Chapter 2 – Nerve Cells And Nerve Impulses Flashcards
Cells that receive information and transmit it to other cells
Neurons
It is estimated that the adult human brain contains approximately 100 billion neurons but the exact number varies from person to person.
Structure that separates the inside of the cell from the outside environment
Membrane
Composed of two layers of fat molecules that are free to flow round one another. Most chemicals cannot cross the membrane, but specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals
Structure that contains the chromosomes
Nucleus
All animal cells have a nucleus except for mammalian red blood cells
Structure that performs metabolic activities in a cell
Mitochondrion
Provides the energy that the cell requires for all other activities. Require fuel and oxygen to function
Sites for cell synthesization of new protein molecules
Ribosomes
Network of thin tubes that transport newly synthesized proteins to other locations
Endoplasmic reticulum
What are the components of larger neurons?
Dendrites, a soma or cell body, an axon, and presynaptic terminals
Neuron that receives excitation from other neurons and conducts impulses to a muscle
Motor neuron
Has its soma in the spinal cord. Receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle
Neuron that is highly sensitive to a specific type of stimulation
Sensory neuron
Stimulation can be light, sound, or touch. Tiny branches lead directly from the receptors into the axon, and the cells soma is located on a little stalk off the main trunk
Branching fibres from a neuron that receive information from other neurons
Dendrites
The branching fibres get narrower near their ends. Comes from the Greek root meaning tree. The surface is lined with specialized synaptic receptors at which the dendrite receives information from other neurons. The greater the surface area of a dendrite, the more information it can receive
Short outgrowths that increase the surface area available for synapses
Dendritic spines
Thin fibre of constant diameter; the neuron’s information centre
Axon
Conveys an impulse toward other neurons or an organ or muscle
Structure containing the nucleus, ribosomes, and mitochondria
Cell body or soma
Most of the metabolic work of a neuron occurs here
Insulating material that covers vertebrate axon
Myelin sheath
Interruptions in the myelin sheath of vertebrate axons
Nodes of Ranvier
point where an axon releases chemicals
Presynaptic terminal
End bulb or bouton
Axon that brings information into a structure
Afferent axon
Neuron that carries information away from a structure
Efferent axon
Every sensory neuron is an afferent to the rest of the nervous system, and every motor neuron is an efferent from the nervous system. Within the nervous system, a given neuron is an efferent from one structure and then afferent to another.
Neuron whose axons and dendrites are all confined within a given structure
Interneuron or intrinsic neuron
For example, and intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus
Describe variations among neurons
Neurons vary enormously in size, shape, and function. The shape of a given neuron determines its connections with other neurons and thereby determines its contribution to the nervous system. Neurons with wider branching connect with more targets.
The function of a neuron relates to its shape.
Type of cell in the nervous system that, in contrast to neurons, does not conduct impulses over long distances
Glia
Derived from a Greek word meaning glue. Reflects early investigators ideas that glia were like glue that held the neurons together
Star-shaped glia that synchronize the activity of the axons
Astrocytes
Wrap around the presynaptic terminals of a group of functionally related axons. By taking up ions released by axons and then releasing them back to axons, and astrocyte help synchronize the activity of the axons, enabling them to send messages in waves. Also remove waste material created when neurons die and control the amount of blood flow to each brain area. Dilate the blood vessels during periods of heightened activity in some brain areas to bring more nutrients into that area
Cells that remove waste material and other micro organisms from the nervous system
Microglia
Very small cells, also remove waste material as well as viruses, fungi, and other microorganisms. Function like part of the immune system
Glia cells that build myelin sheaths. In the brain and spinal cord
Oligodendrocytes
Glia cells that build myelin sheaths. In the periphery of the body
Schwann cells
Cells that guide the migration of neurons and the growth of axons and dendrites during embryological development
Radial glia
When embryological development finishes, most radial glia differentiate into neurons, and a smaller number differentiate into astrocytes and oligodendrocytes
Mechanism that excludes most chemicals from the brain
Blood-brain barrier
A protein-mediated process that expends energy to enable a molecule to cross the membrane
Active transport
A B1 vitamin necessary to use glucose
Thiamine
Why do we need a blood-brain barrier?
When a virus invades a cell, mechanisms within the cell extrude virus particles through the membrane so that the immune system can find them, kill it, and the cell that contains it. Because the vertebrate brain does not replace damaged neurons, to minimize the risk of irreparable brain damage, the body builds a wall along the sides of the brain’s blood vessels which keeps out most viruses, bacteria, and harmful chemicals.
How does the blood-brain barrier work?
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 virtually nothing passes between them. This barrier keeps out useful chemicals as well as harmful ones. Useful chemicals include all fuels and amino acids, the building blocks for proteins. For the brain to function, it needs special mechanisms to get these chemicals across the blood-brain barrier.
Identify one major advantage and one disadvantage of having a blood-brain barrier
The blood-brain barrier keeps out viruses, which is an advantage. It also keeps out most nutrients, a disadvantage.
Which chemicals cross the blood-brain barrier passively
Small, uncharged molecules such as oxygen, carbon dioxide, and water cross the blood-brain barrier passively. So do chemicals that dissolves in the fats of the membrane such as vitamins A and D and all the drugs that affect the brain
Which chemicals cross the blood-brain barrier by active transport?
Glucose the brains main fuel, amino acids the building blocks of proteins, purines, choline, a few vitamins, iron, and certain hormones
A simple sugar. Vertebrate neurons depend almost entirely on it.
Glucose
Because the metabolic pathway that uses glucose requires oxygen, neurons need a steady supply of oxygen. The brain uses about 20% of all the oxygen consumed in the body.
Neurons depend so heavily on glucose because it is practically the only nutrient that crosses the blood-brain barrier after infancy, except for ketones.
To use glucose, the body needs vitamin B1, thiamine
Which kind of glia cells wrap around the synaptic terminals of axons?
Astrocytes
Difference in electrical charges between the inside and outside of the cell
Electrical gradient or polarization
The difference in voltage in a resting neuron where the neuron inside the membrane has a slightly negative electrical potential with respect to the outside, mainly because of negatively charged proteins inside the cell
Resting potential
Ability of some chemicals to pass more freely than others through a membrane
Selectively permeable
Mechanism that actively transports sodium ions out of the cell while drawing in two potassium ions
Sodium-potassium pump
Difference in distribution of ions across the neuron’s membrane
Concentration gradient
Messages sent by axons
Action potentials
Increased polarization across a membrane
Hyperpolarization
Example: using an electrode to apply a negative charge further increases the negative charge inside the neuron. When the stimulation ends, the charge returns to its original resting level
To reduce polarization towards zero across a membrane
Depolarize or depolarization
Example: applying a small depolarizing current to a neuron reduces its polarization towards zero
Minimum amount of membrane depolarization necessary to trigger an action potential
Threshold of excitation
When the potential reaches the threshold, the membrane opens it sodium channels and permits sodium ions to flow into the cell. The potential shoots up far beyond the strength of the stimulus.
The peak of the action potential varies from one axon to another, but is consistent for a given axon
Membrane channel whose permeability to sodium or some other ion depends on the voltage difference across the membrane
Voltage-gated channel
Drugs that attach to the sodium channels of the membrane, stopping action potentials
Local anaesthetics
Principle that the amplitude and velocity of an action potential are independent of the stimulus that initiated it
All-or-none law
Time when the cell resists the production of further action potentials
Refractory period
A time when the membrane is unable to produce an action potential
Absolute refractory period
Time after the absolute refractory period that requires a stronger stimulus to initiate an action potential
Relative refractory period
Describe sodium and potassium channels when the membrane is at rest
When the membrane is at rest, the sodium channels are closed, preventing almost all sodium flow. Potassium channels are nearly but not entirely closed, so potassium flows slowly
Describe the concentration of sodium and potassium ions due to the sodium-potassium pump
The sodium-potassium pump is an active transport that requires energy. As a result of the sodium-potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are similarly more concentrated inside than outside.
The sodium-potassium pump is affective only because of the selective permeability of the membrane, which prevents the sodium ions that were pumped out of the neuron from leaking right back in again. When sodium ions are pumped out, they stay out. However, some of the potassium ion is pumped into the neuron slowly leak out, carrying a positive charge with them. That leakage increases the electrical gradient across the membrane.
Describe the two forces that act on sodium when the neuron is at rest.
The electrical gradient and the concentration gradient which tend to push sodium into the cell
Electrical gradient: sodium is positively charged and the inside of the cell is negatively charged – opposite electrical charges attract, so the electrical gradient tends to pull sodium into the cell
Concentration gradient: the difference and distribution of ions across the membrane – sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell then to leave it. However, very few sodium ions across the membrane except by the sodium-potassium pump
Describe how the electrical and concentration gradients act on potassium when the neuron is at rest.
Electrical gradient: potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in.
Concentration gradient: potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out. If the potassium channels were wide open, potassium would have a small net flow out of the cell. That is, the electrical gradient and concentration gradient for potassium are almost in balance, but not quite. The sodium-potassium pump has more potassium into the cell as fast as it flows out of the cell, so the two gradients cannot get completely in balance.
When the membrane is at rest, are the sodium ions more concentrated inside the cell or outside? Where are the potassium ions more concentrated?
Sodium ions are more concentrated outside the cell; potassium is more concentrated inside
When the membrane is at rest, what tends to drive the potassium ions out of the cell? What tends to draw them into the cell?
When the membrane is at rest, the concentration gradient tends to drive potassium ions out of the cell; the electrical gradient draws them into the cell. The sodium-potassium pump also draws them into the cell
Why does the neuron use considerable energy to produce a resting potential?
The resting potential prepares the neuron to respond rapidly. Excitation of the neuron open channels that allow sodium to enter the cell rapidly. Because the membrane did it’s work in advance by maintaining the concentration gradient for sodium, the cell is prepared to respond vigourously to a stimulus. It is like an archer who pulls the bow in advance and then waits to fire at the appropriate moment
What is the difference between a hyperpolarization and a depolarization?
A hyperpolarization is an exaggeration of the usual negative charge within a cell to a more negative level than usual. A depolarization is a decrease in the amount of negative charge within the cell
What is the relationship between the threshold and an action potential?
A depolarization that passes the threshold produces an action potential. One that falls short of the threshold does not produce an action potential
Three principles helpful for remembering the events behind the action potential:
- At the start, sodium ions are mostly outside the neuron and potassium ions are mostly inside
- When the membrane is depolarized, sodium and potassium channels in the membrane open
- At the peak of the action potential, the sodium channels close
Describe the events behind an action potential and what happens to sodium and potassium, which are regulated by voltage-gated channels
At the resting potential, the sodium channels are close to permitting no sodium to cross and the potassium channels are almost closed allowing only a little flow of potassium.
As the membrane becomes depolarized, both the sodium and potassium channels begin to open, allowing for your flow. At first, opening the potassium channels makes little difference, because the concentration gradient and electrical gradient are almost in balance anyway. However, opening the sodium channels makes a big difference, because both the electrical gradient and the concentration gradient tend to drive sodium ions into the neuron. When the depolarization reaches the threshold of the membrane, the sodium channels open wide enough for sodium to flow freely. Driven by both the concentration gradient and the electrical gradient, the sodium ions into the cell rapidly, until the electrical potential across the membrane passes beyond zero to a reversed polarity.
Because of the persistent concentration gradient, sodium ions should still tend to diffuse into the cell. However, at the peak of the action potential, the sodium gates snapshot and resist re-opening for the next millisecond.
The depolarization of the membrane also opens potassium channels. At first, opening those channels made little difference. However, after so many sodium ions across the membrane, the inside of the cell has a slight positive charge instead of its usual negative charge. At this point both the concentration gradient and the electrical gradient drive potassium ions out of the cell. As a flow out of the axon, they carry with them a positive charge. Because the potassium channels remain open after the sodium channels clothes, enough potassium ions leave to drive the membrane beyond its usual resting level to a temporary hyperpolarization.
At the end of this process, the membrane has returned to its resting potential, but the inside of the neuron has slightly more sodium ions and slightly fewer potassium ions than before. Eventually, the sodium-potassium pump restores the original distribution of ions, but that process takes time.
During the rise of the action potential, do sodium ions move into the cell or out of it? Why?
During the action potential, sodium ions move into the cell. The voltage-dependent sodium gates have opened, so sodium can move freely. Sodium is attracted to the inside of the cell by both an electrical and a concentration gradient
As the membrane reaches the peak of the action potential, what brings the membrane down to the original resting potential?
After the peak of the action potential, potassium ions exit the cell, driving the membrane back to the resting potential. It is important to note that the sodium-potassium pump is not responsible for returning the membrane to its resting potential. The sodium-potassium pump is too slow for this purpose
State the all-or-none law
According to the all-or-none law, the size and shape of the action potential are independent of the intensity of the stimulus that initiated it. That is, every depolarization beyond the threshold of excitation produces an action potential of about the same amplitude and velocity for a given axon.
Does the all-or-none law apply to dendrites? Why or why not?
The all-or-none law does not apply to dendrites because they do not have action potentials
Suppose researchers find that axon A can produce up to 1000 action potentials per second, but axon B can never produce more than 100 per second. What can we conclude about the refractory period of the two axons?
Axon a must have a shorter absolute refractory period, about 1 ms, whereas B has a longer absolute refractory period, about 10 ms
A swelling where the axon exits the cell body
Axon hillock
Transmission of an action potential down an axon
Propagation of the action potential
An insulating material composed of fats and proteins
Myelin
Axons covered with myelin sheaths
Myelinated axons
The jumping of action potentials from node to node
Saltatory conduction
In addition to providing rapid conduction of impulses, saltatory conduction conserves energy: instead of admitting sodium ions at every point along the axon and then having to pump them out via the sodium-potassium pump, a myelinated axon admits sodium only at its nodes
Neurons without an axon
Local neurons
Neurons without an axon exchange information with only their closest neighbors. Because they do not have an axon, they do not follow the all or none law. When a local neuron receives information from other neurons, it has a graded potential, a membrane potential that varies in magnitude in proportion to the intensity of the stimulus. The change in membrane potential is conducted two adjacent areas of the cell, in all directions, gradually decaying as it travels. Those various areas of the cell contact other neurons, which the excite or inhibit through synapses
A membrane potential that varies in magnitude in proportion to the intensity of the stimulus
Graded potential
In a myelinated axon, how would the action potential be affected if the nodes were much closer together? How might it be affected if the nodes were much farther apart?
If the nodes were closer, the action potential would travel more slowly. If they were much farther apart, the action potential would be faster if it could successfully jump from one node to the next. When the distance becomes too great, the current cannot diffuse from one node to the next and still remain above threshold, so the action potentials would stop.
