorphines:
a.
same as above, but with a more potent and longer lasting effect.
* See Clinical Application 10.4, page 382: Opiates in the Human Body.
* See Clinical Application 10.5, page 383: Drug Addiction.
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.7
SYNAPTIC TRANSMISSION
E.
Disorders Associated With Neurotransmitter Imbalances. See Table 10.5,
page 380.
1.
Alzheimer's = deficient ACh
2.
Clinical Depression = deficient norepinephrine/ serotonin
3.
Epilepsy = Excess GABA leads to excess norepinephrine & dopamine
4.
Huntington Disease = deficient GABA
5.
Hypersomnia = excess serotonin
6.
Insomnia = deficient serotonin
7.
Mania = excess norepinephrine
8.
Myasthenia gravis = deficient ACh receptors at NMJ
9.
Parkinson's disease = deficient dopamine
10.
Schizophrenia = deficient GABA leads to excess dopamine
11.
SIDS = excess dopamine
12.
Tardive dyskinesia (uncontrollable movements of facial muscles) =
deficient dopamine.
F.
Drugs that Alter Neurotransmitter Level: See Table 10.6, page 381.
DRUG
Tryptophan
Reserpine
Curare
Valium
Nicotine
Cocaine
Tricyclic Antidepressants
Monoamine oxidase
inhibitors
Selective serotonin
reuptake inhibitors
Selective serotonin
reuptake inhibitors
G.
NT
AFFECTED
Serotonin
Norepi
MECHANISM OF
ACTION
stimulates NT synthesis
decreases packaging of NT
into vesicles
blocks receptor binding
enhances receptor binding
Activates receptors
elevates levels
blocks reuptake
blocks reuptake
blocks reuptake
blocks enzymatic
degradation of NT in
presynaptic cell
EFFECT
Serotonin
blocks reuptake
Serotonin
Norepi
blocks reuptake
Antidepressant,
anti-anxiety agent
mood elevation
Ach
GABA
Ach
Dopamine
Dopamine
Norepi
Serotonin
Norepi
sleepiness
decreases blood
pressure
muscle paralysis
decreased anxiety
increased alertness
sense of pleasure
euphoria
antidepressant
antidepressant
antidepressant
See Table 10.7, page 381: Events Leading to Neurotransmitter Release.
10-15
UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
10.8
IMPULSE PROCESSING: See Fig 10.21, page 382.
A.
Neuronal Pools – neurons that synapse and work together.
1.
B.
Convergence
1.
2.
3.
C.
Working together results in facilitation – a general excitation that makes
stimulation easier to achieve
many neurons come together on fewer neurons (summation occurs)
typical of motor pathways
many inputs from brain, but usually only one motor response
Divergence
1.
2.
3.
fewer neurons spread out to signal many neurons (signal amplifies)
typical of sensory pathways
reason that a stimulus (i.e. odor) can cause many responses
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
NERVE PATHWAY SUMMARY (keyed at the end of this outline)
NS
Division
CNS OR PNS?
TYPE OF
NEURON?
RECEPTIVE
PORTION OF
NEURON
STIMULATED BY
WHAT?
TRANSMISSION
AND DIRECTION
OF IMPULSE
THROUGH
NEURON
TRANSMISSION
OF IMPULSE TO
NEXT NEURON
(OR EFFECTOR)
10-17
UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
OTHER INTERESTING TOPICS:
A.
B.
C.
D.
E.
Brain Banks. See introduction on page 361.
Clinical Application 10.1, Migraine. See page 362.
Clinical Application 10.2, Multiple Sclerosis (MS). See page 366.
Tay-Sachs Disease. See box on page 367.
Neural Stem Cells. See box on page 372.
INNERCONNECTIONS of the Nervous System – see page 384.
CHAPTER SUMMARY – see pages 385-386.
CHAPTER ASSESSMENTS - see page 387.
INTEGRATIVE ASSESSMENTS/CRITICAL THINKING - see page 388.
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UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function
Nerve Pathway Summary
NS Division
CNS OR PNS?
TYPE OF
NEURON?
sensory
PNS
sensory (afferent)
neuron
Integrative
CNS
Interneuron
motor
PNS
motor (efferent) neuron
RECEPTIVE
PORTION OF
NEURON
sensory receptors
at the ends of
dendrites
dendritic ends (or cell
body)
dendritic ends (or cell
body)
STIMULATED
BY WHAT?
A stimulus (i.e.
light, temperature
change, etc)
neurotransmitter (NT)
released by sensory
neuron
neurotransmitter
released by interneuron
TRANSMISSION
AND
DIRECTION OF
IMPULSE
THROUGH
NEURON
from sensory
receptors of
dendrites down
dendrites through
cell body down
axon* and into
axonal terminal
(synaptic knobs)
*if axon is
myelinated =
saltatory
conduction
down dendrites through
cell body (where
integration occurs).
Impulse is passed down
axon* and into axonal
terminal (synaptic
knobs)
*if axon is myelinated =
saltatory conduction
down dendrites through
cell body down axon*
and into axonal terminal
(synaptic knobs)
*if axon is myelinated =
saltatory conduction
TRANSMISSION
OF IMPULSE
TO NEXT
NEURON (OR
EFFECTOR)
from electrical
within sensory
neuron to chemical
between neurons.
NT released from
axonal terminals of
sensory neuron
stimulates the
dendrite (or cell
body) of an
interneuron
from electrical within
interneuron to chemical
between neurons. NT
released from axonal
terminals of interneuron
stimulates the dendrite
(or cell body) of a motor
neuron
from electrical within
motor neuron to
chemical between motor
neuron and effector.
NT/Acetylcholine
released from axonal
terminals of motor
neuron stimulates an
effector to respond.
Effectors = muscle
(contract) or glands
(secretion of hormones)
10-19