Anatomy and Physiology I
Chapter 12
Nervous Tissue
2 Divisions
• Central Nervous System (CNS)
– Brain and spinal cord
• Peripheral Nervous System
(PNS)
– Everything else
– Composed of nerves and ganglia
– Nerves- carry signals to and from
CNS
– Ganglia- swelling in nerve
PNS
• Sensory Division (afferent)
– Signals from receptors to CNS
– Informs CNS of stimuli
• Somatic sensory- signals
from skin, muscle, bones,
joints
• Visceral sensory- signals from viscera
PNS
• Motor Division (efferent)
– Signals from CNS to glands or
muscles
– Effectors
• Somatic motor- signals to skeletal muscles
– voluntary cx and reflexes
• Autonomic (visceral) motor- signals to glands, cardiac and
smooth muscle
– Involuntary actions
– 2 divisions
PNS
• Autonomic Motor Division
– Sympathetic
• Arouse body for action
– Parasympathetic
• Calming effect
Nervous System
• Electrical and chemical
• 3 steps
– Sensory
• Receives info about environment
• Transmits to CNS
– CNS processes
• Determine response
– Commands issued
• Muscles, glands
Properties of Neurons
• Excitability
– Respond to stimuli
• Conductivity
– Produce electrical signals
• Secretion
– Neurotransmitter
Neuron Classes
• Sensory (afferent) Neurons
– Detect stimuli
– Transmits info to CNS
• Interneurons
– Entirely within CNS
– Receive signals, integrate
signals, determine reaction
– 90%
• Motor (efferent) Neurons
– Signals to muscles or glands
– Carry out response to stimuli (effectors)
Neuron Structure
• Soma- control center (cell body)
– Central nucleus
– Nissl bodies
• Dendrites- receive signals from other
neurons
• Axon hillock- axon originates (mound)
• Axon- rapid conduction of nerve signals
– Away from soma
– Nodes of Ranvier in myelinated fibers
– neurilemma
• Synaptic knob- swelling that forms a
junction
– At end of axon
– Synaptic vesicles- neurotransmitters
Neuroglia
• Supportive cells
• Protect and help function
• 6 types of neuroglia
– Oligodendrocytes
– Ependymal cells
– Microglia
– Astrocytes
– Schwann cells
– Satellite cells
Oligodendrocytes
• Form myelin in CNS
• Arm-like processes
• Spirals around nerve fiber
– Myelin sheath- insulates nerve fiber
Ependymal Cells
• Lines cavities of CNS
• Produce and circulates CSF
• Cilia
Microglia
• Phagocytize and destroy
• Wander through CNS
– Multiple times a day
• Concentrated in areas of infection, trauma,
stroke
Astrocytes
•
•
•
•
•
•
•
•
•
Most abundant- 90%
Found everywhere in CNS
Supportive framework
Blood-brain-barrier
Nourish neurons
Promote neuron growth
Communicate with neurons
Absorb excess neurotransmitters
Form scar tissue
Schwann Cells
•
•
•
•
Only PNS
Envelop nerve fibers
Myelin sheath
Regeneration of damaged fibers
Satellite Cells
•
•
•
•
Only PNS
Surround somas
Electrical insulation
Regulate chemical environment
Myelin
• Myelin sheath- insulating layer around nerve
fiber
– Oligodendrocytes- CNS
– Schwann cells- PNS
• Myelination- production
– Dietary fat important
• Myelin sheath is segmented
• Nodes of Ranvier- gaps in myelin sheath
• Speeds signal conduction
Conduction
• Speed of conduction
– Fiber diameter
– Presence or absence of myelin
– Large fibers- fast
• More surface area
– Myelin- fast (skeletal muscles, sensory signals)
– Unmyelinated- slow (secrete stomach acid, pupil
dilation)
Nerve Fiber Regeneration
• PNS- soma must be intact
• 1. normal nerve fiber
• 2. local trauma
– Fiber distal to injury can’t survive
– Macrophages clean up
• 3. Soma swells
– Axon sprouts growth processes
Nerve Fiber Regeneration
• 4. Schwann cells form
regeneration tube
• 5. Regeneration tube guides
growing sprout to target cell
• 6. Reestablishes synapse
– Soma shrinks
• Not perfect
– Functional deficit post-injury
Electrical Potentials
• Difference in the concentration of charged
particles b/t one point and another
– Produce current- flow of charged particles
• RMP- charged difference across PM
– Unstimulated neuron
RMP
• Electrolytes distributed unequally b/t ICF and
ECF
• 3 factors
– Concentration gradient
– Selective permeability
– Electrical attraction
• NaK Pump- 70% of energy requirement
– Pumps 3 Na out for every 2 K it brings in
– equilibrium
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Action Potentials
• Rapid up and down shift in
membrane voltage
• Resting neuron- polarized
• 1. Sodium enters cell
– Depolarization of the membrane
• 2. Must meet the threshold
– Minimum needed to open gates
• 3. Neuron produces action
potential
– Further depolarizes membrane
Action Potentials
• 4. Voltage peaks
– Positive inside
– Negative outside
• 5. Potassium leaves cell and
repolarization occurs
• 6. Potassium gates stay open
longer
– Membrane potential drops
slightly more negative than
original RMP- hyperpolarization
Action Potential
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Action Potential
• All-or-None Law
– If threshold reached- neuron fires at max voltage
– If threshold not reached- neuron doesn’t fire
– Stronger stimulus does not produce stronger action
potentials
• Nondecremental
– Do not get weaker with distance
• Irreversible
– Threshold reached- action potential completes
– Can’t be stopped
Refractory Period
• Period of resistance to
restimulation
– Impossible or difficult to stimulate
same region
• Absolute
– No stimulus will trigger new action
potential
– Lasts from start of AP until
membrane returns to resting
potential
• Relative
– Unusually strong stimulus will
trigger potential
– Lasts until hyperpolarization ends
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Signal Conduction
• Unmyelinated fibers
– Voltage-regulated gates along entire length
– Action potentials triggers new action potential
distally
• Continues until reaches axon end
• Stimulates next one
• Can’t go backwards
– Slower
– Nondecremental
Signal Conduction
• Myelinated fibers
– Voltage-regulated gates are
scarce
– Conduction is decremental
– Recharges at nodes of Ranvier
• Creates new
action potential
• Saltatory Conductionpropagation of nerve signal
that jumps from node to node
– Fast
Synapses
• Presynaptic neuron- releases neurotransmitter
• Postsynaptic neuron- responds to
neurotransmitter
• Synaptic cleft- gap b/t neurons
Neurotransmitters
• 1. Synthesized by presynaptic neuron
• 2. Released in response to stimulation
• 3. Bind to specific receptors on postsynaptic
cell
• 4. Alter physiology of that cell
• Excitatory/ Inhibitory
Excitatory Cholinergic Synapse
• Acetylcholine (ACh)
• 1. nerve signal arrives at synaptic
knob
– Opens Ca gates
• 2. Ca enter knob- triggers vesicles
– Ach released
• 3. Ach diffuses across synaptic cleft
– Bind to gates on postsynaptic neuron
– Gates open: Na enter, K leave
• 4. Na enters cell, depolarizes it,
triggers postsynaptic potential
http://www.youtube.com/watch?v=LT3VKAr4roo&NR=1
Cessation of Signal
• Turn off stimulus
– Prevents postsynaptic cell from firing indefinitely
– 1. Stop new neurotransmitters
• Cessation of signal
– 2. Get rid of old
• Diffusion- astrocytes absorb
• Reuptake- synaptic knob reabsorbs, breaks down
• Degradation in synaptic cleft- AChE breaks down
ACh
Neural Integration
• Ability of neurons to process information,
store and recall it, and make decisions
• Based on postsynaptic potentials
– Excitatory postsynaptic potential (EPSP)- voltage
change that makes neuron more likely to fire
– Inhibitory postsynaptic potential (IPSP)- makes
neuron less likely to fire
– Summation- process of adding up postsynaptic
potentials and responding to their net effect
Postsynaptic Potentials
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0
mV
–20
–40
Threshold
Repolarization
–80
(a)
Resting membrane
potential
EPSP
–60
Depolarization
Stimulus
Time
0
mV
–20
–40
Threshold
Resting membrane
potential
–60
IPSP
Figure 12.24
–80
Hyperpolarization
(b)
Stimulus
Time
12-41
Summation
• Temporal summation
– ESPSs generated so quickly that a new one is generated
before the old one fades
• Spatial summation
– EPSPs from several synapses add up to threshold
Summation of EPSPs
+40
+20
0
mV
Action potential
–20
Threshold
–40
–60
–80
EPSPs
Resting
membrane
potential
Stimuli
Time
Memory
• Physical basis of memory
– Memory trace- pathway through the brain
• Synapses formed to make transmission easier
• Added, taken away, modified
• 3 kinds
– Immediate memory
– Short-term memory
– Long- term memory
Immediate Memory
• Few seconds
• Flow of events and sense of present
• Read
Short-term Memory
• Few seconds to few hours
• Quickly forgotten if stop mentally reciting it,
distracted, or new
Long-term Memory
• Lifetime
• Declarative memory- retention of events and
facts
• Procedural memory- retention of motor skills