Neuron Flashcards
mitochondrion
the structure that performs metabolic activities, providing the energy. Genes different from those in the nucleus.
Ribosomes
the sites within a cell that synthesize new protein molecules. Proteins provide building materials for the cell and facilitate chemical reactions
endoplasmic reticulum
network of thin tubes that transport newly synthesized proteins to other locations
The cell membrane
encase the cell, approx. 8 nm thick, is made of phospholipids which are hydrophobic (thus, insulating), which keeps inside and outside apart, which allows the cell to function and ultimately keeps us alive
soma
cell body, ), which contains nucleus, endoplasmic reticulum, cytoskeleton, mitochondria, golgi apparatus, and other intercellular organelles, suspended in cytoplasm (made up of ions (mostly potassium, sodium, chloride and calcium) and molecules like protein.) neuron itself is situated in extracellular fluids with same types of ions. somas
of neurons range in diameter from 0.005 millimeter (mm) to 0.1 mm in mammals and up to a millimeter in certain invertebrates
Radial glia
guide the migration of neurons and
their axons and dendrites during embryonic development.
When development done, they differentiate into neurons, and a smaller number differentiate into astrocytes and oligodendrocytes
schwann cells
form myelin in peripheral nervous system
Oligodendrocytes
form myelin in central nervous system. Wrap cell membrane around axon during development, squeezes out cytoplasm, leaving lipid bilayer of glial cells sheathing membrane.
Microglial cells
small and irregular shaped phagocytes, act as part of the immune system, removing viruses and fungi from the brain.
They proliferate after brain damage, removing dead or damaged neurons.
Contribute to learning by removing the weakest synapse
Astrocytes
Wrap around the presynaptic terminal and taking up chemicals released by the axon sync activity of closely related neurons = important for rhythm eg. For breathing
Responsible for dilating blood vessels for more nutrients to come into brain tissue
In some brain areas, astrocytes also respond to hormones and thereby influence neurons.
Information processing theory: ”tripartite synapse” the tip of an axon releases chemicals that cause the neighboring astrocyte to release chemicals of its own, thus magnifying or modifying, the message to the next neuron possble contributor to learning/memory
Glial cells
– many different types and different functions: provide structural support and electrical insulation to neurons and modulate neuronal activity. Approx. same amount of glial cells as neurons. Produced throughout the organisms life, in contradiction to neurons.
- Error in glia generation is the main source of abnormal growth in the brain, i.e. brain tumours.
- Glia outnumber neurons in the cerebral cortex, but neurons outnumber glia in several other brain areas
Ramón y Cajal
used Golgi’s method, found neurons where discrete entities
golgi stain
“the black reaction” - impregnating neurons with silver chromate
Dendrites
branches that receive input (through tiny “spines” (tiny knobs attached by necks on dendrites) from other neurons. Vary in form. The greater the surface area of a dendrite, the more information it can receive.
Axon
Output side of neuron, electric signals travel through axon to axon terminals, transmitted through synapses to other neurons. Some form axon collaterals (basically several axons, allow transmission to multiple neurons). Axon covered in myelin, spaces between myelin called nodes of Ranvier
afferent axon
brings (ADMITS) information into a structure (sensory neuron)
efferent axon
carries information away (EXIT) from a structure. (motorneuron)
an interneuron
If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus.
bipolar neuron
has one axon and one dendrite extending from the soma. e.g. sensory neuron
Multipolar neurons
(the most common type of neuron) Each multipolar neuron contains one axon and multiple dendrites. - e.g. motor neuron
Blood brain barrier
BBB is a protective partition between blood vessels and the brain formed by tight junctions between the cells that compose the blood vessels in the brain. The ends of astrocytes attach to blood vessel cells causing the vessels to bind tightly together this prohibits entry an array of substances including toxins in the brain.
• Astrocytes contribute to the structure of the protective partition between blood vessels and the brain. Astrocytes important in maintaining the BBB
Why we need it:
When a virus invades a cell, mechanisms within the cell extrude
virus particles through the membrane so that the immune
system can find them, then the immune system kills the virus and the cell that contain it.
This is fine for e.g. skin cells, easily replicable, but not brain cells (few exceptions, but otherwise no replacement of cells).
BBB to protect vira, chemicals and bacteria from entering the brain.
But some vira CAN pass: rabies virus evades the blood–brain barrier, infects the brain and leads to death. The spirochete responsible for syphilis also penetrates the blood–brain barrier, producing long-lasting and potentially fatal consequences.
How does the blood brain barrier work?
The blood–brain barrier (see Figure 1.11) depends on the endothelial cells that form the walls of the capillaries. Outside the
brain, such cells are separated by small gaps, but in the brain, they are joined so tightly that they block viruses, bacteria, and other harmful chemicals from passage.
But the BBB also keeps out useful chemicals, like amino acids and fuels, which the brain has a special mechanism for transporting.
small, uncharged molecules (e.g. oxygen) and molecules that dissolve in the fats of the membrane cross easily. These include vitamins A and D and all the drugs that affect the brain—from antidepressants and other psychiatric drugs to illegal drugs such as heroin. How fast a drug takes effect depends largely on how readily it dissolves in fats and therefore crosses the blood–brain barrier
water crosses through special protein channels in the wall of the endothelial cells, other chemicals require active transport (a protein-mediated process that expends energy to pump chemicals from the blood into the brain), e.g glucose (the brain’s main fuel), amino acids (the building blocks of proteins), purines, choline, a few vitamins, and iron.
BBB is crucial to health: Alzheimer’s disease or similar conditions, the endothelial cells lining the brain’s blood vessels shrink, and harmful chemicals enter the brain.
But BBB poses difficulty in treating brain cancers pharmacologically as nearly all the drugs used for chemotherapy does not cross BBB.
How are neurons nourished?
Most cells use a variety of carbohydrates and fats for nutrition, but vertebrate neurons depend almost entirely on glucose. Because metabolizing glucose requires oxygen approx 20% of whole body oxygen supply and 25% of glucose supply used in the brain (though brain only makes up 2% of body tissue). Neurons CAN use ketones (a kind of fat) and lactate for
Fuel, but glucose is the only one that crosses the BBB in large quantities.
glucose shortage is rarely a problem, except during starvation. The liver makes glucose from many kinds of carbohydrates and amino acids and glycerol (a breakdown product from fats). A more likely problem is an inability to use glucose, for which the body needs vitamin B1, thiamine. Prolonged thiamine deficiency, common in chronic alcoholism, leads to death of neurons and a condition called Korsakoff’s syndrome, marked by severe memory impairments.
How does neuron signal (internal and external)? (overall, not details)
Info transferred from synapse –> dendrite –> soma –> axon terminals –> synapse –> next cell
Within neuron: information moves because of change in electrical state of neuron.
Between neurons: info transfer mediated by neurotransmitters or electrical currents.
Most neurons are both pre- and postsynaptic.
Resting membrane potential
difference in voltage inside vs outside neuron, dependent on concentration of ions and charged protein molecules - typically -70 mV inside the neuron.
at resting potential there is no net influx/outflux of ions
Inside voltage is more negative since membrane more permeable to K+, more K+ flow out than Na+ flow in – and more Na+ PUSHED out and K+ PUSHED in. The negative inside create the electrical gradient. (opposing the concentration gradient)
The resting potential prepares the neuron to respond rapidly. Excitation of the neuron opens channels that allow sodium to enter the cell rapidly action potential
concentration gradient
concentration gradient = the difference
in distribution of ions across the membrane.
Normally, diffusion of ions happen, areas which a lot will “give some” to areas with less. –> ions flow down concentration gradient, from areas with more of a specific ion, to areas with less.
Membrane “hinders” this (only allows transfer through ion channels and sodium-potassium pump)
transmembrane proteins
channels/pumps facilitating a transfer of ions across membrane. these are ion channels and pumps.
Ion channels
proteins with pore through center, allow ions to flow down concentration gradients - Selective for either Na+, K+, Ca++ and Cl- ions.
- More permeable to K+ than Na+ because there are more K+ selective ion channels, contributing to resting membrane potential since K+ is a positive ion, so when it flows out (due to more channels and there are more K+ on inside normally), positive charge flows out of the cell, creating a more negative inside than outside (-70 mV)
Ion pumps
Active transport proteins - use energy to transport ions from areas with low to high concentration
Action potential
We can hyperpolarize a neuron (increasing the negative inside) or depolarize (bring inside voltage closer to 0) – if depolarization reaches a threshold, this creates a massive depolarization of the membrane. When the potential reaches the threshold, the membrane opens its sodium channels and lets sodium ions flow into the cell. This creates an action potential. Any stimulation to a subthreshold creates a small response with quick decay, but not sufficient for action potential.
All or none law
all action potentials are approximately equal in amplitude
(intensity) and velocity. That is, the intensity of the stimulus cannot cause a neuron to produce a bigger or smaller action potential, or a faster or slower one = all-or-none law = the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it. It is the RATE of action potentials that determines further signaling.
Local Neurons
• Have short (or no) axons, exchange information with only close neighbors, and do not produce action potentials
• When stimulated, produce graded potentials—membrane potentials that vary in magnitude and do not follow the all-or-none law
• Depolarize or hyperpolarize in proportion to the stimulation
A common misconception is, from the focus on large neurons, as they are easier to study, that local/small neurons are “premature” and don’t really do much. The neurons are understudied, so hard to say.
SALTATORY CONDUCTION
Suppose an action potential occurs at the first myelin segment.
The action potential cannot regenerate along the membrane
between nodes because sodium channels are virtually
absent between nodes (Catterall, 1984). After an action potential
occurs at a node, sodium ions enter the axon and diffuse, pushing
a chain of positive charge along the axon to the next node,
where they regenerate the action potential.
PROPAGATION
During action potential: sodium ions enter a point on
the axon. Temporarily, that spot is positively charged in comparison with neighboring areas along the axon. The positive ions flow within the axon to neighboring regions. The positive charges slightly depolarize the next area of the membrane, causing it to reach its threshold and open its voltage-gated sodium channels. Then the membrane regenerates the action potential at that point –> AP travels along axon (saltatory conduction)
How does an action potential occur?
How action potential occur: passive electrical currents sum up at axon hillock, flows across neural membrane of spike-triggering zone, depolarizing membrane. When depolarization makes membrane moves from -70 mV to approx. -55 mV (referred to as threshold for initiating an action potential), an action potential is triggered
(1)Threshold is reached voltage-gated Na+ channels open and Na+ flow in rapidly. further depolarization (2) more Na+ channels open (Hodgkin-Huxley cycle) voltage-gated K+ channels open, K+ flow out down concentration gradient. outflow of positive ions make membrane potential shift back toward resting potential. (3) opening of K+ channels outlast Na+ channels equilibrium potential of K+ (almost same amount of K+ inside and outside) = even more negative than resting potential hyperpolarization (approx. -80mV) (4) K+ channels close, membrane potential slowly returns to resting potential (because of the pumps, potassium is being pumped back in).
