NOTES- Nervous System part 2
Neuron Repair, Synapse, &
Neurotransmitters
Repair of Nerve Fibers
– Mature neurons cannot undergo mitosis so damage to
nervous tissue can be permanent
– Neurons have a limited capacity to repair themselves
and can be repaired if the damage is not extensive
• Neurons can repair themselves if the cell body
and neurilemma (cell membrane) remain intact
and scarring has not occurred
• Stages of the Healing Process in the PNS
– An injury occurs resulting in a cut axon
– Immediately after, the distal portion of the axon
degenerates
– Macrophages (immune system cells) move into the
area and remove the debris
– A tunnel is formed by the neurilemma to the effector
– New Schwann cells grow in this tunnel and leave a
path for the axon
– The axon bud begins to regrow in the path
• It will grow 3-5 mm a day
Fig. 3. SAPNS allows axons to regenerate through the lesion site in brain. The dark-field composite photos are
parasagittal sections from animals 30 days after lesion and treatment. (a) Section from brain of 30-day-old hamster
with 10 µl of saline injected in the lesion at P2. The cavity shows the failure of the tissue healing. The retinal
projections, in light green at the top left edge of the cavity, have stopped and did not cross the lesion. Arrows indicate
path and extent of knife cut. (b) A similar section from a 30-day-old hamster with a P2 lesion injected with 10 µl of 1%
SAPNS. The site of the lesion has healed, and axons have grown through the treated area and reached the caudal part
of the SC. Axons from the retina are indicated by light-green fluorescence. The boxed area is an area of dense
termination of axons that have crossed the lesion. Arrows indicate path and extent of knife cut. (c) Enlarged view of
boxed area in b. The regrown axons, shown in white, were traced with cholera-toxin fragment B labeling by using
immunohistochemistry for amplification of the tracer. (Scale bars, 100 µm.)
• During this time, the muscle stimulated by the
damaged nerve atrophies
– If the neuron fails to repair itself, an adjacent neuron
may form a connection with the muscle
• Neuron repair does not take place in the CNS, so
most damage is permanent!
Photo caption: An adult rat nerve cell called a neuron
normally grows or regenerates only poorly (red-colored
nerve cell at top of photo taken through a microscope).
But when a neuroscientist manipulated an integrin gene,
the adult neuron (green) displayed extensive
regenerative growth by extending nerve fibers called
axons (green fibers). Photo credit: Maureen L. Condic,
University of Utah.
SYNAPSE: junction between
a neuron and another cell;
found between:
-2 neurons
-sensory receptor
& sensory neuron
-motor neuron & muscle cell
-neuron & gland cell
Motor Neuron and Muscle Cell
• most synapses are CHEMICAL SYNAPSES
– these cells are separated by a synaptic cleft &
a series of events converts:
elec. signal  chem.signal  elec.signal
• NEUROTRANSMITTERS: intercellular
chemical messengers
Synaptic Transmission Review…
– An action potential (the electrical signal) reaches the
end of the axon/synaptic knob
– The action potential cannot cross the synaptic cleft
• Causes neurotransmitters to be released from vesicles in
the synaptic knob
– The neurotransmitter reaches receptors in the cell of
the postsynaptic membrane and causes a new
impulse to be initiated
There are Different
Neurotransmitters!
• Some will cause an action potential in the
next cell (excitatory)
• Some will inhibit an action potential in the
next cell (inhibitory)
• About 50 different neurotransmitters have
been identified
Examples of neurotransmitters:
Neurotransmitter:
Functions in:
acetylcholine
Neuromuscular junction; stimulates
muscle contraction
epinephrine/
norepinephrine
epin. & norep. also function as
hormones; “fight or flight response”
dopamine/
serotonin
dop. & ser. both affect sleep, mood,
attention, learning; LSD &
mescaline bind to serotonin receptors
endorphins
Decrease perception of pain by CNS
(inhibitory); heroin & morphine mimic
endorphins
histamine
Release in hypothalamus promotes
alertness