functions of the nervous system 1.sensory input 2.integration 3.motor output
TRANSCRIPT
Nervous System and Nervous Tissue
Chapter 11
Functions of the Nervous System
1. Sensory input2. Integration3. 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 PNS1. 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
Figure 11.2
Central nervous system (CNS)
Brain and spinal cordIntegrative and control centers
Peripheral nervous system (PNS)
Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body
Parasympatheticdivision
Conserves energyPromotes house-keeping functionsduring rest
Motor (efferent) division
Motor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)
Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS
Somatic nervoussystem
Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles
Sympathetic divisionMobilizes bodysystems during activity
Autonomic nervoussystem (ANS)
Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands
StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS
Somatic sensoryfiber
Visceral sensory fiber
Motor fiber of somatic nervous system
Skin
StomachSkeletalmuscle
Heart
BladderParasympathetic motor fiber of ANS
Sympathetic motor fiber of ANS
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
Figure 11.4b
Dendrites(receptive regions)
Cell body(biosynthetic centerand receptive region)
Nucleolus
Nucleus
Nissl bodies
Axon(impulse generatingand conducting region)
Axon hillock
NeurilemmaTerminalbranches
Node of Ranvier
Impulsedirection
Schwann cell(one inter-node)
Axonterminals(secretoryregion)
(b)
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
Table 11.1 (1 of 3)
Structural Classification of Neurons
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 open2. 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
change2. 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
Figure 11.10c
Distance (a few mm)
–70Resting potential
Active area(site of initialdepolarization)
(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.
Mem
bra
ne p
ote
nti
al (m
V)
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
Actionpotential
1 2 3
4
Resting state Depolarization Repolarization
Hyperpolarization
The big picture
1 1
2
3
4
Time (ms)
ThresholdMem
bra
ne p
ote
nti
al (m
V)
Figure 11.11 (1 of 5)
Actionpotential
Time (ms)
1 1
2
3
4
Na+ permeability
K+ permeability
The AP is caused by permeability changes inthe plasma membrane
Mem
bra
ne p
ote
nti
al (m
V)
Rela
tive m
em
bra
ne p
erm
eab
ility
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
Figure 11.13
Threshold
Actionpotentials
Stimulus
Time (ms)
Figure 11.14
Stimulus
Absolute refractoryperiod
Relative refractoryperiod
Time (ms)
Depolarization(Na+ enters)
Repolarization(K+ leaves)
After-hyperpolarization
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
Figure 11.16
Dendrites
Cell body
Axon
Axodendriticsynapses
Axosomaticsynapses
Cell body (soma) ofpostsynaptic neuron
Axon
(b)
Axoaxonic synapses
Axosomaticsynapses
(a)
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
Figure 11.17
Action potentialarrives at axon terminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.
Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.
Ca2+
Synapticvesicles
Axonterminal
Mitochondrion
Postsynapticneuron
Presynapticneuron
Presynapticneuron
Synapticcleft
Ca2+
Ca2+
Ca2+
Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.
Binding of neurotransmitteropens ion channels, resulting ingraded potentials.
Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.
Ion movement
Graded potentialReuptake
Enzymaticdegradation
Diffusion awayfrom synapse
Postsynapticneuron
1
2
3
4
5
6
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 voltage-
gated channels, and cause AP to be generated
© 2013 Pearson Education, Inc.
Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)10 20 30
Mem
bra
ne p
ote
nti
al (m
V)
Excitatory postsynaptic potential (EPSP)
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
© 2013 Pearson Education, Inc.
Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)10 20 30
Mem
bra
ne p
ote
nti
al (m
V) An IPSP is a localhyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.
Inhibitory postsynaptic potential (IPSP)
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
Figure 11.21
Presynaptic(input) fiber
Facilitated zone Discharge zone Facilitated zone
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
Figure 11.23
1
2
3
4
5
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Stimulus
ResponseSpinal cord (CNS)
Interneuron
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