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james14hunter
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What is the CNS?
The Central Nervous System - The brain and spinal chord
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What is the PNS?
- The Peripheral Nervous System
- Somatic - controls the musculo-skeletal system
- Visceral - controls the body's organs
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What are neurons?
The nerve cells that transfer information within the body.
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Communication by neurons largely consists of two distinct type of signals. What are they?
Long-distance electrical signals, and short distance chemical signals.
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How is information transmitted within neurons?
As an electrical current, consisting of the movement of ions
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In more complex animals, what is the high-order processing of signals is carried out by?
By large groups of neurons organised into a brain or into simple clusters called ganglia
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What are sensory neurons?
Neurons which transmit information s from eyes and other sensons that detect external stimuli or internal conditions (blood pressure, blood CO2 levels)
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What are interneurons?
Neurons which only make local connections - the vast majority of the neurons in the brain.
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What are motor neurons?
Neurons which transmit signals to muscle cells, causing them to contract
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Where are neurons which carry out integration organized?
In the CNS
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Where are neurons which carry information into and out of the CNS?
In the PNS
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Where are most of a neuron's organelles, including its nucleus, located?
In the cell body
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What are dendrites?
The highly branched extensions that receive signals from other neurons - a typical neuron has numerous dendrites
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What are axons?
An extension that transmits signals to other cells - a neuron has only one
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What is an axon hillock?
What typically happens there?
The cone-shaped region of the axon where it joins the cell body - this is typically where the signals that travel down the axon are generate
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What is a synapse?
The junction where each branched end of an axon transmits information to another cell.
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What is a synaptic terminal?
The part of each axon branch that forms a synapse.
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What are neurotransmitters?
Where are they synthesized?
Chemical messengers which pass information from the presynaptic neuron to the receiving cell at the synapse. Either excites or inhibits the postsynaptic neuron
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What is the presynaptic cell?
The transmitting neuron
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What is the postsynaptic cell?
The neuron, muscle, or gland cell that receives the signal.
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Are neurons structurally diverse?
Yes, some interneurons have branched dendrites that take part in about 100,000 synapses whereas other neurons with simpler dendrites have far fewer synapses.
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What are glia?
Supporting cells which may nourish neurons, insulate axons of neurons, or regulate extracellular fluid surrounding neurons.
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What is a myelin sheath?
What are they produced by?
What are they made of?
- It is a layer of electrical insulation that surrounds vertebrate axons.
- They are produced by two types of glia - Oligodendrocyctes in the CNS and Schwann cells in the PNS.
- They are made mostly of lipid, which is a poor conductor of electrical current.
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In what ways can neurones vary?
- Cell body - size and shape
- Dendrites - number, branching, length
- Axon - length, diameter, branching
- Axon - myelinated or unmyelinated
- Synaptic terminals - number and structure
- Synaptic transmission - chemical or electrical
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Are action potentials transmitted from neurons to other cells?
Not in most cases.
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What are electrical synapses?
What are they responsible for?
- Synapses which contain gap junctions which do allow electrical current to flow directly from one neuron to another.
- They synchronize the activity of neurons responsible for certain rapid, unvarying behaviors. e.g. escape responses
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What are chemical synapses?
Synapses which involve the release of a chemical neurotransmitter by the presynaptic neuron. - the majority
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How many synaptic terminals might a cell body and dendrites receive inputs from chemical synapses from?
Hundreds or even thousands.
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What are synaptic vesicles?
They are multiple membrane-bounded compartments into which the neurotransmitters are packaged by the presynaptic terminal after the neurotransmitters are synthesized.
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What is the synaptic cleft?
The narrow gap that separates the presynaptic neuron from the post synaptic cell.
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What happens when an action potential arrives at a synaptic terminal?
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- The action potential's arrival depolarizes the plasma membrane, opening voltage gated channels that allow Ca2+ to diffuse into the terminal
- The rise in CA2+ concentration in the terminal causes some synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter.
- The neurotransmitter then diffuses across the synaptic cleft.
- The neurotransmitter binds to the receptor portion of the ligand-gated ion channels in the postsynaptic membrane, opening the channel.
- The neurotransmitter is released from the receptors and the channel closes.
- Synaptic transmission ends when the neurotransmitter diffuse out of the synaptic cleft, is taken up by the synaptic terminal or by another cell, or is degraded by an enzyme
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What are the most common neurotransmitters?
- Acetylcholine (ACh)
- Glutamate (GLU)
- Noradrenaline (NA)
- GABA (gamma-aminobutyric acid)
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What is a neurone required to do?
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- 1. To be stimulated by a neurotransmitter released by another neurone
- 2. To respond to the stimulus, either by being excited or inhibited
- 3. To convey a message, via an action potential, to its terminal
- 4. To release a neurotransmitter from the terminal which will stimulate another neurone (or muscle cell)
- 5. To inactivate the neurotransmitter that it has released.
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How are peptide neurotransmitters synthesized?
- 1. A precursor peptide is synthesized in the rough ER.
- 2. The precursor peptide is split in the Golgi apparatus to yield the active neurotransmitter
- 3. Secretory vesicles containing the peptide bud off from the Golgi apparatus
- 4. the secretory granules are transported down the axon to the terminal where the peptide is stored.
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How are amine and amino acid neurotransmitters synthesized?
- 1. Enzymes convert precursor molecules into neurotransmitter molecules in the cytosol.
- 2. Transporter proteins load the neurotransmitter into synaptic vesicles in the terminal where they are stored.
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Describe how the shape of a neurone depends on its 'cytoskeleton'.
- Actin filaments - 5nm - Shape of neurone
- Intermediate filaments - 10nm - Axonal 'neurofilaments'
- Tubules - 20nm - axonal neurotubules
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What is Orthograde axonal transport?
- Orthograde transport is movement of molecules/organelles outward, from the cell body (soma) to the synapse or cell membrane.
- Orthograde movement of transport vesicles along the microtubules is mediated by kinesins
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What is retrograde axonal transport?
- Retrograde transport is movement of molecules/organelles inward, away from the synapse or plasma membrane towards the cell body or soma.
- It is mediated by dynein, and is used to send chemical messages back from the axon back to the cell
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Why is a large blood supply crucial for neurones?
Because all the events in neurotransmitter synthesis and release require energy, so neurones need to generate ATP constantly
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In 1mm3 of cerebral cortex, it is estimated that there are...
- 100,000 cells
- 4 kilometers of axon
- 500 metres of dendritess
- 1,000,000 synapses
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What are neuroglial cells?
What types are there?
Glial cells, sometimes called neuroglia or simply glia, are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain and peripheral nervous system.
- Oligodendrocytes
- Astrocytes
- Microglia
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What do Oligodendrocytes do?
They form myelin sheaths for several axons
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What do Astrocytes do?
- They control K+, glutamate, and Ca2+ in the extracellular space.
- Astrocytes adjacent to active neurons cause nearby blood vessels to dilate, increasing blood flow to the area and enabling the neurons to obtain oxygen and glucose more quickly.
- They play important roles in the blood-brain barrier and in local blood flow.
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What are microglia?
They are Microglia are specialized macrophages, capable of phagocytosis, that protect neurons of the central nervous system.
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