Neurophysiology I

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Questions and Answers

What triggers the exocytosis of neurotransmitters in response to an action potential?

Influx of potassium ions

Hyperpolarization of the membrane

Opening of sodium channels

Influx of calcium ions (correct)

What happens to small molecule neurotransmitters after they fulfill their role in synaptic transmission?

They bind permanently to receptors

They are released into circulation

They are recycled by the presynaptic neuron (correct)

They are degraded by intracellular enzymes

How does inhibition occur at a neuron-neuron synapse?

Through the influx of calcium ions

By blocking calcium channels

By the outflow of K+ ions (correct)

Through the entry of Na+ ions

What is required for an excitatory postsynaptic potential (EPSP) to reach the threshold for an action potential?

Impulses from multiple presynaptic neurons Signup and view all the answers

What can happen to receptors after neurotransmitter binding in neuromuscular synapses?

They can be desensitized Signup and view all the answers

What is the primary function of sensory neurons?

To carry signals from the PNS to the CNS Signup and view all the answers

Which type of neuron primarily integrates incoming and outgoing information?

Interneurons Signup and view all the answers

Which of the following describes the axon in relation to nerve cells?

Transmits information away from the cell body Signup and view all the answers

What is the role of oligodendrocytes in the nervous system?

To form myelin sheaths around axons Signup and view all the answers

Which classification of neurons is mainly found in sensory organs?

Bipolar neurons Signup and view all the answers

What is the primary characteristic of glial cells compared to neurons?

They are more abundant than neurons Signup and view all the answers

Which statement best explains the function of astrocytes in the nervous system?

To secrete glutamate and modulate excitability Signup and view all the answers

How do neurons primarily obtain glucose and oxygen?

Via transport from glial cells and blood vessels Signup and view all the answers

What does grey matter primarily consist of?

Cell bodies of neurons Signup and view all the answers

What structure is responsible for the white appearance of white matter?

Myelin sheaths Signup and view all the answers

Where are the cell bodies of sensory neurons located?

In ganglia outside the CNS Signup and view all the answers

What is the primary role of the Na+/K+ pump in maintaining resting membrane potential?

Creating a charge imbalance across the membrane Signup and view all the answers

What is the average resting membrane potential in nerve cells?

-70 to -90 mV Signup and view all the answers

What are the interruptions in myelin sheaths known as?

Nodes of Ranvier Signup and view all the answers

What is a characteristic of myelinated fibers compared to unmyelinated fibers?

Faster action potential transmission Signup and view all the answers

Which of the following contributes to the maintenance of resting membrane potential?

Selective permeability of the membrane Signup and view all the answers

What accounts for the positive charges accumulating outside the resting membrane of a neuron?

High permeability to K+ ions Signup and view all the answers

How does the Na+/K+ pump function in maintaining ion concentration within the cell?

Pumps 3 Na+ out and brings 2 K+ in Signup and view all the answers

What is the role of voltage-gated Na+ channels during depolarization?

They open to allow Na+ to rush into the cell Signup and view all the answers

What happens during the refractory period of a neuron?

Resting membrane potential (RMP) is restored Signup and view all the answers

What principle does the all-or-none rule in nerve cells imply?

Action potentials are generated once the threshold is met Signup and view all the answers

What is the primary function of neurotransmitters in the nervous system?

To convert electrical signals from action potentials into chemical signals Signup and view all the answers

How does saltatory conduction differ in myelinated axons compared to unmyelinated axons?

Current jumps between nodes in myelinated axons Signup and view all the answers

What initiates the opening of voltage-gated ion channels in neurons?

Stimulus reducing the membrane potential Signup and view all the answers

Why are K+ channels slower to open compared to Na+ channels during an action potential?

K+ channels are voltage-gated but have delayed response Signup and view all the answers

What characterizes chemical synapses compared to electrical synapses?

Transmission solely through neurotransmitters Signup and view all the answers

What primary factor affects the conduction velocity of a nerve impulse?

Thickness of myelin and diameter of the fiber Signup and view all the answers

What is the primary purpose of the K+ voltage gated channels during repolarization?

To maintain resting membrane potential Signup and view all the answers

What occurs at the synaptic cleft during neurotransmission?

Neurotransmitters bind to receptors on the post-synaptic membrane Signup and view all the answers

Which statement is true regarding neurons' energy requirements?

Constant supply of glucose and oxygen is needed Signup and view all the answers

What effect does Ca2+ influx have during neurotransmitter release?

It triggers exocytosis of neurotransmitters Signup and view all the answers

Excitatory synapses result in the entry of Cl- ions into the postsynaptic cell.

False Signup and view all the answers

What is the process called when neurotransmitters are internally taken up by the presynaptic neuron for recycling?

Endocytosis Signup and view all the answers

After neurotransmitter binding, the receptor can become __________.

desensitized Signup and view all the answers

Match the type of synapse with its function:

Excitatory synapse = Depolarization Inhibitory synapse = Hyperpolarization Neuropeptide = Internalization by postsynaptic cell Small molecule neurotransmitter = Recycled by presynaptic neuron Signup and view all the answers

What type of matter corresponds to bundles of neuron processes?

White matter Signup and view all the answers

The resting membrane potential (RMP) in nerve cells is typically around -70 to -90 mV.

True Signup and view all the answers

What are the cells that form myelin in the central nervous system called?

oligodendrocytes Signup and view all the answers

The ______ allows faster transmission of action potentials in myelinated fibers.

myelin sheath Signup and view all the answers

Match the following components with their function:

Na+/K+ pump = Maintains resting membrane potential Myelin = Insulates axons Ganglia = Contains cell bodies of sensory neurons Nodes of Ranvier = Facilitates rapid action potential propagation Signup and view all the answers

What structure allows depolarization to occur in myelinated fibers?

Nodes of Ranvier Signup and view all the answers

Grey matter only consists of neuron axons.

False Signup and view all the answers

What gives the white appearance to white matter?

myelin sheaths Signup and view all the answers

Which type of neuron is primarily involved in relaying information between other neurons within the CNS?

Interneurons Signup and view all the answers

Glial cells are less abundant than neurons in the nervous system.

False Signup and view all the answers

What are the two main processes of a neuron called?

Axons and dendrites Signup and view all the answers

The structure of neurons that integrates incoming and outgoing information is called the ______.

cell body Signup and view all the answers

Which class of glial cells is responsible for myelin formation?

Oligodendrocytes Signup and view all the answers

Match each neuron classification with its primary characteristic:

Multipolar = Mainly found in CNS Pseudounipolar = Mainly in PNS Bipolar = Found in sensory organs Sensory neurons = Transmit signals to CNS Signup and view all the answers

The axon of a neuron carries information toward the cell body.

False Signup and view all the answers

Name the two primary functions of neurons.

Transmit information and integrate signals Signup and view all the answers

What is the primary role of the Na+/K+ pump?

To maintain ion concentration and resting membrane potential Signup and view all the answers

Neurons can store glucose and oxygen for later use.

False Signup and view all the answers

What occurs during the repolarization phase of an action potential?

K+ channels open and K+ ions flow out of the cell. Signup and view all the answers

The period during which a neuron cannot be stimulated again after an action potential is called the ______.

refractory period Signup and view all the answers

Match the following ion channels with their functions:

Voltage-gated channels = Open in response to changes in membrane potential Ligand-gated channels = Open in response to neurotransmitter binding K+ channels = Facilitate repolarization of the membrane Na+ channels = Facilitate depolarization of the membrane Signup and view all the answers

Which statement best describes the all-or-none rule?

Once threshold is reached, an action potential is generated with a fixed amplitude. Signup and view all the answers

During saltatory conduction, action potentials occur at every point along the axon.

False Signup and view all the answers

What ion primarily contributes to depolarization during an action potential?

Na+ Signup and view all the answers

Neurotransmitters are released into the ______ to transmit signals between neurons.

synaptic cleft Signup and view all the answers

Which type of synapse is most common in vertebrates?

Chemical synapse Signup and view all the answers

Match the types of neurons with their functions:

Sensory neurons = Carry signals from sensory receptors to the central nervous system Motor neurons = Transmit signals from the central nervous system to effectors Interneurons = Connect sensory and motor neurons Efferent neurons = Send signals away from the central nervous system Signup and view all the answers

Myelinated axons transmit action potentials more slowly than unmyelinated axons.

False Signup and view all the answers

What role does acetylcholine play in the neuromuscular synapse?

It serves as a neurotransmitter to transmit signals from nerve to muscle. Signup and view all the answers

The process by which an action potential moves along an unmyelinated axon is called ______.

continuous conduction Signup and view all the answers

Study Notes

Nervous System Organization

The nervous system is composed of the central nervous system (CNS) and the peripheral nervous system (PNS).
The PNS is divided into the afferent and efferent systems.
The afferent system receives sensory information from the body and transmits it to the CNS.
The efferent system carries motor commands from the CNS to the muscles and glands.

Nervous Tissue Composition

Nervous tissue is composed of neurons and glial cells.
Neurons are nerve cells that transmit information.
Neurons have a cell body, an axon, and dendrites.
Axons transmit information away from the cell body.
Dendrites receive information from other neurons.
The cell body integrates information from both the axon and dendrites.
Neurons are categorized by the number of processes they have.
- Multipolar: mainly in the CNS (top)
- Pseudounipolar: mainly in the PNS (middle)
- Bipolar: mainly in sensory organs (bottom)

Neuron Function

Neurons are also classified by their function.
- Sensory (afferent): from the PNS to the CNS
- Motor (efferent): from the CNS to muscles and glands
- Interneurons (association): relay information between neurons within the CNS
- Specialized "receptors": transducers that convert stimuli into signals

Glial Cells

Glial cells are non-neuronal cells that are 10 times more abundant than neurons.
They provide structural support to nervous tissue.
Glial cells participate in myelin formation (oligodendrocytes).
Some glial cells possess phagocytic activity (microglia).
Glial cells contact blood vessels and neurons, transporting nutrients to neurons.

White and Grey Matter

Grey matter corresponds to cell bodies.
White matter corresponds to bundles of neuron processes with myelin sheaths.
Nerves are bundles of axons that run from or to the CNS.
Cell bodies of sensory neurons are located in clusters called ganglia, outside of the CNS.
Cell bodies of motor neurons are located in well-defined areas of the CNS.

Myelin Sheaths

Myelin is a white lipid (sphingomyelin) that wraps around nerve fibers.
It is formed by glial cells wrapped around the axon.
Myelin prevents ion leakage and acts as an electrical insulator.
Gaps in the myelin sheath are called nodes of Ranvier.
Myelinated fibers transmit action potentials faster than unmyelinated fibers.

Membrane Potential

Every cell in the body possesses a membrane potential, known as the Resting Membrane Potential (RMP).
The RMP is the difference in charge across the cell membrane between the cytosol and the extracellular fluid.
The inside of the membrane is negative relative to the outside.
The absolute value of the RMP varies between cell types.
The average RMP in a nerve cell is around -70 to -90 mV.
Intra- and extracellular compartments are electroneutral.

Maintenance of the RMP

The RMP is maintained by a combination of selective permeability and the Na+/K+ pump.
Selective permeability allows for passive leakage of ions through channels based on their concentration gradients.
The Na+/K+ pump pumps 3 Na+ ions out of the cell and brings 2 K+ ions in.
The Na+/K+ pump requires a lot of energy, up to 40% of ATP availability.

Excitable Cells

Cells that can generate electrical impulses (action potentials) are called excitable cells.
They need to be stimulated.
Chemical, electrical or physical stimulation can induce change in membrane potential to reach a threshold.
If Na+ channels open, Na+ rushes inside the cell, depolarizing the membrane.
Subsequent opening of K+ channels results in an outflow of K+ repolarizing the membrane.

Action Potentials in Neurons

An initial depolarization needs to reach threshold to open voltage-gated Na+ channels.
Na+ channels close rapidly after about 0.5 ms.
K+ channels open, causing an outflux of K+.
K+ channels progressively close, causing hyperpolarization.
All gated channels close, ions rejoin their respective compartments by diffusion and the Na+/K+ pump.
Neurons cannot be re-stimulated until the RMP is restored (refractory period).

All-or-none Rule

Nerve cells follow the all-or-none rule.
When threshold is met, an action potential is generated.
The amplitude of the action potential is fixed for each cell.
Intensity is encoded by the frequency of action potentials, not their amplitude.

Conduction of Action Potentials

Depolarization and repolarization processes propagate along the cell membrane.
The change in potential needs to reach threshold on the nearby microdomain to trigger opening of gated channels.
In unmyelinated axons, the action potential propagates along the entire length of the axon.
In myelinated axons, the action potential occurs only at the nodes of Ranvier.
Myelinated axons transmit action potentials faster than unmyelinated axons because less membrane is affected, and less energy is required to transport ions.

Synaptic Transmission

Synaptic transmission allows for continuity of signal between neurons or between a neuron and a target cell.
A gap exists between pre- and post-synaptic cell membranes called the synaptic gap or cleft.
Vertebrate neurons are predominantly chemical synapses.

Neurotransmitters

Neurotransmitters are molecules that transmit information from one neuron to another.
They are released by the pre-synaptic neuron into the synaptic gap, bind to specific receptors on the post-synaptic membrane, and elicit a response.
Neurotransmitters are classified based on their molecular size and composition.
- Small molecules: synthesized in nerve terminals by specific enzymes.
- Neuropeptides: synthesized in the cell body, packaged in secretory vesicles, and transported to the site of release.

General Mechanism of Action

Action potential opens voltage-gated Ca2+ channels, allowing calcium to enter the axon terminal.
Calcium triggers exocytosis of neurotransmitter vesicles.
Neurotransmitter diffuses across the synaptic gap and binds to receptors on the post-synaptic cell.
This binding opens ion channels on the post-synaptic membrane, causing depolarization.
Neurotransmitter inactivation terminates the signal.

Termination of Transmission

For small molecules:
- The neurotransmitter can be picked up by the pre-synaptic neuron via endocytosis and recycled.
- Enzymatic degradation in the synaptic cleft by enzymes released by the post-synaptic cell (e.g., acetylcholine esterase).
For neuropeptides:
- The neurotransmitter can be internalized by the post-synaptic cell via endocytosis and be degraded by cellular enzymes.
- Extracellular peptidase in the synaptic gap can break down the neurotransmitter.
Receptors can be desensitized.

Integration of Multiple Synapses

A single neuron can receive input from multiple other neurons.
Synapses can be excitatory or inhibitory.
A single impulse does not always lead to a response.
Excitatory synapses cause depolarization by the entry of Na+.
Inhibitory synapses cause hyperpolarization by the entry of Cl- and/or the outflow of K+.

Nervous System Organization

Nervous system is composed of afferent (sensory) and efferent (motor) nerves.

Nervous System Composition

Nervous tissue is composed of neurons and glial cells.

Neurons

Neurons are responsible for transmitting information.
Neurons are composed of a cell body, axon and dendrites.
Axons transmit information away from the cell body.
Dendrites transmit information towards the cell body
The cell body integrates incoming and outgoing information.
Neurons are categorized by the number of processes:
- Multipolar neurons have multiple processes, mainly in the CNS.
- Pseudounipolar neurons have one process that branches into two, mainly in the PNS.
- Bipolar neurons have two processes, mainly in sensory organs.
Neurons are also classified by function:
- Sensory neurons (afferent) transmit information from the PNS to the CNS.
- Motor neurons (efferent) transmit information from the CNS to muscles and glands.
- Interneurons (association) relay information between neurons within the CNS.
- Specialized "receptors" are transducers, converting stimuli into a signal.

Glial Cells

Glial cells are non-neuronal cells that provide structural support to nervous tissue.
Glial cells are ten times more abundant than neurons.
Oligodendrocytes participate in myelin formation in the CNS.
Astrocytes secrete glutamate, which can modulate the excitatory level of neurons.
Microglia possess phagocytic activity.
Glial cells contact both blood vessels and neurons, facilitating nutrient transport.
Neurons do not store glucose or oxygen, requiring a constant supply.

White and Grey Matter

Grey matter consists of cell bodies.
White matter consists of bundles of neuron processes, myelinated axons.
Nerves are bundles of axons that run from or to the CNS.
Ganglia are clusters of cell bodies of sensory neurons located outside the CNS.
Cell bodies of motor neurons are located in specific areas of the CNS, like the brain and spinal cord.

Myelin Sheaths

Myelin is a white lipid (sphingomyelin) that wraps around nerve fibers.
Myelin is formed by glial cells, which lose their cytoplasm, leaving layers of lipids.
Myelin is only found in white matter, not all nerve fibers.
Myelin acts as an electrical insulator, preventing ion leakage.
Nodes of Ranvier are interruptions in the myelin sheath, allowing for depolarization and transmission of action potentials.
Myelinated fibers transmit action potentials faster than unmyelinated fibers.

Membrane Potential

Every cell in the body possesses a resting membrane potential (RMP).
The RMP results from a difference in charge across the cell membrane.
The inside of the membrane is negative relative to the outside.
The absolute value of RMP varies between cell types, depending on the amount of charges, ion channels, and membrane thickness.
The average RMP in nerve cells is around -70 to -90 mV.
Intra- and extracellular compartments are electrically neutral.
Negative charges in the cytosol, carried by large organic molecules, are attracted to the membrane by positive charges on the outside.

Maintaining the RMP

The RMP is maintained by a combination of selective permeability, the Na+/K+ pump, and large anions trapped on the inner surface of the membrane.
Selective permeability allows for passive leakage of ions through channels based on diffusion.
The resting membrane is permeable to K+, but barely permeable to Na+, Ca2+, and Cl-, allowing positive charges to accumulate outside.
The Na+/K+ pump pumps 3 Na+ ions out and 2 K+ ions in, compensating for diffusion leakage.
The pump operates against concentration gradients and for both ions, against the membrane polarity.
The Na+/K+ pump requires significant energy, up to 40% of ATP availability.

Excitable Cells

Excitable cells can generate electrical impulses, called action potentials.
Excitable cells require stimulation, which can be chemical, electrical, or physical, to induce a change in membrane potential to reach a threshold.
The threshold triggers the opening of voltage-gated ion channels.
Opening of Na+ channels or Ca2+ channels in certain nerve endings, smooth muscles, and cardiac muscle cells, results in an influx of Na+ or Ca2+ ions, respectively.
This influx depolarizes the cell, making the potential less negative, then positive.
Subsequently, the opening of K+ channels leads to an outflow of K+, repolarizing the cell by returning the potential to RMP.

Generation of Action Potentials in Neurons

An initial depolarization (stimulation) must reach the threshold to trigger the opening of Na+ voltage-gated channels, causing depolarization.
After about 0.5 milliseconds, the opened Na+ channels close rapidly.
K+ voltage-gated channels open with a delay compared to Na+ channels, resulting in an outflow of K+ and repolarization.
The K+ voltage-gated channels progressively close, and K+ outflow continues after reaching the RMP, leading to hyperpolarization.
Once all gated channels are closed, ions rejoin their respective compartments by diffusion and Na+/K+ pump activity.
Neurons cannot be re-stimulated until the RMP is restored, known as the refractory period.

Voltage-Gated Na+ Channels

They have two gates: activation and inactivation.
The activation gate is electrically charged.
In a resting state, the activation gate is closed, and the inactivation gate is open.
During depolarization, the activation gate opens, and the inactivation gate remains open.
After a short delay, the inactivation gate closes.
During repolarization, the activation gate closes, but the inactivation gate takes longer to reopen.

Ion Gated Channels

There are several types of gated channels, including voltage-gated channels and ligand-gated channels.
Ligand-gated channels have binding sites for neurotransmitters.
Each channel is composed of several subunits with varying degrees of specificity.

All-or-None Rule

Nerve cells follow the all-or-none rule.
When the threshold is met, an action potential is generated.
The amplitude of the action potential is fixed for a particular cell.
The intensity of a signal is encoded by the frequency of action potentials, not the amplitude.

Conduction of Action Potential

Depolarization and repolarization processes of action potentials propagate along the cell membrane.
The change in potential must reach the threshold in the nearby microdomain to trigger the opening of gated channels.
In unmyelinated axons, action potentials propagate continuously.
In myelinated axons, action potentials only occur at the nodes of Ranvier, leading to saltatory conduction.
Myelin prevents ion leakage, allowing the current to jump from one node to another.
Saltatory conduction increases the velocity of action potential transmission because less membrane is affected, requiring less energy to transport ions.
Factors influencing nerve velocity:
- Thickness of myelin
- Diameter of the fiber (thicker= faster)
Velocity range for action potential transmission: 100 to 0.5 m/sec, 2500 to 250 impulses/sec.

Synaptic Transmission

Synaptic transmission ensures continuity of the signal between neurons or between a neuron and target cells.
Cell membranes are made of phospholipids, an electrical insulator.
A gap exists between pre- and post-synaptic cell membranes, called the synaptic gap or cleft.
In rare cases, direct continuity of electrical impulses occurs through gap junctions, found in cardiac and some smooth muscles.
In vertebrates, neuronal synapses are predominantly chemical synapses.

Neurotransmitters

Neurotransmitters are molecules that transmit information from one neuron to another.
They convert electrical signals (action potentials) into chemical signals.
Neurotransmitters are released by the presynaptic neuron into the synaptic cleft.
They bind to specific receptors on the postsynaptic membrane.
They elicit a response.
Neurotransmitters are classified based on their molecular size and composition:
- Small neurotransmitters:
  - Synthesized in the nerve terminals by specific enzymes.
  - Amino acid derivatives and biogenic amines are types of small neurotransmitters.
- Neuropeptides:
  - Synthesized in the cell body.
  - Packaged in secretory vesicles.
  - Transported to the site of release.

General Mechanism of Action, Case of the Neuromuscular Junction

Postsynaptic folding is common in the neuromuscular junction, increasing surface area.
Acetylcholine is the neurotransmitter at the neuromuscular junction.
Mechanism of action:
1. Action potential arrives at the presynaptic terminal.
2. The action potential opens voltage-gated Ca2+ channels, allowing Ca2+ to enter the cell.
3. Ca2+ triggers exocytosis of synaptic vesicles containing acetylcholine.
4. Acetylcholine diffuses across the synaptic cleft.
5. Acetylcholine binds to specific receptors on the postsynaptic membrane.
6. This binding opens ion channels on the postsynaptic membrane, leading to depolarization.
7. Acetylcholine is inactivated, ending the signal.

Termination of Synaptic Transmission

Termination occurs for both small molecule neurotransmitters and neuropeptides.
For small molecules:
- They are picked up by the presynaptic neuron via endocytosis and recycled for future use.
- They are deactivated in the synaptic cleft by enzymes released by the postsynaptic cell, for example, acetylcholinesterase.
For neuropeptides:
- After binding to its receptor, they may be internalized by the postsynaptic cell via endocytosis and degraded by cellular enzymes.
- They are broken down by extracellular peptidase in the synaptic cleft.
Receptors can be desensitized.

Integration of Multiple Synapses Between Neurons

In a neuromuscular junction, one neuron's action potential leads to muscle cell depolarization.
In neuron-neuron synapses:
- One neuron can receive impulses from multiple other neurons.
- Synapses can be excitatory or inhibitory.
- One impulse may not always lead to a response; the threshold must be reached.
An excitatory synapse causes depolarization with Na+ entry.
An inhibitory synapse causes hyperpolarization with Cl- entry and/or K+ outflow.

Excitatory and Inhibitory Synapses

Multiple excitatory synapses can summate (add up) to reach the threshold, triggering an action potential.
An inhibitory synapse can counteract excitatory signals, blocking the occurrence of an action potential.
This integration allows for fine-tuning of neuronal communication and complex decision-making processes in the nervous system.

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Explore the organization and composition of the nervous system in this quiz, covering both the central and peripheral nervous systems. Understand the roles of neurons and glial cells, as well as how sensory and motor commands are transmitted throughout the body.