Chapter 5 - Physical and Electrical Properties of Cells in the Nervous System Flashcards
What are the 4 main components of a neuron?
Soma, Dendrites, Axon, and Presynaptic Terminals
Soma
The cell body. It maintains the cell and keeps it functioning.
(e.g. synthesizes proteins used as neurotransmitters)
Dendrites
Branchlike extensions that serve as the main input sites for the cell.
Axon
The output unit of the cell, specialized to send information to other neurons, muscle cells, or glands.
Presynaptic Terminals
Transmitting elements of the neuron
Neurons transmit information about their activity via neurotransmitters from the ___________ to the ___________.
Presynaptic terminals, synaptic cleft
____________ is the space between neurons, and serves as the site for interneuronal communication,
Synaptic cleft
Axoplasm
The cytoplasm within an axon
Axoplasmic transport
Mechanism for transporting neurotransmitters substances within an axon
What are the two directions involved in axoplasmic transport?
Anterograde and Retrograde
Anterograde
From the SOMA toward the PRESYNAPTIC TERMINAL
Retrograde
From the SYNAPSE back to the SOMA
What causes axoplasmic transport to slow down in speed?
Aging and also in neurodegenerative diseases such as Alzheimer’s, Huntington’s, ALS, etc.
In vertebrates, what are the 2 groups of cells that are classified based on number of processes arising from the soma?
Bipolar and Multipolar cells
Bipolar cells
Have two primary processes that extend from the cell body. The dendritic root divides into multiple dendritic branches, and the axon projects to form its presynaptic terminals.
Example: the retinal bipolar cell in the eye
Multipolar cells
Most common cells in the vertebrate nervous system with a variety of shapes and organizations.
- Have multiple dendrites arising from many regions of the cell body and a single axon.
- They are specialized to receive and accommodate huge amounts of synaptic input to their dendrites.
Example: spinal motor neuron, which projects from the spinal cord to innervate skeletal muscle fibers.
“Typical spinal motor cell receives approximately 8,000 synapses on its dendrites and 2,000 synapses on the cell body itself”
What are the subclass of bipolar cells? Describe it.
Pseudounipolar cells appear to have a single projection from the cell body that divides into two axonal roots. They also have TWO AXONS and NO TRUE DENDRITES.
Neurons function via rapid changes in the ______________ across the cell membrane.
electrical potential
What is electrical potential?
The distribution of ions across a membrane which creates a difference in the electrical charge on each side of the cell membrane
Membrane channels
Openings that allow ions to flow across the membrane. May be opened and closed. When open, ions diffuse through the membrane.
Leak channels
A small number of ions leak at a slow, continuous rate
Gated channels
Open in response to a stimulus and close when the stimulus is removed
What are the two types of membrane channels?
Leak and gated channels
What are the 3 types of gated channels?
Modality-gated, ligand-gated, voltage-gated channels
Modality-gated channels
Open in response to mechanical forces, temperature changes, or chemicals
Ligand-gated channels
Open in response to a neurotransmitter binding to the surface of a channel receptor on a postsynaptic cell membrane
Voltage-gated channels
Open in response to changes in the electrical potential across the cell membrane
What are the three types of electrical potentials in neurons that are essential for transmitting information?
Resting membrane, local, and action potential
What is resting membrane potential?
The electrical potential across the membrane that exists when a neuron is at rest (i.e. not transmitting information)
Unequal distribution of the ionic charge across the membrane is essential for the neurons to be excitable.
Steady-state condition
No net flow of ions across the membrane
Cell at its resting membrane potential will have no _______.
Net change in the total distribution of ions across the two sides, although individual ions may move across through leak channels.
What two forces determine ion distribution across the plasma membrane?
Concentration gradient (chemical concentration) and electrical gradient (charged ion distribution)
What controls the movement of ions?
Opposing chemical and electrical forces
When does equilibrium occur?
When there is no net movement ions across the membrane
How is electrochemical gradient maintained?
- Anions trapped inside the neuron that are too large to diffuse through the channels
- Passive diffusion of ions through leak channels
- Na+/K+ pump
What is the Na+/K+ pump?
- Uses energy from ATP to move ions across the membrane against their electrochemical gradient
- Carries two K+ into the cell and three Na+ out of the cell with each cycle
- As long as the cell has ATP, an unequal distribution of K+ and Na+ can exist across the membrane
When sudden, brief changes in membrane potential occur, the membrane can become __________ or ___________.
Depolarized, hyperpolarized
Depolarized
The potential becomes LESS NEGATIVE than the resting potential
INCREASES the likelihood that the neuron will generate a transmittable electrical signal and is excitatory
Hyperpolarized
The potential becomes MORE NEGATIVE than the resting potential
DECREASES the neuron’s ability to generate an electrical signal, and is inhibitory
Changes in membrane potential can result in _________ and __________.
Local potentials, action potentials
Change in membrane potential occurs when…
ion channels open to selectively allow specific ions to pass.
Sensory neurons
receiving sites are the sensory receptors
Motor and interneurons
receiving sites are on postsynaptic membranes (often dendrites)
The information causes the membrane to become _______ or _________.
depolarized, hyperpolarized
Local potential
Initial change in membrane potential at receiving site
Spreads passively…
only a short distance along the membrane
Peripheral receptors have…
modality-gated channels
Local receptor potentials
generated when the peripheral receptors of a sensory neuron are stretched, compressed, deformed, or exposed to thermal or chemical agents
Local synaptic potentials
generated in motor neurons and interneurons when they are stimulated by input from other neurons
Local potentials generally travel __ to __ mm.
Amplitude (strength) _______ with distance traveled.
1, 2
decreases
Strength can be increased and potentials integrated via _______ and ________ summation.
Temporal, spatial
Temporal summation
combined effect of a series of small potential changes that occur within milliseconds of each other
Spatial summation
potentials generated in different regions of the neuron are added together
A significant change in membrane potential can result from…
summation of multiple local potentials
If summation of multiple local potentials causes the membrane potential to depolarize to a certain threshold an ________ is generated.
action potential
Action potential
Brief, large depolarization in electrical potential that is repeatedly regenerated
Action potential is all-or-none
- Every time sufficient stimuli are provided so that threshold is reached, an action potential will be produced.
- Stronger stimuli produce action potentials of the same voltage and duration as the minimally sufficient stimuli
A large, depolarizing signal is actively propagated by…
repeated generation of a signal.
Information is transmitted longer distances than local (receptor or synaptic) potentials
Meaning of the signal is determined by the neural pathway
Threshold stimulus intensity
stimulus intensity sufficient to produce an action potential
Action potential is produced by a sequence of 3 events. List them.
- Rapid depolarization (voltage-gated Na+ channels open)
- Decrease in Na+ conduction (Na+ channels closed)
- Rapid repolarization (voltage-gated K+ channels open)
15 mV depolarization is typically sufficient to trigger an action potential.
Change in membrane potential -70 mV to -55 mV.
Refractory period
Period of hyperpolarization
- membrane potential is even more negative than during rest
- difficult to initiate a subsequent action potential
Refractory period is defined by the…
ion channels
What adaptations make action potential progression faster?
increased diameter of the axon and myelination
What is the water and hose analogy?
A wider hose will pour more water in less time. Tape (myelination) around a leaky hose will concentrate the flow.
Myelin
A sheath containing fat that surrounds an axon
What does myelin provide?
Insulation
What does myelin prevent?
current flow across the axonal membrane
What does myelin increase?
The speed of action potential propagation and the distance a current can passively spread
What type of myelin leads to faster conduction?
Thicker
Nodes of Ranvier
- small patches on myelinated axons that lack myelin
- distributed every 1 to 2 mm along the myelinated axon and contain high densities of Na+ and K+ channels
- allow ion flow across the membrane
Saltatory conduction
quick node-to-node jumping of action potential down a myelinated axon
Saltatory conduction
quick node-to-node jumping of action potential down a myelinated axon
Afferent neurons
carry information TOWARD the CNS
Efferent neurons
carry commands FROM THE CNS outward (to muscles and glands)
Interneurons
transmit impulses between other neurons
process information locally or convey information short distances; largest class of neurons
Convergence
multiple inputs from a variety of cells terminate on a single neuron
Divergence
single neuron with many branches that terminate on a multitude of cells
________ and ________ contribute to the distribution of information through the nervous system.
Convergence, divergence
Glial cells
cells that form a critical support network for neurons; also transmit information
Glial cells are categorized by size and function
Microglia (small)
Macroglia (large)
Microglial cells
- act as the CNS immune system
- activated during CNS development and following injury, infection, or disease
- proteins in dying cells attract microglia to the site; microglia clean up and remove debris
Macroglial cells are classified into 3 groups
Astrocytes, oligodendrocytes, schwann cells
Astrocytes
Star-shaped cells
Act as scavengers, taking up extra K+ ions in the extracellular environment, removing chemical transmitters from synaptic clefts, and cleaning up other debris in the extracellular space
Oligodendrocytes and Schwann cells form __________.
myelin sheaths
Oligodendrocytes myelinate neurons in the _______ and Schwann cells myelinate neurons in the ________.
CNS, PNS
Peripheral neuropathy
- Any pathologic change involving peripheral nerves
- Often involves destruction of the myelin surrounding the largest, most myelinated sensory and motor fibers
- Results in disrupted proprioception (awareness of limb position) and weakness
Guillain-Barre Syndrome
• Involves acute inflammation and demyelination of peripheral sensory and motor fibers
• Typically occurs after a mild infection; in 2/3 of cases it is preceded by an intestinal infection that activates the
immune system causing production of an antibody that mistakenly cross reacts with the myelin sheath
• Weakness is typically greater than sensory loss; may have pain or hypersensitivity to touch
• Patients may have difficulty with chewing, swallowing, speaking, and facial expressions if cranial nerves affected; weakness of respiratory muscles may require
ICU/ ventilator (25%)
• Onset is rapid, but followed by plateau then gradual recovery; recovery is usually complete
• Treatment may include plasmapheresis and
intravenous immunoglobulin therapy; PT and OT are often recommended
Multiple Sclerosis
- Immune system produces antibodies that attack oligodendrocytes, producing plaques in the white matter of the CNS
- Signs and symptoms: weakness, lack of coordination, impaired vision, double vision, impaired sensation, and slurred speech; disruption of memory and emotions also possible
- Diagnosis is difficult; MS usually manifests with one sign that may completely resolve
- Onset common between 20 and 40 years of age; women 3x more frequently affected
- 4 types of MS, named according to the course of disease progression:
- Relapsing/remitting
- Secondary progressive
- Primary progressive
- Progressive relapsing
Neural stem cells
Immature and undifferentiated cells, precursors to both neurons and glial cells
Characteristics of neural stem cells include the ability to:
- self-renew, differentiate into most types of neurons and glial cells
- populate developing and degenerating regions of the CNS
Neural stem cells in the healthy mature brain are involved in forming ________ and _________.
memories, learning new tasks
excitement too over the possible rehabilitation implications
Which of the following is NOT true about action potentials?
They are followed by a period of inability to fire.
They are larger in response to stronger stimuli.
They travel faster in larger axons.
They travel faster in myelinated axons.
They are larger in response to stronger stimuli.
Oligodendrocytes are found in the ________ while Schwann cells are found in the ________.
Central nervous system; autonomic nervous system.
Central nervous system; peripheral nervous system.
Peripheral nervous system; autonomic nervous system.
Peripheral nervous system; central nervous system.
Central nervous system; peripheral nervous system.
