Transcript Power Point
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