BI0005 - Lecture 1 - nerves
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What is the CNS?
The Central Nervous System - The brain and spinal chord
What is the PNS?
- The Peripheral Nervous System
- Somatic - controls the musculo-skeletal system
- Visceral - controls the body's organs
What are neurons?
The nerve cells that transfer information within the body.
Communication by neurons largely consists of two distinct type of signals. What are they?
Long-distance electrical signals, and short distance chemical signals.
How is information transmitted within neurons?
As an electrical current, consisting of the movement of ions
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
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)
What are interneurons?
Neurons which only make local connections - the vast majority of the neurons in the brain.
What are motor neurons?
Neurons which transmit signals to muscle cells, causing them to contract
Where are neurons which carry out integration organized?
In the CNS
Where are neurons which carry information into and out of the CNS?
In the PNS
Where are most of a neuron's organelles, including its nucleus, located?
In the cell body
What are dendrites?
The highly branched extensions that receive signals from other neurons - a typical neuron has numerous dendrites
What are axons?
An extension that transmits signals to other cells - a neuron has only one
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
What is a synapse?
The junction where each branched end of an axon transmits information to another cell.
What is a synaptic terminal?
The part of each axon branch that forms a synapse.
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
What is the presynaptic cell?
The transmitting neuron
What is the postsynaptic cell?
The neuron, muscle, or gland cell that receives the signal.
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.
What are glia?
Supporting cells which may nourish neurons, insulate axons of neurons, or regulate extracellular fluid surrounding neurons.
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.
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
Are action potentials transmitted from neurons to other cells?
Not in most cases.
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
What are chemical synapses?
Synapses which involve the release of a chemical neurotransmitter by the presynaptic neuron. - the majority
How many synaptic terminals might a cell body and dendrites receive inputs from chemical synapses from?
Hundreds or even thousands.
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.
What is the synaptic cleft?
The narrow gap that separates the presynaptic neuron from the post synaptic cell.
What happens when an action potential arrives at a synaptic terminal?
- 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
What are the most common neurotransmitters?
- Acetylcholine (ACh)
- Glutamate (GLU)
- Noradrenaline (NA)
- GABA (gamma-aminobutyric acid)
What is a neurone required to do?
- 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.
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.
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.
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
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
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
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
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
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.
What do Oligodendrocytes do?
They form myelin sheaths for several axons
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.
What are microglia?
They are Microglia are specialized macrophages, capable of phagocytosis, that protect neurons of the central nervous system.
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