Title: Introduction to the Nervous System Chapter 12 Lecture Notes
1Introduction to the Nervous SystemChapter 12
Lecture Notes
- to accompany
- Anatomy and Physiology From Science to Life
- by
- Gail Jenkins, Christopher Kemnitz, Gerard J.
Tortora
2Chapter Overview
- 12.1 System Overview
- 12.2 System Organization
- 12.3 The Neuron
- 12.4 Neuroglia
- 12.5 Neural Communication
- 12.6 Graded Potentials
- 12.7 Action Potentials
- 12.8 Impulse Propagation
- 12.9 Synapse
- 12.10 Repair and Regeneration
3Essential Terms
- neuron
- functional cell responsible for nervous system
signaling - neuroglia
- supporting cells of the nervous system
- nerve
- bundle of neurons
- neurotransmitter
- chemical signal
4Nervous System Overview
- Role Maintain homeostasis
- Sense changes (sensory neurons)
- Integrate information (interneurons)
- Respond (motor neurons)
- Basic Anatomy
- Mass 2 kg or 4.5 lbs 3 total body mass
- Main Subdivisions
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)
5Concept 12.1The Nervous System Maintains
Homeostasis and Integrates All Body Activities
6Nervous System Anatomy
- Structures
- brain
- spinal cord
- nerves
- ganglia
- enteric plexuses
- sensory receptors
7Brain
- Part of CNS
- Enclosed by the skull
- Cranial nerve associations
- Attached to spinal cord
8Spinal Cord
- Part of CNS
- Connects to brain
- Leaves skull through foramen magnum
- Encircled by vertebrae
- Spinal nerve associations
9Nerves
- Part of PNS
- Cranial nerves
- 12 pair
- Emerge from base of brain
- Spinal nerves
- 31 pair
- Emerge from spinal cord
- Each nerve follows defined path
- Each nerve serves specific region of body
10Ganglia
- Mass of nervous tissue in PNS
- Collection of neurons
- Closely associated with cranial and spinal nerves
- Use of term
- Ganglia is plural
- Ganglion is singular
11Enteric Plexuses
- Located in walls of gastrointestinal tract
- Help regulate digestive system function
- Checkpoint
- To which subdivision of the nervous system would
the enteric plexuses belong?
12Sensory Receptors
- Monitor internal environment
- Monitor external environment
13Nervous System Physiology
- Responsible for
- Perceptions (five senses)
- Behaviors
- Memories
- Movements
- Three main functions
- Sensory function
- Integrative function
- Motor function
141. Sensory Function
- Sensory or Afferent neurons
- Carry information from PNS to CNS
- Signal travels through both cranial and spinal
nerves - Detect stimuli
- Internal (blood acidity)
- External (raindrop on arm)
152. Integrative Function
- Primarily Interneurons
- Short axons in CNS
- Contact nearby neurons in brain, spinal cord or
ganglion - Most neurons are interneurons
- Process information from afferent (sensory)
neurons by - Analyzing information
- Storing information
- deciding appropriate response
163. Motor Function
- Motor or Efferent neurons
- Carry information from the CNS to the PNS
- Respond to interneuron decisions
- Cells responding are effectors
- Examples include muscle fibers glandular cells
17Concept 12.2 CNS consists of the Brain and
spinal cord- PNS consists of all other nervous
tissue
18Nervous System Organization
- Central Nervous System (CNS)
- Brain
- Spinal cord
- Peripheral Nervous System (PNS)
- Cranial nerves
- Spinal nerves
- Spinal nerve branches
- Ganglia
- Sensory receptors
19Nervous System Organization
20Central Nervous System
- Integrates and correlates incoming sensory
information - Source of thoughts, emotions, memories
- Most motor signals originate in CNS
21Peripheral Nervous System
- Subdivisions of PNS
- Somatic Nervous System (SNS)
- Motor portion under voluntary control
- Autonomic Nervous System (ANS)
- Motor portion under involuntary control
- Further subdivided
- Sympathetic nervous system
- Parasympathetic nervous system
- Enteric nervous system (ENS)
221. Somatic Nervous System
- A. Sensory input from somatic receptors
- B. Motor output
- voluntary control
- carry impulses from CNS to skeletal muscles only
232. Autonomic Nervous System
- A. Sensory input from autonomic receptors
- B. Motor output to smooth muscle, cardiac muscle,
glands - involuntary control
- carry impulses to
- sympathetic division
- fight or flight response
- parasympathetic divisions
- return to normal
243. Enteric Nervous System
- A. Sensory input from plexuses of GI tract
- B. Motor output to plexuses of GI tract
- innervate smooth muscle, glands, and endocrine
cells of GI tract - involuntary control
25Concept 12.3 Neurons Are Responsible For Most
Unique Functions Of The Nervous System
26Neurons
- Functional cell of nervous system
- Bundled into nerves
- Are excitable
- Respond to stimuli
- Can produce action potentials or impulses
- Supported by neuroglia
- Most are amitotic
27Anatomy of a Neuron
28Basic Anatomy of a Neuron
Cell Body
Dendrites
Axon
29Collect signals
Dendrites
30Dendrites
- multiple processes
- collect signals
- usually short, tapering, highly branched
31Cellular Metabolism
Cell Body
32Cell Body of Neuron
- Cellular Metabolism
- Size from 5µm up to 135µm
- Nucleus surrounded by cytoplasm
- Typical cellular organelles
- Lack centrioles (no mitosis)
- Nissl bodies are prominent clusters of rough ER
- Cytoskeletal elements
- neurofibrils give cells shape and support
- microtubules move materials to and through axon
33Sends signal
Axon
34Axon
- single process
- sends signal away toward target cell
- long, thin, cylindrical
- cytoplasm called axoplasm
- membrane called axolemma
- axon branches axon collaterals
- end at axon terminals
- some swell into synaptic end bulbs
35Axon
- axon joins cell body at axon hillock
- first part called initial segment
- trigger zone
- where impulses arise at junction of hillock and
initial segment - newly synthesized substances moved via motor
proteins and microtubules - toward end bulbs process called anterograde
transport - toward cell body process called retrograde
transport
36Cell Body
Dendrites
Axon Hillock (trigger zone)
Axon
Axon Terminals
Synaptic End Bulbs
37Structural Classification
- multipolar
- several dendrites
- one axon
- most CNS neurons
- bipolar
- one main dendrite
- one axon
- retina of eye, inner ear, olfactory CNS
- unipolar
- embryonic sensory neurons
38Structural Classification
39Concept 12.4 Neuroglia Support, Nourish, And
Protect Neurons And Maintain Homeostasis
40Neuroglia
- Supporting cells of the nervous system
- one-half of the volume of CNS
- 5 to 50 times more numerous
- smaller than neurons
- no action potentials
- highly mitotic
41Neuroglia
- CNS
- astrocytes
- oligodendrocytes
- microglia
- ependymal cells
- PNS
- Schwann cells
- satellite cells
42CNS - Neuroglia
- Astrocytes
- maintain chemical environment, support, and
nourish - Oligodendrocytes
- produce myelin sheath around adjacent axons
- Microglia
- migrating phagocytes
- engulf invaders and injured tissue
- Ependymal cells
- form and circulate cerebrospinal fluid (CSF)
43Astrocyte
44Oligodendrocyte
45Microglia
46(No Transcript)
47PNS - Neuroglia
- Satellite cells
- support cells within PNS ganglia
- Schwann cells
- produce myelin sheath around axons
- participate in regeneration of PNS axons
48Satellite Cells
49Schwann Cells
50Schwann Cells
51Myelination
- multilayered lipid and protein covering
- insulates axons
- greatly increases speed of impulse
- produced by neuroglia
- CNS oligodendrocytes
- PNS Schwann cells
- amount increases from birth to maturity
- increasing coordination and speed of responses to
stimuli as individual matures
52Myelination Schwann Cells
- begin formation in fetal development
- each wraps 1mm of axon
- cytoplasm and nucleus form outer layer
- wraps approximately 100 times around
- neurolemma is outermost layer
- when axon is injured neurolemma aids regeneration
tube guiding and stimulating regrowth of axon - spaces between nodes of Ranvier
53Myelination
54Unmyelinated Axons
- Some axons are said to be unmyelinated
- Schwann Cells are still present but do not wrap
around and around process
55Myelination Oligodendrocytes
- each cell has approximately 15 processes that
each mylenate axons - neurolemma is not present because cell body and
nucleus not at processes - Nodes of Ranvier present, but fewer in number
- Repair and regeneration of axons less in CNS than
PNS due to absence of neurolemma and inhibitory
influence of oligodendrocytes
56Oligodendrocyte
57Gray and White Matter of CNS
- White matter consists primarily of myelinated
axons - white color due to myelin
- Gray matter consists primarily of neuronal cell
bodies, dendrites, unmyelinated axons, axon
terminals, and other neuroglia - Nissl bodies and absence of myelin lead to gray
color
58Gray and White Matter of CNS
- Spinal cord
- Gray matter surrounded by white matter
- Centrally located H shape to gray matter
- Brain
- Mostly white matter surrounded by gray matter
- Thin shell of gray matter on surface of cerebrum
and cerebellum - Study Hint
- Brain Spinal cord have opposite layer pattern
for gray and white matter
59Brain Gray over White
60Spinal Cord White over Gray
61See it?
62Concept 12.5Neurons Communicate with Other Cells
63Neuronal Communication
- Excitable cells generate action potentials
- electrical signals traveling cell membranes as a
result of ionic movement into and out of cells - Graded potentials
- used only for short-distance communication
- Nerve action potential
- both short and long-distance communication
64Graded Potential
- Vary in amplitude according to strength of
stimulus - Travel short distances
- Occur at
- sensory receptors
- dendritic connections
- Can generate or trigger nerve action potentials
65Nerve Action Potential
- Amplitude fixed (all-or-none event)
- Travel both short and long distances
- Occur along axons
- Triggered by
- graded potentials at sensory receptors and
dendritic connections
66Graded Potential
67Ion Channels
- Ion channels open and close in response to
stimuli - when open ions move down electrochemical gradient
- chemical and electrical difference across
membrane - movement constitutes flow of electrical current
that flips the charge - Four main types of ion channels
68Types of Ion Channels
- Leakage channel
- randomly alternate between open closed
- voltage-gated channel
- respond to change in voltage
- ligand-gated channel
- responds to chemicals stimulus
- e.g. neurotransmitters, hormones, etc..
- mechanically gated channel
- responds to mechanical stimulation
- e.g. vibration, pressure, stretching
69Voltage and Ligand Gated Channels
70Resting Membrane Potential
- Recall that there is a separation of charges
across the membrane of excitable cells. - Extracellular fluid contains more sodium ions
than are found inside a cell - Cytosol contains more anions and negatively
charged proteins - Thus sodium ions cling to the periplasmic cell
surface
71Resting Membrane Potential
- Cell somewhat permeable to potassium
- Much less permeable to sodium
- Sodium quick to rush in when gates open
- following both electrical and concentration
gradients - Potassium not quick to rush out
- only has concentration gradient to drive flow
72Resting Membrane Potential
- small build-up of anions in cytosol
- equal build-up of cations in extracellular fluid
73Change in Membrane Potential
- Ligand-gated or mechanical-gated Na channels
open - Fast Na influx
- following electrical concentration gradients
- Inside of cell becomes less negative
74Concept 12.6Graded Potentials are the First
Response of a Neuron to Stimulation
75Graded Potentials
- Most occur at dendrites or cell body
- When ligand-gated or mechanical-gated channels
open or close - Slight deviation from resting membrane potential
76Graded Potentials
- Hyperpolarization
- Increases charge difference between inside
outside of cell - inside made even more negative
- Depolarization
- Reduces charge difference between inside
outside of cell - inside made less negative
77Graded Potentials
- Graded because vary in amplitude (size) depending
on strength of stimulus - larger or smaller signal depending on how many
ion channels have opened or closed - alters the flow of ions
- producing localized current that can trigger
action potential when signal strong enough
78Types of Graded Potentials
- Postsynaptic potential
- ligand-gated channel signal
- neurotransmitters
- hormones
- Generator Potential
- mechanical-gated channel signals
- sensory receptors
79Concept 12.7The Action Potential is an
All-or-None Electrical Signal
80Action Potential Overview
- Signals or impulses of communication
- Travel along axons
- Are all-or-none events
- Unlike graded potentials
- Threshold must be reached
- Two phases
- Depolarization
- Repolarization
- Refractory Periods
- Absolute refractory period
- Relative refractory period
81Primary Location of Gates
Voltage-gated sodium and Potassium Ion channels
Voltage-gated CALCIUM ion channels
Ligand-gated and mechanical-gated channels
82Events in Neural Signaling
Action Potentials
Synapses
Graded Potentials
83Resting Membrane Potential
- Review
- Resting membrane potential in excitable cell is
-70mV - Membrane positive outside negative inside
- Anions inside cell that do pass
- Na concentration high outside low inside
- K concentration low outside high inside
84Membrane Potential Changes
- Graded potentials occur when
- Ligand-gated or mechanical-gated Na channels
open - Fast Na influx
- following both electrical concentration
gradients - Inside of cell becomes less negative
- If change is 15mV action potential occurs
85Depolarization
- When graded potential signal is strong enough
(15mV causing change to -55mV) - Trigger zone at axon hillock depolarizes opening
voltage gated Na channels - Fast influx of Na at axon hillock
- Triggering chain-reaction-like depolarization of
entire axon as voltage-gated Na open - Depolarization peaks at 30mV
- -70mV 15mV -55mV
86Ion Gate Activity
- All voltage-gates closed
- Threshold reached (-55mV)
- Fast responding voltage-gated Na channels open
at activation gate - Slow responding voltage-gated K channels open
- Fast Na influx
- Membrane potential reaches 30mV
- K outflow begins
- Voltage-gated fast Na channels closed at
inactivation gate - Fast Na influx stops
- K outflow continues
87(12.14)
88Action Potential
89Repolarization
- Depolarization peaked at 30mV
- Resting membrane potential must be restored
- Hyperpolarization occurs first as Na influx
stops and K outflow continues reducing membrane
potential - When membrane potential reaches less than -55mV
Na channels return to resting state and K
channels close - Sodium potassium pump cleans up
90Repolarization
- At 30mV
- Na influx stopped
- K outflow continues until membrane potential is
less than -55mV - After-hyperpolarization phase can occur because
K gates are slow - Membrane potential even less than resting
membrane potential of -70mV - All gates start to return to resting state when
membrane potential is less than -55mV
91Refractory Periods
- Absolute refractory period
- Second AP not possible
- Na channels open until Na channels inactivated
- During stimulus response and depolarization
- Relative refractory period
- Second AP possible with larger-than-normal
stimulus - Na channels returned to resting state
- K channels still open
- During repolarization and hyperpolarization
92Action Potential
93Concept 12.8Nerve Impulses Travel as a Chain
Reaction of Action Potentials Along an Axon
94Review Location of Gates
Voltage-gated sodium and Potassium Ion channels
Voltage-gated CALCIUM ion channels
Ligand-gated and mechanical-gated channels
95Events in Neural Signaling
Action Potentials
Synapses
Graded Potentials
96Events in Neural Signaling
Action Potential Impulse Propagation
Synapses
Graded Potentials
97Impulse Propagation
- AP continues as depolarization at one gate region
triggers depolarization down axon - If axon is uninsulated by myelin sheath
- impulse continues smoothly down axon
- termed continuous conduction
- If axon is insulated by myelin sheath
- impulse jumps from node to node down axon
- termed saltatory conduction
- Faster than continuous conduction
98Impulse Propagation
99Axon Diameter
- Recall that axons are also called nerve fibers
- Larger fibers propagate impulses faster
- Larger fibers usually myelinated
- Smallest fibers are unmyelinated and therefore
propagate impulses slower
100A Fibers
- Largest-diameter fibers
- 5-20µm
- Myelinated
- Saltatory conduction
- Speeds of 12-130m/sec (27-280 mph!)
- Examples
- Sensory fine touch, joint position
- Motor impulses to skeletal muscles
101B Fibers
- Mid-size diameter fibers
- 2-3µm
- Myelinated
- Saltatory conduction
- Speeds of 15m/sec (32 mph!)
- Examples
- Sensory viscera to CNS
- Motor ANS to cardiac muscle, smooth muscle, and
glands
102C Fibers
- Smallest-diameter fibers
- 0.5-1.5µm
- Unmyelinated
- Continuous conduction
- Speeds of 0.5 to 2m/sec (1-4 mph!)
- Examples
- Sensory viscera to CNS, skin for pain, general
touch, and pressure - Motor ANS cardiac muscle, smooth muscle, and
glands
103Sensory Stimulus Intensity
- Frequency of impulses and receptor recruitment
determines encoding of stimulus intensity - Light pressure generates low frequency impulses
is perceived as low intensity stimulus - High pressure generates high frequency impulses
is perceived as high intensity stimulus - A low number of sensory receptors stimulated
perceived as low intensity stimulus - A high number of sensory receptors stimulated
perceived as high intensity stimulus
104Comparing Electrical Signals
- Graded Potentials
- cell body dendrites
- ligand mechanical gates
- shorter propagation
- variable amplitude
- longer lasting than AP
- hyperpolarizing to depolarizing
- no refractory period
Action Potentials axon hillock axon voltage
gated Na and K longer propagation all-or-none
amplitude shorter lasting than GP depolarization
to repolarization has refractory period
105Concept 12.9The Synapse is a Special Junction
Between Neurons
106Synapse
- Functional junction
- between two neurons or
- between neuron and its effector
- Focus this chapter on neuron-to-neuron
- Site of action for many therapeutic and addictive
chemicals
107Synapse
108Neuronal Synapse
- Neural Synapse relationships
- Axoanonic synapse axon to axon
- Axodendritic synapse axon to dendrite
- Axosomatic synapse axon to cell body
- Presynaptic neuron sends signal
- Postsynaptic neuron receives signal
- Two types
- Electrical Synapses
- Chemical Synapses
109Electrical Synapses
- Direct cell-to-cell communication
- Cell membranes connected by gap junctions
- Act like tunnels to connect cytosol of cells
- Found in smooth and cardiac muscle
- Rare in CNS
- Fast communication
- Synchronized synapses coordinate effector tissue
response
110Chemical Synapses
- Indirect communication between cells
- Cell membranes separated by synaptic cleft
between Presynaptic end bulb and postsynaptic
cell membrane - Neurotransmitter released into cleft
- Post synaptic cell responds with type of
ligand-gated graded potential called postsynaptic
potential - Exhibit synaptic delay of 0.5milliseconds
111Chemical Synapses
- Indirect communication between cells
- Cell membranes separated by synaptic cleft
between Presynaptic end bulb and postsynaptic
cell membrane - Neurotransmitter released into cleft
- Post synaptic cell responds with type of
ligand-gated graded potential called postsynaptic
potential - Exhibit synaptic delay of 0.5milliseconds
112Chemical Synapse
- AP arrives at synaptic end bulb of presynaptic
neuron - Synaptic end bulb voltage gated Ca2 channels
open resulting in fast Ca2 influx - Triggers fusion of synaptic vesicles plasma
membrane and exocytosis of neurotransmitter into
synaptic cleft - Neurotransmitter fuses with receptors on
postsynaptic neuron
113Chemical Synapse
- Postsynaptic neuron ligand-gated channels open
- Graded potential results as ions in synaptic
cleft flow into postsynaptic cell - Depolarization occurs if channels are
ligand-gated Na channels - Hyperpolarization occurs if channels are
ligand-gated Cl- channels - If depolarization reaches threshold, action
potential triggered
114Signal Transmission
115Postsynaptic Potentials
- Excitatory Postsynaptic Potentials (EPSP)
- Usually open ligand-gated Na, K, and Ca2
channels - Na influx faster Ca2 influx or K outflow and
depolarization results - Low intensity graded potential
- Inhibitory Postsynaptic Potentials (IPSP)
- Usually open ligand-gated K or Cl- channels
- hyperpolarizing postsynaptic neuron
- Preventing or inhibiting depolarization
116Summation of Postsynaptic Potentials
- Summation
- Integration of input from thousands of synapses
in CNS - Spatial Summation
- Neurotransmitter released simultaneously by
several presynaptic axon terminals - Temporal Summation
- Neurotransmitter released by single presynaptic
axon terminal two or more times in rapid
succession
117Summation of Postsynaptic Potentials
118Net Summation Effects
- Occurs when both inhibitory and excitatory
effects are received by one postsynaptic neuron - Possible responses include
- EPSP excitatory effect greater than inhibitory
effects but no threshold reached - Nerve impulse excitatory effect greater than
inhibitory effect and threshold reached - IPSP inhibitory effect greater than excitatory
effect and membrane hyperpolarizes
119Neurotransmitter Removal
- Prevents prolonged or excessive influence of
neurotransmitter on postsynaptic neuron or
effector cell - Three mechanisms for removal
- Diffusion of neurotransmitter away from synaptic
cleft - Enzymatic degradation
- Uptake by cells
- Back into presynaptic neuron (reuptake)
- Into nearby neuroglia (uptake)
120Concept 12.10Neurons In The PNS Have Greater
Capacity For Repair And Regeneration Than Those
Of The CNS
121Plasticity of Nervous System
- neuronal plasticity
- new dendrites
- new proteins
- change in synaptic contacts with effectors
- very limited ability to regenerate
- replicate or repair themselves
122Damage and Repair in PNS
- Neuron and nerve repair can occur if
- damaged axons and dendrites are associated with
neurolemma of functional Schwann cell - scar tissue formation does not block path
123Neuronal Repair in PNS
- Wallerian degeneration
- Distal portion of axon myelin sheath
degenerate - Debris phagocytized by macrophages
- Chromatolysis
- Nissl bodies break up into granular masses
- Accelerating RNA and protein synthesis
- Functional Schwann cells undergo mitosis and form
regeneration tube across injured area - Bud of regenerating axon invades regeneration
tube - can grow at rate of about 1.5mm per day
- eventually connects to effector and some sensory
or motor function returns
124Axon Repair in PNS (12.19)
125Neuronal Repair in CNS
- Most injury in CNS is permanent
- Little or no repair of damage to neurons occurs
- Myelination occurs by oligodendrocytes
- which do not form neurolemma
- therefore no regeneration tubes form
- Following axonal damage astrocytes move in
quickly forming barrier scar tissue
126Ongoing Research
- Improve environment for spinal cord axons to
bridge injury gap - Find ways to stimulate dormant stem cells to
replace lost, damaged, or diseased neurons - Develop tissue cultured neurons that can be used
for transplantation purposes.
127End Chapter 12