Download Nervous System (Ch11)

Functions of the Nervous System
1. Sensory input
2. Integration
3. Motor output
Divisions of the Nervous System
• Central nervous system (CNS)
– Brain and spinal cord
– Integration and command center
• Peripheral nervous system (PNS)
– Paired spinal and cranial nerves carry messages to
and from the CNS
Peripheral Nervous System (PNS)
1. Sensory (afferent) division
•
•
Somatic afferent fibers—convey impulses from
skin, skeletal muscles, and joints
Visceral afferent fibers—convey impulses from
visceral organs
2. Motor (efferent) division
•
Transmits impulses from the CNS to effector
organs
Motor Division of PNS
1. Somatic (voluntary) nervous system
– Conscious control of skeletal muscles
2. Autonomic (involuntary) nervous system
(ANS)
– Visceral motor nerve fibers
– Regulates smooth muscle, cardiac muscle, and
glands
– Two functional subdivisions
•
•
Sympathetic
Parasympathetic
Peripheral nervous system (PNS)
Central nervous system (CNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Brain and spinal cord
Integrative and control centers
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Skin
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body
systems during activity
Sympathetic motor fiber of ANS
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Bladder
Figure 11.2
Histology of Nervous Tissue
1. Neurons—excitable cells that transmit electrical
signals
2. Neruoglia – glial cells
•
•
•
•
•
•
Astrocytes (CNS)
Microglia (CNS)
Ependymal cells (CNS)
Oligodendrocytes (CNS)
Satellite cells (PNS)
Schwann cells (PNS)
Astrocytes
• Most abundant, versatile, and highly branched glial
cells
• Cling to neurons, synaptic endings, and capillaries
• Support and brace neurons
• Help determine capillary permeability
• Guide migration of young
neurons
• Control the chemical
environment
• Participate in information
processing in the brain
Microglia
• Small, ovoid cells with thorny processes
• Migrate toward injured neurons
• Phagocytize microorganisms and neuronal
debris
Ependymal Cells
• Range in shape from squamous to columnar
• May be ciliated
– Line the central cavities of the brain and spinal
column
• Separate the CNS
interstitial fluid from
the cerebrospinal
fluid in the cavities
Oligodendrocytes
• Branched cells
• Processes wrap CNS nerve fibers, forming
insulating myelin sheaths
Satellite Cells and Schwann Cells
• Satellite cells
– Surround neuron cell bodies in the PNS
• Schwann cells (neurolemmocytes)
– Surround peripheral nerve fibers and form myelin
sheaths
– Vital to regeneration of damaged peripheral nerve
fibers
Neurons (Nerve Cells)
• Special characteristics:
– Long-lived ( 100 years or more)
– Amitotic—with few exceptions
– High metabolic rate—depends on continuous
supply of oxygen and glucose
– Plasma membrane functions in:
• Electrical signaling
• Cell-to-cell interactions during development
Cell Body (Perikaryon or Soma)
•
•
•
•
Biosynthetic center of a neuron
Spherical nucleus with nucleolus
Well-developed Golgi apparatus
Rough ER called Nissl bodies (chromatophilic
substance)
• Network of neurofibrils (neurofilaments)
• Axon hillock—cone-shaped area from which axon
arises
• Clusters of cell bodies are called nuclei in the
CNS, ganglia in the PNS
Processes
• Dendrites and axons
• Bundles of processes are called
– Tracts in the CNS
– Nerves in the PNS
Dendrites
• Short, tapering, and diffusely branched
• Receptive (input) region of a neuron
• Convey electrical signals toward the cell body
as graded potentials
The Axon
•
•
•
•
•
One axon per cell arising from the axon hillock
Long axon = nerve fiber
Occasional branches = axon collaterals
Numerous terminal branches
Knoblike axon terminals (synaptic knobs or
boutons)
– Secretory region of neuron
– Release neurotransmitters to excite or inhibit
other cells
Axons: Function
• Conducting region of a neuron
• Generates and transmits nerve impulses
(action potentials) away from the cell body
• Molecules and organelles are moved along
axons by motor molecules in two directions:
– Anterograde—toward axonal terminal
• Examples: mitochondria, membrane components,
enzymes
– Retrograde—toward the cell body
• Examples: organelles to be degraded, signal molecules,
viruses, and bacterial toxins
Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
Myelin Sheath
• Segmented protein-lipoid sheath around most
long or large-diameter axons
• It functions to:
– Protect and electrically insulate the axon
– Increase speed of nerve impulse transmission
Myelin Sheaths in the PNS
• Schwann cells wraps many times around the
axon
– Myelin sheath—concentric layers of Schwann cell
membrane
• Outer collar of perinuclear cytoplasm—
peripheral bulge of Schwann cell cytoplasm
• Nodes of Ranvier
– gaps between adjacent Schwann cells
– Sites where axon collaterals can emerge
Myelin Sheaths in the CNS
• Formed by processes of oligodendrocytes, NOT the
whole cells
• Nodes of Ranvier are present
• No OCPC
• Thinnest fibers are unmyelinated
White Matter and Gray Matter
• White matter
– Dense collections of myelinated fibers
• Gray matter
– Mostly neuron cell bodies and unmyelinated fibers
Structural Classification of Neurons
Table 11.1 (1 of 3)
Functional Classification of Neurons
1. Sensory (afferent)
•
Transmit impulses from sensory receptors toward the
CNS
2. Motor (efferent)
•
Carry impulses from the CNS to effectors
3. Interneurons (association neurons)
•
Shuttle signals through CNS pathways; most are
entirely within the CNS
Neuron Function
• Neurons are highly irritable
• Respond to adequate stimulus by generating
an action potential (nerve impulse)
• Impulse is always the same regardless of
stimulus (Action potential)
Role of Membrane Ion Channels
1. Leakage (nongated) channels—always open
2. Gated channels (three types):
–
–
–
Chemically gated (ligand-gated) channels—
open with binding of a specific
neurotransmitter
Voltage-gated channels—open and close in
response to changes in membrane potential
Mechanically gated channels—open and close
in response to physical deformation of
receptors
Resting Membrane Potential (Vr)
• Potential difference across the membrane of a
resting cell
– Approximately –70 mV in neurons
Membrane Potentials That Act as
Signals
• Membrane potential changes when:
1. Concentrations of ions across the membrane
change
2. Permeability of membrane to ions changes
• Changes in membrane potential are signals
used to receive, integrate and send
information
Membrane Potentials That Act as
Signals
• Two types of signals
– Graded potentials
• Incoming short-distance signals
– Action potentials
• Long-distance signals of axons (outgoing)
Changes in Membrane Potential
• Depolarization
– A reduction in
membrane potential
(toward zero)
• Hyperpolarization
– An increase in
membrane potential
(away from zero)
Graded Potentials
• Short-lived, localized changes in membrane potential
• Depolarizations or hyperpolarizations
• Graded potential spreads as local currents change
the membrane potential of adjacent regions
Membrane potential (mV)
Active area
(site of initial
depolarization)
–70
Resting potential
Distance (a few mm)
(c) Decay of membrane potential with distance: Because current
is lost through the “leaky” plasma membrane, the voltage declines
with distance from the stimulus (the voltage is decremental ).
Consequently, graded potentials are short-distance signals.
Figure 11.10c
Action Potential (AP)
• Brief reversal of membrane potential with a
total amplitude of ~100 mV
• Occurs in muscle cells and axons of neurons
• Does not decrease in magnitude over distance
• Principal means of long-distance neural
communication
The big picture
1 Resting state
3 Repolarization
Membrane potential (mV)
2 Depolarization
3
4 Hyperpolarization
2
Action
potential
Threshold
1
4
1
Time (ms)
Figure 11.11 (1 of 5)
3
2
Action
potential
Na+ permeability
K+ permeability
1
4
1
Relative membrane permeability
Membrane potential (mV)
The AP is caused by permeability changes in
the plasma membrane
Time (ms)
Figure 11.11 (2 of 5)
Coding for Stimulus Intensity
• All action potentials are alike and are
independent of stimulus intensity
• Strong stimuli can generate action potentials
more often than weaker stimuli
• The CNS determines stimulus intensity by the
frequency of impulses
Action
potentials
Threshold
Stimulus
Time (ms)
Figure 11.13
Absolute refractory
period
Relative refractory
period
Depolarization
(Na+ enters)
Repolarization
(K+ leaves)
After-hyperpolarization
Stimulus
Time (ms)
Figure 11.14
Conduction Velocity
• Conduction velocities of neurons vary widely
• Effect of axon diameter
– Larger = faster
• Effect of myelination
– Myelination = faster
Multiple Sclerosis (MS)
• Autoimmune disease that mainly affects young adults
• Symptoms: visual disturbances, weakness, loss of
muscular control, speech disturbances, and urinary
incontinence
• Myelin sheaths in the CNS become nonfunctional
scleroses
• Shunting and short-circuiting of nerve impulses occurs
• Impulse conduction slows and eventually ceases
The Synapse
• A junction that mediates information transfer
from one neuron:
– To another neuron
– To an effector cell
• Electrical or Chemical
• Presynaptic neuron—conducts impulses
toward the synapse
• Postsynaptic neuron—transmits impulses
away from the synapse
Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
(b)
Cell body (soma) of
postsynaptic neuron
Figure 11.16
Electrical Synapses
• Less common than chemical synapses
– Neurons are electrically coupled (joined by gap
junctions)
– Communication = very rapid
• may be unidirectional or bidirectional
– Important in:
• Embryonic nervous tissue
• Some brain regions
Chemical Synapses
• Specialized for the release and reception of
neurotransmitters
• Typically composed of two parts
– Axon terminal of the presynaptic neuron, which
contains synaptic vesicles
– Receptor region on the postsynaptic neuron
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
Axon
terminal
Ca2+
Synaptic
cleft
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
Figure 11.17
Postsynaptic Potentials
• Types of postsynaptic potentials
– EPSP—excitatory postsynaptic potentials
– IPSP—inhibitory postsynaptic potentials
Excitatory Synapses and EPSPs
• Neurotransmitter binding opens chemically gated
channels
• Allows simultaneous flow of Na+ and K+ in opposite
directions
• Na+ influx greater than K+ efflux  net depolarization
called EPSP (not AP)
• EPSP help trigger AP if EPSP is of threshold strength
– Can spread to axon hillock, trigger opening of voltagegated channels, and cause AP to be generated
Membrane potential (mV)
Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
+30
0
Threshold
–55
–70
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
Na+ and K+ to pass
through simultaneously.
Stimulus
10
20
30
Time (ms)
Excitatory postsynaptic potential (EPSP)
© 2013 Pearson Education, Inc.
Inhibitory Synapses and IPSPs
• Reduces postsynaptic neuron's ability to
produce an action potential
– Makes membrane more permeable to K+ or Cl–
• If K+ channels open, it moves out of cell
• If Cl- channels open, it moves into cell
– Neurotransmitter hyperpolarizes cell
• Inner surface of membrane becomes more negative
• AP less likely to be generated
Membrane potential (mV)
Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
+30
0
Threshold
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
that drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
–55
–70
Stimulus
10
20
30
Time (ms)
Inhibitory postsynaptic potential (IPSP)
© 2013 Pearson Education, Inc.
Synaptic Integration: Summation
• A single EPSP cannot induce an AP
• EPSPs and IPSPs can summate to influence
postsynaptic neuron
• Most neurons receive both excitatory and
inhibitory inputs from thousands of other
neurons
– Only if EPSP's predominate and bring to threshold
 AP
Two Types of Summation
• Temporal summation
– One + presynaptic neurons transmit rapid-fire
impulses
• Spatial summation
– Postsynaptic neuron stimulated simultaneously by
large number of terminals at same time
Neurotransmitters
• Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
• 50 or more neurotransmitters have been
identified
• Classified by chemical structure and by
function
Chemical Classes of Neurotransmitters
• Acetylcholine (Ach)
– Released at neuromuscular junctions and some ANS
neurons
• Biogenic amines include:
– Broadly distributed in the brain
– Play roles in emotional behaviors and the biological
clock
• Catecholamines
– Dopamine, norepinephrine (NE), and epinephrine
• Indolamines
– Serotonin and histamine
Chemical Classes of Neurotransmitters
• Amino acids include:
– GABA—Gamma ()-aminobutyric acid
– Glycine
– Glutamate
• Peptides (neuropeptides) include:
• Substance P
– Mediator of pain signals
• Endorphins
– Act as natural opiates; reduce pain perception
• Gut-brain peptides
– Somatostatin and cholecystokinin
Chemical Classes of Neurotransmitters
• Purines such as ATP:
– Act in both the CNS and PNS
– Produce fast or slow responses
– Induce Ca2+ influx in astrocytes
– Provoke pain sensation
Chemical Classes of Neurotransmitters
• Gases and lipids
– Nitric oxide (NO)
• Synthesized on demand
• Involved in learning and memory
– Carbon monoxide (CO) is a regulator of cGMP in
the brain
– Endocannabinoids
• Lipid soluble; synthesized on demand from membrane
lipids
• Involved in learning and memory
Functional Classification of
Neurotransmitters
• Neurotransmitter effects may be excitatory
(depolarizing) and/or inhibitory
(hyperpolarizing)
– Determined by the receptor type of the
postsynaptic neuron
– Acetylcholine
• Excitatory at neuromuscular junctions in
skeletal muscle
• Inhibitory in cardiac muscle
Neurotransmitter Actions
• Direct action
– Neurotransmitter binds to channel-linked receptor
and opens ion channels
– Promotes rapid responses
• Examples: ACh and amino acids
• Indirect action
– Neurotransmitter binds to a G protein-linked receptor
and acts through an intracellular second messenger
– Promotes long-lasting effects
• Examples: biogenic amines, neuropeptides, and dissolved
gases
Neural Integration: Neuronal Pools
• Functional groups of neurons that:
– Integrate incoming information
– Forward the processed information to other
destinations
• Simple neuronal pool
– Single presynaptic fiber branches and synapses
with several neurons in the pool
– Discharge zone—neurons most closely associated
with the incoming fiber
– Facilitated zone—neurons farther away from
incoming fiber
Presynaptic
(input) fiber
Facilitated zone
Discharge zone
Facilitated zone
Figure 11.21
Types of Circuits in Neuronal Pools
•
•
•
•
Diverging circuit
Converging
Reverberating
Parallel after-discharge
Patterns of Neural Processing
• Serial processing
– Input travels along one pathway to a specific
destination
– Works in an all-or-none manner to produce a
specific response
– Example: Reflexes
• rapid, automatic responses to stimuli that always cause
the same response
• Reflex arcs (pathways) have five essential components:
receptor, sensory neuron, CNS integration center,
motor neuron, and effector
Stimulus
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord (CNS)
Response
Figure 11.23
Patterns of Neural Processing
• Parallel processing
– Input travels along several pathways
– One stimulus promotes numerous responses
– Important for higher-level mental functioning
• Example: a smell may remind one of the odor
and associated experiences