Download Human Anatomy & Physiology I

Nervous Tissue
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Unit 1
Chapter 9
Structures of Nervous System
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• Brain- Neurons enclosed in skull
• Spinal cord– connects to brain & enclosed in
spinal cavity
• Nerves- bundles of neuronal axons
• Ganglia- groups of cell bodies outside brain &
spinal cord
• Enteric plexuses- networks in digestive tract
• Sensory receptors- monitor changes in internal
or external environments
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Cranial emerge from brain; spinal nerves- emerge from spinal
cord
Figure 9.1
Function
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• Sensory Receptors & afferent
nerves
Carry information into brain & spinal cord
Perception = awareness of sensory input
Carry by short interneurons
• Motor activity- efferent nerves
Signals to glands and muscles (effectors)
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• Integration- information processing
• Central Nervous System (CNS)
• Peripheral Nervous System (PNS)
• Subdivided: Somatic (SNS) &
Autonomic (ANS) nervous systems
• Also
• INPUT-Afferent or Sensory division
• OUTPUT- Efferent or Motor division
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Organization
Figure 9.2
Nervous System
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• Neuron= nerve cell
Specialized for signal carrying & information
processing
Neuroglia critical for homeostasis of interstitial
fluid around neurons
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• Neuroglia cells-support, nourish &
protect neurons
• Cell body- nucleus, cytoplasm with
typical organelles
• Dendrites- highly branched input
structures emerging from cell body
• Axon- conducts away from cell body
toward another neuron or effector
Emerges at cone-shaped axon hillock
• Axon terminals -at end of axon with
synaptic bulbs
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Neuronal Structure
Figure 9.3
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Myelination
• Axons covered with a myelin sheath
Many layered lipid & protein creating insulations
Increases speed of nerve conduction.
• Nodes of Ravier= gaps in the myelin
• Some diseases destroy myelinE.g. multiple sclerosis & Tay-Sachs
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Nodes are important for signal conduction
Gray and White Matter
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• White matter- primarily myelinated
axons
• Gray matter- cell bodies, dendrites,
unmyelinated axons, axon terminals &
neuroglia
• spinal cord gray matter is centrally
located
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• Gray matter in brain covers surface
of cerebrum & cerebellum – cortex
• deep cluster of neuronal cell bodies
= nucleus
• Bundle of white matter in CNS=
Tract
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Other terms
• ~ half the volume of CNS
• Cells smaller than neurons
• Can multiply and divide and fill in
brain areas
• Do not conduct nerve impulses
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Neuroglia
• Astrocytes- blood brain barrier
• Oligodendrocytes- myelin in CNS
• microglia - defense
• Ependymal cells- CSF production
• Schwann- PNS cell support
• Satellite cells- in PNS ganglia
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Neuroglia-support
Action Potentials
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• Action potentials = nerve impulses
• Require a membrane potential
• Ion Channels- allow ions to move by
diffusion = current
• If no action potential then resting cell
has resting membrane potential
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electrical charge difference across cell
membrane – like a Battery
Ion Channels
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• Allow specific ions to diffuse across
membrane
• Leakage channels
• Gated channels- require trigger to
open
• Voltage- Gated channels respond
to a change in membrane potential
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Move from high concentration to low
or toward area of opposite charge
• Leakage channels
• Cytosol high in K+ & interstitial fluid
high in Na+ (sodium –potassium pumps)
• Leakage lets K+ through easily and
Na+ poorly
• inside is negative relative to outside
• actual value depends on the relative
leakage channel numbers
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Resting Membrane Potential 9
Figure 9.4
Action Potential (AP)
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• Series of active events
• Channels actively open & close
• Some initial event is required to
reach a voltage threshold (~ = - 55
mv)
• Stimulus = any event bringing
membrane to threshold
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Action Potential
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• Then
• Depolarizing phasemembrane potential rises and becomes
positive
potential restored to resting value
May overshoot =hyperpolarizing phase
Then recovery to rest.
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• Repolarizing phase-
• Stimulus to reach threshold
• Na+ channel opens=>
• Na+ ions enter=>
• positive potential=>
• Causes K+ channel opening =>
• repolarization
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Active Events
All- or -None
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• This sequence is always the same
• If threshold then the same size of
changes occur no larger or smaller APs
• Stimulus must reach threshold to start
• After one AP there is a short period
before next can be triggered=
refractory period
Figure 9.5
• Each section triggers next locally
• Refractory period keeps it going the
right direction
• unmyelinated fiber- continuous
conduction
• With myelin- saltatory conduction
Can only be triggered at Nodes of Ranvier
• Myelinated fibers faster & larger
neurons faster
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Conduction of Nerve
Impulses
Figure 9.6a
Figure 9.6b
• Sequence of events at synapse
• Triggered by voltage change of the
Action Potential
• Sending neuron = presynaptic
• Receiving neuron = postsynaptic
• Space between = synaptic cleft
• Neurotransmitter carries signal
across cleft
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Synaptic Transmission
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Events at Synapse
• AP arrives at presynaptic end bulb=>
• Opens voltage gated Ca2+ channels=>
• increased Ca2+ concentration =>
• exocytosis of synaptic vesicles=>
• Neurotransmitter released into cleft
• Diffuse across and bind to receptors
in postsynaptic cell membrane
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Ca2+ flows into cell
• Binding at receptors
• Chemical trigger of ion channels
• May depolarize or hyperpolarize
postsynaptic cell membrane
• If threshold reached at axon hillock
then postsynaptic cell action potential
results
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Synaptic Transmission
• Finally the neurotransmitter must be
removed from the cleft• Diffusion away
• Destroyed by enzymes in cleft
• Transport back into presynaptic cell
• Neuroglia destruction
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Synaptic Transmission
Figure 9.7
Neurotransmitters
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• AcetylCholine (Ach)- common in PNS
May be stimulatory or inhibitory
• Amino Acids• Modified amino acidsNorepinephrine (NE), Dopamine (DA), serotonin
• Neuropeptides – endorphins
• Nitric oxide (NO)
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Glutamate, Aspartate, gamma aminobutyric acid
(GABA), glycine