Chapter11

Nervous Tissue
•
– Controls multiple muscle movements
–
Others movements without voluntary input
• e.g., beating of the heart
•
–
Controls and interprets all these sensations and muscle movements
Body’s primary communication and control system
Integrates and regulates body functions
Uses electrical activity transmitted along specialized nervous system cells

Introduction to the Nervous System:
General Functions
•
–
Collects information
• specialized nervous structures, receptors
• monitor changes in external and internal environment, stimuli
• e.g., receptors in the skin detecting information about touch
– Processes and evaluates information
• then determines if response required
–
Initiates response to information
• initiate response via nerves to effectors
• include muscle tissue and glands
• e.g., muscle contraction or change in gland secretion

Introduction to the Nervous System:
General Functions
•
–
Central nervous system
• anatomic division of the nervous system
• includes brain and spinal cord
• brain protected in the skull
• spinal cord protected in the vertebral canal
–
Peripheral nervous system
• other anatomic division
• includes nerves , bundles of neuron processes
• includes ganglia , clusters of neuron cell bodies

The major components and functions of the nervous system
Central Nervous System
The central nervous system
(CNS) consists of the brain and spinal cord and is responsible for integrating, processing, and coordinating sensory data and motor commands.
Information processing includes the integration and distribution of information in the CNS.
Peripheral Nervous
System
The peripheral nervous system
(PNS) includes all the neural tissue outside the CNS.
The motor division of the
PNS carries motor commands from the CNS to peripheral tissues and systems.
includes
The sensory division of the PNS brings information to the CNS from receptors in peripheral tissues and organs.
The somatic nervous system
(SNS) controls skeletal muscle contractions.
The autonomic nervous system
(ANS) provides automatic regulation of smooth muscle, cardiac muscle, glands, and adipose tissue.
Somatic sensory receptors provide position, touch, pressure, pain, and temperature sensations.
Special sensory receptors provide sensations of smell, taste, vision, balance, and hearing.
Visceral sensory receptors monitor internal organs.
Receptors are sensory structures that detect changes in the internal or external environment.
Skeletal muscle
• Smooth muscle
• Cardiac muscle
• Glands
• Adipose tissue
Effectors are target organs whose activities change in response to neural commands.
Figure 11 Section 1

Introduction to the Nervous System:
General Functions
•
–
Sensory nervous system
• also known as afferent nervous system
• responsible for receiving sensory information from receptors
• transmits information to the CNS
• further divided into somatic and visceral sensory

•
- Somatic sensory
• detects stimuli that we consciously perceive
• receptors include:
– eyes and nose
– tongue and ears
– skin
– proprioceptors (receptors detecting body position)
–
Visceral sensory
• detects stimuli we do not consciously perceive
• receptors include:
– structures within blood vessels
– structures within internal organs
• e.g., detecting stretch of organ wall

•
–
Motor nervous system
• also known as efferent nervous system
• initiates and transmits motor output from CNS
• transmits information to the effectors
• may be further divided into the somatic and visceral parts
–
Somatic motor
• transmits motor output from CNS to voluntary skeletal muscles
• effector consciously controlled
– e.g., pressing on accelerator of your car
–
Autonomic motor
• transmits output from CNS without conscious control
• transmits to cardiac muscle, smooth muscle, glands

Organization of the Nervous System (Figure 12.1)
Central nervous system (CNS)
Structural organization
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Functional organization
Brain
Spinal cord
Sensory nervous system detects stimuli and transmits information from receptors to the CNS
Motor nervous system initiates and transmits information from the CNS to effectors
Peripheral nervous system (PNS)
Nerves
Ganglia
Somatic sensory
Sensory input that is consciously perceived from receptors (e.g., eyes, skin, ears)
Visceral sensory
Sensory input that is not consciously perceived from blood vessels and internal organs
(e.g., heart)
Somatic motor
Motor output that is consciously or voluntarily controlled; effector is skeletal muscle
Autonomic motor
Motor output that is not consciously or is involuntarily controlled; effectors are cardiac muscle, smooth muscle, and glands

Nervous Tissue: Neurons
•
– Neurons
• basic structural unit of the nervous system
• excitable cells that transmit electrical signals
–
Glial cells
• nonexcitable cells that primarily support and protect neurons

•
–
Excitability
• responsive to stimulation
• most respond only to binding of molecules, neurotransmitters
–
Conductivity
• electrical charges propagated along membrane
• can be local and short-lived or self-propagating
– Secretion
• release neurotransmitters in response to electrical charges
• given neuron releasing only one type of neurotransmitter
– may have excitatory or inhibitory effect on target
– Extreme longevity
• most formed before birth still present in advanced age
–
Amitotic
• mitotic activity lost in most neurons
• not always the case (e.g., occasionally in hippocampus)

Nervous Tissue—Neurons:
General Characteristics
•
–
Cell body
• enclosed by plasma membrane
• contains cytoplasm surrounding a nucleus
• neuron’s control center
• conducts electrical signals to axon
• cytoplasm within cell body
• free and bound ribosomes
• gray color of gray matter
– due to chromatophilic substance and lack of myelin

•
– Dendrites
• short processes branching off cell body
• may have one or many
• receive input and transfer it to cell body
• more dendrites = more input possible
– Axon
• longer process emanating from cell body
• makes contact with other neurons, muscle cells, or glands
• first part, a triangular region, axon hillock
• gives rise to side branches, axon collaterals
• branch extensively at distal end into (axon terminals)
• at extreme tips, expanded regions, synaptic knobs
• knobs containing numerous synaptic vesicles
– contain neurotransmitter

•
– Composed of microfilaments, intermediate filaments, microtubules
–
Intermediate filaments, termed neurofilaments
• aggregate to form bundles, neurofibrils
• provide tensile strength through the neuron

Structures in a Typical
Neuron
(Figure
12.2)
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a)
Dendrites
Cell body
Chromatophilic substance Axon hillock
Perikaryon
Nucleolus
Nucleus
Axoplasm
Axolemma
Neurofibrils
Axon (beneath myelin sheath)
Neurolemmocyte
Neurofibril node
Axon collateral
(b)
Myelin sheath
Telodendria
Synaptic knobs
Synapse
Synaptic vesicles containing neuro transmitter
Synaptic cleft
Postsynaptic neuron
(or effector)
Dendrite
Chromatophilic substance
Nucleus
Cell body
Axon hillock
Nucleus of glial cell
Axon b: © Ed Reschke

Nervous Tissue—Neurons:
Classification of Neurons
•
– Structural classification of neurons
• according to number of neuron processes
–
Multipolar neurons
• most common type
• have many dendrites and a single axon
–
Bipolar neurons
• have two processes extending from cell body
• one dendrite and one axon
• limited, e.g., in retina of the eye
– Unipolar neurons
• have single short neuron process
• emerges from cell and branches like a T

Structural
Classification of Neurons
(Table
12.1)

Nervous Tissue—Neurons:
Classification of Neurons
•
– Sensory neurons ( afferent neurons )
• neurons of the sensory nervous system
• conduct input from somatic and visceral receptors
• most unipolar, few bipolar
• cell bodies usually in posterior root ganglia, outside CNS
–
Motor neurons (efferent neurons)
• neurons of the motor nervous system
• conduct motor output to somatic and visceral effectors
• all multipolar
• most cell bodies in CNS

Nervous Tissue—Neurons:
Classification of Neurons
•
–
Interneurons (association neurons)
• entirely within the CNS
• receive stimulation from many other neurons
• receive, process, and store information
• “decide” how body responds to stimuli
• facilitate communication between sensory and motor neurons
•
99% of neurons
• generally multipolar

Skin receptors
Functional Classification of Neurons (Figure 12.3)
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Posterior root ganglion
Cell body of sensory neuron
Spinal cord
Sensory input
Motor output
Sensory neuron
Interneuron
Motor neuron
Skeletal muscle

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Perineurium
Epineurium
Nerve
Blood vessels
Endoneurium
Neurolemmocyte
Axon
Structure of a Nerve and
Ganglion
(Figure
12.4)
Perineurium
Fascicle
Endoneurium
Axon
Neurolemmocyte
(a) Blood vessels
Ganglion
Cell bodies
Axons Nerve
(b)
Fascicle
(c)
Epineurium Blood vessels Axons b: © Dr. Richard Kessel & Dr. Randy Kardon/Tissues & Organs/Visuals Unlimited

Nervous Tissue—Neurons:
Relationship of Neurons and Nerves
•
–
Sensory nerves
• contain only sensory neurons
– Motor nerves
• contain primarily motor neurons
–
Mixed nerves
• contain both sensory and motor neurons
• most named nerves in this category
• individual neurons transmitting one type of information

Synapse
–
Where neuron functionally connected to neuron or effector
–
Two types: chemical and electrical
Chemical synapse
Most common
Composed of presynaptic neuron, signal producer
Composed of postsynaptic neuron, signal receiver
Between axon and postsynaptic neuron -with a dendrite
Knob almost touches the postsynaptic neuron –gapsynaptic cleft
Neurotransmitters released from synaptic vesicles within synaptic knob
Diffusion of neurotransmitter across cleft
Binding of some neurotransmitters to receptors
•
–
Much less common
–
Presynaptic and postsynaptic neuron physically bound together
– No delay in passing electrical signal
–
In limited regions of brain and eyes

Nervous Tissue—Glial Cells:
General Characteristics
•
–
Nonexcitable cells found in CNS and PNS
–
Smaller than neurons
–
Capable of mitosis
–
Far outnumber neurons
– Half volume of nervous system
–
Physically protect and nourish neurons
–
Provide physical scaffolding for nervous tissue
• help guide migrating neurons to their destination
– Critical for normal function at neural synapses

Nervous Tissue—Glial Cells:
Types of Glial Cells
•
–
Starlike shape from surface projections
– Most abundant glial cell in CNS
– Help form the blood-brain barrier with capillaries in the brain
– strictly controls substances entering brain nervous tissue from blood
– protects neurons from toxins
– allows nutrients to pass
– Regulate tissue fluid composition
• control movement of substances between blood and interstitial fluid
– e.g., regulate K + concentration
–
Form a structural network
• cytoskeleton strengthening and organizing nervous tissue
– Assist neuronal development

Types of Glial Cells
•
–
Line internal cavities of brain and spinal cord
– Form choroid plexus with nearby blood capillaries
• helps produce cerebrospinal fluid
•
–
Small cells with slender branches
– Smallest percentage of CNS glial cells
–
Phagocytic cells Engulf infectious agents- removes debris
•
– Large cells with slender extensions
– Processes ensheathing portions of axons of different neurons
–
Insulate axons in a myelin sheath
–
Prevent passage of ions through axonal membrane
–
Allow for faster action potential propagation through CNS

Oligodendrocyte
Myelinated axon
Myelin sheath (cut)
Cellular Organization of Nervous Tissue:
CNS Glial Cells (Figure 12.5a)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microglial cell
Neuron
Astrocyte
Perivascular feet
Capillary
Ependymal cells
Ventricle of brain
(a) CNS glial cells

Types of Glial Cells
•
–
Arranged around neuronal cell bodies in a ganglion
– Physically separate cell bodies in ganglion from surrounding fluid
– Regulate the exchange of nutrients and waste products
• e.g., surrounding bodies of sensory neurons in a posterior root ganglion
•
– Also known as Schwann cells
–
Ensheathe PNS axons to form myelin sheath
–
Allows for faster action potential propagation

Cellular Organization of Nervous Tissue:
PNS Glial Cells (Figure 12.5b)
Axon
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Posterior root ganglion
Satellite cells
Neuron cell body
Neurofibril nodes
Axon
Cell body of sensory neuron Posterior root
Neurolemmocyte
Nucleus
Myelin sheath
Neurilemma
(b) PNS glial cells

Nervous Tissue—Glial Cells:
Types of Glial Cells
–
Neoplasm from unregulated cell growth, tumors
–
Sometimes occur in CNS
–
Tumors originating from the brain, primary brain tumors
–
Typically originate in supporting tissues
• tissues with capacity to undergo mitosis
• from meninges (protective membranes of CNS) or glial cells
–
Gliomas , glial cell tumors
• may be relatively benign
• may be malignant, capable of metastasizing

Nervous Tissue—Glial Cells:
Myelination
•
–
Process by which part of an axon wrapped in myelin
–
Myelin , insulating covering around axon
• consists of repeating layers of glial cell plasma membrane
• has high proportion of lipids
• gives glossy appearance and insulates axon
–
Completed by neurolemmocytes (PNS)
–
Completed by oligodendrocytes (CNS)
–
Layers of plasma membrane form the myelin sheath
– Its cytoplasm and nucleus is pushed to the periphery
• termed neurilemma

Myelination of PNS Axons
(Figure
12.6)
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1
Neurolemmocyte starts to wrap around a portion of an axon.
Axon
Neurolemmocyte
Nucleus
Direction of wrapping
2 Neurolemmocyte cytoplasm and plasma membrane begin to form consecutive layers around the axon as wrapping continues.
3 The overlapping inner layers of the neurolemmocyte plasma membrane form the myelin sheath.
Cytoplasm of the neurolemmocyte
Myelin sheath
4 Eventually, the neurolemmocyte cytoplasm and nucleus are pushed to the periphery of the cell as the myelin sheath is formed.
Myelin sheath
Neurolemmocyte nucleus
Neurilemma

•
–
Can myelinate only
1 mm of single axon
–
Takes many to myelinate entire axon
– Gaps between neurolemmocytes
• neurofibril nodes, or nodes of Ranvier
(Figure 12.7a)
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
PNS
Neuron cell body
(a) Myelination by neurolemmocytes
Neurolemmocytes
Neurofibril node
Neurilemma
Myelin sheath
Axon

•
–
Can myelinate 1 mm of many axons
–
Extensions wrapping around axons
– No neurilemma formed
–
Neurofibril nodes between adjacent
“wraps”
(Figure 12.7b)
Axons
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CNS
Oligodendrocytes
(b) Myelination by oligodendrocytes
Myelin sheath
Neurofibril node

Nervous Tissue—Glial Cells:
Myelination
•
–
Associated with neurolemmocytes
–
No myelin sheath covers them
–
Axon in depressed portion of neurolemmocyte
–
Not wrapped in repeated layers
– In CNS,
• unmyelinated axons not associated with oligodendrocytes

Unmyelinated axons
1 Neurolemmocyte starts to envelop multiple axons.
Unmyelinated Axons (Figure 12.8)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neurolemmocyte
Axons
Neurolemmocyte nucleus
2
The unmyelinated axons are enveloped by the neurolemmocyte, but there are no myelin sheath wraps around each axon.
Unmyelinated axon
Neurolemmocyte
(a) (b) b: © Donald Fawcett/Visuals Unlimited
Unmyelinated axons
Myelin sheath
Myelinated axon
Neurilemma

Nervous Tissue—Glial Cells:
Myelination
–
Multiple Sclerosis
• progressive demyelination of neurons in CNS
• autoimmune disorder
• oligodendrocytes attacked by immune cells
• repeated inflammatory events causing scarring and permanent loss of function
• vision problems, muscle weakness and spasms, urinary and bladder problems, mood problems
- Guillain-Barre syndrome loss of myelin from peripheral nerves due to inflammation muscle weakness that begins in distal limbs advances to involve proximal muscles no specific infectious agent identified most function recovered with little medical intervention

Nervous Tissue—Glial Cells:
Myelination
–
Guillain-Barre syndrome
• loss of myelin from peripheral nerves due to inflammation
• muscle weakness that begins in distal limbs
• advances to involve proximal muscles
• no specific infectious agent identified
• most function recovered with little medical intervention

Axon Regeneration
•
–
• vulnerable to cuts, trauma
– Regeneration possible if
• cell body intact
• enough neurilemma remains
–
Regeneration success more likely if
• amount of damage less extensive
• smaller distance between site of damage and structure it innervates
Extremely limited growth-inhibiting molecules secreted by oligodendrocytes regrowth obstructed by scars from astrocytes and connective tissue

•
– Type of transport protein
–
Move substances against concentration gradient
–
Require energy
• e.g., sodium-potassium and calcium pumps in plasma membr ane
(Figure 12.10a)
Interstitial fluid
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cytosol
Breakdown of ATP
(releases energy)
K +
K +
ATP binding site
ATP
Na +
Na +
P
ADP
Na+/K+ pump
(a) Sodium-potassium (Na + /K + ) pump
Na + /K + pump changes shape (requires energy from ATP breakdown)

•
– Move substances down concentration gradient
–
Leak channels
• always open for continuous diffusion
• e.g., sodium ion and potassium ion channels
(Figure 12.10b)
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interstitial fluid
Cytosol
(b) Leak (passive) channels
Na +

Distribution of Pumps and
Channels in the Plasma
Membrane of a Neuron
(Figure
12.11)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Plasma membrane of entire neuron
Chemically gated cation channel
Receptive segment
Chemically gated K + channel
Chemically gated Cl
– channel
Dendrites
Na + /K + pump
Cell body
Axon hillock
Na + leak channel
K+ leak channel
(b)
Initial segment
Voltage-gated
Na + channel
Voltage-gated
K + channel
Conductive segment
Voltage-gated
Na + channel
Voltage-gated
K + channel
(c)
Entire neuron
Axon
(a)
Synaptic bulb
Transmissive segment
Voltage-gated
Ca 2+ channel
Ca 2+ pump
(d)
(e)

Ultrastructure of Neurons: Distribution of
Substances and Membrane Potentials
•
–
Essential for neuron function
–
More prevalent within cytosol
• negatively charged phosphate ions (e.g., in ATP)
• negatively charged proteins
• K +
–
More prevalent in interstitial fluid
•
Na +
•
Cl -

Ultrastructure of Neurons: Distribution of
Substances and Membrane Potentials
•
–
Chemical concentration gradient
• unequal distribution between two areas
• each substance with own chemical concentration gradient
• e.g., K + with a higher concentration inside the neuron
• e.g., Na + with a higher concentration outside the neuron

Introduction to Neuron Physiology:
Resting Membrane Potentials
•
–
Membrane potential in a resting, excitable cell
–
Relative difference in charge across membrane
–
Measured with a voltmeter
• microelectrodes into neuron and interstitial fluid
– Negative value, typically -70 mV
–
More positive ions outside a neuron than in it at rest
–
A consequence of the plasma membrane permeability to ions

Introduction to Neuron Physiology:
Resting Membrane Potentials
•
+
+
– Play relatively small role in establishing RMP
–
Three Na + pumped out for two K + pumped in
–
More significant role in maintaining gradients of K + and Na +
• following diffusion as part of neuron’s electric current
– Ions pumped back up concentration gradient by pump
– Two-thirds of a neuron’s energy expenditure

Establishing and Maintaining the Resting Membrane Potential
(RMP) (Figure 12.12)
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–70mV
+ + + + + + + + + +
– – – – – – – – – –
– – – – – – – – – –
+ + + + + + + + + +
–70 mV
Voltmeter
Na + leak channel
Na +
Interstitial fluid
Greater concentration of Na + and Cl
–
Plasma membrane
+ + +
– – –
Cytosol
Greater concentration of K + , P i
, and proteins
K +
Cl
–
+ +
– –
ADP
+ +
– –
P i
+ +
– –
–70 mV
Protein
K + leak channel
+ + +
– –
+ + +
– – –
ATP
Na + /K + pump
+ +
Microelectrode
– – – –

Introduction to Neuron Physiology:
Changing the Membrane Potential
•
–
Opening of chemically gated channels or voltage-gated channels
–
Causes change in ion flow across membrane
–
Alters resting membrane potential
–
Depolarization
• inside of cell becomes more positive than RMP
• e.g., from -70 mV to -60 mV
• occurs when gated channels open
• movement of Na + into neuron
• causes inside of neuron to become more positive

Changing the Resting Membrane Potential: Resting
Membrane Potential (Figure 12.13a)
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Interstitial fluid
Plasma membrane
Cytosol
+ +
–70 mV
+ + + + + + + + + +
– – – – – – – – – –
– – – – – – – – – –
+ + + + + + + + + +
Gated Na + channel
Gated K + channel
Na +
+ + + + +
Cl –
+ + + + +
Gated Cl
– channel
+ +
– – – – – – – – – – – – – –
K +
(a) Resting membrane potential e.g., –70 mV

Changing the Resting Membrane Potential:
Depolarization (Figure 12.13b)
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
–60 mV
+ + + + + + + + + +
– – – – – – – – – –
– – – – – – – – – –
+ + + + + + + + + +
Gated Na + channel
Cl
–
+ + + + + + + + + + + + +
– – – – – – –
Na +
K +
– – – – – e.g.,
– – –
–60 mV
(b) Depolarization: Na + flows in

•
– Generated within the initial segment
–
Propagated along axon
–
Due to opening of voltage-gated channels
–
Threshold value
• minimum voltage change to open voltage-gated channel
• any value below this, a subthreshold value
–
If threshold value reached
• channels open and membrane potential reversed
• if Na + channel opens, enters the neuron
• makes inside relatively positive
• opening of voltage-gated channels in these areas
• successive opening down the axon
• followed by sequential opening of voltage-gated K + channels
• movement of K + out of neuron returns membrane to RMP

Introduction to Neuron Physiology:
Changing the Membrane Potential
•
– involve temporary reversal of polarity across plasma membrane
• inside becomes relatively positive
• followed by a return to RMP
– are self-propagated
• maintain intensity as move to synaptic knob
– obey the “all or none law”
• if threshold reached, action potential sent
• if not reached, no action potential sent
See Table 12.3: Graded Potential Versus Action Potential

Release of Inhibitory Neurotransmitter and
Generation of IPSP (Figure 12.15b)
Axons of presynaptic neuron
Synaptic vesicles containing inhibitory neurotransmitter
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Release of inhibitory neurotransmitter and generation of IPSP
1 Inhibitory neurotransmitter binds to either chemically gated K + channels or chemically gated Cl – channels, causing them to open.
Inhibitory neurotransmitter
Postsynaptic neuron
Chemically gated K + channel
Chemically gated Cl – channel
K +
2 Either K + flows out of, or Cl – flows into, the neuron, depending on the type of channel stimulated.
Cl –
3 Inside of neuron becomes more negative; called IPSP (e.g., –72 mV).
Cl –
4 IPSP propagates toward axon hillock.
0
–20
–40
–60
–70
–80
Stimulus
Threshold
IPSP
Time (msec)
Resting membrane potential
(b)

IPSP EPSP
Several Presynaptic Neurons with a Postsynaptic Neuron(Figure 12.16)
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© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Postsynaptic neuron
Synaptic knob
Presynaptic axons
Axons of presynaptic neuron
Dendrites
Cell body of postsynaptic neuron
Myelin sheath
Axon
Axons of presynaptic neuron b: © Science VU/Lewis-Everhart-Zeevi/Visuals Unlimited

Physiologic Events in the Neuron Segments:
Initial Segment
•
–
Addition of graded postsynaptic potentials (IPSPs and EPSPs)
–
Occurs at the initial segment
–
Determines if threshold membrane potential is reached
•
-55 mV, +15 mV from RMP
– If threshold reached
• voltage-gated channels open
• action potential generated that travels along axon
–
Spatial summation
• release of neurotransmitter from multiple presynaptic neurons
• action potential initiated if enough EPSPs generated
–
Temporal summation
• repeated release of excitatory neurotransmitter at same location
• effects added if occur within small timeframe
• action potential initiated if threshold reached

–55
Spatial
Summation at the Axon
Hillock
(Figure
12.7a)
Dendrites
–70
P2
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Spatial summation
+30
Initial segment
0
Action potential
P1
P2 P3
P4
P5
Threshold
Axon hillock
Time (m sec)
Cell body of postsynaptic neuron
P1
Myelin sheath
P3
EPSPs
P4 P5
Axon
Axons of presynaptic neurons (P), (P1
–P5)
(a) Spatial summation

Temporal
Summation at the Axon
Hillock
(Figure
12.7b)
Axon of presynaptic neuron (P2)
+30
0
–55
–70
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Temporal summation
P2
Action potential
Threshold
Time (m sec)
Postsynaptic neuron
EPSPs
P2
Axon
(b) Temporal summation

Physiologic Events in the Neuron Segments:
Initial Segment
•
–
If threshold reached, action potential propagated
–
If threshold not reached, not propagated
–
Same intensity of response to values greater than threshold
–
Similar to what occurs with a gun
• with sufficient pressure on trigger, gun fired
• with insufficient pressure on trigger, not fired
• travels at same velocity even if pressure is greater than needed
•
–
The propagation of an action potential
–
Involves depolarization and repolarization
–
Threshold reached in initial segment
– Initiates action potential along the axon (conductive segment

Physiologic Events in the Neuron Segments:
Conductive Segment
•
–
Occurs only at plasma membrane
• via voltage-gated Na + channels
– Channels triggered to open when threshold reached
–
Rapid entry of Na +
–
Inside of axon made positive
–
Channels open briefly before closing
• change from activation state to temporary inactivation state

Physiologic Events in the Neuron Segments:
Conductive Segment
•
–
Sequential opening of voltage-gated Na + channels along the axon
–
Flow of Na+ into cell
• causes adjacent regions to also reach threshold
• triggers voltage-gated Na + channels in these areas
–
Process repeated rapidly down synaptic knob
• does not go backwards
• voltage-gated Na + channels here in inactivated state

Physiologic Events in the Neuron Segments:
Conductive Segment
•
–
E.g., lidocaine
–
Inhibit action of voltage-gated Na + channels
– Block nerve signal
–
Pain signal blocked from reaching CNS
–
Application of ice
• reduces pain sensation
• slows transmission of sensory action potentials

Propagation of Action Potential Down an Axon (Figure 12.18a)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+
Axon hillock
Nerve signal: propagation of action potential
+ ++ + + + + + ++ + +
– – – – – – – – – – – –
– – – – + ++ + + + + + ++ + + + ++ + + + + + ++ + +
+ ++ +
– – – – – – – – – – – – – – – – – – – – – – – –
– – – – – – – – – – – –
+ ++ + + + + + ++ + +
+ ++ +
– – – – – – – – – – – – – – – – – – – – – – – –
– – – – + ++ + + + + + ++ + + + ++ + + + + + ++ + +
Repolarization Depolarization
+
(a)

Propagation of Action Potential: Depolarization (Figure 12.18b)
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Depolarization: Consecutive voltage-gated Na+ channels go through the following stages: open, closed (inactivation state), closed (resting state)
Interstitial fluid
Na +
+ + + + + + + + + + + + + +
– – – – – – – + + + + + + + + +
– – – – – – – – – – – – – –
Cytosol
Closed
(resting state)
Closed
(inactivation state)
+ + +30 mv
+ –55 mv
Open
(activation state)
–
As threshold is reached
Na + channels open and Na + diffuses in; polarity reversed
–70 mv
– – – –
Closed
(resting state)
(b)

Physiologic Events in the Neuron Segments:
Conductive Segment
•
–
Voltage-gated K + channels normally closed
–
Stimulated to open by threshold
– Not open until depolarization has ended
–
Exit of K + , making inside of axon negative
–
Return to RMP (-70 mV)
–
Triggers voltage-gated Na + channels to return to resting state
•
• Opening of voltage-gated K + channels adjacent
• Open sequentially along length of axon

Propagation of Action Potential: Repolarization (Figure 12.18c)
Copyright
© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Repolarization: Consecutive voltage-gated K + following stages: open and closed channels go through the
K +
+ + + + + + + + + + + + + + – – – – – – – – – + + + + + + + +
– – – – – – – – – –70 mv
–
Closed
– + +
+30 mv
+ + + + + – – – – – – – –
Closed Open
K + channels open and
K + diffuses out; RMP
( –70 mv) is reestablished
(c)

Physiologic Events in the Neuron Segments:
Conductive Segment
•
–
Brief time period after action potential initiated
–
During absolute refractory period
• no amount of stimulus able to generate a second action potential
•
Na + channels opened then closed in inactivated state
• remain closed until potential almost to resting potential
• ensures that action potential moves in one direction only
–
During relative refractory period
• with greater stimulation, action potential possible
• Na + returned to resting state
• neuron hyperpolarized
• due to extended time K + channels remain open

Events of an Action Potential (Figure 12.19)
+30
+10
0
–10
–30
–50
1
Threshold
–70
–90
Resting membrane potential
0
2 3
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4
1
Time (msec)
5
2
6
3
1 The unstimulated axon has a resting membrane potential of –70 mV.
2
3
Graded potentials reach axon hillock and are added together.
Depolarization occurs when the threshold ( –55 mV) is reached; voltage-gated Na + channels open and Na + enters rapidly, reversing the polarity from negative to positive
( –55 mV +30 mV).
4 Repolarization occurs due to closure of voltage-gated
Na+ channels (inactivation state) and opening of voltage-gated K + channels. K + moves out of the cell into the IF and polarity is reversed from positive to negative
(+30 mV –70 mV).
5 Hyperpolarization occurs when voltage-gated K + channels stay open longer than the time needed to reach the resting membrane potential; during this time the membrane potential is less than the resting membrane potential of –70 mV.
6 Voltage-gated K + channels are closed, and the plasma membrane has returned to resting conditions by activity of Na + /K + pumps.

Physiologic Events in the Neuron Segments:
Transmissive Segment
•
–
Calcium concentration gradient established by pumps
• more calcium outside synaptic knob than in
–
Voltage-gated Ca 2+ channels
• triggered by propagated action potential
• movement of calcium ions into synaptic knob
–
Binding of calcium to proteins of synaptic vesicles
–
Neurotransmitters released into synaptic cleft by exocytosis
• facilitated by numerous proteins
• diffuse across cleft
• bind to specific receptors on cell to be stimulated

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Neuromuscular junction
Transmissive
Segment:
Release of
Neurotransmitter
(Figure
12.21a)
Ca 2+
1
+ + + + +
+ +
+
Action potential reaches synaptic knob.
Voltage-gated Ca 2+ channel
Synaptic cleft
2 Voltage-gated Ca 2+ channels open and
Ca 2+ enters the synaptic knob and binds with proteins of synaptic vesicles.
+ + + + +
+ +
+
3 Synaptic vesicles merge with synaptic knob plasma membrane and neurotransmitter is released by exocytosis.
Neurotransmitter
Synaptic knob
Synaptic vesicle
(contains neurotransmitter)
4
Neurotransmitter crosses synaptic cleft and attaches to receptors on a muscle, as shown.
(Or to receptors of a neuron or gland.)
Receptor
(a)

Velocity of a Nerve Signal
•
–
Diameter of axon
• larger diameter, faster the velocity of the signal
–
Myelination of axon
• more important factor
• faster velocity in myelinated axons
•
–
Occurs in unmyelinated axons
– Sequential opening of voltage-gated Na + and K + channels

Velocity of a Nerve Signal: Propagation
•
–
Occurs in myelinated axons
–
Action potentials propagated only at neurofibril nodes
–
Myelinated regions
• with limited numbers of voltage gated Na + and K + channels
• well insulated, preventing ion movement
–
Neurofibril nodes
• with large number of voltage-gated Na + and K + channels
• lack myelin insulation

Velocity of a Nerve Signal: Propagation
•
–
Neurofibril node
• initiation of action potential
• diffusion of Na + into axon
–
Myelinated regions
• diffusion through axoplasm of axon
• relatively fast
• becomes weaker with distance as experiences resistance

Saltatory Conduction (Figure 12.23)
Neurofibril node
+ + +
– – –
+ +
– –
– – –
+ + +
– –
+ +
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Myelin sheath
+ + +
– – –
– – –
+ + +
+ +
– –
– –
+ +
K +
Na +
+ + +
– – –
– – –
+ + +
– – –
+ + +
+ + +
– – –
Action potential
Repolarization Depolarization
Diffusion of Na + through axoplasm
+ + +
– – –
– – –
+ + +

Velocity of a Nerve Signal:
Nerve Fiber Classification
•
–
Nerve fiber: an axon and its myelin sheath
–
Classified into three major groups
–
Group A
• conduction velocity as fast as 150 m/sec
• large diameter myelinated fibers
• e.g., most somatic sensory neurons, somatic motor neurons
–
Group B and Group C
• small in diameter, unmyelinated, or both
• e.g., sensory and motor visceral neurons
• group B: 15 m/sec
• group C: 1 m/sec

Neurotransmitters and Neuromodulation
•
–
Neurotransmitters, various small organic compounds
–
Released at synaptic cleft
–
Approximately 100 known
–
Classified into major groups

Neurotransmitters and Neuromodulation
•
– Acetylcholine
• excitatory or inhibitory neurotransmitter
• released in both CNS and PNS
• molecule released from motor neuron at neuromuscular junction
– Amino acids
• building blocks of proteins
• some also neurotransmitters
• e.g., glutamate, glycine, aspartate
– Monoamines
• derived from certain amino acids
• catecholamines (norepinephrine, epinephrine, dopamine)
–
Neuropeptides
• chains of amino acids
• include enkephalins and somatostatin

Neurotransmitters and Neuromodulation
•
–
Temporary association between neurotransmitter and receptor
–
Necessary to eliminate molecule after stimulation
– Can occur by degradation
• neurotransmitter chemically inactivated in synaptic cleft
• e.g., breakdown of ACh by acetylcholinesterase
–
Can occur by reuptake
• neurotransmitter reabsorbed by transport protein in presynaptic neuron
• “recycled” into another synaptic vesicle for reuse
• e.g., drugs, selective serotonin reuptake inhibitors
– block reuptake of serotonin and used in treatment of depression