Transcript Ch. 48nervous system

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Chapter 48 ~
Nervous System
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Nervous System Overview
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Sensory Input
Integration
Motor Output-signal conducted from processing
center to effector cells
Signals Conducted by Nerves-extensions of nerve
cells
Nervous System Composition:
Neurons and Glia (supporting cells)
Neurons communicate information via electrical and
chemical signals
Both Divisions of the Nervous
System Involved
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1. Central nervous
system (CNS)~ brain and
spinal cord; Integration
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2. Peripheral nervous
system (PNS)~ sensory
(input) and motor neurons (output)
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Effector cells~ muscle or
gland cells
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Nerves~ bundles of neurons
wrapped in connective tissue
Neuron structure
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Neuron- structural and functional unit
– Cell body- nucelus and organelles
– Dendrites- signals to cell body. Short, numerous
– Axons- away from cell body. Long,
 Myelin sheath- supporting, insulating layer produced by Schwann Cells
 Schwann cells-PNS support cells; surround axons
 Axon hillock-Hillock-axon extends from here
 Synaptic terminals~ neurotransmitter releaser
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Synapse- gap / neuron junction
3 Classes of neurons
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1. Sensory neuron: receive & convey from sensory
environment information to spinal cord
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2.Interneurons: information integration; located in
CNS. Synapse only with other neurons.
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3. Motor neurons: convey impulses from CNS to
effector cell. (muscle or gland)
Neurons Grouped into Nerve
Circuit
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The Reflex Arc
– Simplest :
– Knee-Jerk Reflex (Patellar
Reflex)
– Stretch receptor
– simple response; sensory
to spinal cord to motor
neurons—knee contracts
Neural Signaling
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Signal transduction depends on voltages across neuron plasma
membranes.
– Membrane Potential: voltage differences across the plasma membrane).
 Net negative charge of about -70mV
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Ions
Intracellular ( -) ; K+ principal cation
Large organic ions- anions
Extracellular (less negative) Na+- principal cation
Cl- main anion.
Ion channels- ungated, gated; all selective
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K+ diffuses out (Na+ in); large anions cannot follow….selective
permeability of the plasma membrane
Creating & Maintaing the
Membrane Potential
Na + - K + Pumps --pump against their conc. gradients
ATP
K+ pumped back in
Na+ pumped back out
Changes in membrane potential
key to neural transmission
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Only neurons and muscle cells can change their membrane
potentials in response to stimuli
– Excitable Cells
– Sensory neurons-environmental stimuli
– Interneurons stimuli transmitted via other neurons
– Resting Potential: M.P. of excitable cell at rest.
– Change due to flow of ions as gated ion channels open.
– stimuli cause ion channels to open
 Stimuli that open K+ channels HYPERPOLARIZE the
neuron
 Stimuli that open NA+ channels DEPOLARIZE the
neuron
Graded Potentials –these voltage changes
1- Hyperpolarization (outflow of K+);
increase in electrical gradient; cell becomes
more negative
 2- Depolarization (inflow of Na+); reduction
in electrical gradient; cell becomes less
negative
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Mylenation
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Electrical insulation—lipid is poor
conductor
– Increasing speed of nerve impulse
propagation
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Multiple Sclerosis: myelin sheaths
deteriorated-los of coordination
Normal Membrane Potential
Resting Potential: Resting Neuron -70 mV
 Cytoplasm is negatively charged relative to cell
interior
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Resting potential~ the membrane potential o
the unexcited nerve.
– A change in voltage MAY result in an
electrical impulse.
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When the Threshold potential is reached,
usually sl. More positive (-50 to -55 mV)….
The action potential is triggered….
– The rapid change in membrane
potential in an excitable cell
– b/c stimulus triggered the selective opening
and closing of voltage-gated ion channels
Action PotentialAll Or None change in the Membrane Potential
Phases
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1. Resting stage •both channels
closed
2-Depolarization: •a stimulus opens
some Na+ channel gates
Na+ influx reverses membrane polarity.
Threshold reached. (cell interior sl.
positive)
Action potential generated .
3-Repolarization •Na+ channels close.
K+ channels open; K+ leaves
 cell returns to resting potential—then ..
 4-Undershoot •K+ channels still opentemporarily HYPERPOLAR.
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The Action Potential
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Followed by a Refractory period~
insensitive to stimulus.
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Amplitude not affected by stimuli
Intensity
Action Potentials are self-propagating
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Action Potential regenerated along axon membrane
begins at Axon Hillock
“Travel” of the action potential is self-propagating
One direction only.
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Nodes of Ranvier-action potential jumps from one node to the next
– Gaps, ion sensitive channels concentrated here, extracellular fluid
contact here
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Forward direction only
Action potential speed:
1) Axon diameter (larger = faster; 100m/sec)
2) Saltatory Conduction:
– Mylenation
– Nodes of Ranvier (concentration of ion
channels in gaps of the myelin).
– A.P. “jumps” from node to node. 120m/sec
Chemical or Electrical Communication
between cells occurs at synapses
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Synapse-tiny gap
Presynaptic cell: transmitting cell
Postsynaptic cell: receiving cell
1) Electrical Synapses-via gap junctions; no
delay or less in signal strength; less common; fish
tail-swim away quickly from predator
2) Chemical Synapses: synaptic cleft separates
pre and post-synaptic cells.
Not electrically coupled
Synaptic communication
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Synaptic cleft: separation gap
Synaptic vesicles: neurotransmitter
releasers
When an Action Potential arrives at
synaptic terminal of presynaptic cell
 Causes Ca++ influx;
 Synaptic vesicles fuse with
presynaptic membrane and release….
 Neurotransmitter
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Neurotransmitters quickly degraded
Neurotransmitter may
do one of the following
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1. Excite the membrane by
depolarization
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Or
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2. Inhibit the postsynaptic cells by
hyperpolarization
Types of Neurotransmitters
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Acetylcholine (most common)
– may be excitatory or inhibitatory
– skeletal muscle
Biogenic amines (derived from amino acids)
•norepinephrine , epinephrine
•dopamine
•serotonin (from tryptophan)
Amino acids
– GABA—most abundant inhibitory transmitter in
brain
Neuropeptides (short chains of amino acids)
•endorphin-natural analgesics for the brain
Gaseous Signals of the Nervous
System
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NO (nitric oxide)—blood vessel dilation.
– Acetylcholine stimulates blood vessel
walls to release NO; neighboring smooth
muscles relax & dilate heart’s blood
vessels.
– Nitroglycerine is converted to NO—similar
response
Nervous system organization
tends to corrolate with body
symmetry
Vertebrate PNS
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Cranial nerves (brain origin)
Spinal nerves (spine origin)
Sensory division
Motor division
•somatic system
voluntary, conscious control
•autonomic system
√parasympathetic
conservation of energy
√sympathetic
increase energy consumption
The Vertebrate Brain
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Forebrain
•cerebrum~memory, learning,
emotion
•cerebral cortex~sensory
and motor nerve cell bodies
•corpus callosum~connects left
and right hemispheres
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•thalamus; hypothalamus
Midbrain
•inferior (auditory) and superior
(visual) colliculi
Hindbrain
•cerebellum~coordination of movement
•medulla oblongata/ pons~autonomic,
homeostatic functions