Download Chapter 12 – Introduction to the Nervous System

Chapter 12 – Introduction to
the Nervous System
• Organization
• Cell Types
Review
What 3 parts make up the nervous system?
1. Brain
2. Spinal cord
3. Nerves
http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/19588.jpg
Functions of the Nervous System
• Detect changes (stimuli) in the internal or
external environment
• Evaluate the information
• Initiate a change in muscles or glands
Goal – maintain homeostasis
What does this remind you of??
Organization of the Nervous System
• Central nervous system (CNS)
– Brain and spinal cord
• Peripheral nervous system (PNS)
– Nervous tissue in the outer regions of the
nervous system
– Cranial nerves: originates in the brain
– Spinal nerves : originates from the spinal cord
– Central fibers: extend from cell body towards
the CNS
– Peripheral fibers: extend from cell body away
from CNS
http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/8679.jpg
Afferent vs Efferent
Nervous pathways are organized into
divisions based on the direction they carry
information
• Afferent division: incoming information
(sensory)
• Efferent division: outgoing information
(motor)
(Efferent = Exit)
Somatic & Autonomic Nervous
Systems
Nervous pathways are also organized
according to the type of effectors (organs)
they regulate
• Somatic nervous system (SNS)
– Somatic sensory division (afferent)
– Somatic motor division (efferent)
Somatic & Autonomic Nervous
Systems cont…
• Autonomic nervous system (ANS): Carry
information to the autonomic or visceral effectors
(smooth & cardiac muscles and glands)
– Visceral sensory division (afferent)
– Efferent pathways
• Sympathetic division – “fight or flight”
• Parasympathic division – “rest and repair”
Figure 12-2
http://behavioralphys.wikispaces.com/file/view/autonomic%2520nervous%2520system.gif/162748987/autonomic%2520nervous%2520system.gif
Review
What are the two main cell types in the
nervous system?
(Hint: we talked about this when we covered
tissue types)
Answer: neurons and glia
Cells of the Nervous System
Neurons: excitable cells that conduct
information
Glia (also neuroglia or glial cells): support
cells, do not conduct information
– Most numerous
– Glia = glue
Types of Glia
Five major types:
1. Astrocytes
2. Microglia
3. Ependymal cells
4. Oligodendrocytes
5. Schwann cells
Astrocytes (12-3A)
• Star-shaped, largest, most numerous
• Cell extension connect neurons and
capillaries
– Transfer nutrients from blood to neuron
– Help form blood-brain barrier (BBB)
http://astrocyte.info/astrocytes1.jpg
Blood-Brain Barrier
• Helps maintain stable environment for
normal brain function
• “feet” of astrocytes wrap around capillaries
in brain
• Regulates passage of ions
• Water, oxygen, CO2, glucose and alcohol
pass freely
• Important for drug research
– Parkinson’s Disease
Microglia (12-3B)
• Engulf and destroy cellular debris
(phagocytosis)
• Enlarge during times of inflammation and
degeneration
Ependymal cells (12-3C)
• Similar to epithelial cells
• Forms thin sheets that line the fluid-filled
cavities of the brain and spinal cord
• Some cells help produce the fluid that fills
these cavities (cerebral spinal fluid - CSF)
• Cilia may be present to help circulate fluid
http://www.lab.anhb.uwa.edu.au/mb140/corepages/nervous/Images/epen100he.jpg
Oligodendrocytes (12-3D)
• Hold nerve fibers together
• Produce myelin sheaths in CNS
http://4.bp.blogspot.com/_XzEk6ORFLFg/SUQ4IitreiI/AAAAAAAAAD4/XrmtzSv1eGU/s400/article_ms_01.gif
http://blustein.tripod.com/Oligodendrocytes/08-zoom.jpg
Multiple Sclerosis (MS)
• Most common myelin disorder
• Characterized by:
– myelin loss and destruction  injury and
death  plaque like lesions
– Impaired nerve conduction  weakness, loss
of coordination, vision and speech problems
– Remissions & relapses
• Autoimmune or viral infection
• Women 20-40 yrs
• No known cure
Multiple Sclerosis (MS)
http://www.riversideonline.com/source/images/image_popup/ww5r308_big.jpg
Schwann cells (12-3E)
• Only in PNS
• Support nerve fibers & form myelin
sheaths
• Satellite cells (12-3G)
– Types of schwann cell that covers a neuron’s
cell body
http://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image425.gif
Neurons
All neurons have 3 parts:
1. Cell body (soma)
2. Axon
3. One or more dendrites
Neuron Anatomy
• Soma resembles other cells
• Nissl bodies – part of rough ER; contain
proteins necessary for nerve signal
transmission & nerve regeneration
• Dendrites – branch out from soma;
receptors; conduct impulse towards soma
• Axon – process that extends from the soma
at a tapered portion called the axon hillock
– Axon collaterals: side branches
– Telodendria: distal branches of axon
– Synaptic knob: ends of telodendria
http://academic.kellogg.edu/
herbrandsonc/bio201_mckin
ley/f143a_structures_in_a__c.jpg
Neuron Anatomy
• Myelin sheaths: areas of insulation
produced by Schwann cells; increases
speed of nerve impulse
– Myelinated = white matter
– Unmyelinated = gray matter
• Nodes of Ravier: breaks in myelin sheath
btwn Schwann cells
• Synapse: junction btwn two neurons or
btwn a neuron and an effector
http://academic.kellogg.edu/
herbrandsonc/bio201_mckin
ley/f143a_structures_in_a__c.jpg
Structural Classification of Neurons
• Multipolar
– One axon, several dendrites
– Most numerous
• Bipolar
– One axon, one dendrite
– Least numerous
– Retina, inner ear, olfactory pathway
• Unipolar
– Axon is a single process that branches into a central
process (towards CNS) and a peripheral process
(towards PNS)
– Dendrites at distal end of peripheral process
– Always sensory neurons
http://www.google.com/im
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http://www.google.com/imgres?imgurl=http
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Functional Classification of
Neurons
• Afferent
– Sensory
– Towards CNS
• Efferent
– Motor
– Towards muscles & glands
• Interneurons
– Connect afferent & efferent neurons
– Lie within CNS
Reflex Arc
Examples of Reflex Arcs
• Ipsilateral
• Contralateral
• intersegmental
Nerves vs Tracts
• Nerves – bundles of parallel neurons held
together by fibrous CT in the PNS
• Tracts – bundles of parallel neurons in the
CNS
Nerve Fibers
• Remember the difference between nerves
and tracts?
– Endoneurium: surrounds each nerve fiber
– Perineurium: surrounds fascicles (bundles of
nerve fibers
– Epineurium: surrounds a complete nerve
(PNS) or tract (CNS)
Review: Gray vs White Matter
• White matter – myelinated nerve fibers
– Myelin sheaths help increase the speed of an
action potential
• Gray matter – unmyelinated nerve fibers &
cell bodies
– Ganglia: regions of gray matter in PNS
Nerve Fiber Repair
•
•
Nervous tissue has a limited repair
capacity b/c mature neurons are
incapable of cell division
Repair can take place if soma and
neurilemma remain intact
Steps of Nerve Fiber Repair
1. Injury
2. Distal axon and myelin sheaths
degenerates
3. Remaining neurilemma & endoneurium
forms a “tunnel” from the injury to the
effector
4. Proteins produced in the nissl bodies
help extend a new axon down the tunnel
to the effector
Nerve Impulses
• Neurons are specialized to initiate and
conduct signals  nerve impulses
– Exhibit excitability & conductivity
– Nerve impulse  wave of electrical fluctuation
that travels along the plasma membrane
Membrane Potentials
• Difference in charges across the plasma
membrane
– Inside slightly negative
– Outside slightly positive
• Result in a difference in electrical charges
 membrane potential
– Stored potential energy
– Analogy = water behind a dam
Membrane Potentials
• Membrane potential creates a polarized membrane
– Membrane has – pole & + pole
• Potential difference of a polarized membrane is
measured in millivolts (mV)
– The sign indicates the charge of the inside of a polarized
membrane
Resting Membrane Potential (RMP)
• When not conducting electrical signals, a
membrane is “resting”
– -70mV
• RMP maintained by ionic imbalance
across membrane
– Sodium-Potassium Pump
• Pumps 3 Na+ out for every 2 K+ pumps in
• Creates an electrical gradient (more positive on
outside)
Resting Membrane Potential (RMP)
Local Potential
• Local potential - The slight shift away from the
RMP
– Isolated to a particular region of the plasma
membrane
• Stimulus-gated Na+ channels open  Na+
enters  membrane potential to moves closer to
zero (depolarization)
• Stimulus-gated K+ channels open  K+ exits 
membrane potential away from zero
(hyperpolarization)
• **Local potentials do not spread to the end of the
axon**
Local Potentials
Action Potentials
Definitions:
• Membrane potential of an active neuron
(one that is conducting an impulse
• Action potential = nerve impulse
• An electrical fluctuation that travels along
the plasma membrane
Steps of Producing an Action
Potential (table 12-1)
1. A stimulus triggers stimulus-gated Na+
channels to open  Na+ diffuses inside the
cell  depolarization
2. Threshold potential is reached (-59mV) 
voltage-gated Na+ channels open 
depolarization continues
3. Action potential peaks at +30mV, voltagegated Na+ channels close
4. Voltage-gated K+ channels open  K+
diffuses outward  repolarization
5. Brief period of hyperpolarization (below 70mV)  RMP is restored by Na+/K+ pump
Refractory Period
Refractory Period
• Period of time where the neuron resists
restimulation (AP cannot fire)
– Absolute refractory period: half a millisecond
after membrane reaches threshold potential
• Will not respond to ANY stimulus
– Relative refractory period: few milliseconds
after absolute refractory period (during
repolarization)
• Only respond to VERY strong stimulus
Refractory Period – What does this
mean?
• Greater stimulus = quicker another action
potential can take place
• The magnitude of the stimulus does not
affect the magnitude of the AP
– b/c APs are “all or nothing”
– Does cause proportional increase in
frequencies of impulses
Conduction of an Action Potential
• During the peak of an AP, the polarity
reverses
– Negative outside, positive inside
– Causes impulse to travel from site of AP to
adjacent plasma membrane
– No fluctuation in AP due to “all or nothing”
principle
– AP cannot travel backwards on axon due to
refractory periods
Conduction of an Action Potential
How does myelin sheaths affect the speed
of an action potential?
• Sheaths prevent movement of ions
• Electrical changes can only take place at
Nodes of Ranvier
• APs “leap” from node to node (current
flows under sheaths)
• Saltatory conduction
Random Facts
• In nerve fibers that innervate skeletal
muscle, impulses travel up to 130 m/s
(300 mph)
• Sensory pathways from skin  0.5 m/s
(<1 mph)
• Many anesthetics block the sensation of
pain by inhibiting opening of Na+ channels
Types of Synapses
Electrical
synapses: two
cells joined end
to end by gap
junctions
Ex: btwn cardiac
muscle cells,
smooth muscles
cells
Types of Synapses
Chemical synapses: use
neurotransmitter to send a
signal from a presynaptic cell
to postsynaptic cell
3 Parts:
1. Synaptic knob
2. Synaptic cleft
3. Plasma membrane
of postsynaptic
neuron
Mechanisms of Synaptic
Transmission
1. AP depolarizes synaptic knob
2. Voltage-gated Ca2+ channels open 
Ca2+ diffuses inside the cell
3. Ca2+ triggers exocytosis of
neurotransmitter vesicles
4. NTs diffuses across synaptic cleft  bind
w/ receptors on postsynaptic cell
Postsynaptic Potentials (Fig 12-22)
• Excitatory NTs cause Na+ and K+
channels to open  depolarization 
excitatory postsynaptic potential (EPSP)
• Inhibitory NTs cause K+ and Cl- channels
to open  hyperpolarization  inhibitory
postsynaptic potential (IPSP)
Summation
• For every postsynaptic cell there are
usually 1K-100K synaptic knobs
• Both excitatory & inhibitory NTs are
released
– Summation of local potentials (EPSP & IPSP)
occur at axon hillock
• EPSP > IPSP  reach threshold  action
potential
• EPSP < IPSP  threshold not reached  no AP
Neurotransmitters
Small-Molecule Transmitters:
1. Acetylcholine
2. Amines
•
•
•
•
Serotonin
Dopamine
Epinephrine
Norepinephrine
3. Amino Acids
•
•
•
Glutamate
GABA
Glycine
Large-Molecule Transmitters:
1. Neuropeptide
•
Endorphins